Fatigue Initiation Life Research Articles

New crack stop hole shape using structural optimizing technique

Abstract In the present study, the stop-hole method was investigated with creating a noncircular hole. The aim of the present work was to obtain an optimum stop hole shape that gives maximum fatigue crack initiation life by using finite element program. It was found that the fatigue life obtained by using the optimum hole shape ranges from 2 to 9 times the fatigue life obtained by using the circular holes. It was found that the optimum hole shape increased the initial fatigue life for all specimens used. Opposite to the traditional stress concentration factor minimization problem where the nominal area remains constant during optimization, here it is allowed to vary nominal area using design variables resulting in decreasing of the nominal stress in addition to decreasing of the stress concentration factor. This leads to higher fatigue life compared to previous studies.

Software for Optimizing Stop Hole Shape

The stop-hole method is investigated previously with creating a non-circular hole. That hole defines as an optimum stop hole shape that gives maximum initial fatigue life. Optimum stop hole shape decreases stress concentration without increasing the nominal stress, by using optimization technique. The effect of this optimum hole shape on crack reinitiating life for different model geometries is studied. The aim of this work is to introduce a software program to make it easy for everybody to determine optimum shape of stop hole suitable for user’s geometry. This program is called OHS (Optimum Hole Shape). The input of that program will be only the specimen width, length, and crack length in addition to applied load. Then OHS outputs global optimum hole shape parameters. The optimum hole shape for different models are saved as database for OHS program. OHS can be linked directly with EDM machines to create optimum hole with one click.

An investigation of contact stresses and crack initiation in spur gears based on finite element dynamics analysis

Contact fatigue, one of the main failure modes of gear tooth flanks, is caused by repeated compression and shear stress cycles. In this study, the surface and subsurface stresses of gear teeth are investigated using Hertzian theory and the finite element method . The number of loading cycles required for fatigue crack initiation is predicted using the Smith–Watson–Topper method based on the multiaxial fatigue mechanism. The effects of friction and speed on stress cycles and fatigue life are studied. Friction is found to shift the distributions of von Mises stress, change the extreme values of the shear stress cycle, and results in low fatigue life. The stresses near the engagement and recess areas are also found to be greater than the static contact conditions and thus result in low fatigue life, particularly at high speeds. The tip relief of the teeth is introduced to decrease the stresses on these points and improve their initiation fatigue life. • The surface and subsurface stresses of gear teeth are investigated using Hertzian theory and finite element method. • Finite element dynamics analysis is used to obtain the realistic description of a stress cyclic loading of a material point in meshing gears. • The effects of friction and speeds on stress cycles and fatigue lives of gears are studied.

The behavior of welded dissimilar metals structures under rotating reversed cyclic bending

A cantilever type rotating bending testing machine was used to test several welded specimens, these consisted of welding of steel ST60 to stainless steel SS304, and welding of steel ST60 to stainless steel SS316, using different filler types of stainless steel SS308, SS309 and SS310. The effect on the heat affected zone (HAZ) was investigated. Fracture surface examinations were also carried out. The numerical analysis adopted the finite element package ABAQUS® to model the welded specimens. Elasto-plastic non-linear analysis was conducted for stress distribution contours. von Mises stresses were obtained as well as crack initiation fatigue life. Both numerical and experimental studies were carried out at four different levels of applied stress from 0.6Sy to 1.2Sy at steps of 0.2Sy. The developed maximum stresses exhibited linear behavior followed by non-linear behavior with further increase of applied stress. The filler type had a pronounced effect on the developed stress such that higher stresses were obtained for filler SS308 and lower stresses for SS309 and for SS310, respectively. The crack initiation fatigue life decreased non-linearly with increasing the applied stress and it was greatly affected by the type of filler material; filler SS310 had higher fatigue life cycles than SS309 while SS308 had the lowest fatigue life cycles. The crack initiation fatigue life (Ni) and failure life (Nf) exponentially decreased with the increase in the applied stress. The experimental and numerical results were in good agreement. The SS310 filler, which has the highest Cr%, Mn% and Ni% is the best filler for welding ST60 to SS304 and also for welding ST60 to SS304, and it is recommended for such welding applications.

