Interlaminar Shear Properties Research Articles

A review of the effect of stitching on the in-plane mechanical properties of fibre-reinforced polymer composites

This paper reviews over fifty studies into the effect of through-the-thickness stitching on the in-plane mechanical properties of fibre-reinforced polymer composites. Reviewed are the in-plane tensile, compressive, flexure, interlaminar shear, creep, fracture and fatigue properties, although little work has been undertaken on the last three properties. When comparing studies it is apparent that many contradictions exist: some studies reveal that stitching does not affect or may improve slightly the in-plane properties while others find that the properties are degraded. In reviewing these studies it is demonstrated that predicting the influence of stitching on the in-plane properties is difficult because it is governed by a variety of factors, including the type of composite (eg. type of fibre, resin, lay-up configuration), the stitching conditions (eg. type of thread, stitch pattern, stitch density, stitch tension, thread diameter), and the loading condition. The implications of these findings for the use of stitching in lightweight engineering structures are discussed.

Effects of Structure and Microcracks on the Flexure, Interlaminar Shear and Impact Properties of Three-Dimensional Composites

Three-dimensional carbon fibre/epoxy resin composites were prepared, which had either three-axis or five-axis carbon fabrics (3D-3A or 3D-5A) in four woven gauges, and Teflon® was embedded in the fabric structures to initiate various types of microcracks. The flexural, interlaminar shear, and impact properties of these 3-D composites were studied. The results indicated that the flexural properties of 3D-3A were better than those of 3D-5A. However, 3D-5A structures gave better impact strength compared to 3D-3A. The interlaminar shear strength depended on the weave gauge. The effect of microcracks on the properties of 3-D composites was generally kept within 10%, but transverse microcracks lowered the flexural properties of 3-D composites by 10–20%. The 3D-3A composites showed cracks along the interface between the Z axial bundles and the matrix, whereas the 3D-5A showed 45°-axial bundle pull-out.

Effects of Structure and Microcracks on the Flexure, Interlaminar Shear and Impact Properties of Three-Dimensional Composites

Three-dimensional carbon fibre/epoxy resin composites were prepared, which had either three-axis or five-axis carbon fabrics (3D-3A or 3D-5A) in four woven gauges, and Teflon® was embedded in the fabric structures to initiate various types of microcracks. The flexural, interlaminar shear, and impact properties of these 3-D composites were studied. The results indicated that the flexural properties of 3D-3A were better than those of 3D-5A. However, 3D-5A structures gave better impact strength compared to 3D-3A. The interlaminar shear strength depended on the weave gauge. The effect of microcracks on the properties of 3-D composites was generally kept within 10%, but transverse microcracks lowered the flexural properties of 3-D composites by 10–20%. The 3D-3A composites showed cracks along the interface between the Z axial bundles and the matrix, whereas the 3D-5A showed 45°-axial bundle pull-out.

Carbon fibre composites based on polyimide/silica ceramers: aspects of structure-properties relationship

Polyimide/silica ceramers, based on the products of the hydrolysis of tetraethoxysilane (TEOS) and a commercial poly(amic acid) solution, were used to fabricate unidirectional carbon fibre composites, which were subsequently evaluated with respect to thermal and mechanical properties. There is evidence to suggest that the silica component of these ceramers is present as dispersed discrete particles at low silica concentration (i.e. ∼7 wt%) and as fine interconnected domains trapped within the polyimide matrix at higher silica content (i.e. ∼14 wt%). The dimensions of the silica domains were in the region of 7–20 nm. Carbon fibre composites produced from ceramer solutions (CF/ceramers) were found to exhibit lower thermal expansion and a greater retention of flexural and interlaminar shear properties at elevated temperature than the corresponding polyimide-matrix composites (CF/polyimide). The properties of CF/ceramers were generally better for systems containing the higher amount of silica and were improved further by lowering the pH value of the precursor ceramer solution. This is believed to have resulted from the enhanced fluidity of the ceramer gel within the pre-impregnated fibres, giving rise to a higher packing density of the fibres and a more homogeneous distribution of fibres. CF/ceramers were also found to exhibit a better thermal oxidative stability at 350°C than the corresponding CF/polyimide, although a substantial amount of porosity developed in the case of ceramers with the higher silica content.

