Compression After Impact Test Research Articles

In-situ detection of delamination reinitiation in carbon fiber reinforced polymers post barely visible impact damage

In this study, advancements are presented in the in-situ detection of delamination reinitiation from Barely Visible Impact Damage (BVID) in composite materials, utilizing enhancements in Digital Image Correlation (DIC) techniques during a Compression After Impact (CAI) test. The study measured strain fields in the longitudinal, transverse, and shear directions, focusing specifically on the point of highest out-of-plane displacement to identify the onset of delamination propagation from BVID sites generated at different impact energy levels. By correlating the measured strains with the peak out-of-plane displacement, a unique determination of onset damage reinitiation associated with BVID during CAI testing was achieved. This method introduces a refined in-situ assessment technique for structural integrity, allowing for the early detection of critical damage propagation in composite materials.

Multi-functional hybrid sandwich composite structures: Evaluating damage mechanics and tolerance using compression after impacts

Building upon previous researches that introduced an innovative foam core hybrid-sandwich composite structure for radome applications, showcasing promising performance under low-velocity impacts (LVIs), this paper delves into the assessment of damage tolerance and mechanics through Compression After Impact (CAI) testing. The primary objective is to analyze disparities in damage tolerance, damage mechanisms, displacements, deformations, energy absorption, and residual strengths resulting from LVIs followed by CAIs on dissimilar materials on opposite faces of the hybrid structure. The extent of damage is evaluated through Computed Tomography (CT) scans. During LVIs, impacts on the S glass face sheets side demonstrated different energy dispersion and absorption mechanisms, leading to variations in indent damage depths and widths across all impact energy levels, unlike the impact damages observed from the Kevlar side. During CAI testing, this difference becomes more evident, with Kevlar specimens (KS3 & KS5) showing greater indentation depths and narrower widths, and S glass specimens (SK3 & SK5) experiencing buckling. These effects are due to the unique damage absorption and dispersion properties of Kevlar and S glass. These variations prompted an in-depth investigation of the structure using CAI to comprehend damage tolerance and mechanisms under compressive loads. This study, unparalleled in existing literature, proposes hybrid sandwich structures with superior specific impact and residual strength compared to various composite sandwich structures documented in published literature, expanding their utility beyond radomes.

Investigating low velocity impact and compression after impact behaviors of carbon fiber/epoxy composites reinforced with helical multiwalled carbon nanotubes

Pre-damage and undetected micro-defects in fiber-reinforced polymer (FRP) composites are significant challenges compromising the integrity and mechanical performance of composite structures in aerospace applications. To address this challenge, this experimental study investigated the low velocity impact (LVI) and compression after impact (CAI) behaviors of carbon fiber/epoxy (CF/E) reinforced with 0.0 (control sample), 0.2, and 0.4 wt% helical multiwalled carbon nanotubes (HMWCNTs). The LVI test results showed that the control samples, with epoxy as the predominant phase, registered the lowest peak force compared to the 0.2 wt and 0.4 wt% HMWCNT/epoxy composite laminates. The CAI test results showed a decrease in strength with increased impact energy and an increase in the weight percent of HMWCNTs. Consequently, the 0.4 wt% HMWCNT ladened composites could withstand higher load without undergoing significant deformations, exhibiting the highest impact resistances due to the presence of CNTs, which function as load-bearing material. The HMWCNT interleaved CF/E composite laminate samples subjected to 25 J of impact energy achieved a higher maximum force of 9870.25 N compared to 8527.26 N for the damaged samples at the same level of impact energies. This fact indicates that the damaged samples could not withstand sufficiently high impact forces due to local instability arising from existing damages. The results strongly suggest a high potential for HMWCNTs CF/epoxy laminate composites in making structural and substructural components in aircraft and automobiles due to their proven superior properties.

