Experimental characterization of high-strain-rate viscoelastic and damage behavior in anisotropic soft materials...

Characterizing soft materials at ultra-high strain rates (> 10³ s^-1) remains a significant challenge due to their nonlinear, large-deformation, and rate-dependent mechanical behavior. Despite these challenges, understanding material response in this regime is essential for a wide range of engineering and biomedical applications, including laser eye surgery, lithotripsy, and high-rate energy deposition in soft tissues. Laser-induced cavitation (LIC) has recently emerged as a powerful experimental approach for subjecting soft materials to extreme, localized loading rates on the microscale, regimes that are difficult to access using conventional mechanical testing methods. However, most biological tissues are anisotropic, optically opaque, and prone to damage at high strain rates, and their orientation-dependent ultra-high-rate mechanical behavior remains poorly understood.

The objective of this study is to develop and validate an experimental method that is based on laser-induced inertial cavitation and enables quantitative measurement of anisotropic soft materials and biological tissues at ultra-high strain rates, while explicitly accounting for fiber-directional mechanics and damage evolution.

LIC experiments were performed in synthetic anisotropic polyvinyl alcohol (PVA) hydrogels and fresh chicken breast tissue using nanosecond-duration pulsed lasers to generate ultra-high-rate deformation. Simultaneously, cavitation bubble dynamics were captured using ultra-high-speed videography at 1-2 million frames per second. Two nonlinear hyper-viscoelastic constitutive models (a Poynting-Thomson model and a generalized Maxwell model) were developed and integrated with the measured bubble dynamics to quantify fiber-directional material response and investigate rate-dependent damage mechanisms. In addition, complementary quasistatic and oscillatory shear rheometry tests were conducted to provide low-rate mechanical benchmarks and investigate the rate dependency of soft materials' mechanical behavior.

LIC experiments revealed pronounced anisotropic bubble dynamics, with preferential elongation along fiber directions in both PVA hydrogels and chicken breast tissue. Model-based fitting of major-axis bubble radius-time histories enabled the extraction of effective ultra-high-rate directional moduli and critical stretch thresholds for damage initiation. Comparison between constitutive models demonstrated that while viscoelastic relaxation is necessary to capture high-rate damping, incorporation of directional damage is essential to accurately reproduce experimental observations in soft materials and biological tissues.