Thermomechanics and Infra-Red Imaging, Volume 7

The overall motivation for the research described in the paper is an enhanced understanding of the behaviour of fibre reinforced polymer composites subjected to high velocity loading. In particular, the work described here considers a method that allows the collection of synchronised high speed full-field temperature and strain data to investigate the complex viscoelastic behaviour of fibre reinforced polymer composites material that occurs at high strain rates. The experimental approach uses infra-red thermography (IRT) and digital image correlation (DIC). Because high strain rate events occur rapidly it is necessary to capture the images at high speeds. The paper concentrates on the challenges of the use of IRT and DIC at high speeds to obtain temperature and strain fields from composite materials, and in particular using them in a synchronised manner. In the future such data-rich techniques provide the opportunity for detailed investigation into the viscoelastic behaviour and allow in-depth material characterisation for input to future finite element or numerical models.

Composite materials are finding increased use in applications where impact and high strain rate loading form a significant part of a component’s service loads. It is therefore imperative to fully characterise the thermomechanical response of composite materials at high strain rates. The work described in the paper forms part of a project investigating the thermomechanical response of composite materials at high strain rates. To obtain the temperature evolutions during the high strain rate event (thermoelastic, viscoelastic and fracture energy), full-field infrared thermography is used. In contrast to visible light photography, the measurand in thermography is the intensity of the emitted radiation from the specimen surface, as opposed to reflected radiation. At increasing recording rates, the emittance available for measurement reduces proportional to the exposure time; the faster the data capture the less the exposure time. Hence, signal noise and detector calibration present a major challenge. This is accompanied by challenges arising from controlling an infrared detector that has not been optimised for the purpose of high speed data acquisition. The present paper investigates the possibility of applying infra-red thermography to high strain rate events and discusses the challenges in obtaining reliable values of the temperature changes that occur over very short time scales during high strain rate events.

Commercially available polycrystalline nickel (Ni200; grain size: 30 μm) and electrodeposited nanocrystalline nickel (grain size: 30 nm) were analyzed for the phenomena of in-situ heat generation and strain localization during plastic deformation at room temperature. Tensile specimens according to ASTM E8 standard dimensions were tested at a strain rate of 10^-2/s to record the amount of heat dissipated and the change of localized strain using a high resolution infrared detector and digital image correlation (DIC) camera, respectively. For deformation close to ultimate tensile strength, data recorded for the maximum temperature increase and localized strain for nanocrystalline were 110C and 4.5%, whereas polycrystalline nickel showed 170C and 60%, respectively. The amount of heat generated locally by strain is related by the heat conversion factor (i.e. Taylor Quinney coefficient). Polycrystalline nickel showed a decreasing trend of heat conversion due to lattice distortions or defect formation during deformation. In contrast, nanocrystalline nickel showed an increasing trend, likely due to differences in deformation mechanisms.

Rubber-like materials can undergo very large strains in a quasi-reversible way. This remarkable behavior is often called hyper (or entropic) elasticity. However, the presence of mechanical loops during a load-unload cycle is not consistent with a purely elastic behavior modeling. Using Digital Image Correlation and Infra-Red Thermography, the present study aims at observing and quantifying dissipative and coupling effects during the deformation of natural rubber at different elongation ratios. For elongation ratios less than 2, the famous thermo-elastic inversion is revisited within the framework of the irreversible processes thermodynamics, and interpreted as a competition between two coupling mechanisms. For elongation of about 3 or 4, the predominance of entropic elasticity is shown and the relevance of the analogy with perfect gases, at the root of its definition, is energetically verified. For very large elongation ratios (about 5), the energy effects associated with stress-induced crystallization-fusion mechanisms are underlined. The current experiments, performed at relatively slow strain rate, did not exhibit any significant dissipation.

