United States

Department of

Agriculture

Forest Service

Techniques in Experimental

Mechanics Applicable to

Forest Products Research

Southern

Research Station

Proceedings of the Experimental Mechanics Plenary Session at

the Forest Products Research Society Annual Meeting

General Technical

I Report SO-125 Portland, ME - June 29, 1994
The use of trade or firm names in this publication is for reader information and does not imply

endorsement of any product or service by the U.S. Department of Agriculture or other

organizations represented here.

April 1998

Southern Research Station

P.O. Box 2680

Asheville, North Carolina 28802
Techniques in Experimental Mechanics Applicable

to Forest Products Research

Proceedings of the Experimental Mechanics Plenary

Session at the Forest Products Research

Society Annual Meeting

Portland, ME

June 29, 1994

Editors

Leslie H. Groom and Audrey G. Zink

USDA Forest Service, Southern Forest Experiment Station,

Pineville, LA

Conducted by

USDA Forest Service, Virginia Polytechnic Institute

and State University, and the Forest Products Society,

Blacksburg, VA

Contents

Page

Introduction to Techniques in Experimental Mechanics

Leslie H. Groom And Audrey G. Zink . . . . . . . . . . . . . . . . . . . . .

Strain Measurement on Wood Using Extensometers, Clip-on Electrical Transducers,

Electrical Resistance Strain Gauges, and Tell-Tale Gauges

Joseph R. Lojkrski. . . . . . . . . . . . . . . . . . . . . . . . . .

Image Correlation for Measuring Strain in Wood and Wood-based Composites

Audrey G. Zink, Robert W Davidson, and Robert B. Hunna . . . . . .

/Application of Imaging Technologies to Experimental Mechanics

Laurence Mott, Steven M. Shale< and Leslie H. Groom . . . . . . . . . . . .

Full Field Stress/Strain Analysis: Use of Moire and TSA for Wood Structural Assemblies

Ronald W. Wolfe, Robert Rowlands, and C.H. Lin . . . . . . . . . . . .

pharacterizing the Wood Fiber/Polymer Interface

T.G. Rials, M.P Wolcott, and D.J. Gardner . . . . . . . . . . . . . . . . . .

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

V

1

6

17

23

31

41

111

Introduction to Techniques in Experimental Mechanics

Leslie H. Groom and Audrey G. Zink

The title of this publication-Techniques in Experimental

Mechanics Applicable to Forest Products Research-is the

theme of this plenary session from the 1994 Annual Meeting

of the Forest Products Society (FPS). Although this session

focused on experimental techniques that can be of assistance

to researchers in the field of forest products, it is hoped that

the techniques and findings presented are beneficial to the

forest products industry as well. The presentations given

during the plenary session ranged from short-term, applied

research to long-term, fundamental research. Both types of

research are necessary for the health and productivity of the

forest products industry; applied research provides

information that can be immediately implemented to

improve product performance, yield, and/or reliability, while

fundamental research is instrumental in product

development and global competitiveness.

Research has long been at the heart of any type of industry

that relies on engineered products. This has been the case

with the forest products industry. There have been many

changes in technology over the past several years, due

primarily to the rapid advancements in digital processing.

One of the results from these advancements is a new

generation of instruments and algorithms that rely on the

collection and analysis of vast amounts of data, thus

allowing researchers to obtain information on stresses and

strains in wood and wood-based composites in manners

previously unattainable. Although it was impossible to

adequately cover all of the recent advancements in

equipment and techniques, the main objective of this

session was to present an overview of new categories of

equipment, how this equipment is used, what type of results

can be achieved, and where the research is being conducted.

Articles within this report are a compilation of the plenary

session presentations. Speakers and contributors were

Leslie H. Groom is a research technologist, USDA Forest Service,

Southern Research Station, 2500 Shreveport Hwy., Pineville, LA 71360;

Audrey G. Zink is an assistant professor, Department of Wood Science and

Forest Products, Virginia Polytechnic Institute and State University,

Blacksburg. VA 24061-0323.

selected on the basis of their knowledge of the subject

matter and respect given to them among their colleagues.

Due to the wide range of topics covered by the session, the

chairpersons determined that the session would best be

introduced with an overview of traditional strain

measurement. This overview, given by Joe Loferski,

focused on the available types of traditional strainmeasuring devices, the limitations associated with each

type, and attachment considerations. Joe gave many

examples of strain-measurement devices but concentrated

primarily on extensometers, clip-on electrical transducers,

electrical resistance strain gauges, and tell-tale gauges.

Although traditional strain-measurement devices are

generally inexpensive and readily available, they rely on

contact with the substrate under investigation.

An introduction into noncontact, full-field strain

measurement was the focus of the next three presentations.

Audrey Zink began the discussion of full-field strain

measurement with an introduction to digital image

correlation. Equipment and software necessary to produce

full-field strain maps were discussed. Although her

discussion focused on macrostrains of solid lumber, the

methods and techniques are applicable to all levels of scale.

This was elaborated upon by the third speaker, Laurence

Mott. His talk centered primarily on the development of

full-field strain maps of specimens in an environmental

scanning electron microscope. Laurence covered a wide

variety of topics such as conventional light microscopy,

scanning electron microscopy (SEM), environmental SEM,

digital image processing, and image acquisition. He also

discussed converting traditional load-elongation traces to

stress-strain curves by three-dimensional reconstruction

algorithms and confocal laser scanning microscopy. Fullfield strain analysis was the topic of the fourth speaker, Ron

Wolfe. Ron used a completely different approach from the

previous two speakers by measuring strains in a metal plate

connector by attaching a moire grid and digitally capturing

deformations within the grid. He also discussed full-field

stress analysis using stress pattern analysis (by measurement

of) thermal emissions, commonly referred to as SPATE’.

V
This type of analysis indirectly measures stresses by

monitoring minute changes in the temperature of the metal

plate connector under strain. Although SPATE shows much

promise, it should be noted that the apparatus is expensive

and that the technique may not be applicable to wood due

to the inherent low level of thermal emissions in wood.

The final speaker, Mike Wolcott, focused his presentation

not so much on material properties of components within a

composite, but rather on the interface between these

components. He discussed advantages and limitations of

the following micro-mechanical methods commonly used to

measure interfacial properties: the fragmentation test, the

micro-debond test, the fiber pull-out test, and dynamic

mechanical analysis. In addition to the discussion of the

techniques, he also presented results discussing the quality

of various fiber/matrix interfaces.

A handout outlining the presentations in the session was

available during the plenary session, but the supply was not

adequate to handle the large audience that attended. Thus,

the handout has been provided here in the form of an

Appendix. These tables summarize the techniques and

methods respectively discussed by all speakers during this

session. The tables list equipment discussed, relative cost

of the equipment, advantages and limitations of techniques,

and some possible applications. It should be noted that the

session was represented by industry and research. It is

hoped that this session provided useful information for

attendees, gave direction as to research tools, and generated

discussions that further forest products research and the

forest products industry.

vi
Strain Measurement on Wood Using Extensometers,

Clip-on Electrical Transducers, Electrical Resistance

Strain Gauges, and Tell-Tale Gauges

Joseph R. Loferski

Abstract

Laboratory and field experiments in wood engineering often rely on

different types of devices to measure strain. Each type has certain

limitations and characteristics that generally dictate its applicability to

wood. Some of the issues related to using traditional strain measurement

devices on wood and wood-based materials are discussed in this paper.

Introduction

Strain measurements in wood are used for verifying

analytical models of wood structures that monitor long-term

performance, analyze structural components, such as the linger

joints in glue-laminated beams, and, in general, study the

physical response of wood structural members to their loads.

The objective of this paper is to identify traditional

techniques that have been developed for measuring the

strain, especially in wood materials. Strain is a fundamental

engineering phenomenon that is produced as a response to

an applied stress and is defined as the change in length

caused by an applied stress divided by the original length.

From basic mechanics, Hooke’s Law is the constitutive

model that describes the relationship between stress and

strain in the linear range:

where

E=o/E

E = elastic modulus,

cr = applied stress, and

E = strain.

Because wood is nonhomogeneous, hygroscopic, and

anisotropic and has anatomical variability, attention to many

details is required to produce quality strain measurements,

which often is not necessary on isotropic, homogeneous

Joseph R. Loferski is an associate professor, Department of Wood Science

and Forest Products, Virginia Polytechnic Institute and State University,

Blacksburg, VA 24061-0503.

materials. Furthermore, because wood is an electrical and

thermal insulator, attention must be directed to heat

dissipation when using some strain-measurement devices.

A variety of strain-measuring devices is available to

researchers in wood mechanics and wood engineering for

use with wood and wood-based materials. The devices

include extensometers, clip-on electrical transducers

(CET’s), electrical resistant strain (ERS) gauges, tell-tale

gauges, and full-field strain measuring systems.

The first four methods for measuring strain on wood are

described here. Techniques for producing full-field strain

measurements are presented elsewhere in these proceedings.

Extensometers

Extensometers are devices that measure the distance

between two points on the surface of the object being

examined. They include dial gauges, linear variable

differential transducers (LVDT’s), and other similar devices.

The advantages of extensometers are that they are reusable

and they can provide fairly long gauge lengths so that the

average strain over a representative region of a test

specimen can be measured. Disadvantages of extensometers

are that many times they are large, heavy, and difficult to

attach to the specimen. Furthermore, the thermal expansion

of the materials used to construct the extensometer can

affect the strain measurements due to temperature changes

between measurements.

Figure 1 shows an extensometer consisting of an aluminum

bar with a conical point fixed to one end and a dial gauge

fixed to the other end. The plunger of the dial gauge is

fitted with another conical point. In use, the two conical

points are inserted into small metal tubes that are driven

into the surface of the specimen. The two tubes provide

exact locations at which subsequent readings can be taken.

By using an extensometer, gauge lengths of greater than 50

cm have been successfully produced (Lang 1993). The

accuracy of this device is related to the resolution and

accuracy of the dial gauge. Recently, digital dial gauges
SPACER BAR

\ SPECIMEN

TUBULAR INSERT

GAUGE LENGTH

\ TUBULARINSERT

Figure l-Extensometer showing top and side views (from Lang 1993)

with resolutions approaching 1 Oe4 mm have become

available.

Care must be taken when using this device to align the

extensometer perpendicular to the surface of the specimen

so that repeatable and accurate measurements can be made.

Clip-on Electrical Transducers

The CET is another type of extensometer that has many

applications in testing wood materials. Clip-on electrical

transducers are strain gauge transducers, which are

commercially produced in laboratories, and that when

properly fabricated, calibrated, aligned, and attached to the

specimen, are accurate and efficient for measuring strain in

wood structural members.

Loferski and others (1989) describe the development and

construction of CET’s for use in testing wood. The device

described has an accuracy of 1-0.0005 1 mm per each 55.4

mm gauge length. The CET is easily attached to the

specimen and is removable and reusable for testing of

multiple specimens. It is economical, custom built, and

capable of measuring both large and small strains. Figure 2

shows a schematic diagram of the construction and

characteristics of a CET bonded to a specimen.

Figure 3 shows an improved mounting system that utilizes

“metallic shoes,” which are attached to the specimen with

screws to allow precise adjustments of the gauge length and

alignment of the gauge with the geometric axis of the

specimen. Each CET must be calibrated in a micrometer to

produce a displacement vs differential voltage relationship.

A typical relationship is shown in figure 4.

When in use, the differential voltage in the gauge is

measured and used to compute the displacement of the ends

of the gauges. These gauges have been successfully used to

measure strain in the surfaces of glue-laminated beams, in

creep studies both parallel and perpendicular to the grain,

and in bending studies of solid wood elements. To verify

the performance of the CET’s, an experiment was

conducted by Loferski and others (1989) in which bonded,

electrical, strain gauges were mounted to a specimen and

the CET’s were mounted directly over the bonded gauges.

An excellent correspondence was found showing that the

CET and bonded gauges produce similar strain

measurements.

2
4=0.06’

BONDED GAGES

(a) PLAN (WHEN FLAT)

BONDED GAGES

}-227. ______i

ORIGINAL SHAPE

f

1’ HARDENED STEEL NAILS (+‘O.W

I---- GAGE LENGTH = 218’ ----I

ON TEStSPEClMEN

(b) ELEVATION

Figure 2-Details of the CET: (a) top and side views showing strain

gauges bonded to steel arach; (b) side view showing CET mounted to a

wood specimen with two steel nails (from Loferski et al. 1989).

