CAI Strength Research Articles

Experimental study on the mechanical behavior of laminates made by fiber steering: influence of fiber angle and tow width

Abstract This work considers the effects of fiber trajectory and prepreg width on the material strength and failure behavior of variable stiffness composites. The test matrix was designed according to commonly used fiber placement trajectory and width of prepreg in engineering. Different trajectories were compared for tensile, compressive, bending, shear and CAI strengths. For tensile and compressive performance, curved ply specimens have varying degrees of decrease compared to straight ply specimens; For shear and bending performance, the optimal layup sequence is [±45/〈30|45〉]. For impact toughness, curved ply is significantly higher than straight ply, and the CAI strength of straight ply exceeds that of curved ply. For variable stiffness laminate with different fiber angles and tow widths, the maximum tensile, compressive and CAI strengths were reduced by up to 66%, 56%, and 27%, and the maximum bending strength, shear strength and impact toughness were increased by up to 53%, 23%, and 10.3%, respectively. The increase in width of prepreg leads to an increase in defects between fiber bundles which weakens the tensile/compressive performance but enhances the shear/bending performance.

LVI and CAI Analysis of Woven Carbon Fiber Reinforced Composite Laminates with Different Stacking Sequence

The study investigates LVI testing on woven carbon fiber-reinforced composite laminates at three different energy levels and assesses the specimens for their compressive strengths. X-ray CT scans are used to study the damaged envelopes, which allows better insights into the damage progression and failure mechanisms of the composite. The study investigates how post-impact damage propagation is influenced by alterations in impact energy and stacking orientation. The findings demonstrated that several variables, such as the characteristics of the constituent materials, the stacking order, laminate thickness, and ply orientations, had an impact on the CAI strength of the composite laminate. These findings highlight the importance of considering multiple factors when designing composite materials that can withstand impact loading and maintain their structural integrity. Even in specimens showing barely perceptible surface damage, a considerable drop in compressive strength is seen after the LVI testing. When specimens are struck with 25 J of energy, the compressive strength reduction reached a maximum value of 15.68%. In CAI testing, it is typical for sub-laminates to buckle near the impact-induced damage zone, resulting in failure. The magnitude of impact damage area can significantly affect the CAI strength, as the damaged area may create a stress concentration that can lead to buckling or other types of failure. Therefore, it is important to consider the impact energy and damage size when evaluating the CAI strength of composite materials.

Effect of sub-micron glass fiber addition on CAI strength of UD-CFRP

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Towards an improved compression after impact response of thin plies using ply grade hybridization and unsymmetrical laminate designs

With thin structures used in the aircrafts, aeronautical manufacturers are worried about the response of these thin laminates towards impact and post impact loads. Thin laminates studied to show reduced compression after impact strengths with thin plies and hence continues to be a primary concern to the aeronautical industries. In this work, we aim to use ply grade hybridization (mixing different ply grades) and unsymmetrical laminate designs as an effort to improve the low velocity impact response and compression after impact strength of thin laminates. Using different materials, uni-directional and non-crimp fabrics, through experimental, numerical and optimization methods, we demonstrate that hybridization along with unsymmetrical designs can improve the CAI strength by upto 40% over the thin ply baseline laminates.

Vol4 num4-15 X-ray tomography investigation of the use of veils to improve the impact tolerance of thin-ply composite laminates

Interleaving high fibre areal weight veils, typically used to improve the interlaminar fracture toughness and impact tolerance of composite laminates, impairs their tensile and compressive properties. We characterized the mechanical response of a thin-ply quasi-isotropic baseline laminate and laminates interleaved with two types of light (4 g/m2) co-polyamide veil: tension, compression, mode-I interlaminar fracture toughness, impact and compression after impact. X-ray tomography provided comprehensive understanding into the effect interleaving has on the microstructure, impact resistance and fracture process zone of double cantilever beam specimens. The veil fibre diameter was the key-parameter controlling the tensile properties (the veil with thinner fibres avoided resin accumulation at the interfaces and left the baseline tensile properties unaffected). The adhesion between veil fibres and resin controlled mode-I interlaminar fracture toughness (the veil with a suspected higher adhesion deflected crack propagation to the surrounding 0° plies, which enhanced mode-I interlaminar fracture toughness by 43%). Both veils reduced pristine compressive strength by up to 9%; however, the veil with thinner fibres improved CAI strength by up to 28% for impact at 14 J.

