Ali Fatemi

Cumulative fatigue damage and life prediction theories: a survey of the state of the art for homogeneous materials

Abstract Fatigue damage increases with applied load cycles in a cumulative manner. Cumulative fatigue damage analysis plays a key role in life prediction of components and structures subjected to field load histories. Since the introduction of damage accumulation concept by Palmgren about 70 years ago and ‘linear damage rule’ by Miner about 50 years ago, the treatment of cumulative fatigue damage has received increasingly more attention. As a result, many damage models have been developed. Even though early theories on cumulative fatigue damage have been reviewed by several researchers, no comprehensive report has appeared recently to review the considerable efforts made since the late 1970s. This article provides a comprehensive review of cumulative fatigue damage theories for metals and their alloys, emphasizing the approaches developed between the early 1970s to the early 1990s. These theories are grouped into six categories: linear damage rules; nonlinear damage curve and two-stage linearization approaches; life curve modification methods; approaches based on crack growth concepts; continuum damage mechanics models; and energy-based theories.

A literature survey on fatigue analysis approaches for rubber

Rubber components subjected to fluctuating loads often fail due to the nucleation and growth of defects or cracks. The prevention of such failures depends upon an understanding of the mechanics underlying the failure process. This paper reviews analysis approaches that are currently available for predicting fatigue life in rubber. Both crack nucleation and crack growth approaches are considered. A discussion of each approachs strengths and limitations, and examples of how these approaches have been applied in engineering analysis are presented.

Mixed Mode Fatigue Crack Growth : A Literature Survey

The applications of fracture mechanics have traditionally concentrated on crack growth problems under an opening or mode I mechanism. However, many service failures occur from growth of cracks subjected to mixed mode loadings. This paper reviews the various criteria and parameters proposed in the literature for predictions of mixed mode crack growth directions and rates. The physical basis and limitations for each criterion are briefly reviewed, and the corresponding experimental supports are discussed. Results from experimental studies using different specimen geometries and loading conditions are presented and discussed. The loading conditions discussed consist of crack growth under mode II, mode III, mixed mode I and II, and mixed mode I and III loads. The effects of important variables such as load magnitudes, material strength, initial crack tip condition, mean stress, load non-proportionality, overloads and crack closure on mixed mode crack growth directions and/or rates are also discussed.

Strain-controlled fatigue properties of steels and some simple approximations

Abstract In this study, first strain-controlled deformation and fatigue data are reported and compared for several steels most commonly used in the ground vehicle industry. Correlations between monotonic tensile data and constant amplitude strain-controlled fatigue properties are then investigated, and validity of some of the more commonly used methods of estimating fatigue properties is examined. A simple method requiring only hardness and modulus of elasticity is proposed for estimation of the strain–life curve. Prediction capability of this method is evaluated for steels with hardness between 150 and 700 HB and compared with several other methods proposed in the literature. The proposed method is shown to provide good approximations of the strain–life curve.

Factors that Affect the Fatigue Life of Rubber: A Literature Survey

Abstract Many factors are known to influence the mechanical fatigue life of rubber components. Four major categories of factors are reviewed here: the effects of mechanical loading history, environmental effects, effects of rubber formulation, and effects due to dissipative aspects of the constitutive response of rubber. For each category, primary factors are described, and existing literature is presented and reviewed. Rubbers fatigue behavior is extremely sensitive to both the maximum and minimum cyclic load limits. Other aspects of the mechanical load history are also discussed, including the effects of static loaded periods (“annealing”), load sequence, multiaxiality, frequency, and loading waveform. Environmental factors can affect both the short and long term fatigue behavior of rubber. The effects of temperature, oxygen, ozone, and static electrical charges are reviewed. A great range of behavior is available by proper manipulation of formulation and processing variables. Effects of elastomer type...

