FATIGUE OF WELDED JOINTS LOADED IN BENDING · shear stresses in the welds, resulting from the...

TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment

Supplementary Report 84UC

FATIGUE OF WELDED JOINTS LOADED IN BENDING

by

S. J. Maddox Bsc (Eng), Ph D, DIC (The Welding Institute)

The work described in this Report was sponsored by the Transport and Road Research Laboratory.

Any views expressed in this Report are not necessarily those of the Department of the Environment

Bridge Design Division Structures Department

Transport and Road Research Laboratory Crowthorne, Berkshire

1974

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

Abstract

1. Introduction

2. General details of test specimens

3. Test specimens

3.1 Test series 1 -4

3.2 Test series 5 - 1 0

4. Testing details

4.1 Series 1A

4.2 Series 2B

4.3 Series 3B and 4C

4.4 Series 5D to 7D

4.5 Series 8E to 10E

CONTENTS

Page

1

1

2

2

2

3

3

3

3

4

4

5. Presentation of results

5.1 Series 1A

5.2 Series 2B


5.4 Series 5D

5.5 Series 6D and 7D

5.6 Series 8E

5.7 Series 9E and 10E

6. General discussion of results and comparison with data in the literature

6.1 Failure in the main plate

6.2 Failure in the stiffener

6.3 Failure in the weld

6,4 The relevance of the shear stress in the weld

6.4.1 Stiffener attached to plate with two welds 6.4.2 Stiffener attached to plate with one weld

6.5 The effect of the gap between the stiffener and plate

8

8

9

9

10

I0 10

10

7. Conclusions

8. Acknowledgements

9. References

11

11

12

10. Nomenc la tu re

11. A p p e n d i x 1 - The materials and methods of construct ion

Page 1 2

13

© CROWN COPYRIGHT 1974 Extracts f rom the text may be reproduced,

except for commercial purposes, provided the source is acknowledged.

FATIGUE OF WELDED JOINTS LOADED IN BENDING

ABSTRACT

Fatigue tests were carded out on a number of welded joints in an investigation relevant to steel orthotropic road bridges. Specimens consisted of 11 mm thick plates with 6.35 mm thick transverse stiffeners welded, generally at right angles, to the surfac e, with either a single weld or welds On both sides of the stiffener. The loading conditions were selected to investigate the fatigue failure in the main plate, the stiffener and the weld, all in bending. In addition, the joint was subjected to loading which gave shear stress in the weld and bending stress in the main plate, the ratio of the two being varied, to investigate the relative importance of each type of stress. Finally, the effect on the fatigue behaviour of load-carrying joints having one or two welds attaching the stiffener to the main plate and the effects of a gap between the stiffener and the main plate were investigated. Where possible the results were compared with relevant data in the literature.

1. INTRODUCTION

The fatigue behaviour of welded joints subjected to axial loading has been investigated extensively in the past. Such data form the basis of the fatigue design rules in BS 153 (1). However, welded joints loaded in bending have received surprisingly little attention. In many practical situations, for example welded I beams in bending, stress gradients across loaded sections are small enough for stresses in the flange to be regarded as being effectively axial. However, such an approximation is unlikely to be valid in the case of welded structures made from thin plates such as an orthotropic bridge deck. Such a structure may be subjected to fluctuating transverse bending stresses and the possibility of fatigue failure occurring at a weld toe, in the stressed plate, or weld throat must be considered in design.

This report describes experiments to examine the fatigue behaviour of joints in or thotropic bridge decks in support of work at the Transport and Road Research Laboratory. Weld details studied are shown in Figure 1. These are the longitudinal stiffener to deck plate welds (subjected to transverse bending by wheel loads close to the weld and direct wheel loading through the stiffener) and the transverse welds between stiffeners and cross-girders (subjected to direct loading due to longitudinal bending in the stiffener).

Some parts of the investigations have wider applications, e.g. the joint between the web and flange of a crane girder subjected to a combination of shear and bending stresses, welded connections in structures made up from box sections and plates with welded at tachments subjected to bending. How- ever, such factors as plate material, plate and weld sizes, and stress ratio were selected to be relevant to orthotropic bridge decks.

2. GENERAL DETAILS OF TEST SPECIMENS

The specimens tested were intended to model the welded joint between either a deck plate (11 mm thick) and longitudinal stiffener (6.35 mm thick) or a stiffener and cross-girder. I t is convenient to refer to the thicker plate as the main plate and the thinner one as a stiffener. In all but two cases, the stiffener was attached at fight angles to the main plate.

The investigations fall into three main parts, each one being concerned with a particular mode of failure. In practice, the main plate/stiffener configurations used in orthotropic deck sections can be subjected to "bending in the main plate, the weld and the stiffener; to shear in the weld; to direct stress in the stiffener. Thus, fatigue failure could occur in the main plate, the crack initiating at the weld toe or root ; in the weld throat, a crack initiating at the root or surface, depending on the stress concentration; or in the stiffener, a crack initiating at the weld toe. The specimen types used in the investigations, referred to by letters, are summarised in Figure 2.

3. TEST SPECIMENS

The materials and methods of construction are given in Appendix 1. The dimensions and methods of testing the specimens differed from one test series to the next. Anumber ing system is used for the test series and a lettering system for the specimens.

