Frank Zeismann

Influence of Martensitic Phase on Microcrack Propagation in a Ferritic-Martensitic Steel

Hard phases such as martensite regions affect micro-crack extension by blocking the plastic zone ahead of the crack tip, but also by changing the crack opening which can be taken as loading quantity for cracks. This paper deals with the measurement of crack opening for microcracks in a ferrite/martensite dual phase steel. The methods used are in-situ testing in the SEM, X-ray tomography, and digital image correlation. It was found that martensite regions affect the relative displacement of the crack phases both at the crack tip and in the crack wake.

Fatigue Properties of a Graded Steel Component

Rods of a Cr-V alloyed heat treatable steel in a soft annealed condition were locally heated to a temperature above austenization temperature and then subjected to an integrated forming and cooling procedure. The resulting component possesses a graded microstructure consisting of martensite, deformed base material and a transition phase with mixed microstructure. Miniature specimens were cut from this component which allowed assessing the local properties of the graded steel both under tensile loading and under cyclic loading. The martensite region had high strength and showed comparatively long lifetimes. The dominant damage accumulation process was identified to be crack initiation from local defects. The base material, on the other hand, contained numerous crack initiation sites. In-situ observation of the damage accumulation process on the surface showed how macroscopic cracks were formed by microcrack growth and coalescence. It is shown how these findings can be incorporated into a mechanism-based lifetime model.Copyright

Experimental Determination of Crack Driving Forces for Small Cracks

The crack tip field of small fatigue cracks was measured using digital image correlation DIC. For this purpose, smooth specimen were fatigued until a certain amount of damage was visible on the surface. This specimens were then placed in a micro-tensile device in the SEM. The grey value patterns obtained at two different load levels were analyzed with DIC. Fitting the Williams series for the crack tip field to the corresponding DIC-displacement field in the crack tip area yielded the mode-I, and the mode-II stress intensity factors together with the T-stresses. It was found that the experimental values of the stress intensity factor were comparatively high with pronounced mode-II contributions.

Extension of Fatigue Cracks in a Low-Alloy Steel After Massive Plastic Deformation

Fatigue crack extension was studied in a ferritic–pearlitic steel in two different material conditions. The first one was the virgin state with a typical pearlitic microstructure containing comparatively large stacks of cementite lamellae. The second material contained predominantly globular cementite and stacks of very fine cementite lamellae and was found after massive plastic deformation during a thermo-mechanical forming process. These differences in the microstructure lead to significantly different mechanisms in the damage accumulation process under fatigue loading. Stacks of cementite lamellae pearlite were both crack initiation sites and microstructural barriers for small cracks in the virgin material, whereas the fine lamellae in the plastically deformed (cold-worked) material could cause crack deviation, but no crack initiation and no significant crack arrest or crack retardation.