Characterizing the failure of parachute seams : the impact of stitch concentration and strain rate on ultimate tensile strength

Abstract

Parachutes are commonly used in space mission landings. With increasing payloads, parachutes are getting larger, bearing larger loads and operating at faster speeds. Designing these devices requires a knowledge of aerodynamics, fluid flow, and the mechanical properties of cloth. A brief overview of the basics of parachute design are described. There are many possible failure modes, but catastrophic failures are caused by failures at the weakest point of the parachute, the seam. Seams are characterized by a seam efficiency, a percentage of the strength of the cloth used to stitch the seam together. In this study, seams and cloth were tested to failure on an Instron 5582 to experimentally determine their respective Ultimate Tensile Strengths (UTS). The ratio of the seam UTS to the cloth UTS was used to determine seam efficiency. Results indicated no clear relationship between strain rate and seam efficiency in the range tested. However, a strong relationship between stitch concentration and seam efficiency was established. A best fit curve was developed and with an an R2=0.80. In order to better understand the failure mode, the open source Matlab function Ncorr was also used to provide a visualization of the strain on the coupons during testing. The results of the digital correlation analysis performed by Ncorr are also reported, and indicate the importance of transverse and shear strain in causing catastrophic failure.