Defect localization based on modulated photothermal local approach

A new method dedicated to macroscopic-like defect localization in composite materials is presented in this paper. The proposed method is based on non intrusive measurements of the sample temperature resulting from a local periodic low energy heating. In such an approach, the low temperature increases of the investigated material avoid damages which can occur with usual flash techniques. Since thermal waves propagation is modified due to the heterogeneity induced by the defect, analysis of both modulus and phase lag spatial distributions provides relevant knowledge. Up to now, macroscopic-like defect detection based on local periodic heating has not been widely investigated. Thus, differences between the global approach and the local approach have to be pointed out in order to verify the local method’s attractiveness. A mathematical model based on complex temperature is developed and provides a relevant predictive tool. In several configurations interest of local periodic heating is highlighted. For example, while several defects are included in the sample, the method capability to distinguish one from each other is shown considering a scanning approach. In order to validate these results, an experimental device has been developed. Several non destructive inspections are performed and defect detection is achieved using an infra-red camera providing observations of the sample surface.

On the feasibility of defect detection in composite material based on thermal periodic excitation

International audienceImplementation of periodic thermal excitation to identify thermal properties (conductivity, heat capacity, diffusivity) of complex composite materials at different investigation scales (from micrometer to millimetre) presents many advantages. These methods are usually based on the thermal waves phase lag observation compared to a reference signal. In fact, phase lag evolution versus distance to the heating source or versus excitation frequency is quite informative about numerous material characteristics. For example, considering that a structural defect can modify heat propagation inside a material, diagnosis can be performed from phase lag observations and comparisons between samples with and without defects. Numerous studies have been performed considering global heating (a quite large surface of the investigated composite material is heated and defect depth or size can be detected). The proposed approach is original since periodic heating is local and aims to detect defects in the periphery of the excitation. Based on a mathematical model for thermal waves propagations and introducing complex temperature for numerical resolution (finite element method), a feasibility study has allowed a sensitivity analysis. This preliminary study also provides information on the operating protocol, for heating(frequency, power, size of the source), and observation (transmission or reflection). Then, experimental device and early experimental results are briefly exposed

Defect localization in plane composite: a non intrusive automated procedure based on active thermography

This communication is focused on the implementation of a local pulsed thermography method in order to locate a defect in a plane composite. For such a local approach, the investigated plane composite is periodically heated on a small surface (several square centimeters) and thermal waves propagation is observed up to three thermal diffusion length on the same material surface (in reflexion). Both modulus and phase lag of the measured periodic signal are modified by the defect neighborhood and the seek for the most effective area leads to the defect localization. The contrast between the composite thermal behavior with or without defect is a relevant tracker of the defect proximity. Several criteria are proposed in order to quantify the contrast. Issued from cartographies differences, they are based upon usual functional norms and do not induce the same sensitivity to defect neighborhood. A simplex method is proposed for the automated procedure leading to the defect localization. Such method is based on an iterative process in order to explore the material surface (using an infrared camera) considering the investigation of new points (potentially better candidates for the defect location). New point coordinates are calculated from the previous points which are weighted considering the above mentioned criteria. Experimentations are performed according to the following steps : heat flux calibration, reference measurements, heat source shifting for automated scan

On the feasibility of defect detection in composite material based on thermal periodic excitation

Implementation of periodic thermal excitation to identify thermal properties (conductivity, heat capacity, diffusivity) of complex composite materials at different investigation scales (from micrometer to millimetre) presents many advantages. These methods are usually based on the thermal waves phase lag observation compared to a reference signal. In fact, phase lag evolution versus distance to the heating source or versus excitation frequency is quite informative about numerous material characteristics. For example, considering that a structural defect can modify heat propagation inside a material, diagnosis can be performed from phase lag observations and comparisons between samples with and without defects. Numerous studies have been performed considering global heating (a quite large surface of the investigated composite material is heated and defect depth or size can be detected). The proposed approach is original since periodic heating is local and aims to detect defects in the periphery of the excitation. Based on a mathematical model for thermal waves propagations and introducing complex temperature for numerical resolution (finite element method), a feasibility study has allowed a sensitivity analysis. This preliminary study also provides information on the operating protocol, for heating (frequency, power, size of the source), and observation (transmission or reflection). Then, experimental device and early experimental results are briefly exposed

Experimental Study on the Low-velocity Impact Behavior of Foam-core Sandwich Panels

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97064/1/AIAA2012-1701.pd

Soft body impact resistance of composite foam core sandwich panels with unidirectional corrugated and tubular reinforcements

Bird strikes represent a major hazard in the lifecycle of composite aircraft components, due to the low impact resistance of composites. The research presented in this paper investigates soft body impact performance of composite sandwich panels with corrugated and tubular core reinforcements. This type of panel with augmented strength and stiffness in one direction is of high importance for specific aerospace applications. The panels were subjected to high velocity impact with soft gelatine projectile as used in bird strike tests. In addition, the experimental part included non-destructive inspections of the sandwich panel samples. Panel performance was also analysed with the non-linear transient analysis software LS-DYNA with finite element and SPH capability. The panel with corrugated reinforcement showed good impact resistance with damage restricted to the impacted face sheet, foam core and corrugated reinforcement. The panel with tubular reinforcement, of the same thickness, did not suffer any damage at the same impact velocity of 115 m/s, but was damaged at a higher impact velocity of 235 m/s. The numerical studies helped to understand the experimental data, enabling comparison of impact performance of the reinforced sandwich panels and a benchmark conventional sandwich panel. The proposed reinforced sandwich panels, with the desired augmented strength and stiffness in one direction, showed improved impact resistance in comparison to conventional sandwich panels and therefore have potential for application in aerospace structures where these properties are desirable. Keywords: bird strike, composites, sandwich panel, tubular core reinforcement, corrugated core reinforcement, soft body impact, FEM, SPH, CT - computed tomograph

Tenue mécanique de panneaux composites auto- raidis produits par infusion = Self stiffened composite panel mechanical behaviour

National audienceCette étude traite du comportement mécanique de structures innovantes en composites permettant d'associer une grande taille et une raideur en flexion suffisante. Des mini-raidisseurs en mousse sont inclus dans l'empilement des plis avant l'injection. La structure finale est légère comme un monolithique mais rigide comme un panneau sandwich. Le comportement en statique et face à un impact montre que le gain de performance en flexion se fait au détriment de la cohésion de l'empilement suivant les autres types de sollicitation. Ainsi, la ruine de la structure intervient presque systématiquement par un délaminage initié par la présence de noyau en mousse