Hydraulic machine tests for compression of a quasi-brittle material at medium strain rate

Hopkinson bar techniques for the intermediate strain rate testing of bovine cortical bone

“…Material testing in the ISR range has no universally accepted testing method, even though a variety of techniques have been developed [12][13][14][15][16][17]. The unique challenge of the ISR regime is that it lies above the upper limit of standard tensile test machines and below the lower limit of typical SHB arrangements and is therefore also referred to as the medium strain rate regime [13] or the sub-Hopkinson bar regime [16].…”

“…The unique challenge of the ISR regime is that it lies above the upper limit of standard tensile test machines and below the lower limit of typical SHB arrangements and is therefore also referred to as the medium strain rate regime [13] or the sub-Hopkinson bar regime [16]. Furthermore, while ISR test speeds are modest, they must be attained quickly, causing inertia effects in the load frame and load sensors.…”

“…Typically, only the nominal or average strain rates are reported, or strain rate histories are reported that vary significantly. Custom-built lightweight machines are sensitive to changes in the specimen stiffness, which results in nonlinear displacement histories [12,15,17], while the inherent compliance of ESH machines causes a similar effect [13,16]. Therefore, specimen-mounted strain gauges [14,16] or optical techniques [13,21] are often required to capture the strain history.…”

“…In tension tests, this is accomplished by using 'run-up' fixtures, such as 'fast jaw' grips [14] or 'slack adaptors' [16]; while, in compression tests, an initial gap is left between the specimen and the upper anvil [13].…”

“…An alternative approach to dealing with stress wave oscillations is to use Hopkinson bar techniques, where the incident bar is replaced by a flywheel [19] or hydraulic actuator [20], and the extended test durations are accommodated by using atypically long bars [20] or by accounting for stress wave reflections using wave separation techniques [13,21,22].…”