Fatigue life prediction of welded cruciform joints using strain energy density factor approach

The influences of two welding processes, namely, shielded metal arc welding (SMAW) and flux cored arc welding (FCAW), on fatigue life of cruciform joints containing lack of penetration (LOP) defects have been analyzed by using the strain energy density factor (SEDF) approach. Load carrying cruciform joints were fabricated from ASTM 517 ‘F’ grade steel. Fatigue crack growth experiments were carried out in a mechanical resonance vertical pulsator (SCHENCK 200 kN capacity) with a frequency of 30 Hz under constant amplitude loading (R=0). It was found that the fatigue lives of the cruciform joints fabricated by SMAW process were relatively higher than the FCAW counterpart. Moreover, fracture mechanics equations have been developed to predict the fatigue life of the cruciform joints fabricated by the above-mentioned two processes.     

Effect of welding process on fatigue crack growth behaviour of austenitic stainless steel welds in a low alloy (Q & T) steel

Fatigue crack growth behaviour from a lack of penetration (LOP) defect in austenitic stainless steel weld metals of cruciform joints made of a low alloy high strength (Q & T) steel has been studied to understand the effect of two welding processes, namely, shielded metal arc welding (SMAW) and flux cored arc welding (FCAW). Fatigue crack growth studies were carried out at a stress ratio of R = 0 and a frequency of 90 to 110 Hz in a resonant testing equipment (Rumul, Model:8601). Crack growth rates were relatively lower in the weld metal obtained by flux cored arc welding process. Microstructural features observed revealed marked difference in the morphology of delta ferrite for the welded joints obtained from the above two welding processes. Long streaks of delta ferrite in austenite matrix were found in case of SMAW-weld metal which seem to have lowered the resistance to the fatigue crack propagation. A discontinuous network of delta ferrite found in austenite matrix in the case of FCAW-weld metal seems to have contributed to slower propagation of fatigue crack. Fractographic features also substantiate the observed trends in the fatigue crack growth behaviour.     

Constant amplitude and variable load history fatigue test results and predictions for cruciform and lap welds

Cruciform and lap welds were fatigue tested under constant amplitude axial load and SAE Bracket spectrum load conditions. For the cruciform joints, fatigue cracks generally initiate at the root but may initiate at the toe if higher bending stresses are induced by joint distortion. For lap welds, the stress ratio ($\text{R}$) and weld shape are the major factors influencing the fatigue crack initiation site.The fatigue test results were compared with predictions made using an analytical model developed by the authors, and good agreement between experiment and theory was observed. The model for the predictions assumes that the fatigue crack initiation period, which is the number of cycles for the initiation of a fatigue crack and its early growth and coalescence into a dominant fatigue crack, is the main portion of the total fatigue life at long lives.     

Influence of seawater and residual stresses on fatigue crack growth in C---Mn steel weld joints

Results are presented on fatigue crack growth of weld joints made of C---Mn structural steel plates in both air and seawater. Tests were conducted to identify the behavior at different locations of the joints as the frequency, R-ratio and electrochemical potential are varied. Residual stresses in specimens with welds are evaluated to analyse the fatigue crack growth behavior. Satisfactory predictions are obtained by accounting for residual stresses and crack closure.     

Fatigue life estimate of C---Mn steel welded cruciform joints

The fatigue life of a manual metal arc welded cruciform joint failing from a root lack of the penetration region is estimated by the application of crack growth relations. A two-parameter relation was used. The initiation life and propagation life of the joint were taken into account to obtain the total fatigue life of the joint from the crack growth parameters. To test the accuracy of the method, the predicted data was compared with the experimental data for a C---Mn type steel welded joints. The results were in good agreement with the experimental data.     

Fatigue crack shapes development of plate-to-plate welded joints

This paper presents crack shapes development of 3D surface semielliptical cracks which are always found at the weld toe of non-load-carrying cruciform welded joints. The alternating current potential drop technique has been used to measure the eight crack depths along the weld toe where probes are placed at 10-mm intervals. 3D crack shapes can be obtained at any particular cycle during the fatigue test. Subregion boundary element methods incorporating the transition and quarter-point elements along the crack front are used to validate the experimental stress intensity factors along the crack front. A method for evaluating the effective stress intensity factors is proposed. It is found that the numerical results generally agree with the experimental results.     