3.1 Test Series 1 to 4

These tests were to investigate the fatigue properties of the weld between the main plate and a closed stiffener of an orthotropic deck. Calculations indicate that both the main plate and stiffener were subject to transverse bending, the ratio of minimum to maximum stress, R, being different in each case. For simplicity, two general cases were investigated:

(a) The fatigue strength of the main plate with the welded stiffener present bu t carrying no load. This formed the basis of test series 1 and specimerr type A was used (referred to as Series 1A). Details of the specimen are given in Figure 2(a). It was expected that designs would incorporate but t welds between the main plate and stiffeners and so butt welds, made with a 60 ° edge preparation, were used in the test specimens.

(b) The fatigue strength of the welded joint with the bending moment applied through the stiffener, the main plate carrying no load. Two cases were covered, namely bending which gave a fully tensile stress at the weld toe in the stiffener or a fully compressive stress at the same position. Test series 2 with specimen type B (Figure 2B) was used to investigate the tension situation. The weld was made using the same edge preparation as for specimen type A. Test series 3 also made use of specimen type B but in this case the loading gave compression at the weld toe and hence tension at the weld root. Finally, test series 4 made use of specimen type C, loaded in the same way as Series 3B, (Figure 2c), in order to provide comparable data for a fillet welded

joint.

3.2 Test Series 5 to 10

.Test series 5 to 10 were concerned with basically the same main plate/stiffener configuration. This was a single stiffener fillet welded at right angles to the main plate surface with either one or two welds. In each case, the stiffener carried direct loading while the main plate was subjected to bending or no loading along the plate. The general objective in the tests was to investigate the interaction between shear stresses in the welds, resulting from the direct load in the stiffener, and bending stresses in the main plate. Shear stresses in the weld could give rise to failure in the weld throat, while bending stresses in the plate could lead to failure in the plate at the weld toe. In an attempt to cover both cases,

different values of the ratio o p were used. (Symbols given in Nomenclature). Tw

Test series 5 to 7 used test specimen type D (Figure 3d) but with differing loading spans to give

values for the ratio °p of 0, 10 and 100. The specimens were made with a 1.3 mm (0.05 in.) gap Tw

2

between the plate and stiffener to simulate the results of distortion which could occur during welding. Joints of the type used in Series 5D to 7D, in which the stiffener is attached to the main plate using two welds, are relevant to web/flange joints in crane girders, as well as single plate stiffener/deck plate joints in orthotropic bridge decks.

• Test series 8 to 10 used specimen type E. For Series 8E and 9E a gap existed between the stiffener and main plate. The difference between the two series was that the span was varied to give two values

of the ratio °p. Series 10E was constructed such that a gap between the stiffener and main plate may "/'w

have existed but was not introduced intentionally. The purpose was to.compare the fatigue behaviour of joints with and without gaps under the same loading conditions. The specimens used in Series 8E to 10E (Figure 2e), in which the stiffener is attached by a single weld to the main plate, are relevant to stiffener/deck plate and stiffener/cross-girder welds in orthotropic bridge decks with closed stiffeners, as well as single sided fillet welded connections such as would occur in box section structures.

4. TESTING DETAILS.

Electrical resistance strain gauges were used to monitor the strains in the main plate or stiffener, depending on which was relevant. Three gauges were equally spaced across the specimen and the average was taken when analysing the results.

Where possible a system of fine cut-out wires (0.05 mm diameter) glued to the specimen surfaces were used to stop the testing machine when a through-section crack had developed.

4.1 Series 1A

The specimens were subjected to simple four point bending to give a uniform bending stress in the plate in the region containing the welded stiffener. The tests were carried out at a stress ratio R o f - 2 on the plate surface containing the weld. '

The tests were carried out in a I000 KN Illinois testing machine at a frequency of 3.3 Hz. The machine maintains a constant displacement during cycling. The specimen was inverted for testing(Plate 1). Rollers were clamped to the main plate at the four load points and the strain in the region of uniform bending was monitored using 20 mm gauge length strain gauges.

!

4.2 Series 2B

The specimens were subjected to direct loading in the main plate and bending in the stiffeners. The tests were carried out in the Illinois machine with the stiffeners simply-supported on rollers. 5 mm gauge length strain gauges were Fixed near the weld toes in the stiffeners to monitor the strain. The stress ratio at the weld toe was R = 0 and failure was expected to occur at that point.


Apart from three Series 4C specimens, the specimens were loaded in the same way as for Series 2B, except that a fully compressive cyclic stress (R = -oo) at the weld toe in the stiffener was used. The loading gave a fully tensile stress at the weld root and failure was expected to initiate at that point. Plate 2 shows a Series 4C specimen under test.

In addition, as part of a separate investigation (2), three Series 4C specimens were tested under loading which gave R = -1.42 at the weld toe in the stiffener.

3

4.4 Series 5D to 7D

The method of loading varied for each specimen and was controlled by the value of the ratio o p "T w

in an attempt to obtain both the possible modes of failure (main plate or weld throat).

