Impact-induced Delamination Research Articles

Numerical model of curved composite tiles under low-velocity impact loading

In recent years, composite structures have seen widespread application across diverse industries, including automotive and aerospace, owing to their favourable strength-to-weight ratio. However, these structures, particularly high-pressure vessels, are susceptible to extreme loadings such as impact forces, resulting in visible and latent damage, ultimately leading to catastrophic failures. Hence, it is imperative to assess such damages diligently. Given the cost implications associated with experimental investigations, numerical methodologies emerge as a potent means for conducting relevant analyses. This paper will introduce a numerical model approach to conducting virtual tests to assess curved carbon fibre composite tiles subjected to low-velocity impact. The primary focus of this study is to consider the impact-induced delamination on composite tiles through a numerical model. The composite tiles are conducted to 90J impact loading. Specifically, the study evaluates filament winding structures’ interface properties (shear resistance) variability during manufacturing. Notably, the parameter of shear resistance significantly impacts the anticipation of intralaminar damage (delamination) after impact loading. Three-point bending tests were performed to calibrate the composite material’s shear interface properties, followed by low-velocity impact tests on the curved composite tiles. The findings indicate that the shear resistance values, which ranged from 14.25 MPa to 60 MPa, play a critical role in predicting delamination, with lower shear resistance leading to increased damage areas. The results underscore the importance of accurately calibrating interface properties to enhance the predictive accuracy of numerical models for curved composite structures under impact loading.

Thermal and mechanical damage to carbon fibre reinforced composites with metallic fasteners under lightning strike

In this study, lightning strike (LS) damage to carbon fibre reinforced composites (CFRCs) with metallic fasteners was investigated by simulated LS tests and finite element (FE) model. First, the LS damage mode and mechanism of a single-lap composite joint and a CFRC laminate with a metallic rivet were studied. Resin pyrolysis was found on the surface but severe delamination was evident inside the composites due to a gas explosion caused by the sudden burn-off of polymer matrices and thermal expansion inside the laminate. Then, the effect of rivet spacing on the LS damage to CFRCs was examined. The results showed that the damaged area was significantly reduced because the surrounding rivets effectively suppressed impact-induced delamination and consumed more electrical energy. The results also showed that large-area delamination across the rivets was achieved due to high potential at the surrounding rivets when the spacing was short but dynamic deflection was confined by the surrounding rivets. In contrast, delamination was only concentrated at the central rivet when the spacing was large, which can be ascribed to fewer currents flowing into the surrounding rivets, and the confinement of the adjacent rivets was less significant.

Visualization and investigation of healing mechanism in carbon fiber reinforced Elium® composites

In this study, carbon fiber/Elium® composites are manufactured using the resin infusion manufacturing technique. Artificial cracks representing impact-induced delamination are incorporated during layup. The laminates are subsequently healed using several healing parameters. The bonding mechanisms are investigated through microscopic analyses and X-ray Computed Tomography (XCT). The mechanical performance of the healed composites is investigated through four-point bending tests. It is found that as the healing temperature is increased, the cracks are gradually healed by interdiffusion and macroscopic resin flow across each crack interface. The 3D models generated from XCT images show an increase in solid volume fraction from 48% to 70% at the optimum healing parameters. The flexural strength and flexural modulus of the undamaged composites subjected to optimized healing condition are significantly increased relative to the room temperature cured pristine samples. The residual flexural strength of the damaged samples healed at the optimum parameters is 96% of the reference condition. The SEM images confirm that delamination from the previously-healed crack interfaces is the most dominant failure mode in the healed samples, in contrast to fiber fracture and brittle failure in the undamaged reference composite samples. The results show that the damaged composite laminates can be effectively healed and repaired to as good as undamaged laminates using the interdiffusion mechanism.

