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Ever larger high pressure die castings (HPDC) with ever thinner walls raise the issue of casting defects. Properties of components are often strongly influenced by inner porosity. In the case of high pressure die castings, shrinkage and gas porosity occur. Two possibilities of characterizing porosity - by microscopy and acitve thermography - are dealt with in two articles. In the previous article, the microscopic investigations are presented. In this article, first the feasibility of thermographic detection will be discussed based on simulations and, thereafter, the experimental determination of porosity with active thermography will be described.
Material defects in fiber reinforced polymers such as delaminations can rapidly degrade the material properties or can lead to the failure of a component. Pulse thermography (PT) has proven to be a valuable tool to identify and quantify such defects in opaque materials. However, quantification of delaminations within semitransparent materials is extremely challenging. We present an approach to quantify delaminations within materials being semitransparent within the wavelength ranges of the optical excitation sources as well as of the infrared (IR) camera. PT experimental data of a glass fiber reinforced polymer with a real delamination within the material were reconstructed by one dimensional (1D) mathematical models. These models describe the heat diffusion within the material and consider semitransparency to the excitation source as well to the IR camera, thermal losses at the samples surfaces and a thermal contact resistance between the two layers describing the delamination. By fitting the models to the PT data, we were able to determine the depth of the delamination very accurately. Additionally, we analyzed synthetic PT data from a 2D simulation with our 1D-models to show how the thermal contact resistance is influenced by lateral heat flow within the material.
Carbon-fiber reinforced composites are becoming more and more important in the production of light-weight structures, e.g., in the automotive and aerospace industry. Thermography is often used for non-destructive testing of These products, especially to detect delaminations between different layers of the composite.
In this presentation, we aim at methods for defect reconstruction from thermographic measurements of such carbon-fiber reinforced composites. The reconstruction results shall not only allow to locate defects, but also give a quantitative characterization of the defect properties. We discuss the simulation of the measurement process using finite element methods, as well as the experimental validation on flat bottom holes.
Especially in pulse thermography, thin boundary layers with steep temperature gradients occurring at the heated surface need to be resolved. Here we use the combination of a 1D analytical solution combined with numerical solution of the remaining defect equation. We use the simulations to identify material Parameters from the measurements.
Finally, fast heuristics for reconstructing defect geometries are applied to the acquired data, and compared for their accuracy and utility in detecting different defects like back surface defects or delaminations.
Impulse-thermography is an active method for quantitative investigation of the near surface region of various structures which has recently been applied to civil engineering. It is well suited for the detection of voids and honeycombing in concrete up to concrete covers of 10 cm and more. For quantitative analysis, a computer program for numerical simulation of the heating up and cooling down processes was developed based on Finite Differences. With this program parameter studies have been performed for investigating the influence of environmental conditions, material parameters and geometry on the thermal behaviour. The comparison between experimental and simulated results enables the Inverse Solution.
Impulse-thermography has been established as a fast and reliable tool in many areas of non-destructive testing. In recent years several investigations have been done to apply active thermography to civil engineering. For quantitative investigations in this area of application, finite difference calculations have been performed for systematic studies on the influence of environmental conditions, heating power and time, defect depth and size and thermal properties of the bulk material (concrete). The comparison of simulated and experimental data enables the quantitative analysis of defects.
This study evaluates whether subsurface features in rotor blades, mainly made of Glass Fibre Reinforced Plastics (GFRP), can generally be detected with ‘‘solar thermography”. First, the suitability of the sun is tested for acting as a heat source for applying active thermography on a 30 mm thick GFRP test specimen. Second, a defective rotor blade segment is inspected outdoors under ideal natural conditions using the sun as excitation source. Additionally, numerical FEM-simulations are performed and the comparability between experiment and simulation is evaluated for outdoor measurements.