Fatigue Analysis of Welded Joints

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1 ISSN Fatigue Analysis of Welded Joints #1 Tabassum Karajagi, #2 Nitin Ambhore 1 tabskar@gmail.com 2 nitin.ambhore@gmail.com #1 Alard College of Engineering and Management, Pune, India #2 Vishwakarma institute of Information Technology, Pune, India ABSTRACT The present work is concerned with an attempt to predict the fatigue strength of welded joints by means of a fracture mechanics approach that takes into account the fatigue behaviour of short cracks. The methodology evaluates the fatigue crack propagation rate as a function of the difference between the applied driving force and the material threshold for crack propagation, a function of crack length. Analytical fatigue models were developed to estimate the fatigue behaviour of notched plates. Analyses are performed on a cruciform fillet welded joint where parameters such as, load variation, plate thickness and weld size, are varied. The achieved results are then compared to calculations with other fatigue assessment methods, fracture mechanics approach and the nominal method. The results indicate that the theoretical methods or FEA results tends to overestimate the lifespan, especially when the plate thickness is small. This leads to non-promising results that are dangerous to use as guidance when designing. Work done here is by using three different approaches to predict accurate fatigue life. Hence we are combining the results from analytical, theoretical and FEA analysis to predict the useful life for the component. Keywords crack propagation, fatigue, FEA (Finite Element Analysis), fracture mechanics approach. ARTICLE INFO Article History Received :18 th November 2015 Received in revised form : 19 th November 2015 Accepted : 21 st November, 2015 Published online : 22 nd November 2015 I. INTRODUCTION Fatigue life prediction of welded joints is a very complicated process which in spite of lot of research is still not understood till today. As there are joints i.e. change of area exists which also affects its life. Addition of filler material also changes the material composition of the assembly. In addition to abrupt change in area pores, inclusions, cavities etc. also accounts in predicting the life of welded components. Residual stresses and distortions resulting because of welding process hampers also fatigue behaviour. As a result of added effect of these parameters, fatigue failures appear in welded structure mostly in the welded area than in the base metal. In engineering applications even small gaps, flaws etc. can eliminate the fatigue crack initiation. But not with only crack generation in welded joints, its fatigue life can be predicted but crack propagation plays a very important role in it [1]. A lot of literature is presented in this regard as propagation of crack is dependent of various parameters like complexity of joint, loading conditions etc. Improper knowledge of fatigue actions is one of the major sources of fatigue damage. Hence fatigue analysis of welded joint becomes of high interest in areas where the part is subjected to repeated loading. Due to its wide area of application and keeping in mind its complexity, it is not surprising that different methods have been used for fatigue analysis of welded joints. Among all of them for comparison, Nominal stress method, Hot spot stretch method, Effective notch method are some of the popular theoretical methods. We are considering only effective notch method among them for theoretical calculations. It has been seen that effective notch method sometimes tends overestimating the fatigue life of components, which sometimes proves dangerous for the 2015, IERJ All Rights Reserved Page 1

