Short Beam Interlaminar Shear Research Articles

Development of an integrated sacrificial sensor for damage detection and monitoring in composite materials and adhesively bonded joints

Quality assurance of adhesively bonded joints is of vital importance if their benefits are to be exploited across a wide range of industrial applications. A novel lightweight, low-cost, non-invasive embedded sacrificial sensor is proposed, capable of detecting damage within an adhesively bonded joint, which could also be used in a laminated composite structure. The sensor operation uses changes in electrical resistance, increasing as the sensing material area diminishes with damage progression. Initial tests prove the sensor concept by showing that the electrical resistance of the sensor increases proportionally with material removal, mimicking the sensor operation. Thermography is used to verify the current flow through the sensor and that any localised heating caused by the sensor is minimal. Short beam interlaminar shear strength (ILSS) tests show that embedding sensors in a composite laminates did not cause a reduction in material interfacial structural performance. Finally, the in situ performance of the sensor is demonstrated in quasi-static tensile tests to failure of adhesively bonded single lap joints (SLJs) with sensors embedded in the bond line. Prior to crack initiation, an electrical response occurs that correlates with increasing applied load, suggesting scope for secondary uses of the sensor for load monitoring and cycle counting. Crack initiation is accompanied by a rapid increase in electrical resistance, providing an indication of failure ahead of crack propagation and an opportunity for timely repair. As the crack damage propagated, the electrical response of the sensor increased proportionally. The effect of the sensor on the overall structural performance was assessed by comparing the failure load of joints with and without the embedded sensor with no measurable difference in ultimate strength. The research presented in the article serves as an important first step in developing a simple yet promising new technology for structural health monitoring with numerous potential applications.

Modelling of deformation behavior of polymeric woven fabric prepreg-based composite materials in flexure and short beam interlaminar shear testing

The mechanisms of deformation behavior of woven fabric prepreg-based carbon-fiber composites with different reinforcement lay-up schemes are studied. The FEM stress-strain analysis is carried out for specimens in flexure and short beam interlaminar shear testing environments. Both methods are based on a three-point bend model. Theoretical grounds underlying the mechanisms governing the deformation of laminates in bending tests are studied. Tests were performed on specimens having different thickness and with different ratios of the distance between supports to the specimen thickness. The effects of the monolayer properties specified in the model, such as the modulus of elasticity in the z-direction and the interlaminar shear modulus, on the effective characteristics of the material are investigated. The problems of correct experimental determination of the interlaminar shear modulus related to the large thickness of specimens, the difficulty of manufacturing and the peculiarities of measurement and identification of the obtained values of deformation in shear are considered. Flexural modules versus actual monolayer thickness and unsupported lengths versus specimen thickness and width are plotted. The necessity of registration of shear deformation in three-point bend testing for proper design of experiments and execution of calculations is shown. The accuracy and adequacy of the structural and bending characteristics are verified against the testing modes. The results confirm the relevance and validity of any given factor in modeling the stress-strain state of tested specimens.

Short beam interlaminar shear behavior and electrical resistance-based damage self-sensing of woven carbon/epoxy composite laminates in a cryogenic environment

This article investigates the interlaminar shear behavior and damage detection of woven carbon fiber reinforced polymer composite laminates at cryogenic temperatures. Short beam shear tests were performed at room temperature and liquid hydrogen temperature (20 K), and the temperature dependence of the apparent interlaminar shear strength was examined. The electrical resistance of the composite specimens was also monitored during the tests. A detailed observation of the tested specimens was made to assess the damage, and the relationship between the damage and the electrical resistance was discussed. In addition, the stress, strain and current density distributions in the short beam shear specimens were determined by the finite element method. The numerical results were used to better understand and explain the experimental findings.

Effect of Sewing Threads on Interlaminar Shear Strength and Flexural Bending Strength of Stitched Non-Crimp Carbon Fabric Laminates

Interlaminar shear strength and flexural bending strength of stitched and non-stitched carbon fibre reinforced polymer (CFRP) laminates were studied with reference to three different sewing threads. The preforms were stitched through-the-thickness by two different hybrid carbon fibre threads and a Zylon (polyphenylen-2,6-benzobisoxazol, PBO) thread with 16 stitches/cm2 stitch density. The short beam interlaminar shear strength of the laminate increases where as flexural bending strength of the laminate shows mixed results depending on the stitching direction. Carbon fibre threads show relatively less positive influence on the laminate properties than the PBO thread. The results of this study differ partly from the literature studies.

Characterisation of thin silica films deposited on carbon fibre by an atmospheric pressure non-equilibrium plasma (APNEP)

Abstract Low modulus carbon fibres were used to investigate the potential of a low cost novel atmospheric pressure microwave plasma technique for continuously depositing silica coatings onto carbon fibres, in a tow form. The objective was to improve the interfacial properties of aluminium/carbon fibre composites produced by liquid metal infiltration techniques. Amorphous silica coatings, approximately 50–400 nm thick, were successfully produced in a continuous process. The nature and morphology of the coatings were determined using transmission electron microscopy. Squeeze cast and gas pressure infiltrated samples were manufactured to investigate the fibre/matrix interface using tensile tests and a short beam interlaminar shear test. The tensile samples displayed brittle fracture with ultimate tensile stresses of 223 and 251 MPa for the uncoated and coated fibres, respectively. The shear samples did not show any interlaminar shear, but failed in tension. It is concluded that the silica coating did not have a significant effect on improving the carbon fibre/aluminium matrix interface. However, the microwave plasma technique was very successful and can be modified to produce low cost alternative coatings for future development.