The specimens were inverted for testing, with a load applied directly through the stiffener, and the

plate simply-supported. The pitch of the plate supports was varied to suit the ratio °p . The following ~'w

calculation was used to determine the required pitch:

Plate bending stress

Weld throat shear stress

3Pa O p - bT 2

P rw b 1']2"

Op = 4.24al ¥ 7 T 2

...(1)

. . . ( 2 )

In the present case 1 = 6.35 mm (% in.) and T = 11 mm (7/16 in.)

ap = 0 w h e n a = 0 Tw

°--E-P = 10 when a = 46 mm (1.8 in.) Tw

Op = 100 when a = 460 mm (18 in.) Tw

The Series 5D tests (r°@. = 0) were conducted in two testing machines. The Illinois machine was " w

used for the shorter durations and two long duration tests were carried out in a 100 KN (10 ton) Amsler Vibrophore at 200 Hz using 51 mm (2 in.) wide specimens. This machine maintains a constant cyclic load. The joint was subjected t O compressive cyclic loading by supporting the specimen directly above the stiffener and loading the stiffener axially. The test was intended to represent the case of a load on the plate directly above the stiffener, such that no bending occurred in the plate. Strain measurements were made using 5 mm gauge length strain gauges on each size of the stiffener adjacent to the weld toe.

The Series 6D specimens ( ~ P = 10) were tested in either a 100 KN (10 ton) or 400 KN (40 ton)

Losenhausen testing machine, at frequencies of between 5 and 16 Hz. The machines maintain constant cyclic load. Strain measurements (5 mm gauge length) were made at positions adjacent to the weld toe on both the stiffener and the plate. The same arrangement was used to test the Series 7D specimens

(r~- = 100) in the Illinois testing machine (Plate 3). In both cases, the applied load ratio was related

to the expected behaviour of orthotropic bridge decks and gave a stress ratio of -4 at the weld toe in

the main plate.

4.5 Series 8E to 10E

Test Series 8E was carried out in the Illinois machine with the ratio op = 0, to represent a load Tw

directly over the stiffener. The testing arrangement was the same as that used for Series 5D specimens

(see Section 4.4).

4

Tests were also carried out on specimens with single welds, in which there was some bending in the plate (Series 9E). For ease of testing, 2a was 92 mm (3.6 in.) (as in the double weld case where

0.2_P = 10, Series 6D) which gave a value of °p = 5. T w Tw

Finally, some tests were carried out on specimens which had no gap introduced between the stiffener and plate, and a single weld was used to attach the stiffener to the plate (Series 10E). Assuming there was no gap between the stiffener and plate after welding, the shear stress in the weld would have been

zero during the compressive part of the toad cycle, that is for four-fifths of the range, giving o__q_p= 25. Tw

However, if a gap occurred, because of distortion after welding, o p would have been 5. It will be seen Tw

later that each of these possibilities appeared to have occurred.

Series 9E and 10E specimens were tested in a 400 KN Losenhausen testing machine. The loading arrangement was the same as that used for Series 6D specimens (see Section 4.4).

5. PRESENTATION OF RESULTS

5.1 Series 1A

Fatigue cracks initiated in the stressed plate at either the weld toe or the weld root and propagated through the plate thickness. The weld toe and root are both regions of stress concentration and crack- like defects at which fatigue cracks initiate (3,4), normally associated with the weld toe, would be present at both locations. The failure criterion was that a crack had proPagated through the plate thickness. In general, this virtually coincided with complete fracture of the plate. In the majori ty of specimens failure initiated from the weld root, but examples of both modes of failure are shown in Plate 4. The point o f crack initiation was not associated with the stop/start position in the weld.

The locations of failure were in a region of uniform bending moment . Therefore, all the fatigue test results, toe and root failures, are presented in terms of the nominal bending stress in that region, based on the. strain gauge measurements. The results, together with a description of the mode of failure, are given in Table 2 and on an S - N diagram in Figure 3.

5.2 Series 2B

The specimens were loaded to give a pulsating tensile bending stress at the weld toe in the stiffener and, as expected, fatigue cracks initiated along the weld toe and propagated through the stiffener thickness. The criterion of failure was that a crack had propagated through the stiffener thickness. A macrosection of a failed weld is shown in Plate 5.

The results (see Table 3 and Figure 4) are presented in terms of the nominal bending stress in the plane of the toe, as calculated from the strain gauge measurements.


The specimens were loaded to give part or fully tensile cyclic stresses at the weld root and, as expected, fatigue cracks initiated at that point and propagated through the throat. The criterion of failure was that a crack had reached the weld surface.

Since failure occurred as a result of bending in the weld, the results are given (Table 4 and Figure 5) in terms of the nominal range of bending stress at the root, i.e. the point of crack initiation. This

5

stress was calculated on the basis of the bending moment in the stiffener, as determined from strain gauges, and the weld throat cross-section on the plane of fracture.

5;4 Series 5D

In this test series a stiffener, fillet welded at right angles to the main plate, was subjected to a direct load, the main plate being supported on a fiat surface (Op = 0). In view of the gap which existed between the stiffener and plate, the weld carried a shear stress (rw). Four specimens were tested and none failed (see Table 5). However, in some cases fatigue cracks propagated fro.m a weld toe into the stiffener at an angle of approximately 45 °. A macro-section of the largest crack obtained is shown in Plate 6. On the basis of the path of propagation of this crack, the shear stress on the expected plane of failure was

calculated and is included in Table 5.

The result for specimen 5D/4, which was tested in the Vibrophore, does not appear to be consistent with that for specimen 5D/2 tested in the Illinois machine. Under similar stress levels, the extent of fatigue cracking was significantly different. This could be scatter or possibly there might have been a

size or frequency effect influencing the results.

5.5 Series 6D and 7D

In bo th series the specimens were subjected to direct loading in the stiffener, giving shear stress in the welds and bending in the main plate. All specimens failed when a fatigue crack had initiated at the weld toe, and propagated through the plate, as illustrated in Plate 7.

