TWI Technology Briefing 747/2002
Y-H Zhang, P J Tubby and R King
Fatigue cracking in welded components starts almost always from welds and there is a risk that it may extend to the point of catastrophic failure. Therefore, it is desirable to develop a simple fatigue gauge which, when attached to or installed on a component, can give advance warning of structural failure, or provide an estimate of its remaining life.
Although many types of damage monitoring systems have been developed and used in a variety of service conditions, the nature of the fatigue process in welded joints imposes special requirements and limitations on the fatigue gauges which can be applied successfully. In particular, the presence of fine crack-like flaws at the weld toe largely eliminates the crack initiation phase, which is a significant proportion of the fatigue life of unwelded components. Secondly, tensile residual stresses substantially increase the damaging effects of nominally compressive applied loading cycles. In addition, the low fatigue strengths of welded details require that a gauge must be capable of responding to stress cycles of very low amplitude. Thus a device developed, for example in the aerospace industry for application to airframes, may not be appropriate for welded structures. This study has therefore reviewed the attributes of fatigue gauges and other structural monitoring methods currently available and some under development, and examines their suitability for welded structures.
- Examine fatigue monitoring methods currently available or under development
- Assess their suitability as a damage indicator in welded structures
- Where appropriate, to recommend further work required for validation