Experimental determination of shear properties, buckling resistance and diagonal tension field of polyurethane coated nylon fabric

Experimental methods have been employed to acquire shear properties, examine buckling, and postbuckling response, and to characterize out-of-plane deformations of thin orthotropic polyurethane-coated nylon fabric designated for use in the finite element analysis of an inflatable structure. A custom-designed bi-axial pre-tension frame, a picture-fame rig, and a photogrammetry system were used to apply pre-tension, collect the angular and linear displacements, and to capture the buckling and wrinkle-forming mechanisms in the coupons during the shear test. The series of monotonic picture-frame tests provided with an average critical shear stress of 0.16 MPa that was required to buckle the coupons for a small range of the pre-tension force. While an average pre-buckling shear modulus (28.5 MPa) was obtained, only 2.25% of a relative difference between the average post-buckling apparent shear modulus (13.07 MPa) and the average shear modulus obtained from the 45 bias-extension test for the same material (13.36 MPa) was registered. The influence of the pre-tension not only on the initial buckling resistance and the transition point to the fully developed diagonal tension folds but also on a reduced resistance to the material damage during cyclic testing was investigated. A series of cyclic picture-frame tests gave the continuous load/unload response with appreciable hysteresis characteristics of the in-plane stress-strain with an average 10% of material degradation. When compared with the envelope of material resistance in the monotonic tests, during cycling loading, the coupons lost on average 13.9% more of its initial stress-carrying capability. The photogrammetry results allowed for the three-dimensional reconstruction of the coupon's surface and the development of the deformation relationships between the effective force exerted on the coupons, the shear angle, and the geometric deformation variables for the wrinkles formed in the diagonal tension fold. The geometric deformation parameters, such as frequency of wrinkle formation, widths of the base and the tip, depths, the deformation angle, and the distances between the centerlines of the wrinkles occurring in the test coupon, were acquired with photogrametry and analyzed numerically with custom-written Matlab (R) code.