Effect of microstructure on the growth of short cracks
The main purpose of this project is to develop a multi-scale model to quantitatively describe the effect of microstructure constituents on micro crack fatigue growth. Due to the known difficulty when simulating cyclic tests of constant load amplitude close to the material’s endurance strength value, a multiple-step testing technique was deployed together with temperature change and direct current potential drop (DCPD) measurements to indicate earliest microstructure changes due to mechanical loading.
For the tests, round bar specimens of the ferritic pearlitic steel material delivered (type EH36) were used. Specimen as well as chamber temperature profiles were registered. The tests were performed at 40°C and at -20°C. Of special interest is the achieved capability of realizing and detecting earliest micro damage in material at as few as 1-1.4 X 105 load cycles. This extensively reduces the computational duration required when simulating the multiple-step test for cyclic micro crack growth analysis. A series of 4-point-bending fatigue micro crack growth experiments was designed for micro crack growth path analysis. In order to minimize the region of crack detection in the specimen, a transversal notch was machined onto one side of each specimen in the direction of loading. In each test, loading was interrupted every 1-2 x 106 load cycles for metallographic analysis at the crack detection region.
In the simulations, the implementation of XFEM elements into the sub model enables the performing of mesh-independent simulations. More importantly, no quasi-static pre-simulation of the test - as in case of cohesive elements - will be required, since XFEM elements will allow micro cracks to grow freely in any favourable direction through the microstructure. For modelling irreversible fatigue damage accumulation, a kinematic hardening approach will be implemented in the model. Gradual element stiffness degradation will also be applied as a fatigue damage evolution.