Thermoelastic Phase Analysis (TPA): a new method for fatigue behaviour analysis of steels

Abstract

In literature, there are already well‐established thermal methods which allow for the estimation of fatigue limit, in particular for metallic materials such as austenitic steels. These methods are based on heat source generation analysis or on surface temperature evaluation of material subjected to different types of cyclic loading. General application of methodology found limitation in those cases in which temperature changes on material related to fatigue damage were very low and, furthermore, thermal methods require high‐performance equipment and a difficult setup. This is the case, for instance, with brittle materials (such as martensitic steels), welded joints and aluminium alloys. In this work, a new thermal method named Thermoelastic Phase Analysis is used to evaluate the fatigue limit of martensitic steels. This thermal method is based on an empirical approach. The main idea is that phase of thermoelastic response of the material subjected to fatigue loading is influenced by the presence of a heat source due to dissipative phenomena related to damage. Monitoring of the phase parameter provides a more stable setup and an independent means of identifying the fatigue limit of material. The method has also proven to be potentially one order of magnitude faster than traditional thermal methods.

A multianalysis thermography‐based approach for fatigue and damage investigations of ASTM A182 F6NM steel at two stress ratios

“…21 The symbol "π" has been included according to the classic theory of thermoelastic stress analysis, 14 where the temperature and first stress invariant have opposite signs. 21 The symbol "π" has been included according to the classic theory of thermoelastic stress analysis, 14 where the temperature and first stress invariant have opposite signs.…”

“…46 However, the use of all of the quantities to assess the behaviour of metallic materials at two different loading ratios has not previously been proposed. Additionally, in previous works, [21][22][23][24] the physical aspect of the energy variations that govern the temperature variations was not highlighted, while in this paper, the aim is to explain what produces the heat variations and to correlate these heat changes to temperature changes. The adopted indexes are:…”

“…This phenomenon is completely governed by plastic phenomena, which also blur any other effects, such as the presence of residual stresses. 21 Hence, it is possible to draw some considerations about the heat dissipated in the material by assessing the temperature signal during the test. 21 Hence, it is possible to draw some considerations about the heat dissipated in the material by assessing the temperature signal during the test.…”

Energetic approach to study the plastic behaviour in CT specimens

“…It is worth noting that even if heat exchanges cannot be neglected, the A term allows an estimation of the heat dissipated. In the model which provides several indexes to study entirely the components of the thermal signal, appear also the phase shifts and , that in turn represent the thermoelastic phase shift [9] and the phase shift between first and second order harmonic components. Both these two parameters are correlated to local dissipative damage [31], however, it is not in the purpose of the present paper to use these parameters, since they allow for determining the when and where a dissipative process occurs but not to measure the relative energy variation.…”

“…When the damage event is produced in a material, a localised heat dissipation appears and a temperature rise is recorded in that zone. In the specific field of fracture mechanics, a temperature rise is detected in the plastic zone around the crack tip where the plastic volume dissipates energy [9][10][11][12]. Therefore, thermography represents a valid tool to localise the crack tip that is important for both determining in experimental way, the Paris Law [13] as well as fatigue crack propagation [14].…”