Veli-Tapani Kuokkala

A laboratory sized jaw crusher with uniform movement of the jaws, the dual pivoted jaw crusher, was used to determine the relationship between wear and work. Wear was concentrated on the jaw plates opposing each other and was measured as mass loss of the specimens. Work was measured directly from the force and displacement of the instrumented jaw, which allowed work to accumulate only from the actual crushing events. The tests were conducted with several jaw geometries and with two motional settings, where the relation of compressive and sliding motion between the jaws was varied. The tests showed that the relation between wear and work was constant in many of the tested cases. In certain tests with larger lateral and faster contact speed, wear occurred at relatively lower amounts of work. This behavior was more definite when the relation of wear and work was investigated using modified Archards wear equation. The results indicate that the lower amount of needed work could stem from the material reaching a dynamic situation, where the flow stress becomes increasingly strain-rate dependent.

This paper presents an improved experimental setup for high strain rate testing based on the modified Tensile Hopkinson Split Bar device developed previously at TUT. The test setup can be used to study the effects of a sudden large change in the strain rate on the stress flow of the material. The setup allows deforming the sample at a low rate and at isothermal conditions before the high rate loading. During the strain rate jump, the deformation rate is rapidly increased by approximately six orders of magnitude. In this work, the low and high rate deformation of the specimen was recorded with a combination of low and high-speed digital cameras and analyzed using the Digital Image Correlation technique. The measurement provides information about the effects of the strain rate jump on the macroscopic response of the material and allows accurate observation of the deformation of the sample just before, during, and immediately after the strain rate jump, when the conditions change from isothermal to adiabatic. In this paper, we present the results for a metastable austenitic stainless steel and discuss the effects of the strain rate jump on the strain-hardening rate, compare the experimental results with numerical results from a thermomechanical model, and evaluate the effects of the preceding deformation at a low strain rate on the strain localization. We conclude that the strain rate jump results in a clear decrease in the strain-hardening rate, the deformation following the jump is uniform along the gauge section, and that the strain localization is not significantly affected by the strain rate or the amount of pre-strain in the studied conditions.

Rubber covered nip rolls have a number of applications in the paper industry. One of them is the coating process, where color or size is applied on one or both sides of the paper web. Finite element modeling can be an accurate and inexpensive tool in the optimization of the coating process, as long as it reflects the real phenomena. Therefore, the choice of optimal material parameters for the model is one of the most important tasks. In this study, a universal material testing machine and a compressive Split Hopkinson Pressure Bar (SHPB) equipment were used to produce material data for OptiSizer test set-up conditions (strain rate 1 s⁻¹, room temperature, ambient moisture) as well as for OptiSizer production conditions (temperature 60°C, strain rate 1600 s⁻¹, saturated moisture in artificial paper mill water at test temperature). The OptiSizer machine was modeled using the Abaqus software. The nip pressure was calculated for linear elastic and hyper-elastic models of the covers and compared with measured data. The model shows good agreement with the measurements in the test set-up conditions and can therefore be used to simulate the mechanical contact in coating processes in actual production conditions and to optimize the shape and materials of the contacting rolls.