Ceramic Bonding Interface under Shear-Compression Stress: Ultra-High-Speed Imaging Contribution.

The aim of this study is to visualize and characterize by ultra-high-speed imaging (UHSI) the failure phenomena at the resin-ceramic bonding interface of lithium disilicate (LiSi2) samples bonded with gold-standard protocol (Monobond Plus [MB]) and the nontoxic one (Monobond Etch & Prime [MEP]) subjected to mechanical loading. Unprecedented frame rate, image resolution, and recording time were reached by using the most advanced UHSI camera. The finite element analysis (FEA) of the proposed mechanical test confirmed that the specific design of our samples enables a combined shear and compression stress state, prone to test the bonding interface while being close to physiological stresses. Ten LiSi2 samples were pretreated by gold standard (MB, n = 5) and self-etching primer (MEP, n = 5). Axial compression loading gradually increased until catastrophic failure was performed. As shown by the FEA, the angle between the bonding interface and load direction leads to shear-compression stresses at the resin-ceramic bonding interface. Failure was recorded by UHSI at 300,000 fps. All recorded images were analyzed to segregate events and isolate the origin of fracture. For the first time, thanks to the image recording setup, it was observed that debonding is the first event before breakage, highlighting that sample fracture occurs by interfacial rupture followed by slippage and cohesive failure of materials. Failure mode could be described as mixed. MEP and MB showed similar results and behavior.

Constitutive modeling of stress softening and permanent set in a porcine skin tissue: Impact of the storage preservation.

Prior to testing, soft tissues are usually maintained in different media and additives (solution, air, cryopreservant…) under various environment conditions (temperature, storage duration….). In many cases, results from mechanical tests performed on these stored tissues are supposed to be as closed as possible to the fresh ones. In the present work, cyclic tensile tests were performed with increasing values of strain on porcine skin tissues (excised following the Langer's lines) to enhance tissues mechanical nonlinearity such as softening behavior and permanent set. Optical methods were used to follow the in-plane strains evolution. These latest values were used as data to simulate the structural behavior of these heterogeneous materials. The numerical simulation is based on the constitutive pseudo-elastic model accounting for the softening behavior as well as the permanent set. As a result, reliable material parameters were extracted from the experiments/model comparison for each storage solution. The result of this study reveals that preservation conditions must be carefully chosen: at low strain the tissues store in fridge in a saline solution during a short time, or in freezer (-80°C) in water with cryopreservant and the fresh one lead to a similar mechanical response. For larger strain, the freezing (-80°C) in water with cryopreservant is the only procedure for which the tissue recovers its initial behavior.

Effect of sample preservation on stress softening and permanent set of porcine skin.

Skin is a composite material with a complex structure which exhibits a wide range of behaviours such as anisotropy, viscoelasticity, hyperelasticity, plasticity etc. Indeed it remains a great challenge to understand its behaviour as it is involved in many consumer and medical applications. In most studies, experiments are performed in situ or in vitro on fresh tissues but most of the time samples are preserved before testing (fridge, freezer, saline solution etc.). In this paper, the impact of samples conservation on the softening behaviour and on the permanent set is studied in order to select the appropriate conservation protocol. Samples are extracted from several pigs' abdomens (direction parallel to spine) and the mechanical testing consists in loading-unloading uniaxial tension tests instrumented with digital image correlation inducing thus reliable strain measurements in a chosen region of interest. The results of this study revealed that preservation conditions must be carefully chosen; conservation in a saline solution and freezing without any caution alter the irreversible part of the global mechanical behaviour of the tissues.