Abstract
Skin injury is one of the most common wounds experienced by individuals, and these injuries can compromise the skin’s ability to protect the body from external pathogens. Skin wounds can become chronic, and lead to infections that are estimated to affect 4.5 million people per year in the United States, at an annual cost ranging from $28 to $97 billion. Therefore, it is important that we understand how skin injuries occur so that we can identify and design countermeasures to prevent skin injuries in circumstances that involved human body impact, and to ensure that devices such as less-than-lethal blunt projectiles are designed to not penetrate the skin. However, research on the mechanical characteristics of human skin to failure is limited, and we do not have the knowledge necessary to develop biofidelic skin surrogates and computational models that can be used to study open skin injury. Therefore, the goals of this dissertation were to characterize the mechanical properties of human skin in dynamic loading and failure and use this knowledge to develop an advanced computational model for studying skin injury. In particular, this work focuses on understanding the skin mechanics related to impact from blunt objects and investigating the sensitivity of skin injury tolerance to a variety of underlying tissues, impactor types, and skin properties.
A comprehensive test protocol for human skin was performed to characterize the mechanical properties of the tissue in loading modes that include uniaxial tension, compression, and stress relaxation tests to gain an understanding of skin characteristics at dynamic loading rates. The dynamic tensile tests and stress relaxation tests demonstrated the skin’s viscoelastic and nonlinear mechanical response. The skin was observed to be anisotropic at a lower loading rate (1/s), but isotropic at a higher loading rate (180/s). For the application of blunt impact, the experimental results informed the development of an isotropic, hyper-viscoelastic constitutive model, coupled with a damage function to capture the skin’s failure response. Three sets of skin material parameters were generated that represented the average, upper bound, and lower bound of the measured skin properties to demonstrate the biological variations as observed in the dataset. An independent set of destructive dynamic indentation tests of human skin were performed to provide response data for model validation. The damage constitutive model was implemented into a finite element analysis software as a user-defined material, which was verified by simulating the experimental tensile tests and validated against the dynamic indentation tests. Overall, the newly developed skin model simulated the uniaxial tensile response successfully but underpredicted the dynamic indentation failure response. It was found the skin’s through-thickness compression properties were an important factor in capturing the biofidelity of the skin during dynamic indentation. Therefore, a transversely isotropic material model for the skin tissue was recommended for future improvement.
A sub-system model that included skin, adipose tissue, and muscle was developed to investigate the skin failure threshold under various types of blunt impact. Both adipose and muscle models were obtained and calibrated from recent experimental data. Using independent stress relaxation and impact test data, the sub-system model was verified and validated. Lastly, a simulation matrix was designed to include the range of masses and velocities of less-than-lethal blunt projectiles that are used by law enforcement. Six sub-system models representing different body regions, skin properties, and impactor geometry were used to investigate the skin failure threshold. This investigation demonstrated that impactor geometry and the skin properties on the skin failure threshold was negligible except for the body regions where the boundary condition is different due to different amount of underlying tissues.
This dissertation delivers a comprehensive skin failure dataset and a skin computational model with the capability to simulate skin failure responses which are crucial to open skin injury research. The skin model facilitates the development of an engineering tool that can be used to predict skin failure in human body models or with physical skin simulants, which will allow engineers to develop and evaluate the effectiveness of protective systems and the safety of less-lethal blunt projectiles.
