1,721,375 research outputs found
The development of a hybrid technique employing the boundary element method for thermoelastic stress separation
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Thermal NDE of thick GRP panels by means of a Pulse Modulated Lock-In Thermography technique
Full text linkThis work describes the development and implementation of an infrared thermal NDE procedure for the evaluation of subsurface defects. The approach is called Pulse-Modulated Lock-In Thermography (PMLT) and is based on the analysis of the frequency response of the measured temperature and comparison with the carrier frequencies launched by the external heat delivering source. The heat deposited on the object is in particular modulated as a train of square waves. This is easily achieved by periodically shuttering the heat source. The temperature is then sampled throughout the deposition of a few square waves. A lock-in algorithm is then implemented able to selectively filter out components at the main carrier frequencies of the heating signal and evaluate the phase information. Defected areas at different depths can be marked based on phase contrast, by using data from a single experiment as in Pulsed-Phase Thermography. An artificially defected thick GRP panel typical of naval monolithic hull structures, is investigated to validate the proposed technique. Experimental data have confirmed the potentials of PMLT as a flexible IR NDT approach, and its ability to be implemented by means of low cost heating and IR equipments
Quantitative thermoelastic stress analysis by means of low-cost setups
Full text linkA low-cost Thermoelastic Stress Analysis (TSA) experimental setup is proposed which uses an ordinary micro-bolometer and in-house developed signal processing scripts. The setup is evaluated by analysing the thermoelastic signal from a tensile and a SENT specimen made of stainless steel AISI 304L, and the bolometer performances are compared with those of a state of the art photon detector. Signal processing is based on off-line cross-correlation, using a self-reference signal which is retrieved from the acquired thermal data. Procedures are in particular developed to recognise, quantify and correct errors due to spectral leakage and loss of streamed frames. The thermoelastic signal amplitude/phase, the thermoelastic constant and the Mode I Stress Intensity Factor (SIF) from the bolometer and photonic cameras are evaluated considering the influence of loading frequency, sampling frequency, detector array sub-windowing and acquisition interval duration. A camera-specific linear calibration procedure is applied to correct the thermoelastic signal obtained with the bolometer. The procedure is extended to correct also SIF values, finding a good match with the SIFs obtained by the photon detector. An automatic iterative algorithm, based on the least square fitting of Williams’ series functions, is proposed to identify the crack tip position. An estimation of processing times of the developed signal processing scripts has been carried out, finding that a full crack characterisation (TSA maps, crack tip position, SIF) can be performed with a data acquisition time of 10-20 s, a post-processing time of less than 2 s and an overall hardware cost under 10 k€
Evaluation of Vertical Fatigue Cracks by Means of Flying Laser Thermography
No full textThe present paper proposes a new procedure to analyze the temperature field distribution during Flying Laser Spot and Laser Line Thermographic scanning (FLST, FLLT) of metallic components, in order to detect vertical surface cracks. The methodology exploits the changes in the temperature field produced by a vertical crack, acting as a barrier towards heat diffusion, when the laser approaches the defect. A number of small regions of interests (ROIs) is placed nearby and around the laser source. The average temperature from each ROI is then monitored during the laser scanning. Vertical cracks can be detected by analyzing and comparing the temperature fluctuations from each ROI when the laser crosses a crack. The paper, in particular, illustrates how the use of multiple ROIs, placed at different locations, may provide additional information that can be used to characterize the defect, and to identify the crack tip location. The approach is validated on plates made of steel and aluminum alloy, where natural cracks have been introduced by fatigue loading, and whose surface has been painted to enhance emissivity. Scratches in the paint have been artificially made in order to analyze their influence on the defect signature. The proposed experimental setup is further simplified by moving the plate samples, mounted on slits, in front of a still laser source and camera head
Investigations on some analytical stress and strain-based relations of the thermoelastic effect on FRP composite materials
No full textThermoelastic Stress Analysis for composite laminates: A numerical investigation
No full textA Finite Element model of the thermoelastic effect in orthotropic composite laminates subject to cyclic loading is developed. A meso-scale approach is employed to model the thermoelastic effect at the lamina level and the through-thickness heat transfer. The model proved itself capable of modelling the dependency of the thermoelastic response on the loading frequency, presence of a superficial resin-rich layer, material system, and layup. Results show that a steady state response is reached almost immediately, after only a few loading cycles, and that in some cases, even for very high loading frequencies, adiabatic conditions are never fully onset. Numerical predictions also confirm that the presence of a superficial resin-rich layer strongly affects the through-the-thickness heat transfer and the measured thermoelastic signal
A Quantitative Analysis of the Thermoelastic Effect in CFRP Composite Materials
No full textIn this study the thermoelastic signal from carbon fibre-reinforced plastic (CFRP)
laminates is investigated. A comparison between the theoretical and experimental values of the thermoelastic signal is reported, with the theoretical predictions obtained from two different quantitative models. These models are based on the classic thermoelastic effect law extended to the case of orthotropic materials (by using the mesomechanical or bulk approach), and the modified law assuming that the surface resin-rich layer behaves as a strain witness of the laminate. It is found that the theoretical predictions of the two models can be strongly and differently influenced by the intrinsic orthotropy of carbon fibres. Some effects are highlighted in particular such as the influence
of the laminate lay-up and the strong mismatch between the thermal expansion coefficients of the polymer matrix and the fibres. These influences are investigated analytically, predicting the thermoelastic
signal from various lay-ups and using strain-based and stress-based analytical models.
Experimental evidence of some theoretical findings is provided by reporting on tests performed on CFRP tensile samples manufactured from low-crimp unidirectional fabrics
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