Effect of low velocity impact on residual compressive strength of stiffened panel with welding residual stress

Al-Li alloy stiffened panel is the main structural form of aircraft fuselage. Laser welding will produce welding deformation and residual stress, on the other hand, low velocity impact will also affect the residual bearing capacity of stiffened panel structure. In this study, the nonlinear finite element (FE) method is used to simulate the laser welding process of stiffened panel, and the welding residual stress is directly introduced into the subsequent impact after compression (CAI) process of stiffened panel. The effects of different impact positions and different impact energy on the residual CAI strength of stiffened panel are studied. The results show that the impact damage has the greatest effect on the residual compressive strength of stiffened panel when the punch impacts the stiffener, and the greater the impact energy is, the greater the influence is.

Low velocity and compression after impact behaviors of fiber‐reinforced MA‐g‐PP and MA‐g‐ABS matrix composites

AbstractThis study mainly focused on how one layer of fiber fabric can increase the energy absorption and post‐damage behaviors of composite structures. For this purpose, the low velocity and compression after impact behaviors of thermoplastic matrix composite materials were experimentally investigated. A total of 0.1%–0.3% maleic anhydride grafted polypropylene and 0.3%–0.5% maleic anhydride grafted acrylonitrile butadiene styrene polymers were used as the matrix materials, whereas only single‐layer plain weave aramid fiber, carbon fiber, and glass fiber fabric were used as the reinforcement materials. The impact tests were conducted at the energy values of 20, 50, and 80 J. The results showed that the aramid fiber increased energy absorption on every level of impact energy for both matrix materials, while the carbon and glass fiber fabrics only increased the energy absorption of the MA‐g‐ABS matrix. They negatively affected the energy absorption of the MA‐g‐PP polymer. No reinforcement materials had any contribution to the maximum CAI strength value of the two matrix materials, especially in the MA‐g‐ABS composites, while all reinforcement materials increased the maximum strain values of MA‐g‐ABS up to 75%. They did not increase the CAI strain values of MA‐g‐PP due to the flexible structure of polypropylene.

Interlocking thin-ply reinforcement concept for improved fracture toughness and damage tolerance

An original concept for improving the delamination resistance and damage tolerance of a composite laminate is proposed. The concept is to insert interlocked thin-ply reinforcement units between the laminae. Each reinforcement unit consists of two thin-ply layers with tabs cut into one layer, and slits cut into the other layer. The slits, and the long axis of the tabs, are parallel to the fibre direction in their respective layers. The two thin-ply layers are placed together, and the tabs are inserted through the slits, creating an interlocked reinforcement unit.The effect of the reinforcement units was quantified via mode I (DCB) and mode II (4ENF) fracture toughness tests, as well as compression after impact tests. Mode I propagation fracture toughness was increased by 77.6%, while mode II fracture toughness was not affected. In the compression after impact tests, an 11.4% reduction in delamination area was achieved but this only resulted in a 5.1% increase in CAI strength.

Impact damage and CAI strength of a woven CFRP material with fire retardant properties

Abstract This paper presents the interrogation of low velocity impact and compression after impact test results on a woven fibre composite having a fire retardant, syntactic core, two phase epoxy matrix. The results of the study were to be utilized in a decision making process regarding the appropriateness of the material usage in question for a certain aerospace application. The epoxy matrix of the material system had dispersed black-pigmented particles with flame-retarding properties. Impact tests were performed at five impact energy levels. Two different laminate layup configurations were tested. Visual and C-Scan inspection were conducted, in order to observe the extent of the damage in the composite material. Compression tests were performed to study the residual strength after impact. Analytical formulation correlations with the test results presented opportunities for quantifying the interfacial fracture toughness resistance. Micro-graphs of the specimen's cross section were also produced in an effort to observe the fractured sections and characterise the various fracture mechanisms involved. The results exploitation in terms of design decision making are presented.

In-situ toughened CFRP composites by shear-calender orientation and fiber-bundle filtration of PA microparticles at prepreg interlayer

Epoxy matrix toughened by polyethersulfone (PES) and polyamide (PA) microparticles was designed and the in-situ interlaminar toughened carbon fiber/epoxy composites were fabricated. Synergistic toughening effect of PES and PA on epoxy matrix was achieved due to semi-IPN structure of PES toughened matrix and uniform dispersion of PA microparticles. Shear-calender orientation of PA microparticles was found during prepreg processing and the microparticles remained on the surface of prepreg due to fiber-bundle filtration. The in-situ formed toughening interlayer of PA microparticles and interfacial bonding between PA and epoxy matrix were detected, which resulted in enhanced fracture toughness, CAI, and transverse flexural strength of the composite based on the PES/PA synergistically toughened matrix. SEM images of fracture morphology of the composite showed evidence of enhanced plastic deformation created by PES and PA, and crack deflection and bridging by PA microparticles.