In-plane and interlaminar shear properties of carbon/epoxy laminates

Carbon/epoxy laminates of three different lay-ups have been tested to measure both in-plane shear and interlaminar shear (ILS) properties by the Iosipescu shear method. A reasonably pure state of shear strain is achieved in the region between the roots of two V-notches in all the laminates. However, the assurance of proper shear failure modes is extremely difficult for some lay-ups on certain shear planes. It is found in the tests of ILS properties that delamination or ILS cracking initiates at load levels much lower than the ultimate so that uncertainty arises as to which stress should be used as the measure of the material property. It is shown that the ILS stiffnesses deteriorate gradually in a non-linear fashion even after the onset of delamination. It is also found that the in-plane shear properties increase with increase in number of reinforcing fibre directions. The ultimate ILS stresses of specimens with fibres in the length direction (if ultimate loads are used) seem to be independent of material lay-up while the ILS moduli steadily decrease with increase in number of reinforcing fibre directions.

Flexural and Interlaminar Shear Properties of C/C Composite at Elevated Temperatures

Flexural and interlaminar shear properties of carbon fiber reinforced carbon (C/C) composite, fabricated by “the preformed yarn method”, were evaluated at elevated temperatures up to 2300°C in vacuum. Four point flexure test and short beam test were carried out on the C/C composites of unidirectional, cross-ply and quasi-isotropic laminates. Effect of absorbed gas on the composite strength was also studied. The results revealed that flexural strength increase with increasing temperature, and was not profoundly affected by absorbed gas. On the other hand, interlaminar shear strength (ILSS) was remarkably lowered by absorbed gas at room temperature, and the test on the degased specimen showed that ILSS was kept constant up to 2300°C. Hence the absorbed gas is shown to lower matrix strength but not fiber strength. Young's modulus of C/C composites was kept constant up to 1500°C, then decreased at 2000 and 2300°C, that might be originated form temperature dependence of the Young's modulus of the carbon fiber.

The Effects and Non-Destructive Evaluation of Defects in Thermoplastic Compression-Loaded Composite Cylinders

Measurements and predictions of the effects of voids and ply waviness on compression and interlaminar shear properties of thermoplasticmatrix composites are presented. Correlations between ultrasonic Cscan loss to voids and to compression strain-to-failure are described. The data are drawn from a database of seventy-six cylinders and cylinder sections fabricated by Du Pont Advanced Material Systems since 1988 using an insitu consolidation filament winding and tape laydown process. Composites with graphite (AS-4 and IM-7) or S-2 glass fibers in polyetheretherketone (PEEK) or polyetherketone ketone (PEKK) thermoplastic matrices are included. Property measurements are from various mechanical and external hydrostatic compression pressure tests on cylindrical shells intended for marine submersible applications and on autoclave or compressionmolded flat panels. Performance data show the negative impact of voids and ply waviness on mechanical properties and cylinder performance. NDE data show a correlation between ultrasonic loss and void content only over a broad void content range; over the important 0% to 2.0% range, no correlation is observable. Data directly relating C-scan loss with compression strain-to-failure show a continuous loss of performance with increasing void content.

Effect of fuming nitric acid surface treatment of ultra‐high modulus polyethylene fibers on the mechanical properties of their composites

AbstractThe effect of hot fuming nitric acid (FNA) treatment on the adhesion of ultra‐high modulus polyethylene fabrics to an epoxy resin has been investigated. Mechanical and molecular characterization of the interface has been attempted. Fourier transform infrared diffuse transmittance spectroscopy has been used to monitor the chemical changes introduced by the FNA treatment as well as the nature of the interface between the fibers and the epoxy resin on the molecular level. Scanning electron microscopy has been used to examine the morphological consequences of the FNA treatment. Flexural and interlaminar shear properties of the composites have been measured as a function of the extent of surface treatment. Esterification of the FNA treated polyethylene fibers is used to examine the role of surface functionality to the mechanical performance.

