Experimental Fatigue Life Research Articles

Fatigue Life Evaluation of Automotive Engine Mount Insulator

Fatigue life of automotive engine mount insulator made of natural rubber was evaluated. In order to develop an appropriate fatigue damage parameter of the rubber material, a series of displacement controlled fatigue tests was conducted using 3-dimensional dumbbell specimens with different levels of mean displacement. It was shown that the maximum Green-Lagrange strain was a proper damage parameter, taking the mean displacement effects into account. Nonlinear finite element analyses of the rubber engine mount insulator and 3D dumbbell specimen were performed based on a hyper-elastic material model determined from the simple and equi-biaxial tension tests. Fatigue life prediction of the engine mount insulator was made by incorporating the maximum Green-Lagrange strain values, which was evaluated from the finite element analysis and fatigue tests, respectively. Predicted fatigue lives of the engine mount insulator showed a fairly good agreement with the experimental fatigue lives.

Calibration of Electromigration Reliability of Flip-Chip Packages by Electrothermal Coupling Analysis

Electromigration reliability of solder interconnects is dominated by current density and temperature inside the interconnects. For flip-chip packages, current densities around the regions where the traces connect a solder bump increase significantly due to the differences in feature sizes and electric resistivities between the solder bump and its adjacent traces. This current-crowding effect along with induced Joule heating accelerates electromigration failures. In this paper, the effects of current crowding and Joule heating in a flip-chip package are examined and quantified by three-dimensional electrothermal coupling analysis. We apply a volumetric averaging technique to cope with the current-crowding singularity. The volumetrically averaged current density and the maximum temperature in a solder bump are integrated into Black’s equation to calibrate the experimental electromigration fatigue lives.

Uncertainty analysis of a neural network used for fatigue lifetime prediction

The application of interval set techniques to the quantification of uncertainty in a neural network regression model of fatigue lifetime is considered. Bayesian evidence training was implemented to train a series of multi-layer perceptron networks on experimental fatigue life measurements in glass fibre composite sandwich materials. A set of independent measurements conducted 2 months after the training session, and at intermediate fatigue loading levels, was used to provide a rigorous test of the generalisation capacity of the networks. The robustness of the networks to uncertainty in the input data was investigated using an interval-based technique. It is demonstrated that the interval approach allowed for an alternative to probabilistic-based confidence bounds of prediction accuracy. In addition, the technique provided an alternative network selection tool, and also allowed for an alternative to estimating the lifetime prediction error that was found to be a significant improvement over the Bayesian-derived estimate of confidence bound.

Influence of residual stresses from shot peening on fretting fatigue crack growth

ABSTRACTOne method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.

A study on the material properties and fatigue life prediction of natural rubber component

Fatigue life prediction and evaluation are very important in design procedure to assure the safety and reliability of the rubber components. Fatigue lifetime prediction methodology of the rubber component was proposed by incorporating the finite element analysis and fatigue damage parameter from fatigue test. Finite element analysis of rubber component was performed based on a hyper-elastic material model determined from material test. The Green–Lagrange strain at the critical location determined from the finite element method was used to evaluate the fatigue damage parameter of the natural rubber. Fatigue life of rubber components was predicted by using the fatigue damage parameter at the critical location. Predicted fatigue life of the rubber component agreed fairly well with the experimental fatigue lives.

모서리균열이 있는 알루미늄판의 복합재 패치보수시 수명예측 연구

노후항공기의 균열보수 방법 중 복합재를 이용한 균열보수 방법을 이용하여 항공기 알루미늄 재료의 피로수명 예측을 위해 유한요소해석을 이용 하였다. 패치보수의 해석 시에는 접착제 층이 매우 얇기 때문에 모델링의 어려움이 있는데, 본 연구에서는 3층 기법을 이용하여 해석을 수행하였다. 피로수명의 예측 시에는 Paris의 법칙을 적용하였고, 효율적 수명예측을 위해 수정된 균열닫힘법을 적용하였다. 해석에 의한 수명예측 결과는 실험치를 잘 모사할 수 있었으며, 항공기의 피로수명 예측이나 수명연장기법으로 활용될 수 있을 것으로 생각된다. One of the hot issues in composite patching is to reduce the thermal residual stresses between composite patch and aluminum surface which occurs after bonding of composite patch. In this study, the edge crack patching is adopted for different curing cycles. For the analysis, three layer Mindlin plate elements are used, and Paris' law is adopted to predict the fatigue life of composite patch plate. The analysis results show a good agreement with the experimental fatigue life and this technique can be applied for the prediction of fatigue life of aircraft structures.

