The paper, which is a somewhat up-dated version of the author’s 1999 Royal Society/Royal Academy of Engineering Lecture of the same title, presents an overview of the issues involved in the initial design of structures and machines, in material selection and guarantees of quality, in erection and fabrication, in non-destructive examination and through-life “health-monitoring”, and in assessment of the threats to integrity posed by the presence of defects. Attention is drawn to the R6 Failure Assessment Diagram and to the characterisation of fatigue-crack growth. Finally, the issues are set in terms of a risk-based probabilistic approach to the occurrence of failure and to the consequences of such failure. The 1999 Lecture was given to an audience having a non-specialist, general science/engineering background and so was put in more popular form than would be the norm for a specialist audience. This form has been retained in the present paper, but it is hoped that no “integrity of message” has been lost by so doing.
The Integrity and Durability of Structures and Machines
The paper's title relates to the difference in emphasis placed on ensuring the integrity of a structure, contrasted with that relating to the durability of a machine. The former is usually treated as a "one-off" assessment: the latter is more involved with calculating the lifetime and specifying appropriate inspection periods. Integrity assessments do, of course, take account of changes with time and this is illustrated in the paper by considering the temporal variation of the assessment point on a Failure Assessment Diagram (FAD) and the factors affecting the movement in its position: both crack growth and changes in material properties. Probabilistic effects are also treated. Durability issues are addressed by detailed consideration of the "lifing" of a turbine disc in a gas turbine, demonstrating the importance of initial defect size and its control. These principles are generally applicable to a wide range of other machine components.