Compression After Impact Test Research Articles

Low velocity impact and compression after impact behaviour of polyester pin-reinforced foam filled honeycomb sandwich panels

Hybridization of sandwich panels and their different components have drawn huge attention due to the significant improvement in their attributes. Hybrid core of ‘Polyester Pin-reinforced Foam filled Honeycomb Sandwich panels’ (PFHS) were fabricated and compared with unreinforced ‘Foam filled Honeycomb Sandwich panels’ (FHS) in terms of low velocity impact and Compression After Impact (CAI) performance. The impact damage area was calculated by employing MATLAB image processing technique. Incorporating through thickness pins for connecting faces and core is an effectual way to improve interfacial bonding, specific bending stiffness and also imparts out of plane properties for sandwich panels. The low velocity impact tests performed on the sandwich panels revealed that the polyester pin reinforcement in foam filled honeycomb sandwich panel improved the load bearing capacity, total absorbed energy and reduced the impact damage area significantly. In CAI test, debond, wrinkling of face sheet, and buckling of face sheet and core are the major modes of failure. The addition of the pins enhanced the compressive strength for all the impact energy levels.

Design and Validation of a Modified Compression-After-Impact Testing Device for Thin-Walled Composite Plates

Thin-walled fibre reinforced composites like carbon fibre reinforced plastics are very susceptible to strength reductions due to low-velocity impact damages. In aerospace engineering, the dominating failure mechanisms of impact damaged composite structures are usually investigated based on the compression after impact (CAI) test procedure. This enables the determination of the influence of impact damages on the static residual compressive strength. CAI testing procedures are typically applicable to composite plates with thicknesses larger than 3–4 mm. If thinner panels are used, they typically fail near one of the loaded edges of the CAI device, in particular the area of the free edge (which is needed for compressing the panel) and not within the free measuring area. As a consequence, the investigated samples cannot be used as valid tests for the evaluation of the residual strength in CAI testing. In order to enable an investigation of the residual strength of thin-walled plates in CAI testing, a CAI testing device is developed based on an available CAI fixture and a standardized one. For comparability reasons, this new device exhibits the same dimensions as standardized fixtures. It shows a significant improvement with respect to standardized devices concerning the measurement of mechanical behaviour during CAI testing.

Low-velocity impact response and compression after impact behavior of tubular composite sandwich structures

This work investigates the dynamic impact response of tubular composite structures with/without honeycomb sandwich core under transverse low-velocity impact (LVI) and compression after impact (CAI) test. Damage mechanisms, such as matrix cracking, delamination and fiber breakage/rupture in face sheets as well as honeycomb crushing and buckling in the core, are characterized by X-ray micro-computed tomography (μCT) to understand failure processes and their relationship with core material and impactor shape. It is found that sandwich core material helps to absorb impact energy and resist localized damage formation. The benefit of core material includes greater energy absorption capability and higher specific CAI strength. The progressive damage events from damage initiation, failure propagation to final collapse of a tubular composite structure during CAI test are also further discussed.

Impact Damage Resistance and Post-Impact Tolerance of Optimum Banana-Pseudo-Stem-Fiber-Reinforced Epoxy Sandwich Structures

Banana fiber has a high potential for use in fiber composite structures due to its promise as a polymer reinforcement. However, it has poor bonding characteristics with the matrixes due to hydrophobic–hydrophilic incompatibility, inconsistency in blending weight ratio, and fiber length instability. In this study, the optimal conditions for a banana/epoxy composite as determined previously were used to fabricate a sandwich structure where carbon/Kevlar twill plies acted as the skins. The structure was evaluated based on two experimental tests: low-velocity impact and compression after impact (CAI) tests. Here, the synthetic fiber including Kevlar, carbon, and glass sandwich structures were also tested for comparison purposes. In general, the results showed a low peak load and larger damage area in the optimal banana/epoxy structures. The impact damage area, as characterized by the dye penetration, increased with increasing impact energy. The optimal banana composite and synthetic fiber systems were proven to offer a similar residual strength and normalized strength when higher impact energies were applied. Delamination and fracture behavior were dominant in the optimal banana structures subjected to CAI testing. Finally, optimization of the compounding parameters of the optimal banana fibers improved the impact and CAI properties of the structure, making them comparable to those of synthetic sandwich composites.

