An investigation into the damage tolerance of amorphous and biaxially oriented PMMA

Abstract

There has been an increased application of polymethyl methacrylate (PMMA) in the modern aerospace industry due to it being a lightweight replacement for glass. One recent development in PMMA processing has been to biaxially pre-stretch the amorphous plate forcing the polymer chains to adopt an ordered arrangement. The resulting biaxially oriented PMMA (BOPMMA) has improved mechanical properties compared to its amorphous state. This thesis reports on an investigation into the damage tolerance of BOPMMA and in particular, its fracture resistance in the presence of cracks, scratches, indentations and aggressive solvents. The technique of broaching was investigated in detail and a rig has been developed which ensured that the defects were introduced in a precise and controllable way. For material comparison, BOPMMA exhibited significantly greater ductility (+106%), ultimate tensile strength (+11%) and in-plane fracture toughness (+87%) compared to the amorphous PMMA. This was achieved with only a very modest reduction in Young’s modulus (-8%) and an almost identical tensile yield strength (+1%) compared to the amorphous grade. A fracture mechanics approach has been followed to characterise the environmental stress cracking (ESC) behaviour of amorphous and BOPMMA in air, ethanol and methanol. Overall, the effect of liquid environments reduced the fracture toughness for both polymers. BOPMMA was shown to have a greater resistance to ESC than amorphous PMMA by a factor of 3.4 and 1.6 in ethanol and in methanol respectively. The scratches and indentations were introduced using the broaching rig and a custom-built pyramidal indenter respectively. For both types of defect, four defect depths from 0.2 – 1.6 mm were studied in both the quasi-static and fatigue conditions. BOPMMA demonstrated an increased fracture and fatigue resistance in both conditions compared to the amorphous grade. The material characterisation techniques developed are applicable to other engineering materials, contributing towards the advancement in material science