Numerical simulation of fatigue initiation life for notched specimens with gradient surface layer

Based on Tanaka Mura's fatigue model and introduced the elastic strain energy release of the crack initiation, a new fatigue model is proposed for fatigue initiation life under complex loading. Then, with the gradient characteristics of the hardness and residual stress, this new model is used to assess the fatigue life distribution and the crack initiation site for the notched components with a gradient surface layer. The results of numerical simulation indicate that surface hardening treatment increases the fatigue life. And the gradient layer thickness changes the crack initiation site. If the gradient layer thickness is smaller than a critical value, fatigue crack will initiate at the interface of surface and matrix, otherwise at the surface or subsurface. The increase of hardness ratio will lead to an increase of the critical thickness value. Excessive residual compress stress reduces fatigue initiation life. For notched samples with the same stress concentration factor, after the same hardening treatment, the fatigue life and crack initiation site would change with the notch size of the sample.

OS2138 SUS304鋼の高温多軸疲労におけるき裂進展の定量的評価とき裂発生・破断寿命に関する検討

OS2138 SUS304鋼の高温多軸疲労におけるき裂進展の定量的評価とき裂発生・破断寿命に関する検討

Bending Fatigue Initiation Life Estimation of Disposable Spur Gears

A method to estimate the bending fatigue initiation life of disposable gears is proposed in this study. Disposable gears used in disposable aeronautics and space equipment undertake much higher loads than general gears. The main damage type for disposable gears is low-cycle bending fatigue failure. In the study, the finite element method is used to analyze the stress-strain state of disposable gears. The moving load model is established to simulate the real load acting on the teeth. For the purpose of analyzing the multi-axial stress state of teeth roots, the crack initiation life is calculated based on the critical plane theory. As the basis parameter of the critical plane theory, the orientation of critical plane is determined by using maximum variance method. The comparison of the simulation results and the experiment data shows that it is security to predict the bending fatigue crack initiation life of disposable gears using the method provided in the study.

Static and fatigue failure analysis of adhesively bonded thick composite single lap joints

Static and fatigue behavior of thick composite laminate single lap joints with thick adhesive bondline is experimentally and numerically investigated. Stress based analysis is used to understand the uniaxial tensile behavior of thick glass fiber reinforced epoxy laminates bonded with relatively thick epoxy adhesive. The effect of adhesive thickness on relatively thick adherend on the fatigue initiation life was modeled with generalized stress singularity approach. It was observed that the failure onset always occurred at adherend–adhesive interface. Subsequent crack propagation mostly resulted in interlaminar failure of adjacent first angle ply near to this interface. Both static and fatigue strength values decrease with increase of bondline thickness. Evaluation of generalized stress intensity factors of the corner geometry (square edge joint) provided a good correlation between predictions and experimental measurements.

High temperature low cycle fatigue properties of 24Cr ferritic stainless steel for SOFC applications

The low cycle fatigue (LCF) properties of 24Cr ferritic stainless steel for solid oxide fuel cell interconnects were investigated. The fatigue strength of 24Cr stainless steel decreased with increasing temperature, but the fatigue life increased at 600°C and 700°C. The fatigue behavior at room temperature (RT) was characterized as cyclic hardening followed by saturation and cyclic softening while marginal cyclic hardening was observed at 600°C and 700°C. The superior oxidation resistance of 24Cr stainless steel allows preventing the fatal impact of oxidation on the high-temperature fatigue life. Microstructural analysis showed that persistent slip bands (PSBs) developed prevalently at RT but not at 600°C and 700°C. Such temperature-dependent microstructural difference retarded the crack initiation and prolonged fatigue life at high temperatures.

