Notched strength of woven fabric composites with moulded-in holes

The feasibility of enhancing damage tolerance and durability of fibre composites through the design of microstructure has been examined using three woven fabric-reinforced composite systems (carbon, Kevlar and carbon-Kevlar in epoxy matrix). Enhancement in notched strength has been demonstrated by comparing the performance of composites with drilled and moulded-in circular holes. Specimens with moulded-in holes exhibited failure strengths which were 2.7–38.3% higher than those of drilled specimens. Furthermore, for certain lay-ups of Kevlar and carbon-Kevlar hybrid laminates, the presence of moulded-in holes did not reduce the unnotched laminate strength; indeed a strength enhancement of 0.4–22.1% was observed. Comparisons of experimental data with existing notched strength theories are made and directions for future research are indicated.     

Fatigue strength of woven fabric composites with drilled and moulded-in holes

The fatigue strength of glass-fiber woven-roving composites with a circular hole has been examined. Circular holes of two types, drilled and moulded-in, were considered. Experiments on the fatigue life and damage processes in (0, 90)_s and (45, −45)_s woven-roving composites with hole diameters of 5, 10, 14 and 19 mm were conducted. Experimental results show that laminates with moulded-in circular holes exhibit higher fatigue strength than those with drilled holes as a consequence of the altered mode of failure. Photographs of fatigue failure processes and specimen elongation with life are presented. The failure phenomena of laminates with drilled and moulded-in holes are compared and discussed.     

Compression behaviour of woven carbon fibre-reinforced epoxy composites with moulded-in and drilled holes

A hole moulding technique, via opening the fibres in a woven carbon/epoxy composite, was used to enhance the performance of panels with circular holes. The experiments were performed on 25.4 mm wide coupons with 3.175 mm, 6.350 mm, and 9.525 mm hole diameters for moulded-in and drilled hole panels. Both types of panels showed reductions in compressive strength due to the presence of holes compared with the strength of the unnotched panel. However, the strengths of the moulded-in hole panels were always greater than those of the drilled hole ones within the experimental data range. An increase in compressive strength enhancement with increasing hole diameter was observed, and a method for the determination of the average fibre area fraction, at horizontal and vertical cross-sections adjacent to the holes, as well as a ‘strengthening index’, was introduced for the moulded-in hole panels.     

Progressive failure analysis of 2/2 twill weave fabric composites with moulded-in circular hole

Micromechanical three-dimensional finite element models of 2/2 twill weave T300 carbon/epoxy woven fabric composite panels with moulded-in circular hole are established for stress analysis. In these models, the streamline equation is used as a shape function to simulate the fibre configuration. A progressive failure analysis together with a newly developed ‘maximum notched strength method’ are also proposed to predict the failure modes and notched strengths of the fibre dominated laminate with moulded-in hole. Perforated specimens of different hole sizes are prepared using a special procedure. Tension tests are performed to evaluate the stress–strain and failure characteristics. An increase in tensile strength with increasing hole size is observed within the experimental data range. Numerical results from progressive failure analysis provide good prediction to the failure phenomena of the fractured specimens. The notched strengths from the proposed numerical procedure are slightly higher than the experimental results.     

Tensile strength of UD-composite laminates with multiple holes

The residual strength of glass fibre reinforced vinyl-ester laminates with multiple holes was investigated through an experimental programme. Different types of structured hole patterns and hole densities were investigated and analysed using digital image correlation strain measuring technique. Three different failure modes could be observed when the hole patterns and the hole densities were a altered. These three failure modes were used as the foundation for a simple yet effective analytical model in order to predict the residual strength of damaged composite specimens. Finally, a number of laminates with randomly distributed holes were tested experimentally. The analytical model can predict the failure mode and failure strength of the experiments with sufficiently good fidelity.     

Predicting the compressive engineering performance of carbon fibre-reinforced plastics

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%.     