2 designer. Hence it can be seen that it s not a good practice to relay on a single method for the fatigue analysis of welded joints. Hence the combination of analytical, theoretical and FEA results are considered together so as to get reliable life prediction for a welded joint. II. PHASES OF FATIGUE FAILURE Fig3 Example of steel structure where cruciform weld is used Fig.1 Behaviour of fatigue cracks in welded joints. (a) Initiation of cracks, (b) Propagation and coalescence of surface cracks, (c) Propagation of a through-the thickness crack [2] The fatigue failure occurring in any material is undergoing in the steps shown in fig. 1. Crack Initiation- the welded structure which is undergoing repetitive loading, small crack generation starts in it at comparatively weak areas such as welded joints. As we considering fatigue failure for welded joints, the applications where it comes into picture are viz., automobile chassis, bridge structures, and other automobile applications s railcars, ships etc.[3-4] In most of the applications listed here the material widely used is M.S. ; due its strength, durability and other reliable properties. Thus the material chosen for the specimen is M.S. (40C8). Model which is used in the study are 3 plates. One of which is 100 x 5mm, and other two are of 50 x 5 mm respectively. The three plates are welded to each other with oxy acetylin electric arc welding. A. CAD Modelling: Crack Propagation/growth-the small crack generated goes on increasing as the specimen is going through the load cycle again and again. Final breakage- the crack has grown so much that the specimen can t withstand the load anymore and immediate failure occurs. III. METHODOLY It has been seen that the use of cruciform welds finds a wide area of application in practical use. So in this study all the tests have been done on the cruciform weld [4]. The examples of some of the practical applications are shown below. Fig.2 Example of chassis where cruciform weld is used Fig.3 Test model generated using CATIAV5 The welded joint is the critical area in this design. Hence analysis is done with the help of Ansys software for this area. B. Ansys model preparation: For doing analysis for any part in Ansys, it is necessary to first do the meshing for the part being analysed. Meshing is the process of dividing the given pat in small elements compatible for computation as per Ansys program. The type of meshing used for the model is square mesh. The model contains 2445 nodes and 309 elements. The meshed model is as shown below. 2015, IERJ All Rights Reserved Page 2

3 Fig.4 Meshed model for test specimen C. Analytical result calculation: The test for physical model is done. In this the machine is having 2800RPM. Here step by step loading is done for the specimen for load of 10 N for the span of 1minute. After doing so It has been observed that crack initiation starts at 154sec. So for 1 sec approximately 47 cycles are done. Hence for 154 sec- 154*47= 7238cycles Then again the test is carried out for 4 hours with the interval of 1 hour in between. So if we consider total time it s around 250 minutes. 250 x 2800 = cycles. We can say that the practical number of cycles which the specimen carries in 0.7*10^6 cycles. After this much cycles the weld breaks down as seen in below image. Fig.6 Loading Diagram for specimen It has been seen that after applying the load of 10 N repeatedly. The load cycles in Ansys can been seen as below. Fig.7 Constant amplitude load fully reversed As we discussed earlier for analysis main focus is on weld toe. As we can see in below image the maximum stress can be seen in this region only. The maximum number of cycles which Ansys gives for the specimen are 1 x 10^6cycles. Loading diagram obtained in Ansys is as shown below Fig.5 weld failure at cracked area D. FEA result calculations: The loading diagram is as shown below for the load of 10N. Fig.8 Ansys results for the specimen A. Theoretical result calculations by Effective Notch Method: The notch stress is the total stress at the root of a notch taking into account the stress concentration caused by the local notch, consisting of the sum of geometrical stress and nonlinear stress peak. When using the effective notch 2015, IERJ All Rights Reserved Page 3

