Compression After Impact Strength Research Articles

Damage evolution behavior of CFRP laminates under post-impact fatigue with water absorption environment

In this study, the damage evolution behavior was evaluated considering the effect of the textile structure and water absorption. Damage observation was conducted by the integration of non-destructive and direct observation methods. Candidate textile reinforcements were T300-3k plain woven fabric (PW) and T700S-12k multi-axial knitted fabric (MA). The effect of water absorption on the performances of compression after impact (CAI) and PIF were small in PW CFRP laminates. Conversely, PIF properties of water-absorbed MA CFRP laminates drastically decreased than that of dry ones. CAI strength was not affected by water absorption. PIF performance of dry MA CFRP was fairly higher than that of the others. From the precise observation, some evidences of interfacial deterioration caused by water absorption were confirmed in both PW and MA CFRP laminates.

Impact characterization of sandwich structures with an integrated orthogrid stiffened syntactic foam core

This paper presents the manufacturing, testing, and modeling of a new composite sandwich structure with a hybrid grid stiffened core. The hybrid core consists of a continuous fiber reinforced polymer orthogrid skeleton that is filled in with light weight syntactic foam in the bays or cells. The grid skeleton was manufactured by a dry weaving process per the pin-guided filament winding technology. Low velocity impact tests and compression after impact (CAI) tests were conducted to evaluate the impact response and residual strength of the sandwich structure. C-scan and SEM observation were implemented to investigate the impact damage. A finite element analysis using ANSYS was conducted to validate the compression test results. It is found that this integrated core enhances impact energy transfer, energy absorption, and positive composite action, insures quasi-static response to impact, and has higher CAI strength.

Effect of hygrothermal condition on compression after impact strength of CFRP laminates

This study investigated the combined effects of water absorption and thermal environment on compression after impact (CAI) characteristics of CFRP laminates. Specimens were immersed in 71 °C water for over 10,000 h to quantify the ratio of water absorption in the CFRP laminates. Numerical simulation based on Fick’s law was conducted to predict moisture diffusion using the measured orthotropic diffusivities. Compression tests after drop-weight impact tests were carried out at various temperatures, i.e. −54 °C, 22 °C, 82 °C, 121 °C, 149 °C, 177 °C. Delamination area and the number of transverse cracks of the wet specimen were smaller than those of dry specimens. Smaller delamination area of the wet specimens provides higher CAI strength than the dry specimens at mid-high temperatures of 22 °C, 82 °C and 121 °C. The CAI strength of the wet CFRP decreased slightly at 149 °C and significantly at 177 °C because the glass transition temperature deteriorates seriously due to the water absorption. The failure mode changes at high temperature, i.e. impact induced delamination is not the trigger of the failure but buckling induced crippling causes final failure at 177 °C.

Mechanical performance of carbon fibre-reinforced composites based on stitched preforms

A comparative assessment of the influence of pure assembly seams based on a thin (11 tex) polyester yarn in a zigzag geometry on the resulting mechanical performance of a non-crimped fabric (NCF) carbon fibre-reinforced epoxy composite manufactured by vacuum-assisted resin transfer moulding is presented. This study was aimed at generating a solid foundation regarding the overall performance level of stitched NCF composites and at identifying critical property changes. The comprehensive evaluation of the mechanical composite properties includes static as well as dynamic tests of the in-plane properties as well as a characterisation of the interlaminar properties such as apparent interlaminar shear strength (ILSS) and compression after impact (CAI). It is demonstrated that mechanical properties such as the tensile and compression stiffness and CAI strength are not degraded by the chosen stitching parameters, whereas the tensile and compression strength, ILSS as well as the tensile fatigue behaviour are reduced as a result of pronounced localised fibre ondulations. A direct comparison to properties of a commonly used 5H satin woven fabric composite verifies that the overall performance of these particular stitched NCF composites must be enhanced with regard to the identified key criteria to meet the level required for aircraft applications and in order to maintain the performance advantage of NCF composites as compared to standard woven fabrics in general. Promising approaches include the use of different yarn materials based on soluble thermoplastics and/or modified stitching parameters.

