United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL–RP–537

Conditioning Stress

Development and

Factors That Influence

the Prong Test

James Fuller
Abstract

To relieve transverse drying stresses, lumber must be conditioned. The standard prong test has been used for many

decades to determine the duration of conditioning time. However, little work has been directed at the proper procedures or

interpretation of the prong test. The purpose of this study

was to gain additional information about stress development

during conditioning and moisture changes influencing prong

response. Data on stress distribution, moisture gradient, and

prong response were obtained periodically throughout conditioning. A residual strain release method, slicing, was used to

obtain stress distributions. Results demonstrate that the

prong response is a function of prong thickness and stress

distribution. The time-dependent moisture gradient was

shown to influence the final strain pattern. Further studies

concerning conditioning schedules need to be completed

before a desired prong response can be determined.

April 1995

Fuller, James. 1995. Conditioning stress development and factors that influence the prong test. Res. Pap. FPL–RP–537. Madison, WI: U.S. Department

of Agriculture, Forest Service, Forest Products Laboratory. 6 p.

A limited number of free copies of this publication are available to the

public from the Forest Products Laboratory, One Gifford Pinchot Drive,

Madison, WI 53705–2398. Laboratory publications are sent to more than

1,000 libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin.

The United States Department of Agriculture (USDA) Forest Service is a

diverse organization committed to equal opportunity in employment and

program delivery. USDA prohibits discrimination on the basis of race,

color, national origin, sex, religion, age, disability, political affiliation, and

familial status. Persons believing they have been discriminated against

should contact the Secretary, U.S. Department of Agriculture, Washington,

DC 20250, or call (202) 720–7327 (voice), or (202) 720–1127 (TTY).

Acknowledgment

This research was conducted at the North Carolina State

University, Wood and Paper Science Department, Raleigh,

North Carolina.
Introduction

To avoid excessive shrinkage while in service, wood must be

dried prior to remanufacturing. During drying, internal

stresses develop in lumber as a result of unavoidable moisture gradients that set up differential shrinkage patterns

throughout the thickness of the lumber (McMillen 1963,

Youngs 1957). At the end of drying, lumber is casehardened,

which is the state in which the surface of the lumber is

stressed in compression and the center in tension. When

lumber is in this state and resawn or unequally surfaced, the

final shape can be distorted. To minimize or eliminate casehardening, the lumber is conditioned to relieve these stresses.

Most research conducted to examine stress development has

focused on stresses during drying and after conditioning.

Reasons for this are the short time involved in conditioning

compared with drying and the extreme danger of collecting

samples in a kiln containing steam. Knowledge of how

drying stresses are altered during conditioning is important in

developing adequate conditioning schedules. In addition,

knowledge of stress development during conditioning would

enable development of meaningful quality control tests.

The prong test is used by industry as part of its quality

control program to assess the degree of casehardening in

lumber. Currently, commercial kiln operators are advised to

follow procedures set forth in kiln manuals, which includes

using a prong test. The prong test indicates only whether a

stress gradient of a form that causes prong deformation

exists, not the stresses within the whole board. A survey of

kiln manuals, which recommend different standard prong

geometries and response, indicates considerable disagreement

among recommendations (Table 1). Standard prong test procedures ignore the interactions that prong length and thickness have with prong response. Some manuals ignore that

the immediate increase or decrease in the surface layer moisture content (MC) can possibly yield false results (Churchill

1954, McMillen 1963). To consider this issue, authors of

some kiln manuals (Table 1) and instructors of short courses

often advise operators to reevaluate the prong test after a

short period to eliminate any bias in interpretation created by

moisture gradients present when the test was originally

interpreted.

Although the prong test has been used for decades to determine the degree of stress relief, there is no standard way to

report the results. Churchill (1954) quantitatively recorded

casehardening as the post-cut surface and center slice length

difference divided by the nonconditioned, uncut board width.

This value is based on two different measurement times and

does not include possibilities of a complex stress gradient.

Conditioning Stress

Development and Factors

That Influence the Prong Test

James Fuller, Research Scientist

Forest Products Laboratory, Madison, Wisconsin

Table 1—Suggested prong geometry and response

by various authors

Board

thickProng

Reference

ness

(in.)