AOJUSTMENT SCREW (0 = .oS3)

UPPER PLATE

LOWER PLATE

WOO0 SCREWS

ELEVATION

ADJUSTMENT HOLE

COUNTERSUNK HOLES (6 =0.083-j

NAIL

UPPER PLATE

b 0.7.5’ _I

r THREADED HOLE

0

l4=.ow

0

BASE

Figure 3-CET mounted to a wood specimen with special adjustable base

plates (from Loferski et al. 1989)

0.022

g 0.01 7

E1* 0.013

=i d 0.009

z

. 0.004

c

e-o.000

w

2 -0.oou

2g -0.009

* -0.013

!z

z -0.017

DISPLACEMENT(U), INCH ES

Figure &Typical calibration curve for a CET showing the differential

voltage versus the relative displacement of the CET end points (from

Loferski et al. 1989)

3
Commercial extensometers are also available for use in

testing materials. Many such devices can be purchased

through commercial establishments and can be custom

made for specific applications in testing wood materials.

Descriptions of these devices are beyond the scope of this

paper.

Electrical Resistance Strain Gauges

Electrical resistance strain gauges were invented in 1938

and consist of a strain-sensitive metal in which the

electrical’resistance changes in proportion to the strain.

Today, ERS gauges are commercially manufactured and

consist of a metallic foil mounted in a plastic sheathing. In

use, the gauge is bonded with an adhesive to the surface of

the specimen. A wheatstone bridge electrical circuit is used

to measure the change in the electrical resistance of the

gauge. Commercial strain gauges are available in a wide

range of sizes and configurations from as small as 0.4 mm

to greater than 150 mm gauge lengths.

The ERS gauge has a finite surface area, responds to all

strains at all points under its grid, and yields an integrated

average strain measurement (Measurements Group 1983).

According to Yadama and other (1991) “The measured

strain represents the strain at a point from a

macromechanics perspective, provided that the strain gauge

is not located in a strain gradient and the characteristic

length of any material attribute giving rise to

inhomogeneity is small relative to the gauge length.”

The advantages of ERS gauges include high accuracy, ready

commercial availability, and suitability for both long- and

short-term measurements. The disadvantages include a

relatively high cost and difficulty in mounting and aligning

the gauges precisely parallel to the geometric axes of the

specimen. In testing wood materials, experiments typically

involve many replicates. The fact that the ERS gauges are

not reusable from one specimen to the next contributes to

the high cost of these types of gauges.

In general, an ideal ERS gauge is small, lightweight, easy to

attach to the specimen, has high sensitivity, and is

unaffected by temperature, relative humidity, or vibrations.

Furthermore, it should be capable of measuring static and

dynamic strains, should be inexpensive, and should have an

infinitesimal gauge length. However, when considering

wood materials, many of these attributes are not applicable.

Yadama and others (199 1) state that “the gauge length

should be just large enough, but not too large, to record an

average macroscopic strain under the gauge, representative

of the deformation of the wood specimen around the point

of interest.” Furthermore, because wood materials are

affected by relative humidity and moisture content, which

produce swelling of the surface of the material, gauged

insulations are affected by changes in temperature or

relative humidity.

When selecting an ERS gauge for use on wood, the

researcher has many options to consider, including the grid

pattern and orientation, the gauge dimensions, the operating

temperature range, a temperature compensation circuit, the

transverse sensitivity (i.e. the strain perpendicular to the

measurement axis), the strain magnitude of interest, and the

fatigue characteristics of the gauge. Furthermore, ERS gauge

rosettes, which consist of two or three gauges mounted at

angles relative to each other, can also be used to measure the

normal and shear strains on the surfaces of materials.

The use of ERS gauges requires attention to many details

(Measurements Group 1983). A critical detail is bonding,

because the adhesive must transfer the strain from the

surface of the specimen to the gauge. Therefore, the

adhesive must be compatible with both the specimen and

the gauge materials.

The required curing temperature and curing time for the

adhesive must also be considered. For example, some

adhesives must cure at high temperatures, which may be

unrealistic and unachievable for testing large wood

specimens. During adhesive bonding, the gauge must be

clamped to the specimen so that it achieves a rigid and

permanent bond. The test environment, including the

temperature and humidity, must also be considered when

selecting an ERS gauge adhesive. Furthermore, the surface

of the wood must be properly prepared by using an abrasive

paper to expose a fresh wood surface before bonding.

The connection between the ERS gauge and the voltage

measuring device is made by the lead wires, which must

produce a precise, stable, electrical transmission. They must

have low electrical resistance and, ideally, should be

shielded from electrical noise by some sort of shielding and

possibly by a braided wire to reduce electromagnetic flux.

Flourescent lights, coiled extension cords, and radio noise

all contribute to electromagnetic noise, which can affect

ERS gauge readings.

The solder that is used to attach the wire to the ERS gauge

must also have the proper operating characteristics

4
including melting temperature and fatigue. Proper soldering

of the ERS gauge is important in producing a reliable and

accurate measurement, and can only be acquired with

practice. Because wood is a thermo-insulating material,

ERS gauges tend to heat up due to the application of

electricity to the circuit. On metal materials that conduct

heat away from the gauge installation, the self-heating of

the gauge is a minimal problem. On wood, however,

because the heat is not rapidly conducted away from the

gauge installation, the self-heating can appear as an

apparent strain in the material. To minimize this effect for

wood materials, an “active” and a “dummy” specimen are

used. Two ERS gauges are bonded to the active specimen,

which is loaded or stressed according to the user’s needs. A

matched dummy specimen of the same wood is located in

the same test environment, and is also instrumented with

two ERS gauges, but is not subjected to loads. These

gauges are then arranged in the wheatstone bridge circuit so

that the thermo-strains produced by self-heating are the

same on both the active and the dummy ERS gauges and

are automatically cancelled out by the electrical circuit.

Other operating characteristics are described in textbooks

and literature provided by ERS gauge manufacturers

(Measurements Group 1983).

Tell-Tale Gauges

Another type of gauge that may be useful for monitoring

the long-term performance of stmctures is the tell-tale

gauge. This gauge consists of two glass slides with etched

micrometer grids. One end of one slide is mounted to the

left side of a crack and the opposite end of the other slide is

mounted to the right side of a crack. The two gauges are

aligned so that the grid lines coincide as a zero position

reading. If the crack in the building material opens or

closes, or if the left or right side of the crack moves

vertically relative to the other, the motion of the two glass

slides will be apparent and can be manually read from the

gauge. This type of gauge is relatively inexpensive and can

be mounted in critical zones in buildings to discover if a

crack is stable or if it is moving or growing. Applications

include long-term monitoring of buildings, investigations of

the stability of historic buildings, and other similar

monitoring systems. These gauges have been used for years

to monitor the performance of masonry materials and are

relatively simple to use, are inexpensive, and have

relatively high accuracy.

Conclusion

Accurate strain measurements require attention to many

details. The characteristics of some traditional strain

measuring devices, including extensometers, CET’s, ERS

gauges, and tell-tale gauges should be taken into

consideration when deciding which type should be used in

any particular application.

Literature Cited

Lang, E.M. 1993. Modeling the behavior of wood-based composite

sheathing under hygrothermal load. Blacksburg. VA: Virginia

Polytechnic Institute and State University. 177 p. Ph.D. dissertation.

Loferski, J.R.; Davalos, J.F.; Yadama, V. 1989. A laboratory built clipon strain gauge transducer for testing wood. Forest Products Journal.

39(9): 45-48.

Measurements Group. 1983. Student manual for strain gauge technology.

Bulletin 309B. Raleigh, NC: Education Division. 45 p.

Yadama, V.; Davalos, J.F.; Loferski, J.R.; Holzer, S.M. 1991. Selecting

a gauge length to measure parallel-to-grain strain in southern pine.

Forest Products Journal. 41(10): 65-68.

5
Image Correlation for Measuring Strain in Wood and

Wood-based Composites

Audrey G. Zink, Robert W. Davidson, and Robert B. Hanna

Abstract

The suitability of a computer vision technique in experimental mechanics,

known as the digital image correlation technique (DICT), for full-size test

specimens of wood and wood-based composites was evaluated in this

study. The technique utilizes digitized video images of undeformed and

deformed test specimens and an image correlation computer routine to

measure the displacements and strains of any or all points on the surface of

the test specimens.

Evaluation was performed using axial compression and bending tests of

small, clear specimens of wood and flexure tests of plywood and oriented

strandboard specimens. A comparison of strain measurements obtained

using an independent measurement technique and those obtained with the

DICT showed close agreement.

Introduction

Full-field experimental measurements of the distribution of

strain in wood have traditionally been tedious and difficult due,

in part, to equipment limitations. Previously, brittle coatings,

photoelasticity, Moire methods, and laser speckle interferometry

have been investigated. Each of these methods requires

surface treatments and/or modifications that change the

mechanical properties on a local level or require complex,

intensive, and expensive preparation and equipment. Because

of the complexity and expense of these methods, examinations

of only a few specimens are economically possible. Dial

gauges, electrical strain gauges, and linear variable differential

transducers have been the conventional devices for measuring

point-wise strain in a wood specimen or structure. These

devices measure strain over a very limited gauge length and,

as a result, cannot be used for full-field measurements.

Audrey G. Zink is an assistant professor, Department of Wood Science and

Forest Products, Virginia Polytechnic Institute and State University,

Blacksburg, VA 2406 l-0323; Robert W. Davidson is a professor emeritus,

Department of Wood Products Engineering, State University of New York,

Syracuse, NY 13210; and Robert B. Hanna is a professor and Director, Center

for Ultrastructure Studies, State University of New York, Syracuse, NY 13210.

The digital image correlation technique (DICT) is a fullfield measurement method in experimental mechanics

that has been developed recently as an application of

computer vision. It relies on a mathematical correlation

of digital images of the surfaces of test specimens

acquired during mechanical testing. It has been

employed to obtain quantities of interest in such diverse

fields as rigid body mechanics (Sutton and others 1983)

dynamics (Peters and Ranson 1982) experimental

mechanics (Chu and others 1985) fluid mechanics (He

and others 1984), and biomechanics (Ranson and others

1986). The technique has been applied to a vast range of

loading, environmental, and testing conditions and

materials. Because of its advantages and wide spread

application, the technique is rapidly becoming well

established in experimental mechanics. The applicability

of the DICT for measuring strain in full-sized specimens

of wood and wood-based composites is discussed in this

paper.

Background

Peters and others (1989) and Sutton and others (1983)

have shown that the use of white light illumination of a

random, black and white pattern made it possible to obtain

strain measurements with optical techniques in

conjunction with a digital computer. In this research, they

developed extensive mathematical and experimental

techniques for cross-correlating a reference image with a

stored image for application in experimental mechanics.

This cross-correlation technique is similar to area

correlation in pattern recognition methodology. The crosscorrelation of two functions is used to indicate the relative

amount of agreement between the two functions for

various degrees of shifting. In the case of experimental

mechanics, the displacement imposed on the test object

during mechanical loading is measured as the degree of

shifting of the light intensity patterns as found by the

cross-correlation criterion.

6
Recently, Choi (1990) and Choi and others (1991) have

examined the experimental application of DICT by utilizing

video microscopy and obtaining surface randomization

using carbon photocopy toner particles for the measurement

of displacement and strain in small specimens of wood and

paper. This application showed that the DICT could be a

fast, simple, and accurate method when applied to very

small specimens of wood (1 by 1 by 4 mm in size) and to

paper. Full-field deformation of wood test specimens using

laser speckle interferometry and digital image processing

was applied by Agrawal (1989) to study the suitability of

the system for monitoring the creep response of wood.

Excellent comparisons were shown between strain

measurements obtained with the image correlation

technique on wood and several independent measurement

devices such as strain gauges (Agrawal 1989) and a Mann

comparator (Choi 1990).

Experimental Procedures

The Digital Image Correlation Technique

Equipment-The equipment used to acquire and digitize

the video images was a black and white Panasonic’

WV-CD50 CCD camera and a Coreco@ Oculus-200 realtime, gray-level, image digitizer board in an IBM’ personal

computer (PC). The matrix resolution for this particular

board was 480 by 5 12 picture elements, called pixels. The

lens used with the video camera was a Nikon@ 55-mm

macro lens. A standard C-mount adapter was used to attach

the macro lens to the video camera. A high resolution

Panasonic’ video monitor, model BT-S 1300N, was used

display the digital image. The illumination source was a

22-W, circular, fluorescent tube suspended and centered

around the camera. The illumination of the surface was

carefully controlled by surrounding the testing machine

table and the camera with two layers of heavy black cloth

and cardboard, and extinguishing all room lights.

to

The PC and image digitizer board system was used to

digitize the video images, redisplay them on the video

monitor for focusing and brightness control, and then

transfer them to a mainframe computer for analysis with the

image correlation computer routine as described in a

following section.

The CCD video camera recorded the surface light intensity

pattern as varying levels of gray. To digitize the video

signal that represented the light pattern, the video image

was divided into an array of equal-area, rectangular pixels.

