explains a smaller CWS effect for the larger stand-off distance. In contrast to the PC targets, the SC jet did not penetrate through the full depth of the target. However, similarly to the PC targets, the penetration depth increased from the NWS using the wave shaping methods employed in the SWS and CWS charges. For the NWS charge, the SC jet did not completely penetrate through both target types. This allowed a comparison of the jet penetration ability into the plain concrete and UHPC. As measured, the borehole into the plain concrete was 28 % deeper than for the UHPC target. Thus the UHPC, which has a similar density to the plain concrete, offers an improved level of protection to SC jet penetration due to the higher compressive strength. The general width of the borehole was also less for the UHPC targets. As seen from Figs. 39.3 and 39.4, a significant difference in the target material response was the level of spallation. The level of spallation on the plain concrete is extensive, whereas the UHPC target, which has a higher tensile strength, has mostly only cracking on the rear face for the present charge-standoff configurations. However, the extent of the spallation for UHPC was still less than the corresponding test against the PC. Further testing is required for generalising the conclusion on the improved UHPC performance to other charge configurations and standoffs. 39.4 High Strain Rate Characterisation The prediction of a material’s dynamic response requires high strain rate characterisation data for closing the constitutive equations for a material model employed in a hydrocode. The well established technique for obtaining high strain-rate characterisation data involves testing using a Kolsky Bar also known as a Split Hopkinson Pressure Bar (SHPB) apparatus. The conventional design [8] employs a striker rod generating a pulse propagating through a sample sandwiched between two instrumented elastic rods. The stress state achieved within a sample being tested by the technique should be close to homogeneous. Thus, for aggregate containing materials such as concrete the sample cross-section area should exceed that of a representative volume of the material. As a result, for plain concrete without reinforcement the required diameter should exceed at least a few inches. With this arrangement, a striker bar launching system can be difficult to implement. To relieve this requirement, along with the potential to extend the device capability to tensile testing also, an alternative SHPB design has been suggested in [9]. This design exploits hydraulically stored tensile energy quickly released by a breaking section and transformed into a compression pulse. This design has succeeded in a number of facilities such as [10]. Recently, DSTO has realised the concept [9] as a facility shown in Fig. 39.5. The facility consists of a hydraulically actuated 2.4 m long pretension bar made from maraging steel with a breaking bolt connecting the hydraulic actuator with the pretension bar. After the bolt fracture, the tensile energy stored in the pretension bar is released at a calibrated load. The pulse is transmitted into an adjoining 3.5 m long 100 mm diameter aluminium input bar as a compressive pulse that propagates through the sample into a similar 3.5 m long aluminium output bar. This design is suitable for testing plain concrete samples and can be used for UHPC samples by replacing the aluminium input/output bars with 60 mm diameter steel bars. The bars are instrumented with strain gauges placed in the middle of the bars and connected to suitable signal conditioning and data acquisition equipment. Fig. 39.5 DSTO large diameter split Hopkinson pressure bar 272 A.D. Resnyansky and S.A. Weckert
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