Bending fatigue characteristics of nanohoneycomb structures

Nanohoneycomb beam structures of Anodic Aluminum Oxide (AAO) were fabricated by a two-step anodization process. The bending fatigue properties of nanohoneycomb structures were measured using a Nano-Universal Testing Machine (UTM) under cyclic three-point bending. The mean values of the experimental results were compared with several single load level fatigue life prediction equations with good agreement, although there were large deviations of experimental fatigue life data among samples with the same load level. There were small increments of resultant displacements during cyclic loading. The present results can be used as a design guideline in applications of nanohoneycomb structures.

A Study on the Fatigue Life Prediction and Evaluation of the Natural Rubber Components for Automobile Vehicles

The fatigue analysis and lifetime evaluation are very important in design procedure to assure the safety and reliability of the rubber components. The interest of the fatigue life of rubber components such as the engine mount is increasing according to the extension of warranty period of the automotive components. In this study, the fatigue lifetime prediction methodology of the vulcanized natural rubber was proposed by incorporating the finite element analysis and fatigue damage parameter determined from fatigue tests. Finite element analysis of 3D dumbbell specimen of natural rubber was performed based on a hyper-elastic material model determined from the tension, compression and shear tests. The Green-Lagrange strain at the critical location determined from the finite element analysis was used for evaluating the fatigue damage parameter of the natural rubber. Fatigue tests were performed using the 3D dumbbell specimens with different levels of maximum strain and various load. The basic mechanical properties test and the fatigue test of rubber specimens under the normal and elevated temperature were conducted. Fatigue life curves can be effectively represented by a following single function using the maximum Green-Lagrange strain. Fatigue lives of the natural rubber are predicted by using the fatigue damage parameters at the critical location. Predicted fatigue lives of the engine mount agreed fairly with the experimental fatigue lives a factor of two.

Fatigue lifetime under uniaxial random loading with different mean values according to some selected models

In the paper, the experimental fatigue lives of plane specimens made of 10HNAP steel have been compared with the calculated lives for random tension loading with non-zero mean value. The cycle amplitude transformation methods proposed by Goodman and Gerber, the Van Dang criterion, averaging with the Svenson–Lipp method and the strain energy density parameter was used. Cycles were counted by the rain flow algorithm; in the case of the Svenson–Lipp method the fatigue cycles were counted by the rain flow algorithm and the level crossing algorithm. Damages were accumulated according to the Palmgren–Miner hypothesis.

Fatigue Life Evaluation of Rubber Components for Automobile Vehicles

The interest of the fatigue life for rubber components was increasing according to the extension of warranty period of the automotive components. In this study, the fatigue lifetime prediction methodology of the vulcanized natural rubber was proposed by incorporating the finite element analysis and fatigue damage parameter determined from fatigue tests. Finite element analysis of 3D dumbbell specimen and rubber component was performed based on a hyper-elastic material model determined from the mechanical tests. The Green-Lagrange strain at the critical location determined from the finite element analysis was used for evaluating the fatigue damage parameter of the natural rubber. Fatigue tests were performed using the 3D dumbbell specimens and rubber component with different levels of maximum strain and various load. Fatigue life curves can be effectively represented by a following single function using the maximum Green-Lagrange strain. Fatigue lives of the natural rubber are predicted by using the fatigue damage parameters at the critical location. Predicted fatigue lives of the rubber component for automobile vehicle agreed fairly with the experimental fatigue lives.

Fatigue tests on steel–concrete–steel sandwich components and beams

In Bi-Steel steel–concrete–steel sandwich members, the friction welded bar–plate connections are subject to plate tension, bar tension and bar shear with associated bar bending. The relative magnitude of these force components depends on the beam geometry and the type of loading. The force components interact, so component fatigue tests were performed under single and combined loads, each with a sufficient number of stress ranges to enable stress vs. fatigue life ( S / N ) curves and equations to be defined. Fatigue tests were also conducted on eighteen beams, and the experimental fatigue lives are compared with the values estimated by means of a proposed interaction equation based on the experimental S / N equations derived from the component tests.

