Simon Joncas

Comparative study on submillimeter flaws in stitched T-joint carbon fiber reinforced polymer by infrared thermography, microcomputed tomography, ultrasonic c-scan and microscopic inspection

Abstract. Stitching is used to reduce dry-core (incomplete infusion of T-joint core) and reinforce T-joint structure. However, it may cause new types of flaws, especially submillimeter flaws. Microscopic inspection, ultrasonic c-scan, pulsed thermography, vibrothermography, and laser spot thermography are used to investigate the internal flaws in a stitched T-joint carbon fiber-reinforced polymer (CFRP) matrix composites. Then, a new microlaser line thermography is proposed. Microcomputed tomography (microCT) is used to validate the infrared results. A comparison between microlaser line thermography and microCT is performed. It was concluded that microlaser line thermography can detect the internal submillimeter defects. However, the depth and size of the defects can affect the detection results. The microporosities with a diameter of less than 54  μm are not detected in the microlaser line thermography results. Microlaser line thermography can detect the microporosity (a diameter of 0.162 mm) from a depth of 90  μm. However, it cannot detect the internal microporosity (a diameter of 0.216 mm) from a depth of 0.18 mm. The potential causes are given. Finally, a comparative study is conducted.

Pulsed micro-laser line thermography on submillimeter porosity in carbon fiber reinforced polymer composites: Experimental and numerical analyses for the capability of detection

In this article, pulsed micro-laser line thermography (pulsed micro-LLT) was used to detect the submillimeter porosities in a 3D preformed carbon fiber reinforced polymer composite specimen. X-ray microcomputed tomography was used to verify the thermographic results. Then, finite element analysis was performed on the corresponding models on the basis of the experimental results. The same infrared image processing techniques were used for the experimental and simulation results for comparative purposes. Finally, a comparison of experimental and simulation postprocessing results was conducted. In addition, an analysis of probability of detection was performed to evaluate the detection capability of pulsed micro-LLT on submillimeter porosity.

Sustainable Vacuum-Infused Thermoplastic Composites for MW-Size Wind Turbine Blades—Preliminary Design and Manufacturing Issues

This paper addresses the feasibility of using innovative vacuum infused anionic polyamide-6 (PA-6) thermoplastic composites for MW-size wind turbine blades structures. To compare the performance of this fully recyclable material against commonly used less sustainable thermoset blade materials in a baseline structural MW-size blade configuration (box-spar/skins), four different blade composite material options were investigated: Glass/epoxy, carbon/epoxy, glass/PA-6, and carbon/PA-6. Blade characteristics such as weight, costs, and natural frequencies were compared for rotor blades ranging between 32.5 and 75 m in length, designed according to both stress and tip deflection criteria. Results showed that the PA-6 blades have similar weights and natural frequencies when compared to their epoxy counterpart. For glass fiber blades, a 10% reduction in material cost can be expected when using PA-6 rather than epoxy while carbon fiber blades costs were found to be similar. Considering manufacturing, processing temperatures of PA-6 are significantly higher than for epoxy systems; however, the associated cost increase is expected to be compensated for by a reduction in infusion and curing time.

Comparative study of microlaser excitation thermography and microultrasonic excitation thermography on submillimeter porosity in carbon fiber reinforced polymer composites

Abstract. Stitching is used to reduce incomplete infusion of T-joint core (dry-core) and reinforce T-joint structure. However, it may cause new types of flaws, especially submillimeter flaws. Thermographic approaches including microvibrothermography, microlaser line thermography, and microlaser spot thermography on the basis of pulsed and lock-in techniques were proposed. These techniques are used to detect the submillimeter porosities in a stitched T-joint carbon fiber reinforced polymer composite specimen. X-ray microcomputed tomography was used to validate the thermographic results. Finally an experimental comparison of microlaser excitation thermography and microultrasonic excitation thermography was conducted.

Infrared thermography, ultrasound C-scan and microscope for non-destructive and destructive evaluation of 3D carbon fiber materials: a comparative study

3D Carbon fiber polymer matrix composites (3D CF PMCs) are increasingly used for aircraft construction due to their exceptional stiffness and strength-to-mass ratios. However, defects are common in the 3D combining areas and are challenging to inspect. In this paper, Stitching is used to decrease these defects, but causes some new types of defects. Infrared NDT (non-destructive testing) and ultrasound NDT are used. In particular, a micro-laser line thermography technique (micro-LLT) and a micro-laser spot thermography (micro-LST) with locked-in technique are used to detect the micro-defects. In addition, a comparative study is conducted by using pulsed thermography (PT), vibrothermography (VT). In order to confirm the types of the defects, microscopic inspection is carried out before NDT work, after sectioning and polishing a small part of the sample..