T应力对立方准晶中含十字交叉型裂纹的影响

论文研究了立方准晶中含十字交叉型裂纹尖端附近的T应力.采用了积分变换法,将相关边值问题转换为求解奇异积分方程,得到声子场和相位子场的T应力,且可以表示为裂纹张开位移和施加在无穷远处的载荷之和.裂纹张开位移在确定声子场和相位子场的T应力中起着主导作用.同时强调了T应力在立方准晶脆性断裂中的作用.此外,还研究了裂纹臂长度比...     

Fatigue life prediction of out-of-plane gusset welded joints using strain energy density factor approach

Fatigue growth behavior of out-of-plane gusset welded joints is studied using the strain energy density factor approach. Fatigue tests on two types of specimens with curvatures of ρ = 0 and ρ = 30 were performed in order to estimate fatigue strength under tension. Fatigue crack growth analysis is carried out to show the effects of initial crack shape, initial crack length and stress ratio. Fatigue crack growth parameters were obtained from crack growth curves assuming constant crack shapes. The results of analysis for the assumed crack shapes agreed well with the experimental data. Fatigue propagation life of the ρ = 30 specimen was larger than that of the ρ = 0 specimen.     

An ultrasonic technique for nondestructive testing of butt-welded steel plate in ship hulls

This paper proposes an ultrasonic nondestructive testing (NDT) method for underwater-diver inspection of shiphull welds. The specimens used were prepared with various induced cracks in the vicinity of the welds. The type of flow considered is a transverse crack in the heat-affected zone in a butt-welded seam. An automated data analysis is developed to scan, locate and characterize cracks using the pulse-echo technique with a single focus transducer. Specimens and probes were submerged in sea water as an additional exercise in simulating operating conditions. Calibration of the NDT characterization was established by first heat tinting to register the size of the induced crack and subsequently to load the specimen to failure so as to examine the crack. The results show that the actual crack length and the maximum load to failure for each cracked specimen were comparable with the test data obtained from the NDT. presently at mechanical Engineering Department, King Abdulaziz University, P.O. Box 9027, Jeddah 21413, Saundi Arabia.     

Magnetic pulse welding in plane geometry

The results of a study of the feasibility of magnetic pulse welding of flat conductors are given. Various inductor circuits and sheet configurations are investigated experimentally. An optimized inductor circuit for accelerating flat conductors by a magnetic field is presented and the necessary diagnostic equipment is developed. Results of experiments on the acceleration of flat conductors and production of aluminum-aluminum and aluminum-steel welded joints are presented. The characteristics of the welds are investigated.     

Allowable crack sizes for railway wheels and rails

Determined are the critical crzes and crack growth characteristics for train car wheels and rails. Service and test limit size of cracks need to be distinguished in view of the difference between subcritical and the onset of rapid crack propagation. Probabilistic calculations have many advantages, but they cannot accurately predict neither the real critical minimum crack size (a_c)_min nor the real maximum crack growth increment Δa_max. As representative values of the minimum critical crack size a_c^* and maximum crack growth increment Δa^* are chosen for the calculations, therefore, that values which have a survival and crack growth probability, respectively, of 90%. Safety factors S(a) and are needed to account for the scatter of a_c and Δa for probabilities of more than 90%. Probabilistic fracture mechanics is applied to analyze the behavior of transverse cracks in the rim of tread-braked monobloc wheels, transverse head cracks in rails and aluminothermic rail welds. These informations are supplied by the German State Railways (DR) the Office of Research and Experiments (ORE) of the International Union of Railways (UIC).     

