Low Velocity Impact Damage Research Articles

A comprehensive study of low-velocity impact behavior and damage evaluation of angle-ply thin woven GFRP composites under elevated temperatures

The recent climatic changes have resulted in a significant increase in temperatures over 50 °C during summer midday in certain countries. This has motivated the study of the low-velocity impact (LVI) response of angle-ply thin woven glass fiber-reinforced polymer composites under temperatures of 25 °C, 50 °C and 75 °C. The LVI performance of the laminates was assessed by eighteen parameters. A new inexpensive image processing method is developed to accurately measure the damage diameter, area, and perimeter length, which significantly aids in analyzing the damage mechanisms under various test conditions. The damage perimeter length is a new parameter that is used to interprets the change of material behaviors with temperature. The LVI results showed that the impact bending stiffness is increased with impact energy (Ei) due to increasing strain rates, which leads to decreasing the molecular mobility of polymer chains and hence, the materials tend to behave more brittle. Delamination threshold force increased with impact energy and vice versa with temperature. Soaking the specimens for 1 h at 50°C improved post curing and thus, enhanced the cross linking density, which significantly improved most of the impact properties. The penetration energy threshold of 25.5J (85 % of Ei) is observed at 30J-50 °C. Relaxation of the polymer chain within the glass transition region at 75°C leads to full fracture energy threshold at 30J, which correlated with open force-deformation curve, absorbed energy of 100 %, zero elastic energy, zero rebound energy, zero rebound velocity, and increase contact duration. The proposed multivariate regression models accurately predict the impact properties.

Translaminar-envelope ply for CFRP mechanical enhancement

Pursuing the trend of ply-level innovation in the field of high-performance CFRP composites, a pseudo-woven layup configuration has been developed by interlacing a thin-ply prepreg in the translaminar-envelope manner space-orthogonally inside a homogenized 4-ply sub-laminate to acquire mechanical superiorities for the global multidirectional laminate. This evolved layup structure possesses the stiffening feedbacks on in-plane principal properties by the locally-regulated 0° ply amount, and exerts the multi-interface synergy on shear-driven interlaminar fractures by the long-acting binding action. Further, the high robustness of structural performances has been revealed by resistance and tolerance against severe low-velocity impact damage, achieving up to 60 % decrease in the projected delamination area and over 60 % increase in the post-impact compressive strength. The translaminar-envelope layup configuration is expected to enhance the service capability of CFRP laminates.

Low-velocity impact damage of bionic turtle shell sandwich laminates with different suture shapes

Turtle shells have evolved over millions of years, developing exceptional mechanical properties such as high relative strength and toughness, rendering them highly effective in resisting impacts. This study delves into the impact resistance of bionic turtle shell structures with various suture shapes. This article analyzes the low-velocity impact of carbon fiber epoxy resin prepreg (CF/EP) composite sandwich panels with a suture interface by using the finite element simulation. The simulations encompass closed and unclosed models featuring bonded and unbonded tips, each with diverse trapezoidal geometries (triangular, trapezoidal, anti-trapezoidal, and rectangular). The findings reveal that sandwich structures with suture interfaces demonstrate significantly enhanced impact resistance compared to those lacking sutures, displaying 3–9 times greater deformation capacity and 20–30 times higher energy absorption capacity. The impact resistance of the triangular suture interface exceeded that of other bioinspired suture shapes, with trapezoidal and anti-trapezoidal sutures also enhancing stiffness, strength, and toughness. Additionally, a 6° bonded tip angle resulted in optimal performance for the triangular suture interface across all analyzed perspectives. The simulation study in this paper provides comprehensive and reliable data on low-velocity impact results, offering fundamental insights for researchers to design composite material structures that meet specific mechanical requirements effectively. Additionally, it offers novel ideas for the connection of protective structures, such as artificial armor.

