Thickness Effect on Fatigue Strength of Welded Joint Improved by HFMI *

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1 [ 溶接学会論文集第 34 巻第 4 号 p (2016)] Thickness Effect on Fatigue Strength of Welded Joint Improved by HFMI * by IWATA Toshiaki **, NIWA Toshio **, TANAKA Yoshihisa ***, ANDO Takahiro **** and ANAI Yosuke ** Fatigue strength decreases by thickness effect, therefore the improvement treatments of the fatigue strength are important as the structure consisting of the thick material. Post weld treatments are one of useful method to improve fatigue strength. However, the thickness effect on welded joint improved by post weld treatments is not clear because the experimental data is insufficient for quantitative evaluation. The object of this study is to evaluate thickness effect on the fatigue strength which is improved by post weld treatments to reflect it in fatigue design guideline; therefore in transverse no-load carrying cruciform joint which is one of the main welded joints of the ship structure, the improvement effect of the fatigue strength by post weld treatments is evaluated. In this report, the result of HFMI (High Frequency Mechanical Impact treatment) and Shot peening are evaluated with the result of the as-welded condition and conventional Burr grinding by nominal stress approach. In addition, the fatigue strength is evaluated using the equivalent stress approach by the modified MIL-HDBK-5 method that both the stress concentration of weld toe and the weld residual stress are separated from nominal stress. Good correlation between the fatigue test results was achieved using modified version of the equivalent stress approach defined in MIL- HDBK-5. With the equivalent stress that was adopted in this study, both the stress range and the effect of maximum stress including the residual stress are considered. It is considered that both number of cycles to crack initiation and that to failure are evaluated by equivalent stress approach integrally. Key Words: Fatigue strength, Thickness effect, No-load carrying attachment, Cruciform joint, HFMI, Shot peening, Burr grinding, Shipbuilding 1. INTRODUCTION 1.1 Background of this study It is known that the fatigue strength of a welded joint decreases with the increase in the thickness of load carrying member as Thickness Effect. It is required in the Common Structural Rule (CSR 1) ) that the fatigue strength reduction factor set to 0.25 uniformly for welded structures. However, the coefficient was decided empirically because of insufficient knowledge about the mechanism of the effect, therefore, the research 2) was carried out to investigate the factors which affect thickness effect by Class NK with the Shipbuilders Association of Japan. Moreover, economical and rational design production is required to reduce the dimensions and the thickness of welded structures. Post weld treatments are one of useful methods to achieve this requirement; therefore, studies to clarify the fatigue strength * 受付日平成 28 年 5 月 13 日受理日平成 28 年 10 月 23 日 ** 正員国立研究開発法人海上 港湾 航空技術研究所海上技術安全研究所 Member, National Maritime Research Institute; National Institute of Maritime, Port and Aviation Technology *** 国立研究開発法人海上技術安全研究所 ( 現法政大学デザイン工学部 ) National Maritime Research Institute (Present Address: Faculty of Engineering and Design, Hosei University) **** 国立研究開発法人海上 港湾 航空技術研究所海上技術安全研究所 National Maritime Research Institute; National Institute of Maritime, Port and Aviation Technology improvement effect by post weld treatments for weld toe have been increased for reflecting the effect in design rules. However, there is insufficient experimental data for quantitative evaluation about the thickness effect and the fatigue strength improvement effect of post weld treatments for extremely-thick welded joint. In shipbuilding industry, the following members are given as examples; Penetration of longitudinal members, weldment between Upper Deck plating and trans. Web, Lower Stool and so on. Especially, Hatch side coaming is 80mm thick over. 1.2 Object of this study The object of this study is to evaluate thickness effect on the fatigue strength improvement effect by post weld treatments including other economical industrial point of view for reflecting the fatigue design guideline. Joint type for evaluating in this study is transverse no-load carrying cruciform joint that is one of three principal joints for ship structure. Three types of post weld treatment for evaluating in this study are Burr grinding, HFMI (High Frequency Mechanical Impact treatment) and Shot peening. In this report, the result of HFMI and Shot peening are evaluated with the result of the as-welded condition and conventional Burr grinding by nominal stress approach. In addition, the results were assessed using the equivalent stress approach by the modified version 3-5) of the approach MIL-HDBK-5 where both the stress concentration of weld toe and the weld residual stress are separated from nominal stress. Original MIL-HDBK-5 is contained in the US Military Standard usually applied to aircraft.