The test results are presented in terms of the nominal bending stress at the weld toe in the plate (Op') Calculated from strain gauge readings in Table 6 and Figure 6. Other stresses given in Table 6 were determined as follows: ap was calculated by extrapolation using Op'and zero stress at the roller supports, and using Equation 1, the direct force P being calculated from the strain gauge readings; rw was calculated using Equation 2.

I t will be seen from Table 6 that the value of Op calculated from Op' is less than that calculated

from Equation 1, so that the ratio op. is also less than expected. However, in view of the section changes "r w

that occur at midspan, both methods of determining Op must be regarded as approximations. The actual value of Op is not relevant to the results, since the nominal stress in the weld toe region controlled the fatigue strength of the joint.

5.6 Series 8E

The specimens were loaded to give a shear stress in the weld and zero bending stress in the plate. The strain gauge readings indicated that there was bending and direct stress in the stiffener, probably because the specimen was not symmetrical about the loading axis. The corresponding bending moment would have been transmitted to the weld. Thus, the loading gave a bending stress as well as a shear stress in the weld, the former being tensile at the weld surface. The specimens failed when a fatigue crack had initiated on the surface of the weld, and propagated through the weld throat to the root. The criterion of failure was that the crack on the weld surface had grown to a length of 100 mm, at which time it was assumed that the crack had propagated completely through the weld throat. This was confirmed after testing for all but one specimen. The appearance of the fracture surfaces indicated that crack propagation occurred under bending stresses, there being no evidence of a shear mode of

fracture.

6

In view of the mode of failure, the bending stress on the surface of the weld is the relevant stress parameter. However, the complexity of the weld geometry and the stress distribution near the weld suggest that this stress cannot be calculated reliably from the strain gauge readings. A nominal value of the stress range in the weld was calculated on the assumption that the bending moment in the stiffener was transmitted directly to the weld and that the direct force in the stiffener produced a bending mo-

stiffener thickness, the total bending moment being the sum of the ment in the weld of direct force x 2

two. The results, which include the stresses actually measured in the stiffener, are given in Table 7 and plotted in terms of the nominal stress in the weld in Figure 7.

To interpret the results in the context of purely axial loading in the stiffener, as might occur in some practical situations (see Section 3.2), the direct stress in the stiffener to give the same nominal bending stress in the weld as obtained in the tests, was calculated. Values have been included in Table

7.

5.7 Series 9E and IOE

In both series the loading gave bending in the main plate and direct loading in the stiffener. How- ever, due to lack of symmetry, bending also occurred in the weld. The mode of failure appeared to depend on whether or not a gap remained after welding the Series IOE specimens. All Series 9E specimens, in which there was a gap, failed in the weld throat from cracks which initiated at the weld root. Three out of the five Series 10E specimens also failed in the weld, while two failed in the main plate from cracks which initiated at the weld root, as in the Series 1A specimens. The failure criterion was that complete fracture occurred in the plane of cracking. The appearance of the weld throat fracture surfaces indicated that cracks had propagated under bending rather than shear stresses in all but one specimen. The exception was a specimen that actually failed in the plate but contained a large weld

throat crack which had propagated in a shear mode.

For those specimens which failed through the weld throat, the relevant stress parameter is the bending stress at the weld root (Ow). This stress was calculated from the stresses in the stiffener, measured by strain gauges as described in Section 5.6. In addition, the stress ratio at the weld root was determined. This was generally less than the ratio of maximum to minimum load (4 compression to 1 tension equivalent to R = -4 at the weld root on the plate surface) indicating that a greater proportion of the stress range was tensile in the case of the stress in the weld.

For specimens which failed in the main plate, the relevant stress parameter is the bending stress in the plate near the root (Or). This stress was determined by extrapolation from the strain gauge measurements made on the plate (Op'). The stress at midspan, Op, was calculated in the same way.

The results are summarised in Table 8. Those for specimens which failed in the weld are shown in Figure 8 in terms of the bending stress range in the weld and the results obtained from specimens which failed in the plate are included in Figure 3, which refers to specimens which failed in a similar way.

It will be seen from Figure 8 that the results for weld failures are widely scattered and do not fall on a well defined S - N curve. Since the mode of failure is not one which automatically leads to widely scattered results it seems likely that the poor correlation is associated with the calculated value of stress. Errors could arise as a result of the complexity of the geometry and, in the case of Series IOE specimens, uncertainty about how much of the load cycle actually caused fatigue damage. In view of these reserva-

tions, the results should be treated with caution.

6. GENERAL DISCUSSION OF RESULTS AND COMPARISON WITH DATA IN THE LITERATURE

6.1 Failure in the Main Plate

Experimental stress analysis of plates with transverse stiffeners carried out by Navrotskii e t al (5) showed that i f the stiffeners were welded along one side only, the point of maximum stress concentration was at the weld root, while if welds were made on both sides of the stiffener the maximum stress was at the weld toe. Navrotskii determined stresses from strain gauges attached to the models, which were loaded axially. Bel 'chuk et al(6) carried out similar work using photoelasticity and bending loads and produced the same general conclusion. Thus, it is to be expected that fatigue failure of a stressed plate (axial or bending stress) with a transverse stiffener will be from the weld root if a single weld is used or f rom the weld toe if the stiffener is welded down both sides. Fatigue tests carried out by Navrotskii(5) and later by Wintergerst (7), both under axial loading, confirmed this, as do the results obtained in the present investigation for bending.

