ndt thermographic techniques on cfrp structural components for aeronautical application

Abstract This paper describes the application of active pulsed Thermography (PT) as a Non-Destructive Test (NDT) method for the investigation of CFRP aeronautical components. The analyzed specimens include T-shaped stringers, previously monitored by ultrasonic analysis, and laminated flat plates with internal production defects. Several set-up tests allowed to identify optimal configurations for the defect detection, according to specimen geometry and defect location. A custom post-processing algorithm has been developed to improve thermographic data for more precise defect characterization, whilst a successive full-field contrast mapping allows to achieve a reliable defect distribution map and a better definition on larger areas. Detection of defects was studied with a specific thermal contrast evaluation, with a suitable choice of undamaged reference area during the transient cooling phase. The influence of heating time and experimental set-up on the thermal contrast results has also been studied; moreover, the ability of thermographic technique to detect real small production defects with accuracy and reliability is verified for CFRP aeronautical components

Determination Of Porosity Level Of Out Of Autoclave Carbon Epoxy Advanced Composites Using Ultrasonic C-Scan Pulse-Echo Method

Various inspection techniques are available for inspection and quality control in monitoring services of composite structures, which ultrasonic inspection techniques C-scan is the most widely used in industrial manufacturing advanced composite materials. At this time, there are no standards that can be applied universally or procedures for inspection ultrasonic C-scan for advanced composite based materials which may cause the reliability and traceability of these methods has been limited. In response to this need, the study of methods and testing of advanced composite materials has been carried out. This assessment has produced three procedures 1. to operate the equipment C-scan through the pulse echo method, 2. producing the reference panel which reference panel is produced by a method recognized for its quality and usability of advanced composite panel produced aims to simulate defects. 3. Perform advanced analysis with use software such as Matlab®, which aims to analyze the defects in the form of three-dimensional images. This thesis gives an overview of the objectives and research activities to be carried out. As a finding, evaluation performance between destructive and image processing technique shown that comparable results between each other. Sample indicated O1 was determined as a best sample among others with lowest defect (i.e. porosity and void) percentage. It involved in no debulking process with normal vacuum bagging technique assisted by two (2) vacuum sources with polytetrafluoroethylene film on mould surface to enhance flow during fabrication. Supporting analysis should be carried out in order to analyze qualitatively

Design of an Automated Ultrasonic Scanning System for In-Situ Composite Cure Monitoring and Defect Detection

The preliminary design and development of an automated ultrasonic scanning system for in-situ composite cure monitoring and defect detection in the high temperature environment of an oven was completed. This preliminary design is a stepping stone to deployment in the high temperature and high pressure environment of an autoclave, the primary cure method of aerospace grade thermoset composites. Cure monitoring with real-time defect detection during the process could determine when defects form and how they move. In addition, real-time defect detection during cure could assist validating physics-based process models for predicting defects at all stages of the cure cycle. A physics-based process model for predicting porosity and fiber waviness originating during cure is currently under development by the NASA Advanced Composites Project (ACP). For the design, an ultrasonic contact scanner is enclosed in an insulating box that is placed inside an oven during cure. Throughout the cure cycle, the box is nitrogen-cooled to approximately room temperature to maintain a standard operating environment for the scanner. The composite part is mounted on the outside of the box in a vacuum bag on the build/tool plate. The build plate is attached to the bottom surface of the box. The scanner inspects the composite panel through the build plate, tracking the movement of defects introduced during layup and searching for new defects that may form during cure. The focus of this paper is the evaluation and selection of the build plate material and thickness. The selection was based on the required operating temperature of the scanner, the cure temperature of the composite material, thermal conductivity models of the candidate build plates, and a series of ultrasonic attenuation tests. This analysis led to the determination that a 63.5 mm thick build plate of borosilicate glass would be utilized for the system. The borosilicate glass plate was selected as the build plate material due to the low ultrasonic attenuation it demonstrated, its ability to efficiently insulate the scanner while supporting an elevated temperature on the part side of the plate, and the availability of a 63.5 mm thick plate without the need for lamination

