J. Radziejewska

Effects of pre-fatigue on the strain localization during tensile tests of DP 500 steel at low and high strain rates

The analysis involved subjecting DP 500 steel to pre-fatigue loads, and then tension at high strain rates using Hopkinson bar. Digital image correlation method was used to investigate how the pre-fatigue loads change the strains’ distribution on the surface of the sample subjected to tension. The analysis involved both films recorded at low rates of deformation (1.0 × 10−2 s−1) using ARAMIS system and the images captured with a high-speed camera during dynamic deformations with a Hopkinson bar (6.0 × 102 s−1). It was noted, based on the micro-structural analysis, that pre-fatigue loads cause the formation of micro-damages in the examined material. Thus, macroscopically observed stress–strain characteristic as well as Huber–Mises substitute strains’ distribution determined locally by the image correlation method is also subject to changes. The observed effects include the following: reduction of deformation corresponding to the tensile strength, decrease in elongation at break, and increase in yield limit and tensile strength. The observed effects are intensified with an increased stress value and pre-fatigue cycles’ number. Furthermore, these phenomena are more intensive in the conditions of dynamic deformation.

Surface Layer Morphology Due to Laser Alloying Process

Abstract The results of experimental research on the influence of laser alloying parameters on the structure and chemical composition are presented. The alloying process was performed with a continuous CO2 laser, of a 2.5 kW power, at different densities of energy and different interaction times of beam on material. The experiments were done on carbon steel, which was alloyed with powders of tungsten carbide and cobalt stellite. The microstructure, the distribution of alloyed elements, and the microhardness of the surface layer were studied after a laser alloying process. It was shown that alloying layer morphology depends on the laser alloying parameters, especially on interaction time. The research has verified that the motion process of liquid material determines the alloyed layer morphology and indicates a necessity to take into account the convection process.

Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties

Laser Beam Machining (LBM) has been successfully applied for improvement of surface layer properties of machine elements. Some of laser treatments are based on melting of surface with a laser beam. Among them is laser hardening, as well as cladding and alloying. The results of broad scope research have shown that surface roughness of elements which underwent the laser melting is too high to apply the process without an additional abrasive machining, even at the optimum parameters of the laser treatment. In most cases after the surface melting with laser beam the tension stresses are observed. That is demonstrated by the cracks in the surface layer and deterioration of its properties (Dietrich Lepski et al., 2009).

Laser induced shock waves: Pressure waves and residual stress in thin films

Properties of materials under conditions of dynamic deformation significantly differ from those in static conditions. They depend on rate of deformation, microstructure of the material and temperature. The study of processes occurring in materials at very high speed of deformation is possible using absorption of high power, nanosecond laser pulses [1, 2]. Short pulse laser radiation and carefully selected absorption layer as well as inertial layers allow obtaining a shock wave pressure up to a few GPa [1]. The dependence of pressure wave profile on material of the inertial layer, measured using polymer PVDF transducer is shown in Fig.1. The maximum pressure of around 1500 bars was attained for glass, due to its highest acoustic impedance.