Impact and fatigue tolerant natural fibre reinforced thermoplastic composites by using non-dry fibres

This article introduces stiff and tough biocomposites with in-situ polymerisation of poly (methyl methacrylate) and ductile non-dry flax fibres. According to the results, composites processed with non-dry fibres (preconditioned at 50% RH) had comparable quasi-static in-plane shear strength but 42% higher elongation at failure and toughness than composites processed with oven-dried fibres. Interestingly, the perforation energy of flax–PMMA cross-ply composites subjected to low-velocity impact increased up to 100% with non-dry flax fibres. The in-situ impact damage progression on the rear surface of composites was evaluated based on strain and thermal field maps acquired by synchronised high-speed optical and thermal cameras. Impact-induced delamination lengths were investigated with tomography. Non-dry fibres also decreased the tension–tension fatigue life degradation rate of composites up to 21% and altered the brittle failure mode of flax–PMMA to ductile failure dominated by fibre pull-out.

Delamination buckling of a laminated composite shell panel using cohesive zone modelling

Abstract Laminated composite shell panels take part in several engineering structures. Due to their complex nature, failure modes in composites are highly dependent on the geometry, direction of loading and orientation of the fibers. However, the design of composite parts is still a delicate task because of these fiber failure modes, which includes matrix failure modes or other so-called interlaminar interface failure such as delamination, that corresponds to the separation of adjacent layers of the laminate as a consequence of the weakening of interface layer between them. In this work, impact-induced delamination represented as a circular single delamination is investigated, as it can reduce greatly the structural integrity without getting detected. Furthermore, attention is focused on its effect upon the post-buckling response and the compressive strength of a composite panel. The delamination buckling was modelled using the cohesive element technique under Abaqus software, in order to predict delamination growth and damage propagation while observing their effects on the critical buckling load.

Numerical simulation of Lamb wave sensing of low-velocity impact damage in composite laminate

It is difficult to investigate the interaction between the Lamb wave and impact damage within a composite laminate if the interfaces caused by impact-induced delamination and cracks are not modeled precisely. In this study, a novel finite element modeling method was proposed to virtually reproduce multiple damage after observing the distribution and form of different impacts in a cross-ply carbon fiber reinforced plastic laminate. Further, a numerical simulation of acousto-ultrasonic sensing was performed to manifest the tone-burst Lamb wave that propagated in the laminate. The calculated stress and displacement contours distinguished the dispersed modes and exhibited the wave attenuation after it interacted with multiple interfaces. The change in amplitude provided a reference for impact damage evaluation. The proposed method was compared with another existing modeling method and validated by the acousto-ultrasonic sensing experiment, which demonstrated its accuracy and capability to study the phenomena in acousto-ultrasonic sensing for low-velocity impact damage.

In situ capture of impact-induced progressive damage and delamination in fiberglass composite laminate with a high-speed optical imaging method

Delamination is one of the severest failure modes for laminated composite materials during applications, especially when the composites are subjected to dynamic loading like impact. The impact-induced delamination patterns in laminates have been studied before but the progressive delamination evolution during the impact process has rarely been revealed through experimental testing. Capturing the real-time delamination progression is of great significance for understanding the fundamentals of the composite performance and damage resistance during the dynamic loading process. This study investigates the feasibility of using a high-speed optical imaging method for in-situ capturing the impact-induced progressive damage and delamination evolution in fiberglass composite laminate [0/90/0/…]13. The progressive delamination areas are further quantified through a sequential series of captured images using a custom algorithm. The relations between the progressive delamination evolution and the real-time absorbed energy, as well as the impact force and deflection, have been investigated. This study provides insights into the experimental testing method of dynamic damage and failure process in materials and structures.

An experimental study of the damage growth in composite laminates under tension–fatigue after impact

The behavior of damaged laminates under cyclic load is crucial to the damage tolerance of composite structures. In this experimental study, we investigate impact damaged laminates under tension–fatigue loading. For that purpose, a tension after impact (TAI) specimen is designed, which exhibits a similar impact behavior like a specimen of the established compression after impact (CAI) standard. A set of quasi-static TAI tests is conducted as a reference for the investigation under cyclic load. A 30J impact was chosen as the baseline, causing a delamination-dominated barely visible impact damage. For the cyclic test, fifteen TAI specimens were distributed over three load levels. The tests reveal a widespread fatigue effect, edge delamination, and a significant growth of the impact-induced delamination. The damage propagates mainly in the load direction and in the interfaces close to the impact side. The observed delamination growth can be explained through to the fiber cracks caused by the impact. Moreover, the growth rate exhibits severe outliers, whose damage propagates exceptionally fast. These outliers can be traced back to the morphology of the impact damage. The qualitative pattern of fiber fracture massively influences the delamination propagation. Based on this dependency we outline the need for a qualitative evaluation of impact damage and its scatter.

