Fatigue Crack Growth (FCG) Testing

Fatigue Crack Growth (FCG) Testing

SMaRT houses a suite of Fatigue Crack Growth (FCG) test frames which are used to characterize the rate at which a pre-existing crack (starter slit) will grow under time-varying crack tip driving force. The FCG test, also known as crack propagation or da/dN testing, is performed using a DC potential drop crack length measurement technique. The method of fatigue crack growth analysis is conducted on metallic materials under either K (stress intensity factor)
or load control, typically using a corner crack geomety
(Other geometries such a compact tension, or CT, can
also be used).

Isothermal testing is conducted between ambient and 800°C. SMaRT currently use a range of test machines that provide operation across loads ranging from 1.0kN to 100kN. SMaRT has the capability to perform FCG testing under partial pressure or vacuum, as well as a facility to perform FCG testing in an SO2 environment. In addition, SMaRT has developed a method to perform fatigue crack growth under thermomechanical fatigue (TMF) conditions. This is the cutting edge of mechanical testing and has been developed in conjunction with academic staff from Swansea University.

Fatigue crack growth (FCG) testing is used to evaluate the safety and reliability of materials by subjecting samples with a pre-existing crack to repeated loading and unloading. The crack growth rate is monitored and provides information on the resistance of materials to stable crack growth under this cyclic loading. Fatigue crack growth rates, from the very slow threshold regime to the fast growth rate region, are typically developed using constant load or K-controlled parameters.

The typical output of these tests is a curve displaying the crack growth rate as the cyclic stress intensity factor (ΔK) is varied. Fatigue crack growth test reports include the following information:

  • da/dN versus ΔK data
  • A plot of da/dN vs ΔK
  • Threshold, and/or Paris Law values, as appropriate


SMaRT routinely conducts FCG testing in compliance with the following international and industry standards (working to other standards are available upon request):

  • BS ISO 12108 Metallic materials – Fatigue testing – Fatigue crack growth method