United States

Department of

Agriculture Nondestructive Methods

Forest Service

Forest

Products

for Detecting Defects in

Laboratory

Research Softwood Logs Paper

FPL-RP-546 . .

Kristin C. Schad

Daniel L. Schmoldt

Robert J. Ross
Abstract

Wood degradation and defects, such as voids and knots, affect

the quality and processing time of lumber. The ability to

detect internal defects in the log can save mills time and

processing costs. In this study, we investigated three nondestructive evaluation techniques for detecting internal wood

defects. Sound wave transmission, x-ray computed tomography, and impulse radar were used to examine white spruce

and balsam fir logs. Computed tomography resulted in the

highest resolution for voids, knots, and high moisture content areas, but at a very high price. Both sound wave transmission and impulse radar were able to detect large voids and

areas of degradation, and these techniques showed some sensitivity to very knotty logs. None of the methods was able to

detect small pockets of decay. The use of radar requires an

experienced operator because of the difficulty of interpreting

the data.

Keywords: NDE, radar, stress waves, CT, scan, logs

February 1996

Schad, Kristin C.; Schmoldt, Daniel L.; Ross, Robert J. 1996.

Nondestructive methods for detecting defects in softwood logs.

Res. Pap. FPL–RP–546. Madison, WI: U.S. Department of Agriculture,

Forest Service, Forest Products Laboratory. 13 p.

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Contents

Page

Introduction................................................................1

Description of NDE Methods.........................................1

Sound Wave Transmission.........................................2

Computed Tomography.............................................2

Impulse Radar..........................................................2

Study Materials and Methods.........................................2

Materials ................................................................2

Sound Wave Transmission.........................................3

Computed Tomography.............................................3

Impulse Radar..........................................................4

Destructive Verification.............................................4

Results and Discussion.................................................5

Sound Wave Transmission.........................................5

Computed Tomography.............................................5

Impulse Radar........................................................11

Destructive Verification...........................................11

Conclusions .............................................................11

Future Research Needs................................................13

References................................................................13
Introduction

Softwood can be degraded by fungi or other organisms at any

time—either while the tree is growing or after it has been

felled, sawn into logs, and stored. Some degradation makes

whole logs or portions of logs undesirable for lumber.

Knowing the sites of degradation in a log, as well as the sites

of defects such as knots and voids, can improve the processing of logs into lumber. For example, unsound sections of a

log can be consolidated as logs are bucked in the woodlot or

in the log yard. In addition, a better opening face can be

selected during sawing, thus improving yield. However,

current visual methods of assessing logs cannot detect

internal defects.

Researchers have proposed a number of techniques for detecting internal defects in logs. These include x-ray computed

tomography (CT) (Benson–Cooper and others 1982), nuclear

magnetic resonance (Chang and others 1987), microwaves

(Birkeland and Holoyen 1987), and ultrasonic CT (Han and

Birkeland 1992). Other researchers have used sound wave

transmission (Lee 1965) and impulse radar to test large,

wooden components of structures in situ. Each of these

methods is able to detect certain internal features, but they

exact a price in terms of time and expense. Not all of these

tradeoffs are well defined. Nevertheless, as processing waste

becomes an increased liability in mill operations, internal log

scanning will become more common. Therefore, it is necessary to improve our understanding of different flaw detection

methods for logs and to identify their advantages and disadvantages.

Nondestructive evaluation (NDE) methods, by definition, do

not change the end-use capabilities of materials that are

tested. In this study, we compared three NDE methods: sound

wave transmission, x-ray CT, and impulse radar. In sound

wave transmission, the transmission time of a sound wave is

measured as it travels through a specimen. This technique

has been used in a variety of wood evaluation applications,

including veneer grading (Sharp 1985), wetwood detection

(Ross and others 1992, Verkasalo and others 1993), and

evaluation of wooden members in structures (Hoyle and

Pellerin 1978). Computer tomography generates a crosssectional density map of a specimen by passing x-rays

through a plane of the material. The attenuation of x-rays is

linearly related to material density. This technique has been

used extensively in the medical field and, more recently, in

many industrial areas. It has also been shown to be useful for

evaluating logs (Benson–Cooper and others 1982, Funt and

Bryant 1987, Wagner and others 1989). Impulse radar (also

called ground-penetrating radar) sends radio wave pulses

through a material and captures the signal as it echoes back

to the transmitter. This method has been widely used for

geotechnical investigations (Stanfill and McMillan 1985).

