FATIGUE RESISTANCE OF QUARTER-SCALE BRIDGE

STRINGERS IN FLEXURE AND SHEAR1

By

WAYNE C. LEWIS, Engineer

Forest Products Laboratory,2 Forest Service

U.S. Department of Agriculture

----

Digest

Research on the behavior under repeated loading of about 300 specimens

(2 by 4 by 43 inches, loaded at third points of a 39-inch span) of southern

pine and Douglas-fir included the variables of green, air-dry, and

preservative-treated wood and straight grain and 1:12 slope of grain. With

air-dry and treated wood, an additional variable was the use of artificial

checks at midheight to simulate those produced from drying. An equal

number of control specimens matched to the fatigue specimens were loaded

to failure statically to provide information on the static strength of each

fatigue specimen.

Fatigue specimens were loaded repeatedly at the rate of 500 cycles per

minute at selected levels of stress until failure developed or the specimen

withstood 10 million repetitions of stress. The minimum repeated stress

for each cycle was 0.10 of the maximum.

The fatigue strength for 2 million cycles of stress was estimated for each of

the species and the 10 combinations of condition, grain, and checking from

curves of stress to number of cycles to failure (figs. 30, 31, 32, and 33).

These values and those from the static control tests are summarized in

table 41.

1 In cooperation with the Association of American Railroads under the sponsorship of Committee 7, Wood Bridges and Trestles, American Railway

Engineering Association.

2Maintained at Madison, Wis., in cooperation with the University of Wisconsin.

Report No. 2236
Analyses of the research results indicate that, with the present method of deriving working stresses for timber (considerations for duration of load and

margin of safety), fatigue resistance from bending stress is probably adequate

for timbers when green, air-dry, or treated, except for those of treated

southern pine. The fatigue strength values were slightly below 50 percent of

the static strength for the treated southern pine.

The results from the artificially checked specimens, which fail in shear under

repeated loading, suggest further consideration in the method of providing

working stresses in shear because of the possibility of fatigue failures from

repeated stresses. Such failures are particularly possible with southern pine,

as estimated fatigue strength was as low as 30 percent of static strength.

Introduction

Approximately 1,800 miles of short-span and multiple short-span wood

bridges and trestles are located on Class I railroads in the United States.

Practice varies in different areas, but timber stringer bridges are used on

both main and branch lines. Stringers were formerly replaced at fairly

frequent intervals because of decay in timbers. The present universal

practice of treating these timbers with preservatives has extended the life

of bridge stringers. The lengthened service life of the timbers, however,

has raised questions about the fatigue resistance of the stringers, since

bridges are subjected to numerous repetitions of loading.

Fatigue in engineering materials is the phenomenon of failure due to

repetitions of stress of a magnitude less than the static strength. Failure

due to an accumulation of loadings of varying duration is different in character

and is a separate consideration. In present design, a factor is applied in

deriving working stresses for wood members that are subjected to long

accumulations of loadings during the life of a structure. No such factor is

applied for the effect of fatigue, nor is such a factor considered necessary,

since the static strength of a member is not reduced by repetitions of stress

until a failure starts. Unless the magnitude of repeated stresses exceeds

the fatigue strength for the number of repetitions in service, there is no

reduction of the margin of safety.

Members of the technical staff at the Association of American Railroads

Research Center estimate that the number of repetitions of design load or

near-design load will not exceed 2 million during the life of the structure.

The important questions that led to the fatigue work reported herein are the

following:

Report No. 2236 - 2 -
(1) If stringers in service for 30 or 40 years were free of decay, should

they be replaced because of possible fatigue failures in the future, or was

their fatigue resistance sufficient to permit their continued use?

(2) If the fatigue strength of timber was low, as compared to the stresses in

service, what allowances should be suggested in design to overcome the

possibility of fatigue in service?

(3) How do fatigue failures develop? What inspection procedures should be

integrated with regular inspections to forestall possible critical failures that

may interrupt service?

This information was not available for wood stressed in bending or for

relatively short beams with a ratio of bending moment to shear force that

causes critical shear stresses. Further, because of the checking that sometimes occurs in drying, timbers of the size used in many bridges may fail

in service as the result of shear. Therefore, engineers at the Forest

Products Laboratory and the subcommittee on repeated loading of Committee

7, Wood Bridges and Trestles, American Railway Engineering Association

developed a program of testing quarter-scale specimens. It embodied these

variables: Moisture content (green and air-dry wood), straight grain and

1:12 slope of grain, specimens free of checks and with a V-notch to simulate

a drying check, two species (southern pine and Douglas-fir), and the effect

on strength of preservative treatment for protection against decay.

Green, air-dry, and treated specimens were selected to encompass the

range of moisture content and conditions encountered in service. Although

the majority of timbers actually installed are treated, the dry untreated

specimens were included so that any effects of treatment on fatigue behavior

could be isolated. The variable of 1:12 slope of grain was included because

that is the maximum slope allowed in structural timbers used in bridge

stringers. The artificial checks were included because abrupt changes in

cross section introduce stress concentrations, which become more critical

under repeated loads than under static loads. One-half the width of the

timber was selected for the total depth of checks, because in cross sectioning

severely checked timbers of the size used in bridge stringers, the total

depth of checks was approximately one-half the total width of the timbers.

The research evaluation program was supported jointly by the Association

of American Railroads and the Forest Products Laboratory. This is the

final report on the research study.3 It presents the results of the staticcontrol and fatigue study for these kinds of specimens:

3Progress report on tests of green and air-dry southern pine was published

under the title, "Fatigue Resistance of Quarter-Scale Bridge Stringers of

Green and Dry Southern Pine," in AREA Bulletin 538, September-October

1957.

Report No. 2236 - 3 -
1. Green, unchecked, southern pine with straight grain

2 . Green, unchecked, southern pine with 1:12 slope of grain

3. Air-dry, unchecked, southern pine with straight grain

4. Air-dry, unchecked, southern pine with 1:12 slope of grain

5. Treated, unchecked, southern pine with straight grain

6. Treated, unchecked, southern pine with 1:12 slope of grain

7. Air-dry, artificially checked, southern pine with straight grain

8. Air -dry, artificially checked southern pine with 1:12 slope of grain

9. Treated, artificially checked southern pine with straight grain

10. Treated, artificially checked, southern pine with 1:12 slope of grain

11. Green, unchecked Douglas -fir with straight grain

12. Green, unchecked Douglas-fir with 1:12 slope of grain

13. Air-dry, unchecked Douglas -fir with straight grain

14. Air-dry, unchecked Douglas-fir with 1:12 slope of grain

15. Treated, unchecked Douglas-fir with straight grain

16. Treated, unchecked Douglas fir with 1:12 slope of grain

17. Air-dry, artificially, checked Douglas-fir with straight grain

18. Air-dry, artificially checked Douglas-fir with 1:12 slope of grain

19. Treated, artificially checked Douglas -fir with straight grain

20. Treated, artificially checked Douglas-fir with 1:12 slope of grain

Report No. 2236 - 4 -
Preparation of Specimens

All specimens were fabricated in pairs; one of each pair was used for a

static control test and the other for a fatigue test. Each pair of specimens

was longitudinally matched as exactly as possible for such large-sized

specimens.

Material was either from loblolly pine logs from eastern Texas or Coast-type

Douglas-fir logs from Oregon.

Specimens were sawn from log flitches as shown in figure 1. Each specimen

was identified and numbered as to log and position in the log, in order to

make comparisons of the results from different types of specimens. Radial

matching, though less desirable than longitudinal or tangential matching

because different growth rings are represented, was used to observe the

effects of the variables of moisture content, slope of grain, preservative

treatment, and the artificial checks on static and fatigue strength.

A common size for a stringer is 8 by 16 inches by 13 feet, so for this study

individual specimens were scaled down to 2 by 4 by 39 inches. The actual

length of specimens was 43 inches, which provided for a 2-inch extension of

each end beyond the supports when the specimens were tested. Green

specimens were sawed to final dimension and maintained green until tested.

Specimens to be tested in the air-dry condition and those to be treated before

test were cut to a rough size of approximately 3 by 5 by 48 inches while

green. Both kinds were carefully kiln dried to approximately 12 percent

moisture content before being cut to final size. The specimens are shown

in figure 2. The artificial checks were cut with a spindle router and

finished with a special hand tool that made the point of the V-notch sharp to

simulate the edge of a check as closely as possible.

A numbering system for the specimens was used to identify each specimen

as to log origin, species, moisture content or treatment, as a control or

fatigue specimen, whether checked or unchecked, and as to location in the log.

During specimen preparation, pairs were matched as to orientation of the

annual rings, in order that pairs would be loaded on matching faces in both

-the control and fatigue tests. In the identification system used, the first

symbol is the log number; the second is a letter, either P for southern pine or

F for Douglas-fir; the third is either D for air-dry, G for green, or T for

preservative-treated material; the fourth is either C, designating control

specimens, or F, fatigue specimens; and the fifth symbol is either CH for

checked specimens or U for unchecked ones.

Report No. 2236 -5-
Treatment and Conditioning

Material for the specimens was separated into three groups, depending on

whether it was to be used green, air dry, or after preservative treatment.

Green Specimens

The moisture content of the green specimens was maintained by placing

individual fatigue specimens in plastic cocoons in a solution of water and a

mild fungicide to prevent mold or other deterioration during storage. A

specimen encased in a plastic cocoon and positioned in the fatigue testing

machine is shown in figure 3. The static-control specimens were close piled

and covered with wetted cloths for the short period between fabrication and

test.

Air-Dry Specimens

The equilibrium moisture content of air-dry wood is about 12 percent. The

material was dried, conditioned, stored, and tested at 75° F. and 64 percent

relative humidity.

Treated Specimens

Since any deleterious effect on the strength of wood due to treatment may

result from the presence of the preservative itself or from the conditions of

heat and pressure required to force the preservative into the wood, the

quarter-scale specimens were treated under the same conditions as though

they were full-sized timbers, except that the wood was at about 12 percent

moisture content at the start of the treatment. Retentions, therefore, were

greater than would be normal for that size material.