Dynamic Mechanical Analysis of Kissing Bonds in Bonded Joints

Inspection of kissing bond defects in composite structures is a challenge for traditional nondestructive testing (NDT) methods. A kissing bond (KB) is a bondline defect where two surfaces are in intimate contact with each other but with little to no bond strength. New nondestructive testing methods need to be developed that can detect adhesive kissing bonds, and areas within a composite joint where the bondline is compromised. The primary goal of this thesis is to explore if a NDT technique based on Dynamic Mechanical Analysis (DMA) can be established which is capable of detecting kissing bonds within bondlines in composite laminates that have been bonded with epoxy film and paste adhesives. DMA as a test method has the benefit that it can be conducted quickly and is able to test the material at various temperature and frequency ranges. Thirty specimens were prepared and attempts were made to create kissing bonds by introducing different types of contaminants at the bond location. Contamination used within the adhesive joint consisted of introduction of mold release and grease lubricant. Ultrasonic inspection is first performed to demonstrate that the surfaces were in contact in the bondline and no attenuation from porosity or delamination is present. Dynamic testing was performed on each sample in an attempt to detect these kissing bonds using the DMA method. Information on changes in the sample’s stiffness, storage modulus and loss modulus over a range of temperatures is recorded and analyzed. The specimens were subsequently tested in tension to determine the maximum strength of the materials. A geometrically nonlinear, 3D finite element (FE) analysis was performed to determine the stress and strain distributions within the composite single-lap joint (SLJ). The results obtained from the study showed artificial KBs appear to have been successfully fabricated based on tension testing and optical scans of fractured surfaces. Visual inspections of the fracture surfaces showed that contaminated bonds predominately failed via adhesion failure, while the control samples failed via cohesion. The production of KBs within the joints were more successful among the paste adhesive rather than the film adhesive. Analysis of DMA parameters shows a reasonable correlation for some of the parameters and the failure load. The FE results on the tension test showed that the stresses became increasingly higher within the single-lap joint for paste adhesive compared to the film adhesive. It was seen from the FEA models that the maximum shear stress and elastic strain occur near the overlap joint corners ends, suggesting that cohesive crack initiation is most likely to occur at the corners for defect-free samples. The stiffness results obtained from the DMA showed that all the paste samples had stiffness values much larger than the film samples; this elevated stiffness could contribute to the increased stress evident in the FE models. As the stresses were higher in the paste adhesive specimens, it is interpreted to be a contributing factor in the reductions of shear strength within the SLJs when defects are present

Multimodal Imaging for Characterisation and Testing of Composite Materials

Carbon fibre reinforced polymers (CFRP) are widely used across several industries, including aerospace, as they are lightweight and offer superior mechanical properties. Barely Visible Impact Damage (BVID), including cracks, delaminations, fibre debonding, as well as manufacturing defects such as porosity, are detrimental to CFRP structural integrity and detection of such faults is important. Different non-destructive evaluation (NDE) methods exist, including ultrasound, X-ray computed tomography (X-ray CT), infrared, and liquid penetrant testing. Edge Illumination X-ray Phase Contrast imaging (EI XPCi) was benchmarked as a viable NDE method for damage detection in CFRP, as it offers additional information through multimodal imaging. With the acquisition of at least three images, EI XPCi allows for the retrieval of the attenuation, differential phase, and dark field signals, using a pair of apertured masks. EI XPCi CT was compared with ultrasonic immersion C-scan imaging and high-resolution X-ray CT for the detection of severe impact damage in a composite plate (visible indent damage on surface of plate and protrusion on the back). The full extent and scale of the different defects were observed in the phase-based signals to a better standard than ultrasonic immersion imaging, with observations confirmed using high resolution X-ray CT. Planar EI XPCi was then compared to contrast agent X-ray imaging and ultrasonic immersion C-scan imaging on a different, less damaged specimen (only small crack visible on surface), showing that planar EI XPCi can detect a network of cracks across the specimen and overcame some of the limitations of contrast agent X-ray imaging. However, in the planar imaging, delamination damage was only detected by the ultrasonic measurement, showing the necessity of using both ultrasonic imaging and EI XPCi for a complete understanding of the damage in the plate. EI XPCi was used for the quantification of porosity for woven composite plates with varying porosity (0.7% to 10.7%), compared to ultrasonic through transmission imaging and destructive matrix digestion. The introduction of the standard deviation of the differential phase (STDP) showed excellent correlation with the porosity calculated from matrix digestion. The STDP signal quantifies the variation of the distribution of inhomogeneities for features of a scale equal to or above the system resolution (in this case, 12µm along the direction of phase sensitivity), which was advantageous for the investigated set of specimens with larger porosity