4 method a linear-elastic material behaviour is assumed. Then sharp edges in the weld toe and weld root will cause discontinuities in the equations. Therefore the actual radius is replaced by an effective notch root radius. For structural steel, Hobbacher recommends to use a radius of ρ = 1 mm which has been verified to give consistent results, see Figure 7. For fatigue assessment, the effective notch stress is compared with a single fatigue resistance curve. FAT = 225 is recommended by IIW but this has later been questioned and suggested to be lowered for special cases of joints and geometries. Fig.10 S-N curve with 50% failure probability The results from tests are usually obtained with a great dispersion. When many tests have been performed the S-N curve can be established for a certain failure probability. The S-N curve can be assumed to be a straight line in a loglog-plot, described by the equation below. σ r = C.... (3) I. Fig.9 Modified specimen for effective notch method where notches at weld toe and weld root are fictitiously rounded[8]. When using the effective notch method one often refers to the stress concentration factor Kt. This is defined as the ratio of the maximal notch stress and the nominal stress. Kt=... (1) The theory about a fictitious radius has its origin from the micro structural support effect. The actual radius is being increased by the material dependent macrostructural length, ρ*, multiplied by the support factor, s. The effective radius, ρ f, to be used in the model is given by: ρ f = ρ + s ρ*... (2) Where ρ is the actual radius of the notch, see Figure 9. When doing a calculation of the stress concentration close to a notch you often assume that the material is elastic. In fact you will get a plasticizing occurs in the micro structural length. Due to this the actual stress is smaller than the one obtained with a complete elastic model. By increasing the radius of the notch this effect is simulated and the stress concentration is lowered. Fig.9 the actual notch is substituted by a fictitious notch radius to obtain a stress concentration that includes the micro structural support effect It states that the conservative estimates, ρ* = 0.4 mm, s = 2.5 and ρ = 0 mm have proved to be realistic for welded joints in structural steel. This means, a fictitious effective radius of 1 mm. Where C and N0 are constants representing stress range and cycles to failure at a certain point on the curve. m is for welded joints often approximately 3. During the years, a lot of welded joints have been tested for fatigue failure. The results from these tests have been put together for different kinds of joints and many different classification systems and standards have been set up. International Institute of welding (IIW) uses different FATclasses (fatigue classes) and the BSK 99 standard uses a C- value to classify the joints [1][5]. Both of these standards define the classes from the stress range in MPa Then this joint is in the class FAT = x for IIW, or C = x for BSK 99. Equation (4) can then for a certain FAT-class be written as σr= 2.54 Mpa FAT = 95% σ r = FAT.... (4) 2.54 = 0.95 x (2x10^6/N) ^1/3 N = N = 1.4 x 10^6 Where, N = number of cycles to failure occur The number of cycles which are given by effective notch method are much more than that shown by analytical and FEA analysis. IV. II. III. IV. CONCLUSIONS It has been seen that as fatigue is a very complex method and its analysis is very important so as to define the life for welded joints. As seen in different results by different approaches, one can t be able to define useful life by suing a single method as their basis. It has been advised that the life prediction for welded joints should not depend on any one method such as analytical, theoretical or FEA results. But a combination of any two or more methods should be done in order to predict the useful life accurately. REFERENCES 2015, IERJ All Rights Reserved Page 4

5 [1] A.Al Mukhtar, H. Biermann, P.Hübner, S,Henkel, Fatigue Crack Propagation Life Calculation in Welded Joints,Institute of materials Engineering.technische Universität Bergakadamie Freiberg,pp [2] T. Lassen, ph. Darcis, and N. Recho,(December 2005) Fatigue behavior of welded joints part 1 Statistical methods for fatigue life prediction supplement to the welding journal, Sponsored by the American Welding Society and the Welding Research Council,pp [3] Teppei Okawa, Hiroshi Shimanuki, Tetsuro Nose, Tamaki Suzuki,(2013) Fatigue life prediction of welded structures based on crack growth analysis, Nippon steel technical report no. 102, pp.53-55s. [4] Wolfgang Fricke, Fatigue analysis of welded joints: state of development, Marine Structures 16 (2003) , Technical University Hamburg-Harburg, AB 3-06, Lammersieth 90, D Hamburg, Germany Received 21 January 2002; accepted 7 October 2002,pp [5] A.Chattopadhyay, G. Glinka, M. El-zein, J. Qian and R.Formas, Stress analysis and fatigue of welded structures, Welding in the world Peer Reviewed Section, Vol 55,pp [6] C.Acevedo, A. Nussbaumer, Residual stress estimation of welded tubular k-joints under fatigue loads, Swiss Federal Institute of Technology, Lausanne, Switzerland, pp 1-4. [7] Mustafa Aygul,(2012), Fatigue analysis of welded structures using the finite element method, Department of Civil and Environmental Engineering Division of Structural Engineering, Steel and Timber -Structures Chalmers University of Technology, Gothenburg, Sweden, ISSN no ,pp [8] A. Hobbacher, (October 2008), International institute of welding, A world of joining Experience IIW commissions XIII and XV,IIW. document IIW ex XIII-2151r4-07/XV-1254r4-07, pp [9] C.M.Branco,V.Infante,and R. Baptista,(October 2004),Fatigue Fract Engg mater Struct 27,,pp , IERJ All Rights Reserved Page 5