Slow crack growth in multispectral zinc sulfide for use in aircraft window design

Abstract

The slow crack growth rate and inert strength of multispectral zinc sulfide were measured for use in aircraft window lifetime analysis. Dynamic fatigue data for uncoated multispectral zinc sulfide were fit to a power law by linear regression and to an exponential law by computer-aided numerical integration to obtain best-fit slow crack growth rate parameters. For the power law slow crack growth equation, crack velocity ( v ) = A * (KI/KIc)n, we find A * = 0.00769 m / s and n = 14.65, where KI is the stress intensity factor, KIc = 0.72 MPam is the critical stress intensity factor, and we assign the value of the geometric factor to be Y = 2 / π. Parameters for the exponential law slow crack growth equation, crack velocity ( v ) = vo exp ( βKI ) are vo = 3.02 × 10 − 14 m / s and β = 43.64 (MPam) − 1. There were only small differences in the crack growth rate between uncoated and antireflection-coated material. The crack growth rate that we observe is 10 to 200 times faster than previously reported for a stress intensity factor KI = 0.25 MPam, which is a representative value of KI for an aircraft window in service. The faster crack growth rate predicts a correspondingly shorter window lifetime. The inert strength of uncoated biaxial flexure disks (38.1 mm diameter with 15.88 mm load diameter and 31.75 mm support diameter) measured in dry nitrogen exhibited a 50% Weibull failure probability at 115 MPa with an unbiased ASTM C1239 Weibull modulus of 4.68. Antireflection-coated material had a 50% Weibull failure probability at 100 MPa with a Weibull modulus of 5.25.