The treating industry and the railroads were surveyed, and the actual treatments selected were chosen as typical of practice at the time specimens

were treated. For the southern pine specimens, treatment involved the

following steps:

1. Steaming for 1 hour to reach 259° F.

2. Continue steaming for an additional 3 hours

3. Vacuum of 24 to 28 inches for 2 hours

Report No. 2236 -6-
4. Initial air pressure of 70 pounds per square inch for 30 minutes

5. Coal-tar creosote at 175 pounds per square inch of pressure at 200° F.

for 3 hours

6. Final vacuum of 24 to 28 inches at 200° F. for 1 hour

The Douglas-fir was incised before treatment at one-fourth the spacing and

about one-fourth the depth used on full-sized timbers, as shown in figure 2.

The steps in treatment were as follows:

1. Boiled in coal-tar creosote under a vacuum of 24 inches at 190° F. for

8 hours

2. Initial air pressure of 30 pounds per square inch for 30 minutes

3. Creosote at 200° F. and 100 pounds per square inch of pressure for

12 hours

4. Final vacuum of 24 inches at 200° F. for 1 hour

After treatment, specimens were stored in a covered shed for a minimum of

6 weeks; excess creosote was then wiped off the specimen surfaces and they

were conditioned to constant weight at 75° F. and 64 percent relative humidity.

Methods of Test

Specimens were loaded in bending at the third points while simply supported

over a 39-inch span. The procedure for the control tests conformed to the

requirements of American Society for Testing and Materials designation

D 198-27 .4 The specimens were supported and loaded on the 2-inch face, as

shown in figure 4. To conform to the requirement of D 198-27, the specimens

were loaded at a uniform rate of head travel of 0.10 inch per minute.

Maximum loads were obtained and ultimate stresses were calculated with the

formula appropriate for the type of failure. ëThe values of modulus of rupture

ëfor failures in bending were computed with the conventional Mc

I

 formula,

which became

39 P R =

bh2

4American Society for Testing and Materials method, "Static Tests of

Timbers in Structural Sizes."

Report No. 2236 -7-
where R is the modulus of rupture or extreme fiber stress in pounds per

square inch, P is the maximum test machine load in pounds, b is the width of

the specimen in inches, and h is the height of the specimen in inches.

The values of unit shear stress for specimens that failed in shear were

calculated on the basis of the gross cross section, even though the artificial

checks reduced the effective area in shear by a factor of one-half. This

conformed to design practice where no reduction is made in width for checking that may occur in service. The formula for calculating unit shear stress

is:

0.75 P

S sg = bh

where S

sg

is the shear stress on the gross cross section in pounds per

square inch, and P is the maximum test machine load in pounds.

The unit shear stresses based on the net cross section were approximately

double those calculated on the basis of the gross section.

Fatigue tests were made in axial loading fatigue machines adapted for bending,

as shown in figure 5. Two specimens were tested simultaneously. The

amplitude of the load was measured by measuring the deflection of the

cantilever loading arm, which was calibrated and found to be directly proportional to amount of the loading. Reset counters for each side of the machine

counted the repetitions applied to each specimen.

In making each fatigue test, the following steps were followed:

1. With the cam set at zero and the static loading screw in a lowered

position, the specimen was placed in position; a bridge with a 0.0001-

micrometer dial, for measuring deflection of the center of the cantilever

loading arm with respect to its ends, was placed in position and a zero

reading taken.

2. The static loading screw was raised until the cantilever loading arm was

deflected by the amount corresponding to that required for the desired mean

load.

3. The cam was adjusted and slowly rotated by hand until the minimum and

maximum deflections of the cantilever loading arm were the desired amounts.

4. The machine was started and, after a few hundred cycles the machine

was stopped and the loading was adjusted.

Report No. 2236 -8-
5. The machine was restarted, and the electronic shutoff mechanism was

adjusted so that a slight change in load (as indicated by hand pressure on the

cantilever loading arm) shut off the power to the driving motor.

6. Loading was checked periodically until the test was completed.

In practice, it was found that a logarithmic schedule of monitoring loads

worked well unless a progressive failure was developing. Loads were

checked at about 1,000, 10,000, and 100,000 cycles, and daily thereafter

until failure started. When any failure developed, the electronic mechanism

stopped the machine, and it was necessary to adjust the load. With this procedure, indications are that loadings were within 5 percent of those desired

at all times during test.

Because of dynamic effects (loading during the actual running of the machine

was greater than when turning it over slowly by hand), it was necessary to

correct the alternating loads. This correction was found to be proportional

to the amplitude of the alternating load at any speed of machine operation.

At 500 cycles per minute, it amounted to 3 percent for one fatigue machine

used in the program and 2 percent for the other.

Specimens were loaded repeatedly to a previously computed percentage of the

strength of the matched control. Loads were repeated at the rate of 500

cycles per minute. The green specimens were tested while in the plastic

cocoons, as shown in figure 3, to maintain the moisture content. The

moisture content of the air-dry, and treated specimens was maintained because

all fatigue tests were made in a room conditioned and maintained at 75° F. and

64 percent relative humidity. The tests were continued until the specimen

either failed or had withstood 10 million repetitions of stress. The specimen

was considered to have failed when a major tension failure or shear failure

developed, or when it could no longer sustain the maximum repeated load

because of severe compression failures. The end point of the test was

selected as 10 million cycles, to provide a margin beyond the 2 million

cycles estimated for the life of a bridge stringer. The ratio of minimum

repeated stress to maximum repeated stress (stress ratio) was 0.10 for all

tests.

After test, a moisture content coupon 1 inch long was cut from each untreated

specimen near the point of failure. This was weighed, ovendried, weighed

again, and the moisture content calculated. The moisture content is expressed

as a percentage of the weight of the ovendry wood. The specific gravity of

each specimen was also determined on the basis of the weight and volume of

the wood when ovendry. Determination of moisture content of treated

specimens by conventional ovendrying techniques is neither accurate nor

suitable. For accurate determinations of treated specimens, it is necessary

Report No. 2236 -9-
to use extraction methods to remove the treating chemical, but electric

resistance moisture meters were used to give reasonable approximations.

Further, because both the air-dry and treated specimens were conditioned

at the same atmospheric condition before and during test, results are

comparable between the two kinds of specimens.

Presentation and Discussion of Test Results

The data from the research program are presented in tables 1 through 40

for the individual variables. Results of the static control tests are presented

in odd-numbered tables and results of the fatigue tests in even-numbered

tables. The following tabulation lists the various tables presenting the results

of the tests:

Table No.

Static control Fatigue

tests tests

Identification

Unchecked Southern Pine

Green, straight grain 1 2

Green, 1:12 slope of grain 3 4

Air dry, straight grain 5 6

Air dry, 1:12 slope of grain 7 8

Treated, straight grain 9 10

Treated, 1:12 slope of grain 11 12

Artificially Checked Southern Pine

Air dry, straight grain 13 14

Air dry, 1:12 slope of grain 15 16

Treated, straight grain 17 18

Treated, 1:12 slope of grain 19 20

Unchecked Douglas -fir

Green, straight grain

Green, 1:12 slope of grain

Air dry, straight grain

Air dry, 1:12 slope of grain

Treated, straight grain

Treated, 1:12 slope of grain

Report No. 2236 -10-

21

23

25

27

29

31

22

24

26

28

30

32
Table No.

Identification Static control

tests

Fatigue

tests

Artificially Checked Douglas -fir

Air dry, straight grain 33 34

Air dry, 1:12 slope of grain 35 36

Treated, straight grain 37 38

Treated, 1:12 slope of grain 39 40

The results of the fatigue tests are presented graphically in this report with

the values of maximum repeated stress plotted as ordinates and the

corresponding number of cycles to failure plotted logarithmically as the

abscissas as is commonly done for tests of this kind. Because of the

variation in strength properties with specific gravity, values of maximum

repeated stress are plotted both as percentages of the static strength of the

matched control and in pounds per square inch.

These plots of data and estimates of the fatigue strength for 2 million cycles

of stress (maximum stress that can be repeated 2 million times before

failure) are presented and discussed in the following sections.

Green Southern Pine Specimens with

Straight and 1:12 Slope of Grain

The results of the fatigue tests of the green, unchecked southern pine with

straight grain are plotted in figure 6, with values of maximum repeated

stress in percentages of the static bending strength as ordinates in the top

part and in pounds per square inch in the bottom. The results for the

similar material with 1:12 slope of grain are plotted in figure 7.

The plotted data exemplify the difficulty in obtaining a spread of values for

fatigue life (cycles to failure) in green material when it is stressed in bending.

It was apparent during the tests that unless repeated stresses were sufficiently

large to produce compression wrinkles in the fiber, specimens did not fail

in 10 million cycles of stress. Compression failures that developed when

stresses were sufficiently high became apparent early and deepened as the

test progressed. Tension failures developed only part of the time, even in

the specimens with the 1:12 slope of cross grain. In other specimens (tables

2 and 4), either the severe, progressive compression was followed by shear,

or compression became so severe that the specimens could no longer sustain

the maximum repeated loads. Consequently, about one-half of the specimens

at various loading levels sustained 10 million cycles without failure.

Report No. 2236 -ll-
Examination of figure 6 (top) discloses that failures in the straight-grained

material did not develop if the maximum repeated stresses were less than

about 50 percent of the static strength. This level is indicated by the

horizontal line on the figure. The average static strength of the control

specimens for the straight-grained material was 7,450 pounds per square

inch, so the indicated level of stress below which failures would not be

expected in 2 million cycles in green, straight-grained southern pine of the

quality in this study is about 3,700 pounds per square inch. This is shown

by the horizontal line in the bottom part of figure 6.

The corresponding stress for the green material with 1:12 slope of grain

was about 55 percent of the static strength. This difference is logical,

since modulus of rupture is influenced more by slope of grain than is

compressive strength. The effect of slope of grain on static strength of

wood is discussed in the Wood Handbook.5 Data in table 15 of that handbook

indicate that the static modulus of rupture for wood with a 1:12 slope of grain

is about 84 percent of the strength of straight-grained material. The data in

the Wood Handbook also indicate that in compression parallel to grain, the

static strength for wood with a 1:12 slope of grain is about 99.5 percent

that of straight-grained wood. The control strength for fatigue tests on green

wood is based on modulus of rupture but the critical stress, as far as

fatigue is concerned, is compression; therefore, it is reasonable to expect

that fatigue strength values for the specimens with 1:12 slope of grain will be

higher percentages of static strength values than those for straight-grained

specimens.

