Fatigue crack initiation detection by an infrared thermography method

Abstract

A B S T R A C T In this paper, the study of the temperature variation during fatigue tests was carried out on different materials (steels and aluminium alloys). Tests were performed at ambient temperature using a piezoelectric fatigue system (20 kHz). The temperature field was measured on the surface of the specimen, by means of an infrared camera. Just at the beginning of the test, it was observed that the temperature increased, followed by a stabilization which corresponds to the balance between dissipated energy associated with microplasticity and the energy lost by convection and radiation at the specimen surface and by conduction inside the specimen. At the crack initiation, the surface temperature suddenly increases (whatever the localization of the initiation), which allows the determination of the number of cycles at the crack initiation and the number of cycles devoted to the fatigue crack propagation. In the gigacycle fatigue domain, more than 92% of the total life is devoted to the initiation of the crack.So, the study of the thermal dissipation during the test appears a promising method to improve the understanding of the damage and failure mechanism in fatigue and to determine the number of cycles at initiation.Keywords infrared pyrometry; number of cycles at crack initiation; temperature recording; very high cycle fatigue. N O M E N C L A T U R Eb = burger vector E = Young's modulus K eff = effective stress intensity factor K = stress intensity factor σ = stress amplitude σ y = yield stress σ y = cyclic yield stress

Through thickness property variations in friction stir welded AA6061 joint fatigued in very high cycle fatigue regime

“…Therefore, the resistances to fatigue crack initiation, identified as a predominant role on the metallic fatigue failure mechanism in the VHCF regime [10,15,16], are inconsistent across the welds, which result in the fatigue failures occurring just in the weakest zone. The fracture surfaces were examined by OM and SEM to determine the locations of fatigue crack initiation sites.…”

“…In the VHCF domain for metallic materials, it has been confirmed that more than 90% of the total life devoted to the fatigue crack initiation process [15]. For this reason, a lot of work has been performed on the crack initiation mechanisms in qualitative and quantitative ways [26][27][28].…”

Infrared thermography applied to the analysis of material behavior: a brief overview

“…However, dynamic fatigue involves loading frequencies of about several dozens of kHz (e.g. 24 kHz), so self-heating increases up to several hundreds of degrees depending on the material properties and heat exchange intensity [9]. The first studies of thermal effects in fatigue were performed using infrared pyrometers or monodetector cameras.…”

A two-parameter model to predict fatigue life of high-strength steels in a very high cycle fatigue regime

“…It has been shown that almost all of the fatigue life in the VHCF regime is consumed to form FGA [21][22][23][24][25], so it is reasonable to approximate the fatigue life N FGA consumed in FGA as the total fatigue life N f . From the expression of β , it should be at least a function of σ a /σ Y , so that N Thus, a two-parameter model for the fatigue life of highstrength steels with fish-eye mode failure in a VHCF regime is derived as…”

“…It was shown that the stress intensity factor range at the front of the FGA was a constant and was close to the threshold value of the crack propagation K th . In crack initiation and early growth stage of VHCF, the crack growth rate is much lower than 10 −10 m/cycle [13,[17][18][19][20], and more than 90 % of fatigue life is consumed to form the FGA [21][22][23][24][25]. Therefore, it is essential to develop a model to predict the fatigue life in relation to FGA.…”