The gray level of each pixel was assigned an integer

value from 0 to 255 that was proportional to the light

intensity received from the surface of the test object. The

lowest intensity (black) was assigned 0, and the highest

(white) was assigned 255, with values in between

representing different shades of gray. The integers

representing the light intensity pattern of the test object

were assigned pixel locations based on an X-Y coordinate

system that corresponded to the actual location on the test

object.

Correlation Analysis-The digitization process

represented the continuous, light intensity patterns of the

recorded images in a discrete numeric form. Because a

pixel in undeformed coordinates may relocate to positions

between pixels in deformed coordinates, an interpolation of

gray levels between pixels was needed to represent the

original continuous pattern. A bilinear interpolation scheme

was used in this study.

The digitized intensity pattern after deformation was related

to the digitized, interpolated, intensity pattern recorded

before deformation. A mathematical correlation of the two

images to determine image differences allowed for

determining the object displacement. The image correlation

program compared subsets of digitized, light intensity

values of subregions around selected pixels in the

undeformed images with all subregions of the same size in

the deformed image.

In this study, the correlation analysis was carried out on an

IBM@ 3090 mainframe computer and a Gray@ Y-MP

supercomputer at the Pittsburgh Supercomputing Center.

The analysis selected subregions to find the values of six

unknown parameters that minimized the correlation

function, C, C is a nonlinear function dependent on the

subregion size and the unknown parameters and is unique

for each selected pixel location. The correlation function to

be minimized was the sum of the squares of the differences

of the light intensity values of the subregions in the pairs of

images and is a “least squares” correlation to measure how

well the subsets match. It is written:

where

C(U, v, 6U/6X, 6u/6y, &/6x, sv/sy>

=c M & Y) - W,Y’)12 (1) M

u and v = the displacements for the subset centers in the x

and y direction, respectively,

7
A (x, y) = the gray-level value at coordinate (x, y) in the

deformed image,

B (x’, y’) = the gray-level value at point (x’, y’) in the

undeformed image, and

M = the subregion of interest chosen from the digital

images.

The subregion, A4, used for the correlation comparison was

20 by 20 pixels in size and centered around the selected

measurement points. A subregion of this size was chosen to

minimize the effects of local wood-structure deviations, to

minimize image distortion, to reduce correlation time, and

to obtain a pattern that was statistically different from its

neighbors.

The coordinates (x,y) and (x’,y’) are related by the deformation

that occurred between acquisition of the two images as

imposed by the mechanical loading. If the motion of the

object relative to the camera is parallel to the image plane

and the deformations are small, the images are related by

X ’ = x + u + @u/6x)(~) + (&/6y)(Ay), an d (2)

y’ = y + v + (Sv/Sx)(Ax) + (Sv/Sy)(Ay) (3)

where

u and v = the displacements for the subset centers in the x

and y directions respectively, and

h and Ay = the distances from the subset center to the

point (x,y).

The correlation of the images was obtained by determining

the values for the six unknown parameters: u, v, &/6x,

6u/Sy, &~/6x, and SvlSy, which minimize the correlation

coefficient, C. Minimization of functions, or optimization

as this procedure is often called, is a very large area of

numerical research. Various schemes exist for achieving

this minimization in the literature on image correlation. In

general, a coarse-fine search technique for determining the

minimum or best fit value has been used. This procedure

requires a very large number of calculations and is very

computationally intense.

A new method for obtaining the best fit was evaluated in

this study: the quasi-Newton method. This method was

used to provide the successive approximations of the initial

guesses of the six unknown deformation parameters. A

two-parameter model was iterated until all values were

within the tolerance limits. The values of the six unknown

parameters that minimized the correlation function were

returned as the best approximations to the six deformation

parameters. This method provided convergence with fewer

calculations and, therefore, greatly decreased the

computational time.

In this study, a typical correlation analysis was conducted on

a measurement matrix of 23 points parallel to the load axis

and 11 points perpendicular to the load, for a total

measurement grid of 253 points. The results of the

correlation analysis were the displacements parallel and

perpendicular to the load of the virtual measurement grid of

the 253 points. Once the displacement of the selected points

was determined, the strain was calculated as the relative

displacement of the points divided by the original distance

between the points. With this technique, the normal and

shear strains are measured, and from these strains, the

modulus of elasticity and Poisson’s ratio can be calculated.

Distortion from out-of-plane effects was minimized by

working with low magnifications (<3X), a high-quality lens

with a large depth of field, and specimen-to-lens working

distances of >400 mm. Sutton and others (1990) have

determined that magnification of <5X and a distance from

the camera lens plane to the object of >400 mm is sufficient

to reduce to an acceptable range any distortion from out-ofplane movement of the specimen. They determined that, at

these levels, any noise that appears in the displacement

fields is from sources other than out-of-plane effects.

The displacements due to external loading were measured

with an image correlation measurement matrix of 20 points

perpendicular to the load (x-direction) and 15 points

parallel to the load axis (y-direction). Each of the points

was placed at 20-pixel intervals on the computer image for

corresponding gauge lengths of 3.85 mm in the x-direction

and 1.33 mm in the y-direction on the test specimen. The

computer-image measurement grid of 300 points was

located in the central portion of the image to minimize edge

effects and curvature of the field.

The normal strains (E,, and E,J between each measurement

point were calculated as the change in distance between the

points in the x-direction or y-direction divided by the

original distance between the points (AL,IL, and ALJL,).

This change in distance between the points was determined

using the displacements of each point as measured with the

image technique. The shear strain ( E,$ was calculated as

the change in angle between adjacent sides of an

elementary block, defined by four measurement points, as

this block was distorted under shearing stresses. The change

8
in angle was determined using the displacements of each of

the four points as measured with the image technique.

A finite difference method that utilized a forward difference

scheme was employed for the strain field calculations. The

strain values in the following contour plots were calculated

using the image acquired at a slight preload as the reference

image. As a result, the strain values in each figure represent

the strain that accumulated from the onset of that initial load

up to the load at which the images were acquired. The

average standard deviation in strain was 0.00027.

Calibration and Evaluation-Extensive evaluation and

calibration procedures were carried out on the experimental

equipment and image correlation computer program.

Calibration tests were conducted on an aluminum alloy

block and a small, clear specimen of wood. Aluminum was

used for calibration because it is considered a typical,

isotropic, homogeneous, elastic material whose elastic

constants are accurately known and can be used for

comparison. Testing to determine system noise levels

involved acquisition of images at several time intervals and

determination of the differences of the digital gray values

between these images. A carbon speckle pattern was

applied to the surface of an aluminum alloy block, the block

was placed on the table of the testing machine, and images

of the entire surface were acquired over time. No load was

applied to the specimen during this experiment, none of the

equipment was moved, and the light was left on throughout

the experiment. Four 20 by 20 subsets of digital gray values

were chosen at random locations on the surface of the

aluminum block for each time interval and compared to the

reference image acquired at time zero. There were no

apparent strains in the 60 minutes allowed for this testing.

Accuracy and precision estimates were determined by

comparing strains obtained with the vision system and the

strains as measured with a compressometer that had been

carefully calibrated to a secondary standard with known

accuracy. The accuracy of the vision system was calculated

as the maximum deviation of the readings from this known

standard. The precision was calculated as the maximum

deviation of several readings from the mean of those

readings.

The electronic device used for the calibration was a

Daytronics’ DS 19OV linear variable differential transducer

(LVDT). The LVDT was carefully calibrated just prior to

testing using a T63Rl Starrett@ micrometer head. The

Starret@’ micrometer had been calibrated within a year of

use according to master standard National Institute of

Standards and Testing Test 7311241460 and in accordance

with MIL-STD-45662A and MIL-I-45208A. Both the

precision and accuracy of the micrometer were found to be

i 0.127 mm. The full scale of the micrometer was 5.08 cm.

The precision and accuracy of both the micrometer and the

LVDT in terms of their percentage of full-scale reading

were found to be 0.0025 percent. Axial strain was measured

relative to the same gauge length for both techniques. The

test was replicated 10 times. It was determined that both

the precision and accuracy of the computer vision system

was ho.0025 percent of a full-scale reading of 5.08 cm.

Figure 1 illustrates the plot of a typical calibration curve for

the two independent measuring systems.

Specimen Preparation

Fabrication-Small, clear, test specimens of sugar pine

(Pinus lambevtiana Dougl.) and red oak (Quercus spp.)

were prepared for compression and bending testing in

accordance with American Society for Testing and

Materials (ASTM) Dl43-83 Standard Methods of Testing

Small Clear Specimens of Timber (1983). Thirty sugar pine

and 30 red oak compression specimens were prepared. The

test specimens were 2.54 by 2.54 cm in cross section and

10.16 cm along the grain. Sixteen bending specimens were

prepared from the same boards of sugar pine and red oak.

The bending specimens were 2.54 by 2.54 cm in cross

section and 40.64 cm along the grain. The average initial

moisture content of the specimens was 8.5 percent.

Flexure specimens of live-ply, 48124 American Plywood

Association (APA) Rated Sheathing plywood and APA

Rated Sturd-I Floor@ oriented strandboard (OSB) specimens

were prepared in accordance with ASTM D3043-87

(Reapproved 1993) Standard Method of Testing Structural

Panels in Flexure (1987). Four flexure specimens were

prepared from the plywood and the OSB. The plywood

specimens were 5.08 cm across and had a test span of 144

cm. The OSB specimens were 5.08 cm across with a test

span of 76.2 cm.

Carbon Speckle Application-Because the image

correlation technique used to determine the strain

distributions relies on a comparison of discrete, light

intensity patterns obtained before and during deformation to

reveal the shift of the images, a unique, random intensity

pattern is required on the surface being observed. Wood

specimens, in general, do not provide the required, unique,

sharp contrast patterns. In this study, the contrast was

enhanced with toner particles used in standard photocopy

9
01

0.0002 0.0004 0.0006 0.0008 0.001

Strain in Al using Image Carrel. Tech.

Figure l-Calibration curve for two independent systems measuring strain in an aluminum block.

machines. The particles are distinctly black in hue and

contain carbon black and chrome III pigments.

The unique contrast pattern was created on the surface of

the wood and composite specimens with a light speckle of

carbon particles. The particles were sifted through a screen

held above the surface of the specimen and scattered across

the surface by room air currents. Heat was applied to the

carbon particles using a heat lamp. The particles melted

and flowed into the wood structure after 10 minutes in an

air temperature of approximately 70 “C. The specimens

were then allowed to cool. During the cooling process, the

particles solidified and became tightly anchored to the

wood structure. The advantages of this particular speckle

pattern are that it does not mask the underlying wood

structure and it does not modify the material properties of

the test specimen in any manner.

Moisture Conditioning-The test specimens were

conditioned to approximate the equilibrium moisture

content in the testing laboratory using an Amino-Aire””

humidity-temperature control chamber. The specimens were

weighed periodically until no weight change was noticeable

and kept in the chamber at constant conditions until just

prior to testing. The average moisture content of the

specimens just prior to testing was 12 percent as determined

by ASTM Standard D4442-92 Standard Test Methods for

Direct Moisture Content Measurement of Wood and Woodbase Materials (1992).

Mechanical Testing

Compression and Static Bending Tests-A BaldwinEmery@ SR-4 universal testing machine was used for the

testing of the compression and bending specimens,

following ASTM D143-83 methods (1983). The

compression load was applied parallel to the grain. The

bending tests were conducted with a single, center-point

load. The load data were acquired during the tests using an

IBM@ Model 5 150 PC and a Hewlett Packard@ 6621A

system direct-current power supply. The sampling rate was

2 Hz. The displacement data were recorded using the DICT

described earlier in this report.

Flexure of Wood-Based Composite Panels-An InstronB

testing machine was used for the testing of the composite

specimens, following ASTM D3043-87 (1987). The

specimens were simply supported and a center-point load

10
was applied to the wide face of the specimens. The load

data were acquired using the data acquisition system listed

above. The displacement data were recorded using the

image correlation technique over the center 10 cm beneath

the load head.

Results and Discussion

Compression and Static Bending Tests of Solid Wood

Compression-Figure 2 illustrates a typical strain plot

obtained from compression parallel to the grain. Figure 2a

is the plot of the strains parallel to the load, and figure 2b is

the strain plot perpendicular to the grain. These strain plots

were acquired just prior to ultimate failure of the specimen.

An examination of figure 2a indicates that the distribution

of the strain parallel to the load is not as homogeneous as

might be expected. The unique microscopic features of the

wood elements are responsible for the nonhomogeneous

nature of the strain fields. The location of the highest strain

on the strain plots parallel to the load exactly corresponds

to the location of visible failure lines on the test specimen.

An examination of figure 2b indicates that there are strains

perpendicular to the load, which is experimental

confirmation of the Poisson effect in wood. This strain

distribution is also nonhomogeneous due to the underlying,

microscopic, anatomical, arrangements. The location of the

highest strain also corresponds to the location of failure on

the test specimen.