Compression after impact (CAI) failure mechanisms and damage evolution in large composite laminates: High-fidelity simulation and experimental study

This study focuses on developing and validating a high-fidelity finite element model for predicting damage evolution and residual strength in fiber-reinforced composite panels. Impact and compression after impact (CAI) tests were conducted at both barely visible impact damage (BVID) and clearly visible impact damage (CVID) levels. The ASTM D7137 standard 100 mm × 150 mm CAI coupons were inadequate to cover the range of experimental studies required for model validation. Therefore, larger 254 mm × 304.8 mm laminates were investigated under two CAI testing conditions: one a scaled-up version of ASTM standard coupon, and the other with additional anti-buckling support plates to reduce unsupported areas to 127 mm × 177.8 mm. The model captured inter- and intra-laminar failure modes, including fiber breakage, splitting, kinking, pull-out, and crushing as well as matrix cracking, delamination, and their interactions. This was achieved by cohesive zone modeling technique and enhancement of the LaRC05 failure criteria through modeling the fiber damage evolution and utilizing an efficient search algorithm to determine the matrix fracture plane and fiber kink band angle. This study underscores the efficacy of the high-fidelity modeling approach in accurately predicting both impact damage and CAI strength in typical aircraft impact damage scenarios. Additionally, it provides insights into complex CAI failure mechanisms and energy release associated with various damage modes and highlights the effect of global buckling on the failure behavior and compressive strength of composite laminates. Furthermore, it shows that the proposed fixture with support plates is suitable for testing a broader range of impact scenarios without experiencing global buckling.

Couple effects of multi-impact damage and CAI capability on NCF composites

Abstract In this study, the mechanical properties of non-crimp fabric (NCF) composite laminates under low-velocity impact and compression after impact (CAI) tests were studied by Scanning electron microscopy (SEM) and Digital image correlation (DIC) techniques. The impact response under different impact times, impact angles, and impact distance is studied. Similarly, in CAI test, DIC technique is used to reveal the whole process of NCF composite compression failure, and SEM is used to reveal the microscopic failure form. The experimental results show that the impact damage process of NCF composites has strong directivity. The concrete manifestation is that the internal failure will extend along the paving direction at the failure layer. The peak load generated under 20 J impact energy is about 1/2 of that under 40 J impact energy. The impact distance is one of the important factors affecting the coupling effect of multiple impacts, and the impact angle has little effect on the internal damage extension. The proportion of internal damage area also supports the relevant view, that is, the average difference in the proportion of internal damage area under different impact distance is about 5%, while the average difference in the proportion of internal damage area under different impact angles is about 3%. During the compression process, the main failure mode is shear failure and the failure mode is brittle fracture. The oblique fracture occurs only when the oblique is 45° and the impact distance is large (50 mm). The impact angle has little effect on the residual compression performance of NCF.

Experimental evaluation on compression-after-impact behavior of perforated sandwich panel comprised of foam core and glass fiber reinforced epoxy hybrid facesheets

Abstract To study the impact response and compression-after-impact (CAI) behavior of perforated sandwich panels comprised of foam core and glass fiber-reinforced epoxy hybrid facesheets, the hole diameter of specimens is changed in the fabrication via vacuum-assisted resin infusion. Furthermore, low-velocity-impact tests with various impact distances between the impact point and hole are carried out. With the help of the digital image correlation technique, CAI testing is conducted, and the strain evolution of specimens is monitored carefully. The mechanical response history, damage morphology, and compressive process are discussed in detail. The results show that the impact and CAI performance of specimens are weakened because of open holes. Compared with the non-perforated specimen, the maximum force of the specimen with a 6-mm hole and the 5-mm impact distance decreases by 41.21%, and its maximum displacement increases by 38.60%. During the CAI process, in comparison with the impact damage, more significant stress concentration and buckling around the hole are found.

Experimental investigations on the repeated low velocity impact and compression‐after‐impact behaviors of woven glass fiber reinforced composite laminates

