Fatigue limit evaluation of metals using an infrared thermographic technique
Introduction
Metal fatigue of aircraft components was detected very early in the history of powered flight (Mann, 1983), and it was also of major importance in determining the durability of ground vehicles. The failure mode was identified early by railway engineers. Vehicles and tracks are expected to last up to 40 years, and bridges 120 years. A large number of potential opportunities for fatigue failure exists. Even very low failure rates can produce an appreciable number of failures (McLester, 1988). Fatigue phenomena, leading for example to the failure of diesel engines (Habib and Husain, 1994), were poorly understood until recently because they occur unexpectedly at relatively low stress levels. Some progress has been made recently in design methods (Ballard et al., 1995). Nevertheless, these methods must be used in conjunction with reliable experimental procedures. Fatigue criteria and fracture controls are needed, together with practical and economic considerations. Thus fatigue testing is central to the process of design assessment, and is complementary to analytical and numerical methods. Traditionally, experimental methods correlate the fatigue life of a smooth specimen under uniaxial stress conditions with constant amplitude of either plastic strain or stress. Multiaxial fatigue assessment is then carried out with the help of an appropriate rule or criterion that reduces the complex multiaxial loading to an equivalent uniaxial loading. Morrow (1965) investigated the relationship among cyclic stress, plastic strain, and the fatigue damage process. Since fatigue is generally caused by cyclic plastic strain, the plastic strain energy plays a very significant role in the damage process (Ellyin and Kujawski, 1984; Irwin, 1965; Mroz, 1983). Therefore, the idea of relating fatigue to the intrinsic dissipation is highly relevant.
This paper emphasises the application of infrared thermography to detect the occurrence of damage and the fatigue process in metals, and subsequently to evaluate the fatigue limit in a very short time, defined as a drastic change in the rate of intrinsic dissipation.
Section snippets
Quantitative measurement of damage
Damage theories rely on assumed discontinuous phenomena at the microscopic scale (Bui and Stolz, 1987). At the macroscopic scale, damage parameters, considered as internal variables, are introduced according to the following three main approaches.
Heat production mechanism
Various heat production mechanisms have been proposed by various authors and are now discussed. According to Moore and Kommers (1921), the temperature test was suggested and to some extent used by C.E. Stromeyer of Manchester in England as early as 1913. However, their search for some short-time test to predict fatigue resistance, based on the use of thermocouples was not conclusive since they could not identify a critical temperature with the endurance limit of steel specimens subjected to
Infrared thermography background
The development of the thermo-elastic–plasticity equations requires three basic assumptions (Allen, 1985; Dillon, 1963; Kratochvil and Dillon, 1969; Farren and Taylor, 1925).
⋅ The basic thermomechanical quantities describing thermodynamic processes: the motion x, the second Piola–Kirchhoff stress tensor S, the body force per unit mass b, the Helmholtz free energy ψ, the specific entropy s, the heat supply r0, the absolute temperature T, the heat flux vector per unit area q, the elastic strain
Short-time evaluation of fatigue limit
Infrared thermography is a convenient technique for producing heat pictures from the invisible radiant energy emitted from stationary or moving objects at any distance and without surface contact or any perturbation of the actual surface temperature of the objects viewed. The temperature rise ahead of a fatigue crack has been measured and thus proved using an AGA thermovision camera by Attermo and Östberg (1971). Attempts to measure and characterise the heat generated during the cyclic
Concluding remarks
This work has demonstrated that the material dissipativity under fatigue loading is the most sensitive and accurate manifestation of damage. Owing to the thermomechanical coupling, infrared thermography provides a non-destructive real-time test with no contact to observe the physical processes of metal degradation and to detect the occurrence of intrinsic dissipation.
Thus, it readily provides a measure of the material damage and permits evaluation of the sharp limit of a low accumulation of
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