Bending-Figure 3 is an example of the strain distributions

obtained from bending a small, clear specimen of wood.

Figure 3a is a plot of the strains perpendicular to the load

(along the axis of the beam), and figure 3b is a plot of the

strains parallel to the load. The strain plots were acquired

just prior to failure of the beam. The highest strains in

these plots are just beneath the load point, the distribution

of strain across the face of the beam is symmetrical, and the

neutral axis is approximately middepth. None of these

results were unexpected; however, it had never before been

possible to measure the distributions in as much detail and

with as much accuracy prior to this study using the image

correlation technique. The failures of the beam were in

exact correlation with the locations of the highest strains on

the strain plots.

Flexure of Wood-Based Composite Panels

Plywood-Figure 4 is an example of typical strain plots

obtained from flexure of plywood specimens. Figure 4a is

a plot of the strains perpendicular to the load (along the

axis of the beam), and figure 4b is a plot of the shear

strains on the surface of the beam. The load level is

approximately 50 percent of the ultimate load.

Examination of these plots indicates an accumulation of

high strain at a load of only half the ultimate load. These

plots further indicate what would be expected from

theoretical mechanics: compression strains, tension strains,

a neutral axis where expected, a linear distribution of axial

strains, and strain concentrations near the loading point. It

has not been possible until now to verify these

expectations. Traditionally, plywood has been considered

as a solid, homogeneous structure. However it was

discovered in this study that, by overlaying these strain

plots on the actual failed specimen, each ply of the

composite board was acting as an individual beam and

there was shear at each glueline.

Oriented Strandboard-Figure 5 is an example of typical

strain plots from the flexure of OSB. Figure 5a is the plot

of strains perpendicular to the load (along the axis of the

beam), and figure 5b is a plot of the shear strains on the

surface of the OSB. These strain plots were acquired at

approximately 75 percent of the ultimate load. An

examination of the strains along the beam (fig. 5a)

indicates that the distribution is not symmetric, the neutral

axis is not a straight line, most of the strain occurs near the

lower edge of the beam, and regions of compression extend

well below middepth. These results might be explained in

part by the localized compaction of the strands in the

region of the load point and by each strand acting as an

individual beam. An examination of the shear strains (fig.

5b) also indicates that OSB behavior does not follow

expected patterns. The shear strains are maximum near the

surface, and the distribution has left/right symmetry with a

central neutral region from the top to the bottom of the

specimen. When overlaid on the actual test specimen, the

strain distributions indicated some correlation with the

location of the individual strands of the composite. The

test specimen failed due to adhesion failure, allowing the

separation of individual strands, rather than to the cohesion

failure of the wood. Further tests are being conducted on

the composite boards.

Conclusions

The image correlation technique was shown to be an

accurate, simple, and versatile method of measuring strain

distributions in wood and wood-based composites. The

technique is noncontact and is not limited by the size or

11
(a) (b)

Figure 2-Strain contour plots for compression test specimen at a load level of 18.6 kN; (a) strain perpendicular to the grain; (b) strain

parallel to the grain (strains are shown x 10-6).

12
gf.pf.&y500 -GZZ ))’l//h “V

I I I I500:-500’ 0 -50 0

5001 500 \ + ivIL’ ‘5T50c

. ..)..1..1.1...1...1...s z . . . . . c-1 0 0 0 .I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -5oo~ ~ .~ .L....L.t_ 1 (j

Figure 3-Strain contour plots for bending test specimen at a load level of 1.3 15 kN; (a) strain perpendicular to the grain; (b) strain parallel

to the grain (strains are shown at x 10~6).

13
. . . . . . . . . . . . . . ._. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . yd. . . . . _.,...

:“‘,,,,........x...c. . . . . ._. . . . . . . .

. . . . . . .-500. . .

_500i boo (~___~ _

Figure &Strain contour plots for plywood flexure specimen at a load level of 0.243 kN; (a) strain perpendicular to the grain; (b) shear strain

(strains are shown x l0-h).

14
Figure 5-Strain contour plots for oriented strandboard specimen at a load level of 0.285 kN; (a) strain perpendicular to the grain; (b) shear strain

(strains are shown x 10-6).

15
nature of the specimen. The distributions of strain in the

compression tests was nonhomogeneous due to the unique

anatomical structure of wood. The strain distributions

from bending tests on small, clear specimens of wood

indicated close agreement with conventional beam theory

and exact correlation with the failure pattern of the

specimen. However, neither composite board followed

conventional beam theory entirely. Of the two composites

investigated in this study, the plywood exhibited behavior

closer to conventional theory. The OSB exhibited strain

patterns very closely related to composite structure

arrangement.

Acknowledgments

The authors are grateful to Mr. Arnold C. Day and Mr. John

McKeon, State University of New York, Syracuse, New

York, for their technical assistance, Dr. Wayne E. Fordyce,

Syracuse University, Syracuse, New York, for his assistance

with the correlation analysis, and the USDA McIntireStennis Forest Research Program for financial assistance.

Literature Cited

Agrawal, C.P. 1989. Full-field deformation measurements in wood using

digital image processing. Blacksburg: Virginia Polytechnic Institute and

State University. 103 p. M.S. thesis.

American Society for Testing and Materials. 1983. Standard methods of

testing small clear specimens of timber. ASTM Standard Dl43-83.

Philadelphia, PA. ASTM. 57 p.

American Society for Testing and Materials. 1987. Standard method of

testing structural panels in flexure. ASTM Standard D3043-87

(Reapproved 1993). Philadelphia, PA. ASTM. 11 p.

American Society for Testing and Materials. 1992. Standard test methods

for direct moisture content measurement of wood and wood-base

materials. ASTM Standard D4442-92. Philadelphia, PA. ASTM. 5 p.

Choi, D. 1990. Failure initiation and propagation in wood in relation to its

structure. Syracuse: State University of New York. 149 p. Ph.D.

dissertation.

Choi, D.; Thorpe, J.L.; Hanna, R.B. 1991. Image analysis to measure

strain in wood and paper. Wood Science & Technology. 25: 251-262.

Chu, T.C.; Ranson, W.F.; Sutton, M.A.; Peters, W.H. 1985.

Applications of digital image correlation techniques to experimental

mechanics. Experimental Mechanics. 25(3): 232-244.

He, Z.H.; Sutton, M.A.; Ranson, W.F.; Peters, W.H. 1984. Twodimensional fluid-velocity measurements by use of digital speckle

correlation techniques. Experimental Mechanics. 24(2): 117-12 1.

Peters, W.H.; Ranson, W.F. 1982. Digital imaging techniques in

experimental stress analysis. Optical Engineering. 21(3): 427-43 1

Peters, W.H.; Sutton, M.H.; Ranson, W.F.; [and others]. 1989. Wholefield experimental displacement analysis of composite cylinders.

Experimental Mechanics. 29(l): 58-62.

Ranson, W.F.; Walker, D.M.; Caulfield, J.B. 1986. Biomechanics. In:

Computer vision in engineering mechanics: a discussion paper prepared

for the NSF workshop on solid mechanics related to paper; [dates

unknown]. Blue Mountain Lake, NY [Place of publication unknown].

[Publisher unknown]: [pages unknown].

Sutton, M.A.; Chae, T.L.; Turner, J.L.; Bruck, H.A. 1990. Development

of a computer vision methodology for the analysis of surface

deformation in magnified images. ASTM STP 1094 Micon 90.

Philadelphia, PA. ASTM: 109-132.

Sutton, M.A.; Walters, W.J.; Peters, W.H.; [and others]. 1983.

Determination of displacements using an improved digital correlation

method. Image and Vision Computing. l(3): 133-139.

16
Application of Imaging Technologies to

Experimental Mechanics

Laurence Mott, Stephen M. Shaler, and Leslie H. Groom

Abstract

Digital image technologies are rapidly emerging as a powerful adjunct to

traditional experimental mechanics techniques. The essential hardware

components as well as considerations in their selection are described in

this paper. A historical perspective of imaging applications to the testing

of individual wood fibers is essential to understand these technologies.

Additionally, examples of results obtainable from new imaging

microscopes (the environmental scanning electron microscope and the

confocal laser scanning microscope) show quantum leaps in the level of

detail now possible. The combination of these image acquisition systems

with image analysis techniques, such as digital image correlation, is

proving to be a powerful tool for quantifying and understanding the

microstructural character of wood composite materials.

Introduction

As is the case with any productive manufacturing sector,

success and growth of the forest products industry relies

heavily on research. Staying abreast of current technologies

is crucial to developing new forest products, improving

product quality, and increasing the efficiency of processing.

Digital processing advancements in recent years have led to

a myriad of new experimental techniques that take

advantage of this newfound computing power. One of the

fastest growing areas of experimental techniques is that of

digital image acquisition and processing.

Digital image processing in its various forms is now

commonly employed in the forest products industry.

However, image forming and processing technology is

developing at such a rapid rate that only people directly

involved with it can hope to keep abreast of the latest

innovations. Some of the most exciting forest products

Laurence Mott is a graduate research assistant, Department of Forest

Management, University of Maine, Orono, ME 04469; Steven M. Shaler is

an associate professor. Department of Forest Management, University of

Maine, Orono, ME 04469; Leslie H. Groom is a research technologist,

USDA Forest Service, Southern Research Station, 2500 Shreveport

Highway, Pineville, LA 7 1360.

research couples digital image processing with improved

microscopic capabilities. Some of the new image forming

technologies such as environmental scanning electron

microscopy and confocal laser scanning microscopy can

provide precise and accurate quantitative information and

data that were previously unattainable or obtainable only by

some less reliable method. Such techniques allow the forest

products industry to determine fundamental knowledge of

material behavior in composite systems.

Digital Imaging Techniques

There are three basic forms of imaging techniques

applicable to experimental mechanics: analog signals, solid

state/tube cameras, and noncamera systems such as

scanning electron microscopes (SEM’s). First, some of the

early imaging techniques that relied on an analog signal

will be appraised. Second, the establishment of simple

digital imaging systems will be illustrated, with key

considerations involved in hardware/software compatibility

being identified. Finally, noncamera-based, digital, imaging

systems and their usefulness in the field of experimental

micromechanics will be discussed.

Traditional Techniques

Some of the earliest imaging techniques were used

primarily to qualitatively observe failure processes.

McMillin (1974) examined failure mechanism of individual

wood fibers loaded in torsion. The sequence shown in

figure 1 was recorded with Polaroid@ film and was useful

in studying the torsional failure mechanism. Page and

others (1977) used a 16-mm cinemagraphic camera to

acquire images of individual fibers loaded in tension (fig.

2). The images could be matched with load-elongation

traces to ascertain the fiber response to loading. These

early attempts at image acquisition were an important first

step in studying fracture behavior because they allowed the

classification of fracture mechanisms and provided an

17
Figure l-Failure sequence (from A to D) of a latewood tracheid

subjected to torsional loading recorded with Polaroid’ film. The scale bar

in A equals to 50 pm and is also applicable to B, C, and D (photo courtesy

f~f C. Mckfillin).

understanding of basic, fundamental, material properties.

Although cinemagraphic and Polaroid@ films are useful for

observing behavior, they are of limited use due to handling

and processing problems, the analog nature of the image,

and the relative high cost of each image.

Digital Imaging Systems

Digital image processing offers several advantages when

compared to comparable analog systems. The digital

format allows for quantitative measurements between

images as well as further processing after images have been

acquired to improve their quality through brightness and

contrast enhancement. Digital images are inexpensive,

easily obtainable with commercially available equipment,

and limited only by the amount of available disk storage

space.

One of the first considerations for a basic digital

imaging system is the camera. There are two basic

types of cameras: tube (sometimes referred to as

Vidicon’) cameras and solid-state [more commonly

referred to as charge-coupled device (CCD)] cameras.

Vidicon’ tube cameras typically possess high resolution

attributes, such as 1,600+ television lines per 2,000

pixels as well as high sensitivity (ISIT and SIT tubes)

with up to 1,000 times more spectral sensitivity than

conventional CCD cameras. However, the VidiconB

(4

Figure 2-Cinemagraphic images of a single tracheid fiber; (a) subjected

to low load levels; and (b) at prefailure. Load-elongation traces are shown

as a ghost image in each photo @hotos courtesy of D.H. Page).

18
tube cameras also have several major disadvantages:

They require frequent adjustments and maintenance,

they are more expensive while being less rugged and

reliable than CCD cameras, and they have a continuous

analog system that requires a compatible framegrabber,

which can be costly.