The absolute, or pound-per-square-inch values, of fatigue strength are

essentially the same for the green material with 1:12 slope of grain as for

the straight-grained material. Figures 6 and 7 (bottom) indicate that the

fatigue strength value for 2 million cycles of stress is almost the same,

about 3,900 (55 percent of 7,020) pounds per square inch, for material with

the 1:12 slope of grain as for the specimens with straight grain. The average

static strength of the controls with 1:12 slope of grain was 7,020 pounds per

square inch (table 3) and that of the straight-grained controls was 7,450

pounds per square inch (table 1). The static strength of the slope-of-grain

specimens in this test was about 94 percent that of the straight-grained ones.

This reduction is less than those indicated in table 15 of the Wood Handbook,

which are based on tests of dry rather than green wood.

5Forest Products Laboratory, Wood Handbook, U. S. Department of

Agriculture Handbook No. 72, p. 95-97. 1955.

Report No. 2236 -12-
Air-Dry, Unchecked Southern Pine

Specimens with Straight Grain

or 1:12 Slope of Grain

The results of the fatigue tests of the air-dry, unchecked southern pine with

straight grain are plotted in figure 8, with values of maximum repeated

stress in percentages of control bending strength in the top part and in

pounds per square inch for the bottom. Results for the fatigue tests of the

material with 1:12 slope of grain are similarly plotted in figure 9.

The typical fatigue failure in the straight-grained material was compression,

followed by either simple or splintering tension (table 6). The fatigue

specimens with 1:12 slope of grain failed in cross-grain tension (table 8);

there was no visible evidence of compression. Both types of failure were

similar in appearance to failures from the static tests.

The plotted results in figures 8 and 9 are more typical of values usually

obtained from fatigue tests than are the results obtained with the green

material; that is, the lower the repeated stress, the greater the number of

cycles to failure. Sloping lines, showing the approximate relation of stress

to number of cycles (S-N curve) to failure, are drawn for each set of data.

These indicate that the fatigue strength for straight-grained material for

2 million cycles of stress is about 60 percent of the static strength, or about

8,000 pounds per square inch (fig. 8) for material of the same average

quality as that used in these tests. For the material with 1:12 slope of grain,

the corresponding fatigue strength is estimated to be about 50 percent of the

static strength, or about 5,200 pounds per square inch (fig. 9) for material

of the quality and density included in this study.

Treated, Unchecked Southern Pine

Specimens withí Straight Grain

or 1:12 Slope of Grain

The results of the fatigue tests of the treated, unchecked southern pine with

straight grain are plotted in figure 10 in the same way as were the green and

air-dry tests in figures 6 to 9. Examination of figure 10 and tables 9 and 10

indicates that when fatigue life (cycles to failure) was more than about

1 million cycles, failures in fatigue were in horizontal shear and not in

bending, which was the type of failure in all but one of the static controls.

This difference in manner of failure is shown in figure 11 (top pair of

specimens) where the static control was a flexural failure and the fatigue

specimen failed in horizontal shear. It also makes analyses of fatigue

strength on the basis of bending stress less valid than if static and fatigue

Report No. 2236 -13-
failures were similar, because only in beams with equivalent loading (third

point) and span-to-depth ratio is the relation between bending and shear

stress the same. The indicated fatigue strength for 2 million cycles for the

straight-grained material is about 45 percent of the static bending strength,

or about 5,400 pounds per square inch.

Because failures in the region of 2 million cycles were in horizontal shear,

the maximum repeated shear stresses were calculated and plotted against

the cycles to failure in figure 12. The fatigue strength at 2 million cycles

of stress, in terms of the shear stress, is estimated from that figure to be

about 425 pounds per square inch. Because failures in the static control

tests of the straight-grained treated material were mainly in bending, and

because the fatigue failures were in horizontal shear when cycles of stress to

failure exceeded 1 million cycles, shear was more critical in fatigue than in

bending, while the opposite was true in the static tests.

This difference was not apparent when slope of grain in the specimens was

1:12, as shown in figure 11 (bottom pair) and recorded in tables 11 and 12.

The normal failure was in cross-grained tension, with little or no compression

in both the static and fatigue tests. Consequently, both plottings of the data

from the fatigue tests of the sloping grain material in figure 13 are on the

same basis as for the other green and air-dry material, with values of

maximum repeated stress expressed as percentages of the static control

strength and in pounds per square inch of bending stress. From this figure,

fatigue strength at 2 million cycles of stress is estimated to be about 45

percent of the static strength or about 4,100 pounds per square inch for

material of similar quality to that used for the tests.

Air -Dry, Artificially Checked Southern Pine

Specimens with Straight Grain or 1:12

Slope of Grain

Results from the fatigue tests of the air-dry, artificially checked southern

pine specimens are plotted in figures 14 and 15. Values from tests of

straight-grained material are included in figure 14 and those from the 1:12

slope of grain are in figure 15.

The artificial checks were V-notches at midheight (neutral axis), fabricated

to simulate as closely as possible the checks that occur in the drying of

large timbers. At one end of each specimen, the artificial checks extended

from the end of the beam to a point midway between the support and the load

point. This was done to simulate a drying check that terminated in the shear

area. At the other end of the beam, the checks were continuous from the

load point to the end to simulate a check that was continuous for the full

Report No. 2236 -14-
length of the area in shear. The straight-grained control specimens and the

four controls with 1:12 slope of grain failed in shear along the continuous or

long check, indicating that under static loading a continuous check was more

severe than a check that terminated within the zone stressed in shear. The

fatigue failures were also along the long check (tables 14 and 16), showing that

under repeated stress the results were the same.

In the straight-grained material, all of the static and fatigue failures occurred

in shear. In the static tests of specimens with 1:12 slope of grain (table 15),

four specimens failed in shear, eight failed in cross-grained tension, and

four failed in a combination of cross-grained tension and shear so nearly

simultaneously that it was impossible to tell which occurred first. In fatigue

tests of the slope of grain specimens, however, all failures were in shear

(table 16). Thus, it is indicated that shear is a more critical factor under

repeated stress than is the presence of cross grain of a severity of 1:12

or less.

Values of shear stress were calculated on the basis of the gross cross

section rather than the net cross section. This means that the actual shear

stresses were approximately twice the values presented in tables 13, 14, 15,

and 16, so the practice corresponds to that used in design of large timbers.

Thus, no reduction need be made in applying the values to design practice.

The fatigue strength of the straight-grained, artificially checked southern

pine for 2 million cycles of stress is about 45 percent of the static strength

(fig. 14). The average static shear strength, calculated on the basis of

gross cross section, was 885 pounds per square inch; so on this basis, the

fatigue strength for 2 million cycles is estimated at about 390 pounds per

square inch (fig. 14) for specimens of the same average quality as those

included in these tests.

Because most of the static control tests of the specimens with 1:12 slope of

grain failed in bending (table 15) and all of the fatigue specimens failed in

shear (table 16), it was more difficult to make the same kind of an analysis

for these specimens as was possible with those with straight grain. The

ultimate shear strength of the specimens that failed in bending was estimated

on the basis of specific gravity and used for computing the values of maximum

repeated stresses in percentages of control (table 16) plotted in figure 15

(top). On this basis, the indicated fatigue strength at 2 million cycles for the

artificially checked material with 1:12 slope of grain is about 40 percent of

the static shear strength, or an average of about 320 pounds per square inch

computed on the gross section (fig. 15, bottom).

Report No. 2236 -15-
Treated, Artificially Checked Southern

Pine with Straight Grain or 1:12

Slope of Grain

Results from the fatigue tests of the treated, artificially checked southern

pine specimens are plotted in figures 16 and 17 in a manner similar to those

for the air-dry specimens in figures 14 and 15. Values from the tests of

straight-grained material are included in figure 16 and those from the

material with 1:12 slope of grain in figure 17. Specimens were fabricated

exactly as for the air-dry specimens, except that they were given the coaltar-creosote preservative treatment after fabrication.

In the treated, straight-grained controls, all specimens but one failed in

shear (table 17). Shear failures were along the long checks, as were those

for the untreated specimens in most instances. One specimen failed along

the short check and one failed simultaneously along both checks. All

failures in fatigue in the straight-grained material were in shear originating

along the long check. In the static control specimens with sloping grain,

failures were similar to those for the control tests of untreated material.

Some failed in bending, some failed in shear, and some failed in a

combination of bending and shear, In the fatigue tests, however, failures

were in shear, supporting the conclusion made on the basis of the air-dry tests

that shear is a more critical factor under repeated loading than the presence

of cross grain of a severity of 1:12 or less.

The fatigue strength for 2 million cycles of the treated, straight-grained,

artificially checked southern pine is about 40 percent of the static strength

(fig. 16). The average static shear strength, calculated on the basis of

gross cross section, was 690 pounds per square inch; on this basis, the

fatigue strength for 2 million cycles is estimated at about 260 pounds per

square inch.

For artificially checked specimens with sloping grain, the fatigue strength

for 2 million cycles is estimated to be about 30 percent of the static strength

(fig. 17), or about 210 pounds per square inch for material of quality

equivalent to that used in this research.

Green Douglas -fir Specimens with

Straight and 1:12 Slope of Grain

The results of the fatigue tests of the green, unchecked Douglas-fir are

plotted in figures 18 (straight-grained tests) and 19 (1:12 slope of grain tests).

Report No. 2236 -16-
The experience in the fatigue testing of the green Douglas-fir was the same

as that for the green southern pine--unless the stresses repeated were

sufficiently large to produce wrinkling of the extreme fibers on the compression face, no failure developed during 10 million cycles. When failures

did develop during the repeated loading, the initial failure was in compression,

which became progressively more severe as the cycles of stress increased

until either a shear or tension failure occurred.

Examination of the data in table 22 and figure 18 (top) for the green, straightgrained, unchecked Douglas-fir discloses that if the repeated stresses were

less than 60 percent of the static strength, failures did not occur during the

10 million cycles. A horizontal line was located at 55 percent of the static

strength as an estimate of the level of repeated stress where no failure

would likely occur in 2 million cycles (fatigue strength). The average static

bending strength for the material included in this study was 6,760 pounds per

square inch; so the stress corresponding to 55 percent of the strength is

about 3,700 pounds per square inch. This is shown as a horizontal line in

figure 18 (bottom).

The specimens with the 1:12 slope of grain behaved similarly (table 24 and

fig. 19, top) to those with straight grain. The level of stress below which

failures would not occur appeared to be about 60 percent of the static strength.

This is shown by the horizontal lines in figure 19 at 60 percent of the static

control strength (upper part) and at 3,600 pounds per square inch (lower part).