On the influence of low-velocity impact damage on constrained-layer damping in hybrid CFRP-elastomer-metal laminates

Following the principle of constrained-layer damping (CLD), fiber-metal-elastomer laminates (FMELs) offer a high potential for damped lightweight structures, overcoming the undesirable vibration characteristics of conventional lightweight materials. While proven to be versatile and efficient, the damage-tolerance of such laminates is unexplored. This study for the first time in literature addresses the damage-tolerance of this efficient damping mechanism using a combined experimental and numerical approach. Results of experimental low-velocity impact tests on different configurations of FMELs are presented. In subsequent numerical modal analyses, different types of damage, namely delaminations, intra-ply damage and permanent deformation, are modeled and their influence on the vibrational behavior is investigated. While all types of damage influence the natural frequencies and modal damping ratios with a strong mode dependency, all laminates retain a high amount of modal damping with losses typically not higher than 15%. The results obtained reveal, that CLD is an efficient intrinsic damping measure in FMELs even in the presence of different types of damage. The key contributions of this paper include the thorough experimental characterization of low-velocity impact damages in different configurations of FMELs as well as the numerical assessment of those in frequency-domain simulations.

Study on the Low-Velocity Impact Response and Damage Mechanisms of Thermoplastic Composites.

A comparative experimental and numerical study of the impact behaviour of carbon-fiber-reinforced thermoplastic (TP) and thermoset (TS) composites has been carried out. On the one hand, low velocity impact (LVI) tests were performed on TP and TS composites with different lay-up sequences at different energy levels, and the damage modes and microscopic damage mechanisms after impact were investigated using macroscale inspection, C-scan inspection, and X-ray-computed tomography. The comparative results show that the initial damage valve force under LVI depends not only on the material, but also on the layup sequence. The initial valve force of the P2 soft layer with lower stiffness is about 11% lower than that of the P1 quasi-isotropic layer under the same material, while the initial valve force of thermoplastic composites is about 28% lower than that of thermoset composites under the same stacking order. Under the same stacking order and impact energy level, the damage area and depth of TP composites are smaller than those of TS composites; while under the same material and impact energy level, the indentation depth of P2 plies is greater than that of P1 plies, and the damage area of P2 plies is smaller than that of P1 plies, but the change of thermoplastic composites is not as obvious as that of thermoset composites. This indicates that TP composites have a higher initial damage threshold energy and impact resistance at the same lay-up order, while increasing the lay-up ratio of the same material by 45° improves the impact resistance of the structure. In addition, a damage model based on continuum damage mechanics (CDM) was developed to predict different damage modes of thermoplastic composites during low velocity impact, and the analytical results were compared with the experimental results. At an impact energy of 4.45 J/mm, the error of the initial damage valve force is 5.26% and the error of the maximum impact force is 4.36%. The simulated impact energy and impact velocity curves agree with the experimental results, indicating that the finite element model has good reliability.

Damage Assessment of Low-Velocity Impacted Sandwich Composite Structures Using X-Ray Micro-Computed Tomography

Sandwich composite structures offer significant versatility in structural system design but are susceptible to low-velocity impact damage, impacting their structural robustness. This study focused on nondestructive testing, particularly using X-ray micro-computed tomography, to assess damage on these structures, comprised of thin glass fibre reinforced polymer face sheets and a polyvinyl chloride foam core, under low-velocity impacts. Impacts were induced by a constant mass of 5.61 kg, dropped from various heights, generating impact energies between 2 and 22 J. This resulted in varied damage levels, from indentations to full perforations. The X-ray micro-computed tomography technique was chosen for its ability to detect internal damage. However, the system’s efficacy in accurately assessing damage depends on numerous factors like focus-to-detector distance, focus-to-object distance, and spatial resolution of the detector, among others. The system yielded an approximated resolution range of 10–25 μm for a focal spot size of 4 μm and the resolution range of 11–26 μm for a spot size of 7 μm. To this end, the system was able to reveal damage inflicted across the specimen through captured and reconstructed images. The quality of the reconstructed images was validated using ImageJ2 software by comparing with the processed images. The median filter was found to deliver images that closely resembled the original ones, albeit with a slight reduction in quality. Damage types varied based on impact energies. Low-level impacts caused matrix cracking and delamination at the foam interface. Medium-level impacts led to intralaminar and interlaminar damage, fibre fractures, and significant damage to the foam core through shearing and crushing. High-level impacts resulted in near or full perforations, with more pronounced delamination at the bottom interface, and fibre fractures in the impact zone, displaying a distinctive diamond-like damage pattern. These findings can be instrumental in developing a predictive impact damage model.