Ultrasonic detection and identification of fabrication defects in composites

Methods for deliberate fabrication of porosity into carbon/epoxy composite panels and the influence of three-dimensional stitching on the detection of porosity were investigated. Two methods of introducing porosity were investigated. Porosity was simulated by inclusion of glass microspheres, and a more realistic form of porosity was introduced by using low pressure during consolidation. The panels were ultrasonically scanned and the frequency slope of the ultrasonic attenuation coefficient was used to evaluate the two forms of porosity. The influence of stitching on the detection of porosity was studied using panels which were resin transfer molded from stitched plies of knitted carbon fabric and epoxy resin

Enhanced composite plate impact damage detection and characterisation using X-Ray refraction and scattering contrast combined with ultrasonic imaging

Ultrasonic imaging and radiography are widely used in the aerospace industry for non-destructive evaluation of damage in fibre-reinforced composites. Novel phase-based X-ray imaging methods use phase effects occurring in inhomogeneous specimens to extract additional information and achieve improved contrast. Edge Illumination employs a coded aperture system to extract refraction and scattering driven signals in addition to conventional absorption. Comparison with ultrasonic immersion C-scan imaging and with a commercial X-ray CT system for impact damage analysis in a small cross-ply carbon fibre-reinforced plate sample was performed to evaluate the potential of this new technique. The retrieved refraction and scattering signals provide complementary information, revealing previously unavailable insight on the damage extent and scale, not observed in the conventional X-ray absorption and ultrasonic imaging, allowing improved damage characterisation

Nondestructive Testing in Composite Materials

In this era of technological progress and given the need for welfare and safety, everything that is manufactured and maintained must comply with such needs. We would all like to live in a safe house that will not collapse on us. We would all like to walk on a safe road and never see a chasm open in front of us. We would all like to cross a bridge and reach the other side safely. We all would like to feel safe and secure when taking a plane, ship, train, or using any equipment. All this may be possible with the adoption of adequate manufacturing processes, with non-destructive inspection of final parts and monitoring during the in-service life of components. Above all, maintenance should be imperative. This requires effective non-destructive testing techniques and procedures. This Special Issue is a collection of some of the latest research in these areas, aiming to highlight new ideas and ways to deal with challenging issues worldwide. Different types of materials and structures are considered, different non-destructive testing techniques are employed with new approaches for data treatment proposed as well as numerical simulations. This can serve as food for thought for the community involved in the inspection of materials and structures as well as condition monitoring

3D TEXTILE PREFORMS AND COMPOSITES FOR AIRCRAFT STRCUTURES: A REVIEW

Over the last decades, the development of 3D textile composites has been driven the structures developed to overcome disadvantages of 2D laminates such as the needs of reducing fabrication cost, increasing through-thickness mechanical properties, and improving impact damage tolerance. 3D woven, stitched, knitted and braided preforms have been used as composites reinforcement for these types of composites. In this paper, advantages and disadvantages of each of them have been comprehensively discussed. The fabric architects and their specification in particular stitched preforms and their deformation mode for aerospace applications have been reviewed. Exact insight into various types of damage in textile preforms and composite that have the potential to adversely affect the performance of composite structure along with their inspection using NDT techniques have been elaborated. The research review reported in this paper can be very valuable to researchers to release the 3D composite behaviour under different loading conditions and also to get familiar with the manufacture of high quality textile composite for aircraft structures