These values are, therefore the estimates of the fatigue strength for 2 million

cycles of stress for green Douglas-fir with 1:12 slope of grain.

The apparently higher percentage value for fatigue resistance of Douglasfir with 1:12 slope of grain than that of straight-grained Douglas-fir can be

explained in the same way as for the similar difference in southern pine,

although it should be stated also in this instance that the estimate of stress

below which failures would not occur might be 2 or 3 percent higher for the

straight-grained material than the 55 percent shown. With the kind of failures

experienced with fatigue tests of the green material, estimates closer than

5 percent do not seem warranted.

Air -Dry, Unchecked Douglas -Fir Specimens

with Straight and 1:12 Slope of Grain

The results of the fatigue tests of the air-dry, straight-grained Douglas-fir

plotted in figure 20 indicate behavior under repeated stress very similar to

that for the corresponding specimens of southern pine. As the magnitude of

the repeated stress decreased, the number of cycles to failure increased.

The location of the S-N curve was estimated, and on this basis, the fatigue

Report No. 2236 -17-
strength for 2 million cycles is indicated to be about 60 percent of the static

strength (fig. 20, top), or about 6,300 pounds per square inch for material

of the same quality as the average included in this study.

The corresponding results for the air-dry Douglas-fir with 1:12 slope of

grain plotted in figure 21 indicate that the fatigue strength for 2 million cycles

for that material may be about 55 percent of the static modulus of rupture,

or about 4,200 pounds per square inch for Douglas -fir of the average quality

used in this study.

Failures for normal specimens with straight grain were in compression,

followed by tension, both statically (control tests) and from fatigue. In a

few instances, the initial compression was followed by shear, as shown in

figure 22, bottom. The remainder of the figure shows typical static failures

(upper one of each pair) and severe compression followed by tension (next to

top) in fatigue of the air-dry, straight-grained specimens.

Failures in the air-dry specimens with 1:12 slope of grain were typically in

cross -grain tension, with no evidence of any compression.

Treated, Unchecked Douglas-fir Specimens with

Straight Grain or 1:12 Slope of Grain

In the fatigue tests of the treated, straight-grained, unchecked Douglas -fir,

bending failures occurred only when maximum stresses repeated were large

and the fatigue life, therefore, was short (relatively a few cycles before

failure). When the stress repeated was such that the specimen withstood a

large number of cycles before failure, the specimens failed in horizontal

shear after initial compression, or in horizontal shear with no evidence of

compression. This experience is the same as for the tests of treated,

straight-grained, unchecked southern pine, where static failures were predominantly due to bending stresses, and fatigue failures in the zone of

2 million cycles were in shear. The same practice was followed in presenting

the data, which in addition to the plots of data where ordinates are expressed

in terms of bending stress (fig. 23), an additional plot (fig. 24) is provided

with the values of the maximum repeated shear stress as ordinates and the

logarithm of the number of cycles to failure as abscissas.

On the basis of bending stress, the estimated fatigue strength for 2 million

cycles of stress is about 50 percent of the static modulus of rupture (fig. 23,

top), or for material of the average quality included in this study, it is about

5,400 pounds per square inch. On the basis of shear stress, the fatigue

strength for 2 million cycles is indicated to be about 400 pounds per square

inch for material of the quality equivalent to that used in the study.

Report No. 2236 -18-
The treated, unchecked Douglas-fir with a 1:12 slope of grain predominantly

failed in cross-grain tension in both the static control and the fatigue tests,

as was the experience in tests of the treated, similar southern pine specimens.

The values from the fatigue tests plotted in figure 25 indicate that the fatigue

strength for 2 million cycles was about 55 percent of the static strength, or

about 3,900 pounds per square inch on an average basis.

Air-Dry, Artificially Checked Douglas-Fir

Specimens with Straight Grain or 1:12

Slope of Grain

The results from the fatigue tests of the air-dry, artificially checked Douglasfir specimens with straight grain and 1:12 slope of grain are plotted in figure

26 and figure 27.

Tables 33 and 34 show that the normal failures in both the static and fatigue

specimens were in horizontal shear along the long check, as was the predominant experience with the corresponding specimens of southern pine,

indicating that in Douglas-fir, also, the continuous check was more severe

than a check that terminated within the zone stressed in shear. The fatigue

strength for 2 million cycles for the straight-grained Douglas-fir is estimated

to be about 45 percent of the static strength (fig. 26, top), or about 310

pounds per square inch for material of the same average quality that is

included in this study.

Table 35 for the static control tests of specimens with a 1:12 slope of grain

indicates that the predominant static failure was in horizontal shear, although

some specimens failed in bending or a combination of bending and horizontal

shear. In the fatigue tests (table 36), however, all normal failures were in

horizontal shear, as was the experience with the air-dry tests of southern

pine, which supports the observation that shear is a more critical factor

under repeated stress than is the presence of cross grain of a severity of

1:12 or less. This is substantiated also by the fatigue strength of both

southern pine and Douglas-fir. In the unchecked specimens, where fatigue

failures were in bending, the values are indicated between 50 and 60 percent

of the static strength, while for the artificially checked specimens, which

failed in shear, the corresponding fatigue strength values were indicated to

be lower percentages of the static shear strength.

The results of the fatigue tests of the artificially checked specimens with

sloping grain plotted in figure 27 indicate that the fatigue strength for 2

million cycles was also about 45 percent of the static shear strength (top

curve), or about 280 pounds per square inch (bottom curve) for material of

the quality included in this study.

Report No. 2236 -19-
The values for fatigue strength at 2 million cycles, of 310 pounds per square.

inch for the straight-grained wood and 280 pounds per square inch for that

with the 1:12 slope of grain, are based on calculations using the total width of

the specimen without any reduction in width for the artificial checks. Actual

widths of the area in shear were one-half the width, so true values would be

about twice those presented. This corresponds to the common practice in

design where no reduction in width is made for checks that develop in service.

Treated, Artificially Checked Douglas -fir with

Straight Grain or 1:12 Slope of Grain

All of the treated straight-grained, artificially checked specimens, except

two tested statically as controls, and all of those loaded repeatedly until

fatigue failures developed, failed in horizontal shear along the long check

(tables 37 and 38). Similar specimens with a 1:12 slope of grain usually

failed in the same manner, but there were more--two in fatigue and several

in static tests --where bending was predominant (tables 39 and 40).

The plotted results for the fatigue tests of straight-grained, artificially

checked specimens of Douglas-fir, presented in figure 28, indicate a

fatigue strength for 2 million cycles of approximately 50 percent of the control

strength, or about 270 pounds per square inch for treated wood of the quality

used in the study. When the slope of grain was 1:12 instead of straight, the

fatigue strength for the treated, artificially checked wood was indicated from

figure 29 to be about 45 percent of the static strength or about 240 pounds

per square inch in shear, calculated on the gross width of the section with

no reduction in width for the artificial checks that reduced the width by onehalf.

As discussed before, in presenting results of this research, where shear was

critical in the artificially checked specimens, the practice was adopted of

making calculations on gross rather than actual width, so that values would

correspond more closely with actual design practices.

Comparisons of Static and Fatigue Strengths

of Green, Dry, and Treated Specimens

The data on static strength and fatigue behavior presented and discussed in

previous sections are summarized in table 41. This condition is also true of

the values for the estimated fatigue strength at 2 million cycles in pounds

per square inch in column 8, where failures were due to bending forces,

and in column 11, where failures were in shear. Values presented are useful

Report No. 2236 -20-
for comparisons within a species, but should not be used for direct

comparisons between species unless specific gravity differences are also

recognized.

The estimated S-N curves (stress-number of cycles to failure) are combined

for the unchecked southern pine in figures 30 and 31 (top) and for the unchecked Douglas -fir (bottom). In figure 30, the maximum repeated stresses

(ordinates) are in percentages of static strength, and in figure 31, they are

on a pounds-per-square-inch basis. Figures 32 and 33 present similar S-N

curves for the artificially checked specimens.

The conditions of treatment for the southern pine and Douglas-fir apparently

had some effect in reducing static strength also. Static bending strength of the

treated southern pine was about 9 percent lower for the straight-grained

specimens and about 15 percent for those with a 1:12 slope of grain (column

6, table 41). Shear values were about 22 percent less (column 9). The

reductions in static strength due to treatment of the Douglas-fir were not so

large. Bending strength values were reduced about 5 percent and those for

shear about 22 percent on the average. The number of tests is too small to

definitely establish the amounts of these reductions, but they do suggest

further study of the effect of treatment on strength.

Green Southern Pine and Douglas-fir

No material difference was observed in the behavior of the green southern

pine and Douglas-fir specimens under fatigue loading. Because of the

phenomenon of progressive compression when failures developed, any

estimation of fatigue strength was difficult. The differences of about 5 percent

of fatigue strength shown in table 41 cannot be considered significant.

Failures were substantially the same.

Air-Dry and Treated, Unchecked Southern

Pine and Douglas -fir

Examination of the curves of figure 30 and the values for the unchecked

specimens in table 41 for the southern pine ëreveals that the fatigue strength

for the treated, straight-grained material was lower on a percent-of-control

basis than that for untreated southern pine or treated material with 1:12

slope of grain. In the Douglas -fir, there was a difference in the fatigue

strength at 2 million cycles on a percentage basis also, but it was not so

large as for the southern pine. Values for the straight-grained, untreated

southern pine and Douglas-fir were estimated at 60 percent; for the treated,

Report No. 2236 -21-
straight-grained southern pine, it was estimated to be about 45 percent, and

for the ëequivalent Douglas-fir it was estimated at about 50 percent of the

static strength.

The lower fatigue strength of both species for the straight-grained, treated

material are substantiated by the fact that actual failures were in shear in

fatigue and in bending statically, indicating that something occurred during

the treating process that reduced the shear resistance in fatigue as compared

to static loading.

There were no substantial differences in the locations of the S-N curves of

the unchecked specimens with 1:12 slope of grain for either species. Fatigue

strength estimations for 2 million cycles of stress ranged from 45 to 55

percent of the static strength, indicating that fatigue behavior would be

similar whether the material was treated or not, provided adjustments were

made in design stress for any effects of treatment on static strength.