NumberThicknessa Prong response

Bramhill and

 Wellwood

 1976

2 6—b —

Cech and

 Pfaff 1977

Situation 1

Situation 2

1

—

2

2–5

50%

—

All prongs are to

 be of equal length

Cech and

 Pfaff 1979

1 6 — Slight outward

 bow

Page 1973 1-1/2 2 0.25 in. --

Simpson

 1991

<1-1/2 2 — Straight prongs

Wengert

 1990

Situation 1 <1-1/2 2 24% —

Situation 2 >1-1/2 >2 Bow outward by

 0.25 in.

a

Percentage or inches of board thickness; 1 in. = 25.4 mm.

b

—, not stipulated in manual.
2

Therefore, the value is false and cannot be interpreted. Cup

(Nishio 1976, 1977a, 1977b) and slit width (Moren 1994)

are inappropriate reporting methods that use an equation

depicting a linear relationship of prong half-length deflection.

This method does not take into consideration board width and

complex stress gradients.

An equation recently developed by Fuller (1993) and Fuller

and Hart (1994) was selected to provide an indication of an

existing stress gradient as displayed by the prong test and the

true geometry of the prong response. The equation takes into

account that when the prongs are released, they do not merely

shift in or out nor do they bend at the connecting base. The

prongs bow through the entire length following the curve of

a circle that is described by a second degree polynomial.

Therefore, the prong response is a function of the prong

length squared:

PR = (W − W')/L2 (1)

where PR is the degree of casehardening as displayed by the

prongs; W is the precut prong tip distance; W' is the released

prong tip distance; and L is the prong length (Fig. 1).

Prong test results are recorded such that when the prongs bow

inward, displaying casehardening, results are positive. When

the prongs bow outward, displaying reverse casehardening,

results are negative. The prong test response is recorded in

inches-1 (millimeters-1).

Thickness of the prong needs to be addressed when interpreting the prong test. However, before a desired prong response

can be selected, the desired corresponding stress profile needs

to be determined.

The objective of the study reported here was to gain additional information about stress development during conditioning and moisture changes influencing prong response.

This would supply some of the needed data for the development of precise procedures for kiln operators when monitoring the charge being conditioned, thereby obtaining stressfree lumber.

Methods and Materials

For this study, 1-1/2-in. (38-mm) airdried red oak boards were

obtained. Prior to drying, all except four boards were planed

to 1-1/4 in. (32 mm). These four boards were used to obtain

material data for transverse modulus of elasticity (MOE) and

specific gravity. After planing both sides evenly, MC of the

boards was 34 percent. Planing was included to reduce both

the presence of surface checks and the subsequent drying

time. Planing also removed the shell containing any tension

set. Three of the clearest boards greater than 5 in. (127 mm)

wide were selected. These boards were ripped on a straightline-rip saw to 5 in. (127 mm).

A T2-C1 modified drying schedule was used (Table 2). This

schedule is commonly used in the geographic region where

this research was done because of the occurrence of wet wood

infection in red oak. Airflow was reversed every 6 h. Two

W

L

t

W'

Figure 1—Prong test geometry and recorded

measurements: W = precut prong tip distance;

W' = released prong tip distance; L = prong

length; t = prong thickness.

Table 2—Kiln conditions used in the

modified drying schedule

Moisture T2-C1 modified (°F)a Equilibrium

content (%)

or step Dry-bulb Wet-bulb

 moisture

content (%)

 >40 95 92 19.0

40-35 95 91 17.4

35-30 100 94 15.0

30-25 110 100 12.0

25-20 120 95 6.6

20-15 130 90 3.8

15-11 150 110 4.2

 <11 160 120 4.3

Conditioning 160 149 11.0

a °C = (°F − 32)/1.8.
3

columns of boards were stacked such that one pile contained

the kiln sample boards used to monitor the MC during drying

and the other pile contained the three test sample boards. The

first set of slice and prong samples was taken just before

conditioning was initiated, and subsequent samples were

taken at the following hours into conditioning: 1, 2, 4, 6,

14, 22, and 35. At each sampling, a 12-in. (300-mm) block

was taken from each of the three test boards to provide eight

prong and two slice samples. The ends of the board were

sealed immediately with neoprene coating after each block

was cut.

Lumber is usually not remanufactured immediately after conditioning. This delay causes reduction in the moisture gradient. To represent lumber in normal use, half the material

from each block after being pulled for the kiln was wrapped

in plastic for 2 days before being processed. This allowed the

moisture gradient to be reduced and the block to cool. Both

the wrapped material and the immediately processed material

were processed identically. Throughout this report, this material is denoted as wrapped samples and immediately processed

samples.

