An investigation into the fatigue properties of a single spot weld lap joint

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THE COLLEGE OF AERONAUTICS

CRANFIELD

AN INVESTIGATION INTO THE FATIGUE PROPERTIES

OF A SINGLE SPOT WELD LAP JOINT

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CoA REPORT MAT. No. 1 March, 1966

THE COLLEGE OF AERONAUTICS CRANFIELD

An Investigation into the Fatigue Properties of a Single Spot Weld Lap Joint

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-A, Younger, P h . D . , B . S c , A . I . M . ,

L. M. Gourd, B.Sc. (Eng). A . I . M . , M . I n s t . W . , and

J . E. M. Jubb, B . S c , A.M. Inst.W.

SUMMARY

An Investigation has been made of the behaviour, under fatigue loading, of resistance spot welds in mild steel sheet. A single spot weld lap test piece has been used and all the spot welds have been produced under the same conditions. The specimens have been subjected to static tensile loads of 220, 330 and 440 l b . , on to which have been superimposed alternating loads having amplitudes ranging from t50 to ±440 l b . , chosen so that the specimens have been in tension at all t i m e s .

In this work the most important parameter in determining the fatigue life appears to be the amplitude of the alternating load. Failure is initiated at the junction of the weld nugget and the interface between the sheets, and cracks propogate around the periphery of the weld indentation.

It is suggested that the effect of the size of spot, sheet separation and

1 . 2 . 3 . 2 . 1 2 . 2 2 . 3 2 . 4 2 . 5 Summary Introduction Experimental Work Material Test Specimen Welding Procedures The Tensile Shear Test The Fatigue Testing

Fatigue Results and their Analysis

4. 3.1 3.2 3.3 3.4 3.5 3.6 4.1 4.2 4 . 3

Consistency of Fatigue Life for a Given Combination of Static and Fatigue Loads Effect of Grip Separation on Fatigue Life for a Given Combination of Static and Fatigue Loads The Effect of Different Values of the Constant Static Load on the Fatigue Life

The Effect of the Maximum Load in the Cycle on Fatigue Life with Three Different Static Loads Comparison of Fatigue Properties of F i r s t and Second Batch of Specimens

Metallographic Observations Discussio;i Reproducibility Specimen-Grip Separation Fatigue Life 1 1 2 2 2 3 3 3 4 4 5 5 6 6

CONTENTS (continued) 5. 6. 7. 4 . 4 F a ü u r e Path The Relevance Spot Welds F u r t h e r Work Conclusions References Appendix F i g u r e s age 7 7 7 8 TABLES

I Specimen Thickness Around the Spot Weld 9

II Standardised Welding Conditions 9

m Tensile Shear Results 10 IV Consistency of Fatigue Life for a Single Load

Combination 11 V Effect of Grip Separation on Fatigue Life for a

Single Load Combination 12 VI Comparison of Fatigue Lives of Batch 1 and

1. Introduction

A lack of consistency in the manufacture of similar spot welded components has led to a conservative approach by designers. The failure to reproduce the strength of the spot weld without much scatter has caused the designer to include extra welds to offset the weaknesses in the control of the welding p r o c e s s .

Attempts a r e being made to refine the welding process by the control of certain variables and, as these refinements a r e promising, the designer should be in a better position to a s s e s s the strength of a spot welded joint In the future. This should enable the number of spot welds in a given joint to be reduced.

Previously useful developments in spot welding design have been limited by the wide scatter in test results and little work has been reported in relation to the extensive use of the process particularly in naass production. The majority of the r e s e a r c h effort has been concentrated on experimental work '^ • ^' to determine the s t r e s s distribution in the joint, the fatigue performance (3) and methods (4) of improving fatigue strength. Analytically, work has been restricted to the residual s t r e s s e s resulting from the thermal cycle of welding, but to the authors' knowledge the s t r e s s distribution in the lap joint has not been thoroughly examined. Little has been said about the preparation, size and testing of specimens and the condition of specimens after welding and before testing in fatigue. With reference to the latter point the separation of the sheets in the region of the weld contributes to the bending moment acting on the joint and this aspect has not been examined previously. A further point which has been neglected is the indentation at the weld: the indentation locally reduces the sheet thickness and consequently the bending s t r e s s e s a r e higher at the edge of the weld, where the fatigue crack usually originates. Both separation and indentation occur frequently in spot welding and increase the scatter of fatigue r e s u l t s . These factors have yet to be investigated.

It was felt that a programme of t e s t s , where special attention had been paid to consistency in the preparation of specimens and to control in testing, could reveal the important welding and loading parameters from which the control of the spot welding process and methods of design could be improved. The present programme, both analytical and experimental, has been confined to the tension shear specimen in mild steel as affected by repeated loading. This form of test specimen was chosen in preference to the British Welding Research Association's Patch Test, as the lap joint is more common in spot welded fabrications.

This report covers some preliminary work indicating the fatigue performance of tension-shear specimens, and it is concluded that further work along these lines could be most fruitful.

2. Experimental Work

A complete fatigue programme on spot welds even in one material would be an immense undertaking. Therefore, for the purposes of this experiment, an attempt has been made to standardise on a single material, a single set of welding conditions and a single geometry. The material chosen, mild steel, is one

frequently spot welded and therefore the results will have some practical signifi-cance. The welding conditions were chosen to produce a spot weld of good appearance and strength. No attempt was made to achieve an optimum strength. The geometry chosen was the simiplest possible, an equal overlap with the spot at

lts centre. The details of these conditions and the testing procedure a r e described In the following sections.

2.1 Material

All the specimens have been cut from sheets produced from a single cast of commercial mild steel. The cast analysis for this material

was:-Carbon 0.055% Sulphur 0.020% Phosphorous 0.015% Manganese 0.32%

As this was a rimming grade some local segregation might be expected.

