4 - Designing reliable products

Abstract

This chapter focuses on designing reliable products. For many years, designers have applied so called factors of safety in a deterministic design approach. These factors are used to account for uncertainties in the design parameters with the aim of generating designs that will ideally avoid failure in service. Many designs are still based on past experience and intuition, rather than on thorough analysis and experimentation. The factors of safety are initially determined from the sensitivity that is experienced in practice of a part, made from a particular material to a particular loading situation, and in general, the greater the uncertainties experienced, the greater the factor of safety used. The random nature of the properties of engineering materials and of applied loads is well known to engineers. Engineers will be familiar with the typical appearance of sets of strength data from tensile tests in which most of the data values congregate around the mid-range with decreasingly fewer values in the upper and lower tails on either side of the mean. Three statistical distributions that are commonly used in engineering are the Normal –- Tolerances, ultimate tensile strength, uniaxial yield strength and shear yield strength of some metallic alloys; Lognormal –- Loads in engineering, strength of structural alloy materials, fatigue strength of metals; and Weibull –- Fatigue endurance strength of metals and strength of ceramic materials. Some important considerations in use of statistical distributions have been highlighted, both in terms of initial data and importantly when modelling the stress and strength for determining the reliability.