Angelika Brueckner-Foit

Passive vibration compensation in scanning white-light interferometry

We present a passive vibration compensation approach in scanning white-light interferometry (SWLI). A pointwise distance measuring interferometer (DMI) obtains fast temporal distance changes during the white-light depth-scan of an aerial-measuring Michelson white-light interferometer for topography measurement. Both interferometers share a part of the optical path so that the measurement spot of the DMI is within the field of view of SWLI. With the real positions of the interferometer with respect to the measuring object during the depth scan known from DMI measurements, we can compensate for the influence of unintentional distance changes caused by environmental vibrations or scanner nonlinearities. By reordering of the captured image frames and improved correlogram interpolation, we are able to reconstruct correct signals from completely distorted (and unusable) SWLI signals. Although the basic idea of the system already has been published, we improved the signal reconstruction technique so that the specimens topography measurement can be obtained with the same accuracy as without any vibrations or scan distortions influence. In addition, we demonstrate the feasibility of the approach by different practical measurements with and without vibrations.

Lifetime Prediction of Components Including Initiation Phase

The lifetime distribution of a component subjected to fatigue loading is calculated using a micromechanics model for crack initiation and a fracture mechanics model for crack growth. These models are implemented in a computer code which uses the local stress field obtained in a finite-element analysis as input data. Elemental failure probabilities are defined which allow us to identify critical regions and are independent of mesh refinement. An example is given to illustrate the capabilities of the code. Special emphasis is put on the effect of the initiation phase on the lifetime distribution.

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.

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.

Investigation of the Three-Dimensional Shape of Slip Bands in Fatigued Dual Phase Steel

The formation and the three-dimensional shape of slip bands in a fatigued dual phase steel were analyzed with the purpose of understanding the relation between fatigue crack initiation and the topography development on the specimen surface. Fatigue tests with small dog-bone-shaped specimens were conducted under fully reversed axial loading (R = -1) with a constant stress amplitude and were interrupted when the first slip bands occurred and at defined numbers of load cycles, respectively. Subsequently the surface topography of the specimen was investigated with a white light interferometer with hundredfold magnification and high numerical aperture (NA = 0.9) which allows analyzing the surface of individual grains. The results were confirmed by additional atomic force microscopy measurements. Based on this analysis the height, width and length of the slip bands are known at different stages of the fatigue process. The results obtained using white light interferometry and AFM, were checked by cutting individual slip bands with the help of focused ion beam thus revealing the true shape of the slip bands.