Impact properties of laminated composites with stitching fibers

In order to characterize the outstanding performance of three-dimensional (3D) composites, the low velocity impact test has been carried out. 3D fiber structures have been achieved by using the automated tape placement (ATP) process and a stitching method. Materials for the ATP and the stitching process were carbon/epoxy prepreg tapes and Kevlar fibers, respectively. Two-dimensional (2D) composites with the same stacking sequence as 3D counterparts have also been fabricated for the comparison of damage tolerance. For the assessment of damage after impact loading, specimens were subjected to C-scan nondestructive inspection. Compression-after-impact (CAI) tests were conducted to evaluate residual compressive strength. The damage area of 3D composites was greatly reduced (30–40%) compared with that of 2D composites. Although the CAI strength did not show drastic improvement for 3D composites, the ratio of retained strength was 5–10% higher than 2D samples. The effect of stitching on the impact performance was negligible above the energy level of 35 J.

Compression Strength of Stitched Laminates after Low-velocity Impact

Tests are conducted using stitched laminates in order to calculate the unnotched failure stress and the compression after impact (CAI) strength of the stitched laminates. Unnotched laminates, laminates with a hole, and laminates with impact damage are used in the tests. All the specimens are made of two different types of layup sequence of laminates with three different stitching directions - 0, 45, and 90. In this paper, an FEM model is developed using the MSC PATRAN software to predict the CAI strength of stitched laminates. Two failure criteria are applied to the model to predict the CAI strength, the point stress criterion (PS) and the fiber breakage in damage zone (FD) criterion. The influence of the moduli retention ratio (Mr) is investigated. The predicted CAI strengths show a good agreement when compared with the test results.

Compression-after-Impact Strength of Sandwich Panels with Core Crushing Damage

Compression-after-impact (CAI) strength of foam-cored sandwich panels with composite face sheets is investigated experimentally. The low-velocity impact by a semi-spherical (blunt) projectile is considered, producing a damage mainly in a form of core crushing accompanied by a permanent indentation (residual dent) in the face sheet. Instrumentation of the panels by strain gauges and digital speckle photography analysis are used to study the effect of damage on failure mechanisms in the panel. Residual dent growth inwards toward the mid-plane of a sandwich panel followed by a complete separation of the face sheet is identified as the failure mode. CAI strength of sandwich panels is shown to decrease with increasing impact damage size. Destructive sectioning of sandwich panels is used to characterise damage parameters and morphology for implementation in a finite element model. The finite element model that accounts for relevant details of impact damage morphology is developed and proposed for failure analysis and CAI strength predictions of damaged panels demonstrating a good correlation with experimental results.

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.

Impact damage and CAI strength of MR50K/PETI5 carbon/tough-polyimide composite at room and high temperatures

This study determined the compression-after-impact (CAI) behavior and strength of a carbon/tough-polyimide composite material, MR50K/PETI5, whose PETI5 resin has been specially developed for the next generation supersonic transport (SST) at NASA. First, an impact test imposed impact damage on CAI specimens at room temperature, and the load and absorbed energy were measured; in addition, the damage was examined with an ultrasonic C-scanner and a 3D ultrasonic inspection system. Second, CAI strength and failure strain were measured at room temperature and at 180° C. Third, the damage induced by the CAI test was examined with an optical microscope. Test results were 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 were determined.

Strength and Impact Damage Characteristics of Al7075/CFRP Sandwich Panel

Carbon fiber reinforced aluminum layer laminate (CARALL), which were consisted of various numbers of CFRP prepregs, adhesive films and aluminum sheets, were fabricated by an autoclave. Impact damage behaviors of carbon fiber reinforced plastic (CFRP) and CARALL were investigated. The impact damage resistance of CARALL was significantly higher than that of CFRP. Compression after impact (CAI) tests was conducted to investigate the effect of impact damage on the compression strength of CARALL and CFRP specimens. The CAI strength of CFRP and CARALL decreased with increase of impact energy. However, CAI strength of CARALL is higher than that of CFRP, which can be attributed to the excellent impact damage absorbability of aluminum sheet in CARALL.