Experimental study on clamping effects on the tensile strength of composite plates with a bolt-filled hole

An experimental study was performed to assess the effects of clamp-up on the net-tension failure of laminated composite plates with bolt-filled holes. Graphite/epoxy prepreg of T800/3900-2 was selected for fabricating the laminates for the tests. The tensile strength and failure response of specimens with an open hole and a bolt-filled hole were evaluated. Both 100% bypass load (no bolt bearing load) and no bypass load (100% bolt bearing load) were considered during the experiments. X-radiographs were taken for specimens after pre-loading at different stress levels for the purpose of characterizing the failure modes and damage progression inside the composite.Experimental results showed that the bolt clamping force can significantly reduce the notch tensile strength of composite laminates which are prone to fiber-matrix splitting and delamination. A reduction in failure load of up to 20% was observed. Higher clamping pressure resulted in higher reductions of notch strength. However, for bolted joints which failed in a net-tension mode, clamping improved the joint strength regardless of the ply orientation.     

Bearing Behavior of Drilled and Molded-in Holes

Glass/epoxy composites were manufactured using RIFT (Resin Infusion under Flexible Tool), a closed mould process capable of obtaining large and complex forms, by impregnating, under a vacuum, a dry preform placed on a flat rigid mould. At certain points of these composite laminates, molded-in holes were made in the dry perform before the resin infusion phase, using two different methods: displacing or cutting the yarn of the fibers. After the resin treatment, other holes were made in the same laminates by drilling. Single-point pin-loaded specimens, cut from laminates, were tested for different values of specimen width-to-hole diameter ratio (W/D) and edge distance-to-hole diameter ratio. In the results of the experiment, the specimens with molded in holes made by displacing the fiber yarn showed higher bearing strength values.     

Impact of Nd-YAG laser drilled holes on the strength and stiffness of laminar flow carbon fibre reinforced composite panels

The effect of Nd-YAG laser drilled holes on the strength and stiffness of carbon fibre epoxy composite panels has been investigated. The holes, approximately 50 μm in diameter and 500 μm apart, are required to produce a porous skin for a Hybrid Laminar Flow application on an aircraft. Scanning Electron Microscope inspection indicated resin damage around the holes due to the heat of the laser. The damage area was elliptical in shape and measured approximately one hole diameter on either side of the drilled hole in unidirectional carbon fibre panels. The perforations reduced the static strength between 2 and 54% compared to non-drilled specimens, depending on the material type (unidirectional or weave), drilling method (single or multi-pulse) and hole pattern.     

An experimental and numerical investigation into the damage mechanisms in notched composites

Investigations of the effect of size on the tensile strength of composite laminates containing circular holes show that there is a large difference both in failure stress and mechanism due to changes in test configuration. This is particularly true of the ply and laminate thickness, and hole diameter. Interrupted tests have been performed on open hole tensile specimens at different load levels to determine the progressive damage development, evaluated through non-destructive testing (X-ray and C-scanning). The tests were also analysed using a novel Finite Element Modelling technique. This was able to accurately predict the wide range of ultimate strengths measured with variation in test parameters, principally through incorporation of the sub-critical damage in the analysis. A significant damage mechanism was seen to be delamination at the hole edge which generally occurred at a lower stress for a smaller hole diameter to ply block thickness ratio. Delaminations allowed damage to join up through the thickness of the laminate and propagate. In ply-level scaled specimens, the delamination propagation was the ultimate failure mode of most of the specimens. In sub-laminate level scaled specimens, localised damage relieved stress in the 0° fibres at the hole edge, delaying the onset of fibre failure. Less damage was seen for larger holes, thus leading to a decreasing failure stress with increasing hole diameter.     

Application of the point stress criterion to the failure of composite pinned joints

An analysis was performed to evaluate the bearing strength of pin-loaded composite joints using a two parameter characteristic curve model. This model involves determination of characteristic dimensions in tension and compression and based on this model, a two-dimensional stress analysis was used to determine the stress distribution around the fastener hole. In this analysis, characteristic dimensions in tension and compression were evaluated using the point stress failure criterion and joint bearing failure evaluated using the Yamada-Sun failure criterion. Results were compared with available experimental data for joints made from AS4/3501-6 graphite epoxy composite laminates and good correlation observed when evaluated as function of edge distance to hole diameter. However, the analysis yields conservative results when joint strength is evaluated as a function of plate width to hole diameter.     