The sheet was obtained cold rolled with a thickness of 18 S. W.G. (0.048 i n s . ) , and was guillotined into s t r i p s ,

2.2 Test Specimen

Strips measuring 6 inches by 1 inch were held in a jig so that there was an overlap of 2 inches and a single spot weld was nnade in the centre of this overlap. The finished specimen had the dimensions shown in Figure 1.

After welding, a check was made on the consistency of the alignnnent and on the overlap of the two s t r i p s . A further, m o r e detailed check was made on a batch of twelve specimens in o r d e r to study the consistency of the separation of the s t r i p s after welding. The results of this study a r e given in Table 1, The positions referred t o , a r e shown in Figure 1. The average total thickness can be contained within the band 0.1020 ±0.0007 inches. This scatter is little more than that in the thickness of the sheet itself. The average sheet separation is 0.006 Inches. 2. 3 Welding P r o c e d u r e s

All the welds were produced on a Sciaky 300 kVa single phase projection welder fitted with Mallory 3 spot welding electrodes. The welding p a r a m e t e r s of electrode p r e s s u r e , weld t i m e , and power input were varied in a s e r i e s of test welds until a condition (Table II) was obtained which produced a spot weld of good appearance and slug diameter. No attempt was made to achieve the conditions for maximum shear strength of the weld. The standardised conditions were used for every specimen produced.

The initial batch of 100 specimens was welded consecutively in one run without any alteration in the machine setting. After 60 welds had been made it was necessary to d r e s s the electrode t i p s . The whole s e r i e s was completed within two h o u r s . Every tenth specimien in sequence was removed for tensile shear

testing and when these t e s t s had shown no general trend in properties with increasing weld number the remainder of the batch were accepted for fatigue testing.

Subsequently a second batch of 50 specimens was produced one month later with the machine reset to the original settings. The machine, including the electrodes, had in the meantime been used on a variety of m a t e r i a l s . Tensile

shear samples were taken as before and the results of this batch and the e a r l i e r one a r e given in Table III.

2.4 The Tensile Shear Test

The control specimens taken from, each batch were pulled in a Denison 6 ton screw-feed machine at a rate of 0.75 inches per minute. The grips were set 5 inches apart with the specimen held vertically and centrally between them. The Indicated maximum load was recorded,

2.5 The Fatigue Testing

The specimens were gripped between flat jaws in Amsler 2 ton Vibrophore machines. The separation between the jaws was set at 5 inches with the weld central. Two s e r i e s were however carried out with separations of 3-^ inches and

2a inches to investigate end effects from the grips. The fatigue frequency was in

the range 100-150 cycles per second,

Combinations of static and fatigue loads were selected such that even at the minimum load in the cycle the specimen was not in compression. Three basic static loads were chosen, i . e . 440, 330 and 220 lbs. Superimposed on these loads were fatigue amplitudes ranging from ±50 lb. to ±440 lb,

By virtue of the construction of the machine resonance is lost as soon as a fatigue crack is initiated at the interface. This departure from resonance auto-nmatically stops the machine. The life to this point is referred to in this report as

'the life to initial failure'. The naachine can then be r e - s t a r t e d , with the cut-out desensitised, to give the same loading conditions. A second stage of failure can then be detected both on the machine and visibly, when the crack begins to

propagate from the periphery of the nugget into the sheet and appears on the outer surface of the specimen. This condition has been termed 'the life to a visible c r a c k ' .

3. Fatigue Results and their Analysis

A complete table of all the fatigue results is given in Appendix 1. In this section the results will be presented for each variable under consideration. A single test may therefore appear under several headings.

3,1 Consistency of Fatigue Life for a Given Combination of Static and Fatigue Loads A static load of 440 lb. with a superimposed fatigue load of ±280 lb. was

chosen as a standard condition and a total of 14 t e s t s from samples throughout the first batch were conducted under these conditions. The results a r e given in Table IV. As generally the fatigue t e s t s were carried out in nuntierical o r d e r they-were performed over a period of time and not necessarily on the same testing machine. The results show a consistency well within that normally found in

fatigue testing; the. life to initial failure was between 2 . 2 x 1 0 and 9 . 0 x 1 0 cycles, and the life to a visible crack between 8.5 x 10* and 5.9 x lO' , The life to a

visible crack has l e s s scatter than the life to initial failure,

4

-range of life to initial failure o f l . B x l o " - 3 . 3 x 1 0 and of life to a visible crack from 5. 7 to 10* to 1.08 x 10^. While the minimum lives of this smaller batch a r e below those of the first batch, the majority of the results of the second batch lie within the scatter band of the first.

3. 2 Effect of Grip Separation on Fatigue Life for a Given Combination of Static and Fatigue Loads

The same combination of static and fatigue load as was used in Section 3.1 was employed for these t e s t s . The t e s t s in Section 3.1 used a grip separation of 5 inches giving the results in Table IV. Two additional s e r i e s were then conducted with grip separations of 3.5 inches and 2.125 inches, giving the results in Table V. The setting at 2.125 inches brought both grips to within 1/16 of an inch of the edge of the overlap.

Although it was anticipated that the grip separation might have an effect on the life, the results presented here show no large differences.

3.3 The Effect of Different Values of the Constant Static Load on the Fatigue Life Three static loads, namely 440, 330 and 220 lb. , were applied, and S-N curves for fatigue loads superimposed on them were determined. The results a r e shown in Figures 2 and 3. It can be seen that there is no significant separation between the fatigue lives under the same fatigue amplitude but different static s t r e s s . A single s t r e s s amplitude-number of cycles curve can justifiably be drawn through all the r e s u l t s .