Measurements and prediction of the compression-after-impact strength of glass knitted textile composites

Two methods were proposed in order to correlate the test results and predict the compression-after-impact (CAI) strength of glass knitted textile composites. The Linear regression analysis was used to establish a linear relationship between damage width and CAI strength. An important finding was that for a wide range of knit architectures, which had similar failure modes, the slope of the regression line for normalised CAI strength was constant, consequently, the slope of the regression line for actual CAI strength varied linearly with undamaged strength. This implies that increased damage tolerance for these knitted composites can be achieved only at the expense of the undamaged strength and CAI performance will be higher for the material with the lower undamaged strength. The second approach involved idealising the impact damage in the knitted composite as a circular hole. Using the semi-empirical failure criterion (point stress model), which necessitates the use of a characteristic length (determined from experiment), the CAI strength was predicted. Comparisons made between predicted and measured CAI strengths revealed good agreement, with differences less than 5% in most cases. A major finding from this work is that for knitted composites, one characteristic length is applicable for a range of damage geometries and knit architectures. This was not the case with composites made from unidirectional prepreg, where the characteristic length depends on the lay-up configuration and hole geometry. Comparing the predictions of the point stress model with the linear regression analysis, it was shown that the stress model generally provided a more accurate prediction of the CAI strength of the knitted composites.

Compression after impact strength of composite sandwich panels

Two types of sandwich panels with carbon epoxy skins and aluminium honeycomb core were subjected to low velocity impacts and then the damaged panels tested for their compression-after-impact (CAI) strength. One type of panel was found to be a very robust energy absorber, i.e. a thick-skin thin-core option. The other panels with their thin skins and thick core were found to penetrate easily whereupon the impactor forced the back-face to debond massively. These panels then had the worst CAI strength. Both the impact damage and the CAI behaviour––to failure––were simulated using a finite element model of skins and core. The model gave very respectable agreements with the impact tests even though the damage in skin and core were extremely complex mechanisms, involving progressive fibre fracture in the skin and elasto-plastic deformation in compression and shear in the honeycomb core. The model for CAI strength predicted well the failure load and the mode of propagation from the damaged zone.

Morphology and compression‐after‐impact strength relationship in interleaved toughened composites

AbstractMorphology and compression after impact (CAI) strength relationships in interleaved toughened high performance composites are investigated using a quantitative two‐dimensional image analysis approach. A group of six quasi‐isotropic carbon fiber‐epoxy composites with identical compositions, but having variations in CAI strengths, were analyzed to study how the interleaf particle size, particle size distribution, and location of particles in the interlaminar region affect the CAI strength values and the corresponding damage mechanisms. It is found that the CAI strength of interleaved‐toughened composites is significantly affected by the size and size distribution of toughener particles in the interlaminar regions of the composite. In general, high CAI strength composites exhibit more uniform particle size distribution throughout the interlaminar regions. Whereas, for low CAI strength composites, the interleaf particles tend to cluster together and have varied size distribution.

A New Method for Compression After Impact Strength Prediction of Composite Laminates

A new method to determine the compression after impact (CAI) strength of composite laminates was presented. In this method, an impact damage zone was modeled as an equivalent hole. The most outstanding characteristic of the method was that the simplification of the impact damage was based on the compressive failure mechanisms of impacted laminates. Such a simplification has not been discussed in the literature before. A technique was established for determining the shape and size of the equivalent hole. To the authors’ knowledge, it is the first practical method for simplifying the impact damage as a hole, and also the present work was the first attempt to predict the CAI strength by modeling the impact damage as a hole. The stress distribution around damage was calculated using the complex potential method and the classical lamination theory. A lay-up independent failure criterion was used to predict CAI strength. The predictions of the present approach were compared to test results and found to be in very good agreement over a wide variety of materials. The study indicated that the width of damage zone was a key factor governing CAI strength. The influence of damage area, impact energy, impactor shape and dimension, etc. on CAI strength can be essentially characterized by their effects on the damage width. The study provided a very simple and effective approach for CAI strength prediction compared with previous methods.

