Key result
The apron surrounding the gauge area in biaxial testing acts as an annular constraint that overestimates stress by up to 36%, an error that can be reduced to 4% by introducing specific cuts.
Why the study?
Rakes or sutures used in biaxial testing require an apron of excess material, but its influence on estimated mechanical responses and test accuracy was unclear.
Absolute Event Rate: 4% vs 36%
Introducing specific cuts in the apron of specimens during biaxial tensile testing significantly reduces stress overestimation, improving the mechanical characterization accuracy of hyperelastic materials like soft tissues.
Refines biaxial testing of hyperelastic cardiovascular biomaterials; extends lab methods but leaves open clinical translation.
Biological soft tissues and polymers used in biomedical applications (e.g. in the cardiovascular area) are hyperelastic incompressible materials that commonly operate under multi-axial large deformation fields. Their characterisation requires biaxial tensile testing. Due to the typically small sample size, the gripping of the specimens commonly relies on rakes or sutures, where the specimen is punctured at the edges of the gauge area. This approach necessitates of an apron, excess of material around the gauge region. This work analyses the apron influence on the estimated mechanical response of biaxial tests performed by using a rakes gripping system, with the aim of verifying the test accuracy and propose improved solutions. In order to isolate the effect of the apron, avoiding the influence of anisotropy and inhomogeneity typical of most soft tissues, homogeneous and isotropic hyperplastic samples made from a uniform sheet of casted silicone were tested. The stress-strain response of specimens with different apron sizes/shapes was measured experimentally by means of biaxial testing and digital image correlation. Tests were replicated numerically, to interpret the experimental findings. The apron surrounding the gauge area acts as an additional annular constraint which stiffens the system, resulting in a significant overestimate in the stress values. This error can be avoided by introducing specific cuts in the apron. The study quantifies, for the first time, the correlation between the apron size/shape and the experimental stress overestimation, proposing a research protocol which, although identified on homogeneous hyperelastic materials, can be useful in providing more accurate characterisation of both, synthetic polymers and soft tissues.
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Leonardo et al. (2023) studied Hyperelastic materials (silicone specimens) (n=3). Trimming the apron (introducing cuts between grips) vs. Standard apron without cuts (A50, A30, A10) was evaluated on Mean error in stress estimation compared to true material behavior. The apron surrounding the gauge area in biaxial testing acts as an annular constraint that overestimates stress by up to 36%, an error that can be reduced to 4% by introducing specific cuts.
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