Residual Fatigue Life Analysis of Submarine Pressure Structure Based on Probabilistic Fracture Mechanics

The residual fatigue life of a submarine pressure structure is investigated, based on the combination between the methods of conventional Monte Carlo and classical probabilistic fracture mechanics. Firstly, Monte Carlo method is employed to obtain the reliability of given initial fatigue life. Secondly, the two induced factors MA1 and MA2 in the paper are estimated according to the initial fatigue life and the reliability. Thirdly, based on the two factors, the residual fatigue life based on other reliability is obtained by using classical probabilistic fracture mechanics method. Numerical examples show that the proposed method is more efficient without accuracy loss for residual fatigue life compared with Monte Carlo method. This method can also be employed to predict the residual fatigue life on other analogue structures.

도시철도 차륜의 접촉피로 초기수명 평가

Rolling contact fatigue of an urban railway wheel was analysed during its rolling. A FEM analysis was performed using a 3D modelling of rail and wheel, considering the slope of the rail and nonlinear isotropic and kinematic hardening behavior of the rail and the wheel. The maximum von-Mises stress and contact pressure between the rail and wheel were 656.9 MPa and 1111.4 MPa, respectively, under axial load of 85 kN with friction coefficient of 0. The fatigue initiation life prediction relationships by strain-lifetime (<TEX>${\varepsilon}$</TEX>-N) and Smith-Watson-Topper method were drawn for the wheel steel as follows: <TEX>$N_i=7.35{\times}10^6{\times}SWT^{-3.56}$</TEX> and <TEX>$N_i=5.41{\times}10^{-9}{\times}(\frac{{\Delta}{\varepsilon}}{2})^{-5.77}$</TEX>. The fatigue lifetimes of the wheel due to rolling contact were determined to be infinite by <TEX>${\varepsilon}$</TEX>-N and SWT methods.

Corrosion fatigue initiation and short crack growth behaviour of austenitic stainless steels under light water reactor conditions

The corrosion fatigue initiation and short crack growth behaviour of different wrought low-carbon and stabilised austenitic stainless steels was characterised under simulated boiling water reactor and pressurised water reactor primary water conditions by cyclic fatigue tests with sharply notched fracture mechanics specimens. The special emphasis was placed to the behaviour at low corrosion potentials and, in particular, to hydrogen water chemistry conditions.The major parameter effects and critical conjoint threshold conditions, which result in relevant environmental reduction and acceleration of fatigue initiation life and subsequent short crack growth, respectively, are discussed and summarised. The observed corrosion fatigue behaviour is compared with the fatigue evaluation procedures in codes and regulatory guidelines.

Fatigue Life Reliability Analysis of Submarine Cone-Cylinder Shell

A new method is presented in the paper. The fatigue life reliability of submarine cone-cylinder shell is investigated, based on the combination between the methods of conventional Monte Carlo and classical probabilistic fracture mechanics. Firstly, Monte Carlo method is employed to obtain the reliability of given initial fatigue life. Secondly, the two induced factors M1 and M2 in the paper are estimated according to the initial fatigue life and the reliability. Thirdly, based on the two factors, the other fatigue life reliability is obtained by using classical probabilistic fracture mechanics method. Finally, numerical cases show that the proposed method is more efficient without accuracy loss for fatigue life reliability compared with Monte Carlo method. This method can also be applied to predict the fatigue life reliability of analogue structures.

Temperature and Frequency Modified Fatigue Initiation Life of Solder Alloys Predicted by Endochronic Cyclic Damage-Coupled Viscoplasticity