The relatively low cost and commercial availability of CCD

cameras make them the most common camera type in a

digital imaging system (fig. 3). There are several factors to

consider when choosing a CCD camera. The cost of the

camera is proportional to its resolution, so the highest

resolution camera within the available budget should be

chosen. There are several sizes of CCD chip formats

ranging from 6 mm to 25 mm. Generally, smaller sized

chips have better resolutions than larger chips given the

same number of pixels. The lens size must be chosen so as

to complement the chip size of the CCD camera. The

operating light level and signal-to-noise ratio should also be

considered. Although most imaging systems utilize a

black-and-white camera, color cameras are also available.

Color cameras cost about three times as much as

comparable black-and-white cameras. In addition, the

accompanying framegrabber for the color CCD camera is

several times the cost of a conventional black-and-white

framegrabber.

Conventional CCD systems are used primarily to measure

the areas and light intensities of digital images of interest.

An example is the work by Mercado (1992) in which he

modified a standard digital imaging system to measure halffringe photoelasticity. He was able to measure the

interfacial shear stress between a polymer matrix and an

embedded wood fiber by measuring the photoelastic effect

(or light intensity) of the polymer sheet (fig. 4).

Micromechanical studies are generally interested in

specimen deformations. The translation and

deformation of a solid body can be measured by userwritten algorithms referred to as digital image

correlation (DIC) (Chu and others 1985). The accuracy

of in-plane deformations can be corrupted by

translations perpendicular to the camera. Binocular

systems can be designed that use two cameras placed at

a predetermined distance from each other and, when

used in conjunction with DIC, allow a three-dimensional

(3D) analysis of both rigid and deformable bodies (Luo

and others 1993).

Figure 3-Schematic design for a basic digital imaging system.

19
Figure 4-Digital image of an individual wood fiber embedded in a polyurethane sheet as seen under a circular polariscope.

Fringe patterns are indicative of stresses in the polymer sheet.

An advanced digital imaging system is necessary when

obtaining confocal laser scanning microscope (CLSM)

images. A CLSM system uses laser light, a conventional

light microscope, and a controlled XYZ stage to produce

3D images. The 3D image is actually a computer-built

composite of individual images from only in-focus planes.

Laser light can penetrate a translucent material, thus

focusing is possible only if the object is small enough and

is transparent enough. The 3D images can generally be

rotated to any angle in the X, Y, or Z direction. Figure 5

shows a loblolly pine, earlywood fiber that was scanned

longitudinally (XY section), with the image then rotated 90

degrees such that the fiber cross section (YZ section) is

exposed. Morphometric measurements can then be made to

determine the cross-sectional area as well as the cell wall

thickness at any point along the length of the wood fiber.

This technique was recently used by Hamad [in press] to

visualize crack formation within fiber walls due to fatigue

loadings.

Noncamera-based Imaging Systems

The most classic example of a noncamera-based imaging

system is the SEM. Wood-based specimens placed within

the chamber of a standard SEM need to be coated with a

conductive material (e.g., gold) and are subject to very high

vacuums that lead to unrealistic test conditions. Standard

SEM’s are thus limited to static or postfailure specimens.

The environmental SEM (ESEM) has several advantages

over a standard SEM: the specimen chamber can contain a

gaseous environment and may be kept at a pressure and

temperature that permit water to exist in a vapor and liquid

state. In addition, materials can be observed in a natural,

nondehydrated state, no conductive coating is necessary,

and dynamic testing is feasible.

The combination of a high degree of surface contrast, a

high magnification, and a realistic environment make

the ESEM a very useful tool for examining failure

20
Figure S%-Confocal, laser-scanned photomicrograph of XZ section of an individual, loblolly pine, earlywood tracheid.

mechanisms in small wood specimens. Figure 6 shows

an individual, latewood, loblolly pine fiber failing in

tension, with failure initiating at and propagating

around a bordered pit (Mott and others 1995).

Coupling this information with DIC would show local

deformations. Digital image correlation, conducted in

matrix form, would allow mapping the strains of an

individual specimen under known testing conditions.

The potential for determining fundamental information

about the performance of individual wood components

by combining several imaging techniques is

phenomenal.

Conclusion

Fundamental knowledge of the mechanical behavior of

wood components, whether acting individually or in concert

within wood-based composites, is necessary to ensure a

high product quality and to engineer next-generation

composites that are superior in efficiency and strength to

their predecessors. Digital imaging systems are fast

becoming a standard research tool that allow the researcher

to quantify physical and mechanical properties of

component and composite performance. Combining several

of the techniques presented in this paper will allow

researchers to gather information that is accurate and, until

recently, has been unobtainable.

Literature Cited

Chu, T.C.; Ranson, W.F.; Sutton, M.A.; Peters, W.H. 1985.

Applications of digital image correlation techniques to experimental

mechanics. Experimental Mechanics. 25(3): 232-244.

Hamad, W.Y.; Provan, J.W. A novel fatigue-testing experimental

technique for single wood pulp fibers [in press]. Experimental

Mechanics.

Luo, P.F.; Chao, Y.J.; Sutton, M.A.; Peters, W.H. 1993. Accurate

measurement of three-dimensional deformations in deformable and rigid

bodies using computer vision. Experimental Mechanics. 33(2): 123-132.

McMillin, Charles W. 1974. Dynamic torsional unwinding of southern

pine tracheids as observed in the scanning electron microscope. Svensk

Papperstidning. 9: 3 19-324.

21
Figure h---Environmental scanning electron microscope image of an individual latewood, loblolly pine fiber subjected to tensile loading.

Mercado, Julius S. 1992. Using digital image analysis to determine the

reinforcement of wood fiber polyurethane composites. Houghton, MI:

Michigan Technological University. 82 p. M.S. thesis.

Page, D.H.; El-Hosseiny, F.; Winkler, K.; Lancaster, A.P.S. 1977.

Elastic modulus of single wood pulp fibers. Tappi. 60(4): 114-l 17.

Mott, L.; Shaler, S.M.; Groom, L.H.; Liang, B.H. 1995. The tensile

testing of individual wood fibers using environmental scanning electron

microscopy and video image analysis. Tappi. 78(5): 143-148.

22
Full Field Stress/Strain Analysis:

Use of Moir6 and TSA for Wood Structural Assemblies

Ronald W. Wolfe, Robert Rowlands, and C.H. Lin

Abstract

MoirC and thermoelastic stress analysis (TSA) methods were compared on

the basis of an assessment of the stress distribution in metal plate

connectors loaded in axial tension. For the 12.7 mm gauge length

considered, the moirC technique had a strain resolution of 1,000 pi

without computer enhancement and 200 p’~ with computer enhancement

using digital image analysis and Fourier interpolation. Using the

commercial SPATE@ equipment, TSA had a resolution of roughly 145 psi

(1 MPa) in steel, corresponding to a strain of 5 p E.

Results of these tests show the TSA test equipment to be superior for

laboratory testing of metallic materials. For field evaluation of wood

structures, however, the moirk method has the potential to be more

portable, to be less costly, and to have the same or a superior resolution

when enhanced using digital image analysis.

An analysis of metal gusset plates stressed in tension confirm the expected

stress distribution characteristics. Stresses are rarely uniformly distributed

over the width of the plate. The assumption of a uniform distribution of

tooth loads along the length of a plate is reasonable for connections

designed with a balance between tooth withdrawal and steel tensile failure.

For longer plates prone to steel failure, the plate teeth nearest the joint

carry a higher load than teeth closer to the end of the plate.

Introduction

Structural wood components and assemblies have been

evolving for over 200 years in the United States to meet

the ever-changing demands on available energy and

material resources. Over the past 40 years, Americans

have seen major changes in the way wood is used in

structural applications: plywood has replaced board

sheathing, trusses have replaced lumber rafters, and wood

l-joists have regained a significant share of the commercial

building market that was lost to steel in the early part of

this century.

Ronald W. Wolfe is a research engineer, USDA Forest Products Laboratory,

One Gifford Pinchot Dr., Madison, WI 53705; Robert Rowlands is a

professor of engineering mechanics, University of Wisconsin, Engineering

Hall, Madison, WI 53706; and C.H. Lin is a research engineer, Sunoco

Products Co., P.O. Box 160, Hartsville, SC 29950.

To compete with other materials in meeting the continuing

demands for energy and material resources, the use of

improved test and analytical procedures that facilitate the

identification of design imbalances must be explored. Tools

for performing full-field analyses of stress and strain enable

stress concentrations to be identified and the range of

stress/strength ratios to be quantified within a given structure.

These tools provide the potential for further developments

toward optimizing the use of wood fiber resources.

The use of two, full-field, analysis tools for the evaluation

of wood-frame truss connections is examined in this paper.

Moiri: and thermoelastic stress analysis (TSA) methods

have been widely used in machine and tool design and to a

more limited extent in the development of design guidelines

for steel structures and connections. While these tools have

been applied to wood and wood-fiber-based materials at the

basic research level, they are rarely used in product

development or for field evaluation. State-of-the-art

developments in image analysis used in conjunction with

these tools give them valuable potential for a broad range of

applications in wood structures research and development.

Background And Theory

Moire and TSA methods are convenient tools for the fullfield analysis of member stresses. The moirt method is a

more mature technology with the advantage of a much

broader experience base. Its uses range from high-resolution,

laboratory, interferometry techniques to field applications

where it is used simply to assess strain concentrations with

relatively little concern for stress magnitude.

The TSA method has the advantage of enabling

direct/remote assessment of member stresses.

Measurements are calibrated on the basis of stress, giving

some advantage where the modulus of elasticity varies or is

unknown. It does not require any applications of rulings to

the structure surface. The primary disadvantage of TSA is

the higher equipment cost.

2 3
Figure 1-A linear moire line pattern showing that hot interference fringes are generated by grid

overlay. The deformed (specimen) grid blocks light passing through the undeformed (analyzer)

grid at locations where the accumulated mismatch in the spacing of specimen line intervals

equals the analyzer line spacing.

MoirC Fringe Analysis

Full-field strain analysis using moire techniques is a fairly

well-established procedure in the field of experimental

mechanics. Moire analyses employ optical geometric

interference as a tool for assessing strain. While

interferometric moire techniques have tended to be confined

to laboratory conditions, techniques employing film

overlays have expanded its use to less restrictive

applications. Optional techniques used to enhance the

moire technique include grid vs. line patterns for biaxial vs.

uniaxial strain measurement, initial rotational or linear

mismatch, digital image analysis to amplify conventional

resolution, and fractional fringe and interferometry.

The basic theory of the moire method is easy to understand

and to apply. When two, closely spaced, line arrays having

similar, but not identical, patterns are superimposed and

viewed with transmitted or reflected light, interference

fringes are generated where lines from one line array block

the light transmitted between the lines of the second array

(fig. 1). Any distortion of one (active/specimen) array

relative to the other (analyzer) array will result in changes

in this fringe pattern that can be used to assess the location

as well as the magnitude of surface strain.

Two methods of enhancing the interference patterns are to

begin with a slight rotation of the analyzer (fig. 2) and to

use a small mismatch in the analyzer array vs. the specimen

array. An initial rotational mismatch facilitates the

differentiation of tension and compression zones. Initially,

the rotation causes an array of fringes oriented at one-half

the rotational angle from the strain direction. As the

specimen is strained, compression on the specimen will

Figure 2-The rotational mismatch of moire, biaxial, grid patterns shows

initial interference fringes with no distortion of the specimen.

24
cause a decrease in the rotational angle of the fringes while

tension will cause an increase in fringe rotation. A small,

linear mismatch results in an initial fringe pattern running

perpendicular to the principal strain direction. Tension will

cause a gain in the number of fringes, and compression will

cause a reduction in the number of fringes per unit length.

By recording the progression of the moire image using a

charge-coupled device (CCD) camera, the variation in

shades of gray between fringes, quantified using an image

processor, may be characterized as a periodic function

using Fourier series to interpolate strains across the entire

strain field. This technique can be used to divide a visible

fringe into four or five quantifiable increments, thus

increasing strain resolution.

A 2,000 line-per-inch moire film used to assess strain over a

gauge length of 12.7 mm [the approximate length of tooth

slots in metal plate connectors (MPC’s)] will show only a

fringe when displacement in that gauge length reaches

0.0254 mm, which is close to the yield point of steel. Being

able to detect one-third of the fringe displacement within

12.7 mm will give the strain in the range of design load.

In addition to its use for evaluating stresses in MPC’s, the use

of the moire method for evaluating the performance of

composites is being contemplated. The use of the moire

method is not confined to uniaxial strain analysis. By using a

moire grid, one can evaluate biaxial strains, including shear

strain. This technique has been documented by Post (1965).

Thermoelastic Stress Analysis

Belgen (1967) first documented the use of noncontact TSA.

In 1982, a British company, Ometron Ltd., began marketing

the first commercial TSA system, which they called

SPATE’ [Stress Pattern Analysis (by measurement of)

Thermal Emission].

