Computation of and testing crack growth at 20 kHz load frequency

Mohamed Sadek *, Jens Bergstrom *, Nils Hallback * *) Karlstad University, Department of Engineering and Physics, SE-658 88 Karlstad Abstract Fatigue properties are evaluated in a large span of fatigue lives ranging from a few load cycles to more than 10 13 load cycles. If the interest is focused on fatigue lives above 10 7 load cycles, we speak of the very high cycle fatigue (VHCF) range. For evaluation of properties in the VHCF range one often needs to use higher load frequencies to be able to perform testing within a reasonable time. Also, there are technical applications where indeed varying or higher load frequencies exists. Therefore, the influence of load frequency on fatigue strength and fatigue crack growth is an important issue, both from testing and design perspectives. Within an EU-RFCS research project on the frequency influence on high strength steel fatigue properties the present study has been conducted on fatigue crack growth testing to determine threshold values and crack growth material parameters. The testing was analyzed by finite element computation to determine geometry factors for stress intensity determination. The testing is performed in a 20 kHz ultrasound resonance instrument. In such a system the whole load train needs to be designed to run at a resonance frequency of 20 kHz, and it implies that the specimen needs to be designed and computations performed by dynamic computational methods. As the crack grows the dynamic response of the specimen will change, and hence calculation to obtain the geometry factor is made with a progressing crack length. The FEM ABAQUS software was used to compute the deformation and stress state of the specimens and arriving at crack tip stress intensities. A uniaxial tensile load at 20 kHz frequency is applied to a single edged notched side-grooved flat specimen. The specimen dimensions are calculated in order to have a resonance frequency of 20 kHz, which is the frequency that will be used for the experiments. Dynamic FEM computation with a 3D-model and a quarter symmetry was used with one of the symmetry planes parallel to and in the crack growth line. To avoid crack surface interpenetration during the simulations a rigid thin sheet was introduced and used as a counter-face to the crack surface. The solution obtained was then combined with the breathing crack model proposed by Chati, M (et al.) in order to solve for the irregularities observed when simulating crack opening and closure at higher frequencies. Finally, the whole load train was considered. Thus, also the computed frequencies were very close to frequencies observed in experiments. The computation of stress intensities were made for varying crack lengths in a series of simulations. The geometry factor relation was determined and used in 20 kHz crack growth testing to control the actual stress intensity at the advancing crack tip. Comparison of computations and experimental results were made.