Fatigue monitoring

A method for monitoring structure liable to fatigue by sustained loading, which may be repeated or cyclic, under which crack-like flaws may grow and lead to fatigue or fatigue-induced failure, comprises providing in a gauge which may be integral with or secured to the structure an initial flaw in a position from which a crack will grow, under the loading, in a predetermined manner depending upon the loading, and monitoring growth of the crack to determine the condition of the structure. Apparatus for the same includes a tab-like gauge 201 or other appendage which has a flaw, preferably made by a fatiguing process, and means for prestressing the gauge so that it will correctly respond to the in service static load. An example given in a gauge for a wing spar which when the plane is grounded is under opposite static loading to when it is in the air in service. Gauge 201 has an initiated crack 202 and is attached to blocks 203 glued to the structure by pins 204. Prior to attachment, blocks 203 are urged apart to apply a predetermined stress by wedge elements 205. Means may be provided for adjusting the prestress. Alternatively prestressing may be provided by the stress-inducing or stress relieving techniques applied to the gauge body as by local heating. Various gauges are shown, which may be glued welded, bolted or pinned to the structure or to an intermediate plate attached to the structure. Resistance, magnetic or acoustic properties of a gauge may be monitored. Alternatively visual inspection of crack growth may be enhanced by a scale or coloured markings. Optical fibres across the crack growth path may monitor growth.