Low-Velocity Impact and Compression-After-Impact Behaviour of Flax Fibre-Reinforced Composites

This study details the low-velocity impact and compression-after-impact (CAI) behaviour of flax fibre-reinforced polymer (FFRP) composites. The impact resistance, energy absorption efficiency and residual compressive strength as well as the damage pattern of the FFRP composites are compared with the corresponding features of glass fibre-reinforced polymer (GFRP) composites, and the effect of the stacking sequence of FFRP composites is also investigated. The results show that the cross-ply FFRP composites have the highest impact resistance, whereas the multi-directional ply composites have the lowest impact resistance but the highest energy absorption efficiency. The energy absorption efficiency of the FFRP composites is greater than that of the GFRP composites, but the penetration resistance and residual compressive strength of the FFRP composites are lower than those of the GFRP composites with the same stacking sequence, mainly due to the lower tensile strength and elongation at fracture of the FFRP composites. It is also reported that the damage pattern of the FFRP composites is localised cracking and delamination, unlike the overall delamination failure exhibited in the GFRP composites after CAI testing. Finally, the failure mechanisms of the FFRP and GFRP composites are detailed.

The low-velocity impact and compression after impact (CAI) behavior of foam core sandwich panels with shape memory alloy hybrid face-sheets

Abstract Due to the properties of shape memory effect and super-elasticity, shape memory alloy (SMA) is added into glass fiber reinforced polymer (GFRP) face-sheets of foam core sandwich panels to improve the impact resistence performance by many researchers. This paper tries to discuss the failure mechanism of sandwich panels with GF/ epoxy face-sheets embedded with SMA wires and conventional 304 SS wire nets under low-velocity impact and compression after impact (CAI) tests. The histories of contact force, absorbed energy and deflection during the impact process are obtained by experiment. Besides, the failure modes of sandwich panels with different ply modes are compared by visual inspection and scanning electron microscopy (SEM). CAI tests are conducted with the help of digital image correlation (DIC) technology. Based on the results, the sandwich panels embedded with SMA wires can absorb more impact energy, and show relatively excellent CAI performance. This is because the SMA wires can absorb and transmit the energy to the outer region of GFRP face-sheet due to the super-elasticity-behavior. The failure process and mechanism of the CAI test is also discussed.

An Experimental Study on the Low Velocity Impact Response of Repaired Carbon Fibre/Benzoxazine Composites

As structure materials, carbon fibre/benzoxazine (CF/BZ) composites are prone to damage, which compromises their structural performance significantly. Scarf repair is an effective means of restoring structure performance. In this paper, one side scarf repair CF/BZ (OR-CF/BZ) and both sides scarf repair CF/BZ (BR-CF/BZ) were prepared to investigate the influence of repair geometry on low velocity impact response of adhesively bonded scarf repair in damaged CF/BZ laminates. The intent of prepare BR-CF/BZ is to reduce the repair area. The CF/BZ laminates are removed the damage area with a rectangle cutout from one side and two sides respectively and repaired with scarf patches. Low velocity impact with different impact energy levels are performed to investigate the impact performance of repaired composites. Visual and CScan inspection are conducted, in order to observe the extent of the damage in the repaired composites. Compression after impact (CAI) are undertaken to measure their residual strength. Micro-graphs of the specimen’s cross-section are also produced in an effort to observe the fractured sections and characterized the various fracture mechanisms involved. Results show that the damage areas after impact of BR-CF/BZ is larger than that of OR-CF/BZ and CF/BZ. The penetrating damage of CF/BZ, OR-CF/BZ and BR-CF/BZ occur at 40 J, 50 J and 60 J impact energy respectively. The CAI strength of BR-CF/ BZ is higher than that of OR-CF/BZ and CF/BZ. While the CAI modulus does not change greatly. The failure mechanism includes impact damage failure and growth of compressive failure during CAI test. The impact point is the most seriously damaged zone of the CAI specimens.

