S. Henkel

Biaxial fatigue behavior of a powder metallurgical TRIP steel

Multiaxial fatigue behavior is an important topic in critical structural components. In the present study the biaxial-planar fatigue behavior of a powder metallurgical TRIP steel (Transformation Induced Plasticity) was studied by taking into account martensitic phase transformation and crack growth behavior. Biaxial cyclic deformation tests were carried out on a servo hydraulic biaxial tension-compression test rig using cruciform specimens. Different states of strain were studied by varying the strain ratio between the axial strain amplitudes in the range of -1 (shear loading) to 1 (equibiaxial loading). The investigated loading conditions were proportional due to fixed directions of principal strains. The studied TRIP steel exhibits martensitic phase transformation from ?-austenite via ?-martensite into ?‘- martensite which causes pronounced cyclic hardening. The ?‘-martensite formation increased with increasing plastic strain amplitude. Shear loading promoted martensite formation and caused the highest ?‘-martensite volume fractions at fatigue failure in comparison to uniaxial and other biaxial states of strain. Moreover, the fatigue lives of shear tests were higher than those of uniaxial and other biaxial tests. The von Mises equivalent strain hypothesis was found to be appropriate for uniaxial and biaxial fatigue, but too conservative for shear fatigue, according to literature for torsional fatigue. The COD strain amplitude which is based on crack opening displacement gave a better correlation of the investigated fatigue lives, especially those for shear loading. Different types of major cracks were observed on the sample surfaces after biaxial cyclic deformation by using electron monitoring in an electron beam universal system and scanning electron microscopy (SEM). Specimens with strain ratios of 1, 0.5, -0.1 and -0.5 showed mode I major cracks (perpendicular to the axis of maximum principal strain). Major cracks after shear fatigue had partially mode II orientation (tilted 45° to the loading axes) and afterwards bifurcated into two pairs of mode I cracks. Another shear test revealed a major crack of mode I orientation (parallel to the loading axes). These results are in good agreement to the literature. Micro cracks after shear fatigue were longer than those after biaxial fatigue with strain ratios of 1 and 0.5. Major and minor cracks after equibiaxial and shear loading showed crack branching and crack coalescence. The results on fatigue crack behavior support the assumption that the period of stage I (mode II) crack propagation is much longer under shear loading than under other biaxial conditions due to absence of tensile stress normal to the planes of maximum shear strain under shear loading.

Fracture mechanics testing and crack growth simulation of highly ductile austenitic steel

Abstract The present contribution deals with ductile austenitic cast steel. CT-specimens are cut out of cast plates and tested under monotonic and quasi-static conditions. Good ductility, high work hardening and restricted specimen dimensions complicate the fracture mechanical characterization of the material and regular fracture toughness values cannot be determined. Crack tip opening displacement (CTOD) and crack extension are measured optically. Force-displacement data and crack growth resistance curves have been gleaned from the experiments. A cohesive zone model has been utilized to simulate the fracture event (crack initiation and propagation). The results of these simulations have been compared with experimental data. The aim is to determine the cohesive parameters as these can be related to fracture toughness. The transferability of these parameters to specimens of greater thickness remains an open question.

Crack growth behavior of aluminum alloy 6061 T651 under uniaxial and biaxial planar testing condition

The crack growth behavior of the aluminum alloy 6061 T651 was investigated using cruciform specimens with a measurement area of 120 x 120 x 2 mm3 with two center crack configurations of the starting notch parallel to one of the loading axes and under an angle of 45°, respectively. For the case with crack direction in one of the loading axes the load ratio R = ?min / ?max as well as the force parallel to the crack direction (resulting in different T-stresses) were changed. Crack growth rate was studied under varying T-stress. Also the retardation after single overloads was determined for R = 0.1, R = 0.5 and R = 0.8. As a result a change in T-stress does not significantly affect crack growth rate on high R ratios (R = 0.5) for constant ?F loading. In case of lower R-ratios (R = 0.1) crack growth retardation was observed at presence of a static tensile load parallel to the crack growth direction due to higher influence of crack closure. Furthermore, such tensile load results in longer retardation periods after applying an overload at R = 0.1. Less pronounced overload retardation can be assumed with tensile force FX for R = 0.8 and 1.3 times overloads. Non proportional loading with a phase shift in time between the two axes of 45° and 90° results in a mixed mode situation (mode I / mode II) at the crack tip of a crack which is orientated under 45° to the loading axes. Mode I and mode II fractions change during every cycle. A phase change of 45° did not change crack growth significantly compared with proportional load. Crack branching occurred when changing from proportional loading to non-proportional 90° phase shifted loading. The two crack tips of the center crack under 45° divided in 4 crack tips under approximately 90° to the loading axes which were simultaneous propagating for more than 10 mm. Finally, two crack tips propagated faster than the remaining two. The stress intensity factors KI and KII as well as the T-stress where calculated by FEA (ABAQUS). For the 45° crack orientation and the non-proportional load case with 90° phase shift linear elastic FEA calculations show that there are time dependent rotating principal stress axes on the crack tip during one cycle. In the unnotched (uncracked) specimen there are fixed principal stress axes also in the phase shifted loading case. The configuration with 4 cracks has a significant higher ?KI than the configuration with two crack tips while ?KII is significantly lower. In addition uniaxial crack growth measurements were performed on SENB specimen in the size of 10 x 20 x 100 mm3 covering the threshold and Paris-region for loading ratios R = 0.1, 0.3, 0.5, 0.8.