Effect of strain rate on interlaminar shear properties of carbon/epoxy composites

In this paper, the effect of strain rate on interlaminar shear properties of laminates is studied. The material tested was a T300/5208 carbon/epoxy composite, and the range of strain rates explored was about 10 −3 − 10 3 s −1. The specimens used were designed and optimized by finite element analysis, and the calculations are presented here. One of the specimens permitted the determination of the interlaminar shear modulus, G 13, and the other permitted the determination of the interlaminar shear strength, S 13. No influence of testing speed on interlaminar properties was observed at low, intermediate and high strain rates. Fracture surfaces were studied by scanning electron microscopy: a slight difference was observed between specimens tested at low and high strain rates.

Fiber-Matrix Adhesion and Its Effect on Composite Mechanical Properties: I. Inplane and Interlaminar Shear Behavior of Graphite/Epoxy Composites

An experimental investigation was performed to establish the relationships between fiber-matrix adhesion as determined by single fiber interfacial shear strength tests with the inplane and interlaminar shear properties of graphite/epoxy composites. ±45°-tension, Iosipescu, and short beam shear tests were conducted on three identical sets of composites differing only in their fiber-matrix interfacial shear strength. The fiber-matrix interphase and consequently the interfacial shear strength was varied by using the same graphite fiber was different surface modifications, namely untreated, surface treated, and surface treated and coated with a thin layer of epoxy. The surface modification changed the interfacial shear strength by more than a factor of two, while the properties of fibers remained unchanged. The experimental results showed that both inplane and interlaminar shear strengths increased approximately in the same ratio as the interfacial shear strength, however, the inplane shear modulus was relatively insensitive to the fiber surface modifications. The fracture surface analysis revealed that when the fiber matrix interfacial shear strength was increased from low to intermediate to high values, the major failure modes changed from primarily interfacial failure to a combination of interfacial and matrix failure to primarily matrix failure, respectively, in a manner identical to that observed with the single fiber fragmentation tests.

Crosslinking‐property relationships in PMR polyimide composites. Part I

AbstractThis study examines for the first time how matrix crosslinking affects the composite physical and mechanical properties of a graphite fiber reinforced PMR polyimide composite during long‐term isothermal aging. Unidirectional composite specimens of Celion 6000/PMR‐P1 were isothermally exposed at 288°C in air for various time periods up to 5000 h. The matrix crosslink densities were estimated from the kinetic theory of rubber elasticity and shifts in the glass transition temperatures (Tgs). The Tg, coefficient of thermal expansion, density, weight loss, moisture absorption, and elevated temperature flexural and interlaminar shear properties were also determined. Several linear relationships were found between the matrix crosslink density and composite physical and mechanical properties. The Tg, initial weight loss and density, and elevated temperature interlaminar shear strength increase with an increase in crosslink density. Conversely, the initial moisture absorption and coefficient of thermal expansion decrease with increasing crosslink density. As expected, the elevated temperature flexural strength and modulus show no direct correlations with crosslink density. Further, after achieving the highest matrix crosslink density, several of the composite properties begin to decrease rapidly. These findings suggest that time‐temperature dependent nature of attaining the maximum matrix crosslinking is closely linked to the onset of the composite property degradation. Though much more work is needed, a fundamental understanding of the relationships between matrix crosslinking and composite physical and mechanical property can provide a scientific basis for the prediction of the extent of composite service life not only for PMR polyimides but also for other thermosetting matrix resins, such as epoxies and bismaleimides.

