Expansion potential of skin grafts with novel I-shaped auxetic incisions

Severe burn injures lead to millions of fatalities every year due to lack of skin replacements. While skin is a very limited and expensive entity, split thickness skin grafting, which involves the projection of a parallel incision pattern on a small section of healthy excised skin, is typically employed to increase the expansion and cover a larger burn site. To date, the real expansion capacity of such grafts are low (<3 times) and insufficient for treatment of severe burn injuries. In this study, novel I-shaped auxetic incision patterns, which are known to exhibit high negative Poisson’s ratios, have been tested on the skin to investigate their expansion potential. Fourteen two-layer skin graft models with varying incision pattern parameters (i.e., length, spacing, and orientation) were developed using finite element modelling and tested under uniaxial and biaxial tensile loads. The Poisson’s ratio, meshing ratios, and induced stresses were quantified across all models. Graft models tested uniaxially along the orthogonal directions indicated opposite trends in generated Poisson’s ratios, as the length of the I-shape incisions were increased. Biaxially, with a symmetric and closely spaced I-shape pattern, graft meshing ratios up to 15.65 were achieved without overstressing the skin. Overall, the findings from the study indicated that expansion potentials much higher than that of traditional skin grafts can be achieved with novel I-shaped auxetic skin grafts, which would be indispensable for covering large wounds in severe burn injuries.

Mechanical Properties of Compressed Wood with Various Compression Ratios

This paper investigates five groups of compressed wood (CF), four of them made from compressed Japanese cedar with four different compression ratios (CR) of 33%, 50%, 67% and 70% and one without compression (control). The specimens were conditioned in relative humidity (RH) of 60% with moisture content (MC) of 12%. Mechanical properties tested were shear modulus in LR, LT and RT planes by single cube test method, Young’s modulus in the L, R, T directions and poisson’s ratios in all planes. Results showed that in comparison with control specimen, the average improvement on density with CR improvement were 25%, 75%, 175% and 261% corresponding to CRs of 33%, 50%, 67% and 70% respectively. It was also found that Young’s modulus in the L and T directions increased significantly with the increase of CR. Shear modulus of RT plane increased with the rise of CR. Poisson’s ratios tended to decrease with increasing compression ratio of CW.

In-situ estimation of subsurface hydro-geomechanical properties using the groundwater response to Earth and atmospheric tides

Subsurface hydro-geomechanical properties crucially underpin the management of Earth's resources, yet they are predominantly measured on core-samples in the laboratory while little is known about the representativeness of in-situ conditions. The impact of Earth and atmospheric tides on borehole water levels are ubiquitous and can be used to characterise the subsurface. We illustrate that disentangling the groundwater response to Earth and atmospheric tidal forces in conjunction with hydraulic and linear poroelastic theories leads to a complete determination of the whole hydro-geomechanical parameter space for unconsolidated systems. Further, the characterisation of consolidated systems is possible when using literature estimates of the grain compressibility. While previous field investigations have assumed a Poisson's ratio from literature values, our new approach allows for its estimation under in-situ field conditions. We apply this method to water level and barometric pressure records from four field sites with contrasting hydrogeology. Estimated hydro-geomechanical properties (e.g. specific storage, hydraulic conductivity, porosity, shear-, Young's- and bulk- moduli, Skempton's and Biot-Willis coefficients and undrained/drained Poisson's ratios) are comparable to values reported in the literature, except for consistently negative drained Poisson's ratios which are surprising. Our results reveal an anisotropic response to strain, which is expected for a heterogeneous (layered) lithological profile. Closer analysis reveals that negative Poisson's ratios can be explained by differing in-situ conditions to those from typical laboratory core tests and the small strains generated by Earth and atmospheric tides. Our new approach can be used to passively, and therefore cost-effectively, estimate subsurface hydro-geomechanical properties representative of in-situ conditions. Our method can be used to improve our understanding of the relationship between geological heterogeneity and geomechanical behaviour.

PETAL AUXETICS WITH TARGETED POISSON’S RATIO USING ISOGEOMETRIC SHAPE OPTIMIZATION

Auxetic materials with negative Poisson’s ratio have potential applications across a broad range of engineering fields. Several design techniques have been developed to obtain auxetics with targeted mechanical properties. However, many of these finite element based techniques are difficult to use directly for auxetics, particularly during the design optimization stage which involves evolving boundary parts with large curvatures. This paper focusses on a series of smoothed petal auxetics, with lower stress concentrations at connecting parts, compared to the reference star shaped structures. An isogeometric shape optimization framework to achieve target Poisson’s ratios at large deformation is presented. Several smoothed petal auxetic designs with target constant Poisson’s ratios up to an effective tensile strain of 30% are shown to demonstrate the capability of the optimization framework.