2 250 研究論文岩田他 :Thickness Effect on Fatigue Strength of HFMI Welded Joint Table 1 Mechanical properties 2. TEST METOD 2.1 Test materials High tensile strength steel for ship structure such as KA36 (AH36) regulated in ship s classifications is suitable for the test material in this study; however, it was difficult to get required quantity in this study because this material is build-to-order manufacturing from shipbuilder. Therefore, rolled steel for welded structure SM490A under Japanese Industrial Standard G 3106 that is approximately equivalent to KA36 was used as test material. Mechanical properties obtained from tensile tests are shown in Table 1 with the values of inspection certification and standard. Yield strength of the test material whose plate thickness is 22mm or 50mm was less than the lower limitation of that of KA36 (AH36). 2.2 Test Specimens The configuration of no-load carrying cruciform joint type test specimens is shown in Figure 1. WL is the weld line. This figure shows the case that the main plate thickness is 50mm. According to the National Institute for Material Science (NIMS) material database 6,7), fatigue strength decreases until the height of attachment increases twice as much as the main plate thickness and the width of attachment increases same as the main plate width because fatigue strength is influenced strongly in this range by the configuration and the dimensions of the attachment for loadcarrying main plate. However, the fatigue strength hardly decrease with the further size, therefore, the configuration of the attachment for load-carrying main plate was decided as shown in Figure 1. Welding method is semi-automatic CO 2 gas shielding arc welding. The quality of weld leg length was targeted in accordance with CSR-B F2 1). F2 is the requirement for side plate and so on except strength deck. CSR-B F1 1) is required for strength deck. Fig. 1 Configuration of test specimen of no-load carrying cruciform joint

3 溶接学会論文集 第 34 巻 (2016) 第 4 号 251 Table 2 Welding conditions IIW requirement is nearly equal to F1. Welding conditions are shown in Table 2. The orders of weld pass on multi-layer welding method are shown in Figure 2. Welding speed was stable, therefore, the scattering of heat input between test structures A and B is about 2%. The as-welded test specimens and Burr grinding test specimens were cut out from the test structure B. Burr grinding was applied in accordance with IIW Recommendation 8). HFMI test specimens and Shot peening test specimens were cut out from the test structure A. HFMI and Shot peening was applied in accordance with the each standards of constructer. The projection conditions of Shot peening were as follows. Projection method: Air (pressurizing), Projection materials: Cast steel (Spherical), Projection materials size: SB-6 (φ0.6mm), Projection materials hardness: 40~50 HRc, Arc-height: 0.341mmA. The applied conditions of HFMI were as follows. The method of treatment: UIT (Ultrasonic impact treatment), The applied equipment: ESONIX 27 UIS, The configurations of impact pin: Pin diameter is 3mm and Tip radius is 3mm, The blow number of times: approximately over one hundred times per second, The standard treatment speed of HFMI: approximately 300mm/min on straight line part (until the original weld toe lines disappeared). Five pieces of fatigue test specimens and two pieces of test specimens for residual stress measurement were fabricated for every all types of test specimens. Fig. 2 Cross section of weld bead 2.3 Test conditions Fatigue tests were carried out under conditions of stress ratio R =0.05, frequency 1-3Hz. The configurations of four weld toes were measured for local stress concentration factor K w on all test specimens by dental silicon rubber impression material. The configurations of crack were measured using ink permeation method on all test specimens when crack has initiated by the judgment based on the 5% strain drop method.