A new method to estimate the inelastic heat fraction (Taylor and Quinney Beta coefficient) during the deformation of a titanium material is proposed. It is based on (1) thermomechanical full field measurements during the loading and on (2) an inverse analysis. First, two cameras (a visible and an infrared) are used to measure the kinematic and the thermal fields on each face of a notched flat sample loaded in tension. Second, two coupled finite element simulations (a mechanical, then a thermal one) of the same tests are conducted. Associated with a Levenberg-Marquardt optimization algorithm, they are able to give, in a first step, optimized values of the anisotropic elastoplastic model parameters. Then, in a second step, parameters of four strain dependent Beta models are identified. Finally, the thermal responses of these models are compared to the experimental values.

This paper presents an experimental protocol developed to locally estimate different energy balance terms associated with the high cycle fatigue (HCF) of steels. Deformation and dissipated energy are respectively derived from displacement and temperature fields obtained using digital image correlation (DIC) and quantitative infrared thermography (QIRT) techniques. The combined processing of visible and infrared images reveals the precocious, gradual and heterogeneous development of fatigue localization zones. It also highlights the plastic character of dissipative heat sources (i.e. proportional to the loading frequencies), and the progress of fatigue dissipation, observing the drift of the mean dissipation per cycle for a given loading. The substantial of internal energy variations during HCF loading are finally underlined. The paper ends with a discussion on the consequences of such energy balance properties in terms of HCF modeling.

Using kinematical and thermal full-field measurements for identification of mechanical parameters has become a very promising area of experimental mechanics. The purpose of this work is to extend the use of non-conventional tests and full field measurements (kinematical and thermal) to the identification of the fatigue properties of a dual-phase steel. A particular attention is paid to the influence of plastic pre-strain on the fatigue limit. Indeed, an analytical approach is proposed to define the geometry of the specimen permitting to obtain a constant gradient of plastic strain within the zone of interest after a monotonic pre-strain. Then, a self-heating test under cyclic loading is carried out on the pre-strained specimen. During this cyclic test, the thermal field is measured using an infrared camera. Finally, a suitable numerical strategy is proposed to identify a given thermal source model taking into account the influence of a plastic pre-strain. The results show that, with the non-conventional test and the procedure developed in this work, the influence of a range of plastic pre-strain on fatigue properties can be identified by using only one specimen. It is worth noting that a great number of specimens is required to determine this effect by using classical fatigue campaign.

Fatigue characterization is an expensive operation commonly undertaken in industry. Some authors thus developed experimental measurement methods based on the materials thermomechanical behaviour to provide faster fatigue limit estimations. Yet, the physical ground of these methods needs to be understood. In this work, it has been assumed that heat dissipation phenomena are related to dislocation movements in the material lattice (internal friction); changes in the dislocation characteristics (through plastic straining for example) will affect the material dissipative behaviour.

The dissipative energy characteristics of a Dual-Phase 600 grade (DP600) have been experimentally estimated during traction-traction cyclic loadings on thin sheet specimens. The specimens surface temperature variations have been recorded using an infrared camera and analysed using the heat balance equation. Each dissipative energy measurement has been performed for a specific microstructural state of the material (no macroscopic plasticity occurs during the measurement).

The effect of different loading sequences on the material dissipative behaviour has been tested and interpreted using the commonly used specific damping capacity. The dissipative energy (the dislocation mobility) has been proved to increase with the macroscopic plastic strain and to be affected by aging periods at ambient temperature.

Polymer foam cored sandwich structures are often subjected to aggressive service conditions which may include elevated temperatures. A modified Arcan fixture (MAF) has been developed to characterize polymer foam materials with respect to their tensile, compressive, shear and bidirectional mechanical properties at room and at elevated temperatures. The MAF enables the realization of pure compression or high compression to shear bidirectional loading conditions that is not possible with conventional Arcan fixtures. The MAF is attached to a standard universal test machine equiped with an environmental chamber using specially designed grips that allow the specimen to rotate, and hence reduces paristic effects due to misalignment. The objective is to measure the unidirectional and bidirectional mechanical properties of PVC foam materials at elevated tempreature using digital image correlation (DIC), including the elastic constants and the stress-strain response to failure. To account for nonhomogeneity of the strain field across the specimen cross sections, a “correction factor” for the measured surface strain is determined using nonlinear finite element analysis (FEA). The final outcome is a set of validated mechanical properties that will form the basis input into a detailed finite element analysis (FEA) study of the nonlinear thermo-mechanical response of foam cored sandwich panels.

