Outer Plies Research Articles

Piezoresistive nanocomposites for sensing the effectiveness of composite patch repair

Abstract In this paper, piezoresistive nanocomposite is used to detect the effectiveness of bonded composite repair. The hybrid nanocomposites fabricated using wet-layup method with carbon nanotube sheet and graphite platelets as fillers exhibits enhanced piezoresistivity. The nanocomposite is incorporated as outer ply of repair patch in a scarf-repair process of damaged carbon fiber composite. Flexural test of repaired composite with intentionally placed disbond exhibits a different electromechanical response compared to defect-free repair; this concept can be used to detect the repair quality.

Advanced 3D and 2D damage assessment of low velocity impact response of glass and Kevlar fiber reinforced epoxy hybrid composites

This work focuses on quasi-static mechanical (tensile, flexural, indentation), low velocity impact (LVI) and viscoelastic behaviour of different stacking sequences and ] of 16 layers woven roving Kevlar-E-glass/epoxy interply hybrid composites. The outermost layers are Kevlar and remaining 14 E-glass layers are placed as interior for making these hybrid laminates. The viscoelastic properties are measured over the temperature range from 25 °C (room temperature) to 180 °C at three different frequencies (1, 10 and 50 Hz) by using the dynamic mechanical analyzer (DMA). The LVI results indicated that Kevlar-glass/epoxy hybrid laminates exhibit the highest damage threshold load (DTL) whereas laminates possess the higher peak load and threshold and maximum energies. Advanced non-destructive testing techniques (NDT) such as X-ray computed tomography (X-CT) and ultrasonic C-scan are employed to evaluate the micro and macro damage mechanisms of LVI tested specimens, respectively. Detailed observations have been made on the onset and growth of cracks in 30°, 45° and 0° plies using X-CT scan imaging. These images show that the bottommost plies subjected to more impact than the topmost plies, the outer Kevlar plies protect the inner 0° glass plies and delays the delamination propagation. ‘C’ scan images indicate that laminates spread the damage to more area. The scanning electron microscopy (SEM) is used for examining the failure mechanisms of indentation specimens.

Progressive matrix cracking master curves of mid and outer off-axis plies in CFRP laminates

An energy based criteria is developed to predict the propagation of mid and outer-ply matrix cracks in laminates of type 0n/θms and θm/0ns with θ=90,75,60 subject to uniaxial tensile loading. Numerous specimens with 14 different layups are tested and optical microscopy is implemented to detect matrix cracks during tensile tests. Matrix cracking master curve is plotted for over 300 data points obtained from experimental observation of all tested layups. A single critical energy release rate Gmc is derived for off-axis and transverse ply cracking in contrast to previous studies which only included transverse plies. Crack densities of all tested layups are analytically predicted and compared to experimentally obtained crack densities. For the first time in the literature, critical energy release rate is confirmed to be a material property independent of laminate layup as far as analytical predictions of matrix crack densities are in accordance with experimental observations.

Compression Fracture of CFRP Laminates Containing Stress Intensifications.

For brittle fracture behaviour of carbon fibre reinforced plastics (CFRP) under compression, several approaches exist, which describe different mechanisms during failure, especially at stress intensifications. The failure process is not only initiated by the buckling fibres, but a shear driven fibre compressive failure beneficiaries or initiates the formation of fibres into a kink-band. Starting from this kink-band further damage can be detected, which leads to the final failure. The subject of this work is an experimental investigation on the influence of ply thickness and stacking sequence in quasi-isotropic CFRP laminates containing stress intensifications under compression loading. Different effects that influence the compression failure and the role the stacking sequence has on damage development and the resulting compressive strength are identified and discussed. The influence of stress intensifications is investigated in detail at a hole in open hole compression (OHC) tests. A proposed interrupted test approach allows identifying the mechanisms of damage initiation and propagation from the free edge of the hole by causing a distinct damage state and examine it at a precise instant of time during fracture process. Compression after impact (CAI) tests are executed in order to compare the OHC results to a different type of stress intensifications. Unnotched compression tests are carried out for comparison as a reference. With this approach, a more detailed description of the failure mechanisms during the sudden compression failure of CFRP is achieved. By microscopic examination of single plies from various specimens, the different effects that influence the compression failure are identified. First damage of fibres occurs always in 0°-ply. Fibre shear failure leads to local microbuckling and the formation and growth of a kink-band as final failure mechanisms. The formation of a kink-band and finally steady state kinking is shifted to higher compressive strains with decreasing ply thickness. Final failure mode in laminates with stress intensification depends on ply thickness. In thick or inner plies, damage initiates as shear failure and fibre buckling into the drilled hole. The kink-band orientation angle is changing with increasing strain. In outer or thin plies shear failure of single fibres is observed as first damage and the kink-band orientation angle is constant until final failure. Decreasing ply thickness increases the unnotched compressive strength. When stress intensifications are present, the position of the 0°-layer is critical for stability under compression and is thus more important than the ply thickness. Central 0°-layers show best results for OHC and CAI strength due to higher bending stiffness and better supporting effect of the adjacent layers.