AbstractIn this paper, the repeated low velocity impact responses and compression‐after‐impact (CAI) behaviors of woven glass fiber‐reinforced composite laminates were studied at the same total impact energy. Three types of impact energies, 30, 40, and 60 J, were chosen for the repeated impact tests relative to the single 120 J impact event. The impact behaviors including impact contact force–displacement and energy–time curves during the impact testing were recorded. The variations of the impact mechanical characteristics such as peak impact force, maximum displacement, and energy absorption were evaluated. The methods of visual inspection and stereo microscope were applied to identify the damage morphology of the impacted laminates. The damage accumulation was evaluated employing the absorbed energy fraction and normalized maximum displacement. It was found that the influences of impact energy on the repeated impact mechanical response are more remarkable than that of the impact number, and the impact events with fewer impact numbers cause more damage to the laminates for the same accumulative impact energy. Furthermore, the load–displacement and the CAI strength versus impact number curves were compared for the CAI testing. The ultimate load and the resulted CAI strength decrease with the increase of impact number owing to the influence of damage accumulation in repeated impacts.Highlights Repeated impact responses were studied at the same total impact energy. The impact energy makes significant influence on the impact dynamic responses. The impact events with fewer impact numbers cause more damage to the composites. The CAI behavior was explored considering the effect of damage accumulation. The CAI strength of damaged laminates decreases with impact number increasing.

Experimental and Numerical Study of Healing Effect on Delamination Defect in Infusible Thermoplastic Composite Laminates.

The integrity of delaminated composite structures can be restored by introducing a thermally-based healing effect on continuous fiber-reinforced thermoplastic composites (CFRTPC). The phenomenon of thermoplastics retaining their properties after melting and consolidation has been applied by heating the delaminated composite plates above their glass transition temperature under pressure. In the current investigation, the composite is comprised of Methyl methacrylate (MMA)-based infusible lamination resin combined with benzoyl peroxide initiator, which polymerizes into a Polymethyl methacrylate (PMMA) matrix. For the reinforcement, unidirectional 220 gr/m2 glass filament fabric was used. Delamination damage is artificially induced during the fabrication of laminate plates. The distributed delamination region before and after thermally activated healing was determined by using non-destructive testing with active thermography. An experimental approach is employed to characterize the thermal healing effect on mechanical properties. Experimentally determined technological parameters for thermal healing have been successfully applied to repair delamination defects on composite plates. Based on the compression-after-impact (CAI) test methodology, the intact, damaged, and healed composite laminates were loaded cyclically to evaluate the healing effect on stiffness and strength. During the CAI test, the 3D digital image correlation (DIC) technique was used to measure the displacement and deformation fields. Experimental results reveal the difference between the behavior of healed and damaged specimens. Additionally, the numerical models of intact, damaged, and healed composite laminates were developed using the finite element code LS-Dyna. Numerical models with calibrated material properties and tie-break contact constants provide good correlation with experimental results and allow for the prediction of the mechanical behavior of intact, damaged, and healed laminated plates. The comparison analysis based on CAI test results and modal characteristics obtained by the 3D Laser Doppler Vibrometer (Polytec GmbH, Karlsbad, Germany) proved that thermal healing partially restores the mechanical properties of damaged laminate plates. In contrast, active thermography does not necessarily indicate a healing effect.

Low-velocity impact response and compression behaviour after the impact of 3D-printed CCFR self-sensing honeycomb structures

Continuous-carbon-fibre-reinforced (CCFR) honeycomb structures with self-sensing abilities were fabricated by three-dimensional (3D) printing. Pure polylactic acid (PLA) honeycombs of the same size were also fabricated for comparison. Low-velocity impact (LVI) tests and compression after impact (CAI) tests were conducted. Cone beam computed tomography (CBCT) and scanning electron microscope (SEM) were employed to investigate the corresponding damage mechanisms. The LVI test results showed that the presence of continuous carbon fibre effectively inhibited the generation and propagation of the damage in the matrix during impact, which allowed the CCFR honeycomb structure to have better low-velocity impact resistance, especially for repeated impacts with small energies. In contrast to the PLA honeycomb, the CAI test results showed that the CCFR honeycomb still retained good specific compression strength and specific energy absorption properties even after experiencing impacts. Furthermore, the continuous carbon fibre integrated into the CCFR honeycomb could be used as a sensor element to realize the in-situ structural health monitoring during impact and compression. The 3D-printed CCFR honeycomb structure, investigated in this research, demonstrates remarkable mechanical performance and self-sensing capabilities both during LVI and CAI tests, which highlights the vast potential of 3D-printed CCFR honeycomb for diverse applications.

