Introduction to Polymer Science and Technology Introduction

1.2.1 Select the right material and the production process for an application

Selection involves such considerations as the material properties (mechanical, thermal, electrical, optical and chemical); service conditions (e.g., operating temperature and humidity) and service life; impact on the environment and health and safety; economics; appearance (e.g., shape, colour, surface finish, decoration); type of production (injection moulding, extrusion, compression moulding, resin transfer mouldings, etc), and production-related material behaviour (e.g., flow, shrinkage, residual stresses, weld lines, etc).

The selection sometimes can mean life or death. For instance, the Challenger, space shuttle, disaster in January 1986 apparently resulted from not choosing quite the right sort of rubber seal for the fuel system. The O-ring seal became rigid and lost its resilience/pliability at low temperatures and resulted in fuel seepage. The seal was made of silicone rubber, which can crystallise under stress. As the craft waited for launch, the O-ring remained clamped too long and its T increased considerably.

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The Concorde crash, which occurred in July 2000, killed 113 people - all passengers on board the aircraft, nine crew and four people on the ground. The aircraft caught fire, see Figure 1.1, on take-off from Paris Charles de Gaulle Airport when one of its tyres was punctured by a strip of metal (debris from another aircraft) lying on the runway, and the burst tyre possibly piercing through the under carriage into a fuel tank. After the accident, although, the Concorde tyres were modified and the under carriage was reinforced with Kevlar (a high performance aramid fibre) Concorde flights did not quite resume service.

Figure 1.1Concorde undercarriage on flame (source: Google images (Toshihiko Sato/AP))

Rolls Royce, one of the pioneers in the production and application of highly acclaimed carbon-fibre in the 1960s, used carbon-fibre in the manufacture of compressor blades for one of their aero-engines without, in retrospect, a full appreciation/evaluation of the mechanical properties of the material. The blades proved to be vulnerable to "bird strike". Consequently, as stated in Wikipedia "Rolls-Royce's problems became so great that the company was eventually nationalized by the British government in 1971 and the carbon-fibre production plant was sold off to form Bristol Composites", http://bit.ly/jffQtO .

Away from aerospace examples, Ezrin (1996, plOl) cites the example of high density polyethylene (HDPE) aerators in a sewage lagoon that fractured due to unanticipated environmental stress cracking (ESC) under dynamic flexural stress. The design was at fault for the selection of HDPE, which has poor ESC, and for the grade of HDPE selected, since ESC is affected by molecular weight. The failure was at the sharp bend of the four feet, which were bolted to concrete pads.

Date: 2015-12-11; view: 947