Fracture study of a polyacetal resin

Abstract

Fracture experiments have been carried out with unnotched and notched tensile specimens of a polyacetal resin at room temperature at various rates of extension. An increase of approximately 13% in yield stress was observed in the unnotched tests with increased deformation rates from 1–1000 mm min⁻¹, whilst strain to failure was reduced from about 85% to approximately 0.05%. In all specimens, failure appeared to have taken place by initiation of a microscopic flaw upon yielding, and this flaw then slowly grew into a critical size when catastrophic fracture set in. In the slow-growth region, the mechanism of failure was by void growth whereby the lamellar texture was transformed into a fibrillar one. However, on the rapid fracture surface, there was evidence of lamellar texture being retained, but with small voids in the stacks of lamellae, In notched fracture specimens containing sharp razor cut, a fracture toughness, K _lc, of approximately 5 MPa m^1/2 was obtained between crosshead speeds of 0.5 and 50 mm min⁻¹. At speeds of 500 and 1000 mm min⁻¹, the K _lc was reduced to 4 MPa m^1/2. In the absence of a starter crack, blunt notch fracture toughness of about 6.3 MPa m^1/2 was observed at all test speeds. In specimens with a razor cut, the slow-growth region decreased as test speed increased; this can be used to construct an R-curve.