Flow curves and deformation of materials at different temperatures and strain rates

The metal matrix composite materials consist of 10, 15 and 20 vol.% alumina particles in an aluminium alloy 6061-T6 matrix. Specimens were taken from hot-extruded rods and the tensile tests were carried out at various strain rates. High strain rate tests were performed by the split-Hopkinson-bar method. The fracture surfaces were observed with a scanning electron microscope to understand the fracture mechanisms. From the results, it can be concluded that: (1) the alumina-reinforced composites are more sensitive to strain rate than the unreinforced material, (2) the strain rate sensitivity of the ultimate tensile strength of these composites increases with increasing temperature and (3) the addition of alumina particles will reduced the ductility of aluminium alloy 6061-T6, irrespective of the strain rate and temperature.

High strain rate tensile properties of an (Al2O3 particles)-(Al alloy 6061-T6) metal matrix composite

The effects of various thermal treatments,i.e., oil quench and different tempering conditions, on quasi-static and impact fracture toughness, stress-strain characteristics, hardness, and Charpy energy of 5140 H steel were examined. During quasi-static and impact loading notched round tensile specimens were used with a prefatigued crack. A specially designed device together with a pendulum hammer and electronic measuring system was used enabling testing of the opening mode fracture toughness at loading rates up to K1= 3 x 106 MPa√m per second. It has been found that within the region of the lower tempering temperatures, 500 K≤ 650 K, the critical stress intensity factor KIc determined from impact testing is lower than that obtained during slow loading, whereas at the higher tempering temperatures, 650 K ≤T* ≤ 900 K, dynamic KIu values show a tendency to be higher than their quasi-static counterparts. This behavior was analyzed quantitatively using the Hahn-Rosenfield model which relates tensile properties to fracture toughness. A good agreement was found between quasi-static experimental results and the model. The relation between Charpy energy Kv and the critical stress intensity factor KIc was also evaluated. Changes of the fracture toughness are discussed within the framework of SEM fractographs taken after quasi-static and impact tests.

Effect of tempering on quasi-static and impact fracture toughness and mechanical properties for 5140 H steel

This thesis explores localization in geological applications in the context of multi-physics, nonequilibrium systems. The term multi-physics refers to systems that contain a mix of physical processes that are allowed to operate simulateneously. The thesis begins with an investigation of purely mechanical systems and reviews localized structures stemming from material nonlinearities. In later chapters further processes are considered and nonlinear effects due to feedback mechanisms are studied and we describe how these feedback mechanisms can lead to fascinating spatio/temporal structures. The simplest model considered here is that of a strut on a Winkler type foundation and it is well known that depending on the foundation characteristics various localized buckling patterns can emerge if the strut is under compression. Owing to the steady movement of tectonic plates the strut is deformed at a constant rate. We investigate how the buckling pattern evolves over time and offer qualitative interpretations of the observed behaviour. Other localization patterns encountered in geological applications are the formation of shear zones. The chapter on energy based localization criteria introduces a generalization to the slip line field theory for application in such problems. The new formulation proposes a thermodynamic continuum-mechanic framework that fulfils the energy balance under consideration of the second law. The energy balance admits multiple steady states and, thus, this approach allows us to identify a critical mechanical dissipation parameter, here called the Gruntfest number, which separates loading conditions that lead either to homogeneous or localized plastic deformation. The geometry of the localized failure follows the generalized slip line pattern and this is used to simplify the semi-analytical solution of localization problem using the method of characteristics. The result is verified in numerical studies that are performed with the newly developed code REDBACK that is capable to handle thermo-mechanical coupling simultaneously. The structure of REDBACK is detailed here as well.

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Localization phenomena in geological settings

The effects of various thermal treatments, i.e. oil quench and different tempering conditions, on quasi-static and impact fracture toughness, stress-strain characteristics, hardness and Charpy energy of 5140 H steel were examined. During quasi-static and impact loading notched round tensile specimens were used with a prefatigued crack. A specially designed device involving a pendulum hammer and electronic measuring system was used for measurement of the opening mode fracture toughness at loading rates of up to, K I = 3×10 6 . It has been found that within the region of the lower tempering temperatures, 500 K≥T*≥650 K, the critical stress intensity factor K Ic determined from impact testing is lower than that obtained during slow loading. Whereas at the higher tempering temperatures, 650 K≥T*≥ 900 K, dynamic K Ic values have a tendency to be higher than their quasistatic counterparts. This behavior was analysed quantitatively using the Hahn-Rosenfield model which relates tensile properties to fracture toughness. A good agreement was found between quasi-static experimental results and the model. The relation between Charpy energy K V and the critical stress intensity factor K Ic was also evaluated.

Quasi-static and impact fracture toughness of 5140 h steel after different thermal treatments

The influence of microstructure, strain rate, and test temperature on the tensile properties of 18Ni(350) maraging steel was examined. The strain-rate and temperature dependence of the yield strength is less than that observed in low-strength carbon steels. The results suggest that the thermally activated flow process involves both the by-passing of a linear barrier, i.e., the disruption associated with a dislocation core, and the by-passing of a localized obstacle, the latter being specific to the microstructure being examined. The tensile ductility of aged 18Ni(350) maraging steel also undergoes a ductile-brittle transition. The extent of this ductility loss depends upon the matrix solute content and the presence of either reverted austenite or a highly dislocated substructure.

S. Beitscher

Papers

The Strain-Rate and Temperature Dependence of 18Ni(350) Maraging Steel Tensile Properties