Methodology of residual strength prediction of composite structures with low-velocity impact damage based on NDT inspections and numerical-experimental CAI testing

One of the most challenging tasks is to evaluate a structural condition and predict a residual strength when damage is present in a composite structure. Such a prediction is usually performed based on experimental destructive testing on specimens, which often does not reflect the geometrical complexity of a component with damage, or based on numerical simulations, which are difficult to parametrize due to the complexity of fracture mechanisms of composite materials. In this paper, we proposed a hybrid methodology based on evaluation of results of the ultrasonic non-destructive testing technique supported with numerical modeling and compression after impact (CAI) tests to evaluate residual strength of glass- and carbon-reinforced polymer composite structures. Based on ultrasonic C-scans processing and incorporation of impact damage shapes in numerical models and further evaluation of the obtained results in the light of CAI tests, the model for the prediction of residual strength in composites was developed.

Full-field deformation and failure analysis for compression after impact of carbon fibre reinforced polymer laminates

Residual compressive strength after impact is a critical design factor in assessing damage tolerance of carbon fibre-reinforced polymers (CFRP) for structural applications in aircraft. This study investigated compression after impact (CAI) performance of quasi-isotropic CFRP laminates by measuring 3D deformation and strain using three-dimensional digital image correlation (3D DIC). Before the CAI, these test panels were subjected to drop-weight impact testing with energy levels representing visible impact damage (VID). During the CAI testing, deformation behaviours throughout the full loading sequence were captured by twin-camera imaging systems at the front and back faces of panels at standard speed. A single-camera high-speed imaging system captured ultimate failure behaviours at the sample front face. The single-camera high-speed system used four planar mirrors to achieve 3D DIC measurements. This study investigated the effect of boundary conditions on the compressive behaviours of the test panels, where the effect of additional support plates at the front and back surfaces to reduce the unsupported area was compared with the boundary condition of standard anti-buckling guides. The panels with added supports were found to experience deformations dominated by local buckling and final out-of-plane displacements in an S-shaped profile. These panels also exhibited residual strength reductions correlating to impact damage levels and ultimate failure initiating at the damaged region and propagating rapidly along the transverse axis. In comparison, the panels without added supports exhibited failure dominated by global buckling as manifested by large out-of-plane displacements, which had an arc-shaped profile. The DIC measurements were compared against those of mounted biaxial strain gauges. The single-camera high-speed system was shown not only to be feasible for high strain rate testing applications; it also offered a significant cost saving compared to an equivalent twin-camera high-speed system.

Experimental investigation on Compression-After-Impact (CAI) response of aerospace grade thermoset composites under low-temperature conditions assisted with acoustic emission monitoring

Low-velocity impact (LVI) can provoke catastrophic failure depending on the impact energy, lay-up configuration, and the operating temperature in fiber-reinforced composite structures via damage accumulation during the service. Additionally, extreme operating temperature is an important factor affecting the damage initiation and growth mechanism and understanding the compression-after-impact (CAI) response is a promising way to predict the damage-growth dynamics. For this purpose, this study investigates the LVI response and CAI strength of composite laminates under the −55 °C temperature considering the low-temperature (LT) service conditions of aerospace applications. The experiments are performed on cross-ply ([0/90°]4s) and angle-ply ([±45°]4s) lay-up composites for impact energy levels of 1.4 J and 10 J. After LVI tests, the occurred damages are examined by infrared thermography (IRT). Furthermore, the effect of the impact damage on the failure mechanism under compression loading is elaborated using acoustic emission (AE) inspection. The CAI test results reveal that the LT increases the compressive residual strength. However, this change is directly related to the load and lay-up configuration. The alternation in the damage growth mechanism of angle-ply laminates under LT operating temperature is more significant compared to the cross-ply laminates.