Flexural strength and interlaminar shear strength of stitched GRP laminates following repeated impacts

The flexural strength and interlaminar shear strength of stitched and non-stitched glass-reinforced plastic (GRP) laminates were studied under conditions of increasing impact energy and increasing number of repeated impacts. The GRP was stitched through the thickness with Kevlar thread in two orientations with a low or high stitch density. The Mode I interlaminar fracture toughness, GIc, increased with stitch density whereas the Mode II toughness, GIIc, was not changed by stitching. The three-point flexural strength and short-beam interlaminar shear strength of the GRP before impact loading were reduced by stitching as a result of stitching damage. Under short-beam loading, the stitches become sites of stress concentration and this contributed to the reduction in interlaminar shear strength. The strengths of the laminates were reduced slightly with increasing impact energy after one impact. The strengths were reduced considerably with increasing number of impacts. The laminates suffered severe microstructural damage under repeated impacts, including shear cracking of the resin, delaminations, and crushing and fracture of the glass fibres. The impact damage resistance, the post-impact flexural strength and the interlaminar shear strength of the GRP were not improved by stitching, and this finding differs from other impact studies on stitched composites.

Interlaminar shear test by hybrid sandwich specimen under distributed load

AbstractA new method is proposed for the determination of the interlaminar shear strength of composites. The method is particularly pertinent to composites of high interlaminar shear strengths, where the ratio of tensile (compressive) strength to shear strength is relatively low. In such materials, including unidirectional composites with improved fiber/matrix bond strength and angle‐ply laminates, an analysis based on a short beam interlaminar shear test is highly problematic and may, in fact, be erroneous. The test method is based on the use of a sandwich composite structure with a core made of layers of the tested composite and skins made of an elastic, strong unidirectional composite. A proper design procedure determines the choice of the skin material and of the relative thicknesses, so that flexural testing under distributed load leads to the intended core failure in shear. Calculations of the stress profile in a hybrid sandwich beam in bending and of the stress ratios under distributed load are presented. Also presented are experimental results recorded with sandwich hybrids made of unidirectional carbon‐fiber‐reinforced epoxy skins and a ±θ aramid‐fiber‐reinforced epoxy angle‐ply core.

The shear fatigue of unidirectional fibre composites

The shear fatigue of unidirectional composites of carbon, glass and Kevlar 49 fibre, has been studied by low frequency torsion cycling. During either constant shear stress or shear strain amplitude cycling there is initially a slow decrease in shear modulus, strain amplitude increasing and stress amplitude decreasing approximately linearly with log (number of cycles), due to fibre/matrix debonding, until eventually macroscopic cracks initiate and propagate at a clearly definable fatigue life. The rate of change of shear modulus prior to cracking increases, and the fatigue life decreases, with increasing stress or strain amplitude. Fatigue in this mode of testing is considerably more severe than in short beam interlaminar shear fatigue, with fatigue lives of 10 3–10 4 cycles at 50% of ultimate stress or strain.

Shear strength of the carbon-resin bond in carbon-fibre-reinforced epoxies

The shear stiffness and strength of uniaxial specimens of carbon-fibre-reinforced epoxy resin have been measured by use of a torsional testing machine. Both type I (high stiffness) and type II (high strength) fibres were tested in the range 30-70% fibre by volume and the results compared with those for unreinforced resin. A quantitative study was made of the effect on these properties of the hypochlorite surface treatment designed to promote fibre-resin adhesion.The results have been analysed in terms of calculated stress concentration factors and the ideas of fracture mechanics. It is concluded that for type II fibres the surface treatment is fully effective in that resin failure is more likely than bond failure, at least for this particular resin. For type I fibres, however, a stronger treatment should be investigated, and possible improvements in the technique of manufacture of materials with high fibre content are indicated.A comparison has been made of torsional measurements with the results of short-beam interlaminar shear tests. The usefulness and limitations of the latter technique are assessed.

Finite indentation of elastic-perfectly plastic membranes by a spherical indenter.

The beryllium wire epoxy composites displayed a yieldingtype tensile stress-strain curve with a proportional limit 30 to 40% of ultimate strength and yield strengths 80 to 96% of ultimate. In the 0-90° bidirectional composites tested in tension, there was 100% conversion of the strength and modulus of the beryllium wire. In compression, the beryllium wire plastic composites failed by plastic deformation, and in the long span flexure, they failed by excessive flexural deformation followed by filament fracture. Short beam interlaminar shear specimens failed by excessive flexural deformation rather than delamination. Inter laminar shear strength of the beryllium wire epoxy composites was not degraded by a two hour water boil. The ballistic impact resistance of beryllium wire epoxy composites was about 5 times that of glass fiber reinforced epoxy composites having similar lay up and thickness. V. Conclusions Based on preliminary processing and mechanical property investigations of plastic composites incorporating intermediate and high modulus graphite fibers, silicon carbide filaments and beryllium wire reinforcements, all three types of fibers warrant further investigation in composites for aerospace structural use. From the standpoint of further improvement, especially in modulus of elasticity, the graphite fiber composites have outstanding potential because of anticipated future availability of higher modulus graphite fibers.