Effect of Local Strain on Low Cycle Fatigue Measured by ESPI System

Low cycle fatigue cracks are mainly detected at discontinuous welded locations with high stresses under repeated cyclic static loads due to cargo loading and unloading. Theoretical and analytical methods have been used to estimate the local stress and strain, which affect the prediction of fatigue life, but these methods have difficulties considering stress concentration at notched locations and complicated material behavior of welded joints or heat affected zones. Electronic speckle pattern interferometry(ESPI) system is a nondestructive and non-contact measurement system, which can provide relatively accurate full field strain at critical positions such as welded zones and structurally discontinuous locations. In this study, local strain was measured by ESPI system at the welded cruciform joint, and then low cycle fatigue test was performed. Effect of local strain on low cycle fatigue life was examined by using the strain value measured by ESPI. In order to verify the relations between local strains and fatigue lives, after theoretical local strains and stresses were calculated by using Neuber’s rule, the measured local strains corresponding the experimental fatigue lives were compared with the results of Neuber’s rule and established codes of the British standard and DNV curve.

Simulations of Cyclic Plasticity and Fatigue Behavior of Structural Steels under Multiaxial Loading

The local cyclic elastic-plastic stress-strain responses were simulated using the incremental plasticity procedures of ABAQUS finite element code. It is shown a better understanding on the evolutions of the local cyclic stress-strain and the strong interactions between the most stressed material element and its neighboring material elements in the plastic deformations and stress redistributions. Based on the stress/strain states of the stabilized cycle, a new damage parameter, proposed on modification of the ASME code parameter, is applied and improved correlations between the predicted and the experimental fatigue lives are shown. It is concluded that the improvement of fatigue life prediction depends not only on the fatigue damage models, but also on the accurate evaluations of the cyclic elasto-plastic stress/strain responses.

Study of the fatigue failure of an anti-return valve of a high pressure machine

This paper presents a study of the fatigue failure of an anti-return valve, designed to work in the high pressure system (500 MPa) of a high pressure processing machine. To do this, the crack propagation has been simulated by means of the linear-elastic fracture mechanics approach under mixed-mode loading conditions. From an initial crack, which size is related with the microstructure and superficial finish, the crack growth has been simulated using the stress intensity factors K I and K II of the cracked valve axisymmetric geometry. The crack propagation path has been obtained step by step, applying the criterion of the maximum circumferential stress at the crack tip. The experimental and simulated crack propagation paths have been compared and, as a consequence of the reliable results obtained, the fatigue life of the valve has been calculated using the Paris law of the material with an effective stress intensity factor K eff. The good agreement with experimental fatigue life allows to perform new improved designs using the methodology presented.

ESPI System을 이용하여 측정한 국부 변형률이 저사이클 피로수명에 미치는 영향에 관한 연구

Low cycle fatigue cracks are mainly detected at discontinuous welded locations with high stresses under repeated cyclic static loads due to cargo leading and unloading. Theoretical and analytical methods have been used for evaluation of local stress and strain which have an effect on a prediction of fatigue life, but those have difficulties of considering stress concentration at notched location and complicated material behavior of welded joint or heat affected zone. Electronic speckle pattern interferometry(ESPI) system is nondestructive and non-contact measurement system which can get the relatively accurate full field strain at critical positions such as welded zone and structural discontinuous location. In this study, local strain was measured on welded cruciform joint by ESPI system and then low cycle fatigue test was performed. Effect of local strain on low cycle fatigue life was examined by measured values using ESPI system. Moreover, experimental fatigue life was compared with established S-N curves using theoretical local strain and stress calculated by Neuber's rule.

An energy‐based damage parameter for the life prediction of AISI 304 stainless steel subjected to mean strain

This study extends the plastic strain energy approach to predict the fatigue life of AISI 304 stainless steel. A modified energy parameter based on the stable plastic strain energy density under tension conditions is proposed to account for the mean strain and stress effects in a low cycle fatigue regime. The fatigue life curve based on the proposed energy parameter can be obtained directly by modifying the parameters in the fatigue life curve based on the stable plastic strain energy pertaining to fully reversed cyclic loading. Hence, the proposed damage parameter provides a convenient means of evaluating fatigue life on the mean strain or stress effect. The modified energy parameter can also be used to explain the combined effect of alternating and mean strain/stress on the fatigue life. In this study, the mean strain effects on the fatigue life of AISI 304 stainless steel are examined by performing fatigue tests at different mean strain levels. The experimental results indicate that the combination of an alternating strain and a mean strain strongly influences the fatigue life. Meanwhile, it is found that the change in fatigue life is sensitive to changes in the proposed damage parameter under the condition of a constant strain amplitude at various mean strain levels. A good agreement is observed between the experimental fatigue life and the fatigue life predicted by the proposed damage parameter. The damage parameter proposed by Smith et al. (1970) is also employed to quantify the mean strain effect. The results indicate that this parameter also provides a reasonable estimate of the fatigue life of AISI 304 stainless steel. However, a simple statistical analysis confirms that the proposed damage parameter provides a better prediction of the fatigue life of AISI 304 stainless steel than the SWT parameter.

