Dietmar Eifler

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.

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.

Fatigue Behaviour of Ni-base Alloys up to 1273 K

ABSTRACT The cyclic deformation behaviour of two Ni-base alloys was investigated in stress and total strain controlled push-pull tests in the temperature range 295 K ≤ T ≤ 1273 K. The cyclic deformation curves and the fatigue life were determined as a function of the different loading conditions. Characteristic experimental results are presented and discussed with respect to dynamic strain aging effects.

Thermal Fatigue of Pearlitic Nodular Cast Iron

The thermal fatigue behaviour of a high strength pearlitic nodular cast iron (German grade GGG-60) was investigated applying triangular temperature-time-loadings with a cycle time of 2 minutes. The experiments were run with a constant lower temperature Tmin = 150 °C and variable upper temperatures of 350 °C ≤ Tmax ≤ 500 °C. At the beginning of the test after reaching the mean temperature, a defined total mean strain em,t was superimposed and afterwards kept constant, simulating a total suppression of the longitudinal thermal expansion of the cylindrical specimens over the whole test. All tests with varying upper temperatures were performed with em,t = 4%.. To evaluate the influence of total mean strains on the thermal fatigue behaviour in the range -2%. ≤ em,t ≤ 7%., a constant upper temperature Tmax = 450 °C was selected. The materials response to the thermal loadings was determined by measuring the stress amplitude and the mean stress of each cycle. Furthermore the number of cycles to crack initiation and failure were evaluated.

A simple specimen heating system for fatigue tests in the temperature range 20–600°C

Abstract The smallest deviation from strictly axial loading may decrease considerably the lifetime of high strength, notch sensitive materials and the endurance limit in tests with the minimum stress in compression. A grip and heating system for tension-compression loading fatigue tests in servohydraulic material testing machines has been developed. The system is characterized by its excellent loading axiality which permits specimen alignment withour any bending moment. In addition, both the specimen and the heating blocks can be changed easily.