ABSTRACTIn this paper, an evolution equation of cyclically internal damage in the intrinsic damage time scale after the threshold cycles N0 was extended by employing its damage parameters proposed to be dependent of frequency (v) and temperature (T) under cyclic fatigue loading. The resulting damage-coupled endochronic viscoplasticity can drive v and T modified power form equations of cyclic damage and its fatigue initiation life = N1 + N0. Under fatigue tests with T effect and N0 = 0, the power form equation of N1(T)/(Th), named as T-LCM (T modified Lee Coffin-Manson) equation for fatigue initiation life can bederived. The T modified factor (Th) depends on the T dependent material elastic modulus, the cyclicstress-strain response and the damage parameters. Theoretical predictions in the life data ofSn/3.8Ag/0.7Cu solder alloy under cyclic strain test with Tϵ [298,393] K were very well.Also under fatigue tests with v effect only, the power form equation of /v-LCM (v modified Lee-Coffin-Manson) equation for fatigue initiation life can be derived. The v modified parameter depends on the v dependent cyclic stress-strain response and the damage parameters. Theoreticalpredictions in the life data of 96.5Sn/3.5Ag solder alloy with surface cracking effect i.e. N0 ≠ 0 during cyclicstrain tests with v ϵ [0.001,1] Hz were quite well.Obviously, the values of power exponents C in the T-LCM and the v-LCM equations can not be determinedsimply by the least square method as in the Coffin-Manson empirical formulae. Also, they must bejustified by constrains imposed in the material parameters defining in the cyclic stress-strain response andthe accumulation behavior of cyclic damage.The resultant equations derived here and the Δ-LCM equation derived under Δ angle proportional cyclicstrain tests can be combined together to form a T-v-ΔLCM equation for fatigue life studies in the solderalloys using bulk specimens or BGA solider joint specimens.

Endochronic Fatigue Life Prediction of SN/3.5AG/0.75CU BGA Solder Joints Under Oblique Displacement Cyclic Tests

ABSTRACTIn this paper, cyclic damage behavior of cyclically load drop curves and their fatigue initiation life of Sn/3.5 Ag/0.75Cu BGA solder joint specimens under oblique displacement cyclic tests were investigated by the theory of damage — coupled endochronic viscoplasticity.By linearly unloading with damage elastic modulus and the linearly damage-free behavior of grip system, the damage loops of force-Φ angle oblique displacement of BGA solder joint specimen were converted successfully into damage loops of the representative solder ball under cyclically proportional straining, which can be predicted by the endochronic constitutive equations. These results established the relationship of the BGA oblique displacement amplitudes da(Φ) and the effective inelastic strain amplitudes of solder ball: da (Φ)= . Based on the phenomena of cyclic damage and its fatigue life, a Φ dependent degree of damage in the evolution equation of damage under proportional strain path was proposed to depend positively on and N cycles. Using this parameter in the damage per cycle computed by the endochronic theory, a Φ modified cycles N(Φ)/β(Φ) can be defined and then derive the Φ modified Lee-Coffin-Manson (Φ-LCM) equation for the fatigue initiation life of solder ball:Finally, a Φ modified Lee's BGA (Φ LBGA) equation for BGA solder joint specimens can be derived:This equation can predict quite well the life data of Sn/3.5Ag/0.75Cu solder joint specimens under Φ ϵ [0π/2]. As a consequence, a vehicle to study the fatigue initiation life of BGA solder joint specimens is constructed completely by the workable methodology and the theory discussed in the paper.

Influence of the contact pressure on rolling contact fatigue initiation of 1070 steel

Rolling contact fatigue initiation life of 1070 steel was predicted by the FE method with an advanced cyclic stress-strain plasticity model and a general multiaxial fatigue criterion. The 2D repeated rolling contact processes were carried out by translating the normal pressure and the tangential traction across the contact surface step by step. The normal pressure and the tangential traction were applied to the nodes through the time-dependant amplitude functions as the concentrated nodal forces. With the detailed stress-strain responses outputted from the FE analysis after 20 rolling passes, the multiaxial fatigue criterion was used in the prediction of the initiation life. Several cases were simulated in this work. The P0/k ratio ranges from 6.0, 7.0 to 8.0, where P0 the maximum Hertzian pressure, k is the yield stress in shear of the material. Comparing these cases, the initiation life was reduced with the increase of the P0/k ratio, and crack initiation tends to subsurface.

Fatigue crack initiation life of fine grain brass H62

Fine grain alloys possess excellent properties entailing high strength and toughness. Fine brass H62 is made by re-crystallization with grain size ranging from 5 to 10 μm. Fatigue initiation life is investigated from specimens tested on Instron 1341 machine a frequency of 30 Hz. Furthermore fatigue crack initiation life of this fine brass H62 is predicted by the energetic approach. It is found that: (1) the finer the grain size, the longer the fatigue life, which is due to its higher toughness; (2) intergranular cracking is the main mechanism of fatigue failure. Concave pits were found in the zone of fatigue crack propagation; and (3) the energetic approach gave acceptable fatigue crack initiation life estimation.

