Estimation of the dissipative heat sources related to the total energy input of a cfrp composite by using the second amplitude harmonic of the thermal signal

Theories for predicting the fatigue behaviour of composite laminates often make strong assumptions on the damage mechanisms that strongly depend on the designed laminate lay-up. In this regard, several physical and empirical models were proposed in the literature that generally require experimental validations. The experimental techniques, such as thermography, also provide useful tools for monitoring the behaviour of the specific material so, that they can be used to support the study of the damage mechanisms of materials. In this research, the second amplitude harmonic of the thermal signal has been investigated and used to assess the relationship with the total energy input in order to estimate the fatigue strength of the material. A thermal index was assessed by monitoring the constant amplitude tests (S/N curve) that were performed on a quasi-isotropic carbon fibre reinforced polymer (CFRP) laminate obtained by the automated fibre placement process. The obtained results demonstrated the capability of the second amplitude harmonic of the thermal signal to describe and monitor the fatigue damage

Study of the plastic behavior around the crack tip by means of thermal methods

AbstractIn this work, the behaviour of two cracked stainless steel AISI 410 and C3FM was studied by means of a new procedure based on thermographic methods. A temperature model in time domain was considered in order to obtain information about the first and the second order harmonic of the temperature signal. Interesting results were obtained in term of possibility to describe the plastic phenomena at the crack tip

Fatigue behaviour assessment of automated fiber placement composites by adopting the thermal signal analysis

Infrared Thermography has been successfully used as an experimental, non-destructive, real-time and non-contact technique both to perform non-destructive evaluations and to study the fatigue behaviour of materials. However, the temperature is a very sensitive parameter to the environment conditions such as the thermal heat exchanges. It follows that, heavy and time expensive algorithms have to be setup to accurately filter out the overall âdisturbing' heat sources. Otherwise, an in-depth analysis of thermal signal allows the assessment of different indexes related to physical processes of fatigue damage and failure. Theoretical and experimental framework becomes complicated in case the material is a composite due to the layups of lamina or due to the viscous properties of the bulk of the matrix, or due to the pattern described by the yarns or fibers making difficult any quantitative and qualitative analysis. In fact, anisotropy and heterogeneity of composites influences unavoidably the mechanical response of the material to external excitation and the failure mechanisms. In all the cases, the study of thermal heat sources related to dissipative phenomena becomes complicated. Thermal signal analysis provides a localised analysis for assessing qualitatively and quantitatively the state of degradation of material in terms of stiffness or in term of damage detection, by extracting temperature components related to the appearance of plastic zones or cracks or in general to dissipative heat sources. The focus of the present research is to provide an innovative method and algorithm for processing the signal from innovative composites obtained by Automated Fiber Placement process in order to assess the fatigue behaviour and damaged regions qualitatively and quantitatively

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)

Analysis of Intrinsic Dissipations and Fatigue Behaviour ofSteelsby Measuring Thermal and Mechanical Signals DuringFatigueTests

The fatigue behaviour and intrinsic dissipations are studied in the present research by using different approaches: a thermography-based one leading to the use of a second harmonic amplitude temperature component as a damage parameter and the one using the mechanical energy input related to the energy dissipated during fatigue processes. Moreover, the relation between second amplitude harmonics of the heat-converted energy and of instantaneous power density is investigated for C45 steel undergoing fatigue tests by using stepwise loading sequences at two stress ratios. The analysis allows to assess the mathematical relation between second amplitude harmonics of instantaneous power density and temperature. Once these calibration coefficients are assessed the area under the hysteresis loop can be finally determined. It will be shown that the relation between second amplitude harmonics of the heat converted energy and instantaneous power density is independent from the damage level and cycles run, but depends on material only. The approach leads to a local analysis and does not require specific loading conditions because no hypothesis is made on the percentage of energy converted in heating. Furthermore, the analysis does not require material stabilization. The proposed procedure involves the estimation of the area under the hysteresis loop at any loading condition by simply assessing second harmonic temperature variations. The verification of the procedure is presented on extra samples

An experimental procedure based on infrared thermography for the assessment of crack density in quasi-isotropic CFRP

The crack density is a very well-known damage parameter representing the actual mechanical state of the material in terms of stiffness degradation. In effect, for laminates presenting off-axis laminae, crack density is useful for determining the “characteristic damage state” (CDS) that is related the load carrying capability of the laminate. In literature, analytical and empirical models in addition with experimental procedures are used for the assessment of crack density. However, in all cases, accurate experimental setups and time-consuming analyses are required. In this work, a novel procedure is proposed for performing contactless measurements of crack density during constant amplitude fatigue tests by using temperature second amplitude harmonic without any material properties assessment. The results of the experimental campaign in terms of crack density assessed at different stress level are in good agreement with those provided by analytical models and demonstrate the capability of the proposed parameter provided by the thermal signal analysis to describe damage mechanisms affecting the specific material. The proposed procedure leads to estimate the crack density in those applications where it is difficult to detect transverse crack with a direct measurement using common experimental techniques

Fatigue damage analysis of composite materials using thermography-based techniques

Composite materials are nowadays used in many fields of industry, especially for producing large structural components in many applications ranging from naval to aerospace. Beside to the capability and versatility of uses, the study of damage in composites is not easy due to the different failure mechanisms that can occur simultaneously or in different conditions. The characterisation of composites represents then a critical stage of assessing mechanical properties and resistance and a careful attention has to be put in the study and damage analysis. Due to this, the fatigue performances imposed by Standards have to be verified by means of experimental techniques involving experimental campaign in laboratory on samples or directly on large components. However, classical procedures for evaluating the fatigue resistance of materials present two issues: the expensive and time-consuming tests because of the high number of specimens being tested, and the totally absence of information on occurring damage. In the last few years, great efforts have been made to develop a number of methods aimed at reducing testing time and, subsequently, the cost of the experimental campaign. Among the different techniques, for instance, thermographic methods are considered as a useful tool for the rapid evaluation of fatigue damage and fatigue resistance at specific cycles number (endurance limit). The capability of thermography, is not only, related to the experimental procedure providing specific tests capable of assessing fatigue resistance in accelerated way, but also to study the energy involved in the fatigue processes. As previously said, damage mechanisms in composite materials are difficult to be understood and even a small scale anomaly can lead the failure of the material without visible damage or visible signs of the onset of failure phenomena. For this reason, energy intrinsically dissipated can be another point of view to face up to a sudden failure. In this way, energy-related parameters assessed by the analysis of thermographic signal can be useful for assessing information related to the onset of irreversible damage. The focus of the present research is to provide an innovative method for process thermal signal from innovative composites obtained by Automated Fiber Placement process in order to understand the fatigue behaviour qualitatively and quantitatively