Abstract
Multiaxial stress states frequently occur in technical components and, due to the multitude of possible load situations and variations in behaviour of different materials, are to date not fully predictable. This is particularly the case when loads lie in the plastic range, when strain accumulation, hardening and softening play a decisive role for the material reaction. This study therefore aims at adding to the understanding of material behaviour under complex load conditions. Fatigue tests conducted under cyclic torsional angles (5°, 7.5°, 10° and 15°), with superimposed axial static compression loads (250 MPa and 350 MPa), were carried out using smooth specimens at room temperature. A high nitrogen alloyed austenitic stainless steel (nickel free), was employed to determine not only the number of cycles to failure but particularly to aid in the understanding of the mechanical material reaction to the multiaxial stresses as well as modes of crack formation and growth. Experimental test results indicate that strain hardening occurs under the compressive strain, while at the same time cyclic softening is observable in the torsional shear stresses. Furthermore, the cracks’ nature is unusual with multiple branching and presence of cracks perpendicular in direction to the surface cracks, indicative of the varying multiaxial stress states across the samples’ cross section as cross slip is activated in different directions. In addition, it is believed that the static compressive stress facilitated the Stage I (mode II) crack to change direction from the axial direction to a plane perpendicular to the specimen’s axis.
Highlights
Conventional uni-axial fatigue tests are very important and have been used for a long time to predict the fatigue life of industrial components, aircraft and car parts, structures, etc
According to Equation (2), τ is the shear stress, T is the torque and D start is the diameter of the smallest cross section of sample
While the two applied compressive stresses lie well below the yield limit of the material, the superposition of the torsional rotation yields significantly higher von Mises effective stresses, which for two experiments even exceed the tensile strength when torsion approaches high angles close to the reversal points. This is relevant, e.g., for a case reported in [18], where an Aesculap stem of an artificial hip joint for the human body made of stainless steel with an asymmetrical cross section was found to initiate fracture at the thinner side of the stem even though the tensile stress is higher at the lateral side
Summary
Conventional uni-axial fatigue tests are very important and have been used for a long time to predict the fatigue life of industrial components, aircraft and car parts, structures, etc. These tests have aided in saving many lives and properties because the structural integrity of components can be determined. Several stress components can be simultaneously in operation at a point in a material. This leads to either proportional or non-proportional multiaxial stress states. Non-proportional stress states have a rotation of principal strain and stress axes which results to somewhat different material dependent responses and different fatigue damage modes [2]
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