Figure 9 shows results obtained from Series IA and 10E, which refer to plate failures from the weld root, compared with those obtained by Navrotskii (5) and Wintergerst (7) for the same mode of failure. I t will be seen that the present results, although widely scattered, indicate a slightly higher fatigue strength. This may simply reflect the difference in stress ratios ( -2 and - 4 compared with 0 and - 1 ) o r it could be associated with the difference in loading (bending compared with axial), as discussed

below.

Figure 10 shows results obtained from Series 1A, 6D and 7D which refer to failure in the main plate from the weld toe. I t also includes results for the same failure mode from the literature. Senior and Gurney(8) tested specimens similar to type D in which the main plate was 37 m m (17116 in.) thick and the stress ratio was approximately -6.7. Ouchida and Nishioka (9) also tested type D specimens but the loading gave bending (R = 0 at the weld toe) in the stiffener, which was 16 or 33 mm thick. Finally, Bykov (10) and Asnis (11) obtained results from plates with transverse stiffeners. The plate was loaded in bending, with R = - 1 at the weld toe. I t will be seen that, in spite of the wide variation in stress ratios, 0 to -6 .7 , the results are not widely scattered. The results for Series 6D and 7D tend to lie on the upper side of the scatterband.

I t will be seen in Figure 6 that Series 7D specimens gave higher fatigue strengths than Series 6D. This is surprising since the specimens were of similar design, apart from the difference in loading span, and were tested under the same loading conditions (R = -4) . The difference may have arisen as a result of the different testing machines used. For Series 6D a constant cyclic load Losenhausen machine.was used, while Series 7D specimens were tested in the constant deflection Illinois testing machine. In the latter series, therefore, the bending stress on the specimen decreased as a crack propagated from the weld toe while in the former it did not. Thus, the stress plotted for the Series 7D results would not have been operative throughout the test. This factor may explain the apparently superior fatigue properties indicated by the Series IA results (Figure 9), which were also obtained in the Illinois machine.

Finally, the results obtained from Series 1A and 10E for plate failures from the root are included in Figure 10 in order to indicate the difference between the two modes of plate failure. In view of the possible effect of the testing machine on the results, a comparison of Series 1A and 7D is most meaning- ful. I t will be seen that they are similar. This is consistent with the results obtained by Wintergerst (7), who found that failure sometimes occurred f rom the root and sometimes from the toe; the results of Series IA also included both modes of plate failure. Thus, although the weld root is favoured as a site for crack initiation in plate/stiffener configurations in which only one weld attaches the stiffener to the plate, the resulting fatigue strength is virtually the same as for weld "toe failure.

8

6.2 Failure in the Stiffener

Results which refer to failure in the stiffener from the weld toe (Series 2B) are considered separately from main plate failures because they illustrate the significant difference that can exist between results obtained in bending and under axial loading conditions. The results are given in Figure 4 together with two scatterbands. The first is that which encloses all the data given in Figure 10, which refers to plate failures obtained in bending for several stress ratios including R = Ofas used for Series 2B tests) and plate thicknesses of at least 11 mm. The second is the 95% confidence limits of data obtained from plates with transverse non-load carrying stiffeners under axial loading O2).

It will be seen that a very high fatigue strength was obtained from the Series 2B tests. Also, all the results obtained in bending showed higher fatigue strengths than those obtained under axial loading. The high fatigue strength may have arisen as a result of the method of loading, which was constant deflection, as discussed above. However, tests carried out on Series 7D specimens under the same loading conditions, which failed in the thicker plate, gave lower fatigue strengths. I t seems more likely that the enhanced fatigue strengths arose as a result of the different stress gradients that exist on the planes of fracture. In the Series 2B specimens, in which the plate that failed was 6.35 mm thick, the stress gradient would be high, so that only a small volume of material was subjected to the max imum fibre stress. It would be less in the case of the thicker specimens subjected to bending while in the case of theaxially loaded specimens, theoretically there would be uniform stressing through the thickness. I t is known that the severity of a notch or crack is affected by the volume of material subjected to high stress, such that it decreases if the volume is small. Hence, the fatigue behaviour illustrated in Figure 9 is as

expected.

6.3 Failure in the Weld

All the weld failures occurred under bending stresses. In Series 8E the bending stress was wholly tensile on the weld surface and consequently cracks initiated there and propagated through the throat. In Series 3B and some specimens from Series 4C the stress was wholly tensile at the weld root, so that cracks propagated through the weld throat from the root. The same mode of failure occurred in the remaining Series 4C specimen and those of Series 9E and 10E, although the loading gave a greater cyclic tensile stress on the weld surface than at the weld toe. The more severe stress concentration at the root

accounts for this mode of failure.

All the results are compared in terms of the range of bending stress in the weld throat in Figure 11. Results obtained b y Ouchida and Nishioka (9) are also included. These used specimens similar to type D loaded to give bending in the stiffener, in which failure occurred in the weld from the root. Some specimens were welded with zero root penetration while others had some root penetrat ion and weld sizes were between 5 and 18 mm leg length. Also shown are results obtained in an investigation carried out by Chapman* in which specimens which modelled the joint between a stiffener and cross- girder in an orthotropic bridge deck panel were tested. The specimens consisted of rectangular hollow sections welded at right angles to a plate and loaded to give bending in the plane of the plate with R = +0.5. Failure occurred after crack propagation from the root through the weld throat. In all cases, the stress,ratio at the weld root has been deduced and is quoted in the figure. Finally, F!gure 11 includes the scatterband enclosing the results obtained from full-scale orthotropic bridge deck sections in which failure occurred in the weld throat from the root (2). In those specimens a single weld at tached the stiffener to the plate. The results cover stress ratios at the root between -1 and +0.7. No data relevant to weld failure in which initiation was on the weld surface could be found in the literature.

