An interferometric-based tensile tester to resolve damage events within reconstituted multi-filaments collagen bundles

Understanding how mechanical damage propagates in load-bearing tissues such as skin, tendons and ligaments, is key to developing regenerative medicine solutions for when these tissues fail. For collagenous tissues in particular, damage is typically assessed after mechanical testing using a broad range of microscopy techniques because standard tensile testing systems do not have the time and force sensitivity to resolve mechanical damage events. Here we introduce an interferometric detection scheme to measure the displacement of a cantilever with a resolution of 0.03% of full scale at a sampling rate of 5000 samples/s. The system is validated using collagen fibers engineered to mimic mammalian tendons. The system can detect sudden decrease in force due to slippage between collagen filaments, one to five microns in diameter, within a fiber in air. It can also detect yield events associated with local collagen unfolding or sliding within collagen fibrils within a fiber in liquid. This is opening the road to the sub-failure study of damage propagation within a broad range of hierarchical biomaterials.

Large deformation analysis of a backside-supported snap-fit with nonlinear behavior

In this study, the large deformation of a backside-supported snap-fit was analyzed. The backside snap-fit pair consisted of mating part and base part. The mating part was a simple cantilever, and the base part had an opening with a supporting bar. The reaction force of the supporting bar was found to be an important parameter for the assembly and separation of the snap-fit. During our analysis, the supporting bar experienced a large deformation with nonlinear elasticity or plastic damage. Finite element analysis was performed. Stress concentration was observed at the root of the supporting bar and at the bent edge of the base part. Three types of specimens were designed and fabricated for experimental verification. The first specimen was a reference design that was fabricated according to the same design concept as the actual product. The second specimen was designed to reduce the stress concentration. The third specimen had an enriched design to increase the supporting force. The reaction force corresponding to the applied displacement was measured using a testing machine. The load exhibited a highly nonlinear behavior and reached a maximum peak value without causing any apparent damage, after which it decreased with plastic damage. Through numerical and experimental analyses, it was found out that the design of the backside-supported snap-fit could be improved by reducing the stress concentration and increasing the stiffness of the supporting bar.

Theoretical and experimental evaluation of the health status of a 1018 steel I-beam using nonlinear Rayleigh waves: Application to evaluating localized plastic damage due to impact loading

This study proposes a sensitive and baseline-free method to evaluate the health status of a 1018 steel I-beam by measuring its material nonlinearity using a new nonlinearity parameter defined for Rayleigh waves. This parameter yields a true value of material nonlinearity using the Rayleigh wave harmonics obtained from the experiments carried out at the intact and impacted states of the I-beam. Accordingly, the evaluated nonlinearities are inherent and damaged induced respectively. The results show that, for an intact state, the nonlinearity obtained using the new parameter and the experimental results for different propagation distances, consist of several peaks and the first peak reaches the true material nonlinearity. Whereas, in case of damaged state, the nonlinearity parameter at the impacted location shows a sudden increase and reaches a value higher than that of the nonlinearity evaluated at the same location for intact state. Thus, the health status can be easily tracked by comparing the nonlinearity obtained from the current state of the I-beam at its first peak with that of a physics based nonlinearity parameter evaluated at the intact state using the higher order elastic coefficients of the material. Therefore, this method is termed as baseline-free. Lastly, a novel concept of evaluating the population of dislocations formed in the material as a result of impact loading, using the new nonlinearity parameter is introduced and an equation for its estimation is given. The trend of the results given by this new equation are in accordance with those reported in the literature. In contrast, deviation between the linear parameter such as the wave velocity at the intact and impacted state remains marginal. Thus, by using the new nonlinearity parameter, it has been proven that the inspected steel specimen can be easily differentiated whether it is at the intact or damaged state.