The theoretical basis for thermoelastic stress analysis lies in

the first and second laws of thermodynamics. The most

commonly referenced derivation for the relationship

between temperature change and material stress relates only

to elastic deformation (Rauch and Rowlands 1993). Under

cyclic loading, this derivation can be simplified to a direct

linear relation (Eq. 1) that estimates the change in

temperature as a function of the change in stress times a

thermoelastic material constant and ambient temperature.

The thermoelastic material constant is inversely related to

material density and specific heat, and directly related to the

coefficient of thermal expansion. If the material is

orthotropic, the coefficient of thermal expansion is likely to

vary with the principal axis of the material, causing the

thermoelastic material constant to be different in each

direction.

A limiting factor in the application of TSA techniques is the

required control on the loading frequency, which must be

acceptable for adiabatic material response; that is, the

thermodynamic process must be reversible, requiring a

balance between the thermal and the mechanical energy.

For most metals, this requires a frequency above 2 Hz. For

materials of decreasing thermal conductivity, however, the

necessary frequency for adiabaticity increases. For

example, wood requires 18 Hz or more.

SPATE provides digital output, which, when color

calibrated, gives a graphic interpretation of how stresses

vary over the stressed field. While an observer cannot pick

up the resolution of the color thermograph when it is copied

in black and white, figure 3 gives some idea of the kind of

output obtained from SPATE. In this figure, the lighter

strands are heavily stressed over the region of the butt joint.

Above the third row of strands, there is evidence of

nonsymmetric load as stresses are larger for areas on the

right side of the plate.

Numerical techniques were used to smooth the digitized

output and give stress distribution plots as shown in figures

4 through 7. A slight compressive stress shown near the

end of the plots in figures 4 and 5 was caused when the row

of teeth farthest from the joint began to peel out of the

wood, bending the plate and compressing the surface.

An analysis of the digitized data suggests a fairly uniform

distribution to the various rows of teeth for a 7.6 by 12.7

cm plate designed with a balance between tooth-holding

and steel tensile stresses (fig. 5). As would be expected,

stress in the steel strands is greatest between the first and

second rows of teeth. The drop in stress is due to the load

being transferred by the teeth into the wood. The drop in

surface stress is roughly the same across each double row

of teeth, suggesting that the load transferred to the wood is

the same for each double set.

The TSA analysis of a 7.6 by 25.4 cm plate (fig. 8) at 4.45

kN shows that the drop in axial stress is not uniform over

the length of the plate. This analysis suggests that for longer

25
Fille d dhange mmle AutCk?ZCl Set Colr Intcrmg Exit

6rid Colors Datptr X ColrBarh Scale Sca n Display I

Figure 3-A black-and-white copy of a color thermogram showing the distribution and variation of stress in a metal connector plate stressed in axial

tension.

plates, the tooth load decreases from the joint to the ends of

the plate.

When looking at a more confined region, along steel

strands between rows of teeth, the effects of stress

concentration at the root of the strands can be seen (fig. 6).

Axial stress appears to decrease toward the middle of the

strands. Shear stress, however, increases toward the

middle of the strand, where the failure occurs in a steel

plate under axial stress.

While the popular use of TSA is for stress evaluation in

metal parts, it can be applied directly to wood. Figure 9

shows a piece of wood containing a knot stressed by

compression. This application has not received much

attention, but as this technology evolves, it has potential for

use in lumber grading, product development, and structural

evaluations. as well as in basic research.

Conclusion

Both the moir and TSA methods have potential for future

developments that will make use of growing computer

technology. While moirC technology is more mature than

TSA and has been applied to many different problems,

developments in the area of digital image correlation can

enhance its value as a precision tool for full-field strain

analysis.

2 6
SPATE SIGNAL ALONG SYMMETRIC LINE

2060-

1500-

3 -

&000-

ii :

~5586-

rJY -

0-

-500 I I I I, I I I, I I I, I I I I, 1

0.00 50.00 100.eJa 150.00 200.00

Data Location (pixe l)

Figure &The measured distribution of stress in plate strands and cross bars over half

the length of a 7.6 by 12.7 cm plate extending from the joint to the end of the plate.

STRESS ALONG SYM M ETRIC LINE

Data Location (pixe l)

Figure 5-A smoothed, axial stress distribution plot over half the length of the plate.

27
STRESS ALONG SYMMETRIC LINE

17000

16000

GAP

11000/

-0.20 -0.15 -0.10 -0.05 -0.00 0.0 5 0.1 0 0.1 5 0.2 0

Data Location (inch )

Figure 6-The distribution of axial stress along the length of a single strand shows that

the highest axial stresses were near the roots of the strand due to stress concentrations.

STRESS ALONG SYMMETRIC LINE

.,,,rn -0.2 0 -0.1 5 -0.1 0 -0.0 5 -0.0 0 0.0 5 0.1 0 0.1 5 0.2 0

Data Location (inch )

Figure 7-The variation of the average transverse stress along the strand length.

28
1

STRESS ALONG SYMMETRIC LINE

ii

--n-n- ________....____........________......____._.....____________._

E

c J---r

,"",""1~"',"~',"~',~~'~,~'~','~'~,"~~,~~'~,~'1

0. 0 0.5 1.0 1.5 2.0 2.5 3. 0 3.5 4.0 4.5 5.0 5.5 6. 0

LOCATION (INCH)

Figure 8-The distribution of axial stresses over half the length of a 7.6 by 25.4 cm plate.

This study demonstrated the first known application of TSA

to wood. While it suggests some serious limitations, it

shows TSA’s potential for future applications in lumber or

veneer stress grading.

The application of both the moire and TSA methods to

MPC joints has shown variations in stress along the length

of the plate and at stress concentration points. Either of

these techniques would be useful in optimizing connector

plate configurations.

Advantages of the moire method over the TSA method

include cost, portability, and the ability to easily take a

reading at any load. The ease of using moire film overlays,

along with development of CCD cameras and digital image

correlation techniques, provides the potential for taking fullfield strain measurements in the field. By permanently

attaching a moire pattern to highly stressed members in a

structure, periodic checks can be made to assess changes in

the state of stress. Availability of a new portable

interferometric camera should greatly increase the

effectiveness and versatility of moire applications to wood.

The primary advantages of TSA are its direct measurement

of member stress and its resolution. The TSA measurements

have a resolution comparable to that of strain gauges with the

advantages that nothing has to be attached to the specimen

and that the measurements are directly correlated to stress.

Companies concerned with the development of products

whose performance is sensitive to stress concentrations may

choose to conduct studies using the moire method on their

own, or contract with a private testing lab or university such

as the University of Wisconsin, to take advantage of the

capabilities of TSA.

Acknowledgments

This project was funded through a USDA Competitive

Grant (Project # 9202563) awarded to establish a

verification data base for plated connection analytical

models. We are grateful to S.H. (Ted) Lin for his assistance

in performing some of the SPATE analyses included in this

project.

Literature Cited

Belgen, M.H. 1967. Structural stress measurements with an infrared

radiometer. Instrument Society of America Transcripts. 6:49-53.

Post, D. 1965. The moir grid analyzer method for strain analysis.

Experimental Mechanics. 5( 11): 368-377.

Rauch, B.J.; Rowlands, R.E. 1993. Thermoelastic stress analysis. In:

Kohayashi, AS., ed. Handbook on experimental mechanics. [Place of

publication unknown]: VCH Publishers. Chapter 14.

2 9
30
Characterizing the Wood Fiber/Polymer Interface

T.G. Rials, M.P. Wolcott, and D.J. Gardner

Abstract

The last several years have seen an extensive research effort addressing a

relatively novel composite system composed of wood fibers and

thermoplastic polymers. In large part, this research has helped to define

the problems associated with the development of this type of composite

and to identify a significant void in our current knowledge. For example,

one major consideration for this composite is the limited compatibility that

commonly exists between these two dissimilar materials. Consequently,

poor adhesion between the phases is commonplace leading to poor stress

transfer from the plastic matrix to the stronger wood fiber. In short, an

inadequate fiber/polymer interface significantly limits mechanical property

development. Not surprisingly, because of the experimental challenges

presented by the small fiber size, very few studies have focused on the

direct evaluation of the interfacial characteristics of the wood

fiber/polymer composite system. Recently, research in a number of

laboratories has demonstrated the usefulness of several analytical

techniques that are available to investigate this important component. The

advantages and limitations of several of the more promising micromechanical methods under development are discussed, including: (1) the

fragmentation test, (2) the fiber pull-out test, (3) the micro-debond test,

and (4) dynamic mechanical analysis. In addition, examples of their

application to lignocellulosic, fiber-based composites will be described in

order to highlight the value of the fundamental information that is derived

by defining the physico-chemical parameters controlling the quality of the

fiber/matrix interface.

Introduction

The use of wood fibers as a reinforcing phase in thermoplastic

polymers has been identified as an attractive alternative use

for low-value timber species and recycled fibers. These

renewable fibers offer such favorable characteristics as high

strength and toughness, low weight, and reduced abrasion of

compounding equipment. There are, however, several

challenges that need to be overcome before significant

advances in wood fiber reinforced polymers (WFRP’s) can be

realized. One of the more prevalent issues is the relatively

widespread incompatibility that has been observed between

wood fibers and the synthetic thermoplastic polymers that

are logical matrix candidates (Kokta 1988). The inherent

T.G. Rials is a research physical scientist, USDA Forest Service, Southern

Research Station, 2500 Shreveport IIwy., Pineville, LA 71360; M.P.

Wolcott and D.J. Gardner are assistant professors. Division of Forestry, West

Virginia University, 206-B Percival Hall, Morgantown, WV 26506-6125.

chemical dissimilarities (e.g., hydrophilic vs. hydrophobic)

between the two phases of the composite generally result in

poor adhesion. As a consequence, stress transfer from the

matrix to the fiber is inadequate, and the performance

properties of the resulting composite suffer.

Given the significant contribution of the fiber/matrix

interface to composite properties, it is not surprising that a

substantial research effort has focused on improving the

compatibility of wood fibers with various thermoplastics like

polyethylene, polypropylene, and polystyrene. While this

research has typically involved relatively well-defined and

well-understood chemical modifications of the fiber’s surface

(or the plastic matrix), evaluating the effect of individual

treatments on interfacial strength and quality has presented a

tremendous challenge. Early investigations relied primarily

on evaluating the effect of fiber content on composite

properties, thereby defining a “fiber efficiency factor”

(Dalvag and others 1985). It became obvious that processing

variables, such as fiber damage and orientation, greatly

influenced the interpretation of the data in terms of the

quality of the fiber/matrix interface. An alternative approach

that has recently been reported is to evaluate the effect of

surface modifications on adhesion using a conventional peel

test (Kolosick and others 1992), or a modified pull-out

experiment with small wooden dowels embedded in a

thermoplastic matrix (Sanadi and others 1992). These

approaches have effectively eliminated the previously

described complications and have demonstrated considerable

value as approaches for the rapid and convenient evaluation

of different surface-modifying treatments. However, it is

virtually impossible to extrapolate this data to the fiber

composite system due to the complications introduced by the

morphological and macroscopic features of the wood

substrate. In addition, the information provided by these

experimental methodologies fail to provide the fundamental

information on the fiber/matrix interface that is needed to

adequately model this hybrid composite material.

In response to the shortcomings of previous experimental

approaches designed to study the strength of the wood

fiber/polymer interface, several methods have been adopted

31
and refined from outside the wood science arena. Four of

the more promising analytical techniques that have been

applied to this novel composite system include: (1) the

fiber fragmentation test, (2) the fiber pull-out test, (3) the

micro-debond test, and (4) dynamic mechanical analysis.

In this discussion, particular emphasis will be given to the

type of information that can be derived from these different

micro-mechanical methods, as well as to the limitations

presented by each approach.

Background

The quality of the fiber/matrix interface is not an entirely

new issue because it is relevant to virtually any type of

composite system including polymer blends, copolymers,

and filled polymers. The significance of the interface in any

composite material originates from the different chemical

and physical environments that are present at the boundary

between the different phases (Lipatov and others 1980).

Strong interaction between two materials can dramatically

alter the properties of the interfacial region by restricting

the mobility of the local polymer molecules. As a result,

the interface region is typically characterized by a property

gradient between the two pure phases (fig. 1). It is

important to recognize that strong adhesion between phases

can also negatively affect the properties of the composite

material. Consequently, it is important to define and

understand interfacial, structure-property relationships to

optimize the composite’s performance.

The contribution of the fiber/matrix interface to the

composite’s performance does seem to be more critical in

wood fiber/thermoplastic composites than in conventional

wood composites bonded with thermosetting resins.