Effect of stacking angles on crashworthiness of CFRP square tubes considering low-velocity impact damage

To enhance the residual energy‐absorption (EA) of low-velocity impact-damaged CFRP square tubes, this study investigates the influence of stacking angles on the damaged tubes' residual EA. Through simulation and theoretical analysis, a well-designed stacking angles is proposed to optimize the residual EA. Initially, a simplified method of low-velocity impact damage of composite materials is proposed, by which a discontinuous damage mechanics finite element model is established and experimentally verified. Subsequently, various layup sequences are examined, and their impact on energy absorption is evaluated using several performance indices, including initial peak load, average load, energy absorption, specific energy absorption, and crushing load efficiency. The findings demonstrate that the designed stacking angles significantly enhances the residual energy absorption of low-velocity impact-damaged square tubes. Moreover, the study verifies the efficacy of the stacking angles design by investigating actual damage conditions, encompassing factors such as damage size, location, and stress concentration.

Effect of graphene nano-platelets on the low-velocity-impact and compression-after-impact strength of the glass fiber epoxy composite laminates

Detecting the low-velocity impact damage on the FRP composite structures can be challenging because the visible signs on the surface may be subtle. Mitigating the effects of BVID (Barely Visible Impact Damage) is crucial in ensuring the safety and reliability of composite components, especially in industries where these materials are commonly used, such as aerospace and automotive. This article explores the experimental investigations of the impact and post-impact damage propagation under compression in the Graphene Nano-Platelets (GNPs) infused glass fiber epoxy composite laminates. A comparative study of varying the percentage of the nano-platelets (0, 0.25, 0.5, and 0.75%) in the matrix material of the composite laminate is presented. The impact tests at low energies (15 J, 20 J, and 25 J), followed by the quasi-static compression tests, are carried out as per the procedures mentioned in the ASTM D7136/D7137. The digital image correlation technique (DIC) is used to measure strains and plot the strain field on the impacted face of the specimen under compression. Damage envelopes of the impact and compression tests are studied using X-ray CT scans. Micro-level failures are observed through Scanning Electron Microscopy (SEM) to get better insights into the failure mechanism of the composite materials under impact and compression loadings. Upon completing the study, it is found that the addition of the GNP into the matrix materials not only increased the impact performance but also increased the load-carrying capacity of the laminate under compressive loading. A decrement of 2–21% in the energy absorbed in the damage formation is seen with the incorporation of GNPs into composite materials. A maximum increment of 17% in the compressive strength is observed for the composite laminates with the 0.75% GNPs. However, for a particular percentage of the GNP, the compressive strength of the laminate is reduced with an increase in impact energy compared with its virgin counterpart. From X-ray CT scans, it is seen that the laminate under the compression tests fails due to excessive ply-splitting and the delamination at the damage site created during the impact. Fiber breakage, fiber brooming, matrix cracking, fiber-matrix debonding, etc., are some of the other failure modes observed in SEM studies.

Finite element analysis of the effect of crystallinity on low-velocity impact performance of poly (aryl ether ketone) composites

A new copolymer poly (aryl ether ketone) (PAEK) resin was used as a matrix to prepare carbon fiber (CF) reinforced composites with different crystallinity by slow and rapid cooling. Finite element (FE) models were constructed according to micromechanics, and the progressive damage method based on the three-dimensional (3D) PUCK damage criterion was used to analyze the low-velocity impact damage mechanism of the CF/PAEK composites. The experimental results show that the composites with lower crystallinity have higher impact resistance and smaller impact damage area. In the FE simulated impact damage evolution process, the damage of the impact side is mainly compressive damage of the matrix and the fiber, while that of the impact back side is mainly tensile damage of the matrix. Moreover, due to their high interfacial bonding strength, the main damage modes of rapid-cooled composites with lower crystallinity are severe matrix tensile damage and slight fiber tensile damage in the impact back side. However, the slow-cooled composites show more delamination. In addition, for the damage evolution of compression after impact, the FE simulation shows that fiber compression, matrix tension, matrix compression and delamination expand along the transverse to the compression load. Furthermore, the in-plane damage expansion occurs earlier in the rapid-cooled composites, which also have graver interlaminar damage.