Fatigue life prediction of in-plane gusset welded joints using strain energy density factor approach

Fatigue crack growth behavior of in-plane gusset welded joints is studied using the strain energy density factor approach. Fatigue tests were performed in order to estimate fatigue strength under tension. Fatigue crack growth analysis was carried out to show the effects of the initial crack shape, the initial crack length, and the stress ratio on the crack types of in-plane gusset welded joints. The assumed crack types were edge crack, semi-elliptical crack, and corner crack. Fatigue crack growth parameters were obtained from crack growth curves assuming constant crack shapes for the given crack types. The results of analysis for the assumed crack types agreed well with the experimental data. The fatigue life did not change as initial crack shape varied for a given initial crack length.     

反应堆贯穿件J形焊缝残余应力研究1）

焊接残余应力是引起应力腐蚀开裂的主要原因,准确地掌握残余应力的分布是对结构完整性评定的关键.为了探明反应堆压力容器顶盖控制棒驱动机构（control rod drive mechanism,CRDM）贯穿件J形焊缝残余应力的分布,本研究加工两个CRDM贯穿件实验模拟件,采用盲孔法测试两个J形接头的焊接残余应力.通过实验探明了CRDM贯穿件焊缝残余应力的分布规律,为反应堆压力容器结构完整性的研究提供必要参考.     

Monitoring short fatigue cracks with miniature strain gages

An experimental technique to monitor the length and the opening level of a short fatigue crack is presented. It is based on the progressive decrease with crack length of the response of miniature strain gages installed on the surface near the crack plane. A first gage installed close to the crack plane can monitor cracks from 10 μm in depth to half a millimeter where the response saturates. Other gages at larger distances from the crack plane are less sensitive but can monitor longer cracks. The response is measured so that it is independent of strain-gage calibration, Young's modulus and Poisson's ratio. The paper first presents the basic principles and possibilities of the technique as well as a finite-element analysis performed on automatic welded joints with straight-fronted cracks for which the technique has been developed. The results give a correlation between gage response, crack length and gage location and the conditions of replacement of a gage reaching saturation. The practical exploitation of the technique has required further work to derive a continuous calibration of the gage response that includes corrections to account for the gage finite dimensions and the crack-plane inclination. This calibration is shown to give crack lengths that compare well with fractographic marks and typical results that have been obtained on short crack growth at the weld toe are presented. In particular, the resolution of the technique is put into evidence with results on the initial growth of a 0.1 mm nonpropagating crack. The paper finally points out the distinctive features that appear in current works to adapt the technique to the growth of semi-elliptical cracks of low and high aspect ratio.     

Numerical simulation of debonding of adhesively bonded joint

This paper describes a numerical method to simulate the debonding of adhesively bonded joints. Assuming that the adhesive thickness and the adhesive Young’s modulus are small with respect to the characteristic length of the joint and to the Young’s modulus of the adherents, a simplified model is derived in the case of large displacements using the asymptotic expansion technique. Then, the problem of the crack growth is stated, in the case of a stable growth, as the search of the local minima of the total energy of the joint, sum of the mechanical energy and the Griffith’s fracture energy. This is made using the Newton’s method. To this end, the expressions of the first and second derivatives of the mechanical energy with respect to a crack front displacement are derived analytically. Finally, numerical examples are presented, highlighting the unstable character of the crack growth at initiation.     

Dissimilar ultrasonic spot welding of Mg-Al and Mg-high strength low alloy steel

Sound dissimilar lap joints were achieved via ultrasonic spot welding (USW), which is a solid-state joining technique. The addition of Sn interlayer during USW effectively blocked the formation of brittle al₁₂Mg₁₇ intermetallic compound in the Mg-Al dissimilar joints without interlayer, and led to the presence of a distinctive composite-like Sn and Mg₂Sn eutectic structure in both Mg-Al and Mg-high strength low alloy (HSLA) steel joints. The lap shear strength of both types of dissimilar joints with a Sn interlayer was significantly higher than that of the corresponding dissimilar joints without interlayer. Failure during the tensile lap shear tests occurred mainly in the mode of cohesive failure in the Mg-Al dissimilar joints and in the mode of partial cohesive failure and partial nugget pull-out in the Mg-HSLA steel dissimilar joints.     

