Eckard Macherauch

Residual stress relaxation in an AISI 4140 steel due to quasistatic and cyclic loading at higher temperatures

Abstract Residual stresses can be relaxed by supplying sufficiently high amounts of thermal and/or mechanical energy, which converts the residual elastic strains to microplastic strains. In order to better understand this relaxation behavior, shot peening induced residual stresses in normalized condition and in quenched and tempered condition of the steel AISI 4140 (German grade 42 CrMo 4) were investigated in annealing experiments, quasistatic loading experiments and bending fatigue experiments at 25, 250 and 400°C. The residual stress relaxation during alternating bending occurs in different regimes. First, thermal relaxation reduces the residual stresses during specimen heating. The relaxation during the first cycle can be discussed on the basis of the effects due to quasistatic loading, if the inhomogeneous distribution of the loading stress is taken into account. Differences in the behavior after the two heat treatments result from the Bauschinger-effect and effects of dynamic strain ageing. Owing to cyclic creep effects, the interval between the first cycle (N=1) and the number of cycles to crack initiation Ni is characterized by residual stresses which decrease linearly with the logarithm of N. Finally for N>Ni the reduction of residual stresses with the logarithm of N is stronger than linear.

An analysis of the growth of short fatigue cracks

Abstract The behavior of short fatigue cracks cannot be analysed by linear elastic methods because of large scale plasticity effects as well as a breakdown in the stress intensity factor correlation of fatigue crack growth rates for crack sizes less than a millimeter in length. In the present paper a number of modifications are introduced to the linear elastic approach to establish a new parameter which is capable of correlating both long crack and short crack fatigue crack growth data. These modifications include the use of a material constant, r e ; an allowance for large scale plasticity effects; allowance for the development of crack closure; and the incorporation of the fatigue crack growth threshold. The new parameter is used with reasonable success in calculating fatigue crack growth behavior of short cracks in notched and unnotched specimens, and comparing with experimental data. In addition the analysis leads to a rationale for the dependence of the stress amplitude at the endurance level on the mean stress in a manner consistent with Gerbers parabolic relationship.

Determination of residual stresses in ceramics and ceramic-metal composites by x-ray diffraction methods

Abstract A survey is given of the research done in the field of X-ray residual stress determinations on ceramics and ceramic-metal composites. The principles of X-ray residual stress determinations are described and helpful hints are given for X-ray stress determinations in ceramic materials. X-ray elastic constants of common ceramics are included. A number of interesting examples of X-ray residual stress determinations on ceramics and ceramic-metal composites are presented.

Microstructure and cyclic deformation behaviour of plain carbon and low-alloyed steels

Abstract Stress- and strain-controlled push-pull tests have been performed to study fatigue phenomena in several plain carbon steels and a low-alloyed steel after different heat treatments with special consideration being given to the microstructural processes. The cyclic deformation behaviour is characterized in terms of plastic strain amplitudes which can be regarded as the basic condition for fatigue damage. Depending on the loading amplitudes and temperatures, the cyclic deformation curves reveal characteristics of cyclic softening and cyclic hardening. In stress-controlled tests with non-zero mean stresses, normalized as well as quenched and tempered material states show cyclic creep effects. TEM investigations indicate distinct differences in the dislocation structures of materials which have been fatigued under different loading and temperature conditions.

Determination of inhomogeneous residual stress states in surface layers of machined engineering ceramics by synchrotron x-rays

Abstract Ceramic components for engineering applications must in most cases be ground after sintering in order to achieve a sufficient precision of the final dimensions as well as a sufficient surface quality. Since the surface layers influenced by grinding are very shallow, the determination of near-surface grinding residual stresses and their gradients within the penetration depth of the X-rays is difficult. In this paper, a procedure is applied which allows the investigation of residual stress states in ground ceramic surfaces by X-ray diffraction using synchrotron radiation and parallel beam optics. Three different ceramics were examined. In all cases, compressive residual stresses were found in the surface layers of the materials investigated. The magnitudes and gradients of the grinding residual stresses markedly depend on the grinding parameters and on the resulting mechanism of materials removal.

Effect of tempering on the microstructure and strength of martensitically hardened plain carbon steels

Abstract The X-ray interference line profile analysis was applied to plain carbon steels with 0.4 wt.% carbon after martensitic hardening and tempering in order to investigate the mean strains and domain sizes. From these parameters micro residual stresses and dislocation densities have been estimated. The mean strains of the hardened samples are only caused by the stress fields of dislocations. Tempering leads to decreasing mean strains and micro residual stresses as well as increasing domain sizes due to rearrangements and annihilations of dislocations. The hardening contributions of dislocations and carbide particles in the yield strength of the quenched and tempered states which increase with increasing carbon content, are evaluated.

Thermal-Mechanical Fatigue Behaviour of NiCr22Co12Mo9

Every start up and shut down of a system operating at high temperatures causes in the components transient temperature gradients through which complex strain and stress fields occur and local damage may be produced. For example, the lifetime of combustion chambers in gas turbines is often limited by damage resulting from start-stop-cycles and changes of the operating temperature [1,2]. Frequently, data from isothermal strain controlled fatigue tests are used to predict the deformation behaviour and the lifetime of components exposed to thermal-mechanical fatigue (TMF) [3]. This procedure, however, is connected with large uncertainties, especially if the cyclic stress-strain response and the microstructural changes are different under isothermal and thermal-mechanical fatigue. In the present study, the TMF behaviour of the solid solution and carbide precipitation hardened nickel base superalloy NiCr22Col2Mo9, which is commonly used as sheet material in gas turbines, is investigated. The TMF life determined is compared to the lifetime in isothermal fatigue tests.

Creep behavior of dispersion-hardened aluminum materials

AbstractDispersion-hardened aluminum materials of pure aluminum with extremely fine oxide and carbide dispersions and very fine grain sizes were creep-deformed under compressive loadings between 573 and 773 K. The creep behavior of the investigated materials is influenced by time, temperature, stress level and microstructure. An increasing content of dispersions causes increasing threshold stresses σthand resistances against creep. The Norton plots of the minimum creep rate

Influence of the Mechanical Strain Amplitude on the In-Phase and Out-of-Phase Thermo-mechanical Fatigue Behaviour of NiCr22Co12Mo9

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