Umberto Galietti

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.

Energy analysis of fatigue damage by thermographic technique

The main aims of this paper are to describe the thermographic methodologies currently used in Italy for the rapid evaluation of the fatigue limit and to describe the local energy approach actually under development by the authors. Thermographic methodologies currently used in Italy for the rapid evaluation of the fatigue limit were applied to two stainless steels (AISI 304 and AISI 409). All the experimental results here obtained are in good agreement with the respective values reported in literature. An experimental program for the local energy approach is under development: its main characteristic consists in doing, besides the usual thermal measurements made by thermography, mechanical measurements in order to evaluate the mechanical energy locally dissipated inside the material. This experimental research is part of an interuniversity research program and it is made on stainless steel (AISI 304) notched specimens.

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.

Investigation on the Influence of the Surface Resin Rich Layer on the Thermoelastic Signal from Different Composite Laminate Lay-Ups

This work presents a set of experimental results based on the measured thermoelastic signal from GRP composite coupons adopting different lay-ups. A comparison is made with the thermoelastic signal predicted by two different analytical models: one based on the classical law of the thermoelastic effect for orthotropic materials, and the other based on a novel theory accounting for the presence of a resin layer on the external surface of the composite structure. The composite coupons were designed such to determine a significant difference in the predictions made by the two theoretical models. Experimental results have shown a far better match with the predictions based on the novel theory accounting for the presence of a surface resin rich layer.

A novel signal processing method for TSA applications

A novel development of the thermoelastic stress analysis (TSA) technique for testing mechanical components under the random load condition is presented. The method proposed does not need a reference signal to reduce the amount of noise in the thermoelastic images, allowing an easier application of the TSA to the real working condition of mechanical components. Noise is filtered out by means of a numerical algorithm based on the hypothesis that a random thermoelastic signal can be approximated with a harmonic signal. Some notched specimens under the pseudo-random load condition have been tested using either the proposed method or a commercial one (DeltaTherm data processing procedure). Results obtained with both methods are in good agreement.

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).

An application of the differential thermographic technique for welded joints fatigue evaluation

The paper deals with the possibility of using the differential thermography (Thermoelastic Stress Analysis) to predict the fatigue resistance of welded joints on the basis of the local stress/strain field at the weld toe. The study is inspired by a local strain method, the WEL.FA.RE. method, based on the local amplitude of strain εa measured by 3mm grid-length strain gauges bonded with the axis at a 2.5mm distance from the real weld toe. The WEL.FA.RE. method suggests to determine the fatigue limit of welded joints simply by means of an experimental curve and the measurements of the local amplitude of strain εa to the weld toe directly on the structure under service conditions. In this work, both strain gauge and TSA techniques have been used to this purpose. In order to understand the development of the fatigue phenomena, the entire local strain field to the weld toe has been monitored by means of the thermoelastic stress analysis (TSA) technique and the results have been compared to those obtained with strain gauge. Structural steel specimens have been fatigue tested under alternate symmetric loads (tension-compression) and the local strain amplitude to the weld toe has been measured with two experimental technique (strain gauge and thermoelastic stress analysis) and compared in view of choose which one is more suitable for the WEL.FA.RE. method. The analysis of the thermoelastic data has showed that TSA is able to provide adequate spatial resolution to describe the complexity of the strain field along the cord. Furthermore the phase image has turned out to be an effective parameter to assess the crack initiating and growth. So, thermoelasticity has the capacity to be used as a non destructive technique for the evaluation of the structural integrity of the welded joints.

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.

Analysis of thermoelastic and dissipative effects related to the fatigue of 2024 T3 aluminium alloy

In this paper the fatigue phenomena of 2024 T3 aluminium alloy were studied in terms of thermal and calorimetric effects during uniaxial cyclic loading. Thermoelastic coupling sources and dissipation were separately estimated by using infrared thermal data and a local simplified form of the heat equation. The simplifications are essentially based on the assumption that the uniaxial fatigue test remains homogeneous until a macroscopic fatigue crack occurs within the gauge section of the specimen. Heat sources were then compared to predictions derived from mechanical data by assuming a linear isotropic thermoelastic behaviour of the material and by neglecting the influence of thermomechanical couplings on the hysteresis area of fatigue cycles.