A modified servo-hydraulic machine for testing at intermediate strain rates

Othman, Ramzi; Guégan, Pierrick; Challita, Georges; Pasco, Franck; Lebreton, Daniel

doi:10.1016/j.ijimpeng.2008.06.003

Cited by 63 publications

(45 citation statements)

References 28 publications

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“…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].…”

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confidence: 99%

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

Cloete

Paul

Ismail

2014

Phil. Trans. R. Soc. A.

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Detailed knowledge of the dynamic viscoelastic properties of bone is required to understand the mechanisms of macroscopic bone fracture in humans, and other terrestrial mammals, during impact loading events (e.g. falls, vehicle accidents, etc.). While the dynamic response of bone has been studied for several decades, high-quality data remain limited, and it is only within the last decade that techniques for conducting dynamic compression tests on bone at near-constant strain rates have been developed. Furthermore, there appears to be a lack of published bone data in the intermediate strain rate (ISR) range (i.e. 1–100 s −1 ), which represents a regime in which many dynamic bone fractures occur. In this paper, preliminary results for the dynamic compression of bovine cortical bone in the ISR regime are presented. The results are obtained using two Hopkinson-bar-related techniques, namely the conventional split Hopkinson bar arrangement incorporating a novel cone-in-tube striker design, and the recently developed wedge bar apparatus. The experimental results show a rapid transition in the strain rate sensitive behaviour of bovine cortical bone in the ISR range. Finally, a new viscoelastic model is proposed that captures the observed transition behaviour.

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“…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.…”

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confidence: 99%

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

Cloete

Paul

Ismail

2014

Phil. Trans. R. Soc. A.

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“…Hence, the servo-hydraulic machine is equipped with a second force transducer; it consists in a threestrain-gauges instrumented tubular aluminum bar. This is inspired from the work of Othman et al [7], although there are two differences. In our case a tubular aluminum bar is used instead of a steel rod in order to increase the signal-to-noise ratio for soft materials.…”

Section: Strain and Stress Measurementmentioning

confidence: 99%

“…These solutions, however interesting, needs a high accuracy on the design of the pulleys and cables in order to achieve an acceptable synchronisation between loads at specimen extremities: the delay between the two loading time must be insignificant compared to the test duration. Recently, Othman et al [7] proposed a hybrid servo-hydraulic machine for intermediate strain rate testing. Namely, the conventional piezo-electric transducer is replaced by a three-gages instrumented elastic bar in order to capture transient effects.…”

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confidence: 99%

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Uniaxial tension test on Rubber at constant true strain rate

Petiteau

Othman

Verron

et al. 2012

EPJ Web of Conferences

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Abstract. Elastomers are widely used for damping parts in different industrial contexts because of their remarkable dissipation properties. Indeed, they can undergo severe mechanical loading conditions, i.e., high strain rates and large strains. Nevertheless, the mechanical response of these materials can vary from purely rubber-like to glassy depending on the strain rate undergone. Classically, uniaxial tension tests are made in order to find a relation between the stress and the strain in the material at various strain rates. However, even if the strain rate is searched to be constant, it is the nominal strain rate that is considered. Here we develop a test at constant true strain rate, i.e. the strain rate that is experienced by the material. In order to do such a test, the displacement imposed by the machine is an exponential function of time. This test has been performed with a high speed hydraulic machine for strain rates between 0.01/s and 100/s. A specific specimen has been designed, yielding a uniform strain field (and so a uniform stress field). Furthermore, an instrumented aluminum bar has been used to take into account dynamic effects in the measurement of the applied force. A high speed camera enables the determination of strain in the sample using point tracking technique. Using this method, the stress-strain curve of a rubber-like material during a loading-unloading cycle has been determined, up to a stretch ratio λ = 2.5. The influence of the true strain rate both on stiffness and on dissipation of the material is then discussed.

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Characterization of High‐Strain Rate Mechanical Behavior of AZ31 Magnesium Alloy Using 3D Digital Image Correlation

Wang

Erdman

et al. 2011

Adv Eng Mater

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Material response under dynamic crash loading is an essential aspect for automobile design to assess vehicle crashworthiness and improve safety performance. However, due to challenges in instrumentation, mechanical property characterization at strain rates of 0.1-10 3 s À1 is not standardized, although these conditions are most relevant to automobile collision scenarios. [1][2][3][4] First, in order to perform high-speed mechanical tests in this strain rate regime, the testing machine actuator is required to have speed capabilities on the order of several hundred in s À1 and necessary dynamic load capacity of several thousand pounds. Second, upon high-speed loading, load cells used for force acquisition exhibit significant ringing and the noise in the signal due to wave propagation, inertia effects, and the frequency response of the transducers. [1,3,[5][6][7][8] This often results in a great deal of uncertainty in stress analysis. Third, accurate strain measurements in the gage section become more difficult as strain rate increases. The most common method to estimate strains is based on the changes in actuator displacement, i.e., through the linear variable differential transformer (LVDT). However, LVDT is not accurate at high speed due to their limited frequency response. Further, LVDT measurements consist of displacements of the loading train and deformations of the entire specimen, hence the contributions of the machine/ loading train compliance to the outputs. [3] Another popular technique for direct measurement of strains of using resistance strain gages is successful other than the limitations of localized measurements and limited measurement range. Recent advances in non-contact optical displacement/strain measurement methods are a solution to the challenges encountered by LVDT and strain gages. [9][10][11][12] Digital image correlation (DIC) as one of the successful technique among the COMMUNICATION Characterization of the material mechanical behavior at sub-Hopkinson regime (0.1 to 1 000 s À1 ) is very challenging due to instrumentation limitations and the complexity of data analysis involved in dynamic loading. In this study, AZ31 magnesium alloy sheet specimens are tested using a custom designed servo-hydraulic machine in tension at nominal strain rates up to 1 000 s À1 . In order to resolve strain measurement artifacts, the specimen displacement is measured using 3D Digital Image correlation instead from actuator motion. The total strain is measured up to % 30%, which is far beyond the measurable range of electric resistance strain gages. Stresses are calculated based on the elastic strains in the tab of a standard dog-bone shaped specimen. Using this technique, the stresses measured for strain rates of 100 s À1 and lower show little or no noise comparing to load cell signals. When the strain rates are higher than 250 s À1 , the noises and oscillations in the stress measurements are significantly decreased from % 250 to 50 MPa. Overall, it is found that there are no significant differences in the elo...

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A modified servo-hydraulic machine for testing at intermediate strain rates

Cited by 63 publications

References 28 publications

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

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

Uniaxial tension test on Rubber at constant true strain rate

Characterization of High‐Strain Rate Mechanical Behavior of AZ31 Magnesium Alloy Using 3D Digital Image Correlation

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