Defects In Composite Laminates Research Articles

Identification of defects in composite laminates by comparison of mode shapes from electronic speckle pattern interferometry

A novel technique for identifying defects in carbon fibre reinforced plates has been developed. Firstly, modal analysis by impact excitation was performed to obtain the first five resonant frequencies for three defect-free and three defective specimens with in-plane fibre waviness. Then amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) was used to obtain the mode shapes at these frequencies. The contours of the nodal regions visible in the mode shapes were extracted using a specially-developed algorithm employing density-based spatial clustering of applications with noise (DBSCAN). Fourier descriptors, that are invariant to rotations, translations, and scaling, were used to decompose the contours to reduce data dimensionality and make comparisons. The differences in contours between the two sets of specimens showed that the 4th and 5th mode shapes can be used for identifying the presence of the waviness defects. This technique for nodal region comparison was found to greatly simplify the comparison of fringe patterns for the purpose of damage assessment and could potentially be used as part of validation procedures.

Micro-CT analysis of process-induced defects in composite laminates using AFP

ABSTRACT Carbon fiber reinforced polymer (CFRP) composites have been gathering a lot of interest in several engineering applications because of their favorable properties including lightweight and ease of manufacturing. Robotic manufacturing techniques using automated fiber placement (AFP) for aerospace and automotive applications present a growing trend toward error-free manufacturing. AFP manufactured laminates can be prone to internal flaws due to improper selection of manufacturing process parameters. Additionally, internal damage in composite laminates can occur during the operational service of the laminate due to fatigue or foreign object impacts. Identifying and characterizing at the microscopic level may allow optimization of process parameters, leading to higher quality laminates. In this paper, a direct 3D imaging approach to characterize global and local deformation-induced defects in AFP manufactured CFRP laminates using X-ray CT techniques is presented. The investigations are conducted on two sets of thermoplastic PEEK composite laminates (undeformed and deformed) manufactured using a set of processing conditions. The presented approach to quantify the defects provides local and global statistics including the evolution of local 3D fiber orientation as a methodology to detect the degree of deformation.

Defect identification in composite materials via thermography and deep learning techniques

Composite materials are widely used in aircraft, vehicle, and various industries due to their excellent mechanical properties. A thermography-based nondestructive test is often employed for diagnosis of defects in composite laminates, while the test results are largely affected by the environmental conditions, and show significant dependence to the inspector, instruments being used, and the test objectives. To overcome the limitation, the present study proposes a framework for identifying defects in composite materials by integrating a thermography test with a deep learning technique. A dataset of thermographic images of composite materials with defects were collected from literatures and were used for training the system to identify defects from given thermographic images. The versatile application of the proposed technique was validated by testing it on composite specimens produced by resin transfer molding and thermoplastic injection molding, using a combination of carbon/organo fabrics and thermoset/thermoplastic resins. The performance of the proposed system was evaluated by assessing its ability to identify defects from the specimens with artificial defects, and is discussed in light of average precision for identification of defects.

The effect of residual stress induced by manufacturing on strength on free edge of carbon-epoxy composite with [00/900]n layup

Last decades, increased attention is paid to well-known stress concentration phenomenon rich in research tradition is so-called free-edge effect. Despite the fact that the problem is not new there is no well-known approaches in engineering practice to take residual stress and free edge effects simultaneously for fracture analysis. The presence of residual stresses combined with mechanical loads can cause several defects in composite laminates and structures such as fiber and tow misalignment, transverse cracking (known as micro-cracking) delamination and warpage. In the present study, the effect of free edge on residual stresses inherited during manufacturing of thermoset multilayered composites was investigated.The purpose of this study can be formulated by following steps:- predict the value of residual stresses in composite sample during the polymerization process;- determine the change in residual stress states of polymerized composite sample after straight line cut into two parts. Compare the stress distribution in a sample after cut without considering the influence of residual stresses- determine the change in stress distribution near free edge after tensile loading after cut (along the notch line). Compare with the stress distribution in sample after tensile loading along the notch line without considering residual stresses.- compare damage distribution near the free edge with the presence of manufacturing residual stresses and without one in case of uniaxial loading of composite material.