Observations of the Constitutive Response and Characterization of Filled Natural Rubber Under Monotonic and Cyclic Multiaxial Stress States

This work explores the monotonic and cyclic behaviors of filled, natural rubber. Results of stress-strain experiments conducted under stress states of simple, planar, and equibiaxial tension are presented. The ability of hyperelastic models to capture the observed response, as well as recent developments in constitutive modeling of filled rubber such as the consequences of the Mullins effect, are discussed. Monotonic and cyclic multiaxial experiments were also conducted using a short, thin-walled, cylindrical specimen subjected to a wide range of combined axial and twist displacements. Experiments included pure axial tension, pure torsion, combined loading in which the axial and torsion displacements varied proportionally, and combined loading in which the axial and torsion displacements varied non-proportionally (phase between axial and torsion channels of f50 deg, 90 deg, 180 deg). Results from these tests are presented and discussed, including evolution of stress-strain behavior with load cycles, and the effects of a short period of initial overloading on the subsequent evolution of the stress-strain response. @DOI: 10.1115/1.1631432#

Effects of mean stress on fatigue behaviour of a hardened carbon steel

Abstract The effects of mean stress on the cyclic deformation and fatigue life of smooth uniaxial fatigue specimens made from SAE 1045 steel hardened to 55 HRC were investigated. Both compressive and tensile mean stresses were imposed through strain-controlled tests with strain ratios of −2, 0 and 0.5, resulting in fatigue lives ranging from 550 to 106 cycles. Insignificant degrees of mean stress relaxation were observed except at the very short lives. The tensile mean stress was found to decrease life by as much as two orders of magnitude, while compressive mean stress increased life by as much as a factor of five. The experimental mean stress data were used to compare the life prediction capabilities of several fatigue damage parameters that are sensitive to mean stress.

Connecting Rod Optimization for Weight and Cost Reduction

An optimization study was performed on a steel forged connecting rod with a consideration for improvement in weight and production cost. Since the weight of the connecting rod has little influence on its total production cost, the cost and the weight were dealt with separately. Reduction in machining operations, achieved by change in material, was a significant factor in manufacturing cost reduction. Weight reduction was achieved by using an iterative procedure. Literature survey suggests cyclic loads comprised of static tensile and compressive loads are often used for design and optimization of connecting rods. However, in this study weight optimization is performed under a cyclic load comprising dynamic tensile load and static compressive load as the two extreme loads. Constraints of fatigue strength, static strength, buckling resistance and manufacturability were also imposed. The fatigue strength was the most significant factor in the optimization of the connecting rod. An estimate of the cost savings is also made. The study results in an optimized connecting rod that is 10% lighter and 25% less expensive, as compared to the existing connecting rod.

Cumulative fatigue damage mechanisms and quantifying parameters : A literature review

Cumulative fatigue damage analysis plays an important role in fatigue life prediction of components and structures which are subjected to field loading histories. Understanding of cumulative damage mechanisms is essential since it provides the necessary physical bases for modeling the cumulative damage process. A damage measure that can reflect and quantify the real damage state the material undergoes is also a key issue for successful modeling of cumulative fatigue damage. This review paper provides a comprehensive overview of research activities highlighting the recent findings and progress on phenomenological observations and mechanisms, as well as quantification measures of cumulative fatigue damage. Depending on the definition of failure or the characteristics of failure experienced in a material, the effectiveness of a damage parameter could vary from case to case. Many damage parameters have been proposed and many of them are in use. Those to be reviewed are sorted into categories of metallurgical parameters, surface crack quantifications, mechanical measures, and physical parameters. Early studies on cumulative damage mechanisms and quantifying measures are reviewed only briefly, since they have been covered in the existing literature.

Fatigue Behavior of Stainless Steel 304L Including Strain Hardening, Prestraining, and Mean Stress Effects

This paper discusses cyclic deformation and fatigue behaviors of stainless steel 304L and aluminum 7075-T6. Effects of loading sequence, mean strain or stress, and prestraining were investigated. The behavior of aluminum is shown not to be affected by preloading, whereas the behavior of stainless steel is greatly influenced by prior loading. Mean stress relaxation in strain control and ratcheting in load control and their influence on fatigue life are discussed. Some unusual mean strain test results are presented for SS304L, where in spite of mean stress relaxation fatigue lives were significantly longer than fully-reversed tests. Prestraining indicated no effect on either deformation or fatigue behavior of aluminum, while it induced considerable hardening in SS304L and led to different results on fatigue life, depending on the test control mode. Possible mechanisms for secondary hardening observed in some tests, characterized by a continuous increase in the stress response and leading to runout fatigue life, are also discussed. The Smith-Watson-Topper parameter was shown to correlate most of the experimental data for both materials under different loading condition.