Repairing damaged composite laminates using carbon fiber reinforced bulk mold compound

This work used short carbon fibers (SCFs) of different lengths to manufacture epoxy bulk mold compound (BMC) composites. Drop-weight-impact tests were performed on the intact carbon-fiber-reinforced-polymers (CFRPs) laminates to generate artificial damage. The damaged CFRP laminates were then repaired using the SCF-BMC composites. Compression after impact (CAI) tests were conducted to measure the compressive strength of the repaired laminates. Tensile properties and compressive properties of the SCF-BMC composites were also measured. The results show that 48-mm carbon fibers result in the largest tensile modulus, tensile strength, and compressive strength, while 12-mm carbon fibers result in the largest compressive modulus. CAI test results show that the compressive strength of the repaired laminate can be restored to 80.3% of that of the undamaged laminate. Therefore, SCF-BMC composites significantly contribute to repairing damaged composite laminates.

Compression-After-Impact analysis of carbon/epoxy and glass/epoxy hybrid composite laminate with different ply orientation sequences

This article is a continuation study of the LVI and CAI response of the plain carbon fiber composite laminates and explores the experimental & numerical study of the post-impact damage propagation in carbon/epoxy and glass/epoxy hybrid laminate under compression. LVI (Low-Velocity Impact) tests are first carried out at three different energy levels, and the same specimens are put under a displacement-controlled compression test. The damage initiation and propagation in both impact tests and compression tests are studied using X-ray computed tomography. The results matched well with the results obtained from the finite element model, simulated using ABAQUS/explicit. The effect of the changing impact energy and the influence of the stacking orientation on post-impact damage propagation is studied. The strain maps during the compression tests are studied through Digital Image Correlation (DIC) technique based on which the stress–strain histories are plotted by using well-calibrated strain values. Upon completing the study, it is observed that the CAI (Compression-After-Impact) strength depends on the factors like properties of the constituent material, ply orientations and stacking sequence of the laminate. It is observed that even when the damage is barely visible on the surface of the specimen, compressive strength has been reduced drastically. The observed mode of damage propagation in CAI testing is buckling of the sub-laminates in the vicinity of the damage zone formed by the impact event. The size of the impact damage zone depends on the impact energy, which actually governs the CAI strength.

Developing Test Methods for Compression after Lightning Strikes

Research into residual strength after lightning strike is increasing within the literature. However, standard test methods for measuring residual compressive strength after lightning strikes do not exist. For the first time, a systematic experimental study is undertaken to evaluate modifications necessary to standard Compression After Impact (CAI) specimen geometry and test jig design to induce specimen failure at the lightning damage region. Four laboratory generated lightning strike currents with peak amplitudes ranging from 25 to 100 kA have been studied. Test set-up modifications were made considering the scale of the lightning damage and its potential proximity to specimen edges. Specimen geometry and anti-buckling guides were adjusted for each peak current to induce specimen failure at the lightning damage. The Compression After Lightning (CAL) strength was 28% lower than the pristine CAI strength even at a relatively low peak current of 25 kA. This study shows that the standard CAI test setup has the potential for CAL application, however, careful modifications are required depending on the peak amplitude of the applied lightning current waveform.

Glass/polypropylene hybrid knitted fabrics for toughening of thermoset composites

This paper investigates tensile, flexural, impact and compression after impact (CAI) performance of weft knitted fabric reinforced thermoset composites containing glass/polypropylene (PP) hybrid yarns. Glass and PP yarns were combined at three different PP ratios and converted to supreme hybrid knitted structures using a circular knitting machine. Using PP yarns helped to decrease glass fibre breakages during the knitting process. It was observed that the addition of PP yarns decreased overall density of composites up to 22%. Fabric tensile test indicated that using PP yarns enhanced ductility of the knitted fabrics. Flexural strength of composites was decreased by addition of PP yarns for both wale and course direction at 3 and 4-point bending tests. However, hybrid composites had higher specific flexural strength when they all had same glass fibre volume fraction for 3-point bending strength. Similar results were achieved for tensile properties whilst hybrid composites had higher values after normalising the same fibre volume fractions. Impact and CAI test results exhibited that the addition of PP yarns enhanced impact and damage tolerance of composites as well as reducing impact induced areas.