A model of low-velocity impact damage of composite plates subjected to Compression-After-Impact (CAI) testing

The paper presents a simplified model of damage of composite plates (SDM) subjected to low-velocity impact testing. The damage model assumes reduced thickness of laminate plies versus impact energy. The decrease in strength of the lay-up was measured by Compression-After-Impact (CAI) tests. The numerical analysis was performed on composite plates made of CFRP laminate subjected to uniform compression, with represented regions of damage caused by different impact energies. The numerical results show the behaviour of the laminate during compression, taking into account the damage process evaluated by a progressive damage criterion. The numerical analysis was performed using the ABAQUS® programme. The results were verified based on experimental findings reported in the literature and the numerical results of the tested composite plates subjected to CAI testing.

Compressive strength of composite laminates with delamination-induced interaction of panel and sublaminate buckling modes

Compression After Impact (CAI) strength is critical to the safety and weight of carbon fibre aircraft. In this paper, the standard aerospace industry practice of using separate analyses and tests for panel buckling and CAI strength is challenged. Composite panels with a range of stacking sequences were artificially delaminated and subject to compression testing in a fixture that allowed local sublaminate and global panel buckling modes to interact. Compared to panels without delamination, interaction of buckling modes reduced panel buckling strains by up to 29%. Similarly, compared to delaminated panels restrained against panel buckling, interaction reduced delamination propagation strains by up to 49%. These results are the first to indicate that restriction of interaction during CAI testing is unconservative and therefore potentially unsafe. A novel integration of an analytical Strip model, for sublaminate buckling driven delamination propagation, and a Shanley model, for determining increased local strain due to sublaminate-buckling-induced panel curvature, is used to calculate the reduction in strength due to buckling mode interaction. Assuming a typical sublaminate post to pre-buckling stiffness ratio of 0.65, the difference in integrated model and experimental results is<11% – a level of accuracy that will allow the integrated model to drive initial design studies.

Experimental study of compression after impact of asymmetrically tapered composite laminate

This paper presents an experimental study of CAI (compression after impact) of tapered composite laminate. Two types of layup with thickness changing from 4mm to 6mm are considered. A new CAI testing rig, suitable for the specimens and a brief description of impact damage are presented first. Then, the behavior of impacted specimens under compressive loading is discussed along with the effects of ply drop-off parameters (taper angle, ply drop-off disposition and configuration) and impact point location on the failure mechanism. Finally, the residual strength in CAI of tapered laminates is compared with equivalent flat laminates. The results show that compressive behavior of the specimens is mostly governed by a coupling between compression and bending, generated by the discontinuity of the neutral axis in the tapered region. Despite this difference of behavior with flat laminates, the presence of ply drop-off has little effect on the residual strength in CAI.

Assessment of residual strength of repaired solid laminate composite materials through mechanical testing

Robust residual strength test methods are critical for assessing the validity of new repair techniques for restoring strength to damaged composite laminates. In this study, the strength of pristine, damaged and repaired solid laminate composite material has been investigated by performing four different mechanical tests. Impact damage was imparted to carbon fibre epoxy resin specimens using a drop tower instrument. Specimens were repaired using a novel resin injection technique employing a low viscosity adhesive as the repair resin. The compressive strength of specimens was determined using compression after impact (CAI) and large-scale CAI test methods. Tensile tests were performed to obtain tensile strength. Flexural strength was evaluated by four-point bend tests. The thoroughness of each test method in determining the success of the repair was investigated. This study demonstrates that the strength restoring capacity of a repair method is subject to the test method used to assess the residual strength. The four-point bend test was shown to be the best preliminary test method for screening repair techniques as it is not affected by loading anomalies. However, design criteria should always be considered when choosing a residual strength test method for assessing repair validity.