Transient thermography non-destructive evaluation methods are being used in aerospace industry to inspect flaws and damages for various composite materials. The purpose of this paper is to establish a set of guidelines for testing Carbon Fiber Reinforced Panels (CFRP) panels using infrared thermography. These guidelines insure that the inspection process is efficient and effective. Samples with simulated defects were made and modeled using a finite element program. Heat will be applied to the models and the temperature profiles analyzed. Along with changing the heat and time, different post-processing techniques were used to improve the method in determining defects in the sample. Once this has been optimized, actual CFRP panels with the same simulated defects were experimentally tested using the conditions from the analytical model. The analytical and experimental data was compared to insure that the testing process has been optimized. A standardized process was developed for evaluating the CFRP panels using infrared thermography.

Polymer foam cored sandwich structures are commonly used in applications where mechanical loads and elevated temperatures form the normal service conditions. The temperature sensitivity of the mechanical properties of the polymer foam cores leads to compromised mechanical performance of the overall sandwich structure at elevated temperatures. So far this phenomenon has primarily been investigated using analytical techniques. The present paper provides a basis for experimental studies of the temperature sensitivity of sandwich structures through the design of a test rig that can simulate the mechanical and thermal conditions experienced in service. The sandwich structure is modelled as a simple beam specimen with the mechanical load introduced using a standard servo-hydraulic test machine. A fixture has been specially designed that can apply a variety of constraints. A through thickness temperature gradient is introduced to the beam via an infrared (IR) radiator applied to one face sheet. The rig is designed to accommodate non-contact full-field techniques. An infrared detector is used to obtain the temperature field and high resolution white light cameras to capture the displacement using digital image correlation (DIC). An arrangement of mirrors enables both the face sheet and through-thickness surfaces to be viewed. The paper presents the design and evaluation of the rig, together with initial data obtained from a PVC foam cored sandwich specimen with aluminium face sheets.

This research developed a reliable intelligent non-destructive evaluation (NDE) expert system for Carbon Fiber Reinforced Panels (CFRP) panels based on infrared thermography testing (IRT) and post processing by means of fuzzy expert system technique. Data features and NDE expert knowledge are seamlessly combined in the intelligent system to provide the best possible diagnosis of the potential defects and problems. As a result, this research help ensure CFRP panels’ integrity and reliability. Specimens with simulated defects were evaluated to demonstrate the usefulness of the intelligent IRT NDE expert system in NDE inspection. The testing data pattern corresponding to feature and quantification of defects were found. This fuzzy expert system not only eliminates human errors in defect detection but also functions as NDE experts. In addition, fuzzy expert system improves the defect detection by incorporating fuzzy expert rules to remove noises and to measure defect size more accurately. In the future, the expert system model could be continuously updated and modified to quantify the size and distribution of defects. The system developed here can be adapted and applied to build an intelligent NDE expert system for better quality control as well as automatic defect and porosity detection in CFRP production process.

A new remote nondestructive inspection technique based on thermoelastic temperature measurement by infrared thermography was developed for the detection of fatigue cracks in steel bridges. Fatigue cracks were detected from localized thermoelastic temperature changes at crack tips due to stress singularities generated by wheel loading from traffic on a bridge. Self-reference lock-in data-processing technique and motion compensating technique were developed to improve the thermal images obtained in the crack detection process. Advantages and limitations of the proposed nondestructive evaluation technique were discussed based on results of field experiments for highway bridges. Thermoelastic stress analyses in the vicinity of crack tips were also carried out after the crack detection process by self-reference lock-in thermography. The stress distribution under wheel loading by traffic was measured by infrared thermography. Stress intensity factors were evaluated from measured stress distribution. It was found that these fracture mechanics parameters can be evaluated with reasonable accuracy by the proposed technique, enabling the assessment of structural integrity based on the evaluated fracture mechanics parameters.