Impact and post impact (CAI) behavior of stitched woven–knit hybrid composites

In this study, the effect of stitch pattern on the impact and post impact (CAI) behavior of eight ply woven–knit hybrid composite plates consisting of 2D woven fabrics as outer layers and rib knitted fabrics as inner layers was investigated. Layers of the woven–knit hybrid composite plates were sewed in circular, square and diamond shape through thickness in order to prevent delamination and slippage between the layers. Impact and post impact testing were carried out in order to determine the impact and post impact resistance of three different stitched composite structures. Results showed that the impact strength of composite laminate increased 5.5%, 11% and 22% in the case of square, diamond, and circular stitching, respectively. It was also found that the woven–knit hybrid composites with square pattern have the highest CAI strength.

Analytical study on low compressive strength of composite laminates with impact damage

This paper analytically investigates the mechanism of compressive strength reduction of composite laminated plates due to impact damage in quasi-isotropic laminates. Simplified mechanistic models are proposed. Explicit form solutions including the upper and lower bounds of applied load against end-shortening are derived for the local post-buckling stage by using the numerically derived load vs. end-shortening relationship of a square plate whose circular portion is allowed to deflect like a plate with circular damage. Based on the solution, explicit expressions of the average energy release rate for self-similar delamination growth are also given for the applied load as a function of the dimensions of the plate and the circular damage. Through the solutions we can roughly estimate the failure load due to delamination instability. The meaning of low CAI strength is discussed through the present solution and previous CAI experiments.

薄層化によるCFRTP積層板のCAI強度向上とそのメカニズム

薄層化によるCFRTP積層板のCAI強度向上とそのメカニズム

Influence of Fibre Architecture on Impact Damage Tolerance in 3D Woven Composites

3D woven composites, due to the presence of through-thickness fibre-bridging, have the potential to improve damage tolerance and at the same time to reduce the manufacturing costs. However, ability to withstand damage depends on weave topology as well as geometry of individual tows. There is an extensive literature on damage tolerance of 2D prepreg laminates but limited work is reported on the damage tolerance of 3D weaves. In view of the recent interest in 3D woven composites from aerospace as well as non-aerospace sectors, this paper aims to provide an understanding of the impact damage resistance as well as damage tolerance of 3D woven composites. Four different 3D woven architectures, orthogonal, angle interlocked, layer-to-layer and modified layer-to-layer structures, have been produced under identical weaving conditions. Two additional structures, Unidirectional (UD) cross-ply and 2D plain weave, have been developed for comparison with 3D weaves. All the four 3D woven laminates have similar order of magnitude of damage area and damage width, but significantly lower than UD and 2D woven laminates. Damage Resistance, calculated as impact energy per unit damage area, has been shown to be significantly higher for 3D woven laminates. Rate of change of CAI strength with impact energy appears to be similar for all four 3D woven laminates as well as UD laminate; 2D woven laminate has higher rate of degradation with respect to impact energy. Undamaged compression strength has been shown to be a function of average tow waviness angle. Additionally, 3D weaves exhibit a critical damage size; below this size there is no appreciable reduction in compression strength. 3D woven laminates have also exhibited a degree of plasticity during compression whereas UD laminates fail instantly. The experimental work reported in this paper forms a foundation for systematic development of computational models for 3D woven architectures for damage tolerance.

Damage control using discrete thermoplastic film inserts

A technique for controlling delamination growth in a GFRP panel subjected to a low velocity impact is presented. Deploying discrete thermoplastic film rings at selective interfaces centred around the impact site has been shown to arrest delamination thus allowing damage control. Using six different layup configurations the thermoplastic film rings have been shown to suppress delamination crack growth at the desired interfaces. This resulted in manipulation of the damage footprint areas. Configurations that reduced delamination area by up to 38% demonstrated an improved CAI strength of up to 18% relative to a baseline panel. However, two configurations which saw an increase in delamination area by up to 30% showed similar CAI strengths to the baseline panel. This finding would confirm that it is the delamination interface location that influences the CAI strength not the damage footprint area.

Low velocity impact and compression after impact characterization of woven carbon/vinylester at dry and water saturated conditions

The low velocity impact response and compression after impact strength of dry and water saturated plain weave carbon/vinylester composites have been determined. The composites employed T700 carbon fibers and vinylester 510A and 8084 resins. Quasi-static impact tests were conducted on dry C/VE510A and C/VE8084 to estimate the threshold impact force required to initiate damage in the composites. Falling-weight impact tests were conducted on the composites over a range of impact energies from 6.7 to 47J. Destructive inspection of damaged panels revealed damage in the form of matrix cracks as well as delamination between fiber bundles. The quasi-static estimation of the threshold impact force was in reasonable agreement with that measured in the impact test. To examine structural degradation due to impact loading, impacted panels were tested in compression (CAI). The CAI strength decreased with increasing impact energy. Absorbed moisture caused further reductions of the CAI strength.

