Rosa De Finis

Mechanical Behaviour of Stainless Steels under Dynamic Loading: An Investigation with Thermal Methods

Stainless steels are the most exploited materials due to their high mechanical strength and versatility in producing different alloys. Although there is great interest in these materials, mechanical characterisation, in particular fatigue characterisation, requires the application of several standardised procedures involving expensive and time-consuming experimental campaigns. As a matter of fact, the use of Standard Test Methods does not rely on a physical approach, since they are based on a statistical evaluation of the fatigue limit with a fixed probabilistic confidence. In this regard, Infra-Red thermography, the well-known, non-destructive technique, allows for the development of an approach based on evaluation of dissipative sources. In this work, an approach based on a simple analysis of a single thermographic sequence has been presented, which is capable of providing two indices of the damage processes occurring in material: the phase shift of thermoelastic signal φ and the amplitude of thermal signal at twice the loading frequency, S2. These thermal indices can provide synergetic information about the mechanical (fatigue and fracture) behaviour of austenitic AISI 316L and martensitic X4 Cr Ni Mo 16-5-1; since they are related to different thermal effects that produce damage phenomena. In particular, the use of φ and S2 allows for estimation of the fatigue limit of stainless steels at loading ratio R = 0.5 in agreement with the applied Standard methods. Within Fracture Mechanics tests, both indices demonstrate the capacity to localize the plastic zone and determine the position of the crack tip. Finally, it will be shown that the value of the thermoelastic phase signal can be correlated with the mechanical behaviour of the specific material (austenitic or martensitic).

Optimization and Characterization of the Friction Stir Welded Sheets of AA 5754-H111: Monitoring of the Quality of Joints with Thermographic Techniques

Friction Stir Welding (FSW) is a solid-state welding process, based on frictional and stirring phenomena, that offers many advantages with respect to the traditional welding methods. However, several parameters can affect the quality of the produced joints. In this work, an experimental approach has been used for studying and optimizing the FSW process, applied on 5754-H111 aluminum plates. In particular, the thermal behavior of the material during the process has been investigated and two thermal indexes, the maximum temperature and the heating rate of the material, correlated to the frictional power input, were investigated for different process parameters (the travel and rotation tool speeds) configurations. Moreover, other techniques (micrographs, macrographs and destructive tensile tests) were carried out for supporting in a quantitative way the analysis of the quality of welded joints. The potential of thermographic technique has been demonstrated both for monitoring the FSW process and for predicting the quality of joints in terms of tensile strength.

Capability of infrared thermography for studying the friction stir welding process

The Friction Stir Welding (FSW) is an innovative solid-state welding method based on frictional and stirring phenomena, discovered and patented by TWI Ltd in 1991, providing high quality components for aerospace, marine and automotive industrial fields. In this process, a rotating non-consumable tool that plunges into the work piece and moves forward produces the heat necessary to weld the parts together. The much lower temperatures compared with those achieved in traditional welding processes by melting, determine the following main advantages of FSW: minimal mechanical distortion, excellent surface finish, absence of splash, no crack formation and porosity after welding, thanks to the low input of total heat. This work deals with the use of thermographic techniques for monitoring the friction stir welding process applied on AA 5754-H111 plates, in order to evaluate the quality of the produced joints in terms of presence of defects and Mechanical strength. The adopted experimental approach was addressed to study and optimizing the FSW process by analyzing the thermographic sequences and extracting several indexes related to the heating involved in the process. Such the indexes, the maximum temperature, the heating and cooling rate of the material, correlated to the frictional power input and the presence of defects respectively, have been investigated for different process parameters (the travel and rotation tool speeds) configurations. The results of the research have been quantitatively supported and characterized by destructive and non-destructive techniques.

Potentialities of thermal signal analysis approach for a rapid mechanical characterisation of high diffusivity materials

One of the most important advantages of using high-diffusivity alloys like aluminium, in industry, is to reduce the weight without renouncing to high strength components. To accelerate the time of the mechanical characterisation, frequently experimental methods based on temperature measurements are adopted, even if in this case, these methods could involve in wrong estimations. In particular, the study of energy dissipations could produce some assessment errors of fatigue limit due to the fact that the fraction of the detected energy dissipated could be lower if compared to the effective energy intrinsically dissipated in the material due to damage. Furthermore, the fatigue life assessment of Aluminium alloys is problematic due to a non-distinct ‘knee’ in the S-N curve. To take into account these issues and to estimate the fatigue strength in rapid and accurate way, in this work, a method providing a specific thermal signal analysis is presented applied to an aluminium alloy 5754 H-111. Firstly, the well-known methods based on direct temperature measurements for estimating fatigue strength of metals, were applied on an aluminium alloy 5754 H111 in order to demonstrate their problematic application for high-diffusivity materials. Furtherly, a specific thermal signal analysis was adopted for extracting first and second order temperature variations as better parameter for fatigue strength assessment. This work questions the use of direct temperature evaluation in high diffusivity materials and fully replaces it in favor of an approach based on in-depth analysis of thermal signal by using thermoelastic and dissipative temperature variations.

Thermographic signal analysis of friction stir welded AA 5754 H111 joints

Aluminium alloys present some criticalities in terms of fatigue life characterisation due to the absence of a point representing the ‘fatigue limit’, the topic becomes complicated when the material is welded. In this case, the fatigue characterisation lies on design specifications which have to clearly explain the guidelines for the performing the tests and for evaluating the failures, in order to design tailored welded joints. However, the fatigue of welded joints is a difficult subject since the welding process makes the material different, introducing residual tensions, defect, etc. Also, the standard test methods provide only the estimation of the strength at fixed loading cycles but no information on the damage processes occurring in the material. Prompted by these issues researchers deal with the study of other approaches to achieve not only information on fatigue resistance but also damage information. In particular, the thermography can be used for thermal signal analysis of dissipative heat sources involved in fatigue of material undergoing cyclic test. In this paper, this approach is adopted to study the fatigue behavior of friction stir welded joints of AA5754-H111 during specific loading conditions. The component of the temperature related to intrinsic dissipations is assessed and the fatigue strength is evaluated together with a graphical study of the location of damaged areas.

Correlation between Thermal Behaviour of AA5754-H111 during Fatigue Loading and Fatigue Strength at Fixed Number of Cycles

The characterization of the fatigue behaviour of aluminium alloys is still capturing the attention of researchers. As it is well known in literature, for certain alloys, in a specific range of cycles number, the S-N curves do not present any asymptote. So that, problems result in the assessment of the fatigue life. In these conditions, the concept of the fatigue limit has to be replaced by the fatigue strength at a fixed number of loading cycles. Temperature acquisitions during fatigue tests allow for a specific analysis that can support the researchers in understanding the complex processes that are involved in fatigue and their influence on fatigue life, even for aluminium alloys. In fact, the analysis of the surface temperature signal that was detected during a self-heating test provides a curve that is characterized by a distinct slope-change point at a specific stress value. Even though researchers have been investigating fatigue life characterisation and temperature variations for more than a decade, it is not clear what this point represents in terms of fatigue strength. The aim of the present paper is to find out a possible correlation between the thermal behaviour of AA5754-H111 undergoing self-heating testing procedure and fatigue strength at a specific loading cycles.

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.