CRACK-TIP SHIELDING AND ANTI-SHIELDING BY A BIMATERIAL INTERFACE

Abstract Spacial variations of the mechanical properties have a shielding or anti-shielding effect on the crack tip by inducing an additional crack driving force term, the material inhomogeneity term, C inh . This paper explores this effect by studying the influence of a sharp bimaterial interface on the effective crack driving force in a fracture mechanics specimen. Linear elastic or elastic -ideally plastic materials are assumed with a mismatch in the elastic modulus and/or yield stress at the interface. Following a numerical stress analysis, the material inhomogeneity term, C inh , is obtained by post-processing. This parametric study is especially focused on the effect of the distance between the crack tip and the interface

Hall characterization of epitaxial GaSb and AlGaAsSb layers using p-n junctions on GaSb substrates

The Hall Van-der-Pauw method is widely used to assess the electrical properties of GaSb based semiconductor layers. Semi-insulating GaSb substrates are not available, and therefore, Hall structures are generally grown on semi-insulating GaAs. The lattice mismatch of 7.8% between GaAs and GaSb results in high defect densities, which may influence the measurement. We investigated an alternative approach for Hall effect measurements using a p-n junction for the electrical isolation of the test layer from layers below. This allows antimonide based test layers with low defect density grown lattice-matched on GaSb substrates to be analyzed. Negligible leakage currents across the p-n junctions are key to ensure significant measurement results. n- and p-GaSb layers show similar carrier concentration if grown on GaSb or semi-insulating GaAs, with the exception of highly n-doped layers >5 x 1017 cm−3. However, majority carrier mobilities were systematically higher on GaSb substrate, explained by a lower density of structural defects. Furthermore, the sample design with p-n junction enabled Hall effect measurements of quaternary p-Al0.2Ga0.8As0.02Sb0.98 layers, which was impossible for those same layers grown on GaAs due to strain induced phase-separation. The methodology is presented for antimonides, but it is applicable to a wide range of material systems including metamorphic structures

Crack driving force in twisted plywood structures

Twisted plywood architectures can be observed in many biological materials with high fracture toughness, such as in arthropod cuticles or in lamellar bone. Main purpose of this paper is to analyze the influence of the progressive rotation of the fiber direction on the spatial variation of the crack driving force and, thus, on the fracture toughness of plywood-like structures. The theory of fiber composites is used to describe the stiffness matrix of a twisted plywood structure in a specimen-fixed coordinate system. The driving force acting on a crack propagating orthogonally to the fiber-rotation plane is studied by methods of computational mechanics, coupled with the concept of configurational forces. The analysis unfolds a spatial variation of the crack driving force with minima that are beneficial for the fracture toughness of the material. It is shown that the estimation of the crack driving force can be simplified by replacing the complicated anisotropic twisted plywood structure by an isotropic material with appropriate periodic variations of Young’s modulus, which can be constructed based either on the local stiffness or local strain energy density variations. As practical example, the concepts are discussed for a specimen with a stiffness anisotropy similar to lamellar bone

An attempt to OHL dynamic line rating by strain-vibration detection of tower

There are many on-line overhead line (OHL) thermal monitoring systems, which are commonly known as dynamic line rating (DLR) systems using different sensors and approaches. Interaction between sag, tension and conductor temperature of OHL enables different approaches using sensors for detecting needed parameters for line ratings. The ambition of this project was to consider deformation and vibration behaviour or structural critical components of tower as a source of information for conductor tension parameters. Vibration behaviour or structural critical components is an important source of information for deciding on the mode of exploitation by a rapid adaptation to existing conditions currently encountered. The concept of monitoring is based on stress-strain analysis, vibration and deflection behavior of structural component. Correlation between tower deflections and conductor tensions are well-defined in case of large angle of tower. In tested polygon of 220 kV OHL there was section with large tower angle and situation of right-of-way in forest, which played barrier for wind. So, condition for hot spot section of line was likely fulfilled. The monitoring system based on algorithm which incorporates the geometric characteristics via calibration function. Dynamic line rating (DLR) systems is supported by numerical analysis of the structural components of the model enables simulation of mechanical behaviour and deflection of conductor and tower