Dynamic response and energy absorption of aluminum foam sandwich under low-velocity impact

Aluminum foam sandwich (AFS) are widely used in energy absorption because of their light weight and excellent energy dissipation capabilities. In this study, energy absorption, deformation modes, and dynamic response of AFS under low-velocity impact are investigated using experimental and numerical approaches. Dynamic impact tests are performed utilizing a drop hammer impact test on AFS with different core densities, face sheet thicknesses, and impact energy. Based on the 3D Voronoi foam model, a full-scale finite element model (FEM) is developed to simulate the mechanical response of AFS under low-velocity impact, and verified by experimental results. The contributions of different AFS components to energy absorption at various impact velocities are further analyzed, along with the effects of face sheet thickness distribution. The results indicate that the performance of the face sheet and core and the impact energy have a remarkable influence on the low-velocity impact response and damage modes of the AFS. The type of structure configuration, “thin top and thick bottom,” enables the AFS better play its energy absorption while ensuring the overall light weight. This study provides a reference for the design and optimization of the face sheet thickness of AFS when it is used as an energy-absorbing member and improves their design efficiency.

Study of Low-Velocity Impact Behavior of Hybrid Fiber-Reinforced Metal Laminates.

In this paper, the low-velocity impact behavior and damage modes of carbon/glass-hybrid fiber-reinforced magnesium alloy laminates (FMLs-H) and pure carbon-fiber-reinforced magnesium alloy laminates (FMLs-C) are investigated using experimental, theoretical modeling, and numerical simulation methods. Low-velocity impact tests were conducted at incident energies of 20 J, 40 J, and 60 J using a drop-weight impact tester, and the load-displacement curves and energy-time curves of the FMLs were recorded and plotted. The results showed that compared with FMLs-C, the stiffness of FMLs-H was slightly reduced, but the peak load and energy absorption were both greatly improved. Finally, a finite element model based on the Abaqus-VUMAT subroutine was developed to simulate the experimental results, and the damage modes of the metal layer, fiber layer, and interlayer were observed and analyzed. The experimental results are in good agreement with the finite element analysis results. The damage mechanisms of two kinds of FMLs under low-velocity impacts are discussed, providing a reference for the design and application of laminates.

Multi-scale numerical analysis of damage modes in 3D stitched composites

3D stitching composites have already sparked great interest among researchers due to their ability to effectively improve the weak inter-laminar properties in composite laminate structures. In this paper, a systematic analysis on mode I fracture and low-velocity impact damage of 3D stitched composites has been demonstrated via a novel multi-scale model which is capable of efficiently characterizing the distribution of stitching threads. The damage modes including intra-laminar cracks, inter-laminar delamination and stitching thread breakage are identified via a combination of continuum damage mechanics (CDM), cohesive zone model (CZM) element and truss element. This novel modeling approach enables an association analysis of stitching thread breakage status and other damage morphologies, promoting an in-depth study for the influence of stitching thread on damage evolution, which has not been reported yet. The introduction of stitching threads effectively suppresses the sharp propagation of delamination after peak load, avoiding the catastrophic destruction during mode I fracture. The mechanism of damage inhibition from stitching threads is well revealed by the numerical simulation. Furthermore, the capability of 3D stitched composites to resist low-velocity impact damage is effectively predicted in terms of peak load, damage area and damage morphology, finding that the increase of stitch density inhibits the propagation of matrix crack, as well as delamination. The universality and the expanding application of the model are also evaluated, demonstrating that it is suitable for various 3D stitched composites.