Effects of cold temperature, moisture and freeze-thaw cycles on the mechanical properties of unidirectional glass fiber-epoxy composites

´The durability of composite materials used for wind turbines blades exposed to a northern climate presents uncertainties as their behavior for very cold temperature applications is not completely understood. The goal of this project is to confront actual theories to experimental results for mechanical properties of unidirectional glass-epoxy composites exposed to moisture, cold temperature and freeze-thaw cycles. Tests were made at ambient temperature and -40℃ on four sample families. The families consisted of either dry or moisture saturated specimens, of which half of the families were further conditioned with 100 freeze-thaw cycles between -40℃ and 40℃. Tensile, compressive and short beam shear tests were conducted. Results showed the inadequacy of classical theories for predicting strength of the specimens exposed to low temperature and/or moisture. Contrary to the results presented in most of the literature, freeze-thaw cycles did not significantly change the strength or modulus of the test specimens. However, low temperature provided an important increase in strength while modulus was retained. Moisture had a stronger effect on properties than models would predict but there was no evidence of a synergistic effect between moisture and temperature. Comparison of the results with those presented in the literature shows that the behavior of composites exposed to low temperatures or freezethaw cycles is very sensitive to the nature of the constituents and the molding process used to produce the parts.

A comparative study of experimental and finite element analysis on submillimeter flaws by laser and ultrasonic excited thermography

Stitching is used to reduce dry-core and reinforce T-joint structure. However, it might cause new types of flaws, especially submillimeter flaws. In this paper, new approaches including micro-VT, lock-in micro-LLT and micro-LST based on both lock-in and pulse methods are used to detect submillimeter flaws in stitched CFRP. A comparison of laser excitation thermography and micro-VT on micro-porosities is conducted. Micro-CT is used to validate the infrared results. Then, a finite element analysis (FEA) is performed. The geometrical model needed for finite element discretization was developed from micro-CT measurements. The model is validated for the experimental results. Finally a comprehensive experimental and simulation comparison of micro-LLT and micro-LST based on both lock-in and pulse methods is conducted.

Interfacial Shear Strength Properties of Vacuum-infused Anionic Polyamide-6 Glass-fiber Composites

A research project is on going at the Design and Production of Composite Structures research group of Delft University of Technology to investigate the possibility of using fully recyclable thermoplastic composites (TPC) for wind turbine blade structures. Since classic TPC melt processes are obviously unsuitable for large wind turbine blade manufacturing (their length now exceeding 50m), the development of reactive processing techniques is essential to enable the production of such large TPC components. This paper aims at benchmarking anionic polyamide-6 (APA-6) thermoplastic composites manufactured with an innovative infusion process against commonly used epoxy composites based on matrix dominated material properties. Balanced and symmetric laminates composed of 12 plies satin weave glass-fiber fabric were chosen for specimens to be manufactured. Both unsized fibers and fibers coated with a sizing commonly known to be compatible with both epoxy and polyamide-6 (PA-6) matrices were used. To address the well known vulnerability of PA6 to water absorption, dry as molded (DAM) and moisture conditioned samples were included in the test program. Results showed that in DAM conditions, APA-6 composites could yield a 15% higher interlaminar shear strength (ILSS) than their epoxy-based counterpart. On the other hand, APA-6 composite moisture conditioned samples proved to be more vulnerable to moisture that epoxy based samples showing a retention level of 40% compared to 64%. When investigating the effect of fiber coatings, it was observed that using a sizing for APA-6 composites helped reduce water uptake by 25 to 40% depending on the conditioning parameters used.

Modelling the storage modulus, transition temperatures and time–temperature superposition characteristics of epoxies and their composites

Epoxies are widely used as adhesives and matrix material for composites in civil infrastructure. As such structures are likely to be exposed to a wide variety of environmental conditions over long service lives, knowledge of their time–temperature sensitivity is desirable. The present study proposes a model describing the evolution of storage modulus for epoxies and their composites subject to forced dynamic excitations over wide temperature and frequency ranges. The model is tested against results for one rubber toughened epoxy and one carbon–epoxy composite. Results show a good agreement between the model and experiments, both in terms of temperature and frequency effects. Moreover, the model is shown to provide an unambiguous definition of the frequency dependent glass transition temperature, which is found to naturally follow the expected Arrhenius relationship with regards to frequency. Activation energies for the glass transition temperature evaluated by the new approach are in good agreement with results from the literature. It is also shown that when accounting for the effect of frequency on the glass transition, the evolution of the time–temperature shift factor is continuous across the glass transition.

Modelling the effect of temperature on the probabilistic stress–life fatigue diagram of glass fibre–polymer composites loaded in tension along the fibre direction

Predicting the fatigue performance of composites has proven to be a challenge both conceptually, due to the inherent complexity of the phenomenon, and practically, because of the resource-intensive process of fatigue testing. Moreover, mechanical behaviour of polymer matrix composites exhibits a complicated temperature dependence, making the prediction of fatigue performance under different temperatures even more complex and resource intensive. The objective of this paper is to provide a method for the prediction of fatigue life of glass–polymer composites loaded in the fibre direction at various temperatures with minimal experimental efforts. This is achieved by using a static strength degradation approach to fatigue modelling, where only two parameters (including static strength) are temperature dependent, in conjunction with relationships for these two fatigue model parameters temperature dependence. The method relies on fatigue data at a single temperature and simple static tests at different temperatures to predict the effects of temperature on the material’s fatigue behaviour. The model is validated on experimental data for two unidirectional and one woven glass–epoxy composites and is found to accurately predict the effect of temperature on fatigue life of composites. A method to obtain probabilistic stress-life ( P - S - N ) fatigue diagrams including temperature effects is also presented.