Strain Analysis by a Total Generalized Variation Regularized Optical Flow Model

In this paper we deal with the important problem of estimating the local strain tensor from a sequence of micro-structural images realized during deformation tests of engineering materials. Since the strain tensor is defined via the Jacobian of the displacement field, we propose to compute the displacement field by a variational model which takes care of properties of the Jacobian of the displacement field. In particular we are interested in areas of high strain. The data term of our variational model relies on the brightness invariance property of the image sequence. As prior we choose the second order total generalized variation of the displacement field. This prior splits the Jacobian of the displacement field into a smooth and a non-smooth part. The latter reflects the material cracks. An additional constraint is incorporated to handle physical properties of the non-smooth part for tensile tests. We prove that the resulting convex model has a minimizer and show how a primal-dual method can be applied to find a minimizer. The corresponding algorithm has the advantage that the strain tensor is directly computed within the iteration process. Our algorithm is further equipped with a coarse-to-fine strategy to cope with larger displacements. Numerical examples with simulated and experimental data demonstrate the very good performance of our algorithm. In comparison to state-of-the-art engineering software for strain analysis our method can resolve local phenomena much better

Fatigue behavior and phase transformation in austenitic steels in the temperature range -60°C≤T≤25°C

AbstractFatigue behavior and phase transformation in the metastable austenitic steels AISI 304, 321 and 348 were investigated in the temperature range from -60°C to 25°C by means of stress-strain hysteresis, electrical resistance and magnetic measurements. The steels show differences in austenite stability, which lead to significant changes in deformation induced martensite formation and fatigue behavior in total strain controlled low cycle fatigue tests. Dependent on the type of steel and testing temperature similar values of martensite fraction but different strengths developed

Influence of Morphology of Deformation Induced α´-Martensite on Stress-Strain Response in a Two Phase Austenitic-Martensitic-Steel

In this research work specimens of the metastable austenitic steels AISI 304 and AISI 347 with one phase (fully austenitic) and two phase (austenitic-α ́-martensitic) microstructure were monotonically loaded at ambient temperature. Using stress-strain and temperature measurements the deformation behavior was characterized in detail. To study the influence of morphology of deformation induced α ́-martensite on the stress-strain response a phase field model for α ́-martensite transformations was developed. With this approach it was possible to model the two phase austenite-α ́-martensite microstructure and investigate the deformation behavior on the micro level. With optical microscopy, magnetic and x-ray measurements the microstructure characterization of fully austenitic and austenitic-α ́-martensitic steels was realized

A combined phase field approach for martensitic transformations and damage

A combined continuum phase field model for martensitic transformations and fracture is introduced. The positive volume change that accompanies the phase transformation from austenite to martensite leads to an eigenstrain within the martensitic phase, which is considered within the present approach. Since the eigenstrain leads to both tensile and compressive loads, the model accounts for the sign of the local volume change. With aid of this model, the interactions between microcrack propagation and the formation of the martensitic phase are studied in two dimensions. Martensite forms in agreement with experimental observations at the crack tip and thus influences the crack formation. The numerical implementation is performed with finite elements. For the transient terms, an implicit time integration scheme is employed