A single-lap shear specimen for determining the effect of strain rate on the interlaminar shear strength of carbon fibre-reinforced laminates

A new design of single-lap shear specimen for determining the effect of loading rate on the interlaminar shear strength of laminated composites is described. Finite element analyses are used to optimize the specimen geometry and minimize the variation in the shear stress and the magnitude of the normal stress along the interlaminar failure plane. Experimental results are obtained at a quasi-static and an impact rate of loading for the interlaminar shear strength parallel to the fibres in both unidirectional carbon/epoxy and unidirectional carbon/polyetheretherketone (peek) laminates and at interfaces across which the fibre orientation is 0°/90° and ±45°. Results for carbon/epoxy laminates are compared with those from an earlier investigation using a double-lap specimen geometry and show a similar small dependence on loading rate. No significant effect of loading rate was observed for the carbon/peek laminates.     

Mechanical properties,fatigue damage and microstructure of carbon/epoxy laminates depending on fibre volume content

This paper investigates the effect of fibre volume fraction on the fatigue behaviour and damage mechanisms of carbon/epoxy laminates. Epoxy resin and unidirectional carbon/epoxy specimens with two different fibre volume fractions are tested under quasi-static tensile and tension–tension fatigue loads at angles of 0°, 45° and 90°. Fracture surfaces are studied with scanning electron microscopy. The results show that stiffness and strength increase with increasing fibre volume fractions. The damage behaviour of off-axis specimens changes with increasing fibre volume content and the height of the applied cyclic load. While matrix cracking and interfacial debonding are dominating damage mechanisms in specimens with low fibre content, fibre bridging and pull out are monitored with increasing fibre content. The higher the applied load in fatigue tests transverse to fibre direction, the more similar behave specimens with different fibre volume fractions.     

Bearing behavior of triaxially braided flat and tubular composites

An experimental study was carried out to evaluate the bearing behavior of glass fiber reinforced epoxy composites made from the triaxial braiding process. The bearing strengths of braided and machined holes in flat as well as tubular samples were compared. The effects of specimen geometry, braid angle, and specimen consolidation condition were investigated. Comparison between specimens with braided and machined holes indicated a higher bearing capacity for the braided joint hole compared with the machined one in tubular specimens whose thickness was not precisely controlled. For flat samples with uniform thickness, the braided hole showed similar or even lower bearing capacity compared with the machined hole, possibly due to the disturbance in fiber arrangement around the braided hole.     

On the influence of weave structure on pin-loaded strength of orthogonal 3D composites

Orthogonal three-dimensional (3D) carbon fibre fabrics with different weave structures were obtained by varying the yarn spacing and number of carbon filaments per tow in the x-, y- and z-directions during weaving. These weave structures were impregnated with epoxy resin to produce orthogonal 3D carbon/epoxy composites. In addition, one-dimensional (0° and 90° unidirectional) and two-dimensional (cross-ply and plain fabric) laminates were prepared from the same carbon fibres and epoxy resin. Single-hole pin-loaded specimens of each material were tested in tension, and the influences of reinforcement type, weave structure, specimen width-to-hole diameter ratio and edge distance-to-hole diameter ratio evaluated. Various modes of failure were observed in the specimens. The effect of in-plane and out-of-plane fibres on the pin-loaded strength of orthogonal 3D composites is discussed.     