4 252 研究論文 岩田他 Thickness Effect on Fatigue Strength of HFMI Welded Joint Weld residual stress measurement was carried out by the HoleDrilling Strain-Gage Method based on ASTM E837-13a 9,10). The 1/16 inch size type A rosettes (gage length: 1/16 inch = 1.59 mm, concentration factor and Sr is the weld residual stress. 3.2 Stress concentration measurement * gage base diameter: 5.13 mm) were adopted, therefore the depth of In this paper, the stress concentration factor K due to the welded the total measurement is 1.0 mm and the depth of the one step joint is separated into two categories Ks and Kw. One is that caused measurement is 0.05 mm. The residual stresses in each Hole- by the local configuration of welded section. The other is that Drilling step are calculated by solving the matrix equation of the caused by the arrangement of the structural members. Fatigue measurement of strains using integral method. evaluation including allowance for this structural stress concentration is usually called the Hot-spot stress method. In this method, no-load carrying cruciform joint is considered as one of 3. TEST RESULTS AND DISCUSSION the basic joints, for which the structural stress concentration factor 3.1 Analysis method Ks is set to 1.0. Therefore, *K is set to Kw on no-load carrying The US military handbook for aircraft materials (MIL11) cruciform joint. HDBK-5), expresses the S-N relation in terms of an equivalent On the other hand, the local stress concentration varies stress Seq (equation (1)). This results in the relation between life N according to the local configuration of welded section. The local and Seq given in equation (2). stress concentration factor Kw of weld toe on no-load carrying Seq = Smax (1 - R ) (1) cruciform joint is often calculated using the equation of Heywood/ log Seq = α + β log N (2) Nishida m 12) by the actual survey shapes 13,14). In this equation, Kw is Here, Smax is the maximum applied stress, R = Smin /Smax is the stress calculated by the bending theory of a strip-like plate having the ratio and m is the exponent to optimize Seq -N relation. protuberance on both sides. MIL-HDBK-5 refers specifically to aircraft materials where the included welded joint is limited to a spot welding lap joint, Kw = 1 + f (θ) (a - 1) (6) Where, 0.65 therefore the effect of both the weld residual stress and the stress a = 1 + [ (l /ρ) / (2.8 B / b - 2) ] concentration of weld toe are not considered in the equivalent f (θ) = [ 1 - exp{ θ (B/l ) }] / [1 - exp{ π (B/l )}] (8) stress of MIL-HDBK-5. On a welded structure, for example ship Here, ρ is the weld toe radius measured by concentric circles and θ where a lot of as-welded butt joint or fillet welded joint is is the maximum angle of tangent of fillet weld as shown in Figure constructed, the fatigue damage initiates mainly at the toe of 3. l is the weld leg length, t is the plate thickness, B = t + l and welded joints. It is influenced by both the weld residual stress and b = t / 2. Figure 3 shows an example of the configurations of weld the stress concentration due to the welds. Therefore, the fatigue toe measured by dental silicon rubber impression material. 11) (7) 1/2 analysis method given in MIL-HDBK-5 was modified to allow it The stress gradient is relieved by the redistribution of the to be used to evaluate the present results. This involved plasticity field on the crack tip with the crack propagation when establishing an equivalent stress that allowed for the effect of both the stress gradient is steep; therefore non-propagation crack is 3-5) weld residual stress and stress concentration. In this method, generated if the weld toe radius is smaller than the limit radius. equation (1) is rewritten to give equation (3). Moreover, stress The limit radius is known as approximately 0.5 mm in the case of concentration and residual stress are considered by S max and SM490 S = Smax Smin using equation (4) and (5). local stress concentration coefficient being saturated in Kw > 3. Seq = Smax (1 - R) 15-17). This limit radius supports with the influence of the 18) m The results of the configurations of weld toe are shown in Table 3. m = Smax (Smax /Smax - Smin /Smax) = Smax 1-m 1-m m (Smax - Smin) = Smax S m Smax = KSn, max + Sr * S = K (Sn, max - Sn, min) * (3) (4) (5) Here, Sn, max is the maximum nominal (applied) stress and Sn, min is the minimum nominal (applied) stress. On the other hand, Smax and Smin are the local stresses. Moreover, *K is the representative stress n The S-N curve of the IIW Recommendation doc IIW : Δ S N = C can be determined from equation (2) parameters as follows: n = -1 / β and log (C ) = - α / β {(1-m) / β}log (1 - R). Fig. 3 Configurations of weld toe