A novel methodology to research the residual compressive mechanical properties of composite stiffened panel after low velocity impact based on quantification damage simulation

AbstractComposite stiffened panels have become one of the most usual components in the design process of the aircraft's structure due to their splendid mechanical properties in the last decades. The main purpose of this work is investigating and presenting a more precise method to forecast the residual compressive mechanical behavior of the composite stiffened panel. First of all, an axial compression after impact (CAI) test is performed to understand the failure mechanism of different type of L‐shaped stiffened carbon fiber reinforced polymer panels with barely visible impact damage, which is introduced by the low velocity impact test. Subsequently, a finite element model, which contains a quantification damage model, is established in ABAQUS software based on the CAI test results to predict the mechanical behavior of these composite stiffened panels during the CAI test process. Both of the experimental and predicted results demonstrate that the propagation of the delamination caused by LVI is going to exert a destructive influence on the total failure of the composite stiffened panel, and the prediction results of the residual compressive strength of these composite stiffened panels are well consistent to that measured in the CAI test.

Experimental study on the panel size effects of the low velocity impact (LVI) and compression after impact (CAI) of laminated composites, part II: CAI

This paper is part II of a study on the CAI responses of laminated composite panels. Panels with three in-plane sizes, two thicknesses, and two stacking sequences were studied using CAI fixtures based on and modified from the industrial standard. The three in-plane sizes are 152.4 mm × 101.6 mm, 177.8 mm × 177.8 mm, and 330.2 mm × 330.2 mm. The two thicknesses are 24-ply and 48-ply. The two stacking sequences are [45/-45/0/90/0/0]ns and [45/0/-45/90]ns. The material system is IM7/977-3. Load–displacement curves, 2D/3D DIC characterization, and ultrasound C-scans are presented. Two types of CAI deformation patterns have been observed. The first type of deformation is mostly in-plane with local buckling, and its corresponding CAI-induced failure concentrates around the impacted plate center. The second type of deformation is with global buckling, and the corresponding CAI-induced failure is unrelated to the LVI-induced damage footprints. Three driving forces for the CAI failure have been identified: LVI-induced damage, global buckling-induced excessive bending, and delamination of the lateral edges. The dominant driving force is dependent on the panel’s thickness-to-length/width ratio, which is found to be the most important geometric parameter affecting the CAI behavior. The experimental results imply that the standard coupon-size (152.4 mm × 101.6 mm) CAI test results do not accurately represent the compressive load-carrying capacity of a composite panel subjected to impact damage. The results reported in this paper call for a re-examination and revision of the existing industrial standard ASTM D7137 (2017) to assess CAI load-carrying capacity.

Impact and CAI behavior of SGF mat filled sandwich panels made from foam core and FRP facesheet

The interlaminar bonding property between the facesheet and foam core plays an important role in enhancing the impact damage resistance ability of sandwich panels. In this paper, we filled short glass fiber (SGF) mat between the facesheet and foam core to enhance their bonding strength. The newly designed sandwich panels were manufactured by vacuum-assisted resin injection processing. A series of SGF mat filled sandwich panels with glass and/or carbon fiber facesheets were manufactured. The low-velocity impact as well as the compression-after-impact (CAI) performence were investigated. Failure modes of the sandwich panels were analyzed. Low-velocity impact results show that SGF mat filled sandwich panels with pure glass fiber facesheet displayed the maximum contact force. In the CAI test, wrinkling of the facesheet, buckling of the foam, and debonding between them are the major failure modes. Compared with contrast samples, SGF toughened sandwich panels show the highest strength increase rate.

Predicting compression-after-impact behavior of thermoplastic composite laminates by an experiment-based approach

An experiment-based approach for predicting the compression-after-impact (CAI) behavior and compressive residual strength of carbon fibre reinforced thermoplastic (CFRTP) composite is presented. The three-dimensional finite element model with an impact-induced damage region obtained from X-ray computed tomography analysis is developed. The unknown stiffness reduction of materials in the damaged region is determined by CAI experiments. The experiment-based bimodular nonlinear constitutive law together with a strain-based failure initiation criterion from the compressive test is developed during the simulation. The virtual CAI test is firstly validated through the comparison work between the results from finite element analysis and the experiments on the CFRTP laminate impacted by 20 J energy. Numerical investigations show the effects of plate geometries and boundary conditions on CAI behavior and residual strength of the CFRTP laminates with the same impact-induced damage.