3D finite element analysis of tensile notched strength of 2/2 twill weave fabric composites with drilled circular hole

This paper presents the micromechanical three-dimensional finite element models of the 2/2 twill weave T300 carbon/epoxy woven fabric composite laminates with drilled circular holes of different sizes. A fiber breakage failure criterion for predicting the ultimate tensile notched strength of fiber dominated composites is also proposed. It is found that the location of failure initiation for laminates with large hole size is different from those for laminates with smaller holes while the stress concentration may not occur at the notch roots for the fiber dominated laminates. Based on the uniaxial, shear and von Mises stress distributions in the yarn and matrix, the influence of hole-size on the stress distributions and stress concentration is discussed. Standard tensile tests with modifications are performed for this particular type of woven fabric composites. The apparent strain concentration factors and notched strengths determined by experiments are presented and the finite element models are verified by satisfactory correlation between prediction and experiment.     

Effects of fibre/matrix adhesion on carbon-fibre-reinforced metal laminates—I.: Residual strength

The role of interfacial adhesion between fibre and matrix on the residual strength behaviour of carbon-fibre-reinforced metal laminates (FRMLs) has been investigated. Differences in fibre/matrix adhesion were achieved by using treated and untreated carbon fibres in an epoxy resin system. Mechanical characterisation tests were conducted on bulk composite specimens to determine various properties such as interlaminar shear strength (ILSS) and transverse tension strength which clearly illustrate the difference in fibre/matrix interfacial adhesion. Scanning electron microscopy confirmed the difference in fracture surfaces, the untreated fibre composites showing interfacial failure while the treated fibre composites showed matrix failure. No clear differences were found for the mechanical properties such as tensile strength and Young's modulus of the FRMLs despite the differences in the bulk composite properties. A reduction of 7·5% in the apparent value of the ILSS was identified for the untreated fibre laminates by both three-point and five-point bend tests. Residual strength and blunt notch tests showed remarkable increases in strength for the untreated fibre specimens over the treated ones. Increases of up to 20% and 14% were found for specimens with a circular hole and saw cut, respectively. The increase in strength is attributed to the promotion of fibre/matrix splitting and large delamination zones in the untreated fibre specimens owing to the weak fibre/matrix interface.     

The effect of moisture on the shear properties of carbon fibre composites

A study was made of the effect of moisture on the mechanical properties of composites (carbon fibre-reinforced epoxy resins) which are dominated by the matrix or matrix/fibre interface. Such properties are the interlaminar shear strength in unidirectional laminates and tensile strength in (± 45) laminates. Unidirectional material was either immersed in boiling water or aged in a hot-humid atmosphere and then the interlaminar shear strength was measured at room temperature in short beam bending. The values obtained were found to be independent of the mode of exposure but depended on the amount of moisture present in the composite. The (± 45) material was aged at 70°C and 95% relative humidity to accelerate the moisture uptake and then tested at 20°C, 70°C, 110°C and 130°C. At test temperatures above 70°C the tensile strength decreased as the composite absorbed moisture. Plasticization, swelling and debonding were identified as the factors affecting the failure mechanisms in these laminates.     

The strength of bolted joints in glass fibre/epoxy laminates

The results obtained from an experimental study on glass fibre-reinforced epoxy laminates are described. Single-hole bolted joints were tested in a variety of lay-ups with two resin systems — Fothergill Code 69 and Ciba-Geigy 913. A small number of tests carried out on carbon fibre laminates compared closely with data from other workers. The general behaviour of the two fibre systems was found to be similar, the optimum lay-ups for bearing strength being only slightly different. The failure modes seemed to be more dependent on the lay-up than the fibre/resin combination, although more delaminations were seen with the glass fibre/epoxy laminates which also showed stronger interaction between modes.     

碳纤维织物/环氧复合材料销钉连接实验研究

针对1K缎纹碳纤维织物/环氧复合材料层板销钉连接,实验研究了几何参数对连接性能的影响.选取了不同的端距和边距,测试了碳纤维织物复合材料层板W/D(试样宽度/孔直径)及E/D(孔端距/孔直径)对销钉连接强度的影响,并分析了销钉连接的破坏模式.实验结果表明:1K缎纹织物/环氧复合材料层板销钉连接承载能力随着边距E和端距W的增加而增加,当E/D=2,W/D=3时,销钉连接强度达到稳定值,此后增加边距E和端距W值连接强度不会有明显的增加.