Designing against fracture in brittle plastics

Abstract

The fracture toughness of a variety of sharply notched tension, bending and rotating disc specimens of PMMA is examined using linear fracture mechanics. It is observed that rapid fracture with a brittle glassy appearance usually follows a period of slow crack growth, denoted by fan shaped markings of local ductility, though still brittle overall. In this near brittle regime the fracture toughness is sensitive to strain rate so that high values of effective surface energy are easily induced by rapid testing or notch bluntness. At impact rates the toughness increases again. For design purposes, in the absence of environmental effects, the onset of slow cracking and rapid (glassy) fracture, can be associated with fracture toughness K1, of about 800 lbf/in3/2 (90 kg/cm3/2) and 1800 lbf/in3/2 (180 kg/cm3/2) respectively. Detailed studies have not been made on other materials but a guide to the levels of notch toughness and notch brittle temperatures are given for several plastics.

Stable crack growth, instability, fatigue and fracture of a 50/50 blend of poly(2,6-dimethyl-1,4-phenylene oxide) and polystyrene

“…This, of course, reflects the initiation process from a starter crack, which was usually inserted by a sharpened razor (see, e.g. [1,5,16]). This crack initiation process was obviated in our tests.…”

“…The stable crack growth behaviour in many polymers can be described by a fracture map, which gives the relationship between crack velocity and crack driving force [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Such fracture maps are schematically illustrated in Fig.…”

“…At a critical crack velocity, typically between 1 and 10 cm sec 1, cracks become unstable and a sudden transition to very rapid fracture, with speeds in excess of 100 m sec-~, can be observed. Fracture maps have been identified for PMMA [1][2][3][4][5][6][7][8][9][10][11][12], PC [13,14], PES [15], HDPE [16] and PVC [17].…”

Theoretical prediction of temperature rise at the tip of a running crack

“…The fracture toughness (Kc) versus crack speed curves for polymers demonstrate that during stable crack growth the resistance to cracking increases, and that the stable propagation is followed by a crack instability resulting in a peak in these curves [4][5][6]. Crack instability has been explained as being due to an isothermal-adiabatic transition [5,7,8].…”

“…Crack instability has been explained as being due to an isothermal-adiabatic transition [5,7,8]. At low crack speeds the heat generated by plastic work at the crack tip has enough time to be conducted away, so that there will be no temperature change.…”