Monday  |  Salon 9  |  09:20 AM–09:40 AM

#15655, Assessment of the Fatigue Crack Closure Phenomenon through Thermometry

The total fatigue life of an industrial part can be described as two stages: the initiation of fatigue cracks and their propagation. The share of each stage is dependent on the component material and geometry as well as the nature of the cyclic loading. To ensure the structural integrity of the component, engineers must be provided with an accurate predictive model. The building of such model and its assessment requires an efficient experimental method adapted to full scale structure testing. This study proposes a novel experimental method based on the use of infrared thermography. This method aims at determining the share of the crack propagation phase as well as in-situ monitoring of crack growth.
Infrared thermography, and more specifically Thermoelastic Stress Analysis (TSA), is used for the detection and in-situ monitoring of cracks in many applications, from civil engineering to aeronautics materials. Under adiabatic conditions, the field of the temperature’s first harmonic at the surface of a structure submitted to cyclic loading is proportional to that of the stress tensor’s first invariant’s amplitude. During a fatigue test, the initiation of a crack affects the nearby stress field, and thus the thermal signature of the specimen. Using this principle, cracks can be detected and monitored thanks to infrared thermography.
However, considering a compressive mean stress in the vicinity of the crack, in Mode I, a crack closing phenomenon can occur which imply that the stress is partly transmitted through the crack. The previously described TSA crack detection technique based on the temperature’s first harmonic can no longer be used, as a non-linearity arises. New indicators are studied to detect a closing crack. Moreover, the magnitude of the crack closing phenomenon can directly be determined from the temperature signal, which gives insight on the local stress state (e.g. residual stresses).
This study proposes the demonstration of the observation and characterization of the crack closure phenomenon through thermometry. Firstly, a case study is proposed in order to allow a controlled fatigue crack propagation under different load ratios. Then, a method to detect and monitor the initiated fatigue cracks for all load ratios is proposed. Finally, a discussion between numerical and experimental results demonstrates the applicability of thermometry to characterize crack closure and its relevance towards modelling.

Corentin Guellec   ENSTA Bretagne
Lorenzo Bercelli   ENSTA Bretagne
Bruno Levieil   ENSTA Bretagne
Cedric Doudard   ENSTA Bretagne
Sylvain Calloch   ENSTA Bretagne