Effects of specimen dimensions and impact energy on energy absorption and damage of glass/epoxy composite plates

Abstract In this study, the impact response of laminated composite plates of various specimen dimensions under various impact energies has been investigated, experimentally and numerically. Glass/epoxy composites with [02/902/02/902]s orientations were manufactured by the hand lay-up technique. Low velocity impact tests were conducted using a CEAST-Fractovis Plus impact test machine for specimens with net impact areas; SQR76 (square with 76 mm edge), SQR150 (square with 150 mm edge) and CIR76 (circle with diameter of 76 mm). Numerical analysis was also carried out via 3DIMPACT transient finite element code including matrix cracking and impact-induced delamination criteria. Absorbed energy and the damage size of specimens were investigated for various impact energies. Delamination areas obtained by finite element were in good agreement with those experimentally obtained up to 50 J of impact energy.

Influence of low-velocity impact-induced delamination on electrical resistance in carbon fiber-reinforced composite laminates

In this paper, experiments have been performed and finite element models have been developed for studying the influence of low-velocity impact damage on the four-probe electrical resistance of carbon fiber-reinforced polymer matrix laminates. Sixteen-ply and 32-ply AS4/3501-6 laminates with quasi-isotropic layup were analyzed. Electrical resistance was evaluated using a four-step procedure. First, finite element models were created in Abaqus Finite Element Analysis (FEA) for simulating low-velocity impact using a quasi-static loading approach. Second, matrix rupture in the inside plies was evaluated, and delamination analysis was performed at the corresponding interfaces to determine delamination patterns. Third, four-probe electrical finite element models were developed in Abaqus FEA for specimens before and after impact using the concept of effective conducting thickness and the delamination patterns obtained from the delamination analysis. Effects of the low-velocity impact delamination on four-probe top and oblique electrical resistance were studied. Electrical resistance predictions were compared to the experimental data. Both top and oblique resistance planes were sensitive to presence of delamination with the oblique resistance measurement being more sensitive as compared to the top resistance measurement. In addition, the resistance of the 16-ply specimens was more greatly affected by the delamination compared to the 32-ply specimens. The proposed analysis can be utilized for design of carbon fiber-reinforced polymer matrix composites with optimized damage sensing capabilities.

Real-time estimation of delamination occurrence induced by low-velocity impact in composite plates using optical fiber sensing system

Low-velocity impact is the one of the most critical events to reduce the reliability of composite structures, because it can generate hidden damages such as delamination inside the structures. In order to address this problem, impact monitoring systems have been suggested using various types of built-in sensors. Generally, such impact monitoring systems are consisted of impact localization and damage assessment. In this paper, a methodology of impact-induced delamination assessment was studied using fiber Bragg grating (FBG) sensors. Because of its multiplexing capability, FBG sensors are advantageous of monitoring large areas of target structures. However, it is hard to be applied in impact monitoring systems due to its low sensitivity and narrow bandwidth. In this study, a quantification method of delamination was examined with maintaining the merits of FBG sensors. For the acquisition of impact-generated acoustic emission (AE) signals, the commercial FBG system was adopted with a sampling frequency of 100 kHz. Then, the acquired AE signals during the low-velocity impact fracture tests were analyzed with the wavelet transform (WT) method for measuring the delamination areas due to different energies. Finally, it can be concluded that the quantities of WT detailed portions are linearly correlated with the delamination areas.