It will be seen that the results of the present investigation which refer to failure originating at the weld root fall in a well defined scatterband and are in reasonable agreement with the results obtained from orthotropic bridge deck specimens. It is of interest to note that results obtained from butt and

*Unpublished work at Imperial College by Dr J C Chapman

9

fillet welded joints gave similar fatigue strengths when plotted in terms 6f the bending stress in the weld. This" indicates that the stress concentration at the root of the butt weld (see Plate 5) is as severe as that at the root of a f'filet. In addition, it will be noted that the applied stress ratio did not have a strong effect on the fatigue strength, stress range being the important parameter.

Compared with the results discussed above, those obtained by Ouchida and Nishioka indicate surprisingly poor fatigue properties. The reason for this is not known. However, it is possible that the actual and calculated stresses differed widely, not only as a result of the simplified method of calculation, but also because of variations in the actual weld sizes. Alternatively, it is possible that it is due to the difference in stress gradient through the weld, as discussed earlier. Compared with the present results and those obtained f rom full-scale bridge deck specimens, the stress gradient would be lower in Ouchida and Nishioka's specimens because the weld sizes were generally larger and two welds were used to attach

the stiffener to the plate.

As would be expected in view of the lower severity of the stress concentration on the weld surface, failures originating on the weld surface gave higher fatigue properties than the root failures.

6.4 The Relevance of the Shear Stress in the Weld

6.4.1 Stiffener attached to plate with two welds

Originally it was thought that the variation of the ratio Op would produce both plate failures, due -,rw

to op, and weld failures, due to rw. However, all failures occurred in the plate. When°P was zero, no ?'p

failures were obtained after considerable numbers of stress cycles. Examination of specimens revealed t h a t some cracks were formed and their orientation suggested that they propagated under the shear stress in the weld. From the practical point of view, the high fatigue strength of the joint for the loading conditions used indicates that it should never present a fatigue problem in practice.

6.4.2 Stiffener attached to plate with one weld

When a single weld was used to attach the stiffener to the plate, direct loading in the stiffener inevitably produced bending as well as shear stress in the welds. The bending stress turned out to be the important stress parameter. From fracture surface observations it was possible to state that no weld failures occurred as a result of only shear stresses in the weld, although two specimens showed evidence of shear mode fracture when a crack, propagated under bending stresses, had reached an appreciable size.

6.5 The Effect o f the Gap Between the Stiffener and Plate

When the stiffener carried a direct stress and the plate a bending stress, the mode of failure appeared to be influenced by the presence of a gap between the two members. In Table 8, it will be seen that

failure was in the plate if O r was greater than unity and in the weld if it was less. It seems likely that the Ow

value of Or is influenced by the presence of a gap between the plate and stiffener, such that it is increased Ow

as the gap is reduced. Clearly, with no gap Ow = 0. This suggests that if there is a gap, failure is likely to occur in the weld and that if there is no gap, failure will occur in the plate. In the present tests the results obtained for plate failure agreed with the Series 1A results in which the same mode of failure

occurred.

10

7. CONCLUSIONS

1. The results obtained from specimens which failed in the plate after crack propagation from the weld root or weld toe (specimen types A, D and E) indicated that the fatigue strength was approximately the same for both modes of failure. The results were in agreement with others in the literature when plotted in terms of stress range, for a range of stress ratios between 0 and - 6 . 7 .

. Specimens which failed in the stiffener after crack propagation from the weld toe (Series 2B) gave comparatively high fatigue strengths. The results were generally above the scatterband for failures in thicker plate. This was attributed chiefly to the difference in stress gradient, the higher stress gradient in the thinner plate giving the higher fatigue strength. This conclusion is supported by the fact that the bending tests gave higher fatigue strengths than the axial tests.

. Butt and fillet welded specimens loaded to give a predominantly tensile bending stress at the root (Series B and C) gave the same fatigue strength when p lo t ted in terms of the range of bending_ stress in the weld. In addition, the results agreed with others from the literature.

4.

.

Fillet welded specimens loaded to give a tensile bending stress on the weld surface (Series 8E) failed in the weld from the surface. The resulting fatigue strength was considerably higher than that corresponding to weld failure by crack propagation from the root, reflecting the difference in

severity of stress concentrations.

The fatigue strength of a fdlet weld loaded to give a shear stress transverse to the weld (Series 5D) was very high. For example, with a shear stress of 245 N/mm2 a specimen endured 5.4x107 cycles without failing.

. The mode of failure of specimens consisting of plates with stiffeners attached by only one fillet weld loaded directly through the stiffener to give bending in the plate, depended on whether or not there was a gap between the edge of the stiffener and the plate surface. When a gap was present, failure occurred in the weld from the root and the resulting fatigue strength, expressed in terms of the bending stress in the weld, was similar to that obtained f rom other specimens which failed in that way. When a gap was not present, the failure was in the plate from the weld root.

. For the specimens which failed in the weld from a crack which initiated from the root, fatigue strength was strongly dependent on stress range in the weld, applied stress ratio having little effect.

. A comparison of results obtained from constant deflection or constant load range cycling indicated that the former may give a higher fatigue strength than the latter.