Void reduction in VARTM composites by compaction of dry fiber preforms with stationary and moving magnets

Voids are the most common process-induced defects in composite laminates fabricated by vacuum assisted resin transfer molding (VARTM). Reduction or total elimination of these defects is essential for the improved performance and long-term durability of the structural composites. This study introduces a novel method that reduces the void content in VARTM laminates to below 1% by compacting the fibrous mat before infusion. The compaction is achieved by applying magnetic pressure on the vacuum bag by either stationary or moving magnets which are removed before the resin infusion. To assess the effectiveness of the proposed method, 6-, 12-, and 18-ply random mat glass/epoxy laminates are fabricated by VARTM using compacted and uncompacted mats and their properties are compared. In addition, different sets of magnets are used to investigate the effect of compaction levels on the resin flow and the quality of the final part. The placement of stationary magnets on the entire vacuum bag surface is practical for fabrication of small parts. For medium to large parts, however, magnets with a smaller footprint can be moved to apply the compaction pressure over a larger vacuum bag surface. The results show that by applying compaction pressure of 0.2 MPa or higher either by stationary or moving magnets on the dry preforms, the void volume fraction was decreased by 65%–95% to 0.1%–0.8% in all laminates.

Infra-Red Thermography based Inspection of Hybrid Composite Laminates under Flexure Loading

Generally, the aircraft structural parts are economically high in cost so the materials need to be inspected for defects or damages using various non-destructive testing (NDT) methods like ultrasonic, thermography and acoustic emission. The aim of this project is to characterize the defects in composite laminates before and after the flexural loading using infra-red thermography NDT method. GFRP and hybrid (GFRP+CFRP) composite laminates are fabricated with different orientation such as uni-directional, cross ply, anti-symmetric and angle ply and then tested under flexural loading according to ASTM D790 standard. The volume fraction of the fibre and matrix needs to be found out to know the void content and the mixing ratio of reinforcement and binder.

Моделирование технологических дефектов и оценка их влияния на статическую прочность композитных фланцев

This work presents an approximate methodology based on the numerical simulation aimed at assessing how defects affect static strength of polymer composite flanges in load bearing elements used in aeronautical engineering. Various defects may occur in the process of flange forming in the areas of bending, e.g. curvature of the layers, resin pockets, voids, delamination and others. Thus, there is an actual problem of assessing how technological defects affect the strength of this part. The main defects in composite laminates are defined and a review of literature related to strength problems for such structures are presented in this work. The numerical stress-strain analysis of composite flange with main types of defects under force loading condition was carried out with ANSYS software. A two - dimensional axisymmetric finite element model was used. For the development of the structural model a parametric modelling approach was applied, including defect size, configuration and location parameters. The problem was solved in a general statement for an anisotropic elastic body. In order to keep the original part profile, we reduced the overall thickness of the layers in the local area near the defect by the size of the defect, so the overstated value of structural strength was obtained in simulation. The safety factor of the flange was estimated by stress components using the maximum stress criterion. The stresses in the material principal directions and interlaminar shear and normal stresses were determined for analysis. A comparison of the obtained data with the results for a defect-free flange allows estimating the impact of each defect or its combination on the static strength of flange.

Velocity Correction of Total Focusing Method and its Application to Ultrasonic Inspection of Composite Laminate

The reliable detection of composite material is dramatically important with its wide application in the aerospace industry. A new post-processing mode of ultrasonic array inspection, entitled total focusing method(TFM), offers significant advantages including high sensitivity and greater inspection coverage. Based on this mode, a wave velocity correction method for the elastic anisotropic of composites is put forward. Subsequently, practical application is carried out to image and locate the defect in a composite laminate sample to demonstrate the advantage of the described method. The results show that the proposed method can effectively improve the ability to detect and evaluate defects in composite laminates.

