An investigation into the damage tolerance of amorphous and biaxially oriented PMMA
File(s)
Author(s)
Ng, Sze Ki
Type
Thesis or dissertation
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
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
Version
Open Access
Date Issued
2021-09
Date Awarded
2022-03
Copyright Statement
Creative Commons Attribution NonCommercial Licence
Advisor
Blackman, Bamber
Dear, John
Sponsor
AECC Beijing Institute of Aeronautical Materials (BIAM)
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)