Defect and Damage Characterization in Composite Materials

A Nondestructive Testing Method for Wind Turbine Blade Multilayer Composites Based on Inverse Heat Transfer Problem

Effect of workability characteristics on the hardened performance of FRSCCMs

The distribution of fiber or polymer lengths during breakage is considered. It is shown that under certain conditions the distribution of lengths is given by a self‐preserving form which is independent of the initial distribution and is not explicitly dependent on time. Some self‐preserving distributions are calculated. The theory is compared with an experiment on the breakage of glass fibres caused by milling.

Features the latest information on methods for predicting how composite materials and composite material structures fail. 27 peer-reviewed papers from the world's leading professionals in the field. Five key areas: Delamination Characterization -- Eight papers explore the challenges of characterizing delamination fracture of laminated composite materials in terms of interlaminar fracture toughness. Damage Modeling -- Developing appropriate models for predicting composite damage such as matrix cracking, fiber failure and delamination are considered in five papers. Material Damage Characterization -- Five papers investigate how environmental exposure and mechanical loads damage composite materials. Impact -- Methods to predict the effect of impact on laminated composite materials are explained in four papers. Structural Damage Characterization -- Five papers describe failure or damage issues in specific structures.

Composite materials play important roles in multifunctional applications, and thus, the diagnosis of damage patterns in composite materials becomes crucial to avoid critical events such as structural or functional failures. The impact of an individual damage in composite materials has been extensively studied, however, the interaction of defects/cracks, which leads to critical fracture paths, has not been understood well. In this paper, we develop a Bayesian estimation based statistical analysis technique that estimates the damage pattern of a composite material, in particular, the relative positions of defects in the material, by measuring its through-thickness dielectric properties. We first explain the fundamental dielectric principle that leads to the detection of defect patterns. A capacitance model is then built to measure the material permittivity, and the relationship between the dielectric permittivity and relative positions are found using COMSOL Multiphysics. The interaction effects between defects observed in the simulation are interpreted using the fundamental dielectric principle. A Bayesian estimation based statistical analysis model is then developed to estimate the relative positions of defects in composite materials from the measured global dielectric properties.

Non-homogeneous, bi-directional glass fabric reinforced thermoplastic (GFRT) composites are used in the automobile industry to produce economic and environmentally friendly vehicles. Despite the fact that GFRT materials have superior mechanical characteristics and lower manufacturing cost compared with traditional materials, they are prone to significant deformation and defect emergence during the forming/manufacturing of parts from prefabricated laminates. Some defects, especially hidden ones like delamination and consolidation defects, can significantly reduce the mechanical properties of the final parts. It is essential to use 3D non-destructive testing (NDT) technology to detect and identify such hidden defects. This research investigates the ability of 3D THz imaging to detect hidden defects (i.e., delamination and consolidation) in GFRT composite materials. Frequency modulated continuous wave (FMCW) technique is used to generate 3D-THz cross-sectional images. Non-defective and defective samples with a thickness of 1.5mm have been measured with two 3D THz imaging systems operating in the frequency range from 238 to 316GHz and from 499 to 733 GHz. In addition, a simulation of the reflected FMCW spectrum is presented to analyze the measurements and intercompare the performance of the two imaging systems. A systematic analysis in a large sample set demonstrates that a delamination defect with a thickness of 125μm as well as a consolidation defect with a difference in thickness less than 0.4mm can be successfully detected by the 700-GHz imaging system.

The pressure rise in a pultrusion die inlet can have a significant impact on the quality of a pultruded product; therefore, a model to predict the pressure rise would be a very useful and effective tool for enhancing the pultrusion process and the pultruded product. There are several parameters that can strongly affect the pressure rise in the pultrusion die inlet. One particular parameter is the permeability. In this study, three permeability models are employed to investigate their effect on the pressure rise in the pultrusion die inlet. A numerical model, based on Darcy's law for flow in a porous media that employs the finite volume solution method, is developed to predict the pressure and velocity fields in the pultrusion die. The numerical results are compared with some preliminary experimental data for both glass and graphite composites to determine which permeability model agrees best with the data. An effective fiber diameter is introduced to account for nonuniform fiber diameters and nonuniform spatial fiber distributions.

Acoustic emission and multiscale computation-guided tensile damage identification in woven composite laminates at cryogenic temperatures as low as 20 K

Damage and failure of the fiber reinforced composites remain as a challenging research subject in the area of material science and engineering. In this study a novel particle assembly model is developed using two dimensional Discrete Element Method (DEM) for the purpose of simulating the damage and failure process of the single-fiber composite (SFC) under axial tension. Fiber (SiC) and matrix (Epoxy) are represented by particles bonded together through elastic parallel bonds which are calibrated by a series of numerical tests. The contacts between the fiber particles and matrix particles are directly accounted for the fiber/matrix interface which is represented by the contact softening model similar to the cohesive zone model (CZM) in the continuum mechanics. The single-fiber composite tensile test is carried out using the developed DEM model in order to evaluate the interactions between fiber breakage, interfacial debonding and matrix cracking. The numerical results have demonstrated the capability of the developed DEM model in simulating the entire failure process of each individual constituent of the single fiber composite. This study has also confirmed that the DEM model has unique advantages over the conventionally numerical models in terms of dealing with the evolution of microscopic damages in composite materials.

