Research On Dynamic Mechanical Properties Of Soft Muscle Tissue

Soft materials of human tissues and bullet-proof vests will inevitably be subject to impact loading. In order to design to a body-protection system that can withstand impacts from various directions, the protective ability of the system must be simulated and evaluated accurately, then be optimized. In such simulation-based evaluation and optimization, dynamic mechanical properties of used materials are required. These materials include both biological materials of human tissues and soft materials of body-protection systems. However, compared to metals and ceramics used in a body-protection system, dynamic mechanical properties of soft materials, especially biological materials of human tissues are poorly understood, mainly because of the difficulty in testing their dynamic behavior under impact loadings.With regard to the difficulty, this study mainly based on experimental research and has proposed a set of accurate, reliable Split Hopkinson Pressure Bar (SHPB) and Split HopkinsonTensile Bar (SHTB) experimental techniques to test dynamic mechanical properties of biological materials. Using these techniques, we have obtained dynamic compression and tensile stress-strain curves of muscles, along their fiber direction and perpendicular to the fiber. Further, a transverse isotropic hyperviscoelastic constitutive model has been built to describe the behavior of muscles at a wide range of strain rate.In this paper, analysis has been performed systematically on weak transmission signal and stress uniformity in SHPB and SHTB experiments on soft materials. With respect to weak transmission signal, we selected bars of small wave impedance and very sensitive sensors. Viscoelastic bars of small wave impedance show advantage in improving the transmission signal, but the dispersion and attenuation of a stress wave occur in viscoelastic bars. A code to compute propagation coefficient in viscoelastic bars has then been compiled based on DFT algorithm. The code has been validated by comparing calculation results with propagation coefficient and phase velocity obtained experimentally. Semiconductor gauges and quartz piezoelectric sensors have bee applied to detect weak signals, particularly the latter not only be used to detect weak signal but also to detect the stress uniformity of the specimen in SHPB experiments. Solutions to the problems in using these sensors, such as the calibration of various sensors, non-linearity of semiconductor gauge and signal interference in due to inertia signal in detecting the stress uniformity, have been proposed in this thesis.Stress uniformity of a sample is a necessary to the validity of SHPB and SHTB. In this study, we analyze the influence of five dimensionless parameters (β,θ/τs, Eα/Es, tr/τs, vr/Cs) on stress or strain uniformity of viscoelastic materials, based on dimensional analysis and numerical simulation using LS-DYNA. We find that the loading with trapezoidal wave of rise time of 2τs easily leads to stress or strain uniformity of a viscoelastic specimen of small wave impedance. To a certain material loaded with trapezoidal wave of rise time of 2τs, the smallerβ, the largerθ/τs and Ea/Es, the shorter duration to achieve stress uniformity. And there is a limit of vr/Cs, when loading with trapezoidal wave of rise time no longer than 2τs, it is much easier to achieve stress uniformity. Both stress or strain uniformity is detected by using sensors.To analyze the properties of muscle tissues of living creatures, fresh porcine muscle being properly preserved and processed has been taken as the research object of this study, and the effect of the duration after the death on experiments has been neglected. Quasi-static compression and tensile experiments have been conducted by using Shimadzu testing machine and biological testing machine. It has been found that the mechanical properties of the porcine muscle preserved in kreb liquid are almost unchanged after the death of 17-hour. Combining existing researches and experiments of this study, we consider that the experimental data is comparable although some difference. Our experiments show that compression strength vertical to fibers is much larger than that along them wherever opposite result will be obtained in tensile. The deformation of specimens has been analyzed based compression/tensile along fibers or vertical to them. Fracture of specimens under tensile has also been discussed.Using improved SHPB and SHTB devices, we have conducted a series of experiments on the muscles of porcine hams. The compression and tensile stress-strain curves of the muscles along their fiber and perpendicular to the fiber at high strain rates have been obtained. The experiments shows that the muscle has a significant strain rate effect at a very wide range of strain rates, flow stress at a high strain rate is much larger than that under a quasi-static loading. Loading direction also affects their mechanical behaviors. Compression strength vertical to fibers is much larger than that along them. Tensile strength along fibers at a low strain rate is larger than that vertical to fibers. But at high strain rates tensile strength vertical to fibers is larger than that along fibers at a small strain. When strain increases, the strength of these two directions are not obviously different. Flow stress along fibers seems to become larger than that vertical to fibers.This thesis firstly developed a transverse isotropic hyperviscoelastic constitutive model to describe the behavior of soft tissues like muscles under quasi-static loadings or high strain-rate loadings. Through experiments and experimental data processing, the parameters of the constitutive have been obtained and well cover the experimental results. Consequently, this study can provide material data to numerical simulation of the behavior of the tissue of human muscles.