Introduction

The selection of materials for aircraft structures and engines is assessed according to a multitude of parameters such as cost, ease of manufacture, weight and a host of other factors. Central to the selection of materials is their mechanical properties such as stiffness, strength, fatigue resistance and creep performance. The durability properties of structural and engine materials in the aviation environment is also critical to their selection. Metals must be resistant to corrosion and oxidation whereas fibre–polymer composites must resist absorbing an excessive amount of moisture from the atmosphere. Aerospace materials should be durable enough to resist degradation and damage over the design life of the structural component, which may range from several hours for rocket engine parts to longer than thirty years for airframes.

Materials are selected by matching their properties to the design specifications and service conditions of the aircraft structure or engine component. The preliminary design of new structures or engines requires the aerospace engineer to analyse the performance requirements for the materials. Key information on the property requirements for the materials is determined early in the design process. For example, in the design of an aircraft wing, the minimum stiffness, strength and toughness properties needed by materials used in the spars, stringers, skins and other load-bearing components must be known. The environmental conditions in which the materials operate must also determined during the design process. For example, materials used in satellites and spacecraft must be built using materials that have high mechanical properties at extremely low temperatures and are not damaged by ionising radiation, micro-meteor impact or the low pressure conditions of the space environment.

The aerospace engineer must understand the mechanical and durability properties of materials to ensure they function over the design life of the aircraft component without the need for excessive maintenance and repair. Unfortunately, many of the mechanical and durability properties of materials cannot be calculated using mathematical models and therefore must be measured. For example, it is not possible to calculate the strength and hardness of metals or the fracture toughness and fatigue life of fibre–polymer composite materials. Likewise, the corrosion resistance of metals or the durability of composites in hot and moist environments cannot be calculated. No theory exists to determine most of the mechanical or durability properties of metals. Calculating the properties of metals is too difficult because they are dependent on too many factors, such as their alloy content, crystal structure, microstructure, heat treatment and processing conditions. Similarly, the complexity of the microstructure, residual stress state and damage modes of composite materials make it difficult to calculate many of their mechanical and durability properties. It is possible to calculate the elastic properties of composites using theoretical models, but many other important properties, strength, fracture toughness, fatigue, creep, and so on, cannot be accurately calculated. Because many of the mechanical and durability properties of metals and composites cannot be calculated, they must be measured under standardised, controlled test conditions.

The testing procedures used to measure the properties of materials are performed under conditions specified by standards organisations, such as the American Society for Testing and Materials (ASTM) or the International Organization for Standardization (ISO). The aerospace industry uses these standards and, in some cases, uses their own specialist test procedures when a standardised method does not exist.

Introduce the mechanical and durability properties of materials and describe the tests used to measure the properties. The main mechanical properties for aerospace materials, including the elastic modulus, yield strength, ultimate strength, ductility, hardness and fracture toughness are described as well as the test methods used to measure these properties. The methods examined include the tension, compression, flexure, fatigue, hardness, fracture toughness and creep tests. Several test methods used to measure the durability properties of materials are introduced. Tests to measure the corrosion resistance of metals and the moisture resistance of fibre–polymer composites are described. We examine how test data on the mechanical and durability properties of materials is used for certification of new aircraft and the structural modification of existing aircraft. The information given in this provides an understanding of the engineering properties of materials; how the properties are measured; and how the property data is used in aircraft certification.