Search

This paper describes a new single-shear specimen (SSS) and method to characterize the dynamic shear behavior of bulk metals using a traditional Split Hopkinson Pressure Bar (SHPB). By this method, the shear behavior of materials can be tested conveniently over a wide range of strain rates within 105 s−1. This technique was applied to a 304 austenitic stainless steel (ASS) under shear strain rates from 0.001 s−1 to 38700 s−1 at room temperature. Based on finite element (FE) simulations, it was found that the deformation of the specimen shear zone was dominated by shear stress/strain components. Stress state parameters represented by stress triaxiality η and Lode angle parameter θ - were found very close to zero, indicating a deformation mode of simple shear. Besides, an obvious gap existed between the local deformation behavior in the specimen shear zone and the macroscopic stress-strain relations measured by the strain gauges on the SHPB bars. A correction coefficient method was adopted to extract the real shear behavior from the experimentally obtained force-displacement data. Through comparisons between the tested and simulated stress-strain curves, a good agreement was obtained.

International audience; This paper presents a systematic study of the thermo-viscoplastic behavior of a 304 austenitic stainless steel (ASS). The experiments were conducted over a wide range of strain rates (10 − 3 s − 1 to 3270 s − 1 ) and temperatures (-163°C to 172°C), for which the deformation behavior of 304 ASS becomes more complex due to the strain- induced martensitic transformation (SIMT) effect. Dynamic tests at low/elevated temperatures were conducted using the Hopkinson technique coupled with a cooling device/heating furnace, and temperature distribution within the specimen was verified to be uniform. Experimental results showed that the strain hardening rate of 304 ASS was strongly affected by SIMT effect. For quasi-static tests (10 − 3 s − 1 to 1 s − 1 ) at low temperatures (-163°C to -20°C), the stress-strain relations exhibited an S-shape and a second strain hardening phenomenon. The strain rate sensitivity and temperature sensitivity of 304 ASS were also different from metallic materials deformed by dislocation glide. Several unexpected phenomena including the negative strain rate sensitivity and the changing temperature sensitivity from quasi-static to dynamic tests were observed. Based on experimental results, an extension of the Rusinek-Klepaczko (RK) model considering SIMT effect was used to simulate the deformation behavior of 304 ASS: it predicted flow stress curves of 304 ASS above -60°C correctly. In addition, to validate the extended RK model and the identified model parameters, numerical simulations of ballistic impact tests of 304 ASS plates at various temperatures were carried out, showing a good agreement with experiments.

This paper presents a novel methodology to find out canting errors in the facets, i.e. mirror modules, of heliostats. An optimization procedure is established to fit simulated heliostat flux distributions to those captured on a white target. On the basis of a convolution-projection optical model, a deterministic algorithm - named DIRECT - has been successfully implemented, reaching correlation coefficients up to 95.8%. In this instance, the procedure has been applied to a THEMIS heliostat presenting canting errors of its, faceted modules. From the optimization results, the heliostat modules were accordingly readjusted. And the heliostat optical quality has been significantly increased, validating the proposed methodology. The authors thank the help provided by Antoine Pérez, Yann Volut and William Baltus during the experimental phase at THEMIS solar power plant. A. Sánchez-González is also indebted to Universidad Carlos III de Madrid for the mobility grant that funded the stay at PROMES-CNRS during spring-summer 2016.