Fatigue initiation behaviour throughout friction stir welded joints in AA2024-T3

This paper presents the results of an experimental and numerical investigation on the fatigue crack initiation behaviour of friction stir welded joints in AA2024-T3. These welds exhibit a complex material behaviour. In this study the influence of the different weld zones on the fatigue crack initiation behaviour was investigated. Therefore, fatigue experiments were conducted on a variety of welded central notched specimen configurations. As a result, the fatigue initiation life as function of the location in and the orientation of the weld was obtained. Numerical analyses were performed to investigate the influence of residual stress and yield strength on the stresses at the notch.

Insights beside assessments into small volume cyclic response of elastic plastic systems

In contrast to monotonic load studies, the engagement with a cyclic time dependent processes of small volume material response has been restrained. Crack tolerance in thin films almost by definition is restricted. Nevertheless, in terms of structural stability the fatigue resistance remains a critical concern regardless the scale. The cyclic displacement amplitudes can be enhanced by remote mechanical stress/strain or thermal origins resulted in similar outcome of cumulative irreversible micro-plasticity damage. Thus, the cyclic life in nano applications is mainly dominated by the crack nucleation controlled process. The current study is centered on the fatigue crack initiation stage as a first order design pillar and therefore require additional experimental input. Activities in elastic-plastic polycrystalline and single crystal systems in various crystal structures are described. Beside findings regarding the fatigue cracks initiation life, the role of compressive residual stress on the cyclic response has been also analyzed. The fatigue crack initiation criteria have been associated to other constitutive assessments that are related mainly to the crack stability equation elements. In addition, completely different insights are introduced into the concept of the critical film thickness, indicating the susceptibility to the crack formation. As addressed in heteroepitaxial layers, specific analogous characterization exists either in dislocation activities or by triggering crack formation. The physical aspects of threading dislocations growth and glide behavior and local decohesion ramifications are highly dependent on the layer thickness. Experimentally based, under strain control conditions at ambient temperatures the aforementioned fatigue crack initiation stage in small volume segments, obeyed a cumulative damage mechanism. It was also established that the low energy dislocation structures and the initiation life were dramatically dependent on the strain amplitude range. The role of compressive residual stress in prolonging the fatigue initiation life was also consistent. Even in silicon based materials, dislocation activities prevailed. It became apparent that categorical generalization in nano-silicon thin film fatigue behavior is beyond the susceptibility to the native oxide or environmental interaction. Issues like film processing methodology, film/substrate interfaces, residual stresses and micro-cracking feasibility occurrence as wall as critical sites still require further design insights.

Fatigue Failure of Extrusion Dies: Effect of Process Parameters and Design Features on Die Life

Analysis of die failure plays an important role in the prediction and prevention of die failure, and subsequently in improving economics of any metal-forming process. Industrial experience has shown that fracture is the most common mode of failure in the case of hot aluminum extrusion dies. The purpose of the present work is to implement fatigue damage models in a Finite Element code for identification of critical process parameters and die design features in the case of fatigue being the dominating failure mode. For the maximum number of billets extruded by the die before fatigue crack initiation (fatigue life cycles in extrusion), Morrow’s stress- and strain-life damage models are implemented for axisymmetric flat extrusion die. With the help of finite element software ABAQUS, extrusion process is simulated and dynamic stress and strain values were obtained by first identifying the potential fatigue location in the die. The evaluation of applicability of the damage models is done for specific hot extrusion die made of H13 steels with Al-6063 as billet material. By considering temperature and strain rate as process parameters and bearing length and fillet radius of the die as geometric features, different simulation runs are performed to investigate the effect of process and design features on the useful die life. Morrow’s stress life model shows a good correspondence between computed and actual failure of dies. By establishing correlations of die life with process and design parameters under different conditions, it was shown that the present investigation is a useful guideline at die design and extrusion process stages.