D. Palumbo

Characterisation of steel welded joints by infrared thermographic methods

Despite the large number of proposals in the field of fatigue prediction of welded joints, there is no worldwide accepted and unified theory which can be easily applicable to any load condition. Real life components, in fact, differ in geometry and/or type of load from the structural design considered by the Standards, hence a lot of precautionary safety factors are used, leading to an underestimation of the actual fatigue life of joints. Infrared thermography has a great potential in this field. In fact, it enables a full-field stress analysis with an adequate spatial resolution so that the complexity of the stress state at the weld toe and its time evolution are taken into account, emphasising anomalies that may predict structural failure. This paper presents a new method for the evaluation of the fatigue limit, focusing on interesting results derived from the analysis of thermoelastic signal phase evolution. Variations in the value of signal phase indicate a non-elastic behaviour and plastic dissipation in the material.

Monitoring of the friction stir welding process by means of thermography

Abstract This work is a study of the thermal behaviour of aluminium alloy 5754-H111 sheets welded with the friction stir welding (FSW) process. In particular, the feasibility of infrared thermography for monitoring of the FSW process is presented. This process has different advantages compared to those of traditional welding, such as very low welding temperature and low mechanical distortion. Usually in the literature, destructive tests are carried out to evaluate the quality of joints, but this approach is time-consuming and off-line. Results have shown that the thermal behaviour of joints is correlated to process parameters and that thermography can be used to perform the online monitoring of the FSW process.

Effect of Friction Stir Process Parameters on the Mechanical and Thermal Behavior of 5754-H111 Aluminum Plates

A study of the Friction Stir Welding (FSW) process was carried out in order to evaluate the influence of process parameters on the mechanical properties of aluminum plates (AA5754-H111). The process was monitored during each test by means of infrared cameras in order to correlate temperature information with eventual changes of the mechanical properties of joints. In particular, two process parameters were considered for tests: the welding tool rotation speed and the welding tool traverse speed. The quality of joints was evaluated by means of destructive and non-destructive tests. In this regard, the presence of defects and the ultimate tensile strength (UTS) were investigated for each combination of the process parameters. A statistical analysis was carried out to assess the correlation between the thermal behavior of joints and the process parameters, also proving the capability of Infrared Thermography for on-line monitoring of the quality of joints.

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.

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.

Automatic detection of subsurface defects in composite materials using thermography and unsupervised machine learning

This paper presents a complete framework aimed to nondestructive inspection of composite materials. Starting from the acquisition, performed with lock-in thermography, the method flows through a set of consecutive blocks of data processing: input enhancement, feature extraction, classification and defect detection. Experimental results prove the capability of the presented methodology to detect the presence of defects underneath the surface of a calibrated specimen made of Glass Fiber Reinforced Polymer (GFRP). Results are also compared with those obtained by other techniques, based on different features and unsupervised learning methods. The comparison further proves that the proposed methodology is able to reduce the number of false positives, while ensuring the exact detection of subsurface defects.

Coating defect evaluation based on stimulated thermography

Thermal Barrier Coatings are used to protect the materials from severe temperature and chemical environments. In particular, these materials are used in the engineering fields where high temperatures, corrosive environments and high mechanical stress are required. Defects present between substrate material and coating, as detachments may cause the break of coating and the consequent possibility to exposure the substrate material to the environment conditions. The capability to detect the defect zones with non-destructive techniques could allow the maintenance of coated components with great advantages in terms of costs and prediction of fatigue life. In this work, two different heat sources and two different thermographic techniques have been used to detect the adhesion defects among the base material and the coating. Moreover, an empirical thermographic method has been developed to evaluate the thickness of the thermal coating and to discriminate between an unevenness of the thickness and a defect zone. The study has been conducted on circular steel specimens with simulated adhesion defect and on specimens prepared with different thicknesses of thermal barrier coating.

Early Detection of Damage Mechanisms in Composites During Fatigue Tests

Conventional procedures and methods used for evaluating the fatigue performance of materials require time consuming tests with high number of specimens. In the last years different methods have been developed with the aim to reduce the testing time and then the cost of the experimental campaign. In this regard, thermographic methods represent a useful tool for rapid evaluating the fatigue damage and the fatigue limit.

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.