Although sound wave transmission and impulse radar NDE

methods have not been previously used for logs, their application to large wooden components of structures, such as

buildings, bridges, and utility poles, indicates that they

might be successfully used to examine logs.

In this paper we briefly describe sound wave transmission,

x-ray CT, and impulse radar and report the results of tests

using these NDE methods.

Description of NDE Methods

All NDE methods use some form of energy propagation

through or around a material to infer some important characteristics about the specimen being examined. This energy

may result from static or dynamic loading or may be generated by electromagnetic or elastic waves. Characteristics

frequently of interest are the external geometry or orientation

of the specimen; its global integrity, such as strength; and

local features, such as the location and size of surface or internal defects.

Nondestructive Methods

for Detecting Defects in

Softwood Logs

Kristin C. Schad, General Engineer

Daniel L. Schmoldt, Research Forest Products Technologist

Robert J. Ross, Supervisory General Engineer

Forest Products Laboratory, Madison, Wisconsin
2

Sound Wave Transmission

Sound waves can be generated within a material in one of

several ways. First, sound can be generated by the material

itself as a result of the sudden movement of stressed portions.

Sounds are emitted and propagation is related to the dynamic

effects of any defects within the material. In a technique referred to as acoustic emission inspection, a piezoelectric sensor is used to detect the sound. Second, sound can be generated externally by a contacting piezoelectric transducer. Sound

waves generated in this manner are received by a second

transducer (pitch–catch or through-transmission) or are reflected to the generating transducer (pulse–echo). These waves

are often above the audible frequency range, and hence, this

inspection method is referred to as acousto-ultrasonic testing

or ultrasonic testing. Third, sound waves can be produced by

impacting the material with an instrument, such as a hammer, which generates an elastic wave within the specimen.

The frequency of this wave may or may not be ultrasonic.

A timer connected to the point of impact and some other

location on the surface of the specimen can be used to measure the transmission time of the sound wave and hence its

velocity. This method is referred to as sound wave transmission, and it is the method used in our study.

Computed Tomography

High-energy electromagnetic radiation, in the form of x-rays

or gamma rays, can be used to generate highly detailed images of materials. X-ray (or gamma ray) scanning indirectly

measures the density of a material through the attenuation of

the rays as they pass through a specimen. As photons are

absorbed and scattered by incident matter in a specimen, the

intensity of the measured x-rays is reduced. The greater the

mass of the specimen within a volume (that is, the greater

the density), the greater the attenuation of the x-rays or loss

of x-ray energy.

Tomographic x-ray scans of logs are produced by sending xrays through the two smallest dimensions of the specimen.

Because the attenuation of each ray is the cumulative effect

of all the small volumes in its path, the contribution of each

volume cannot be determined from a single ray. Therefore,

many individual rays are sent through each small volume in

the material. The attenuation resulting from each volume can

then be computed from all the x-rays passing through it.

Since the resulting tomograph, or image, must be computed,

the technique is called computed tomography (CT). This

procedure generates a two-dimensional scan plane image or

cross section. Successive scan planes are generated along the

longest dimension to produce a detailed scan of the entire

specimen.

Impulse Radar

Radar waves reside in the microwave portion of the electromagnetic spectrum. Because of their long wavelengths, they

are able to penetrate deeply into materials. Radar waves reflect from any boundaries contained in the material. Because

clear wood provides relatively insignificant boundaries,

abrupt changes caused by defect interfaces with clear wood

generate more noticeable reflections. The relative time delays

between the reflections from such interfaces can create a map

of where discontinuities exist in the material.

An early NDE application of radar was the location of subterranean objects, such as buried drums. Extensive use in this

area led to the term “ground-penetrating radar.” A later NDE

application of impulse radar has been the inspection of structures in situ. An antenna can be passed along a specimen to

generate a specimen length–depth map of internal reflections.

This map can be interpreted to indicate the sites of various

interfaces and the types of internal objects represented by the

reflections.

Study Materials and Methods

Materials

Five logs, three balsam fir (Abies balsamea, L.) and two

white spruce (Picea glauca, Moench), were shipped from the

Pukall Lumber Company in Woodruff, Wisconsin, to the

Forest Products Laboratory in Madison, Wisconsin, for testing. The logs varied in length and diameter (Table 1). Log 1

had a large hole in the butt end (Fig. 1), and log 3 had a few

small voids on one end. These were the only visible signs of

degradation on any logs.

