Rear-face Temperature Research Articles

Experimental Investigation on Ablation Behaviors of CFRP Laminates in an Atmospheric Environment Irradiated by Continuous Wave Laser

In order to understand the ablation behaviors of CFRP laminates in an atmospheric environment irradiated by continuous wave laser, CFRP laminates were subjected to a 1080-nm continuous wave laser (6-mm laser spot diameter), with different laser power densities carried out in this paper. The internal delamination damage in CFRP laminates was investigated by C-Scan. The rear- and front-face temperature of CFRP laminates were monitored using the FLIR A 655 sc infrared camera, and the rear-face temperature was monitored by K type thermocouples. The morphology of ablation damage, the area size of the damaged heat affected zone (HAZ), crater depth, thermal ablation rate, mass ablation rate, line ablation rate, etc., of CFRP laminates were determined and correlated to the irradiation parameters. It is found that the area size of the damage HAZ, mass ablation rate, line ablation rate, etc., increased as the laser power densities. The dimensionless area size of the damaged HAZ decreased gradually along the thickness direction of the laser irradiation surface.

Experimental investigation on ablation characteristics of coated and uncoated steel under 30/80 μs impulse current

The ablation tests on coated and uncoated Q235B steel sheets were conducted under 30/80 μs impulse current simulating the lightning first return stroke current, aimed at further understanding the ablation characteristics of steel and investigating the impact of anti-corrosion coating on these characteristics. Ablation characteristics were investigated through the macroscopic morphology and x-ray diffraction patterns on the surface of damaged zones, the microstructure and micro Vickers hardness on the cross-section of damaged zones, and the maximum rear-face temperature of sample sheets. It can be concluded that the ablation areas of uncoated sheet consist of the melted layer and the heat-affect layer. These ablation areas include not only the area ablated directly by the arc root, of which the depth is deeper, but also the area forming due to the splashing of molten steel, of which the depth is shallower and decreases when the area’s distance from the arc attachment point increases. For coated sheet, coating materials have decomposed and evaporated forming an ablation pit on the sheet surface, in which the steel surface is exposed, and zinc filler of coating primer has infused into the exposed surface. The ablation diameter of uncoated sheet relates to the amplitude of the 30/80 μs impulse current in quadratic function, while for coated sheet, the relation is linear. In general, under the 30/80 μs impulse current, the coating can decrease the energy injected from the arc to the steel sheet and reduce the melting and splashing of steel. As a result, the ablation severity of uncoated sheet is severer than that of coated sheet.

Numerical inversion analysis on front‐face temperature rise of Al alloy suffered from long continuing current in lightning

The front-face temperature rise of Al alloy subjected to the high-energy long continuing current (LCC) component in lightning is important but difficult to measure. Therefore, a numerical inversion model based on the control volume finite element method is proposed to analyse the front-face temperature rise of Al alloy deduced from the rear-face temperature rise measured in simulated LCC experiment. In this model, Al sample is divided into discrete control-volume units with continuous medium by grids, forming internal nodes, surface nodes, edge nodes, and corner nodes. Temperature governing equations for different nodes are derived from Fourier's heat conduction law and solved with finite difference method in time and spatial domain. The secant method is used to make the optimal approximation to experimental data. The results show that subjected to LCC with amplitude 404 A and duration 520 ms, the highest front-face temperature rise of Al alloy is 10,800 K. The damaged area calculated by the proposed model is 298.7 mm2. The relative error is 4.8% compared with the experimental result 314.0 mm2. The energy consumed in all processes is 6710.1 J calculated by this model, in contrast to the total energy 7140.9 J generated from the arc and Joule heat.

Signal noise ratio improvement technique for bulk thermal diffusivity measurement

This paper presents an improvement of the classical flash technique that allows measuring the thermal diffusivity of highly diffusive materials without the need of blackbody coatings. The method consists in heating the front face of the sample using a periodic pulses sequence with suitable period and pulse duration. The rear face temperature is recorded simultaneously. An inverse approach is then used to estimate the thermal diffusivity. The underlying model is completely analytical and includes heat transfer and analog signal processing which has been specifically designed for this experiment. A sensitivity analysis and an experiment optimization are performed. Applied to an uncoated copper sample, this method appears to be reliable even in the case of very low optical emission/absorption. Theoretical expectations have been confirmed from experimental data obtained considering copper. Thermal diffusivity has been estimated with less than 4% on both the standard and absolute deviation.