3.4 The Effect of the Maxinnum Load in the Cycle on Fatigue Life with Three Different Static Loads

If the results reported in Section 3.3 a r e re-plotted (Figures 4 and 5) in t e r m s of the maxinmunn load in the cycle versus the number of cycles to failure, separation into t h r e e curves for the different static loads o c c u r s . F o r a given maximum load, the fatigue life increases with an increasing static component. 3. 5 Comparison of Fatigue P r o p e r t i e s of F i r s t and Second Batch of Specimens

Curves of the cycles to initial failure and to visible crack for both batches under the same conditions a r e shown in Figures 6 and 7. While the initial failure curve for batch 2 may occur at a slightly smaller number of cycles than that for batch 1, the curves for visible crack life a r e s i m i l a r . A more detailed comparison for selected load combinations is given in Table VI. The average lives of the second batch to initial failure a r e inferior to those of the first batch. The figures for the second batch a r e approximately 50 per cent that of the first. The lives to visible cracks however do not show such a variation and in most cases the lives for the second batch a r e within 20 per cent of those for the first.

3.6 Metallographic Observations

Microscopic examination of welds both in the initial failure and visible crack conditions showed that in all cases the crack started at nugget-parent metal inter-face at position A o r C (Figure 1). There was some evidence, from the specimens

In the 'initial failure' condition that the crack initiated at the junction between the weld metal and the interface between the two sheets. F r o m this point it propagated outwards to the edge of the indentation. It then spread equally in both directions around the periphery until the crack subtended an angle of about 60 -to the centre of the weld. Subsequently the crack proceeded to propagate through the sheet perpendicular to the longitudinal axis of the specimen, i. e. at right angles to the direction of maximum tensile loading. In no case did the crack path enter any part of the weld metal.

4, Discussion

The purpose of this work, and this discussion in particular, can be divided into two broad categories. F i r s t l y , there is the actual significance of the experi-mental results presented h e r e , and secondly there is the general significance of the single spot lap joint for the fatigue testing of spot welds. The first of these topics will be covered In this section, the second in Section 5,

4.1 R eproducibility

Two facets of the reproducibility need to be considered, firstly the reproduci-bility within a single continuously produced batch and secondly the reproducireproduci-bility between this batch and other produced under apparently the sanae conditions at other t i m e s . F o r the first batch produced for these t e s t s the breaking strength in t e n s i l e - s h e a r testing varied from 1793 to 1955 lb, with an average of 1864 lb, The scatter is therefore from -71 to +91 lb. o r -4% to +5% of the average value, The breaking strengths of the second batch were slightly higher, from 1913 to 2065 lb. with an average of 1989 lb. , although the number of tests were less for the second batch. Thus before a s s e s s i n g fatigue results the inherent scatter of the weld strengths, as shown by the static t e s t s , must be taken into consideration. In Table IV, based on the first batch, the life to initial failure varied between 2.2 x 10* and 9 . 0 x 1 0 * cycles with an average of 5.5 x 10 cycles. The r e s u l t s , therefore, lie within a band from -60% to +63% of the average life. The life to a visible crack lay between 8. 5 x 10* cycles to 1.88 x lO' cycles with an average of 1.52 X l O ' cycles. If the rather exceptional life from specimen number 17 is d i s

-4 5

counted, the range is from 8.5 to 10 cycles to 1.88 x 10 cycles, with an average of 1.09 x 10^ , i . e . the results lie within a band from -22% to +65% of the average life. While these scatter bands a r e quite wide, such scatter is not uncommon in fatigue. The scatter must also be considered with regard to the difference in life produced by a change in s t r e s s level. Thus, as seen in Table VI a reduction of 100 lb. in the total fatigue load amplitude from 660 to 560 lb. produced a doubling of the average life. When the results a r e presented graphically they do lie within reasonable limits enabling a representative S/N curve to be drawn through them. It is proposed therefore that the degree of scatter within a single batch is not unreasonable and that the results obtained do enable a useful picture of the fatigue properties of a spot weld to be drawn.

The second batch had tensile shear properties higher than the first batch. This improvement is not reflected in the fatigue properties which a r e generally inferior to those of the first batch. However on plotting the lives to a visible crack a single S/N curve will fit both sets of results (Fig. 7). It would be of great interest therefore to produce a number of batches of specimens under

between t e n s i l e - s h e a r strength and fatigue life, if such a relationship exists. 4. 2 Specimen-Grip Separation

It was anticipated that the distance of separation between the specimen grips, i . e . the effective specimen length, would have a pronounced effect on the fatigue p r o p e r t i e s . As the grips a r e moved apart more rotation can be expected to occur at the weld and the bending moment due to the eccentricity of the load in the region of the weld would fall. The experimental results do not support this reasoning. The change in the length of the specimens in these t e s t s naust only give a marginal change in the bending moment and this change is probably lost in the scatter of the r e s u l t s .

4. 3 Fatigue Life

F o r a given amplitude of the fatigue component of the total load, changes in the value of the static component do not appear to influence the number of cycles to failure. Therefore within the limitations of the tests reported here it is only the actual value of the alternating part of a conabined load that controls the fatigue life of the spot weld. As this statement would seem improbable if the static s t r e s s should exceed the yield s t r e s s , two tensile-shear s t r e s s tests were carried out on an Instron machine (at 0.1 i n s / m i n . strain rate) utilising the autographic load-extension facilities (Fig. 8), These show that yield occurs at about 600 lb. in these specimens. Thus in a large number of the tests reported h e r e , the yield load was exceeded for a part of each cylce. It would seem, therefore that for tensile conditions whether above o r below the yield load the alternating r a t h e r than the static com.ponent is by far the most dominant factor determining fatigue life. However it is felt that further experimental evidence, particularly at high maximum loads is needed to substantiate fully these conclusions,

All the S/N curves so far established show a continuous increase in fatigue life with decreasing fatigue load amplitude. There is no evidence, in so far as these t e s t s have continued, of an endurance limit.