Although many considerations, such as wettability, are

common to both types of composite systems, the interface

between wood fibers and thermosetting resins has been

studied more from a microscopic level than the interface in

wood fibers/thermoplastic composites. The relatively low

X

,- Average

material

Matrix

Fiber

Figure 1-A schematic illustration of a composite model with the contribution of the fiber/matrix interface

32
molecular weight and chemical reactivity of the resin mean

that adhesive bonding can be achieved through penetration

and mechanical attachment, and possible chemical reaction,

between the resin and the wood fiber. By contrast, the

thermoplastic system is characterized by high molecular

weight and limited chemical reactivity, suggesting that the

adhesion between phases is dominated by secondary

interactions (i.e., hydrogen bonding). Because these

interactions are of relatively low energy, the fiber/matrix

interface may only consist of several molecular layers,

presenting a significant analytical challenge. For this

reason, it is imperative to evaluate the wood fiber/

thermoplastic polymer interaction under conditions that

realistically simulate the actual environment of the

composite. Recent developments in micro-mechanical test

methods are beginning to make a real contribution to this

experimental problem and are providing the fundamental

information needed to understand and model the

contribution of the fiber/matrix interface to the ultimate

properties of WFRP composites.

Analytical Techniques

It is worth emphasizing from the outset of this discussion

that, while each of the micro-mechanical tests presents its

own challenges and limitations (table 1), there are several

cautions that are common to all of them. First, given the

small size of the fiber and the sample as a whole, handling

and sample preparation represent the most significant

challenges to be overcome in the application of these

techniques. Second, given the small interfacial area being

investigated, the presence of any voids or defects at the

interface will effectively invalidate any results. This can be a

Table l-Summary of the advantages and disadvantages of fiber/polymer interfacial test methods

Test Method Advantages Disadvantages

Fiber Fragmentation

(Critical Fiber Length)

Direct measure of fiber/polymer

interface strength

Stress optical analysis

Fiber Pull-out Indirect measure of fiber/

polymer interface strength

Information on interfacial failure

mechanism

Micro-debond Eliminates problem from tapered

fiber ends

Fiber strength information not

required

Relatively fast and simple

Dynamic Mechanical

Analysis

Links molecular structure to

properties

Requires ductile matrix polymer

Requires information on in-situ

fiber strength

Sample preparation (with fibers)

is difficult

Tapered fiber end complicates

data analysis

Fiber failure may be dominant

for strong interface

No direct measure of interfacial

strength

Provides a measure of interfacial

volume

33
Figure 2-A schematic illustration of the fiber fragmentation test showing applied stress, , and critical fiber length, 1,.

tremendous problem when using a reactive polymer system

where the evolution of volatile reaction products often occurs

(e.g., phenolic resins and polyurethanes). Finally, it is

important to recognize that frictional stress transfer, rather

than adhesive stress transfer, can be important and can

complicate analyses and comparisons between the various

methods. In short, attention to detail at all stages of

experimentation is critical to generating valid data.

Fiber Fragmentation Test-The fiber fragmentation (Drzal

1985), or critical fiber length test, involves embedding a

single fiber in a polymer matrix and stressing the sample in

the direction of the fiber’s longitudinal axis (fig. 2). As the

tensile load increases, the fiber will fracture, decreasing the

segment size until a critical length, I?, is reached, which is

too short to transfer the stress needed to break the fiber.

The critical fiber length can be related to the shear strength

(z,) at the fiber/polymer interface by:

where

o/d (1)

r,=Z- ‘

/ = the fiber tensile strength and d the fiber diameter.

Additional information on interfacial strength and stress

transfer efficiency can be obtained for translucent,

birefringent, matrix materials by viewing the deformation

process with polarized light microscopy to enhance the

image. This allows the observation of the stress field in the

matrix polymer, providing some insight into adhesion

mechanisms. Figure 3 is a stress optical micrograph of a

wood fiber embedded in polyethylene and shows the

concentration of stress around a natural fiber defect,

3 4

illustrating the utility of this approach. Note the stress

concentration in the matrix near the fiber ends. Samples

with poor interfacial properties would show little variation

in the stress field along the fiber length.

Samples can be produced by some relatively simple

approaches including dispersing a few fibers in a polymer

solution and casting a film. In order to eliminate solvent

effects, the fibers can be placed between two polymer films

and compression molded. Individual samples can then be

isolated by punching test specimens with appropriate fiber

orientation from the polymer sheet. Perhaps the greatest

restriction on the widespread use of the fiber fragmentation

test is the requirement of relatively ductile matrices that are

capable of strain levels above 3 percent. However,

alternative test designs are under investigation to minimize

this problem (Lee and Holguin 1990). Promising results

have been reported using this test procedure for wood fiber

composites of polyethylene (Tai and others 1992).

Fiber Pull-Out Test-The fiber pull-out test (Wang and

others 1988) involves embedding fibers to different depths

in the plastic matrix of interest as illustrated in figure 4 and

provides a critical fiber length similar to the fragmentation

test described above. Alternatively, individual fibers can be

embedded in small buttons of the polymer, and the force

required to extract the fiber can then be determined

(Lawrence 1972). This experiment is attractive because it

can also provide information on the interfacial failure

mechanism. Although the exact form of the relationship

depends on the type of failure that is observed for the

system, the interfacial debonding shear strength (Q) for the

fracture process can be obtained as follows:
Figure 3-An optical micrograph showing stress in a wood fiber embedded in polyethylene.

Figure 4-A schematic diagram of the critical fiber length pull-out test

where

V

E, G, zd=n -

Y

(2)

n = a constant related to the properties of the two components.

This test has been used to evaluate the effects of different

surface chemical modifications on the adhesion of

polyethylene to wooden dowels (Sanadi and others 1992).

E, = the fiber modulus, Unfortunately, it has not been applied extensively to wood

G, = the work of fracture of the interphase, fiber/polymer systems due in part to the difftculties associated

r = the fiber radius, and with accurately controlling the embedded length of the fiber.

35
In addition, the characteristic tapered ends of lignocellulosic

fibers greatly complicate the experiment and interpretation of

the data. Regardless, this approach holds considerable

promise because it is much more tolerant of the polymer

matrix properties and is sensitive to the type of failure.

Micro-Debond Test-The micro-debond test (Gaur and

Miller 1989) or micro-droplet test, is similar to the fiber

pull-out test in its approach and the type of information it

provides. The preparation of the sample involves first

solvent casting a thin film of the polymer matrix and

cutting a rectangle that is slit in the middle (similar to a

pair of trousers). The polymer “trouser” film is then hung

on a horizontal fiber mounted on a support and melted to

form a droplet. A small vise (fig. 5) is mounted on the

testing machine to grip the droplet, and the force required

to detach the polymer from the fiber is measured. The

fiber/matrix interfacial shear strength (r,,,) is estimated

from the relation:

Pmax

z =-

max TEDI

where

P,,, is the maximum load, D is the fiber diameter, and 1 is

the embedded fiber length.

The micro-debond test offers several important advantages

over the previously discussed methods including the fact that

the fiber strength or fiber strength/embedded length

relationships are not a consideration in this approach. Also,

the shear strength distribution can be assumed to be uniform

over a wide range of the embedded fiber aspect ratio. The

most critical factor in the successful use of this experiment

is control of the droplet size and of the interfacial area. If

the area is too large, then fiber failure prior to debonding can

become dominant. These problems have largely been

overcome by Liu and others (1995). Their investigations on

wood fiber/polystyrene composites indicate that the microdebond test provides a relatively simple and rapid method

for evaluating interfacial properties. Figure 6 shows typical

force-displacement results for polystyrene on rayon, cotton,

and wood fibers. From these experiments, a linear

relationship between the surface energy of the fiber and the

interfacial shear strength was established.

Dynamic Mechanical Analysis-Dynamic mechanical

analysis (Rosen 1982) provides a sharp contrast to the

previously discussed analytical approaches in that it is a

nondestructive test. In this experiment, the sample is

subjected to a small sinusoidal strain that allows the storage

and loss components of the complex modulus to be

resolved (fig. 7). By monitoring the variation in the storage

Microcaliper

t

- bFiber

Figure 5-A schematic illustration of the micro-debond test geometry.

36
Rayon/polystyrene

Cotton/polystyrene

Wood/polystyrene

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Displacement (in)

Figure 6-Typical results from the micro-debond test for polystyrene on wood, cotton, and rayon fibers.

and loss modulus over a wide range of temperatures and

frequencies, the viscoelastic spectrum of a material can be

generated. The interest in dynamic mechanical analysis lies

in the sensitivity of this method to molecular motion and

the relaxation properties of the composite material. Given

that strong interactions between the fiber and polymer can

substantially alter the mobility of polymer chains at the

interface, this technique can provide insight into the

mechanics of interfacial interactions and the relative

volume of the interface or interphase. Consequently, this

test method provides complementary information that is

necessary to completely characterize the properties of

WFRP’s.

The dynamic mechanical data presented in figure 8 clearly

illustrates the value of this nondestructive procedure

(Nassar 1994). Polyurethane composites were prepared

with thermomechanical pulp (TMP) fiber, mechanically

regenerated newsprint fiber, and acetylated TMP fiber (the

chemical reactivity decreases in that order), and a synthetic

polyol. The intense tan peak at approximately -40 “C,

common to all of the spectra, represents the glass transition

of the purely synthetic urethane phase. The unmodified

TMP fiber, which is considerably more reactive in this

system, exhibits a dramatically lower peak height as well as

a high temperature shoulder from about -25” to 10 “C. This

suggests that a high chain density interphase is established

more effectively with this fiber than with either of the other

fiber composite systems. The influence of this fiber/

polymer interphase on the material properties of the

composite results in a considerably higher storage modulus

(E’) at higher temperatures. It should be clear from this

example that dynamic mechanical analysis provides a

critical link between the molecular and phase structure and

the mechanical properties of wood fiber/polymer

composites.

Conclusion

The ultimate properties of wood fiber/thermoplastic

polymer composites are determined not only by the

37
?f I ---*

I L4LI

6 I

G0

z’ y*+

Rotating

Vector

Diagram

G” = G’ + iG” (compIex modulus)

G ’ = z I/y ’ (storage modulus)

G ’ = z “/y ’ (loss modulus)

tan 6 = z ‘/z’ = G ‘/G ” (loss tangent)

Figure 7 -Parameters provided by dynamic mechanical analysis.

1 0 ”

3 10’0

F4

b

g IO9 A

108

n Recycled

-100 -50

A Acetylated

0 50 loo I50 200 250

0'

-100 -50 0 50 loo 150 200 250

Temperature (OC) Temperature (“c)

Stress

and

Strain

Figure X-Dynamic mechanical spectra (log E’ and tan 6) of polyurethane composites prepared with thermomechanical

pulp fiber, acetylated thermomechanical pulp fiber, and recycled newsprint fiber.

3 8
properties of the fiber and the plastic, but also by the

characteristics of the fiber/matrix interface. As such, it is

imperative that reliable experimental techniques be

available for the determination of interfacial properties

under conditions that realistically simulate the stress

distributions of the use environment. Although

considerable progress has been made, it is apparent that no

single technique can satisfy this requirement at present.

Rather, an integrated approach is required that carefully

considers the characteristics of the fiber/polymer system

under investigation in selecting the appropriate micromechanical test method.

Literature Cited

Dalvag, H.; Klason, C.; Stromvall, H.E. 1985. The efficiency of cellulosic

fillers in common thermoplastics: Filling with processing aids and

coupling agents. International Journal of Polymeric Materials. 11: 9-38.

Drzal, L.T.; Rich, M.J.; Koenig, M.F. 1985. Adhesion of graphite fibers

to epoxy matrices. III. The effect of hygrothermal exposure. Journal of

Adhesion. 18: 49-72.

Gaur, U.; Miller, B. 1989. Microbond method for determination of the

shear strength of a fiber/resin interface: Evaluation of experimental

parameters. Composites Science and Technology. 34: 35-51.

Kokta, B.V. 1988. Use of wood fibers in thermoplastic composites

Polymer News. 13(11): 331-333.

Kolosick, P.C.; Myers, G.E.; Koutsky, J.A. 1992. Polypropylene

crystallization on maleated polypropylene-treated wood surfaces:

Effects on interfacial adhesion in wood/polypropylene composites. In:

Rowell, R.M.; Laufenberg, T.L.; Rowell, J.K. eds., Materials

mteractions relevant to recycling of wood-based materials: Materials

Research Society Symposium Proceedings; April 27-29, 1992; San

Francisco, CA. Pittsburgh, PA: Materials Research Society. 266: 137-154

Lawrence, P. 1972. Some theoretical considerations of fibre pull-out from

an elastic matrix. Journal of Material Science. 7(l): l-6.

Lee, S.M.; Holguin, S. 1990. A new single fiber/resin interface test for

highly cross-linked resin systems. Journal of Adhesion. 31: 91-101.