There was an apparent reduction in static strength for both the straightgrained and sloping-grained unchecked southern pine (column 6, table 41)

when treated, and it is possibly explained by the severity of steaming

exposure for the southern pine during treatment. Similar differences have

been observed in studies of the strengths of treated poles.6

These differences account for the lower position of the curve in figure 31 (top)

for treated southern pine with a 1:12 slope of grain as compared to its

untreated equivalent, and for the much lower position of the curve for treated,

straight-grained material than that indicated by the differences in fatigue

strength on a percentage basis. There was no such difference for the Douglasfir, as is shown by figure 31 (bottom), but the conditions of treatment were

milder and probably the effect of treatment was not so great.

Because of the fatigue failures in shear in the treated, straight-grained

specimens of both species, fatigue strength at 2 million cycles was calculated

on the basis of shear stress on the measured width and presented in column 11

of table 41. While these values are presented on a pound-per-square-inch

basis, values should only be considered as relative to material of equivalent

specific gravity to that included in the study. Again, the density of the

Douglas-fir used was less than that for the southern pine, and lower values

are expected for that reason. That both values, 425 and 400 pounds per

square inch for the southern pine and Douglas-fir, respectively, were

substantially greater than design stresses in shear indicates a comfortable

margin in service between actual stresses in shear and those which would

produce fatigue failures in service.

6 Wood, L. W., et. al. Strength and Related Properties of Wood Poles,

ASTM Wood Pole Research Program, Final Report, Sept. 1960.

Report No. 2236 -22-
The reason for fatigue failures in shear in the straight-grained Douglas-fir

and southern pine, when stresses were low enough so that a large number of

cycles were sustained before failure, is largely one of conjecture. Minor

checking was observed after treatment, and it is reasonable to assume that

if some checking was visible, other invisible strains and damage created

internal stresses that reduced the fatigue resistance in shear. These same

internal stresses were not so critical in the slope-of-grain specimens,

because principal failures were in cross-grain tension.

Air -Dry and Treated, Artificially Checked

Southern Pine and Douglas -fir

Examination of the values for static shear strength (column 9, table 41) of

the artificially checked specimens with the notches, which reduce the width in

shear by one-half to simulate drying checks, shows that in each instance the

average shear strength values of the treated specimens were materially

lower than the corresponding values for untreated material, The static

strength of the treated southern pine specimens was apparently reduced

about 22 percent by the treatment, and the Douglas-fir from 15 to 19 percent.

This difference between species is probably reasonable because of different

treatment conditions.

The values for fatigue strength at 2 million cycles based on a percentage of

static strength (column 10, table 41, and figure 33) indicate that there was

no material difference in the fatigue behavior of Douglas-fir in shear,

whether the material was treated or not. Because values of fatigue strength

were about the same percentages of static strength for these two variables,

it is reasonable to conclude that, if any necessary reductions in the static

strength for treatment are made, no additional reduction is necessary for

considerations of repeated loading.

In the southern pine, fatigue strength apparently was definitely lower for the

treated material than for the untreated material for wood with both straight

grain and 1:12 slope of grain; this suggests that the fatigue resistance of

the treated wood was proportionately lower than that for the untreated wood.

Values of fatigue strength on the pound-per-square-inch basis (column 11,

table 41 and the curves of figure 33) show the cumulative effects of reduced

static strength and lower percentage fatigue resistance for the southern pine

and the much closer grouping of values for the Douglas-fir.

Report No. 2236 -23-
Analysis of Results in Terms of Design Practice

No data are available by which the results of small tests, such as those of

the quarter-scale bridge stringers herein reported, can be directly applied

to full-sized timbers used in structures, with full confidence of their

applicability. Some difficulty always exists in transferring results from a

study of small tests to large full-sized members. A research program on

full-sized members with the variables controlled as they were in this study,

however, is impractical if not impossible, because of the difficulty in obtaining large members matched to each other so that pairs of specimens could be

tested, one statically and one in fatigue.

The data obtained from such a study as this are valuable, however, in

appraising the possibilities of fatigue failures in service. In any structural

part that undergoes a large number of repetitions of stress, such as is

possible in a railroad bridge stringer, the important consideration is: Are

the stresses which are repeated in service below those that will cause either

failure of that part of the structure or excessive maintenance from fatigue

produced by repetitions of stress?

In arriving at working stresses for normal loading of timber structures,

factors for (1) variability, (2) duration of load, (3) a combination of other

intangibles, and (4) the strength ratio based on the maximum defect permitted

in a certain stress grade are applied to the strength of clear wood. A

working stress derived in this way gives a near minimum factor of safety of

1-1/4 to 1-1/2, with an average more nearly equal to 2 to 2-1/2. 7 If the

factor for duration of stress of 5/8 is combined with the near minimum factor

of safety of 1-1/4, the combined factor is 0.50.8 This value can then be

compared directly to the various values for fatigue strength for 2 million

cycles on a percentage basis in columns 7 and 10 of table 41.

Of the unchecked specimens, green, dry, or treated of either species, only

the treated southern pine did not have a fatigue strength of 50 percent or

greater, indicating that except for treated, unchecked southern pine, the

reductions made in arriving at working stresses were probably adequate

from the standpoint of members withstanding the repeated loads expected in

service.

7

Wood, L. W., Safety Factor in Design of Timber Structures, American

Society of Chemical Engineers, Proceedings Vol. 84, ST 7, 1958.

8 Lewis, Wayne C., Design Considerations for Fatigue in Timber Structures,

American Society of Chemical Engineers, Proceedings Vol. 86, ST 5, 1960.

Report No. 2236 -24-
The artificially checked tests indicate a greater possibility for fatigue failures

to develop in service. The fatigue strength values of the artificially checked

southern pine with straight and sloping grain were found to be substantially

less than 50 percent of the static strength, while those for the Douglas-fir

were generally slightly less than 50 percent. Those results indicate that

possible revision of working stresses in shear may be desirable to provide

a margin against possible fatigue failure in service for bridge stringers.

The previous analyses do not take into account any reduction in static strength

for treatment, such as was indicated for all of the southern pine and for the

artificially checked Douglas -fir. If there is such an effect of treatment, it

should be established and the necessary reductions in static working stress

made. Some of the reduction in static shear strength due to treatment may

have been the results of further checking along the line of the artificial check.

The original assumption for this study was that checking in service in big

timbers could reduce the cross section by as much as one-half. If the total

cross section damage from both drying and treatment is no more than that

amount, the above consideration for reductions in static shear strength due

to treatment is conservative.

Examination of the values for fatigue strength in pounds per square inch for

the different kinds of material and specimens discloses a comfortable margin

between the values of fatigue strength and working stresses used usually in

design. These are, however, average values for average material and are

not representative of the minimum quality possible in a structure.

The fatigue strength of an assembly or material is influenced by the difference

between the maximum and minimum loads for each cycle (repeated stress

amplitude). The greater this amplitude, the lower is the fatigue strength

for any specific number of cycles. The amplitude of the repeated stress is

usually indicated by the stress ratio, the ratio of minimum to maximum

repeated stress. Stress ratio for this research was 0.10, which in effect

implied that the dead load was equal to one-tenth of the ìlive plus the dead

load," a reasonable assumption for short-span structures with timber

stringers. If the actual amplitude of the repeated stress in service was a

smaller percentage of the maximum (stress ratio more than 0.10), fatigue

strength obtained would be a higher percentage of the static strength than

reported here.

Summary and Conclusions

Definite conclusions based on approximately 15 evaluations of each variable

are difficult for a property, such as fatigue, where there is an inherent

Report No. 2236 -25-
scatter of values from test, added to the difficulty produced by the variability

of wood itself. The interrelations of the different variables, however,

support each other, and it is believed that the following conclusions are

warranted:

1. Fatigue Resistance, Green Unchecked Specimens

Green, unchecked southern pine and Douglas-fir, whether straight-grained or

with a 1:12 slope of grain, did not fail in fatigue in 10 million cycles if maximum repeated stresses were not high enough to produce wrinkling of the

compression face. When failures did develop, compression failures became

apparent early in test and became progressively worse, with final failure in

horizontal shear originating at the bottom of the compression failure or in

tension. The levels of stress below which no failures developed in 2 million

cycles were about 50 percent of the flexural strength for straight-grained

southern pine, 55 percent for southern pine with 1:12 slope of grain and

straight-grained Douglas-fir , and 60 percent of the static strength of the

Douglas-fir with 1:12 slope of grain.

2. Fatigue Resistance, Air-Dry, Unchecked Specimens

Southern pine and Douglas-fir that were air-dry, unchecked, and with straight

grain or 1:12 slope of grain developed fatigue failures as was expected for

fatigue tests --the lower the level of the maximum repeated stress, the

greater the number of cycles to failure. Fatigue failures in the straightgrained material were usually in compression followed by tension. Those for

the material with 1:12 slope of grain were usually in cross-grain tension with

no evidence of compression. Fatigue strength values for 2 million cycles of

stress were estimated to be the following percentages of the static strength

in flexure:

Southern pine with straight grain, 60 percent

Southern pine with 1:12 slope of grain, 50 percent

Douglas-fir with straight grain, 60 percent

Douglas-fir with 1:12 slope of grain, 55 percent

Report No. 2236 -26-
3. Fatigue Resistance, Treated, Unchecked Specimens

Treated, unchecked, straight-grained southern pine and Douglas-fir developed

horizontal shear failures in fatigue when the number of cycles of stress before

failure was about 1 million or more. As a consequence, fatigue strength values

were lower percentages of the static flexural strength for the treated specimens

(the static control specimens failed in bending) than for the untreated specimens--45 percent for southern pine and 50 percent for Douglas-fir.

The treated specimens of both species with 1:12 slope of grain failed in fatigue

in cross-grain tension, with indicated fatigue strength for 2 million cycles of

45 percent of the static flexural strength for southern pine and 55 percent of

the static flexural strength for Douglas-fir.

4. Fatigue Resistance, Air-Dry, Artificially Checked Specimens

Air-dry, artificially checked specimens with straight grain or 1:12 slope of

grain failed in fatigue in horizontal shear along the V-notches, cut parallel

to the axis of the beam to simulate a drying check at midheight, that reduced

the total width by one -half. Failures were predominantly along the checks that

extended from load point to support, rather than along the shorter check that

originated halfway between the load point and support, indicating the continuous check was more severe than the other one. Fatigue strength for 2 million

cycles was estimated at 45 percent of the static shear strength for the

straight-grained southern pine, 40 percent of the static shear strength for the

southern pine with 1:12 slope of grain, and 45 percent of the static shear

strength for Douglas-fir with straight or sloping grain.

