The structural integrity of metals are often limited by their mechanical performance under cyclic loads. These loads are generally the result of cyclic plasticity, sliding or physical contact , environmental damage, or elevated temperatures. The detection of crack initiation and the observation of crack growth are of great importance for evaluating the fracture mechanics and fatigue behavior of metals. It helps in the development and realization of most mechanical components and structures, especially in applications that demand high levels of safety such as the development of railway vehicles. Therefore, measurement of crack propagation of metals is adapted as a routine materials testing tool by industries.
The propagation speed of a fatigue crack in a metal specimen can be measured with the help of strain gauges. In this case, FAC-5 and FAC-20 strain gauges from TML are applicable. The gauges are bonded with an adhesive on the position where the crack is initiated or the crack initiation is expected. During this course, the grids of gauges aligned at intervals of 0.1mm or 0.5mm are disconnected one by one with the propagation of the crack. For this measurement, the crack gauge adaptor CGA-120B is used along with the strain gauges. The disconnection of one grid corresponds to the change of approx. 45 or 40×10-6 strain by a strainmeter.