Static Strength of Adhesively Bonded ARALL-1 Joints

ARALL® Laminates are a family of structural composite materials which consist of thin aluminum alloy sheets alternating with aramid fiber/epoxy prepreg layers developed for fatigue-critical applications requiring light gage sheet. To obtain the benefits associated with ARALL Laminates, the joint regions must be designed to carry the required loads without incurring a substantial weight penalty. This paper considers the static strength of adhesively bonded ARALL-1 single and double lap joints. Single and double lap joints with various overlap lengths were fabricated using two standard ARALL thicknesses: 3/2 ARALL-1 (three layers of aluminum with 2 prepreg layers) and 5/4 ARALL-1 (five layers of aluminum with four prepreg layers). These joints were tested in uniaxial tension. A shear lag analysis based on failure of the adhesive was used to predict the failure load as a function of the joint configuration and overlap length. A finite element analysis was used to determine the interlaminar shear and tensile stresses within the laminate. The results of these two analysis techniques were compared to the experimental results. The results show that the maximum adhesive and adherend stresses calculated using the shear lag analysis agree with those calculated using finite element techniques. While the shear lag analysis gave accurate predicted failure loads for 312 ARALL-1 single and double lap joints, the predicted failure loads for 5/4 ARALL-1 single lap joints were generally higher than the experimental failure loads. For the 5/4 ARALL-1 specimens, the experimental failure loads were dependent on the observed failure mode. The premature failure can be attributed to interlaminar tensile and shear stresses in the prepreg layers closest to the bondline. The interlaminar failure observed in these joints can be predicted using the interlaminar shear and tensile properties and an appropriate interaction equation.

Analyses of the mechanical properties and microstructure of bamboo-epoxy composites

Bamboo reinforced epoxy possesses reasonably good properties to waarrant its use as a structural material, and is fabricated by utilizing bamboo, an abundant material resource, in the technology of fibre composites. Literature on bamboo-plastics composites is rare. This work is an experimental study of unidirectional bamboo-epoxy laminates of varying laminae number, in which tensile, compressive, flexural and interlaminar shear properties are evaluated. Further, the disposition of bamboo fibre, the parenchymatous tissue, and the resin matrix under different loading conditions are examined. Our results show that the specific strength and specific modulus of bamboo-epoxy laminates are adequate, the former being 3 to 4 times that of mild steel. Its mechanical properties are generally comparable to those of ordinary glass-fibre composites. The fracture behaviour of bamboo-epoxy under different loading conditions were observed using both acoustic emission techniques and scanning electron microscopy. The fracture mode varied with load, the fracture mechanism being similar to glass and carbon reinforced composites. Microstructural analyses revealed that natural bamboo is eligibly a fibre composite in itself; its inclusion in a plastic matrix will help solve the problems of cracking due to desiccation and bioerosion caused by insect pests. Furthermore, the thickness and shape of the composite can be tailored during fabrication to meet specific requirements, thereby enabling a wide spectrum of applications.

In-Plane and Interlaminar losipescu Shear Properties of Various Graphite Fabric/Epoxy Laminates

The Iosipescu shear test method was used to measure the in-plane and interlaminar shear properties of four T300 graphite fabric/Fiberite 934 epoxy composite materials. Weave geometries tested included an Oxford weave, a 5-harness satin weave, an 8-harness satin weave, and a plain weave with auxiliary warp yarns. Both orthogonal and quasi-isotropic layup laminates were tested. In-plane and interlaminar shear properties were obtained for laminates of all four fabric types. Overall, few differences in shear properties attributable to the fabric weave pattern were observed. However, the auxiliary warp material was significantly weaker and less stiff in interlaminar shear parallel to its fill direction.

Effects of 0.5-MeV electrons on the interlaminar shear and flexural strength properties of graphite fiber composites

Composite samples of graphite fibers in both an epoxy matrix and a polyimide matrix have been prepared with different fiber arrangements and irradiated with 0.5-MeV electrons up to 104 Mrad. Interlaminar shear strengths decreases significantly with radiation dosage while little change was observed in the flexural strengths and moduli. The results indicate that radiation causes significant degradation at the interface but not much change in the fiber or matrix.