Low-velocity Impact Damage Behavior and Failure Mechanism of 2D Plain C/SiC Composite and 3D Needle-punched C/SiC Composite

Low-velocity Impact Damage Behavior and Failure Mechanism of 2D Plain C/SiC Composite and 3D Needle-punched C/SiC Composite

Low-velocity Impact Damage of Foam-core Sandwich T-joints: Numerical Study

T-joint has an important function in the lightweight structure to connect and transfer loading from one component to the other component. Failure analysis of the T-joint is necessary to obtain the optimum geometry. Since experiments require complex preparation and large costs, numerical modeling is needed to reduce the number of experiments that must be carried out. Numerical modeling in this study was performed to observe the failure modes of foam-core sandwich T-joint under low velocity impact loading. Three failure modes were considered in the current model: delamination, core crushing, and core shearing. To capture these failure modes, traction-separation response and ductile damage were implemented in the adhesive layer and foam material, respectively. The validation process was carried out by comparing the simulation results with experimental results from the literature. The simulation results showed similar failure modes as the experimental results from the literature, which confirms that the parameters used in the current numerical were accurate.

A comparison of damping-based methods to identify damage to carbon-fiber-reinforced polymers laminates subjected to low-velocity impact

A method for detecting low-velocity impact damage in carbon fiber reinforced polymer (CFRP) is presented. It involves the use of the Impulse Excitation Technique (IET) and hysteresis loops to calculate the damping parameter of T700/NCT304-1 carbon/epoxy samples subjected to various low-velocity impact energies. The value of the coefficient of restitution (COR) is determined for each impact, ranging between 0.62 for the lowest impact energy to 0.48 for the highest one. The results reveal that a three-step increase in the damping parameter exists in all cases as the impact energy on the specimen increases. An abrupt jump in the damping parameter value is observed for impact energies exceeding ∼0.9 of the material's maximum capacity. Overall, at the highest impact energy equal to 3.65 J, the damping parameter increased by 43.3% compared to the pristine specimen. Additionally, two cases of cyclic tension-tension loading were applied to the specimens, with maximum stresses set at 150 MPa and 200 MPa. The measured values of plastic and elastic strain energy were used to determine the damping ratios. For both cases, the damping of the specimen subjected to the highest impact energy was ∼1.2 times greater than that of an intact specimen, with an increase pattern similar to the findings of the IET method. Optical microscope images of the specimens are provided to illustrate various damage modes observed in the composite materials.

A comparative study on the low velocity impact behavior of UD, woven, and hybrid UD/woven FRP composite laminates

This study is aimed at comparing the response and damage of unidirectional (UD), woven fabric (WF) and hybrid UD/WF fiber reinforced polymer (FRP) laminates subjected to low velocity impact. The unidirectional tape and/or woven fabric (plain weave) carbon/epoxy prepregs are laminated and hot-pressed to produce UD, WF and sandwich-like hybrid UD/WF specimens. Impact responses of specimens are determined through low velocity impact (LVI) tests with impact energies of 10 J, 17 J and 25 J. After the LVI tests, the damage of specimens is characterized and analyzed using a combination of visual inspection, ultrasonic phased-array inspection, micro-computed tomography (CT) inspection, cross-sectional microscopic observation, and thermal de-ply test. Also, the LVI damage mechanisms of the three types of specimens are quantitatively compared by using the inter fiber crack volume ratio, total delamination area and fiber fracture length. It is concluded that the fiber architecture plays an important role in determining low velocity impact behavior of composite laminates. Especially, for the sandwich-like hybrid UD/WF laminates whose surface is a WF layer and the core is a UD layer, the WF layer on the surface plays an important role in reducing matrix cracking, delamination and fiber fracture, thus improving its LVI resistance.