5 溶接学会論文集 第 34 巻 (2016) 第 4 号 253 Table 3 Configurations of weld toe 3.3 Residual stress measurement The residual stress distribution in the depth direction is calculated from the measured value of 20 steps in the depth direction by the Hole-Drilling Strain-Gage Method based on ASTM E837-13a. According to the measurement result to 1mm in depth in this study, the difference between the First step ( mm in depth) and the Second step ( mm in depth) varied in several times in many test specimens. Thus, only the First step ( mm in depth) was adopted as the value on the surface. Figure 4 shows an example of the configurations of the Hole-Drilling Strain-Gage Method based on ASTM E837-13a. The weld residual stress measured using the 1/16inch size type A rosettes is the mean value in the circle of a radius of 2.57mm that the center is away from weld toe 5.13mm. Therefore, the value that is reduced about 30% from the value right above the weld toe was measured. In consideration of the measurement accuracy, the mean value Fig. 4 Configurations of the Hole-Drilling Strain-Gage Method based on ASTM E837-13a of three measurements points on each welding line was adopted as the representative value because it was considered that the error of dozens of MPa degree occurred. In addition, the upper limit of the

6 254 研究論文岩田他 :Thickness Effect on Fatigue Strength of HFMI Welded Joint absolute value of the residual stress was assumed the yield stress. The results of residual stress measurement are shown in Table 4. The residual stress inside the load carrying member should be adopted for the evaluation of crack propagation life; however, because the measurement of the residual stress distribution through all thicknesses in the load carrying member is very expensive, the cost-effectiveness was considered, and the measurement through all thickness was carried out in only some specimens. On the other hand, as shown by N i/n f in Table 5 and Table 6, the ratio of crack initiation life N i among failure life N f is large, and N i is more than half of N f in the case of both Burr grinding and HFMI where the configuration of weld toe is smooth. In addition, even in the case of the as-welded condition where N i is 20% to 50% of N f, it is thought that the residual stress on the surface layer is able to be adopted as the representative value for evaluating until the crack tip reaches to some depth in the load carrying member because the weld residual stress is updated by the limitation that the maximum local stress is not exceed the yield stress. Therefore, in addition to the evaluation for N i by the modified MIL-HDBK-5 method, the evaluation for N f by this technique was carried out adopting the residual stress on the surface layer because the experimental data of N f has been accumulated widely in other studies. 3.4 Fatigue test results The results of the as-welded condition referred to B, Burr grinding referred to G, HFMI referred to U and Shot peening referred to S are shown in Tables 5 and 6. The relation between nominal stress range and number of cycles to failure is shown in Figure 5. The fatigue strength S-N curves were obtained on condition that the slope n was assumed a variable. The variables were determined by least-squares method. The lines drawn for the experimental results according to the plate thickness of each post weld treatment are the mean line. The experimental results should be evaluated normally using the 97.5% survival probability lines calculated by subtracting two standard deviations of log N from the mean lines. However, in this study, there were only three experimental results except run out data according to the plate thickness of each post weld treatment; thus, the precision of the standard deviation of some series was insufficient to process statistics. Therefore, in this study, the experimental results were evaluated using the mean line and the coefficient of thickness effect was calculated based on these lines. The database based on many experiments is available relating to the as-welded joint. The slope of the fatigue strength S-N curves for details assessed on the basis of typical stresses is n =3 on fatigue design recommendation 19) for constant amplitude loading. As shown in Figure 5, the values of slope n obtained from the experimental results of the as-welded condition in this study were the range of 4.5 from 2.7. The values of slope n obtained from the experimental results of Burr grinding were the range of 5.6 from 3.0, though the value of the slope n for Burr grinding is set 3 on IIW Guideline 8) as the as-welded condition. It is considered that the finish of Burr grinding in this study was smoother than the IIW requirement. The values of slope n obtained from the experimental results of HFMI were the range of 8.0 from 4.1. The values of slope n obtained from the experimental results of Shot peening Table 4 Results of residual stress of the First step ( mm in depth)

7 溶接学会論文集 第 34 巻 (2016) 第 4 号 255 Table 5 Fatigue test results (The as-welded condition referred to B, Burr grinding referred to G) Table 6 Fatigue test results (HFMI referred to U, Shot peening referred to S)

