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Fatigue life prediction for toe ground welded joints

Yan-Hui Zhang and Stephen J Maddox

Structural Integrity Technology Group
TWI Limited

Paper published in International Journal of Fatigue, Vol. 31, Issue 7, July 2009, 1124-1136.

Abstract

The paper presents the results of an investigation of the effect of weld toe burr grinding on the fatigue performance of non-load-carrying transverse fillet welded joints. Crack initiation and propagation were monitored by a modified replica method. It was found that, although the average life increase due to toe grinding was in agreement with published data, the majority of the fatigue cracks in specimens that gave fatigue lives <~106 cycles initiated at flaws just beneath the ground surface. Both the experiments and calculations based on fracture mechanics suggested that the fatigue lives of the toe ground joints in this life regime were dominated by the crack propagation process. However, in the long life regime (>106 cycles), crack initiation became significant. Reasonable estimates of the crack initiation period were made using the local stress approach proposed by Lawrance et al. The investigation suggested that more benefit from weld toe grinding could be claimed in the long (N > 106 cycles) than the short life regime.

Key words:

Welds, toe grinding, LCF, crack initiation, crack growth.

Nomenclature and definitions

Bottom welds: The two welds below the attachments in the present specimens that were held vertically for fatigue testing (welds toe ground and then needle peened)

Crack aspect ratio: Ratio of crack depth to crack length, a/2c

Equivalent (constant amplitude) stress range ε

1 Introduction

With respect to the process(es) leading to fatigue failure of welded joints, there are two different opinions. One view is that, because small, sharp, slag intrusions are unavoidably present at the weld toe and act as crack initiation sites, the fatigue life of a welded joint is predominantly controlled by the crack propagation process.[1] Indeed, metallurgical examinations showed that the average depth of these flaws is 0.15mm and typically the maximum depth is approximately 0.4mm.[2,3] A review by Grover[4] suggested that even high-quality welds contain flaws up to a depth of about 0.1mm. One approach then available for calculating the fatigue life is to integrate the fracture mechanics-based fatigue crack growth law for the material concerned between the limits of flaw size and critical crack size corresponding to failure. [5,6]

A different view is that fatigue endurance of welded joints is composed of both crack initiation and crack propagation processes.[7] Lawrence et al[7] applied a local approach developed by Morrow[8] to estimate the fatigue crack initiation period as part of the evaluation of the fatigue strength of welded joints. This was defined as the number of cycles required to produce a crack of a certain size. It was estimated using a low-cycle fatigue (LCF) approach which utilised a Coffin-Manson type equation. The LCF properties of the heat affected zone (HAZ), where crack initiation occurred, were estimated by applying the empirical relation between hardness and tensile strength of steels. A significant crack initiation period, about 40-50% of the total life, for endurances between 105 to 106 cycles was reported. [7,9] The percentage of the total life spent initiating a crack was predicted to increase with increasing fatigue endurance. A weakness in the approach is that the transition crack size, at which the fatigue damage process is assumed to change from crack initiation to crack growth, has been defined arbitrarily, ranging from 0.1mm,[9] to 0.15mm[10] to 0.25mm.[7] It will also be noted that the assumption that part of the fatigue life of a welded joint is governed by the tensile strength of the material is in direct contradiction of the well established finding that the fatigue lives of welded joints are independent of the material's tensile strength. Consequently, the approach is not generally used to assess as-welded joints.

Weld toe grinding is a well-established technique for improving the fatigue strength of welded joints. The main aims of the operation are to reduce the local stress concentration and to remove crack-like flaws at the weld toe. To obtain significant improvement in fatigue life, it is recommended[11] to grind to at least 0.5mm below any visible undercut to ensure that the intrusions and crack-like flaws are removed. Such treatment justifies an increase in fatigue design endurance of at least 2.2 times according to[12].

Since all flaws are expected to have been removed after grinding, the proportion of the fatigue endurance spent initiating a crack is expected to be significant. Consequently, the local approach for evaluating fatigue crack initiation in welded joints mentioned earlier[7] may be suitable for application to toe ground joints. Although many attempts have been made to calculate the fatigue endurance of welded joints by considering crack initiation and propagation, work on ground joints is limited.[10] Furthermore, there is little convincing data in the literature to verify any model for calculating the fatigue crack initiation period. The work described in this paper addressed this lack of data and analysis. On the basis of experimental results and fatigue life calculations for welds in steel plate, the aim was to develop a method for predicting the fatigue lives of toe ground welded joints allowing for both the fatigue crack initiation and propagation processes.

2 Experimental details

Specimens of the design shown in Figure 1 were manufactured from steel plate complying with EN 10025 S355JR. The chemical composition of the material is given in Table 1. The yield and tensile strengths of the material were determined by tensile testing to be 406 and 539MPa, respectively. The transverse non-load-carrying joints with nominally 10mm leg length fillet welds were made in two passes using 3.25 and 5.0mm diameter electrodes complying with AWS 5.1:E6013. All the weld toes were dressed by burr grinding following the standard TWI practice, which is in accordance with the recommendations given in BS 7608.[11] The radius of the burr was 5.0mm and the maximum depth of grinding was limited to ~0.8mm. After grinding, each weld was carefully examined visually to ensure that no flaws were present on the ground surface. As seen in Figure 1, in each test specimen there were four fillet welds, two above the attachments (termed 'top welds') and two below (termed 'bottom welds'). As will be described later, crack initiation and growth in welds were determined by a replica method. Since it was difficult to make a replica on the bottom welds when the specimens were installed in the fatigue testing machine, it was decided to confine attention to the top welds. There is evidence that peening after weld toe burr grinding can increase fatigue performance further by introducing compressive residual stresses.[13] Therefore, the bottom welds were also needle peened after being burr ground to delay crack initiation there.