5. Fatigue Resistance, Treated, Artificially Checked Specimens

The treated, artificially checked specimens with straight or sloping grain of

both species developed fatigue failures under repeated loading in the same

manner as did the air-dry specimens. The percentages of matched shear

strength estimated to represent fatigue strength at 2 million cycles were:

Southern pine with straight grain, 40 percent

Southern pine with sloping grain, 30 percent

Douglas-fir with straight grain, 50 percent

Douglas-fir with sloping grain, 45 percent

Report No. 2236 - 27 -
6. Species Effect, Green Material

Comparisons of the fatigue strength of the green, unchecked southern pine

and Douglas-fir indicate no significant differences in fatigue behavior between

species.

7. Species Effect, Straight-Grain Unchecked Treated vs. Air-Dry

Comparisons between the results of the tests for air-dry and the treated unchecked specimens indicate that the fatigue strength at 2 million cycles for

the treated, straight-grained southern pine was substantially lower, based on

a percentage of static strength, than that for the untreated, equivalent airdry material. There was a similar, but not so large, difference between

treated and untreated straight-grained Douglas-fir. This lower fatigue resistance for treated, straight-grained material suggests that the conditions of

treatment reduce the resistance to fatigue in addition to any reduction in static

strength.

8. Treatment Effect, 1:12 Slope of Grain, Unchecked

The fatigue tests of unchecked specimens with 1:12 slope of grain showed slight

or no difference between treated and untreated material. Fatigue strength

values were not substantially different when comparisons were based on

percentages of static strength. There were differences between the static

strength of the treated and air-dry southern pine with sloping grain that made

the fatigue strength lower for the treated material when compared on a basis

of pounds per square inch.

9. Treatment Effect, Artificially Checked

There was no material difference in the fatigue behavior of Douglas-fir in

shear (artificially checked specimens), whether the material was treated or

not. Values for fatigue strength were about the same percentages of the static

strength for both straight- and sloping-grained specimens, regardless of

whether or not they were treated. In southern pine the fatigue strength values

were a lower percentage of the static strength when specimens were treated

than when they were not, which suggests that the conditions of treatment

reduced the fatigue resistance of the southern pine somewhat over the

reduction in static strength.

Report No. 2236 -28-
10. Effect of Treatment on Static Strength in Bending and Shear

The conditioning and treatment for the southern pine and Douglas-fir apparently

had some effect in reducing static strength. Static bending strength of the

treated southern pine was 9 to 15 percent less than that of corresponding untreated material. Shear strength (artificially checked specimens) was about

22 percent less. Bending strength of the Douglas-fir was not reduced so

much --about 5 percent --and shear strength was reduced about 17 percent on

the average. These reductions were not considered in previous conclusions

because all comparisons were based on percentages of static strength.

11. Fatigue Resistance in Terms of Present Design

The results of the fatigue test research program indicate that with present

methods of deriving working stresses for timber, the fatigue strength for

2 million repetitions of stress in flexure is probably adequate for all unchecked

material, except treated southern pine, providing proper reductions are

made in deriving working stresses for reductions in static strength due to

treatment, and slope of grain does not exceed 1:12. The flexural fatigue

resistance of the treated southern pine was less than the factor of 0.50

obtained by combining the factors for the near minimum factor of safety and

the factor for duration of stress.

All of the estimated fatigue strength values for shear in the artificially checked

specimens of southern pine were less than 50 percent of the static strength

and the corresponding values for Douglas-fir were 50 percent, or slightly less,

of the static strength, suggesting that some reduction in shear design

stresses should be considered where fatigue may be important.

12. Limitation of Results on a Pound-Per-Square-Inch Basis

Analyses of the results of fatigue strength for 2 million cycles of stress in

pounds per square inch are only indicative of average values for material of

the same density as included in this study and cannot be used to determine

the margin of fatigue resistance for material of the lowest quality permitted

in either species.

Report No. 2236 -29- 1.-104
Table .1 .--Summary of results of static control tests of green,

straight-grained, unchecked, quarter-scale,

1 southern pine bridge stringersñ

1Specimens 2 by 4 by 43 inches tested in bending by loading at third

points of 39-inch span.

2

Based on volume and weight when ovendry.

Report NO. 2236
Table 2 .--Summary of results of fatigue tests of green, straight-grained, unchecked, quarter-scale,

southern pine bridge stringer1

2 Based on volume and weight when ovendry.3 Static strength of matched specimen.4 Fiber stress in bending, minimum repeated stress 0.10 of maximum.

1 Specimens 2 by 4 by 43 inches loaded at third points of 39-inch span at 500 cycles per minute.
stringers 1 with 1:12 slope of grain

Table 3.--Summary of results of static control tests of green,

unchecked, quarter-scale, southern pine bridge

1Specimens 2 by 4 by 43 inches tested in bending by loading at the

third points of 39-inch span.

2 Based on volume and weight when ovendry.

Report NO. 2236
Table 4.--Summary of results of fatigue tests of green, unchecked, quarter-scale, southern pine

bridge stringers1 with 1:12 slope of grain

1Specimens 2 by 4 by 43 inches loaded at third points of 39-inch span at 500 cycles per minute.

2 Based on volume and weight when ovendry.

3Static strength of matched specimen.

4Fiber stress in bending, minimum repeated stress 0.10 of maximum.
Table 5.--Summary of results of static control tests of airdry, straight-grained, unchecked, quarter-scale,

southern pine bridge stringers1

1Specimens 2 by 4 by 43 inches tested in bending by loading at

the third points of a 39-inch span.

2Based on volume and weight when ovendry.

Report No. 2236
Table 6.--Summary of results of fatigue tests of air-dry, straight-grained, unchecked, quarter-scale,

1 southern pine bridge stringers

1Specimens 2 by 4 by 43 inches loaded at third points of a 39-inch span at 500 cycles per minute.

2Based on volume and weight when ovendry.

3 Static strength of matched specimen.

4Fiber stress in bending, minimum repeated stress 0.10 of maximum.
Table 7.--Summary of results of static control tests of air-dry,

unchecked, quarter-scale, southern pine bridge

stringers 1 with 1:12 slope of grain

1

Specimens 2 by 4 by 43 inches tested in bending by loading at the third

points of a 39-inch span.

2 Based on volume and weight when ovendry.

3 Not included in average.

Report NO. 2236
Table 8 .--Summary of results of fatigue tests of air-dry, unchecked, quarter-scale, southern pine

1 bridge stringers with 1:12 slope of grain

Fiber stress in bending, minimum repeated stress 0.10 of maximum.

5 Estimated on basis of specific gravity because matched control was not typical. Not included in average.2Based on volume and weight when ovendry.

3Static strength of matched specimen.

1

4

Specimens 2 by 4 by 43 inches loaded at third points of a 39-inch span at 500 cycles per minute.
Table 9. --Summary of results of static control tests of treated, straight

grain, unchecked, quarter-scale, southern pine bridge

stringers 1

2

Determined by resistance-type moisture meter.

3

From adjacent untreated specimen (based on volume and weight when ovendry)

4

Value not included in average.

5Control value not used; not included in average.

points of a 39-inch span.

1

Specimens 2 by 4 by 43 inches tested in bending by loading at the third

Report No. 2236
Table 10.--Summary of results of fatigue tests of treated, straight-grained, unchecked, quarter-scale

southern pine bridge stringers1

1 Specimens 2 by 4 by 43 inches loaded at third points of a 39-inch span at 500 cycles per minute.

3 From adjacent untreated specimen (based on volume and weight when ovendry).

2 Determined by resistance-type moisture meter.

4 Static strength of matched specimen. 5 Fiber stress in bending, minimum repeated stress 0.10 of maximum. 6 Estimated on basis of specific gravity, not included in average.7 Not included in average.
Table 11 .--Summary of results of static control tests of treated, unchecked

quarter-scale, southern pine bridge stringers1 with 1:12 slope

of grain

1Specimens 2 by 4 by 43 inches tested in bending by loading at the third

points of a 39-inch span.

5

Control value not used; not included in average.

2

Determined by resistance-type moisture meter.

3

From adjacent untreated specimen (based on volume and weight when ovendry).

4

Value not included in average.

Report No. 2236
Table 12 .--Summary of results of fatigue tests of treated, unchecked, quarter-scale, southern pine bridge

1 stringers with 1:12 slope of grain

1Specimens 2 by 4 by 43 inches loaded at third points of a 39-inch span et 500 cycles per minute.

2Obtained by resistance-type moisture meter.

3 From adjacent untreated specimen (based on volume end weight when ovendry).4Static strength of matched specimen.

5Fiber stress in bending, minimum repeated stress 0.10 of maximum

6

Estimated on basis of specific gravity; not included in average.
Table 13.--Summary of results of static control tests of air-dry,

straight-grained, artificially checked, quarterscale, southern pine bridge stringers1

1Specimens 2 by 4 by 43 inches with artificial checks et midheight, reducing width by one-half, were tested in bending by

loading at third points of 39-inch span.

2

Based on volume and weight when ovendry.

3

Sheer strength on actual net cross section approximately 2 times

value shown.

Report No. 2236
Report NO. 2236
Table 15. --Summary of results of static control tests of air-dry,

artificially checked, quarter-scale, southern pine

bridge stringers1 with 1:12 slope of grain

1

Specimens 2 by 4 by 43 inches with artificial checks at midheight,

reducing width by one-half, were tested in bending by loading at third

points of 39-inch span.

2

Based on volume and weight when ovendry.

3

Shear strength on actual net section approximately 2 times value shown.

Report No. 2236
Table 16 .--Summary of results of fatigue tests of air-dry, artificially checked, quarter scale,

southern pine bridge stringers1 with 1:12 slope of grain

1Specimens 2 by 4 by 43 inches with artificial checks at midheight, reducing width by 0.5, loaded

at third points of 39-inch span at 500 cycles per minute.

2 Based on volume and weight when ovendry.3Shear stress calculated on gross cross section; stress on actual net section was approximately

2 times values shown.

4Minimum repeated stress 0.10 of maximum.

5 Estimated on basis of specific gravity.

6 Because of high specific gravity, shear strength was not estimated.
Table 17 .--Summary of results of static 'control tests of treated, straight-grained, artificially

1 checked, quarter-scale, southern pine bridge stringers

3From adjacent untreated specimen; based on volume and weight when ovendry.

