The origins of residual stress and changes during fatigue are discussed, A new mechanism for fading is proposed, Pracitcal (destructive and non-destructive) methods for measuring this stress are critically reviewed. Each technique has major problems requiring further study, but acoustic, magnetic and x-ray methods are all poised for more widespread use.

The Nature of Residual Stress and its Measurement

The fatigue damage criterion and evolution equation containing material microparameters

The metal magnetic memory (MMM) method, based on the magnetomechanical effect, provides an effective and simple approach for fatigue evaluation of train axles in laboratory tests as well as in service on railway systems. In lab tests carried out to analyse the fatigue process, the distribution of the surface magnetic field on axle model specimens under different cycle numbers (N) was investigated. Good correlation between resulting fatigue level and MMM signals has been demonstrated. In the M (magnetic)-N (cycle) curve derived from the experimental data, three stages corresponding to the fatigue process can be clearly distinguished. A similar situation was found in the practical inspections of train axles in service. This technique could be further studied for fatigue evaluation.

Magnetic evaluation of fatigue damage in train axles without artificial excitation

Purpose: The problem addressed in the paper is the description of an effort and durability of components under the conditions of mechanical and thermal interactions. The problems of proper material testing methods have been shown as the important part of the components life assessment methodology. Design/methodology/approach: The FEM modelling has been used to determine the stress and strain fields in the components and to describe their behaviour under mechanical and thermal loading. Findings: An appropriate models description has been developed. So far, experimental verification of the usefulness of the model description to determine the stress and strain patterns in particular object and for chosen operation conditions has been made. Research limitations/implications: The developed description should be useful in problems of behaviour predictions of high temperature components and their durability assessment under different mechanical and thermal loadings in industry practical applications. Originality/value: The method, which more precise description of power industry components behaviour makes possible have been shown in the work. The work is addressed to researchers interested in problems of component behaviour prediction under different loadings that we can meet in the operation practice and to power industry engineering maintenance staff.

Life and operational safety of power systems and chemical plants

There are two problems when investigating the effects of shot-peening on fatigue process. The first one is characterizing residual stresses (RS) and strain hardening (WH) on the sample surface. The second one is the evolution of RS and WH during fatigue. In order to solve the first problem, this thesis proposes a “simultaneous function method” to measure RS and WH with nanoindentation. Accumulated plastic strain (PP) is used to represent WH. Then, by establishing functions of normalized indentation responses through a series of finite element method (FEM) simulations, normalized indentation responses obtained from nanoindentation experiments can be used to extract RS and PP values, assuming that the constitutive behavior is known. Although the simultaneous function method shows fairly high accuracy from a pure numerical view point, experiments performed on T40 commercial pure titanium are not completely satisfying. However, if the residual stress profile is known, the method can be used to derive the work hardening profile.In order to study the second problem, a series of strain-controlled fatigue tests are performed on Ti-18 alloy. Fatigue specimens of 4 material states, including raw, shot-peened, prestrained and prestrained + shot-peened, were tested under 3 different strain amplitudes over 100 cycles. X-ray diffraction tests on the sample surfaces showed that the RS relaxation and the cyclic softening increase together with the strain amplitude. IIT tests showed that shot-peening may induce a softening of the surface of Ti-18 alloy samples

Shot-peening and low-cycle fatigue of titanium alloys : instrumented indentation and X-ray diffraction

Changes in surface residual stress during fatigue process of annealed specimen are investigated adopting X-ray stress measurement. It is found that in torsional fatigue a tensile residual stress appears in the early stage of fatigue process, which is insensitive to the alternation of applied stress range. Meanwhile in bending fatigue, the residual stress produced during stress alternations is fairly dependent on the range of applied stress. For instance, in the case of completely reversed bending fatigue, compressive residual stress is produced at the early period of fatigue process. Considerations are made on the mechanism of the changes in residual stresses, in connection with the behavior of microscopic lattice defects during fatigue process. The effect of residual stress on fatigue life is also discussed.

X-Ray Study of Surface Residual Stress Produced in Fatigue Process of Annealed Metals

In the previous papers, the authors have investigated the behaviour of half-value breadth of X-ray diffraction line, residual stress and micro-structure under completely reversed stress amplitude, and have discussed on the changes in half-value breadth in relation to the basic mechanism of fatigue. In the present paper, as a continuation of the previous studies, the authors carried out some investigations on the changes in half-value breadth, residual stress and micro-structure in a case of fatigue under bending stress amplitude combined with mean stress, for the purpose of studying the mechanism of fatigue failure of metallic materials under mean stress. It has been found that, in the early stage of fatigue which corresponds to the period of fatigue hardening, there is little difference between the changes caused by stress alternation with positive mean stress and those with negative mean stress. However, in the second stage of fatigue which corresponds to the period of the growth of micro-crack, the changes in half-value breadth and residual stress become rapid, while they become slow with compressive mean stress. On the other hand, from microscopic observation, it has been confirmed that tensile mean stress promotes the growth of macro-crack and compressive one suppresses it. Then, it is suggested that the mean stress affects the growth rate of micro-crack just the same as macro-crack, and consequently influences the fatigue life.

X-Ray Investigation on Fatigue Damage of Metals under Mean Stress : (1st Report, Plane Bending Fatigue)

X-Ray Investigation of Surface Residual Stress Produced in Fatigue Process

Following the previous studies under cyclic bending stresses, the changes in half-value breadth, residual stress, hardness and microstructure have been investigated under completely reversed torsional stress amplitude and under alternating torsional stress with mean stress. It is found that the mean stress has little effect on the aforesaid changes. So it is deduced that, in the case of torsional stresses, the fatigue mechanism is not influenced essentially with mean stress, and consequently mean stress has little effect on fatigue life. It is also found that in torsional fatigue, tensile residual stress appears on the specimen surface at the initial stage of fatigue, and then it decreases with the number of stress cycles. It seems that the tensile residual stress is due to the absorption of excess vacancies, which are produced with stress cycles, by specimen surface.

X-Ray Investigation on Fatigue Damage of Metals under Mean Stress : 2nd Report, Torsional Fatigue

It was observed that the propagation rate of fatigue crack is faster under the presence of tensile mean stress. The purpose of the present study is to give a theoretical basis of the observed characteristics of fatigue crack. At first, the energies of a crack are evaluated, the elastic energy, the surface energy, the interaction energy with external stress field and the accumulated strain energy with stress cycles. Then, the energetic conditions of crack growth are discussed. It is suggested that the process of crack propagation is devided into nearly three stages, according to the size of crack, that is, the initiation, the growth of dislocation-type crack and the propagation of semi-macroscopic crack. It follows that, the mean stress have influence on every stages, which is approximately expressed in the form pV, where p and V are the hydrostatic component of stress and the volume of crack, respectively. Various other characteristics of fatigue crack are also discussed based on the energetic consideration.

An Energetic Consideration of Fatigue Mechanism of Metal under Mean Stress

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