Biaxial tensile tests to study mechanical behaviour of thin soft materials

Abstract

The mechanical properties of soft materials such as biological tissues, membranes, fabric, and polymers are anisotropic. For example, young’s Modulus of above materials could be different in different directions. Study of mechanical behaviour of soft materials, thus, involves multi-axial tensile tests. This paper deals with uniaxial and biaxial tensile tests on several different sheets of paper. The experiments were carried out on an indigenously developed benchtop biaxial tensile testing instrument. It has four independent actuators that apply stress in orthogonal directions. Each actuator is actuated using a stepper motor and the stretching force is measured using a load cell. The strain in the central part of the specimen is monitored using Digital Image Correlation (DIC) technique that uses digital images to track the deformation and the strain of a material under load. The instrument is designed for handling thin samples of soft materials and uses an indigenously developed gripping system. It has provision for specimen to have a sample environment that allows temperature variation or submersing of the specimen in suitable liquids (e.g. water, biological fluids). The instrument has a foot print of 400 mm x 400 mm and is portable. Technical details of the above biaxial tensile testing instrument and the results of uniaxial and biaxial tensile tests, both, on dry and wet sheets of paper are reported.