4.4 Failure Path

An important practical feature of these results is the two-stage nature of the failure. The microscopic observations showed that the crack began on the inside of the periphery of the nugget and then spread some way round this periphery before propagating into the sheet. F a i l u r e on the inside of the periphery of the nugget would be expected from an approximate analysis of the s t r e s s distribution in that area due to a combination of the mean tensile s t r e s s and the maximum tensile s t r e s s due to bending which is caused by the offset load in the lap joint.

The number of cycles to the Initial failure was, in general, about one half of the number of cycles to the onset of propagation into the sheet. Even at the second stage of failure some joint strength still exists, although in certain materials the crack already initiated may propagate catastropically through the whole sheet. The existence of an initial stage of failure may be of practical con-sequence offering a s it does a chance of detection before any parent metal failure can occur.

5 . T h e R e l e v a n c e of t h e s e T e s t s t o I n d u s t r i a l Spot Welds

E x a m i n a t i o n of t h e r e s u l t s of t h e s e t e s t s d o e s i n d i c a t e w h e r e t h e a d v a n t a g e of c o n t r o l in spot welding can be b e n e f i c i a l in the d e s i g n of spot welded c o m p o n e n t s in t h e f o r s e e a b l e f u t u r e . T h e c o m b i n a t i o n of a spot weld of g u a r a n t e e d m i n i m u m s i z e and q u a l i t y with a b e t t e r u n d e r s t a n d i n g of i t s p e r f o r m a n c e u n d e r r e p e a t e d loading conditions will give t h e d e s i g n e r a m o r e l o g i c a l b a s i s on which t o a s s e s s t h e numiber of welds which a r e r e q u i r e d in a given component,

E x i s t i n g p h i l o s o p h i e s of d e s i g n a r e e x t r e m e l y c r u d e . C e r t a i n a i r c r a f t c o m -p o n e n t s h a v e b e e n d e s i g n e d on t h e a s s u m -p t i o n that four out of five w e l d s a r e sound and m o t o r c a r b o d i e s often h a v e double t h e m i n i m u m n u m b e r of welds r e q u i r e d on a s t r e n g t h b a s i s ; t h i s d u p l i c a t i o n i s n e c e s s a r y to offset t h e v a r i a t i o n in q u a l i t y d u r i n g p r o d u c t i o n . Whilst it will not p r o v e p o s s i b l e t o e l i m i n a t e a l l t h e v a r i a t i o n s , a s u b s t a n t i a l i m p r o v e m e n t would lead to a r e d u c t i o n in t h e n u m b e r of w e l d s to be m a d e and in t u r n a significant e c o n o m y In t h e cost of t h e welding m a c h i n e s f o r a given m a s s p r o d u c e d c o m p o n e n t . A f u r t h e r gain would follow in t h e p r o b a b l e d e c r e a s e in l a b o u r c h a r g e s a s s o c i a t e d with a s n a a l l e r n u m b e r of w e l d s p e r c o m p o n e n t .

6. F u r t h e r Work

Initial work h a s b e e n c a r r i e d out f o r a given type of joint m a d e u n d e r s t r i c t l y c o n t r o l l e d conditions and it would be a d v i s a b l e t o d e t e r m i n e t h e effect of c e r t a i n v a r i a b l e s on t h i s joint b e f o r e t h e r a n g e of t e s t i n g i s expanded to e x a m i n e different joint c o n f i g u r a t i o n s and t h e effects of change in s t r e s s a m p l i t u d e d u r i n g t e s t i n g .

T h r e e v a r i a b l e s which a r e of i m m e d i a t e I n t e r e s t a r e spot d i a m e t e r , s h e e t s e p a r a t i o n and s h e e t i n d e n t a t i o n . I n f o r m a t i o n on t h e s e v a r i a b l e s would i n d i c a t e w h e r e e m p h a s i s should be placed in t h e d e v e l o p m e n t of t h e c o n t r o l e q u i p m e n t . It would a l s o e n a b l e a m i n i m u m a c c e p t a n c e l e v e l to b e e s t a b l i s h e d f o r c r i t i c a l w e l d s , which can b e u s e d w h e r e post welding i n s p e c t i o n i s u n d e r t a k e n .

T h e r e i s a difficulty t o be r e s o l v e d In t h e m e a s u r e m e n t of t h e spot weld d i a m e t e r . An e n l a r g e d view of a s e c t i o n t h r o u g h the weld s h o w s a zone at the I n t e r f a c e , o u t s i d e t h e weld nugget, w h e r e p r e s s u r e welding m a y o r m a y not h a v e p r o d u c e d a continuous bond b e t w e e n t h e s h e e t s . A m e t a l l o g r a p h i c e x a m i n a t i o n of a l a r g e n u m b e r of s e c t i o n s might h e l p t o r e s o l v e t h i s difficulty.

7. C o n c l u s i o n s

It i s concluded f r o m t h i s work t h a t :

-1) A s i n g l e spot weld l a p joint p r o v i d e s a s u i t a b l e t e s t p i e c e f o r fatigue t e s t i n g . T h e d e g r e e of s c a t t e r in t h e fatigue r e s u l t s i s low.

2) F o r a given a m p l i t u d e of t h e fatigue component in t h e t o t a l l o a d , c h a n g e s in t h e v a l u e of t h e s t a t i c load do not a p p e a r t o influence t h e n u m b e r of c y l c e s to f a i l u r e .

3) F a i l u r e i s i n i t i a t e d in t h e s h e e t at t h e junction of t h e weld nugget and t h e i n t e r f a c e b e t w e e n t h e s h e e t s .

