Xu Nie

Dynamic Tensile Response of Porcine Muscle

The stress-strain response of a porcine muscle along and perpendicular to the muscle fiber direction was characterized over a wide range of strain rates under uniaxial tension. A modified Kolsky tension bar was used to conduct the experiments at high strain rates. Tubular specimen geometry was used to achieve uniform loading within the specimen and to minimize lateral inertia effect. Loading pulse was controlled to facilitate constant strain rates and dynamic stress equilibrium. Quasi-static experiments were also performed to explore the rate effects over a wider range of strain rates. The results show that the nonlinear tensile stress-strain responses in both directions along and perpendicular to the fibers are highly sensitive to strain rates. Compared with high-rate compression response, the strain rate sensitivity in the tensile test is less dependent on the fiber orientation to the loading direction.

Inertia effects on characterization of dynamic response of brain tissue

Modeling and simulation of traumatic brain injury (TBI) resulted from collision or blast loading requires characterization of mechanical response over a wide range of loading rates under valid testing conditions. In this study, mechanical response of fresh bovine brain tissue was studied using the two modified Kolsky bar techniques. Radial deformation behavior of annular specimens, which are typically used to characterize the dynamic uniaxial compressive response of biological tissues, was examined using a modified Kolsky bar and a high speed camera to collect images while the specimen deforms at an axial strain rate of 2000s(-1). The high-speed images revealed inhomogeneous specimen deformation possibly brought about by radial inertia and causing a multi-axial stress state. To acquire valid stress-strain results that can be used to produce constitutive behavior of the soft materials, a novel torsion technique was developed to obtain pure shear response at dynamic loading rates. Experimental results show clear differences in the material response using the two methods. These results indicate that the previously demonstrated annular specimen geometry aimed at reducing inertia induced stress components for high rate soft materials uniaxial-compressive testing may still possess a significant component of radial inertia induced radial stress which consequently caused the observed inhomogeneous deformation in brain tissue test samples.

High Strain Rate Pure Shear and Axial Compressive Response of Porcine Lung Tissue

In this study, both the dynamic shear (torsion) and axial compressive responses of porcine lung tissue were examined using modified Kolsky bar techniques. High-rate compression data were collected using a Kolsky bar with a hollow transmission bar on annular specimens at strain rates between 1000-3000 s(-1). The radial deformation of the annular specimen was recorded on a modified single loading Kolsky bar using highspeed imaging capabilities. The collected images and analysis of boundary movement indicated inhomogeneous specimen deformation induced by radial inertia, which significantly altered the desired uniaxial stress state in such high-rate compression test techniques. A novel torsion experimental technique was developed to obtain the dynamic pure shear behavior of lung tissue at shear strain rates above 500 s(-1) without inertia effects. The pure shear response was found to be two orders of magnitude weaker than the uniaxial compressive response when compared by equivalent stress-strain relations. [DOI: 10.1115/1.4007222]