The five sample logs were harvested in winter. They were

stored outside in much the same way as they would be stored

at a sawmill. Sound wave measurements were taken in June

and July 1993, the logs were scanned by CT in February

1994, and impulse radar was performed in June 1994.

Table 1. Description of study logs

Log Length Diameter (in. (mm))

Species no. (in. (mm)) Butt end Small end

Balsam fir 1 101 (2,565) 14 (357) 12 (305)

2 124 (3,150) 12 (305) 11 (279)

3 198 (5,029) 15 (381) 12 (305)

White spruce 4 200 (5,080) 15 (381) 12 (305)

5 195 (4,953) 15 (381) 13 (330)
3

During this time span, some drying of the logs occurred.

However, because all three techniques have been shown to be

effective for dry and wet material, the only effects introduced

by this time delay were additional cracking of the logs and

possibly more advanced decay.

Sound Wave Transmission

The first test was sound wave through-transmission (Figs. 2

and 3). The logs were supported off the ground by two sawhorses. Six-inch (152-mm) spikes were driven into the log

for approximately 1 in. (25.4 mm) to mark the start and stop

locations for measurements of sound wave transmission time

through the diameter. Care was taken to locate the spikes

away from obvious defects. An accelerometer was attached to

each spike and then connected to a Metriguard Stresswave

Timer (Model 239A). A sound wave was sent through the

wood to the receptor spike via a hammer tap on the end of

the impact spike. The timer displayed the travel time of the

sound wave between accelerometers. The impact spike was

then moved 6 in. (152 mm) and sound transmission time

was measured again; this process was repeated to the end of

the log. A log-length set of times was taken in three locations on each log, approximately 120° apart (Fig. 4).

Computed Tomography

Logs were scanned via an arrangement with the Advanced

Computed-Tomography Inspection System located at the

NASA Marshall Space Flight Center in Huntsville, Alabama. The industrial-scale CT unit consists of a scanning

table that rotates and that also translates in two dimensions,

vertical and front to back. Therefore, to obtain log crosssections, it was necessary to stand the logs on end. To faciliFigure 1—Decay and loss of wood material on

butt end of log 1. (M93 30157-4)

Stress

wave

received

Impact ➞➞➞ ➞applied

Figure 2—Schematic depiction of sound wave

transmission test.

Figure 3—Positioning of log for sound wave transmission test. Each log was supported by sawhorses.

Figure shows placement of impact and receptor spikes,

attachment of accelerometers, and connection to timer.

(M93 0157-12)
4

tate this process, the logs were cut into sections 4 to 5 ft

(1.2 to 1.5 m) in length, with one end flat enough so that the

log could stand on end with little vibration when rotated. For

logs 2 to 5, scans were taken every 6 in. (152 mm) to coincide with the sound wave measurements taken previously

(Fig. 5). Because the sound wave transmission measurements

obtained for log 1 were highly erratic, this log was scanned

every 3 in. (76 mm).

Each CT image slice is 0.12 in. (3 mm) thick. Images have a

pixel size of 1024 by 1024, where each pixel represents

0.016 in. (0.415 mm). Each pixel is represented by a 16-bit

number and therefore requires 2 MB of storage. Image files

were transferred electronically over the Internet to Virginia

Tech University.

One limitation of this study and its results with regard to CT

scanning is that the scanner was calibrated for industrial

applications. That is, the materials commonly scanned with

this unit are large specimens of very dense metals, such as

rocket engines and entire automobiles. Consequently, the CT

unit was calibrated to give CT number values for materials

much denser than wood. Whereas a typical range of CT numbers for any scanned material (including wood) would be

0 to 2047, without recalibration the CT numbers for the

wood samples in our study typically ranged from 0 to 65.

Therefore, although the spatial resolution in this study was

high (0.016 in. [0.415 mm] per pixel), the density resolution

was very low (approximately 66 density levels). This means

that our ability to distinguish different density areas in logs

was severely limited—by a factor of 30 (2048 divided by

66)—compared to the differences in density that can typically

be obtained by CT imaging.

Impulse Radar

The impulse radar test was performed at the Forest Products

Laboratory by the staff of Detection Sciences, Inc., of Carlisle, Massachusetts. A 1.2-GHz radar antenna was run along

the length of the log. A hand-held switch was used to electronically mark the log position of the antenna on the

graphic chart recorder, which produced a printed scan of the

data.