The effect of drilling-induced delamination on tensile strength and prediction of residual strength of carbon fiber-reinforced polymer laminate

An energy-based continuum damage mechanics model was employed to investigate the effect of drilling-induced delamination on tensile strength of carbon fiber-reinforced polymer laminates and the residual strength was predicted. The decrease in tensile strength caused by delamination was investigated in detail by a finite element method (FEM) model. Experiments were also conducted to validate the numerical results. The drilling-induced delamination was measured with ultrasonic C-scan technique and modeled as deletions of corresponding cohesive elements in the FEM model. Different sizes of delamination zones were investigated and the predicted strengths were compared with experimental results. The FEM model showed that the decrease in strength is mainly caused by early failures of outer plies. Good correlation between numerical simulations and experimental results was also obtained.

Numerical Modeling Mechanical Delamination in Laminated Glass by XFEM

This paper deals with the three-dimensional modeling of laminated glass failure due to delamination of the glass. The extended finite element method is used here (XFEM) to input discontinuity to the approximation field. The regarded glass consists of two outer glass plies and thin inner layer of the polyvinyl butyral (PVB). The PVB inner layer is very thin in comparison to the laminated glass thickness. That is why in the model, presented in this paper, the PVB layer is not spatially discretized. However, the inner layer exists in the problem formulation. The constitutive law for PVB is written with the help of two dimensional inner layer traction vector. Finally it results in three-dimensional effective numerical model for delamination analysis. The model is illustrated with an example.

Damage characterisation of cryogenically cycled carbon fibre/PEEK laminates

A unique insight into damage formation in CF/PEEK laminates before, during and after cryogenic cycling, using optical microscopy and 3D X-ray computed tomography (CT), is presented. Thicker laminates were found to exhibit significantly greater microcrack density and delamination when compared to thinner laminates, with lay-up and material type also being important contributing factors. Thermal residual stress induced microcracking was also found in thicker laminates post-processing. 3D rendering software was used to prove the presence of through thickness crack networks within the laminates, as well as the extent of cracking through the specimen width. Crack opening in inner and off-axis ply groups was found to be significantly less than outer plies, implying the importance of these plies in limiting laminate permeability. The presence of voids was found to influence crack nucleation and growth paths within the laminates, with full void volume characterisation presented.

Effects of slip on eccentrically loaded multiple-ply girder timber trusses

Recent roof failures in South Africa, of multiple ply nail plated trusses, have necessitated a rethink about the way in which timber trusses are analysed. The authors believe that the relative slip between plies caused by a torsional moment as a result of eccentric loading is a possible reason for some of the failures. The design assumption in South Africa and America is that all the plies share the applied load equally; Eurocode 5 (BS EN 1995-1-1:2004) does not cover the design of multiple ply trusses; and BS 5268-3:2006 recommends additional load factors to be applied to eccentrically loaded truss components with the outer ply being loaded more than the inner plies, as a result of the torsion induced by an out-of-plane eccentricity. In this paper, a matrix stiffness method is used to show that, by ignoring the eccentric loading and relative slip between the trusses, the member and plate force may be underestimated by a factor in excess of two for a three ply girder truss. A single three-ply girder truss test is used to illustrate the merits of the method. The required load factors calculated also show that the magnification factors in BS 5268-3:2006 are lower than they should be.

A finite element unit-cell method for homogenised mechanical properties of heterogeneous plates

This paper presents a 3D unit-cell approach which enables estimations of all bending, in-plane and coupling properties in the sense of the classical laminated plate theory for heterogeneous plates. For this reason, periodic boundary conditions which simultaneously allow in-plane as well as bending/twisting deformation modes are introduced. In contrast to existing approaches, the 3D unit-cell is connected with the macroscopic plate theory by means of prescribed loads and the extraction of strains/curvatures. Novel bending experiments at large curvatures of E-glass-fibre/epoxy cross-ply laminate with cracking outer ply at the bending tension side are conducted. Subsequently, numerical results are validated by comparing them to the measured bending stiffness degradation. Further, the effects of the single-sided transverse cracks on relevant stiffness properties are computed.