Lipatov, Y.S.; Rosovitsky, V.F.; Babich, B.V.; Kvitka, N.A. 1980. On

shift and resolution of relaxation maxima in two-phase polymeric

systems. Journal of Applied Polymer Science. 25(4): 1029-1037.

Liu, P.F.; Gardner, D.J.; Wolcott, M.P.; Rials, T.G. 1995.

Characterization of the interface between cellulosic fibers and a

thermoplastic matrix. Composite Interfaces. 2(6): 419-432.

Nassar, J.M. 1994. Production and analysis of wood fiber polyurethane

composites. Morgantown, WV: West Virginia University. M.S. thesis.

Rosen, S.L. 1982. Fundamental principles of polymeric materials. 2d ed.

New York, NY: Wiley-Interscience. Chapter 18.

Sanadi, A.R.; Rowell, R.M.; Young, R.A. 1992. Estimation of fiber-matrix

interfacial shear strengths in lignocellulosic-thermoplastic composites. In:

Rowe]], R.M.; Laufenberg, T.L.; Rowell, J.K., cds., Materials interactions

relevant to recycling of wood-based materials: Materials Research

Society Symposium Proceedings; 1992 April 27-29; San Francisco, CA.

Pittsburgh, PA: Materials Research Society. 266: 81-92.

Tai, W.C.; Quarles, S.L.; Rials, T.G. 1992. The effect of compatibilizers

on interfacial bonding in lignocellulosic fiber/polyethylene composites.

In: Kennedy, J.F.; Phillips, G.O.; Williams, P.A., eds. Cellulosics:

chemical, biochemical and material aspects, part 3. New York: Ellis

Horwood. Chapter 74, 507-5 12.

Wang, Y.; Backer, S.; Li, V.C. 1988. A special technique for determining

the critical lengths of fiber pull-out from a cement matrix. Journal of

Material Science. 23: 842-844.

3 9

Appendix
R Table l-Summary of presentation discussing strain measurement using traditional devices (Speaker: Joe Loferski)

Category

Extensometers

Strain Transducers

(a) User-built

Equipment

LVDT

Digital Dial Gauges

Mechanical Dial

LVDT’s, Dial Gauges,

Strain Gauges

cost

$500

$450

$50

Variable

Advantages

Accurate, high resolution

Accurate, easy to use

Accurate, east to use, inexpensive

Developed for specific uses, low

cost, reuseable, large or small

length

Limitations Possible applications

Heavy, bulky, involves electronics Measuring relatively displacements

Relatively expensive, heavy, bulky Measuring dimensions, deflections

Limited resolution, cumbersome Measuring dimensions, deflections

Development time, reliability and Measuring strains, distances, dimensions,

accuracy questionable and displacements

(a) Commercial

Electrical Strain

Gauges

Variety of special

test devices

Strain gauges, clamps,

wire, adhesive,

High

$5-$125

(Size

Development is already done, accuracy

guaranteed, industrial support

Accurate, reliable, industry support

Expensive

Cost, gauge length, patience

Measuring strains, distances, dimensions,

and displacements

Bonds directly to specimen for accurate

strain measurements

electronics, solder dependent)

Table 2-Summary of presentation discussing the digital image correlation technique in experimental stress analysis(Speaker: Audrey Zink)

Category

Image Acquisition

Image Correlation

Equipment cost Advantages Limitations Possible applications

CCD camera $500- Non-contacting Portability limited Unlimited, any material, any load type,

$2,000 any environmental conditions

Illuminators $500- Necessary for consistent lighting Must be unchanging Provide uniform lighting

$1,000

Image Capture Board $1,500. Allows digital acquisition of images Must be coupled w/computer and Image acquisition and subsequent storage

$3,000 CCD camera

Computer and Software $2,000. Full-field and point-to-point strain Limited by computer power, Measure strain distributions, concentrations,

LVDTs, Dial Gauges, $20,000 measurement, non-contact memory elastic constants, dimensional changes

Strain Gauges
Table Sa--Summary of presentation discussing the application of new imaging technologies to experimental mechanics (Speaker: Laurence Mott)

Category Equipment cost Advantages Limitations Possible applications

Light microscopes Transmitted/Reflected/

Incident Light

Microscope

$6,000 Inexpensive, low-maintenance, easy No contrast, limited surface Morphological measurements (when

$30,000 to use; very high resolution information connected to CCD camera); general

observations; used as polariscope for

ohotoelastic observations

Confocal Scanning

Laser Microscope

(CSLM)

$80,000- Allows 3D observations and recon- Expensive, specimen often requires 3-D morphological measurements of various

$250,000 structions (rotation about all axes); staining to flouresce; image acquisr- x-, y-, and z- planes; micromechanics (esp.

highest resolution of various confocal tion time may be slow; no real time fracture process); fiber and paper sections

microscopes image building

Deconvolution-type $20,000-

confocal microscope $30,000

Inexpensive 3D imaging; given microscope, only stage and software needed;

out-of-plane blur removed with

deconvolution software

NIPKOW Disk type $20,000- 3D imaging of materials; morphometric

confocal microscope $30,000 measuring in x-, y-, and z-directions

Direct Video Microscope $30,000- Large focal field, versatile, high

$60.000 maonification oossible

Limited sectioning available;

computationally intensive

Microbiology; limited use in micromechanics

Limited sectioning available; difficulty

in obtaining cross-sectional areas

Expensive

Microbiology; limited use in micromechanics

General observations; good for large,

soecimens

Electron Microscopes Scanning Electron $60,000- Very high magnification, good surface High-vacuum environment, specimens Morphology of individual fibers; study of

Microscopes (SEM) $170,000 detail (non-conductive) need coating in a failure mechanisms in composites, adhesive/

brittle medrum (gold) coating distribution on wood components

Environmental SEM

(ESEM)

$150,000- No conductive coating needed, water Expensive; maintenance Intensive, Observation of failure mechanisms of small

$250,000 can exist in liquid state (due to higher generally requiring agreement of specimens (individual fibers, adhesion)

pressures); dynamic testing approximately $10,000 year during dynamic testing; fiber hydration

Digital Image

Processrng

Digital Image

Enhancement

$200-

54,000

Allows user to increase visual contrast

within a digital image

If not careful, may alter original digital Used to show areas of high contrast; useful

image; time intensive in studying fracture mechanisms and resin

distributions

Morphometric

Measurements

51,500- Measurements can be made qurckly

56,000 using macros; helps reduce bias

Requires some training, and access

to digital images; macros generally

soecific for certain samole

Excellent for determining length, width, and

thickness distributions within a sample

Digital Image

Correlation

50-

$3,000

Allows remote sensing of non-contact

strains; allows for measurement of

Need capability to acquire successive Determination of local stiffnesses within a

digital images during testing; code solid or composite specimen

active and passive strains needs to be written or modified by user

Image Acquisition CCD camera

Tube cameras

$1,500- Allows images to be stored digitally for Resolution of image proportional to Necessary for digital assessment of

$30,000 morphological measurements or cost of camera and frame grabber; morphological characteristics, and for

contrast enhancement; flexible best CCD camera cannot match the acquisition of digital images (archival and

images, quantitative imaging quality of standard analog camera comparison purposes)

$1,500- Very high resolution and spectral Analog images need framegrabbers to High magnification and high resolution

$15,000 sensitivity be used quantitatively; supporting microscopy (e.g. CSLM); microbiological

resolution framegrabbers tend to be sciences

expensive, physically less robust

Frame Grabber 5800- Necessary hardware which interfaces Resolution of image proportional to Necessary for digital assessment of

$10,000 CCD camera with processor cost of camera and frame grabber; morphological characteristics, and for

resolution of frame grabber should acquisition of digital images (archival and

approximate that of the CCD camera comparison ourposes)
%

Table 3b-Supplemental information for presentation on applying new imaging technologies to experimental technologies (Speaker: Laurence Mott)

Technique Setup cost Technical considerations

ESEM microtensile

stage

ESEM facilities can be purchased on an hourly basis. Few facilities have the

microtensile stage assembly which costs about $7,000. Need to manufacture

own tensile grips. A LaB6 filament can also be purchased for about $1,000.

Crosshead movement typically 1-l 00 microns/second. Maximum load around 100 Ibs.

Fast (near TV) scan rate can be used when testing large solids. Condenser lens can be

adjusted to avoid burning the sample over time. Images can be used for digital image

The filament increases contrast and base quality. correlation. Possible to condense water droplets on sample.

Basic CCD imaging

systems

Alternative Scanning

Micrographs

Cameras of medium resolution cost approximately $1,000. RGB high resolution Matching chip and lens size is important. Using a larger lens size than chip is preferable.

monitor costs $1,000~. Framegrabber cost = $1 ,OOO+. A computer w/sufficient Smaller chip size leads to better resolution. Chip size from 5 to 25 mm. Good signal

memory essential. 640x640 image = 300k. Lenses cost anywhere from $500+. to noise ratio of 52dB+. Higher resolution = lower light sensitivity. Color cameras

Extension tubes can be used to convert the system into a video microscope. may have 1, 2, or 3 chips. Signal can be extracted from these individually.

None A 600 dip scanner can digitize a high resolution micrograph. The image can then be used

in the same fashion as a digitally acquired image. Copy centers generally offer a high

resolution scanning service.

Free software None Kohroe is one of the most powerful digital image processing packages available. This

UNIX-based system is available thru ftp from the University of New Mexico. For

Macintosh computers, NIH is one of the best digital imaging packages available

free on the public domain.
Table 4--Summary of presentation discussing full-field stress analysis using moire and thermo-elastic stress analysis (Speaker: Ron Wolfe)

Category

Full-Field Stress

Analysis

Equipment

Moire Master Grid

Photo Developing

Equipment

cost

$900

$1,000

Advantages

Portability

Photographic prints adhered to test

surface or direct exposure of photo

Limitations

Strain measurement

Requires knowledge of material

MOE for stress analysis

Possible applications

Lab or field evaluation of stress around

connections

Stress patterns in engineered composites

used as webs or shear diagrams

sensitive emulsion

CCD camera $2,000 Improved resolution; no need to

convert analog to digital

Full-Field Stress

Analysis

S.P.A.T.E. $120,000 Resolution comparable to strain Expensive, not easily used in field Connections, quality control for engineered

gauges; direct evaluation of components

material stresses

Table 5--Summary of presentation discussing the characterization of the interface in wood fiber/polymer composites (Speaker: Mike Wolcott)

Category

Thermo-Dynamic

Equipment

Dynamic Mechanical

cost

$70,000-

Advantages

Links molecular structure and

Limitations Possible applications

Small samples; difficult to incorporate Viscoelastic characterization: determination

Properties Analyzer

Mechanical Properties Miniature Materials

Tester

$120,000 physical behavior; provides thermal moisture control; best for solid of glass transition temperature; determination

relations of mechanical properties systems of polymer morphology: composite structure

$18,000 Produces load-elonation traces of Small specimen size; low load Stress optical analysis; determination of

small specimens, thus providing capacity; monotonic loading of mechanical properties of small specimens

mechanical properties; small enough

to fit under conventional microscope

for material observation

R

Groom, Leslie I-I.; Zink, Audrey G. 1998. Techniques in experimental mechanics

applicable to forest products research-Proceedings of a Plenary Session at the

Forest Products Society Annual Meeting; 1994 June 26-29; Portland, ME: Gen.

Tech. Rep. SO-125 Asheville, NC: U.S. Department of Agriculture, Forest

Service, Southern Research Station. 45 p.

Presents opening remarks and presentations of five speakers from a plenary session

on techniques in experimental mechanics presented at the FPS Annual Meeting in

Portland, ME on June 26-29, 1994. Information presented in this report is aimed at

forest products researchers and the forest products industry.

Keywords: Strain measurement, extensometers, transducers, digital image

correlation, strain mapping, enviromnental scanning electron microscope, confocal

scanning laser microscope, imaging, moire, SPATE@, metal plate connectors,

interface, micro-debond, fragmentation, critital fiber length.
The Forest Service, U.S. Department of

Agriculture, is dedicated to the principle of multiple

use management of the Nation’s forest

resources for sustained yields of wood, water, forage, wildlife, and

recreation. Through forestry research, cooperation with the States

and private forest owners, and management of the National Forests

and National Grasslands, it strives-as directed by Congress-to

provide increasingly greater service to a growing Nation.

The United States Department of Agriculture (USDA) prohibits

discrimination in its programs on the basis of race, color, national

origin, sex, religion, age, disability, political beliefs, and marital or

familial status. (Not all prohibited bases apply to all programs).

Persons with disabilities who require alternative means for

communication of program information (braille, large print,

audiotape, etc.) should contact USDA’s TARGET Center at 202-

720-2600 (voice and TDD).

c

To file a complaint of discrimination, write, USDA, Director,

Office of Civil Rights, Room 326-W, Whitted Building, 14th and

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