Delaminations growth in compression after impact test simulations: Influence of cohesive elements parameters on numerical results

The compressive strength of impact damaged composite laminates may be reduced by ply damage and delaminations, which may grow under load. At coupon level, the residual strength of impacted laminates is assessed through compression after impact (CAI) tests; as a complement to the experiments, numerical tools are sought that predict the behavior of delaminated composites.In this context, the paper investigates the influence of the cohesive elements parameters – the inter-facial strengths and the cohesive mesh size – in CAI test simulations. The study is carried out using a recently developed initialization technique that allows CAI simulations be run independently from the impact simulations (used to calculate induced delaminations).The first part of the study uses a conventional sequential approach (CAI analyses follows the impact simulations which share the same parameters) to asses the sensitivity of force time histories and in-plane delaminations growths to parameters variation. Then, for a selected initial damage scenario, the inter-facial strengths sensitivity is performed also evaluating the computational efficiency.The study shows how the cohesive parameters affect both the accuracy and the computational cost of the analyses and contributes to establish guidelines for an effective use of cohesive elements in CAI tests simulations.

Study on the low-velocity impact response and CAI behavior of foam-filled sandwich panels with hybrid facesheet

Foam-filled sandwich panels with six types of facesheets were manufactured by vacuum-assisted resin injection (VARI) process. Low velocity impact test and compression after impact (CAI) test of the prepared panels were performed. Failure modes of different sandwich panels were analyzed by observing the contact surface and the cross section through damage region. Scanning electron microscope (SEM) was carried out to study the microstructure of the impacted specimens. It is found that matrix cracking, fiber breaking, foam cracking and debonding are the damage modes of sandwich panel with pure carbon facesheet, while other specimens merely show some of these modes after impact test. Panels with pure carbon facesheet have poor impact resistance properties, while panels with pure glass facesheet have preferable properties among all the specimens. Hybrid panel with ply mode of [C2/G2/Foam core/G2/C2] could provide higher maximum contact force and absorb more impact energy by carbon fiber breaking on the contact surface. In CAI test, wrinkling of facesheet and buckling of foam and skin are the two main damage modes. [C4/Foam core/C4] shows highest compress strength decline rate, while [G4/Foam core/G4] shows lowest rate. Decline rate of [C2/G2/Foam core/G2/C2] is very low but the compress strength of its undamaged specimen is not high.

A new device for determining the compression after impact strength in thin laminates

In this work a new device has been developed to estimate compression-after-impact (CAI) strength. This device allows the testing of laminates thinner than those recommended by CAI test standards. The proposed device is composed of a support structure, with a set of vertical ribs that stabilize the specimen during the test, increasing the buckling load. A numerical analysis was made to ensure that global buckling does not occur in the laminate during the CAI test, and that there is no interference with the damage area. Laminate specimens were tested with the proposed device and the ASTM device. For specimens 4.416mm thick (thickness according to ASTM D7137 standard), the test results were similar with both devices. For thinner laminates, higher CAI strength was estimated with the proposed device than with the ASTM device, showing that the global buckling was delayed.

Characterization of failure modes in compression-after impact of glass–epoxy composite laminates using acoustic emission monitoring

This paper investigates the effect of impact velocity on compression-after impact (CAI) strength, while the different failure modes arising during CAI test are characterized using the acoustic emission (AE) technique. For this purpose, CAI GFRP laminates manufactured as per ASTM D7137 were subjected to impact velocities such as 3.5, 5 and 6 m/s at room temperature. Impacted laminates were then subjected to compression tests using a specially fabricated fixture: AE activity was monitored during loading. Frequency analysis and location analysis were performed on the AE data obtained during CAI test of GFRP specimens impacted at different velocities to investigate the nature and extent of damage.