This continuous scanning method was performed longitudinally for each log section, coincident with the path of the

sound wave measurements and perpendicular to the CT scans

(Fig. 6). The process yielded a total of three graphic charts

per log section.

Based on prior experience, the testing staff knew that impulse

radar detects the nonuniformity of material on the basis of

signal velocity. Much nonuniformity along the length of a

log results from the presence of knots. Therefore, the testing

staff first created a simulated log scan by holding the antenna

stationary and recording a radar chart. This chart served as a

reference for the comparison of other scans.

Destructive Verification

After the NDE tests were completed, the logs were sawn

every 6 in. (152 mm) to coincide with the locations of the

sound wave and CT tests. Soon after sawing, log crosssections were photographed to provide a basis for comparison

to the NDE results.

Stress wave paths

6 in.

3 ft

1 ft

2 ft

3 ft

2 ft

1 ft

Figure 4—Scan lines for sound wave transmission test.

Longitudinal scan lines were spaced 120o apart. Measurements were taken at 6-in. (152-mm) intervals along

scan lines.

6 in.

Figure 5—Location of CT scans.
5

Results and Discussion

Figures 7 to 11 show the results of NDE and destructive tests

on the five sample logs. Each figure shows (a) sound wave

transmission time per unit distance for three scan lines for all

log scan positions, (b) representative CT scans, showing

high density (light shading) and low density (dark shading)

areas, and (c) representative photographs of sawn logs. In the

photographs of sawn logs, the numbers indicate approximate

locations of sound wave tests.

Sound Wave Transmission

Sound wave transmission time (or velocity) appeared to distinguish between areas of high degradation and sound wood

and to locate large voids. Defects were indicated by longer

transmission times (lower velocity). Sound wave transmission did not appear sensitive to less extensive voids or pockets of decay, as evident by comparing the transmission time

at the 36-in. (914-mm) position with its corresponding CT

image (Fig. 7).

Transmission times were also somewhat sensitive to

very knotty areas of logs, as shown by highly varying

transmission times (Fig. 8a) for the knotty areas at the

30- in. (762-mm) and 72-in. (1,829-mm) positions (Fig. 8b).

In other instances, this relationship was not as clear cut. For

example, at the 18-in. (457-mm) position, the knots nearly

lie in a single plane in contrast to the almost total absence of

knots at the 102-in. (2,591-mm) position. For these log positions, there did not seem to be any noticeable difference in

transmission times, although the two positions varied greatly

in the number of defects. Although large voids, severely degraded material, and extremely knotty areas increase sound

transmission time, it can be difficult to identify the type of

defect or to determine if multiple defects are present. However, a pattern is sometimes formed, as in the butt end of

log 1 (Fig. 7a), which enables the operator to make an educated guess that this is highly degraded material or a void.

To obtain transmission time, the bark on the log must be

penetrated or removed. Once this is accomplished, the technique can be performed quickly and requires little training.

The equipment is relatively inexpensive.

Computed Tomography

The CT scanner used in this study was able to locate knots,

large and small voids, and areas of high moisture content.

(Note the caveat of scanner calibration and density resolution

mentioned previously.) Voids were displayed as dark areas

(similar to background) on the CT images, corresponding to

the density of air (Fig. 7b). Areas of high density such as

knots, including high moisture content areas, were displayed

by very light shading on the CT images (Fig. 9b). One problem in interpreting these CT images was that knots are also

higher in density than the surrounding wood, as are areas

with high moisture content. Therefore, it is sometimes difficult to determine where an area of high moisture content

stops and a knot begins (Fig. 9b). Birkeland and Holoyen

(1987) noted the same problem with high moisture areas

when scanning Norway spruce (Picea abies, L.), as did Som

and others (1992) and Taylor and others (1984). With CT

images of higher density resolution, it is possible that knots

and high moisture content areas would have different CT

numbers and, consequently, would appear differently in the

images. If a log does not have large areas of high moisture

content, a CT scan will locate knots with extreme accuracy,

as shown in Figure 10b.