8. A C K N O W L E D G E M E N T S

The author thanks laboratory staff of The Welding Institute, in particular Messrs. W. J. Blacklock, R. A. Males and C. O. Martin, for the high standard of experimental work.

The'encouragement and interest shown by Mr. D. E. Nunn of the Transport and Road Research Labora- tory is gratefully acknowledged.

The investigation was sponsored by Transport and Road Research Laboratory and Mort, Hay and Anderson. The work at Imperial College by Dr J. C. Chapman was sponsored by Mott, Hay and Anderson and permission to quote the results is acknowledged.

11

1.

.

.

4 .

5 .

.

.

.

.

1 0 .

1 1 .

1 2 .

9. R E F E R E N C E S

British Standards Institution. Specification for Steel Girder Bridges. Part 3B Stresses. British Standard 153, (British Standards Institution).

MADDOX, S. J. The fatigue behaviour of trapezoidal stiffener to deck plate welds in orthotropic bridge decks: Department of the Environment, TRRL Supplementary Report 96UC (in the press).

SIGNES, E. G., R. G. BAKER, J. D. HARRISON, and F. M. BURDEKIN. Factors affecting the fatigue strength of welded high strength steels. British Welding Journal, Vol. 14, No. 3, 1967,

pp 108-116.

WATKINSON, F., P. H. BODGER, and J. D. HARRISON. The fatigue strength of welded joints in high strength steels and methods for its improvement. Proceedings of Conference on Fatigue of Welded Structures, The Welding Institute, 1971.

NAVROTSKII, D. I., V. N. SAVEL'EV, and G. V. LAVOCHKIN. Determination of the stresses at points where transverse stiffening ribs are welded on. Welding Production, No. 5, May 1963.

BEL'CHUK, G. A., S. I. REPIN, and O. N. LYCHEV. Research into the stress concentration factors in certain types of welded joint. Trudy LKI, No. XVIII, Sudpromgiz, 1956.

WlNTERGERST, S. and K. HECKEL. Die Dauerfestigkeit von Flachstahl aus ST37 mit aufgesch- weisster Querversteifung. Der Stahlbau, Vol. 35, No. 12, 1966, p. 353.

SENIOR, A. G. and T. R. GURNEY. The design and service life of the upper part of welded crane girders. The Structural Engineer, Vol. 41, No. 10, October 1963.

OUCHIDA, H. and A. NISHIOKA. A study of fatigue strength of fillet welded joints. IIW Trans-

lation X I I I - 3 3 8 - 6 4 , February 1964.

BIKOV, V. A. The fatigue strength of welded steel members made from section and plate. Welding

Production, Vol. 3, No. 2, February 1957, p. 6.

ASNIS, A. E. and G. A. IVASHCHENKO. Improving the resistance of welded joints to alternating

loads. Automatic Welding, Vol. 20, No. 10, October 1967.

GURNEY, T. R. and S. J. MADDOX. A re-analysis of fatigue data for welded joints in steel.

Welding Research International, Vol. 3, No. 4, 1973.

O

O p

12

10. N O M E N C L A T U R E

2a pitch of simple supports b specimen length (in direction of welding) I weld leg length P direct load

R stress ratio defined as Ornha where Omin is the arithmetical minimum stress and Oma x is the arith- Ornax' --4

metical maximum stress, e.g. Omtn = --4 (compressive), Omax = +1 (tensile), R = +1 = -4 .

stress bending stress in main plate directly above stiffener

opl bending stress in main plate at weld toe, as measured using strain gauges Or bending stress in main plate at weld root Ow bending stress in weld throat rw shear stress in weld throat T main plate thickness

13

11. APPENDIX 1

The Materials and Methods of Construction

The specimens were constructed from structural steel to BS4360 Grade SOB, the main plate being 11 mm thick and the stiffener 6.35 m m thick. The chemical analysis and tensile properties of the steel are

given in Table 1.

All the plates were flame-cut to size and straightened after cooling. The specimens were 254 mm long (i.e. measured along the weld) in the rolling direction.

All welds were made manually in the fiat position using rutile, coated class 3 electrodes. The nominal weld leg length was 6.35 mm. Stop/start positions were included half-way along each weld since they would certainly occur in an actual structure welded manually and may affect the fatigue strengths of the welds. The ends of the welds and edges of the plates were hand machined and ground, using deburring tools and emery bands, to remove tack welds or other edge discontinuities that might initiate

fatigue cracks.

The specimens used for Series 1A, 2B and 3B were welded using three runs; all other welds were single run f'filet welds. In general, stiffeners were tack welded in position, at the ends, before finally welding. The tack welds were removed by machining.

In order to weld the specimens in Series 5D to 7D, in which welds were made on both sides of t he stiffener, two spacing strips 1.3 m m thick were placed between the edge of the stiffener and the plate, and tack welds made at each comer of the edge of the stiffener. The spacing strips were then removed. The weld sequence varied according to the distortion that occurred after the first run, but in general four passes were made from each end and towards the centre.

In the specimens used in Series 8E and 9E the same procedure was adopted, except that the stiffener was set at a small angle to the vertical to compensate for distortion. This angle was determined by trial and error and was set after the stiffener had been tack welded to the plate.

The specimens used in Series 2B, 3B, 4C and 10E were welded with the stiffener resting directly on to the plate, again at a pre-set angle to the plate to allow for distortion.