4) T h e c r a c k p r o p a g a t e s t o the s u r f a c e of the s h e e t and then follows the p e r i p h e r y of the weld indentation for a l i m i t e d d i s t a n c e b e f o r e d i v e r t i n g into the s h e e t . R e f e r e n c e s 1. W e l t e r , G. Welding J o u r n a l , 1950, 2 9 , p . 565s 2 . B e r g h o l m , A . D . , Welding J o u r n a l , 1950, 29, p. 217s S w a r t z , P . W, and H o e l l , G . S . 3 . W e l t e r , G. Welding J o u r n a l , 1955, 34, p . 153s 4 . W e l t e r , G. Welding J o u r n a l , 1949, 2 8 , p , 414s

T A B L E I S p e c i m e n T h i c k n e s s A r o u n d t h e Spot Weld S p e c i m e n N u m b e r 53 54 55 56 57 58 59 61 62 63 64 65 A v e r a g e 0 , 1 0 2 0 0 , 1 0 1 5 0 . 1 0 2 4 0 . 1 0 2 6 0 . 1 0 2 4 0 . 1 0 2 1 0 . 1 0 2 7 0 . 1 0 2 5 0 . 1 0 2 6 0.1014 0 . 1 0 2 2 0 . 1 0 1 7 T o t a l t h i c k n e s s (ins) A 0 , 1 0 1 5 0 , 1 0 0 0 0 . 1 0 2 0 0 . 1 0 2 5 0 , 1 0 2 0 0 . 1 0 1 0 0 . 1 0 2 5 0 . 1 0 2 5 0 . 1 0 2 5 0 . 1 0 0 0 0 . 1 0 1 5 0 . 1 0 1 0 B 0.1010 0 , 1 0 1 0 0,1005 0 . 1 0 2 5 0.1015 0.1005 0 . 1 0 2 0 0.1040 0 . 1 0 1 8 0 , 1 0 0 0 0 . 1 0 3 0 0 . 1 0 1 0 C 0 . 1 0 2 5 0 . 1 0 2 5 0 . 1 0 3 5 0 . 1 0 2 5 0 , 1 0 2 5 0 . 1 0 3 5 0 . 1 0 3 0 0 , 1 0 2 5 0 . 1 0 2 5 0 . 1 0 2 5 0 . 1 0 3 0 0 . 1 0 3 0 D 0 , 1 0 3 0 0 . 1 0 2 5 0 . 1 0 3 5 0 , 1 0 3 0 0 . 1 0 3 5 0 . 1 0 3 5 0 . 1 0 3 5 0 . 1 0 1 0 0.1035 0 , 1 0 3 0 0 . 1 0 1 5 0 . 1 0 2 0 Sheet t h i c k n e s s 0 . 0 4 8 - 0 . 0 0 0 5 I n s . T A B L E U S t a n d a r d i s e d Welding Conditions Sciaky Setting for

a) Weld 18 b) Weld Heat C o n t r o l 45 c) Weld P r e s s u r e 2 0 p . s . l . d) B a c k p r e s s u r e l O p . s . i . T i p D i a m e t e r : - 7 / 3 2 i n . No p r e - w e l d o r post weld h e a t i n g M a t e r i a l t e m p e r a t u r e b e f o r e welding: 20 C A s r o l l e d s u r f a c e , d e g r e a s e d

10

-TABLE III

Tensile Shear Results

a) F i r s t Batch

Specimen Number Breaking Strength (lbs)

11 20 30 40 50 60 70 80 90 100 1912 1815 1882 1880 1793 1800 1955 1943 1825 1840 Average 1865 b) Second Batch 111 121 127 131 132 141 146 147 148 1913 1965 1970 1980 1978 1989 2025 2029 2065 Average 1990

TABLE IV

Consistency of Fatigue Life for a Single Load Combination

Static Load + 440 lb. Fatigue Load - 280 l b . Grip Separation 5 i n s . S p e c i m e n N o . B a t c h 1 3 4 5 17 53 54 55 56 57 58 93 94 97 98 B a t c h 2 112 113 125 126 128 130 Life t o I n i t i a l F a i l u r e ( c y c l e s ) 3 . 9 x 1 0 * 8 . 0 X 1 0 * 4 . 7 x l O * 3 . 3 x l O * 7 . 0 X 1 0 * 4 . 3 x l O * 6 . 4 X l O * 8.2 X 1 0 * 6 . 0 X 1 0 * 9 . 0 X 1 0 * 2 . 2 x l O * 4 . 4 X l O * 3 . 2 x l O * 6 . 5 x l O * 2 . 1 9 X 1 0 * 1.8 x l O * 2 . 9 x l O * 3 . 0 x 1 0 * 3 . 3 x l O * 2 . 6 x l O * Life t o V i s i b l e C r a c k ( c y c l e s ) 8.9 X l O * 9 . 5 x l O * 8 . 5 x 1 0 * 5.9 x i o ' 1.23 x i o ' 1.34 x l O ' 1.02 x l o ' 1.25 X l o ' 1.17 X l o ' 1.25 X l O ' 1,88 X l o ' 1 , 4 0 X l o ' 9 . 5 x l O * 1.2 x i o ' 9 . 2 x l O * 1.08 X 1 0 ' 5.7 x l O * 9 . 4 x l O * 8 . 0 x l O * 6 . 6 x l O *

12

-TABLE V

Effect of Grip Separation on Fatigue Life for a Single Load Combination

Separation 5"

3i"

ti i« II II

2i"

II II •1 II II II II 11 Specimen N o .