Slice Test

The slice test (Fig. 2) was used to obtain both the moisture

and strain gradients. These data were used to determine the

level of stress. As each sample was processed, its width was

measured. On the end grain, slices 1 to 4 were marked on the

pith side and slices 7 to 10 on the bark side. A slice, including the bandsaw kerf, one-tenth of the board thickness, was

then sawn from both surfaces. The slices were immediately

weighed individually and measured to the nearest 0.001 in.

(0.025 mm). This procedure was repeated until the center

two-tenths of the board thickness remained. This process was

completed within 15 min from when the sample was taken

from the kiln. All slices were ovendried for 24 h and then

reweighed. The approximate average board MC was determined by adding the weights of all slices before inserting the

weight values into the MC equation.

Prong Test

Figure 1 depicts prong geometry and recorded measurements.

Four prong tests were performed on each board at each sampling time such that influence of prong thickness could be

monitored. The four prong thicknessess were 10, 20, 35,

and 50 percent of the board thickness. Prong response was

recorded simultaneously with strain and MC data.

Results and Discussion

Moisture Content Gradient

As a result of scheduling conflicts, the step prior to conditioning in the drying schedule was prolonged an extra day,

which helped flatten the moisture gradient. The set points for

conditioning, dry-bulb 160°F (71°C), wet-bulb 149°F (65°C),

and equilibrium moisture content (EMC) 11 percent, were

achieved more rapidly than sometimes can be achieved in

commercial kilns, which are large and sometimes cause the

dry-bulb temperature to overshoot the desired set point. This

is called kiln-overheat. The MCs of the slices are shown in

Figure 3. (For clarity, only three layers are shown.) The

other two layers, intermediate 3,8 and hyper-center 4,7 are

situated between subsurface 2,9 and the center 5,6 shifted

close to the center. Throughout this report, this pattern can

1 2 3 4 5 6 7 8 9 10

Figure 2—Slice test with the slices numbered.

12

10

8

6

4

2

0 5 10 15 20 25 30 35

Conditioning duration (h)

Moisture content (%)

Surface 1,10

Sub-surface 2,9

Center 5,6

Board average

✱

✱ ✱✱ ✱ ✱ ✱

✱

✱

Figure 3—Average moisture content within three

layers across the board thickness for the immediately processed samples from three 1-1/4-in.

(32-mm) red oak boards during conditioning.
4

be assumed when only three curves are shown for MC and

stress levels. Most moisture gained was within the first 5 h

of conditioning. Figure 3 shows at least a 1.5-percent MC

difference between the surface and the other slices after 6 h,

with little change in the total MC. In this run, monitoring of

the kiln samples was not continued into conditioning. In the

second run, this moisture gain pattern was repeated in the

slices but at a slower rate for a different conditioning schedule. The peak occurred after about 20 h. However, the kiln

samples in the second run showed a continued increase for

25 h up to 7.9-percent MC. This is explained by the loss of

moisture while cutting the slices and a different stress profile

between the two runs.

After the second hour, the decrease in MC resulted from kiln

controls not being turned on immediately after removal of

one sample, thereby reducing the EMC for about 30 min.

Figure 4 shows the average MC of each layer for the wrapped

samples. Compared to Figure 3, Figure 4 shows a flatter

gradient. The wrapped samples also showed a slightly increased average MC. Because samples were wrapped, they

should not have gained moisture. The apparent gain in moisture was probably due to a greater moisture loss during

slicing in the immediately processed samples, because these

were warmer than the wrapped samples during processing.

Figure 4 shows a surface MC of 8.5 percent, with the EMC

during conditioning at 11 percent. Only a small portion of

this discrepancy was from moisture loss during cutting and

weighing. The MC was low and the temperatures of the air

and sample were close; therefore, little moisture was lost in

this manner. The discrepancy was from hysteresis. The EMC

curve is desorption, which occurs during conditioning. A

hysteresis factor of 0.75, a plausible value, would be required

to account for this difference. This indicates that both surface

MC and strain for the immediately processed samples can be

corrected to a fairly accurate degree.

Transverse Stresses

Stresses are used to maintain static equilibrium for calculation purposes. However, it is the released strains that are

measured, assumed to result completely from mechanosorptive deformation, and the source of the stresses. Figures

5 to 7 show the strains that do not necessarily sum to zero.

In a report by Fuller (Prong Test Prediction for Red Oak, in

preparation), stresses used to successfully predict prong

response were calculated from the recorded strains in the slice

test and the MOE–MC curves obtained from the preliminary

test.