4Shear strength on actual net cross section approximately 2 times value shown.

5 Not included in average.1Specimens 2 by 4 by 43 inches with artificial checks at midheight, reducing width by one-half

were tested by loading at third points of 39-inch span.

2Determined by resistance-type moisture meter.
Table 18 .--Summary of results of fatigue tests of treated, straight-grained, artificially checked, quarter-scale,

southern pine bridge stringers1

5 Minimum repeated stress 0.10 of maximum.

6 Estimated on basis of specific gravity.

points of 39-inch span at 500 cycles per minute2Determined by resistance-type moisture meter.

3From adjacent untreated specimen; based on volume and weight when ovendry.4Shear stress calculated on gross cross section; stress or actual net section was approximately 2 times value

shown.

1Specimens 2 by 4 by 43 inches with artificial checks at midheight reducing width by one-half, loaded at third
Table 19.--Summary of results of static control tests of treated, artificially checked, quarter

scale, southern pine bridge stringers1 with 1:12 slope of grain

Shear strength on actual net section approximately 2 times value shown.

5 Not included in average.

6 Not used as control or included in average.

1Specimens 2 by 4 by 43 inches with artificial checks at midheight, reducing width by one-half

were tested in bending by loading at third points of 39-inch span.

2Determined by resistance-type moisture meter.

3 From adjacent untreated specimen; based on volume and weight when ovendry.4
Table 20.--Summary of results of fatigue tests of treated, artificially checked, quarter-scale, southern pine

bridge stringers1 with 1:12 slope of grain

points of 39-inch span at 500 cycles per minute.

2 From adjacent untreated specimens; based on volume and weight when ovendry.

3Shear on actual net section approximately 2 times value shown.

4 Minimum repeated stress 0.10 of maximum.

5 Estimated on basis of specific gravity.

6 Stress in control when bending failure occurred.

7Estimated but because of high gravity, value in doubt.

1Specimens 2 by 4 by 43 inches with artificial checks at midheight reducing width by one-half, loaded at third
Table 21 .--Summary of results of static control tests of green, straightgrained, unchecked quarter-scale Douglas-fir bridge stringers1

of 39-inch span.

2 Based on volume and weight when ovendry.

3

From specimen 2-F-D-F-U-7A.

4

Not included in average.

5

Control value not used, not included in average.

1

Specimens 2 by 4 by 43 inches tested in bending by loading at third points

Report No. 2236
Table 22.--Summary of results of fatigue tests of green, straight-grained, unchecked, quarter-scale, Douglasfir bridge stringers1

1Specimens 2 by 4 by 43 inches loaded at third points of 39-inch span at 500 cycles per minute.2 Based on volume and weight when ovendry.3Static strength of matched specimen.4Fiber stress in bending, minimum repeated stress 0.10 of maximum.5 Estimated for value of specific gravity; see note on control test; not included in average.
quarter-scale, Douglas-fir bridge stringers1 with 1:12 slope

of grain

Table 23 .--Summary of results of static control tests of green, unchecked,

1

Specimens 2 by 4 by 43 inches tested in bending by loading at third points

at 39-inch span.

2

Based on volume and weight when ovendry.

3

Control value not used, not included in average.

Report No. 2236
Table 24 .--Summary of results of fatigue tests of green, unchecked, quarter-scale, Douglas-fir bridge

stringers1 with 1:12 slope of grain

Estimated for value of specific gravity; see note on control test.

Local slope of grain (1:4) at failure; not representative. 6

Fiber stress in bending, minimum repeated stress 0.10 of maximum.

5

Static strength of matched specimen.

2

4

Based on volume and weight when ovendry.

13

Specimens 2 by 4 by 43 inches loaded at third points of 39-inch span at 500 cycles per minute.
Table 25. --Summary of results of static control tests of air-dry, straightgrained, unchecked, quarter-scale, Douglas-fir bridge stringers1

1Specimens 2 by 4 by 43 inches tested in bending by loading at the third

points of a 39-inch span.

2

Based on volume and weight when ovendry.

3Not included in average.

4Value not used as control value for fatigue test; knot in specimen adversely

affected strength.

Report No. 2236
Table 26 .--Summary of results of fatigue tests of air-dry, straight-grained, unchecked, quarter-scale, Douglasfir bridge stringer1

1 Specimens 2 by 4 by 43 inches loaded at third points of a 39-inch span at 500 cycles per minute.2 Based on volume and weight when ovendry. 3 Static strength of matched specimen.4 Fiber stress in bending, minimum repeated stress 0.10 of maximum. 5 Estimated for value of specific gravity; see note on control test.

6 Not included in average.
Table 27.--Summary of results of static control tests of air-dry,

unchecked quarter-scale , Douglas-fir bridge stringers1

with 1:12 slope of grain

1

Specimens 2 by 4 by 43 inches tested in bending by loading at the third

points of a 39-inch span.

2

Bed on volume and weight when ovendry.

3

Actual slope on tension face 1:16.

4 Combined slope of grain 1:6.5.

5 Actual slope on tension face 1:9.

6 Combined slope of grain 1:9.

7

Not included in average.

Report No. 2236
Table 28 .--Summary of results of fatigue tests of air-dry, unchecked, quarter-scale, Douglas-fir, bridge

1 stringers with 1:12 slope of grain

Estimated for specific gravity; see note on control; not included in average.

1Specimens 2 by 4 by 43 inches tested in bending by loading at third points of a 39-inch span.2 Based on volume and weight when ovendry.

3Static strength of matched specimen.

4 Fiber stress in bending, minimum repeated stress 0.10 of maximum.5Combined slope of grain at failure 1:7.5.6Combined slope of grain at failure 1:6.5.7
Table 29.--Summary of results of static control tests of treated,

straight-grained, unchecked, quarter-scale, Douglasfir bridge stringers1

1

Specimens 2 by 4 by 43 inches tested in bending by loading at the

third points of a 39-inch span.

2 Based on volume and weight when ovendry; because specimens were

treated, values are from adjacent untreated specimens.

3

Not included in average.

4

Not included in average or used for control.

Report No. 2236
Table 30 .--Summary of results of fatigue tests of treated, straight-grained, unchecked, quarterscale, Douglas-fir bridge stringers1

1Specimens 2 by 4 by 43 inches loaded at third points of a 39-inch span at 500 cycles per minute.

2Based on volume and weight when ovendry; because specimens were treated, values are from adjacent

untreated specimens.

3Static strength of matched specimen (modulus of rupture).4Fiber stress in bending, minimum repeated stress 0.10 of maximum.5Estimated on basis of specific gravity.6Estimated from adjacent static specimen.
Table 31 .--Summary of results of static control tests of treated,

unchecked, air-dry quarter-scale, Douglas-fir bridge

stringers 1 with 1:12 slope of grain

1Specimens 2 by 4 by 43 inches tested in bending by loading at the

third points of a 39-inch span. Material conditioned and tested at

75° F. and 64 percent relative humidity.

2

Based on volume and weight when ovendry; because specimens were

treated, values are from adjacent untreated specimens.

3

Not included in average.

4

Not included in average or used for control.:

Report No. 2236
Table 32 .--Summary of results of fatigue tests of treated, unchecked quarter-scale. Douglasfir bridge stringers1 with 1:12 slope of grain

1Specimen 2 by 4 by 43 inches loaded at the third points of a 39-inch span at 500 cycles per minute.2 Based on volume and weight when ovendry;

untreated specimens.

because specimens were treated, values are from adjacent

3Static strength of matched specimen.4Fiber stress in bending, minimum repeated stress 0.10 of maximum.5Estimated on basis of specific gravity.6Estimated from adjacent static specimen.
Table 33 .--Summary of results of static control tests of air-dry, straight-grained, artificially

1

checked, quarter-scale, Douglas-fir bridge stringers

1Specimens 2 by 4 by 43 inches with artificial checks at midheight reducing width by one-half,

Not used as control or included in average.

were tested in bending by loading at third points of 39-inch span.

2Based on volume and weight when ovendry.3Shear strength on actual net cross section approximately 2 times value shown.4

Not included in average.

5
Table 34 .--Summary of results of fatigue tests of air-dry, straight-grained, artificially checked, quarter-scale,

Douglas-fir bridge stringers1

4 Minimum repeated stress 0.10 of maximum.

5 Estimated on basis of specific gravity; not included in average.

1Specimens 2 by 4 by 43 inches with artificial checks at midheight, reducing width by one-half, loaded at third

points of 39-inch span at 500 cycles per minute.

2Based on volume and weight when ovendry.3Shear stress calculated on gross cross section; stress on actual net section was approximately 2 times values

shown.
Table 35 .--Summary of results of static control tests of air-dry, artificially checked, quarter1 scale, Douglas-fir bridge stringers with 1:12 slope of grain

1Specimens 2 by 4 by 43 inches with artificial checks at midheight, reducing width by one-half, were tested in bending by loading at third points of 39-inch span.

2Based on volume and weight when ovendry.3 Shear strength on actual net cross section approximately 2 times value shown.4From matched fatigue specimen,5 Not included in average.6 Not used as control for fatigue.7 Not used as control for fatigue because of effect of tension failure on shear, nor included in

average.
Table 36 .--Summary of results of fatigue tests of air-dry, artificially checked, quarter-scale, Douglas-fir bridge

stringers1 with 1:12 slope of grain

1Specimens 2 by 4 by 43 inches with artificial checks at midheight, reducing width by one-half, loaded at third

points of 39-inch span at 500 cycles per minute.

2Based on volume and weight when ovendry.3Shear stress calculated on gross cross section; stress on actual net section was approximately 2 times value

shown.

4 Minimum repeated stress 0.10 of maximum.5 Estimated on basis of specific gravity.
Table 37 .--Summary of results of static control tests of treated, straight

grained, artificially checked, quarter-scale, Douglas-fir

bridge stringers1

1

Specimens 2 by 4 by 43 inches with artificial checks at midheight, reducing

width by one-half, were tested in bending by loading at third points of 39-

inch span.

2From adjacent untreated specimen; based on volume and weight when ovendry.

3Shear strength on actual net cross section approximately 2 times value shown.

4 Not included in average.

Report No. 2236
Table 38 .--Summary of results of fatigue tests of treated, straight-grained, artificially

checked, quarter-scale, Douglas-fir bridge stringers1

1

Specimens 2 by 4 by 43 inches with artificial checks at midheight, reducing width by

one-half, loaded at third points of 39-inch span at 500 cycles per minute.