Ductile–brittle transitions in the fracture of plastically-deforming,adhesively-bonded structures. Part I: Experimental studies

Rate effects for adhesively-bonded joints in steel sheets failing by mode-I fracture and plastic deformation were examined. Three types of test geometries were used to provide a range of crack velocities between 0.1 and 5000 mm/s: a DCB geometry under displacement control, a wedge geometry under displacement control, and a wedge geometry loaded under impact conditions. Two fracture modes were observed: quasi-static crack growth and dynamic crack growth. The quasi-static crack growth was associated with a toughened mode of failure; the dynamic crack growth was associated with a more brittle mode of failure. The experiments indicated that the fracture parameters for the quasi-static crack growth were rate independent, and that quasi-static crack growth could occur even at the highest crack velocities. Effects of rate appeared to be limited to the ease with which a transition to dynamic fracture could be triggered. This transition appeared to be stochastic in nature, it did not appear to be associated with the attainment of any critical value for crack velocity or loading rate. While the mode-I quasi-static fracture behavior appeared to be rate independent, an increase in the tendency for dynamic fracture to be triggered as the crack velocity increased did have the effect of decreasing the average energy dissipated during fracture at higher loading rates.     

Initiation and propagation of fatigue crack in pure aluminium single crystals

The fatigue crack initiation and propagation in the pure aluminum single crystals with different orientations are investigated. Acoustic Emission (AE) technique is used to monitor the fatigue crack initiation and propagation data. The results show that the acoustic emission technique could be used for detecting fatigue crack initiation and propagation. The total counts of AE are plotted as a function of the number of fatigue cycles, and change in slope of the curve corresponded to the initiation of the fatigue crack. The fatigue crack initiation and propagation are related to the specimen crystalline orientation. Specifically, they depend on the ratio K of the Schmid factor of the secondary slip system, to that of the primary one. The greater the ratio K, the easier the fatigue crack initiates and the faster the fatigue crack propagates. When the ratio K is equal or close to one, the fatigue crack grows either along a cleavage plane which intersects both the primary plane and the secondary one or along the primary slip plane and the secondary one. Growth along alternating planes can also occur giving rise to a zigzag path. When the ratio K is much less than one, the fatigue crack grows along the primary slip plane only. The results are explained with a model for the faceted mode of fatigue crack growth.     

Transformation toughening of a ceramic in stable crack growth under plane stress conditions

A finite element study of stable crack growth in ceramics that can undergo a stress-induced martensitic phase transformation is performed under plane stress and small scale transforming conditions. The finite element method is based on the continuum model developed by Budiansky et al. (Int. J. Solids Structures 19, 1983). To guarantee the subcritical transformation behavior without loss of ellipticity of the governing equations, the possibility of strain localization is first re-examined. It is found that the plane stress conditions greatly favor transformation instability in that supercritical transformation occurs when the bulk modulus ¯B during transformation is below –G/3, instead of –4G/3 for three-dimensional or plane strain cases, whereG is the shear modulus. Next, transient crack extension under continuously increasing tensile load is simulated by a node release technique. Transformation zones and crack growth resistance curves are obtained.     

Fatigue and fracture of bovine dentin

In this paper, the fatigue and fracture properties of bovine dentin are evaluated usingin vitro experimental analyses. Double cantilever beam (DCB) specimens were prepared from bovine maxillary molars and subjected to zeroto-tension cyclic loads. The fatigue crack growth rate was evaluated as a function of the dentin tubule orientation using the Paris law. Wedge-loaded DCB specimens were also prepared and subjected to monotonic opening loads. Moiré interferometry was used to acquire the in-plane displacement field during stable crack growth, and the instantaneous wedge load and crack length were acquired to evaluate the crack growth resistance and crack tip opening displacement (CTOD) with crack extension. The rate of fatigue crack growth was generally larger for crack propagation occurring perpendicular to the dentin tubules. The Moiré fringe fields documented during monotonic crack growth exhibited non-linear deformation occurring within a confined region adjacent to the crack tip. Both the wedge load and CTOD response provided evidence that a fracture process zone contributes to energy dissipation during crack extension and that dentin exhibits a risingR-curve behavior. Results from this preliminary investigation are being used as a guide for an evaluation of the fatigue and fracture properties of human dentin.