Immediate strain magnitudes as a function of conditioning

duration are shown in Figure 5. Initially, all three boards

showed high levels of compression strain in the surface

layers (slices 1, 10), nearly zero strain in the subsurface

layers (slices 2, 9), and moderate levels of tension strain in

the center layers (slices 5, 6). Within 6 h, strain in the surface layers decreased to about zero, while strain in the subsurface layers increased to high levels of compression. This reversal of the stress gradient means that a prong that included

only these two layers would turn outward rather than inward.

The strain values changed little after 15 h into conditioning,

indicating that prolonged conditioning does not cause reverse

casehardening.

Figure 6 shows the strain gradient across board thickness at

different times for one board. For clarity, only time intervals

of major developments are shown. Strain values were asymmetric. The maximum compression strain in the surface

occurred at different times but at about the same magnitude

within the same board. This was partially due to the growth

geometry of the tree and was noted by Moren (1994). As a

result, Figure 5 shows a lower average maximum strain than

the actual maximum strain and a quicker reversal in the surface strains. However, Figure 5 does show the true general

pattern of the strain history.

Two factors need to be considered in interpreting and understanding strain history: mechano-sorptive response and moisture migration. The sum of these influencing factors produces

the altered strain profiles. Mechano-sorptive relaxation occurs

with reversed stress and a change in MC. The reversed stress

must increase to at least a critical level (Fuller 1993, Moren

1994). This level must be present during conditioning and

achieved either during drying of the wood or when wood absorbs moisture during conditioning. Relaxation, which starts

at the surface during conditioning, relieves high compression

strain. Figure 5 shows a transition of the surface compression stress to tension and is explained as follows. When the

surface stresses were relieved by the sudden increase in MC,

12

10

8

6

4

2

0 5 10 15 20 25 30 35

Conditioning duration (h)

Moisture content (%)

Surface 1,10

Center 5,6

Sub-surface 2,9

Board average

✱

✱ ✱ ✱ ✱

✱✱

✱

✱

Figure 4—Average moisture content within three

layers across the board thickness for the

wrapped samples from three 1-1/4-in. (32-mm)

red oak boards during conditioning.
5

two things occurred. One, the moisture migrated to the next

inner layer (where the strain was near zero), causing it to

swell. Two, the remaining layers contained mechano-sorptive

strain. To maintain static equilibrium, the neutral stress axis

must shift. Together, this puts the surface in tension. Checks

may or may not open, depending on depth of the tension

zone and degree of tension. In this study, only a small number of open surface checks were present after conditioning.

The strain resulting from the reduction of the moisture gradient by wrapping the board for 2 days is shown in Figure 7.

Strain differences can be seen when comparing the immediately processed with wrapped samples. As expected, the surface moisture migrated to the inner slices. With the migration of moisture from the surface to the center, the strain

gradually changed. The initial strain magnitudes in the surface were more negative (tension) in the wrapped samples

than the immediately processed samples.

Prong Test

The immediately processed prong test samples were remeasured after a 45-min delay. These results consistently showed

the same general pattern and similar magnitudes as the

immediate results, with the 10-percent prong test results

displaying the greatest dissimilarity. Only the 10-percent

delayed prong results are shown in Figure 8. Even though the

10-percent prong results showed the greatest difference, they

were not of such a magnitude that a kiln operator would be

mislead in decisionmaking.

The average prong response for the wrapped samples, used to

simulate lumber in actual use, is shown in Figure 9. For the

short conditioning times, the wrapped sample response was

approximately the same as the immediately processed

response. However, for the lengthier conditioning times, the

values were markedly less. This is important to remember

when interpreting the prong test results that are processed

immediately after conditioning.

Conclusions

Strain data were obtained by slicing the board and did not

indicate that prolonged conditioning causes reverse casehardening. In this study, stresses were not completely eliminated.

With the gain of moisture during conditioning, surface

stresses were found to attain a maximum value and then subside to near zero. This was accomplished through mechanosorptive recovery. With a reduction in the moisture gradient

after a 48-h delay before processing, the general patterns

remained about the same except for a reduced magnitude of

stress and a transition from tension to compression in the

center or core of the board.

0.004

0.003

0.002

0.001

0

-0.001

-0.002

0 5 10 15 20 25 30 35

Conditioning duration (h)

Strain (in./in.)

Surface 1,10

Sub-surface 2,9

Center 5,6

Figure 5—Immediately processed sample strain

magnitude within three layers cross the

thickness of the board during conditioning

(1 in. = 25.4 mm).

0.006

0.004

0.002

0

-0.002

-0.004 Strain (in./in.)

1 2 3 4 5,6 7 8 9 10

 0 h

 1 h

 4 h

 6 h

21 h

35 h

Figure 6—Strain profile across the thickness of

the 1-1/4-in. (32-mm) red oak boards during

conditioning with the sampling time and numbered

slices (1 in. = 25.4 mm).

