Application of fracture mechanics to the prediction of the ductile-brittle transition

Abstract

Eleven different laboratories have collaborated in a research programme on factors affecting the fracture resistance of injection moulded plaques, as measured in dart impact tests. Factors investigated were the presence of weld lines and ejector pin marks, the test temperature and the yield stress σy and fracture toughness KIC of the polymer over a range of temperatures. Materials studied were high density polyethylene (HDPE), nylon 66 (PA 66), and two grades of polypropylene (PP), a homopolymer and a copolymer. A fracture mechanics analysis showed that welds formed in PA66 by head-on impingement of melt fronts behaved like surface cracks of length 250 μm. Polarised light microscopy revealed the presence in the PA66 mouldings of a distinctive layer of large spherulites running normal to the surface to a depth of 500 μm. This weld-line defect structure was associated with low-energy fractures in the dart impact test on PA66 plaques. Correlations between weld-line defects and low impact energy were also observed in HDPE and PP. Large defect sizes, low KIC, and high σy all contributed to brittle fracture. © 1986, Walter de Gruyter. All rights reserved.