Life to Initial Failure ( c y c l e s ) See Table IV 69 71 72 73 74 59 61 62 63 64 65 66 67 68 1 . 0 2 X 1 0 ' 5 . 3 x l O * 8 . 2 x l O * 6 . 5 x l O * 9 . 9 X 1 0 * 3 . 7 7 X 1 0 ' 4 . 4 X 10 * 4 . 3 X 1 0 * 4 . 3 X 1 0 * 5 . 0 X 1 0 * 5 . 2 X 1 0 * 5 . 2 X 1 0 * 2 . 8 X 1 0 *

Life to Visible Crack ( c y c l e s ) 1 . 2 X l o ' 1 . 4 9 x 1 0 ' 1 . 5 0 X 1 0 ' 8 . 9 x l O ' 1.55 X 1 0 ' 4 . 3 9 X 1 0 ' 6 . 0 x l O * 1 . 6 8 x 1 0 ' 2 . 4 3 x 1 0 ' 8 . 0 x l O * 8 , 2 x l O * 8 . 2 x l O * 8 . 7 X l O *

T A B L E VI C o m p a r i s o n of F a t i g u e L i v e s of B a t c h 1 and B a t c h 2 at S e l e c t e d Load C o m b i n a t i o n s a) 440 l b . s t a t i c - 330 l b . f a t i g u e . C y c l e s t o I n i t i a l F a i l u r e B a t c h 1 B a t c h 2 1.4 2 . 7 2 . 2 X 10 X 10* X 10* 5 . 2 x l O 2 . 3 X 10* 1.8 x l O * 2 . 2 X 1 0 * 2 . 9 x l O * A v e r a g e 2 . 1 x l O * 2 . 8 8 x 1 0 * b) 440 l b . s t a t i c - 280 l b . f a t i g u e . T h e d e t a i l s a r e given in T a b l e IV C y c l e s to V i s i b l e C r a c k B a t c h 1 4 . 2 x l O * 5 . 3 x l O * 4 . 3 x l O * 4 . 6 x l O * B a t c h 2 5 , 9 x 10* 3 . 6 X 1 0 * 2 . 8 x l O * 3 . 9 X 1 0 * 4 . 7 X 1 0 * 4 . 1 8 X 1 0 * C y c l e s to I n i t i a l F a i l u r e B a t c h 1 B a t c h 2 A v e r a g e 5 . 5 x l O * 2 . 7 5 x 1 0 * If S p e c . N o . 17 excluded c) 440 l b , s t a t i c - 220 l b . f a t i g u e . C y c l e s to I n i t i a l F a i l u r e B a t c h 1 B a t c h 2 1.02 X 1 0 ' 8 . 9 x l O * 1,42 X 1 0 ' 1.64 X l o ' A v e r a g e 1.28 x 10 ' 5 . 0 x l O * 1.03 X l O ' 3 , 2 X 1 0 * 6 . 1 X 10* 6 . 1 5 X 1 0 * d) 330 l b . s t a t i c - 330 l b . f a t i g u e . C y c l e s to I n i t i a l F a i l u r e B a t c h 1 B a t c h 2 1.4 x l O * 2 . 6 x l O * 2 . 7 . x ' l O * 3.o" x i O * 3 . 7 X 10.* A v e r a g e 2 . 6 8 x 10 * 1.64 X 10* 1.6 X 1 0 * 1.2 X 10* 1.2 x l O * 1,41 X 1 0 * C y c l e s t o V i s i b l e C r a c k B a t c h 1 B a t c h 2 1.52 X 1 0 ' 8 . 2 8 X 10* 1.09 X 1 0 ' C y c l e s to V i s i b l e C r a c k Batch 1 B a t c h 2 18 X 1 0 ' 08 X 1 0 ' 35 X 1 0 ' 52 X 1 0 ' s 2 . 5 4 X 1 0 ' 2 . 0 8 X 1 0 ' 1.74 X l O ' 5.89 X 1 0 ' 2 . 0 2 X 1 0 ' 2 . 9 4 X 1 0 ' C y c l e s t o V i s i b l e C r a c k B a t c h 1 B a t c h 2 5 . 4 5 . 4 X 1 0 * X 1 0 * 8.4 x l O * 1.13.X l O ' 1.11 X 1 0 ' 4 . 5 X 1 0 * 5 . 5 X 1 0 * 4 . 0 X 10* 4 . 0 X 1 0 * 8 . 3 2 x 1 0 * 4 . 5 X 10*

14 TABLE VI (continued) e) 330 l b . static - 280 l b . fatigue. C y c l e s to Initial F a i l u r e Batch 1 Batch 2 6 . 0 x l O * 8 . 5 x l O * 2 , 8 x l O * 4 , 0 x l O * 3 . 7 X 1 0 * 2 , 4 x l O * 3 , 1 X 1 0 * 4 , 3 x l O * A v e r a g e 5 . 3 x l O * 3 , 3 7 x 1 0 * C y c l e s to Visible Crack Batch 1 Batch 2 1,23 X 1 0 ' 9 . 5 x l O * 7 . 3 x l O * 1.2 x l O ' 7 . 5 x l O * 4 . 1 x l O * 1 . 0 3 X 1 0 * 8 , 5 x l O * 1,03 X l O ' 7 . 6 X 1 0 *

APPENDIX 1 Complete Results A) Batch No. 1

Static Load Fatigue Load No. of Cycles No, of Cycles Specimen to Initial to Visible

