F. Ancona

Fatigue Behaviour of Stainless Steels: A Multi-parametric Approach

In recent years different experimental methods have been experienced to enhance the fatigue characterisation of materials with the aim to overcome the Standard long-lasting tests, i.e. Wohler curve determination. Standard fatigue treatment requires at least 15 specimens being tested to get an estimation of material fatigue limit and it is worth noting that this kind of tests do not provide any information on damage phenomena occurring in the material. Thus, topic to be addressed in this research have to do with development of lock-in infrared measurement based thermal method for rapid evaluation of fatigue limit. By performing a single test , the adopted method leads to match different parameter information. The Assessed parameters are in number more than the ones provided by TSA, as well. Moreover, the adopted technique points to study damage by analysing the different phenomena involved in fatigue and in this regard, the aim of this paper is to show how a thermal technique can attain an early assessment of the failure processes during a cyclically loading test. The author is, also, focused on to illustrate the strong points of a method based on infrared measurements for assessing endurance limit for both austenitic and martensitic stainless steels while considering, as reference, the Standard Test methods.

Energetic approach based on IRT to assess plastic behaviour in CT specimens

In this work, the Thermographic technique (IRT) was used to characterize the fracture mechanics behaviour of stainless steels. In particular, IRT is proposed for evaluating the dissipated energy and the plastic area around the crack tip in order to study the fatigue crack growth. Experimental approaches used for the measure of dissipated energy require an accurate equipment and suitable techniques that may restrict the applications just to laboratory tests. The proposed approach is based on thermal signal investigation in the frequency domain in order to separate the two heat sources related to the material behaviour during fracture mechanics test: thermoelastic sources and intrinsic dissipations. These latter are directly related to the plastic phenomena around the crack tip and occur at the twice of the loading frequency. Both amplitude and phase signals at the twice of the loading frequency can be used for evaluating the crack growth rate. In particular, the first index through an estimation of the heat dissipated while the second due to the effects occurring at the crack tip. It was also demonstrated as the proposed approach is capable of monitoring the crack growth over time and in automatic way by means of such the contactless and full field technique.