8 256 研究論文岩田他 :Thickness Effect on Fatigue Strength of HFMI Welded Joint Fig. 5 Relation between nominal stress range and number of cycles to failure (The as-welded condition referred to B, Burr grinding referred to G, HFMI referred to U and Shot peening referred to S) were the range of 6.1 from 2.7. Figure 6 shows the relation between plate thickness and nominal stress range based on the fatigue strength at 2 x10 6 cycles obtained from the mean lines according to the plate thickness in Figure 5. Though the relation between the fatigue strength and the weld leg length is not made clear in ship s classifications, the relation between FAT (2 x10 6 cycles) and the weld leg length of the IIW requirement is made clear. The weld leg length of the IIW requirement is nearly equal to that of CSR-B F1 20) that is the severe requirement for important part of ships including the strength deck. The weld leg length of F1 requirement is more 10% than that of F2 requirement. The IIW requirements that are prescribed by FAT of standard plate thickness 25mm and the coefficient of thickness effect 0.3 (the as-welded) or 0.2 (Ground) are shown in Figure 6 for reference. In this study, the fatigue strength of Burr grinding was times higher than that of the as-welded condition. However, the fatigue strength improvement effect decreases with the increase in the thickness of load carrying member, therefore, the thickness effect on Burr grinding is steeper than that on the as-welded condition. The fatigue strength of HFMI was times higher than that of the as-welded condition. Moreover, the fatigue strength improvement effect increases with the increase in the thickness of load carrying member, therefore, the thickness effect on HFMI is Fig. 6 Relation between nominal stress range and plate thickness (EXP Mean Base) gentler than that on the as-welded condition. The fatigue strength of Shot peening was times higher than that of the aswelded condition. However, the fatigue strength improvement effect decreases with the increase in the thickness of load carrying member, therefore, the thickness effect on Shot peening is steeper than that on the as-welded condition.

9 溶接学会論文集 第 34 巻 (2016) 第 4 号 257 As shown by N i /N f in Table 5 and Table 6, there is little influence of the thickness of load carrying member in the ratio of N i to N f in the case of the as-welded condition and Burr grinding, on the other hand, the ratio of N i to N f decreases with the increase in the thickness of load carrying member in the case of HFMI and Shot peening. Therefore, in the case of HFMI and Shot peening where compressive residual stress is added by the improvement treatment of weld toe, the compressive residual stress in the load carrying member is contributed to not only improvement of crack initiation life N i but also improvement of crack propagation life N f -N i, and it is thought that the effect increases with the increase in the thickness of load carrying member. By this influence, the thickness effect on HFMI is gentler than that on the as-welded condition. In the case of HFMI, this is shown as the value of slope n on both the plate thickness 22mm and 50mm that is larger than the value of slope n on the plate thickness 10mm in Figure 5. On the other hand, this tendency in HFMI is confirmed in Shot peening; however, the improvement effect on the radius of weld toe by the projection condition in this study is insufficient in the case of the plate thickness both 40mm and 50mm where a part of the radius of weld toe after the projection is remained less than 0.5mm as shown in Table 3. That is to say, in the case of Shot peening, the improvement effect on the radius of weld toe in the plate thickness 22mm is not provided in the plate thickness both 40mm and 50mm. As a result that both the improvement effect on the radius of weld toe and the effect of the compressive residual stress addition are combined, the thickness effect on Shot peening is steeper than that on the as-welded condition. In the case of Shot peening, this is shown as the value of slope n on both the plate thickness 40mm and 50mm that is considerably smaller than the value of slope n on the plate thickness 22mm in Figure 5. In the case of Burr grinding, the similar tendency in the aswelded condition is confirmed. It is not clear that the thickness effect on Burr grinding is a little steeper than that on the as-welded condition in the test result in this study. 3.5 Fatigue strength of no-load carrying cruciform joint improved by post weld treatment In the case of steel welded joints, it is normally assumed that yield magnitude residual stresses are induced with the result that S max has the upper limit of yield strength and S min has the lower limit of yield strength 3,5,21). S max = * KS n, max + S r S Y (9) -S Y S min = * KS n, min+s r (10) When S max no less than a previous maximum is loaded in a certain load cycle, a condition limited by equation (9) is generated. The weld residual stress S r is updated in S r = S Y - * KS n, max (11) and keeps the value then until the limitation by equation (9) is assigned next. Similarly, When S min no more than a previous minimum is loaded in a certain load cycle, a condition limited by equation (10) is generated. The weld residual stress S r is updated in S r = -S Y - * KS n,min (12) and keeps the value then until the limitation by equation (10) is assigned next. Therefore, the upper limit of S max assumed it the yield stress and the lower limit of S min assumed it the negative yield stress. S max and S were calculated by equations (4) and (5). The coefficient m, α and β were determined by least-squares method. As for all data, least-squares method was applied without distinguishing both the plate thickness and post weld treatments of weld toe including the as-welded condition. The results are as following. m f = 0.892, m i = (13) α f = 4.66, α i = 4.41 (14) β f = , β i = (15) σ f (α f) = 0.107, σ i(α i) = (16) Here, subscript f means failure based on Number of cycles to failure N f and subscript i means initiation based on Number of cycles to crack initiation N i. The parameter σ(α) of equation (16) is the standard deviation of the difference between logn obtained directory from test results and logn obtained from the S eq-n relation, equation (2), and the test results calculated from equations (13), (14) and (15). IIW parameter n and log(c) are as following. n f = 2.69, n i = 2.87 (17) log(c f) = 12.5, log(c i) = 12.7 (18) The fatigue test results in Figure 5 are presented in terms of S eq in Figure 7. The solid line is the mean line based on equation (2). The broken line is the 97.5% survival probability line calculated by subtracting two standard deviations of logn from the mean line. Comparing Figure 5 with Figure 7, it is clear that the modified MIL-HDBK-5 method has successfully correlated most of test results from the various types of post weld treatment without the distinction of the plate thickness including the result of the aswelded condition. The relation between equivalent stress and number of cycles to crack initiation is shown in Figure 8. The result in reference 5 was shown in dotted line in addition for reference. Comparing Figure 7 with Figure 8, it is as follows. 1) n f is slightly steeper than n i 2) The effect of S max=s Y on N i is slightly larger than that on N f ; (1-m i)>(1-m f) 3) The standard deviation (σ i (α i) = 0.149) of N i is larger than that (σ f (α f) = 0.107) of N f However, each of them is not a significant difference. Therefore, it is considered that both number of cycles to crack initiation and that to failure are evaluated by equivalent stress approach integrally.