ε are local true stress and true strain ranges, σre values were assumed in a sensitivity study, equal to the yield strength of the material, in accordance with the conservative recommendation given in BS 7910,[17] or about half of the yield strength (200MPa). The resulting estimates of the crack initiation endurances are shown in Figure 11 for the parent metal and Figure 12 for the weld metal. The limited experimental data are also included for comparison. These data include the following:

  • The initiation endurances of those cracks which did not obviously initiate from flaws.
  • The fatigue lives of those top welds, which did not show any fatigue cracking when the specimens failed from the bottom welds. They are shown as run-outs in the two figures.
  • The fatigue endurance of Specimen 05, which was a run-out at a stress range of 180MPa.
ΔK, when compared at the same nominal stress range and crack size. This was attributed again to two factors, the stressintensity magnification factor Mk and crack shape. Firstly, by toe grinding, Mk was significantly reduced. Secondly, because of the severe stress concentration alongthe weld toe, cracks in as-welded joints tend to adopt a lower aspect ratio. This was often enhanced by multiple crack initiation and coalescence of these cracks, Figure 7(b). On the other hand, the aspect ratio of cracks inground joints was significantly higher and the fatigue life, or at least the majority of the fatigue life, of a specimen was dominated by growth of a single crack. This difference can be readily seen by comparing the fracture surfacesof the as-welded and ground joints in Figure 7. When approaching a through-thickness crack, the aspect ratio of the crack in the as-welded joint was 0.12, but was about 0.35 for the crack in the ground weld. ΔK decreases with increase in a/2c and consequently that for the ground joints was less than that for the as-welded joints at the same crack depth and applied stressrange.

  • Crack growth rates. The average growth rate for cracks growing from the ground weld toes was significantly lower than that for the as-welded joints. As described before, the crack growth parameter A for the as-welded joints wasestimated to be A=2.5x10-13, significantly greater than the average value of A=1.3x10-13 found in the ground joints. This difference can be explained by the possibledifference in residual stresses, and hence effective stress ratio, between the two types of joint, crack growth rate being influenced by stress ratio. Through residual stress measurements,[24] it has been estimated that there is a sharp decrease in residual stresses through the plate thickness. This finding was related to a similar type of welded joint to the one tested in the present work but in20mm thick by 250mm wide steel plate with a yield strength of 530MPa. Residual stress reached the maximum value of about 270MPa near the surfaces at the weld toes and decreased sharply with increasing distance from the surface,becoming compressive at the centre of the plate. It then increased again towards the back surface. At a depth of 0.8mm from the surface, which was the grinding depth in the current investigation, the residual stress decreased to about150MPa (reduced by 44%). By grinding, a further reduction in residual tensile stresses or even the creation of compressive stresses can be expected.[25] Thus, although the actualmagnitudes of the residual stresses in the present specimens may have been lower than those investigated in [24], it does seem likely that the reduced crack growth rates observedin the ground joints are attributable to a more favourable residual stress state.

The above discussion can also explain the behaviour of ground joints in high strength steels. As toe grinding reduces the chance of crack initiation from flaws, crack initiation life is expected to be important in the total fatigue endurance. As a result, high strength steels (yield strength of about 700MPa) benefit more from toe grinding: a fatigue strength increase of over 100% was observed in the review by Booth.[12] On the other hand, some specimens of high strength steels only saw an increase of about 30%, similar to the average increase found in low and medium strength steels (yield strength of 240-400MPa). It is likely that the high strength steel ground joints contained flaws, which offset any potential benefit from tensile strength. The large data scatter in S-N curves typically observed for ground joints[12,15,25] also supports this speculation.

The present results highlight the importance of welding flaws, which would be innocuous in an as-welded joint, in toe ground welds. Clearly, every effort should be made to avoid them or grind them out but a serious practical limitation is the problem of detecting such small flaws by current NDT methods. However, if they can be avoided it seems that a significant crack initiation period will be required before a fatigue crack starts to propagate, potentially leading to much greater improvement in fatigue life than that obtained from the present test specimens, and that the parent metal at the base of the groove produced by toe grinding is the most likely crack initiation site. Furthermore, on the basis of the rather limited current experimental data on crack initiation free from any flaws, it seems that a reasonable estimate of the crack initiation endurance can be obtained using the local approach proposed by Lawrence et al. [7]

Conclusions

Based on a study of fatigue crack initiation and growth in toe ground fillet welded joints, the following conclusions were drawn:

  1. A replica method for detecting crack initiation from ground joints was successfully developed. It enabled the detection of surface cracks as little as 0.1mm in length.

  2. Burr grinding increased the fatigue endurance of the fillet welds by a factor of at least 4.6.

  3. It was revealed by the replica method that, even in ground joints, most cracks initiated from flaws on or just beneath the ground surface, consistent with the observation of initiation of these cracks in the weld metal.

  4. The fatigue performance of ground joints was predicted well using fracture mechanics fatigue crack growth analysis for endurance ≤106 cycles.

  5. Compared to as-welded joints, the increased fatigue performance of ground joints was attributed to i) reduced stress intensity magnification factor Mk; ii) reduced ΔK due to more favourable fatigue crack front shapes; and iii) slower crack growth rates possibly related to reduced tensile residual stresses.

  6. Reasonable estimates were made of the crack initiation endurances for those joints where crack initiation did not occur from flaws using the Lawrence approach.

Acknowledgements

This work was supported by the Industrial Members of TWI. In addition, the authors would like to thank the staff of the Fatigue Laboratory in carrying out the experimental work.

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