As examined in this study, CT scanning has several drawbacks. First, obtaining enough scans to produce a representative set of images for a log can be time-consuming. However, a scanner that is optimized for this type of scanning

could generate images very quickly and therefore greatly

mitigate this problem. Second, highly skilled personnel are

required to maintain and operate CT equipment. Finally, the

cost of a CT scanner is approximately two orders of magnitude greater than that of sound wave transmission equipment.

These limitations indicate that the information gained from

the use of CT must be carefully weighed against the high

cost of this technology.

▲

▲

▲

1

2

3

▲

= scanning motion of radar equipment

1 ft

2 ft

3 ft

Figure 6—Scan lines for impulse radar tests.
b c

d e

Time (

μs/ft)

Velocity (m/s)

Log position (in.)

▲ ▲ ▲

▲ ▲ ▲ ▲ ▲ ▲

▲ ▲

▲

▲

▲

▲

▲

900

800

700

600

500

400

300

200

100

350

550

750

950

1150

1350

1550

1750

1950

2150

2350

2550

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102

a

Figure 7—Log 1 tests. (a) Sound wave transmission times and corresponding velocity values;

(b,c) CT scans at 6-in. (152-mm) and 36-in. (914-mm) positions; (d,e) photographs of sliced log

corresponding to CT scans. 1 in. = 25.4 mm; 1 ft = 0.3048 m.

6
Time (

μs/ft)

Velocity (m/s)

Log position (in.)

▲ ▲ ▲

▲

▲ ▲

▲

▲ ▲

▲ ▲ ▲

▲ ▲

▲ ▲

900

800

700

600

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950

1150

1350

1550

1750

1950

2150

2350

2550

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120

126

▲ ▲ ▲ ▲

a

b c

d e

Figure 8—Log 2 tests. (a) Sound wave transmission times and corresponding velocity values;

(b,c) CT scans at 30-in. (762-mm) and 72-in. (1,829-mm) positions; (d,e) photographs of sliced

log corresponding to CT scans.

7
Time (

μs/ft)

Velocity (m/s)

Log position (in.)

▲ ▲

▲ ▲ ▲ ▲ ▲ ▲

▲ ▲

▲ ▲

▲

▲ ▲

900

800

700

600

500

400

300

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1150

1350

1550

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2150

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2550

0 12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204

▲ ▲ ▲ ▲ ▲ ▲ ▲

▲

▲ ▲ ▲ ▲ ▲ ▲

▲ ▲ ▲ ▲ ▲ ▲

a

b c

d e

Figure 9—Log 3 tests. (a) Sound wave transmission times and corresponding velocity values;

(b,c) CT scans at 6-in. (152-mm) and 36-in. (914-mm) positions; (d,e) photographs of sliced log

corresponding to CT scans.

8
Time (

μs/ft)

Velocity (m/s)

Log position (in.)

▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲

900

800

700

600

500

400

300

200

100

350

550

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1350

1550

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1950

2150

2350

2550

0 6 18 30 42 54 66 78 90 102 114 126 138 150 162 174 186 198 210

▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲

Figure 10—Log 4 tests. (a) Sound wave transmission times and corresponding velocity values;

(b,c) CT scans at 48-in. (1,219-mm) and 120-in. (3,048-mm) positions; (d,e) photographs of sliced

log corresponding to CT scans.

a

b c

d e

9
Time (

μs/ft)

Velocity (m/s)

Log position (in.)

▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲

900

800

700

600

500

400

300

200

100

350

550

750

950

1150

1350

1550

1750

1950

2150

2350

2550

0 6 18 30 42 54 66 78 90 102 114 126 138 150 162 174 186 198

▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲

Figure 11—Log 5 tests. (a) Sound wave transmission times and corresponding velocity values;

(b,c) CT scans at 90-in. (2,286-mm) and 162-in. (4,115-mm) positions; (d,e) photographs of sliced

log corresponding to CT scansn

a

b c

d e

10
11

Impulse Radar

Operators of the impulse radar equipment were able to distinguish between basically sound wood and wood with major

defects or voids (Detection Sciences, Inc. 1994). When a severely degraded log was tested, the chart was wavy, with depressed or elevated reflection signals (Fig. 12a). On the other

hand, the chart from the test of a relatively sound log was

relatively smooth (Fig. 12b). Impulse radar was not able to

locate small voids or pockets of decay or to distinguish between different types of internal degradation. Using the simulated radar chart as a reference, the investigators were able to

rank the log sections subjectively on the basis of uniformity

of radar charts (Table 2).