Low velocity impact and compression-after-impact response of hybrid pseudo-woven meso-architectured carbon/epoxy composite laminates manufactured via automated fiber placement

An experimental study is presented on the low velocity impact (LVI) and compression after impact (CAI) response of T800-SC carbon/epoxy laminates manufactured via automated fiber (AFP) placement manufacturing. Two hybrid laminate configurations consisting of unidirectional and pseudo-woven meso-architectured composite (MAC) sub-laminates are compared against a traditional quasi-isotropic control laminate. MAC sub-laminates are manufactured via a modified AFP process implementing tow skips. The MAC architecture is macroscopically heterogeneous with discontinuous and spatially varying fiber orientations both in-plane and through thickness resulting in multiple interfaces and an expanded design space. The laminates are tested under LVI energies of 15–55 J using a 25.4 mm diameter hemispherical projectile according to ASTM D7136. The results show similar load-displacement responses but significant difference in the LVI damage profiles and CAI strengths for the different configurations. The hybrid laminate configurations exhibit increased damage resistance with up to 37% higher critical delamination load and increased damage tolerance with 26% higher residual compressive strength after an impact of 55 J compared to control laminate.

Low-velocity impact damage detection in CFRP composites by applying long pulsed thermography based on post-processing techniques

ABSTRACT Carbon fibre reinforced polymer (CFRP) composites are widely used in the aerospace industry; they are susceptible to impact damage from low-velocity impacts throughout their service life. Long pulse thermography (LPT) was proposed to detect defect in low-velocity impact test (LVIT). In this framework, the defects of matrix fractures, fibre cracks and interface delamination were produced by LVIT in the CFRP specimens. The halogen lamps LPT test system was applied to detect the low-velocity impact damage, and an A655sc infrared camera was applied to collect the surface temperature. In order to improve the efficiency of the impact damage detection, the infrared image sequences were processed by applying different post-processing techniques such as pulse phase thermography (PPT), thermal signal reconstruction (TSR), principal component analysis (PCA), total harmonic distortion (THD), linear discriminant analysis (LDA) and quadratic discriminant analysis (QDA). The results show that the signal-to-noise ratio (SNR) was improved by PPT, TSR, PCA, THD, LDA and QDA post-processing techniques. The proposed supervised learning (SL) QDA algorithm achieves better defect identification results than the above post-processing techniques. The detectability of real defects can provide a realistic assessment by the above post-processing techniques.

Numerical simulation on addition of Kevlar as reinforcement in CFRP (Carbon fiber reinforced plastic) laminates under low velocity impact

Fibre Reinforced Polymer (FRP) laminates are used extensively in automotive and aerospace components because to their reduced weight, increased design freedom, increased durability, and reduced susceptibility to corrosion. The research of impact damage is crucial for composite materials utilised in these components. Impact circumstances and laminate designs play the biggest roles in determining how laminates react to impacts. The hybrid composite laminates in this work (made of CFRP and Kevlar) are subjected to an explicit numerical analysis of the low velocity impact damage behaviour for three distinct thicknesses. Modelling is done using ABAQUS/CAE on the composite laminate and impactor. Modelling is done using ABAQUS/CAE on the composite laminate and impactor. The laminates' characteristics were coded in a VUMAT subroutine, and the analysis was done using ABAQUS/Explicit. Numerical analysis results for the absorbed energy are compared to laminates reinforced simply with carbon. In comparison to bare CFRP laminates, the simulation results indicated that hybrid laminates had a better energy absorption capability.

Terahertz pulsed imaging of low velocity impact damage in woven fiber composite laminates

Recently, terahertz pulse imaging has been used extensively to characterize internal defects in various electrically insulating materials. In this work, terahertz pulse imaging is used to identify the damage induced by low-velocity impact on woven glass-fiber reinforced polyamide laminates. Several impact energies are considered to study the damage initiation and propagation. The permanent indentation, known to be a relevant damage indicator, is extracted from the terahertz results and validated through comparison with profilometry. Further, the criticality of the damage, in terms of the number of composite plies in which the cracks propagate, can be clearly determined from terahertz imaging. These observations are validated through X-ray tomographic observations and analysis. Finally, a strong connection is established between the evolution of the permanent indentation and the appearance, and criticality, of the low-velocity impact-induced damage. These observations suggest that terahertz imaging is a reliable technique for the nondestructive assessment of impact damage in glass fiber composites.