Compressive Strength of Composite/Concrete Cylinders after Low Energy Impact

This paper deals with the repair of the concrete cylinder by fiber reinforced composites fabricated through the filament winding method. Composites fabricated by the method can reinforce concrete cylinders efficiently because they provide comprehensive and multi-axial reinforcing effects. In this work, three winding angles were adopted to wind six layers of glass/epoxy composite jacket onto the concrete cylinder. These three winding angles combined with different lay-up sequence form seven types of composite/concrete cylinders. The compression after impact (CAI) test was conducted on the seven composite/concrete cylinders. These experiments were designed to investigate the influence of winding angles and lay-up sequence of the composite on the CAI strength. It is found that the highest CAI strength among the composite wound concrete cylinder is 4.81 times higher than that of the concrete cylinder without the reinforced composite. In addition, the former has higher failure strain than the latter. The impact resistance can be evaluated by a compression after impact/compression before impact (CAI/CBI) strength ratio. It is also detected that although impact damage has been introduced, the fracture of composite/concrete cylinder under compression may not be initiated from the impact site. Moreover, the failure strengths are influenced by damage mechanisms of the cylinders, and the damage modes for the concrete cylinder and wound composites are related. In this paper, the damage mechanisms of different wound concrete cylinders under CAI test were discussed.

Compressive Strength of Composite/Concrete Cylinders after Low Energy Impact

This paper deals with the repair of the concrete cylinder by fiber reinforced composites fabricated through the filament winding method. Composites fabricated by the method can reinforce concrete cylinders efficiently because they provide comprehensive and multi-axial reinforcing effects. In this work, three winding angles were adopted to wind six layers of glass/epoxy composite jacket onto the concrete cylinder. These three winding angles combined with different lay-up sequence form seven types of composite/concrete cylinders. The compression after impact (CAI) test was conducted on the seven composite/concrete cylinders. These experiments were designed to investigate the influence of winding angles and lay-up sequence of the composite on the CAI strength. It is found that the highest CAI strength among the composite wound concrete cylinder is 4.81 times higher than that of the concrete cylinder without the reinforced composite. In addition, the former has higher failure strain than the latter. The impact resistance can be evaluated by a compression after impact/compression before impact (CAI/CBI) strength ratio. It is also detected that although impact damage has been introduced, the fracture of composite/concrete cylinder under compression may not be initiated from the impact site. Moreover, the failure strengths are influenced by damage mechanisms of the cylinders, and the damage modes for the concrete cylinder and wound composites are related. In this paper, the damage mechanisms of different wound concrete cylinders under CAI test were discussed.

Cooling rate influences in carbon fibre/PEEK composites. Part III: impact damage performance

The effect of cooling rate on impact damage performance of carbon fibre/polyether ether ketone (PEEK) matrix composite is characterised based on the instrumented drop-weight impact test, scanning acoustic microscopy (SAM) damage evaluation and compression-after-impact (CAI) test. Judging from the incipient impact load, incipient impact energy and total damage area, the ability to resist damage initiation upon impact was higher in the order of fast-cooled carbon/PEEK, slow-cooled carbon/PEEK and carbon/epoxy laminates. Furthermore, the threshold impact energy was higher and the CAI strength reduction rate was lower for the fast-cooled specimen than the slow-cooled counterpart, strongly indicating higher impact damage tolerance of the former system. The present study demonstrates that the impact damage performance and other important properties of carbon/PEEK composites can be optimised, if not maximised, by proper control of processing conditions, especially the cooling rate.