14

Chemical Analysis

TABLE 1

Material properties

Plate

11 mm main plate 6.35 mm stiffener

Element

C S Si P Mn Nb

0.16 0.20

0.028 0.023

0.03 <0.01

0.014 0.006

1.50 1.15

0.037 0.034

Mechanical Properties

Plate

11 mm main plate 6.35 mm stiffener

Yield strength, N/mm2

437 423

UTS, N/ram2

568 574

Elongation, %

19 30

TABLE 2

Fatigue test results for series 1A (R = --2)

Specimen Number

IA/1

IA/2

1A/3

1A]4

1A/5

1A/6

Stress range in plate, N/mm2

300

285

253

240

227

193

Endurance cycles

447,000

458,000

523,500

378,000

1,772,000

1,943,000

Details of failure

Crack propagation from weld toe. Crack present at weld root.

Crack propagation from weld root.


Crack propagation from weld root. Crack present at weld toe.



15

TABLE 3

Specimen Number

2B/1 2B/5 2B/6 2a/3 2B/4 2B/2

Fatigue test results for series 2B (R = 0)

Stress range at weld toe in stiffener, N/mm2

304 289 279 277 270 250

Endurance, cycles

250,000 310,000 582,500

1,250,000 2,000,000

3,000,000(unbroken)

TABLE 4

Fatigue test results for series 3B and 4C (R = 0 at weld root)

Specimen Number

3B/7 3B/8 3B/9

4C/1 4C/2

Stress range at weld toe in stiffener, N/ram2

191 223 257

191 136

Calculated stress range in weld at root, N/ram2

139 160 199

314 289

Endurance, cycles

3,400,000(unbroken) 945,500 290,500

74,000 168,000

TABLE 5

Fatigue test results for series 5D (r~w = 0, zero to compressive direct load through stiffener,

double weld)

Specimen Number

5D/1

5D/2

5D/3

5D/4

Direct stress in stiffener

N/mm2

246

287

167

283

Shear stress on expected plane of fracture N/mm2

222

248

145

245

Endurance cycles

2,172,000 (unbroken)

2,143,000 (unbroken)

76,000,000 (unbroken)

54,000,000 (unbroken)

Testing machine

Illinois

Illinois

Vibrophore

Vibrophore

Remarks

Crack 0.8 mm long al weld toe in stiffener

Crack 4.5 mm long propagated from welt toe in stiffener (see Figure 12)

No cracks-found

Small cracks (up to 0.25 mm) present at weld toes and roots

16

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PLATE 1 Neg No 3388/1/74

Series 1A specimen under test

PLATE 2 Neg No 3388/1/74

Series 4C specimen under test

PLATE 3

Series 7D specimen under test

\

PLATE 4(a) Neg No 3388/1/74

Macrosection of Series 1A specimen which failed from toe of weld

~ _ ~ , ~ - ~ - ~ ' - :

Neg No 3388/1/74

PLATE 4(b)

Macrosection of Series 1A specimen which failed from root of weld

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PLATE 5

Macrosection of Series 2B specimen showing crack which propagated from

toe of weld

Neg No 3388/1/74

k PLATE 6

Fatigue crack in Series 5D s~oecimen tested at 286 N/mm z

Neg No 3388/1/74

PLATE 7

Specimen which has failed from the weld toe through the plate (as in

Series 6D and 7D)

Neg No 3388/1/74

(1717) D d 6 3 5 2 4 7 300 9 /74 H P L t d S o ' t o n G1915 P R I N T E D IN ENGLAND

ABSTRACT

Fatigue of welded joints loaded in bending: S J MADDOX (The Welding Institute): Depart- ment of the Environment, Supplementary Report 84 UC: Crowthorne, 1974 (Transport and Road Research Laboratory). Fatigue tests were carried out on a number of welded joints in an investigation relevant to steel orthotropic road bridges. Specimens consisted of 11 mm thick plates with 6.35 mm thick transverse stiffeners welded, generally at right angles to the surface, sometimes with either a single weld or welds on both sides of the stiffener. The loading conditions were selected to investigate fatigue failure in the main plate, the stiffener and the weld, all in bending. In addition, the joint was subjected to loading which gave shear stress in the weld and bending stress in the main plate, the ratio of the two being varied, to investigate the relative importance of each type of stress. Finally, the effect on the fatigue behaviour of load-carrying joints having one or two welds attaching the stiffener to the main plate and the effects of a gap between the stiffener and main plate were investigated. Where possible the results were compared with relevant data in the literature.

ABSTRACT

Fatigue of welded joints loaded in bending: S J MADDOX (The Welding Institute): Depart- ment of the Environment, Supplementary Report 84 UC: Crowthorne, 1974 (Transport and Road Research Laboratory). Fatigue tests were carried out on a number of welded joints in an investigation relevant to steel orthotropic road bridges. Specimens consisted of 11 mm thick plates with 6.35 mm thick transverse stiffeners welded, generally at right angles to the surface, sometimes with either a single weld or welds on both sides of the stiffener. The loading conditions were selected to investigate fatigue failure in the main plate, the stiffener and the weld, all in bending. In addition, the joint was subjected to loading which gave shear stress in the weld and bending stress in the main plate, the ratio of the two being varied, to investigate the relative importance of each type of stress. Finally, the effect on the fatigue behaviour of load-carrying joints having one or two welds attaching the stiffener to the main plate and the effects of a gap between the stiffener and main plate were investigated. Where possible the results were compared with relevant data in the literature.

FATIGUE OF WELDED JOINTS LOADED IN BENDING · shear stresses in the welds, resulting from the...

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