Real-time detection of low-velocity impact-induced delamination onset in composite laminates for efficient management of structural health

Abstract Carbon fiber reinforced polymer (CFRP) laminates have been becoming primary structures in the aerospace industry because of their high specific strength and stiffness. However, CFRP laminates have susceptibility to low-velocity impact events which can easily induce internal or hidden damages such as delamination. Such impacts frequently arise during maintenance, flight operation or in-service events. Thus, composite structures have to be irregularly inspected in addition to the periodic maintenance for ensuring the structural health. However, such irregular inspections can inherently incur the overall maintenance cost because it has to be performed in all suspicious cases of damages. For this reason, the methodology for accurately realizing the onset of delamination induced by low-velocity impact events is required for reducing the operating cost of composite structures. In this paper, the potential of using high speed fiber Bragg grating (FBG) sensing system for detecting the delamination onset was studied for thick CFRP laminates. Because FBG sensors can be simply multiplexed to capture the structural responses, the proposed method in this study can be quite attractive for an efficient impact monitoring system. To obtain the impact response signals and contact force histories, several low-velocity impact experiments were performed in a range of 1–30 J. From the signal processing of these experimental data, the meaningful damage index was introduced using the detail components of wavelet transformed sensor signals. Although this result is in the preliminary step, such damage index can be useful for applying an in-situ impact damage assessment system to the real composite structures in the near future.

Rapid guided wave delamination detection and quantification in composites using global-local sensing

This paper presents a rapid guided ultrasonic wave inspection approach through global inspection by phased array beamforming and local damage evaluation via wavenumber analysis. The global-local approach uses a hybrid system consisting of a PZT wafer and a non-contact laser vibrometer. The overall inspection is performed in two steps. First, a phased array configured by a small number of measurements performs beamforming and beamsteering over the entire plate in order to detect and locate the presence of the damage. A local area is identified as target damage area for the second step. Then a high density wavefield measurement is taken over the target damage area and a spatial wavenumber imaging is performed to quantitatively evaluate the damage. The two-step inspection has been applied to locate and quantify impact-induced delamination damage in a carbon fiber reinforced polymer composite plate. The detected delamination location, size and shape agree well with those of an ultrasonic C-scan. For the test case studied in this work the global-local approach reduced the total composite inspection (damage detection and characterization) time by ∼97% compared to using a full scan approach.

Micro–macro FE modeling of damage evolution in laminated composite plates subjected to low velocity impact

A phenomenological-based micro–macro mechanical method, comprising the coupling of a 3D continuum damage mechanics (CDM) model with an original micromechanical FE model, is presented to predict low velocity impact response of composite laminates. The key point of the proposed method is the definition and computation of saturation crack density, which is used to scale impact-induced delamination instead of via expensive cohesive zone model based FEM. In order to gain the specific value of the saturation crack density, a micromechanical FE model is established based on the RVE technique. Additionally, the initiation of intralaminar failure including fiber fracture (FF) and inter-fiber fracture (IFF) is evaluated by Puck criteria, and its evolution, with considering effects of the fracture plane angle, is assumed to be controlled by equivalent strains on the fracture plane rather than the classic material principal axis plane. A good agreement between the experimental and simulated results shows correctness and effectiveness of the developed micro–macro mechanical method for predicting impact response and damage using a relatively coarse mesh. Furthermore, significant reduction of computational cost, compared with existing techniques, can be achieved with the novel mesh-independent method.

Impact-Induced Delamination Detection of Composites Based on Laser Ultrasonic Zero-Lag Cross-Correlation Imaging

This paper presents impact-induced delamination visualization by zero-lag cross-correlation (ZLCC) imaging computed using fully noncontact laser scanned ultrasonic wavefields. The proposed technique enables instantaneous visualizing of invisible delamination of composite materials without any sensor installation. Moreover, it provides robust damage diagnosis without comparing with baseline data obtained from the undamaged condition of a target structure, making it possible to minimize false alarms. First, the existence of internal delamination-induced standing waves is proven by employing a finite element analysis. Then, how ZLCC can physically isolate and visualize only the standing wave feature from the measured ultrasonic wavefields is shown. To experimentally validate the proposed technique, a fully noncontact laser ultrasonic imaging system is introduced, and the internal delamination is visualized by laser scanning in a graphite fiber composite plate. The experimental results reveal that hidden delamination is successfully and automatically visualized and quantified without any users’ intervention.