10 258 研究論文岩田他 :Thickness Effect on Fatigue Strength of HFMI Welded Joint Fig. 7 Relation between equivalent stress range S eq and number of cycles to failure N f Fig. 8 Relation between equivalent stress range S eq and number of cycles to crack initiation N i

11 溶接学会論文集 第 34 巻 (2016) 第 4 号 CONCLUDING REMARKS Thickness effect on fatigue strength of welded joint improved by post weld treatments was investigated. The summarized results are as following. 1) Good correlation between the fatigue test results was achieved using modified version of the equivalent stress approach defined in MIL-HDBK-5. With the equivalent stress that was adopted in this study, both the stress range and the effect of maximum stress including the residual stress are considered. It is considered that both number of cycles to crack initiation and that to failure are evaluated by equivalent stress approach integrally. 2) The standard treatment speed 300mm/min of HFMI was fast more than 3 times for the standard treatment speed 100mm/min of Burr grinding by IIW guidelines. In addition, the fatigue strength improvement effect of HFMI is more than that of Burr grinding and the thickness effect coefficient of HFMI was also improved more than that of Burr grinding. The cost-effectiveness was the highest. 3) The fatigue strength improvement effect of Shot peening was insufficient. In this study, it was less than the requirement value in the case of Burr grinding in IIW guidelines in the plate thickness 40mm or more. 4) The compressive residual stress in the load carrying member is contributed to not only improvement of crack initiation life N i but also improvement of crack propagation life N f -N i ; the thickness effect on HFMI is gentler than that on the as-welded condition. 5) As a result that both the improvement effect on the radius of weld toe and the effect of the compressive residual stress addition are combined, the thickness effect on Shot peening is steeper than that on the as-welded condition. 6) In the case of Burr grinding, the similar tendency in the aswelded condition is confirmed. It is not clear that the thickness effect on Burr grinding is a little steeper than that on the as-welded condition in the test result in this study. 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