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In this work a fatigue damage evaluation of martensitic steels was performed by means of thermal
methods. The stainless steel analyzed was ASTM A182 grade F6NM, a martensitic steel with low carbon
content. In addition to the corrosion resistance, very important is the fatigue behavior of this material
considering structural applications in offshore platforms and for the valve bodies and pumps for energy
production.
Classical methods for evaluating the fatigue limit require very time consuming tests, so accelerated methods for
analyzing the fatigue behaviour of materials are of great interest.
The aim of this study is the evaluation of the fatigue limit of martensitic stainless steel with thermal methods
based on temperature surface monitoring and phase variation of thermographic signal.
During fatigue tests the temperature trend allows to detect when the plastic deformations occur in the material
because high temperature value are measured. The thermal methods present in literature are based on the
monitoring of the temperature that provides the fatigue limit through a statistical approach that can cause some
problem with materials characterized by a high coefficient of thermal conductivity such as aluminum. Moreover,
temperature, depends of the amplitude stress, of the test frequency, of the specimen geometry and
environmental conditions that affect the heat exchange conditions between the specimen and the environment.
In this work was used the Thermoelastic Stress Analysis (TSA) to evaluate the thermoelastic sources and the
new approach based on the thermoelastic phase signal (TPA: Thermographic Phase Analysis) was used to
evaluate plastic behavior of the material. Thermoelastic stress analysis provides a signal S proportional to the
peak-to-peak variation in temperature during the variation peak to peak of the sum of principal stress Δσ, so for
an isotropic material, in linear elastic and local adiabatic conditions, is possible to link surface temperature
variation with the variation of the trace of stress tensor for a plane stress state.
The loss of adiabatic conditions are due to the presence of heat generation due to the high gradient stress or
local plasticity due to dissipative sources. When this conditions occur, the classical thermoelastic equation can’t
be used to assess the stress field on the surface of materials. Nevertheless, the evaluation of these phenomena
that arise with the heat conduction can be performed by means the thermoelastic phase signal. In this case a
phase shift occurs in thermographic signal that can be used to monitor the fatigue damage occurring in the
material and evaluate the fatigue limit.
The experimental setup provide an hydraulic loading machine, three infrared cameras (one in front of load
machine and two back of this one), an insulated wooden case preserving environmental influences around
loading zone. In this case only the data obtained from two thermocamera will be shown in particular, the
thermoelastic data have been acquired via the differential IR camera DeltaTherm 1560 (thermal sensitivity
(NETD) < 18 mK) and based on a InSb photonic detector with 320×256 pixels while thermographic data have
been acquired with Flir A20 (detector 160x140).