Fatigue limit evaluation of a stainless steel using thermal data analysis

Fatigue is an irreversible process, accompanied by microstructural changes, localized plastic strains and energy dissipation. The temperature increase of a metallic material undergoing a fatigue test is a manifestation of the thermal energy dissipation and it was experimentally observed that the higher the applied stress amplitude the more pronounced the temperature increase of the material. Moreover, the trend of the observed temperature increase versus the applied stress amplitude presents a more or less abrupt positive change at a certain stress level. This characteristic stress amplitude was associated to the material fatigue limit [1-7]. Since the load-increasing tests are fast and a single specimens can be used to estimate the material fatigue limit, Risitano and co-workers set-up a rapid experimental methodology to determine the fatigue limit based on load-increasing tests monitored with an infrared camera, according to the so-called Risitano method [5]. It is known that due to plasticity, the stress amplitude and the material temperature waves present a phase shift. Therefore, taking advantage of the lock-in thermography, temperature oscillations due to dissipated energy, T2, can be obtained as the component having double frequency of the load signal sine wave by using the classical Fourier analysis, as suggest by Krapez et al [8]. They proposed to use T2, instead of the stabilized material temperature measured during a fatigue test in order to estimate the material fatigue limit. However, they noticed that the applicability of the method depends on the material under analysis. In particular, satisfactory results were found in the case of AISI 1050 steel and AISI 316L stainless steel, while the approach was useless while investigating the fatigue behavior of 7010 aluminum alloy, since temperature signal associated with the dissipated energy was very weak in relation to the available experimental equipment. Aaki et al [9] proposed to use the dissipated energy, evaluated by using the lock-in thermography, to assess the material fatigue limit. They pointed out that satisfactory results were found in the case of AISI 316L, while in the case of AISI 304 the fatigue limit estimated by dissipated energy measurements provided a conservative value as compared to that evaluated by means of conventional fatigue test. The phase shift angle was proposed as a suitable damage index for the rapid determination of material fatigue limit by Shiozawa et al [10]. They validated the approach against experimental results obtained from AISI 306L stainless steel specimens and noticed a satisfactory agreement with the fatigue limit determined by the conventional stair-case tests. Recently, Shiozawa et al [11] proposed a new experimental technique for fatigue limit estimation based on the phase 2f lock-in infrared method, where a single fatigue test was performed on AISI 316 steel specimens by increasing the applied stress amplitude, a, and by measuring the in-creasing rate of the dissipated energy, dq/da. By plotting dq/da versus a, an abrupt increase is seen when a exceeds the material fatigue limit. Meneghetti [12] proposed an experimental technique for the direct evaluation of the heat energy density per cycle dissipated by a material undergoing a fatigue tests (the Q parameter). Q can be easily evaluated by stopping the fatigue test at t = t* after thermal equilibrium has been reached and by measuring the cooling gradient immediately after t*. Originally adopted to rationalise the notch effect in finite-life fatigue of stainless steel specimens [13], it is envisaged that the specific heat loss Q may be used also to estimate the fatigue limit. This paper analyses all previous thermal methods for the rapid experimental estimation of the fatigue limit. In particular, the rapid engineering techniques proposed by La Rosa and Risitano [5], by Shiozawa et al [10] as well as by using the Q parameter [12] will be applied to estimate the fatigue limit of cold-drawn AISI 304L stainless steel bars and critical issues in practical applications of the analysed techniques will be singled out.