Elevated temperature testing for comparison of glass/resin composites

AbstractStatic test methods were used to evaluate and compare the thermal and mechanical properties of several glass/thermoset laminated composites between 25° and 400°C. The unidirectional matrix composites consisted of phenolic‐modified epoxy, epoxy novolac, epoxy, and modified phenolic resins. These materials were selected as potential alternative materials for rotary compressor vanes. Dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), and thermogravimetric analysis (TGA) techniques were selected to evaluate elevated temperature performance. The short‐beam shear test was chosen to measure interlaminar shear properties. The results indicated that an elevated‐temperature matrix, such as the modified phenolic resin, may not result in optimum composite strengths. Instead, an epoxy resin reinforced with glass fibers provides a better balance between elevated‐temperature performance and interlaminar shear strength. The test results of this study, in addition to being adequate for discriminating the materials for initial selection purposes, were obtained quickly and easily. Moreover, the thermal results provide a more realistic understanding of composite elevated‐temperature characteristic than do those of the present standard test.

Interlaminar shear properties of graphite fiber, high‐performance resin composites

AbstractShear properties of laminates consisting of graphite fiber (Celion 6000, Celion 3000, and T‐300) and several resins (epoxy, hot melt‐bismaleimide, solvent bismaleimide, polyimide, and polystyrylpyridine (PSP)) were measured using the short‐beam test. The interlaminar shear strength of epoxy composites was hightest. All other resin composites showed considerably lower shear properties. The shear strengths deereased with temperature, with the loss up to 100°C being more pronounced. The PSP composites did not show a loss in strength in the temperature range investigated (room temperature to 250°C). Boiling the composite samples in water for 24 h resuled in negligible reduction in shear strength in all cases. The interlaminar shear properties depended on the fiberresin interfacial bond, the wetting characteristics of the resin, and the resin meechanical properties. Attempts were made to determine to what extent each of these factors controlled the shear properties of the composites studies. Thus, the interfacial bond between the resin and the single fiber was determined. Results showed that the lower shear strength of polyimide, bisimides and PSP composites, as compared to epoxy resin composites, seemed to be due to their corresponding low interfacial bond strengths. The subsequent decrease in shear strength with temperature appeared to be directly correlated to the decrease in bond strength. The wettability of carbon fiber tow with all these resins was also determined. The small variations in wetting characteristics of the resin did not seem to justify the rather large differences in shear‐strength properties observed.

Interlaminar shear properties of some glass‐fiber‐reinforced phenolic and polyester laminates

AbstractChopped strand mat and glass‐fabric‐reinforced thin and thick laminates were made from phenolic and polyester laminating resins using three modifications of a hand lay‐up method. Short‐beam interlaminar shear strengths were determined in three‐point bending at a span‐to‐depth ratio 3:1. Plane and perpendicular specimens were used. It was found that the perpendicular specimens were more useful. Interlaminar delamination energies were determined statically and dynamically. It was found that the results for the phenolic and polyester laminates cannot be simply compared because of differences in the fracture mechanism.

Predicting shear and internal bond properties of flakeboard

Relations defining interlaminar shear and rail shear in terms of specific gravity and flake alinement of flakeboard have been developed for several flake types. Equations relating internal bond to specific gravity are also given. Prediction of rail shear values using the above equations was relatively accurate. Considerable variation existed in predictions of interlaminar shear and internal bond properties.

THE EFFECT OF THE MARINE ENVIRONMENT ON STRESSED AND UNSTRESSED GRAPHITE/EPOXY COMPOSITES

ABSTRACTThree types of graphtte/epoxy composite materials were subjected to stressed (static and fatigue) and unstressed prolonged immersion in tap and synthetic seawater. One material consisted of a Iaminate comprising undirectlonally oriented high tensile strength graphite “prepreg” tape, the second material consisted of a combination high tensile strength and ultra high modulus “prepreg” tapes oriented In a [0/ ± 45] pattern, and the third comprised a high tensile strength “prepreg” woven fabric laminate. The data developed show the effects of a marine environment on the interlaminar shear and flexural properties of the composites.