Effect of Water Absorption on Compressive Strength after Impact and Post Impact Fatigue Behavior of Woven and Knitted CFRP Laminates

The effect of water absorption on the compressive strength after impact (CAI strength) and the post impact fatigue (PIF) properties of CFRP laminate were evaluated. Materials used were plain woven T300B-3K CFRP laminates and multi-axial knitted T700S-12K CFRP laminates molded by the vacuum assisted resin transfer molding (VARTM) method with vinylester as the matrix. Coupon specimens, which were applied the impact energy (1 J per unit thickness), were soaked in a water bath at 95oC for 120 hours up to the saturation water content of 0.6 wt%. CAI and PIF strengths of ‘Dry’ and ‘Wet’ specimens of plain woven CFRP showed almost same values. Thus, for T300B plain woven CFRP laminates, the effect of water absorption on PIF properties was small. On the other hand, multi-axial knitted CFRP laminates showed the large effect of water absorption, and Wet specimens showed lower PIF properties than Dry ones. However, net CAI and PIF strengths of multi-axial knitted CFRP were larger than that of plain woven CFRP.

Experimental characterization of strength and damage resistance properties of thin-ply carbon fiber/toughened epoxy laminates

Composite laminates manufactured from thin-ply prepregs are expected to have superior damage resistance properties compared to those from standard prepregs. Although the use of thin-ply prepregs leads to increase in manufacturing cost, the damage resistance properties against matrix cracking and delamination increase. This study investigates several strength properties as well as the damage resistance properties of carbon fiber/toughened epoxy composite laminates for the applicability of thin-ply prepregs to aircraft structures. Specifically, compressive strengths of open hole laminates (OHC strength) and laminates after impact loadings (CAI strength) are investigated by the comparison of results between laminates manufactured from the thin-ply prepregs and the standard prepregs. It is shown that laminates using thin-ply prepregs have superior strength compared to those using standard prepregs. It is expected that the design limit of composite aircraft structures increase by using thin-ply prepregs.

ＣＦＲＰ積層板の衝撃後残留圧縮強度特性に及ぼす吸水・環境温度の影響

Effects of water absorption and thermal environment on compression after impact (CAJ) charac-teristics of CFRP laminates are studied. Specimens were immersed in 71°C water for 10, 000 hours (410 days) to quantify the ratio of water absorbed in the CFRP laminates. Water content of the specimen was 1.4 wt% after the water absorption test. Numerical simulation using FEM code was conducted to predict moisture diffusion based on the measured orthotropic diffusion constants. Then impact tests by the drop-weight were carried out at room temperature at normalized energy level of 3.3 J/mm. Compression tests after impact tests were carried out at various temperatures, i.e., RT, 32°C, 177°C, for water absorbed specimens, and -54°C, RT, 82°C, 121°C, 149°C, 177°C for dry specimens. Delamination area and the number of transverse cracks of the water absorbed CFRP were smaller than those of dry specimens. Such smaller delamination area of wet specimens provides higher CAI strength than dry specimens. CAI strength of the water absorbed CFRP decreased significantly at high temperature because the glass-transition temperature deteriorates seriously due to the water absorption.

Effect of Stitching on Impact and Interlaminar Properties of Graphite/Epoxy Laminates

Effects of through-the-thickness stitching on the impact damage resistance, impact damage tolerance, and interlaminar fracture toughness (Mode I and Mode II) of plain woven and uniweave textile graphite/epoxy laminates were investigated. The uniweave textile was formed by weaving dry carbon-fiber tows with fiberglass fill tows. The content of fiberglass fill tows was 2.5% by weight. The plain woven laminates were manufactured using resin infusion molding and the uniweave laminates by resin transfer molding. Kevlar and glass yarns of various yarn numbers were used for stitching. Static Indentation-Flexure, Compression-After-Impact, Double Cantilever Beam and End-Notched Flexure tests were conducted. Stitching did not have any effect on the onset of impact damage. However, stitching leads to significant improvement (25-40%) in impact damage tolerance as measured by CAI strength and impact damage area. Mode I fracture toughness as characterized by critical strain energy release rate (G,Ic) was found to increase by at least an order higher (15-30 times) than the unstitched laminates. Mode II fracture toughness (GIIc) increased by 5-15 times over the unstitched laminates. New methods to estimate the Mode II critical strain energy release rate in the stitched laminates are presented. The stitched textile advanced composites are considered potentially superior to prepregs for high-volume, low-cost and high-performance structural materials.