Compression-after-impact (CAI) performance of epoxycarbon fibre-reinforced nanocomposites using nanosilica and rubber particle enhancement

One of the problems in the design of automotive structures and body parts made by fibre reinforced composites is that these materials are susceptible to a small energy impact caused by for instance, accidental tool drop during maintenance or stone strike while in operation. This often lead to a barely visible impact damage which causes reduction in compressive strength of the composite part. To increase the impact tolerance of the composites, toughening agents like silica nanoparticles and rubber particles can be utilized to toughen the resin. To understand the effect of the particles enhancement, the impact tolerance was evaluated utilizing Compression After Impact (CAI) test after the impact induced by gas- gun impacting equipment. The results from CAI test after 20 J impact (high energy stone strike) shows about 30% improvement in residual compressive strength for the nanosilica enhanced composite compared to unmodified CFRP. Also C-scan results after 7 J impact shows about 50% smaller delamination area for the nano-enhanced composite.

Observations of damage development from compression-after-impact experiments using ex situ micro-focus computed tomography

Abstract The development of damage mechanisms leading up to compression-after-impact (CAI) failure is studied in particle-toughened and untoughened systems. Microfocus computed tomography (μCT) enabled non-destructive monitoring of the internal damage development in three-dimensions (3D) by taking scans after impact, after an application of near failure compression loads and after coupon failure. In combination with μCT work, mechanical CAI testing and ultrasonic C-scans were conducted to determine the effect of the projected damage area on residual CAI strength and to complement the observations made from μCT scans. The important role of the undamaged “cone” of material immediately under the impact site for out-of-plane sublaminate stability is identified. The implication of delamination growth into this region is discussed. It was found that where particle-toughened systems suppressed delamination growth into this region, greater residual CAI strength was maintained on a like-for-like projected damage area.

Deformation Measurement of Composite Laminate with Impact Damage under Compressive Loads

The out-of-plane displacement fields for a cross-ply laminate [03/903]s with impact damage under axial compression are investigated using carrier electronic speckle pattern interferometry (carrier-ESPI) in this paper. The compression after impact (CAI) test is conducted on rectangular specimen 65mm × 87.5mm in size using an anti-buckling fixture. In order to apply a uniform load to the top of composite laminate, a self-designed device insures that an accurate loading is given. The experimental results reveal the deformation characteristics of laminate under CAI test.

Impact damage resistance of CFRP with nanoclay-filled epoxy matrix

The influence of nanoclay on the impact damage resistance of carbon fibre–epoxy composites has been investigated using the low-velocity impact and compression after impact (CAI) tests. The load–energy vs. time relations were analyzed to gain insight into the damage behaviours of the materials. The CFRPs containing organoclay brought about significant improvement in impact damage resistance and damage tolerance in the form of smaller damage area, higher residual strength and higher threshold energy level. The presence of nanoclay in the epoxy matrix induced the transition of failure mechanisms of CFRP laminates during the CAI test, from the brittle buckling mode to more ductile, multi-layer delamination mode. Addition of 3 wt% clay was shown to be an optimal content for the highest damage resistance.

炭素繊維／高靭性ポリイミド樹脂複合材料ＭＲ５０Ｋ／ＰＥＴＩ５の衝撃後圧縮（ＣＡＩ）強度特性について

In this study CAI (Compression After Impact) strength of a carbon/tough-polyimide composite material, MR50K/PETI5, where PETI5 resin was newly developed for the next generation SST at NASA was investigated. First, impact damage was given to CAI specimens at room temperature and the load response was measured during the impact test and the damage of the specimens was observed by an ultra sonic C-scanner and 3-D ultrasonic inspection system. Second, CAI strength and failure strain at room and high temperatures were measured. Third, the damage caused by the CAI test was observed using an optical microscope. Test results are compared with those of T800H/PMR-15 carbon/polyimide composite, where PMR-15 is a typical brittle resin, and the characteristic CAI behavior and strength of a carbon fiber/highly-tough resin composite was clarified.