2

From adjacent untreated specimen; based on volume and weight when ovendry.

3Shear stress calculated on gross cross section; stress on actual net section was

approximately 2 times value shown.

4

Minimum repeated stress 0.10 of maximum.

5Not included in average but used as estimate of control strength because of high

specific gravity.

Report No. 2236
stringers1 with 1:12 slope of grain

Table 39. --Summary of results of static control tests of treated,

artificially checked, quarter-scale Douglas-fir bridge

1

Specimens 2 by 4 by 43 inches with artificial checks at midheight, reducing width

by one-half, were tested in bending by loading at third points of 39-inch span.

2 From adjacent untreated specimen; based on volume and weight when ovendry.

3 Shear strength on actual net cross section approximately 2 times value shown.

4 Not included in average.

Report No. 2236
Table 40 .--Summary of results of fatigue tests of treated, artificially checked, quarter scale Douglas1

fir bridge stringers with 1:12 slope of grain

3 Shear stress calculated on gross cross section; stress on actual net section was approximately 2 times

value shown.

1Specimens 2 by 4 by 43 inches with artificial checks at midheight reducing width by one-half, loaded

at third points of 39-inch span at 500 cycles per minute.

2 From adjacent untreated specimens; based on volume and weight when ovendry.4 Minimum repeated stress 0.10 of maximum.5 Control shear strength estimated on basis of specific gravity; not included in average.
Table 41 .--Summary of estimated fatigue strength values at 2 million cycles of stress for quarter-scale bridge stringers

3Percentage values rounded to nearest 5 percent.

4From adjacent untreated specimens.

5 Based on actual width of unchecked specimen; footnote2 does not apply.

6 Estimated on basis of average specific gravity.

1Based on volume and weight when ovendry.

2 Calculated on gross cross section with no reduction In width for artificial checks; actual

stresses about 2 times.

Report No. 2236
Report No. 2236
Figure 2.--Specimens for fatigue tests of quarter-scale bridge stringers.
Figure 3. --Assembly for fatigue test of quarter-scale bridge stringers.

Yoke and load blocks distribute loads to third points of the specimen.

The plastic cocoon is used to maintain the specimen in a green

condition.

Figure 4.--Assembly for static control test of quarterscale bridge stringers.

Z M 120 730
Figure 5.--Axial loading fatigue machine for repeated loading of quarter-scale bridge

stringers. Essential parts of machine are: A, Specimen; B, Beam and yoke to

support specimen at ends; C, Static loading screw to apply and adjust mean load

on specimen; D , Loading yoke to apply alternating stress to specimen at third

points; E, Linkage and flex-plate system to transfer loading from cantilever arm

to loading yoke; F, Cantilever loading arm; G, Adjustable cam for required

alternating load; H, Electronic adjustable cutoff switch to control machine or

shut off drive when specimen failed or load was reduced.

ZM 97280
Figure 6. -- (Top) Relation of stress to number of cycles to failure for fatigue tests of green straight-grained, unchecked southern pine. Values for maximum repeated stress are expressed as percentages of static

strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum repeated stress

expressed in pounds per square inch.

Report No. 2236
Figure 7. --(Top) Relation of stress to number of cycles to failure for fatigue tests of green, unchecked

southern pine with 1:12 slope of grain. Values for maximum repeated stress are expressed as percentages

of static strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum repeated stress

expressed in pounds per square inch.

Report No. 2236
Figure 8.--(Top) Relation of stress to number of cycles to failure for fatigue tests of air-dry, straight- grained, unchecked southern pine. Values for maximum repeated stress are expressed as percentages

of static strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum repeated

stress expressed in pounds per square inch.

Report No. 2236
Figure 9. -- (Top) Relation of stress to number of cycles to failure for fatigue tests of air-dry, unchecked southern pine with 1:12 slope of grain. Values for maximum repeated stress are expressed as percentages

of static strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum repeated stress

expressed in pounds per square inch.

Report No. 2236
Figure 10.--(Top) Relatlon of stress to number of cycles to failure for fatigue tests for treated, straightgrained, unchecked southern pine. Values for maximum repeated stress are expressed as percentages

of the static bending strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum

repeated flexure stress expressed in pounds per square inch.

Report No. 2236

Figure 12. --Relation of stress to number of cycles to failure for treated, straight-grained, unchecked southern

pine. Since specimens that withstood a large number of cycles before failing developed horizontal shear

failures, values of maximum repeated stress are expressed in shear in pounds per square inch.
Figure 13. --(Top) Relation of stress to number of cycles to failure for fatigue tests of treated, unchecked southern pine with 1:12 slope of grain. Values for maximum repeated stress arc expressed as percentages

of the static bending strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum

repeated stress expressed in pounds per square inch.

Report No. 2236
Figure .4. --(Top) Relation of stress to number of cycles to failure for fatigue tests of air-dry, straightgrained, artificially checked southern pine. Values for maximum repeated stress are expressed as

percentages of static shear strength. Stress ratio 0.10. (Bottom) Same relationship with values of

maximum repeated stress expressed in pounds per square inch.

Report No. 2236
Figure 15. --(Top) Relation of stress to number of cycles to failure for fatigue tests of air-dry, aritficially checked southern pine with 1:12 slope of grain. Values for maximum repeated stress are expressed as

percentages of static shear strength. Stress ratio 0.10. (Bottom) Same relationship with values of

maximum repeated stress expressed in pounds per square inch.

Report No. 2236
Figure 16. --(Top) Relation of stress to number of cycles to failure for fatigue tests of treated, straightgrained, artificially checked southern pine. Values of maximum repeated stress are expressed as

percentages of static shear strength. Stress ratio 0.10. (Bottom) Same relationship with values of

maximum repeated stress expressed in pounds per square inch.

Report No. 2236
Figure 17.--(Top) Relation of stress to number of cycles to failure for fatigue tests of treated, artificially checked southern pine with 1:12 slope of grain. Values for maximum repeated stress are expressed as

percentages of the static shear strength. (Bottom) Same relationship with values of maximum repeated

stress expressed in pounds per square inch.

Report No. 2236
Figure 18. --(Top) Relation of stress to number of cycles to failure for fatigue tests of green, straightgrained, unchecked Douglas-fir. Values for maximum repeated stress are expressed as percentages of

static strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum repeated stress

expressed in pounds per square inch.

Report No. 2236
Figure 19. --(Top) Relation of stress to number of cycles to failure for fatigue tests of green, unchecked

Douglas-fir with 1:12 slope of grain. Values of maximum repeated stress are expressed as percentages

of static strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum repeated stress

expressed in pounds per square inch.

Report No. 2236
Figure 20. --(Top) Relation of stress to number of cycles to failure for fatigue tests of air-dry, straightgrained, unchecked Douglas-fir. Values for maximum repeated stress are expressed as percentages of

static strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum repeated stress

expressed in pounds per square inch.

Report No. 2236
Figure 21. --(Top) Relation of stress to number of cycles to failure for fatigue tests of air-dry, unchecked Douglas-fir with 1:12 slope of grain. Values are expressed as percentages of static strength. Stress

ratio 0.10. (Bottom) Same relationship with values of maximum repeated stress expressed in pounds per

square inch.

Report No. 2236
Figure 22. --Typical static and fatigue failures of air-dry, straight-grained

Douglas -fir. Upper specimen of each pair is from static control test;

lower one of each pair from fatigue test.

ZM 119 403
Figure 23. --(Top) Relation of stress to number of cycles to failure for treated, straight-grained unchecked

Douglas -fir. Values for maximum repeated stress are expressed as percentages of the static strength.

Stress ratio 0.10. (Bottom) Same relationship with values of maximum repeated flexure stress expressed

in pounds per square inch.

Report No. 2236
Report No. 2236
Figure 25. --(Top) Relation of stress to number of cycles to failure for fatigue tests of treated Douglas-fir

with 1:12 slope of grain. Values of maximum repeated stresses are expressed as percentages of static

strength. Stress ratio 0.10. (Bottom) Same relationship with values of maximum repeated stress

expressed in pounds per square inch.

Report No. 2236
Figure 26. --(Top) Relation of stress to number of cycles to failure for fatigue tests of air-dry, straight- grained, artificially checked Douglas-fir. Values for maximum repeated stress are expressed as

percentages of static shear strength. Stress ratio 0.10. (Bottom) Same relationship with values of

maximum repeated stress expressed in pounds per square inch.

Report No. 2236
Figure 27. --(Top) Relation of stress to number of cycles to failure for fatigue tests of air-dry, artificially checked Douglas-fir with 1:12 slope of grain. Values for maximum repeated stress are expressed as

percentages of static shear strength. Stress ratio 0.10. (Bottom) Same relationship with values of

maximum repeated stress expressed in pounds per square inch.

Report No. 2236
Figure 28. --(Top) Relation of stress to number of cycles to failure for fatigue tents of treated, straightgrained, artificially checked Douglas, -fir. Values for maximum repeated stresses are expressed at

percentages of static shear strength. Stress ratio 0.10. (Bottom) Same relationship with values of

maximum repeated stress expressed in pounds per square inch.

Report No. 2236
Figure 29. --(Top) Relation of stress to number of cycles to failure for fatigue tests of treated, artificially checked Douglas-fir with 1:12 slope of grain. Values for maximum repeated stress are expressed as

percentages of static shear strength. Stress ratio 0.10. (Bottom) Same relationship with values of

maximum repeated stress expressed in pounds per square inch.

Report No. 2236
Figure 30. -- Approximate S-N curves for unchecked, quarter-scale bridge stringers; (top) Southern pine and

(bottom) Douglas-fir. Values of maximum repeated stress are expressed as percentages of static bending

strength. Stress ratio 0.10.

Report No. 2236
Figure 31. --Approximate S-N curves for unchecked, quarter-scale bridge stringers; (top) southern pine and

(bottom) Douglas-fir. Values of maximum repeated bending stress are in pounds per square inch. Stress

ratio 0.10.

Report No. 2236

Figure 33. --Approximate S-N curves for artificially checked, quarter-scale bridge stringers; (top) southern pine and (bottom) Douglas-fir. Values of maximum repeated shear stress are in pounds per square inch,

calculated on the basis of the gross width with no reduction in width for notches, so actual stresses were

about twice those shown. Stress ratio 0.10.

Report No. 2236