The labeling of signal reflections as “wavy” or “depressed”

can be quite subjective. Therefore, the impulse radar method

provides only a rough indication of where defects may be

found. All radar transmission planes typically pass through

the log pith, which is generally degraded material and registers as a defect. Therefore, interpretation of a impulse radar

chart needs to account for this omnipresent degradation.

An operator can perform impulse radar without touching the

wood; only moderate skill is required to conduct testing at a

fairly quick pace. The price of the equipment is also moderate, about twice as much as that of sound wave transmission

equipment.

Destructive Verification

Figure 13 shows degradation in a small block of wood removed from the 6-in. (152-mm) position in log 4. Fungal

infection, bacterial growth, and pit degradation in this area

were severe. Other samples tested from areas similar in

appearance to that marked on Figure 13 showed no degradation. Therefore, at the high spatial and low density resolution

used in this study, CT imaging could not reliably detect this

type of decay. Likewise, sound wave transmission and impulse radar were not sensitive to small areas of degradation.

Conclusions

The results of our study led to the following conclusions:

• Sound wave transmission, x-ray computed tomography

(CT), and impulse radar can be used to detect large areas of

internal degradation and large voids in logs.

• None of the nondestructive evaluation methods tested

can locate small pockets of decay. Computed tomography

is able to detect small areas of void and advanced degradation. However, in our study, failure to recalibrate the CT

scanner for wood material, which reduced our ability to

distinguish density differences, could have been partially

responsible for the insensitivity of CT in our tests.

• All three techniques can detect the presence of knots in

very knotty wood. CT can also precisely locate regions of

high moisture content and other high density areas.

• The use of impulse radar may be limited by the difficulty

of interpreting the data. Although the scientist from Detection Sciences, Inc. was able to use the data to distinguish between basically sound wood and wood with major

defects or voids, none of the current investigators was able

to do so. The pulse-echo charts do not generate a repeatable numerical value (as does sound wave transmission),

nor do they provide a detailed or easily interpretable image

(as does CT). Consequently, we have considerable doubt

concerning the use of impulse radar for the nondestructive

evaluation of logs.

• Impulse radar equipment is about twice as expensive as

equipment used for sound wave transmission, and CT

equipment can be at least 100 times as costly. The high

cost of CT equipment and operation demands careful

examination of its potential benefits for the operation of a

particular mill.

Table 2—Ranking of log sections on

basis of uniformity of radar chart

Log sectiona Rankb

4–2 1

4–1 2

4–3 3

4–4 4

3–3 5

3–4 6

1–2 7

5–1 8

5–4 9

5–2 10

5–3 11

3–2 12

3–1 13

2–2 14

2–1 15

a

Section 1–1 not included because of

 degradation level.

b

Low values indicate greater uniformity;

 that is, relative freedom from large

 knots. Conversely, high values

 indicate presence of many large knots.
12

a b

Figure 12—Pattern of impulse charts depended on the amount of wood degradation. (a) Impulse chart

from severely degraded log, showing depressed or elevated reflection signals; (b) impulse chart from

relatively sound log.

a b

Figure 13—Extensive degradation at 6-in. (152-mm) position of log 4. (a) CT image; (b) micrograph of wood

removed from log at places indicated in CT image (750x). Removed wood was approximately 0.005 by

0.006 in. (0.13 by 0.15 mm).
13

Future Research Needs

We recommend further experiments to refine the testing techniques used with sound wave transmission on large members.

These could include an investigation of the size and extent of

defects that are discernible by sound wave transmission time.

Alternative arrangements for sound wave transmission tests

could also be tried, such as the use of a fixed impact spike

with a movable receptor spike. Future studies might also

focus on relating aspects of sound wave transmission to the

quantity and quality of lumber recovered, as an alternative to

visual log grading.

Whereas CT scanning can provide a great deal of information

about the internal features of a log, there has not been a concerted effort to merge the latest CT hardware with image

processing and log processing software. Advancements in

these areas should be monitored in hopes of obtaining rapid

scans at a much lower cost and being able to automatically

apply that information to estimating lumber yield or optimizing log breakdown at the headrig.

A full-scale study of logs should be performed using a systematic approach to determine the type of degradation present

and the extent to which different NDE methods respond to

individual types of degradation. However, based on our experiences with impulse radar in this study, we cannot recommend that this technique be included in further investigations

on NDE methods for evaluating logs.

References

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