Computation of transient-temperature profiles in objects exposed to simulated lightning currents

This paper presents a theoretical analysis of transient temperature distribution in metals and composites struck by different lightning currents to simulate objects of interest, especially aircrafts. A computer program based on the finite volume technique is written to compute the transient-temperature distributions in three dimensions due to both high-amplitude impulse currents (IC) and the relatively low-amplitude long-duration currents (LDC). Typical waveform of the discharge current during lightning strikes to objects near/on ground (negative downward flash), at high altitudes (negative upward flash) and in-flight (e.g. aircrafts) are simulated with different amplitudes and durations. The simulation considers the IC as first negative, or superimposed or subsequent return strokes with respect to the LDC. Different types of metals and carbon-fiber composites are also investigated to compare their thermal transient responses. Finally, the theoretical results are verified by using a sophisticated high-speed infrared camera to measure the rear-face temperature profiles of metals as a function of the coordinates and time due to both IC and LDC.

Diffusivity measurement of semi-transparent media: model of the coupled transient heat transfer and experiments on glass, silica glass and zinc selenide

The aim of this theoretical and experimental study is to provide a complete methodology to estimate the intrinsic diffusivity of semi-transparent media from flash method experiments. A semi-analytical model describes the coupled conductive–radiative transient heat transfer in a slab. The relevance of the model used for the inversion is then investigated. Experimental results are presented for several semi-transparent samples: float glass, SiO 2 and ZnSe. Measurements of rear face temperature are obtained by optical way (infrared detection) for a wide temperature range (293–700 K), for several radiation boundary conditions (black or gold coatings). The results are compared with those obtained through an other set-up and an other experimental procedure.

New logarithmic technique in the flash method

The logarithmic transformation of the rear face temperature rise issued from a flash experiment can be interpreted as adding up a heat loss and radial effect component to the one for the adiabatic model. Two data reduction methods based on this transformation are proposed with the common main goal to identify one parameter depending upon the sample's thickness and thermal diffusivity, not upon the other experimental conditions. They allow one to determine thermal diffusivity with good accuracy for nonuniform pulses and high Biot numbers.

Finite pulse time effects in flash diffusivity measurements

Analytical short-time solutions of one-dimensional heat conduction problems associated with flash thermal diffusivity measurements are given for the case of triangular pulse shape. The heat conduction problem is solved by the use of Laplace transforms taking advantage of the fact that the convolution of the temperature response curve at the back face of the sample with a triangular pulse function reduces to a simple multiplication in the Laplace domain. Instead of using the usual inversion, an expansion of the transform, followed by a term-by-term inversion, yields the solution in the time domain. This leads to a rapidly converging series for short times. Even using only one term, the relative error from the exact solution does not exeed 0.1 % at the double half-rise time. The given solutions describe the rear-face temperature response after illumination of the front surface of a mono-layer slab with a triangular shape pulse for the case of heat losses from the sample faces as well as for adiabatic conditions. Numerical approximations are given for less common higher functions appearing in the analytical solutions.

Analysis of heat pulse thermal diffusivity measurements on four-layer samples

The heat pulse technique for the measurement of the thermal diffusivity of materials is well known. For multilayer specimens the analysis of the rear-face temperature transient is difficult, and a computational method is reported for use in such cases. Experiments on three- and four-layer specimens have been made to test the method. It is concluded that the method for analysing the transients of multilayer specimens works well provided that the thermal diffusivities of the layers are not too dissimilar.

Optimizing the cooling block for the quick freeze method.

It is important for future ultrastructural preservation studies to freeze biological specimens as rapidly as possible. Finite element numerical techniques have been used to compare different cooling block materials used in the quick freeze 'slamming' method. It is concluded that a pure silver block at an optimum initial temperature of about 15.6 K will produce the quickest cooling and will give rates approximately 50% higher than if the block were initially cooled to 4.2 K. A copper block will produce the fastest cooling rates when the block is initially at liquid nitrogen temperature of 77.3 K provided the section being studied is taken no more than about 30 microns from the specimen-block interface. Composite metal blocks can further reduce the total specimen cooling time compared to a mono-block at the same initial temperature but the cooling rate near the interface will be slower. Future methods for specifying cooling rates within thin biological samples should be standardized and must be based on a combination of finite element numerical analysis and the measured temperature at the rear surface of the sample. Thermal contact resistance between cooling block and specimen and between specimen and the rear face temperature sensor must be minimal.

Modeling and Measurement of the Initial Anode Heat Fluxes in Pulsed High-Current Arcs

An anode heat flux model has been developed for pulsed high-intensity dc arcs. The model is based on temperature-time-history measurements of the rear face of a very thin plane anode and high-speed streak photographs of the arc. The arc heat flux model is derived from a comparison of experimental data with an analytical solution of the one-dimensional heat conduction equation and the arc intensity and timing information obtained from high-speed photographs. A simplified input heat flux model consisting of connected segments of linearly varying heat fluxes with respect to time is used. Duration of the individual segments is determined from the streak photographs and the graphical match of measured rear-face temperature history and the numerical solution. Results using argon gas at atmospheric pressure indicate an initial transient heat flux regime of 100-?s duration with a peak heat flux of 2 × 109 W/m2 followed by a quasi-steady heat flux regime with a heat flux of 1 × 108 W/m2.