0.004

0.003

0.002

0.001

0

-0.001

-0.002

-0.004

0 5 10 15 20 25 30 35

Conditioning duration (h)

Strain (in./in.)

Surface 1,10

Sub-surface 2,9

Center 5,6

Figure 7—Wrapped sample strain magnitude

within three layers across the thickness of the

board during conditioning (1 in. = 25.4 mm).
6

The overall objective of this study was to gain additional

information about stress development during conditioning

and moisture changes influencing prong response. This study

supplies some of the needed data to develop precise procedures for kiln operators when monitoring the charge being

conditioned, thereby obtaining stress-free lumber.

Further research is needed on the influence of different kiln

schedules, board thickness, and species before an overall conclusion can be made concerning standard prong test procedures. Although results of this study do not suggest a standard prong thickness, kiln operators are advised to maintain a

constant prong thickness from one charge to the next for

consistent results. Also, operators should be suspicious of

prong tests that display reverse casehardening and should not

take seriously sharp changes in prong response early in the

conditioning process. The wood probably would still be

casehardened early in the conditioning process, even if the

prong test indicates otherwise, unless the EMC in the kiln

was extremely high.

References

Bramhill, G.; Wellwood, R. 1976. Kiln drying of western

Canadian lumber. Inf. Rep.. VP–X–159. Vancouver, British

Columbia: Canadian Forestry Service. Western Forest Products

Laboratory.

Cech, M.; Pfaff, F. 1977. Kiln operator's manual for eastern

Canada. Rep. OPX192E. Ottawa, Ontario: Eastern Forest Products Laboratory.

Cech, M.; Pfaff, F. 1979. A quick guide for the dry kiln

operator. Rep. OPX217E. Ottawa, Ontario: Eastern Forest Products Laboratory.

Churchill, J. 1954. The effect of time, temperature and relative humidity on the relief of casehardening stresses. Forest

Products Journal. 4(5): 264–282.

Fuller, J. 1993. Lumber drying stress development and prong

test implications. Thesis. Raleigh, NC: North Carolina State

University.

Fuller, J.; Hart, C. 1994. Factors that influence the prong

test. In: Proceedings, 4th international IUFRO drying conference; 1994 August 9–13; New Zealand.

McMillen, J. 1963. Stresses in wood during drying. Res.

Rep. 1652. Madison, WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory.

Moren, T. 1994. Steam conditioning after low temperature

drying. Holz als Roh– und Werkstoff. 52: 77–82.

Nishio, S. 1976. Estimating drying stress in wood by cup

Method II comparison of cup method with slice method. Journal

of Japan Wood Research Society. 22(11): 626–631.

Nishio, S. 1977a. Estimating drying stress in wood by cup

Method III variation of the cup quantity under different drying

conditions and with different conditions of thickness of boards.

Journal of Japan Wood Research Society. 23(4): 180–186.

Nishio, S. 1977b. Estimating drying stress in wood by cup

Method IV drying stresses in Tanoak. Journal of Japan Wood

Research Society. 23(4): 180–186.

Page, R. 1973. High-temperature versus conventional methods

of drying southern pine lumber. Res. Pap. 73. Macon, GA:

Georgia Forest Research Council. 29 p.

Simpson, W. 1991. Dry kiln operator's manual. Ag. Handb.

188. Madison, WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory. 274 p.

Wengert, E. 1990. Drying oak lumber. Madison, WI:

Department of Forestry, University of Wisconsin–Madison.

167 p.

Youngs, R. 1957. The perpendicular-to-grain mechanical

properties of red oak as related to temperature, moisture content,

and time. Res. Rep. 2079. Madison, WI: U.S. Department of

Agriculture, Forest Service, Forest Products Laboratory. 111 p.

0.06

0.04

0.02

0

-0.02

-0.04

-0.06

-0.08

0 5 10 15 20 25 30 35

Conditioning duration (h)

Prong response (in.-1)

Prong thickness

10%

20%

35%

50%

+

+ +

+

+ after 45-

 min delay

Figure 8—Average prong test immediate

responses (1 in. = 25.4 mm).

0.06

0.04

0.02

0

-0.02

-0.04

-0.06

-0.08

0 5 10 15 20 25 30 35

Conditioning duration (h)

Prong response (in.-1)

Prong thickness

10%

20%

35%

50%

Figure 9—Average prong test wrapped response

 (1 in. = 25.4 mm).