Numerical life prediction method for fatigue failure of rubber-like material under repeated loading condition

Predicting fatigue life by numerical methods was almost impossible in the field of rubber materials. One of the reasons is that there is not obvious fracture criteria caused by non-standardization of material and excessively various way of mixing process. But, tearing energy as fracture factor can be applied to a rubber-like material regardless of different types of fillers, relative to other fracture factors and the crack growth process of rubber could be considered as the whole fatigue failure process by the existence of potential defects in industrial rubber components. This characteristic of fatigue failure could make it possible to predict the fatigue life of rubber components in theoretical way. FESEM photographs of the surface of industrial rubber components were analyzed for verifying the existence and distribution of potential defects. For the prediction of fatigue life, theoretical way of evaluating tearing energy for the general shape of test-piece was proposed. Also, algebraic expression for the prediction of fatigue life was derived from the rough cut growth rate equation and verified by comparing with experimental fatigue lives of dumbbell fatigue specimen in various loading condition.

Multiaxial fatigue and failure analysis of helical compression springs

Multiaxial fatigue criteria are applied to the analysis of helical compression springs. The critical plane approaches, Fatemi–Socie and Wang–Brown, and the Coffin–Manson method based on shear deformation, were used to predict fatigue lives of the springs under constant amplitude loading. Experimental fatigue lives are compared with the multiaxial fatigue criteria predictions. The stress analysis was carried out in the finite element code ANSYS, and the multiaxial fatigue study was performed using the fatigue software nCode. A failure analysis was conducted in order to determine the fatigue crack initiation point and a comparison of that location with the most damaged zone predicted by the numerical analysis is made. The Fatemi–Socie critical plane approach gives a good prediction of fatigue life. While the Wang–Brown criterion overestimates spring fatigue life, the Coffin–Mason model gives conservative results.

How to reduce the duration of multiaxial fatigue tests under proportional service loadings

This paper deals with a technique to transform a multiaxial stress and strain-time sequence (in service recorded) in a simplified sequence. This simplified sequence is shorter than the original one and equivalent in terms of damage and lifetime: the number of simplified sequences to crack initiation is equal to the number of original sequences. The proposal is based on an energy threshold, below which no micro-crack can initiate or grow in the material. This technique was validated with real loading sequences recorded by strain gauges pasted on a car suspension arm. Fatigue tests were carried out on smooth specimens made of spheroidal graphite cast iron loaded in bending, in torsion and in combined bending and torsion. Experimental fatigue lives under the original sequence and under the simplified one are in very good correlation. Fatigue test duration is reduced up to a factor of 10 for the severe stress–strain sequences tested in this study.

Fatigue Strength Predictions of Notched Specimens of Spring Steel

This paper presents the study on fatigue strength and fatigue life predictions of surface notched specimens of spring steel. The validity of the fatigue limit prediction and the fatigue crack growth prediction based on Haddad's equation, where the hardness of material and stress ratio were taken into considerations, were studied, compared with the experimental results. The specimens with semicircular notch by electrical discharge machining were prepared. The conditions of tests were without decarburization, with decarburization and with shot peening. The main results can be summarized below:(1) The calculated fatigue limits were slightly smaller than the experimental results for the without decarburization specimens. It can be said that the fatigue limit prediction is evaluated within the safe side in this case.(2) The calculated fatigue limits were between the experimental fracture and non-fracture results for the decarburized specimens. Therefore, the fatigue limit prediction can be said to be effective.(3) The fatigue limit prediction for the shot peeed specimens was evaluated within the safe side. It can be said that the prediction is effective.(4) The calculated fatigue crack growth behavior with da/dN-ΔK diagrams showed good agreements with experimental results.(5) The calculated fatigue life was slightly shorter than the experimental fatigue life without decarburization on the S-N curves. The calculated fatigue life showed good agreements with the experimental results with decarburization. Therefore, it can be concluded that the fatigue crack growth prediction is very effective.