Preparation and properties of quartz cloth–reinforced Xylok composites fabricated by vacuum bag only process

Xylok resin was synthesized from 1,4-bis(methoxymethyl)-benzene and phenol through a condensation reaction catalyzed by p-toluenesulfonic acid. The effects of molar ratio of phenol to 1,4-bis(methoxymethyl)-benzene, reaction time and washing process on the properties of Xylok resin were investigated. Xylok composite laminates reinforced by quartz cloth were fabricated by vacuum bag only (VBO) process. Softening point of Xylok resin and volatile content of prepreg and voids in composites were also studied to better control process defects in composite laminates. Void content was measured by image analysis and calculating parameters. Experimental results showed that the Xylok composite laminates manufactured by VBO process had good thermal and dielectric properties. The flexural strength and flexural modulus of laminates could be 733 MPa and 23 GPa, respectively.

Damage detection of unidirectional carbon fiber–reinforced laminates with embedded magnetostrictive particulates: A preliminary study

Defects in composite laminates are difficult to detect because of the conductive and paramagnetic properties of composite materials. Timely detection of defects in composite laminates can improve reliability. This research illustrates the preliminary analysis and detection of delaminations in carbon fiber laminate beams using a single layer of magnetostrictive particles and noncontacting concentric magnetic excitation and sensing coils. The baseline analytical models also begin to address the intrusive nature of the magnetostrictive particles as well as relate the applied excitation field with the stress and magnetic flux densities induced in the magnetostrictive layer. Numerical methods are used to begin to characterize the presence of magnetostrictive particles in the laminate and the behavior of the magnetostrictive particles in relationship to the magnetic field used during sensing. Unidirectional laminates with embedded delaminations are used for simulations and experimentations. A novel, yet simplified fabrication method is discussed to ensure consistent scanning and sensing capabilities. The nondestructive evaluation scanning experiments were conducted with various shapes and sizes of damages introduced into carbon fiber–reinforced polymeric composite structures. The results demonstrate high potential for magnetostrictive particles as a low-cost, noncontacting, and reliable sensor for nondestructive evaluation of composite materials.

Numerical simulation of void formation in LCM

Abstract The existence of void type defects in composite laminates manufactured using the liquid composite molding process alters the mechanical characteristics of the final product. The object of this paper is to present a procedure to simulate mold filling and to incorporate void formation. The model is composed of a unique combination of robust and accurate numerical algorithms for solving the transport equation. The saturation ratio is a macroscopic entity yet it is clearly the consequence of microscopic phenomena and especially of air entrapment within tows, hence the presence of micropores. In the model, the source term is dependent on the capillary number and is related to the micro–macro scale effects with the dual-porosity. The unsaturated flow and transport model was applied to a problem and was found to produce accurate, mass conserving solutions when compared to experimental results in void content.

Location and Geometry of Defects in Composite Laminates from Infrared Images

An infrared scanning system is employed to analyze defective glass-epoxy laminates. Several specimens, manufactured with insertion of pieces of different materials to simulate either delaminations or inclusions are considered, and tests are performed for varying material, shape, depth, and size of inserts. Each specimen is heated by quartz lamps and viewed by an infrared camera, and thermograms are acquired in time sequence in the transient heating phase. Thermograms are quantitatively analyzed in terms of time temperature profiles along lines through the center of defects. The center of a defect is individuated with high certainty; the size instead is measured (from the temperature profile) much well as much as the defect is positioned at small depth and is large. For each test condition, the measured dimension is related to the real one. This ratio, plotted against time, shows an asymptotic distribution and allows for a simple evaluation of defective laminates.

Selection of Lamb modes for detecting internal defects in composite laminates

In this paper an investigation is carried out to predict the most efficient leaky Lamb mode and associated frequency to detect defects in a specific layer of a multilayered composite plate. To this end the stress and displacement fields inside a plate are theoretically calculated for different propagating leaky Lamb modes. It is observed that for every leaky Lamb mode the stress field inside the plate varies significantly with depth. A five-layer fiber reinforced composite plate with pre-existing defects, such as missing or broken fibers, and delamination is ultrasonically scanned with different leaky Lamb modes for which internal stress and displacement fields have been theoretically computed in absence of any defect. In presence of defects such as broken fibers and delamination the stresses on the defective surface are significantly reduced. Hence, if these defects are located in a region where the stress level is high for certain leaky Lamb modes then the presence of the defects will affect these modes significantly by releasing the high stress. These specific modes would then be most effective in producing the image of the defect. Theoretical predictions are then experimentally verified.