Damage in composite laminates evolves through complex interactions of different failure modes, influenced by load type, environment, and initial damage, such as from transverse impact. This paper investigates damage growth in cross-ply polymeric matrix laminates under tensile load, focusing on three primary failure modes: transverse matrix cracks, delaminations, and fiber breaks in the primary loadbearing 0-degree laminae. Acoustic emission (AE) techniques can monitor and quantify damage in real time, provided the signals from these failure modes can be distinguished. However, directly observing crack growth and related AE signals is challenging, making numerical simulations a useful alternative. AE signals generated by the three failure modes were simulated using modified step impulses of appropriate durations based on incremental crack growth. Linear elastic finite element analysis (FEA) was applied to model the AE signal propagating as Lamb waves. Experimental attenuation data were used to modify the simulated AE waveforms by designing arbitrary magnitude response filters. The propagating waves can be detected as surface displacements or surface strains depending upon the type of sensor employed. This paper presents the signals corresponding to surface strains measured by surface-bonded piezoelectric sensors. Fiber break events showed higher-order Lamb wave modes with frequencies over 2 MHz, while matrix cracks primarily exhibited the fundamental S0 and A0 modes with frequencies ranging up to 650 kHz, with delaminations having a dominant A0 mode and frequency content less than 250 kHz. The amplitude and frequency content of signals from these failure modes are seen to change significantly with source–sensor distance, hence requiring an array of dense sensors to acquire the signals effectively. Furthermore, the reasonable correlation between the simulated waveforms and experimental acoustic emission signals obtained during quasi-static tensile test highlights the effectiveness of FEA in accurately modeling these failure modes in composite materials.

Advanced composite materials play vital structural roles in automotive, aerospace and marine industries. Recently, reduction in arctic sea ice region over the last three decades has opened new sailing routes which are more efficient and economical. This has resulted in the increased use of marine and naval vessels in extreme low temperature arctic conditions. The fundamental challenge of operating in such cold and harsh environment lies in the understanding of how materials and structures behave and perform in extreme low temperature. Composite sandwich structures far exceeds classical composite laminates in terms of flexural capability and performance. In this study, we experimentally investigate the impact and post-impact bending response of Divinycell H-100 foam core sandwich panel with woven carbon fiber reinforced polymer (CFRP) facesheets. Specimens were conditioned and impacted over a temperature range (from room temperature down to -70°C). Results show that exposure to low temperature generally causes more severe damage in the specimens. Post-mortem inspection using x-ray micro-computed tomography revealed complex failure mechanisms in the composite facesheets (such as matrix crack, delamination and fiber breakage) and foam core (core crushing, core shearing and interfacial debonding).

This study presents a novel methodology that integrates piezoelectric actuation and sensing with regression models and neural networks for high-accuracy detection and localization of multiple delamination and crack defects in composite laminates. An eight-layer graphite/epoxy composite plate, instrumented with piezoelectric patches, is excited using random voltage stimuli, generating structural responses captured by sensors. The proposed framework employs six regression techniques and artificial neural networks, achieving localization accuracy with R^2 values exceeding 99.6%. Additionally, combining five signal decomposition methods with four classifiers enables defect type identification with up to 98.26% accuracy. The methodology demonstrates exceptional precision in detecting previously unseen delamination and crack defects down to the lamina level, optimized through grid search cross-validation. A comparative analysis highlights that piezoelectric sensor voltage signals outperform acceleration signals for defect characterization. This integrated piezoelectric-regression/neural network framework establishes a robust foundation for non-destructive evaluation, offering significant potential for real-time structural health monitoring applications.

This study is continuation of the authors’ previous work on examining early failure events in unidirectional polymer matrix composites under transverse tension or under axial shear. This paper considers the combined effect of applying these two loading modes. The early failure events consist of brittle cavitation or yielding depending on the local stress states resulting from the applied loading. The criterion for brittle cavitation is attainment of the critical dilatational energy density when the stress state is hydrostatic tension (or nearly so), while that for initiation of yielding is attainment of the critical distortional energy density. As in the previous works, the nonuniform fiber distribution is described in terms of the degree of nonuniformity, which is implemented in a representative volume element (RVE). Several realizations of the RVE for different fiber volume fractions and different degrees of nonuniformity are analyzed using a finite element model. The results show that the occurrence of brittle cavitation versus yielding depends on the biaxiality ratio, i.e., the ratio of the axial shear stress to the transverse tension

Characterization of impact damage in polymer matrix composites (PMCs) continues to be a challenge due to the complex internal structure of the material. Nondestructive characterization approaches such as normal incident immersion ultrasound and flash thermography are sensitive to delamination damage, but do not provide information regarding damage obscured by the delaminations. Characterization of material state below a delamination requires a technique which is sensitive to in-plane damage modes such as matrix cracking and fiber breakage. Previous studies of the lateral heat flow through a composite laminate showed that the diffusion time was sensitive to the depth of the simulated damage zone. The current study will further evaluate the lateral diffusion model to provide sensitivity limits for the modeled flaw dimensions. Comparisons between the model simulations and experimental data obtained using a concentrated heat source and machined targets will also be presented.

Composite materials are of great importance in many industries due to their unique properties, strength and durability, but they also include disadvantages that can affect their use and must be studied. In this article, we simulate the acoustic interference field received by a delay line following an interaction between an acoustic field and a multilayer carbon fibre composite material using the ray approach. In this case, the simulation is performed using carbon fiber composite plates, where some plates have lamination defects such as delamination, while others are free of these defects. Experimental measurements are performed on carbon fiber composite plates with and without delamination defects introduced beforehand, without knowing their thickness. The thickness of each ply is 0.125 mm. The transducer used is a focusing immersion transducer type parametric (V 312) that operates at a frequency of 10 MHz The simulation results almost exactly corroborate those of the experiment. The simulation model was employed to determine the thickness of the induced delamination, which is 0.2 mm, found in the second and third layers.