No. l b . - l b . Failure Crack

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 440 440 440 440 440 440 440 440 440 440 Tensile 440 440 440 440 440 440 440 440 Tensile 440 330 330 330 - broke - broke Metallographic 440 440 440 330 Tensile 330 330 -330 330 220 220 220 220 Tensile 220 110 280 280 280 440 440 330 330 330 at 1912 lb. 220 220 220 110 180 280 180 180 at 1815 lb. 180 330 330 330 Specimen 220 220 280 280 -broke at 1882 lb. - broke 280 280 -180 220 220 220 220 180 at 1880 lb. 1.02 N 3.9 8.0 4.7 8.0 1,0 1,4 2.7 2.2 8.9 1.42 1.64 9,48 1.82 3.3 8.65 4,22 2.00 1,4 2.6 2,7 9.5 6.3 6.0 8.5 2.8 4,0 N. 1,11 1,51 1.49 5,10 2.20 X 1 0 ' .D. X 10* X 10* X 10* X 1 0 ' X 10* X 10* X 10* X 1 0 * X 10* X 1 0 ' X 1 0 ' X 1 0 ' X 10» X 10* X 1 0 ' X lo' X 1 0 ' X 1 0 * X 10* X 10* X 10* X 1 0 * X 10* X 1 0 * X 1 0 * X 1 0 * D. X 1 0 ' X lO' X 1 0 ' X l o ' X 1 0 ' 3.18 X l O ' 3. 04 X 10 ' 8.9 x l O * 9.5 x l O * 8.5 X l O * 2.3 x l O * 3.9 x l O * 4.2 X l O * 5.3 X l O * 4.3 x l O * 1.08 X 1 0 ' 3.35 X 1 0 ' 2.52 X 1 0 ' 2,30 X 10* 6.62 X 1 0 ' 5,90 X 1 0 ' 1,94 X 10* 5,09 X 1 0 ' 9.62 X 1 0 * 5,4 x l O * 5.4 X l O * 8,4 x l O * 1,96 X 10» 1.69 X 1 0 ' 1.23 X 1 0 ' 9.5 x l O * 7,3 X l O * 1,20 X 1 0 ' -3.60 X 1 0 * 3,14 X 1 0 ' 3.13 X 1 0 » 2.51 X 1 0 ' 8,32 X 1 0 ' 4.53 X 1 0 '

16

-Complete Results (continued)

Static Load Fatigue Load No, of Cycles No, of Cycles Specimen to Initial to Visible

No, l b , - l b . F a u u r e Crack 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59* 60 61* 62* 63* 64* 65* 66* 67* 68* 691 70 711 721 73i 741 75 76 77 78 79 80 81 82 83 84 85 220 220 220 220 220 440 220 220 -Tensile - broke Metallographic 440 440 440 440 440 440 440 440 Tensile 440 440 440 440 440 440 440 440 440 Tensile 440 440 440 440 -220 220 440 440 Tensile 440 440 330 330 330 - broke - broke 180 180 110 110 110 50 110 110 -at 1793 lb, Specimen 50 280 280 280 280 280 280 280 at 1800 lb, 280 280 280 280 280 280 280 280 280 at 1955 lb. 280 280 280 280 -220 220 110 110 - broke at 1943 lb. 110 110 110 110 330 4,41 X 1 0 ' 1,02 X l o ' N.D, N,D. 7,55 X 1 0 ' N,D. 1.53 X 1 0 * N.D. -N.D. 7.0 X 1 0 * 4.3 x l O * 6.4 x l O * 8.2 x l O * 6.0 x l O * 9.0 x l O * 9,9 x l O * 3.77 X l O ' 4,4 x l O * 4,3 x l O * 4,3 x l O * 5.0 X 1 0 * 5.2 x l O * 5,2 x l O * 2.8 x l O * -1.02 X l o ' 5,3 x l O * 8.2 x l O * 6.5 X 1 0 * -2.25 X 1 0 ' 2.12 X 1 0 ' 1.21 X l O ' 4.7 X 1 0 * 1.99 X 10 * 1.80 X 10 • 2.09 X 10 * 3.49 X 10 * 3.0 X 10 * 5.43 X l O ' 7.08 X10» 2,72 X 1 0 * 2.83x10* 2.95 X 1 0 ^ 2.82 X 1 0 ^ 2.10 X 1 0 * 2.53 X 1 0 * -1.02 x l O ' 1.23 X 1 0 ' 1.34 X l O ' 1.02 X 1 0 ' 1.25 X10» 1.17 X l O ' 1.25 X l O ' 1,55 X10» 4.39 X 1 0 ' 6.0 X l O * 1.68 X l o ' 2.43 X l O ' 8.0 X 10* 8.2 X 1 0 * 8.2 xlO* 8.7 x l O * -1.2 X l o ' 1.49 X l O ' 1.50 X l O ' 8.9 X 1 0 ' -2,75 X 1 0 ' 4.00 X 1 0 ' 8.94 X 1 0 * 9.90 X 1 0 '

4.97 xio*

9,92 X 1 0 * 6,52 X 1 0 * 9.36 X 1 0 * 1.13 X l O ' . •Jtvt

Complete Results (continued) S p e c i m e n N o . 86 87 88 89 9 0 9 1 9 2 9 3 94 9 5 9 6 97 98 9 9 1 0 0 B) B a t c h N o . 1 0 1 1 0 2 1 0 3 1 0 4 1 0 5 106 1 0 7 1 0 8 1 0 9 1 1 0 1 1 1 1 1 2 1 1 3 1 1 4 1 1 5 116 1 1 7 1 1 8 119 1 2 0 1 2 1 1 2 2 1 2 3 1 2 4 1 2 5 126 Static Loac l b . 3 3 0 3 3 0 3 3 0 2 2 0 T e n s ü e -2 -2 0 3 3 0 4 4 0 4 4 0 1 F a t i g u e L o a d

tlb.