This research developed a reliable intelligent non-destructive evaluation (NDE) expert system for Carbon Fiber Reinforced Panels (CFRP) panels based on infrared thermography testing (IRT) and post processing by means of fuzzy expert system technique. Data features and NDE expert knowledge are seamlessly combined in the intelligent system to provide the best possible diagnosis of the potential defects and problems. As a result, this research help ensure CFRP panels’ integrity and reliability. Specimens with simulated defects were evaluated to demonstrate the usefulness of the intelligent IRT NDE expert system in NDE inspection. The testing data pattern corresponding to feature and quantification of defects were found. This fuzzy expert system not only eliminates human errors in defect detection but also functions as NDE experts. In addition, fuzzy expert system improves the defect detection by incorporating fuzzy expert rules to remove noises and to measure defect size more accurately. In the future, the expert system model could be continuously updated and modified to quantify the size and distribution of defects. The system developed here can be adapted and applied to build an intelligent NDE expert system for better quality control as well as automatic defect and porosity detection in CFRP production process.

This paper presents an effective way to determine the individual stresses in arbitrarily-loaded multiply-perforated finite plates whose various size cutouts are randomly distributed. Thermoelastic stress analysis (TSA) is used such that the recorded temperature data are processed utilizing an Airy’s stress function. Advantages of TSA include full-field, non-contacting, nondestructive, no surface preparation other than a paint is needed, unnecessary to differentiate the measured information and has a resolution comparable to that of commercial foil strain gages. The present analysis was inspired by previous studies involving the TSA stress analysis of an incline-loaded clamped plate containing a single cutout [1] and a tensile plate containing multiple holes which were located collinearly to the external load [2]. The individual stresses in vertically- and incline-loaded aluminum plates containing two side-by-side circular holes are determined here. The holes are sufficiently close together that their stress fields interact with each other. The absence of any universal loading fixturing causes the inclineloaded plate to be subjected to some unknown in-plane bending as well as the tension. This unknown loading of the inclined plate hampers stress analyzing the plate numerically, and theoretical solutions to finite geometries are extremely difficult. Notwithstanding the aforementioned statement, TSA results agree well with those from commercial strain gages and ANSYS. Load equilibrium is also satisfied.

Thermoelastic Stress Analysis (TSA) is based on the principle that under adiabatic and reversible conditions, a cyclically loaded structure experiences temperature variations that are proportional to the sum of the principal stresses. These temperature variations may be measured using a sensitive infra-red detector and thus the cyclic stress field on the surface of the structure may be obtained. In order to achieve the adiabatic and reversible conditions, the test specimen must be cyclically loaded at a high enough frequency to prevent heat transfer, which is not only dependent of the loading frequency, but also stress gradients in the specimen and the thermal conductivity of the material. TSA has been found to be an ideal method to study crack tip strain fields, due to its non-contacting, full-field, data collection capabilities, however the adiabatic assumption breaks down close to the crack tip due to the steep stress gradients. Hence the majority of crack tip studies have focussed on the determination of linear elastic parameters, where data are recorded from regions surrounding the crack tip where the adiabatic and reversible assumptions are valid.

More recently the non-adiabatic region close to the crack tip has been explored, with the aim of gaining a greater understanding of crack-tip plasticity. Infrared thermography has been used to correlate the energy dissipation at the crack tip to the plastic zone [1,2], whereas others [3,4,5] have explored using the TSA phase data to quantify the size and shape of the crack-tip plastic zone. The latter method allows the elastic and plastic strain field information to be recorded simultaneously, and thus has the potential for near real-time studies of fatigue crack growth. Thus far, the phase method has only been applied at fairly low frequencies on aluminium alloys. Since the method considers non-adiabatic effects at the crack tip and heat transfer is known to be dependent on material and frequency, it was decided to investigate the extent of plasticity at the tip of propagating fatigue cracks in two different aerospace materials at a range of frequencies.