Predicting the compressive engineering performance of carbon fibre-reinforced plastics

Abstract This paper examines the compressive strength data of a recent experimental study [Smith FC. The effect of constituents’ properties on the mechanical performance of fibre-reinforced plastics. PhD thesis. Centre for Composite Materials, Imperial College, April 1998] concerned with the evaluation of a range of engineering properties of continuous carbon fibre/epoxy composites subjected to static tensile and compressive loading. A plastic fibre kinking analysis [Budiansky B. Micromechanics. Comput Struct 1983;16(1):3–12] and a linear softening cohesive zone model (CZM) [Soutis C. Compressive failure of notched carbon fibre–epoxy panels. PhD thesis. Cambridge University Engineering Department, UK, 1989; Soutis C, Fleck NA, Smith PA. Failure prediction technique for compression loaded carbon fibre–epoxy laminates with an open hole. J Comp Mat 1991;25(5):1476–1498] are used for the prediction of the unnotched and open hole compressive strength (OHC) of unidirectional and multidirectional laminates made of six different commercially available CFRP prepregs. Damage introduced by drop-weight (low-velocity) impact is modelled as an equivalent open hole and the cohesive zone model [Soutis C. Compressive failure of notched carbon fibre–epoxy panels. PhD thesis. Cambridge University Engineering Department, UK, 1989; Soutis C, Fleck NA, Smith PA. Failure prediction technique for compression loaded carbon fibre–epoxy laminates with an open hole. J Comp Mat 1991;25(5):1476–1498] is applied to estimate compression-after-impact (CAI) strength. The unnotched strength is accurately predicted from the knowledge of initial fibre misalignment and the shear yield stress of the composite, while the difference between the theoretical and experimental OHC and CAI strength results in most cases is less than 10%.

Compression after impact (CAI) strength of concrete cylinders reinforced by non-adhesive filament wound composites

Concrete cylinders reinforced by filament wound composites were fabricated, and the compressive strength of the composite/concrete cylinders was tested after low-energy impact. In this study, a glass fiber woven cloth wrapping method and a filament winding technique were adopted to wrap the concrete cylinder. In order to investigate the influence of composite/concrete interfacial bonding on the compression after impact (CAI) strength, aluminum foil was introduced into the composite/concrete interface; thus, the compressive behavior of the composite/concrete system with or without inserting the aluminum foil was compared. The effects of aluminum foil in the curing process were also revealed. Based upon the results of this study, the placement of aluminum foil can significantly enhance the compressive strength of the composite/concrete cylinder. The CAI strength depicts that the winding angle used in the filament winding process can significantly influence the reinforcing effects. Among the tested cylinders, the FWIC (defined in the text) [902/±60/902] cylinder shows the highest CAI strength—129.4 MPa—which is 4.5 times higher than the impacted concrete cylinder and 4.1 times higher than the pure concrete cylinder. Fracture mode was also investigated on the cylinder reinforced by the composites wound with different winding angles. The placement of aluminum foil reduces shear stress transfer across the composite/concrete interface, which affects not only the impact response but also the compressive strength.

Impact resistance and tolerance of interleaved tape laminates

This paper presents and discusses the results of low-velocity impact and compression-after-impact (CAI) tests conducted on interleaved and non-interleaved carbon/epoxy tape laminates. Olefin film interleaves provided a strong interface bond, resulting in a reduction in projected damage area. These interleaves changed the stress distribution under impact and restricted delamination formation at the ply interface. An investigation into the compression behaviour of these laminates revealed a reduction in undamaged strength using olefin interleaves. This was attributed to the lack of lateral support for fibres at the fibre/interleaf interface, allowing fibre microbuckling to occur at a low load. Low modulus copolyamide web interleaves resulted in an increase in damage area and minor changes to CAI strength. Examination of laminate cross-sections revealed that this was due to both the open structure of the interleaf and poor resin/interleaf adhesion. High shear modulus polyethylene interleaves resulted in a significant decrease in damage area at various impact energies, with CAI strength improved compared to the non-interleaved laminates.