3 3 0 2 2 0 2 2 0 1 8 0 b r o k e at 1825 l b . 1 8 0 7 0 2 8 0 2 8 0 N o . of C y c l e s t o I n i t i a l F a ü u r e 3 . 7 X 1 0 * 9 . 7 x l O * 1 . 6 9 X l o ' N . D . 3 . 2 7 x l O ' N . D . 2 , 2 X 1 0 * 4 , 4 X l O * I n s t r o n t e n s i l e t e s t - b r o k e at 1920 l b . I n s t r o n t e n s i l e t e s t - b r o k e at 1900 l b . 4 4 0 4 4 0 M e t a l l o g r a p h i c T e n s i l e

-J

2 8 0 2 8 0 S p e c i m e n b r o k e at 1840 l b , M e t a l l o g r a p h i c 4 4 0 4 4 0 4 4 0 4 4 0 4 4 0 3 3 0 3 3 0 3 3 0 3 3 0 T e n s i l e -4 -4 0 4 4 0 S p e c i m e n 3 3 0 3 3 0 3 3 0 3 3 0 3 3 0 2 8 0 2 8 0 2 8 0 2 8 0 b r o k e at 1913 l b , M e t a l l o g r a p h i c Not u s e d 4 4 0 4 4 0 4 4 0 4 4 0 4 4 0 T e n s U e -4 -4 0 Not u s e d Not u s e d 4 4 0 4 4 0 2 8 0 2 8 0 S p e c i m e n 4 4 0 4 4 0 4 4 0 4 4 0 4 4 0 b r o k e at 1965 l b . 2 2 0 2 8 0 2 8 0 3 , 2 X 1 0 * 6 , 5 X 1 0 * 5 , 2 X 1 0 * 2 . 3 x l O * 1 , 8 x l O * 2 , 2 X l O * 2 . 9 x l O * 3 . 7 X 1 0 * 2 . 4 X 1 0 * 3 . 1 X 1 0 * 4 . 3 X 1 0 * 2 . 1 1 X 1 0 * 1 . 8 X l O * N . D . N . D . N . D . N . D . N . D . 5 . 0 X 1 0 * 2 , 9 X 1 0 * 3 . 0 X 1 0 * N o , of C y c l e s t o V i s i b l e C r a c k 1 . 1 1 X 1 0 ' 3 . 3 7 X 1 0 ' 4 . 5 5 X 1 0 ' 1 . 2 0 X 1 0 ^ 7 . 1 8 X 1 0 » 7 . 0 0 X 1 0 ' 1 , 8 8 X l o ' 1 . 4 0 X l O ' 9 . 5 x l O * 1 . 2 0 X l o ' 5 . 9 x l O * 3 . 6 x l O * 2 . 8 x l O * 3 . 9 X l O * 4 . 7 X l O * 7 . 5 x l O * 4 . 1 x l O * 1 . 0 3 x l o ' 8 , 5 x l O * 9 . 2 X l O * 1 . 0 8 x 1 0 » 1 . 4 X l O * 1 . 2 X l O * 1 . 1 x l O * 2 . 6 x l O * 5 X 1 0 » 2 . 0 8 X 1 0 » 5 , 7 x l O * 9 , 4 X 1 0 *

18

-Complete Results (continued)

Specimen N o . 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150

Static Load Fatigue Load

lb. t i b . T e n s i l e - b r o k e at 1970 l b . 440 280 330 330 440 280 T e n s i l e - b r o k e at 1980 l b . T e n s i l e - b r o k e at 1978 l b . 440 220 330 330 330 330 Not u s e d 440 220 330 330 M e t a l l o g r a p h i c S p e c i m e n Not u s e d T e n s i l e - b r o k e at 1989 l b . 440 220 440 220 440 440 Not u s e d T e n s i l e - b r o k e at 2025 l b . T e n s i l e - b r o k e at 2029 l b . T e n s i l e - b r o k e at 2065 l b . Not u s e d Not u s e d N o , oi • C y c l e s to Initial F a U u r e 3 . 3 1.6 2 . 6 1.03 1.6 1.2 1.54 1.2 3 . 2 6 , 1 N X 10* X 10* X 10* X 10» X 10* X 10* X 10» X 10* X 10* X 10* D . No. of C y c l e s t o V i s i b l e C r a c k 8 , 0 4 , 5 6 , 6 1.74 5 . 5 4 . 0 3 . 4 8 4 . 0 5.89 2 . 0 2 1,3 X 10* X 10* X 10* X ICP X 10* X 10* X l O ' X 10* X 10» X l o ' X 10* N . D . Not Determined

* 2,125 Ins. grip separation i 3.5 Ins, grip separation

O a* A • O ^ B -C lO" -J.

FIG.2. CYCLES TO INITIAL FAILURE VERSUS FATIGUE AMPLITUDE AT 3 DIFFERENT STATIC LOADS « 4 4 0 LBS STATIC « 330 LBS STATIC * 2 2 0 LBS STATIC

a SOO •M u o K * 0 0 2 2 0 0 lOO STATIC LOAD X 4 4 0 LBS STATIC . 3 3 0 LBS STATIC & 2 2 0 LBS STATIC I O ' I O * IO « 5 I O " CYCLES i^T-10 10»

RG.5. CYCLES TO VISIBLE CRACK VERSUS MAXIMUM LOAD AT 3 D I F F E R E N T STATIC LOADS. X 4 4 0 L B S STATIC o 3 3 0 LBS STATIC A 2 2 0 LBS STATIC

STATIC LOAD 4 4 0 L B S + FIRST BATCH • SECOND BATCH

IO°

CYCLES

-tr

FIG.6.CYCLES TO INITIAL FAILURE FOR BATCH I AND BATCH 2 UNDER THE SAME STATIC LOAD 0 F 4 4 0 L B S .

• riRST BATCH • SECOND BATCH

tr

tj-10" CYCLES

FIG.7. CYCLES TO VISIBLE CRACK FOR BATCH I AND BATCH 2 UNDER THE SAME STATIC LOAD OF 4401.BS.

aooo ISOO SOO SP! /

J

1

•JC N«»« !

w

1 \

\ \

TRAIN RATE O-l IN S/MIN SPEC

f

/

J

( N«»S ) \ 0-3 O 0 1 OVERALL EXTENSION-INS