Guided wave imaging for detection and evaluation of impact-induced delamination in composites

In this paper, guided wavefield interactions with delamination damage in laminated composite panels are investigated. The frequency–wavenumber representations of the guided wavefields show that different wavenumbers are present in the delaminated plate, compared to a pristine case. The wavenumbers are correlated to trapped waves in the delamination region. Novel approaches for imaging the composite panels using guided waves are discussed and demonstrated for quantitative evaluation of the delamination damage. A filter reconstruction imaging method is shown to provide a rapid technique to locate delamination damage by showing where guided wave energy is trapped. A spatial wavenumber-based imaging algorithm is applied to calculate wavenumber values at each spatial location and highlights the delamination damage as regions with larger wavenumber values. The imaging approaches are demonstrated using experimental data from a plate with a simulated delamination (teflon insert) and from a plate containing impact-induced delamination damage. The methods are also applied to a multiple mode guided wave case to demonstrate application to complex wave cases.

Further investigation of Delamination Reduction Trend for stitched composites

Abstract This paper seeks to further investigate a recently proposed novel empirical-based Delamination Reduction Trend (DRT) for stitched composites. The DRT is capable of predicting the effective reduction in impact-induced delamination area due to the influence of stitching. DRT simply relates two parameters: normalised delamination area and stitch fibre volume fraction, to characterize the effectiveness of stitching in impact damage suppression. In this work, quasi-static indentation (QSI) test is performed on stitched composites with the aim to offer new observations, provide validations and understand limitations in DRT. Test data coupled with experimentally observed damage mechanisms reveal limitations of DRT, particularly across two regions: first, when damage is initiated and delamination growth has not been developed; second, when delamination propagation is influenced by boundary conditions and saturation of delamination damage has occurred. Validation of DRT is demonstrated in domain outside of these regions. The investigation is further analysed by statistical approach using a three-factorial design of experiments (DoE). The main influence and coupling interaction of stitch density, stitch thread thickness and indentation displacement on indent-induced delamination area are established and discussed. Graphical and statistical results provide further insights and validation to DRT. The novelty of this work lies in showing validation and understanding limitation of DRT for its effective use and accurate prediction of delamination damage in stitched composites.

Finite element model for compression after impact behaviour of stitched composites

A finite element (FE) model using coupling continuum shell elements and cohesive elements is proposed to simulate the compression after impact (CAI) behaviour and predict the CAI strength of stitched composites. Continuum shell elements with Hashin failure criterion exhibit the composite laminate damage behaviour; whilst cohesive elements using traction-separation law characterise the laminate interfaces. Impact-induced delamination is explicitly modelled by reducing material properties of damaged cohesive elements. Computational results have demonstrated the trend of increasing CAI strength with decreasing impact-induced delamination area. Spring elements are introduced into the model to represent through-thickness stitch thread in the composite laminates. Results in this study validate experimental finding that CAI strength is improved when stitching is incorporated into the composite structure. The proposed FE model reveals good CAI strength predictions and indicates good agreement with experimental results, making it a valuable tool for CAI strength prediction of stitched composites.

Delamination propagation analysis in tufted carbon fibre-reinforced plastic composites subjected to high-velocity impact

Impact-induced delamination propagation on both tufted and untufted carbon fibre-reinforced plastics composite laminates was investigated. High-velocity impacts were performed on a ballistic gas gun. Initial and residual velocities were monitored with optical sensors especially developed for this study. Energy-absorption abilities were compared for different tufting parameters. The perforated specimens were inspected with an original technique allowing the observation of interlaminar delamination area for each interface. The existence of preferential propagation direction is highlighted. It is concluded that tufting helps to contain delamination and provides improved energy absorption depending on the tufting pitch.

Low-velocity impact-induced delamination detection by use of the S0 guided wave mode in cross-ply composite plates: A numerical study

Finite element method based numerical simulations are performed to identify low-velocity impact-induced asymmetrically-located delamination in the [0/903]S and [0/90]2S composite plates, respectively, using a fundamental symmetric guided wave mode (S0). The wave attenuation effect due to the viscoelasticity of the composite material is modeled by calculating the Lamb wave attenuation constants and using the Rayleigh proportional damping model. The estimated sizes and locations of the delamination in both plates were in good agreement with the experimental measurements. Moreover, the analysis of wave structure of the impacted plates shows that when the S0 mode propagates through the damaged region, the delamination mouth opens up due to the presence of standing waves, which are generated as a consequence of multiple reflections of trapped waves with the delamination boundaries.