Experimental and Numerical Investigations of Textile Hybrid Composites Subjected to Low Velocity Impact Loadings

In the present study experimental and numerical investigations were carried out to predict the low velocity impact response of four symmetric configurations: 10 ply E Glass, 10 ply AS4 Carbon, and two Hybrid combinations with 1 and 2 outer plies of E Glass and 8 and 6 inner plies of Carbon. All numerical investigations were performed using commercial finite element software, LS-DYNA. The test coupons were manufactured using the low cost Heated Vacuum Assisted Resin Transfer Molding (H-VARTM©) technique. Low velocity impact testing was carried out using an Instron Dynatup 8250 impact testing machine. Standard 6 × 6 Boeing fixture was used for all impact experiments. Impact experiments were performed over progressive damage, that is, from incipient damage till complete failure of the laminate in six successive impact energy levels for each configuration. The simulation results for the impact loading were compared with the experimental results. For both nonhybrid configurations, it was observed that the simulated results were in good agreement with the experimental results, whereas, for hybrid configurations, the simulated impact response was softer than the experimental response. Maximum impact load carrying capacity was also compared for all four configurations based on their areal density. It was observed that Hybrid262 configuration has superior impact load to areal density ratio.

Formability of Fiber-Reinforced Thermoplastics in Hot Press Forming Process Based on Friction Properties

High performance composites are used in commercial applications in a steadily growing degree. This increase of advanced materials is accomponied with the development of fully automated fabrication processes. It aims to drive down the time and costs of the production while ensuring a high quality of the product. This can achieved by considering the process of hot press forming with continuous fiber reinforced thermoplastics. The development of the process is, however, accompanied with a few difficulties, which require more research. For example, composite materials with different architectures, lay-ups, and constituents, show large differences in formability. This research examines the effect of friction on the formability of thermoplastic composites. Both experiments and simulations were conducted. Demonstrator products have been press-formed from laminates with different materials and architectures (UD-carbon PEEK, UD-carbon-PEI, 8hs-glass PPS, 5hs-carbon PEEK and UD-glass PPS), to investigate their effects on formability. Creating a doubly curved shape from a flat laminate requires at least three deformation mechanisms, namely in-plane shear, bending and inter-ply slippage This paper focuses on the sliding mechanism and the corresponding friction. In order to quantify the amount of sliding in the press-formed product, a dot pattern has been applied to both surfaces of the laminate. The slip between the outer plies can be analyzed by means of photogrammetry. Besides, the friction coefficient of each material is measured in a special designed friction test set-up. It can be seen that the composite formability is directly linked to its friction properties. FE simulations of the press-form process will be performed based on the measured material properties, to demonstrate the influence of the materials friction coefficient.

INVESTIGATION OF HYBRID AND DIFFERENT CROSS-SECTION COMPOSITE DISC SPRINGS USING FINITE ELEMENT METHOD

In this study the effectiveness of composite disc springs with different cross-section and hybrid type are determined by taking into account load capacities, masses, hybridization characteristics and costs of composite disc springs. The disc springs are analyzed with ABAQUS finite elements program by compressing between two rigid plates. The load-deflection characteristics obtained as a result of the analysis are compared with the analytic and experimental studies. Then different cross-section and hybrid composite disc springs were modeled. The trapeze A disc spring were confirmed to be more advantageous in terms of load capacity and mass by investigating the modeled disc springs. The effect of hybridization on hybrid disc springs with standard cross-section was investigated and optimum hybrid disc spring was determined according to cost and maximum loading capacity. Consequently, it is determined that carbon/epoxy plies used for outer layers are more advantageous. But the outer ply subjected to force was damaged thus this layer should be particularly reinforced.

Size effects on the tensile and compressive failure of notched composite laminates

An experimental and analytical investigation of the effect of size on the strength of composite laminates with central holes loaded in tension and compression is presented. Specimens with different hole sizes and with constant width-to-diameter ratios were tested in tension and compression under quasi-static loading and the strength reduction for increasing sizes was quantified for two lay-ups and for the two loading conditions. The first-ply failure load of the outer ply was identified using a new method that post-processes the displacement field obtained using the digital image correlation technique. The accuracy of the available strength prediction methods (point and average stress methods, inherent flaw model, semi-analytical cohesive zone model and finite fracture mechanics) to simulate the effect of size on the strength of notched composites is discussed. It is shown that the finite fracture mechanics model is the most accurate method.

Analysis of Laminated Architectural Glazing Subjected to Wind Load and Windborne Debris Impact

During windstorms and hurricanes, architectural glazing is subjected to wind loading and windborne debris impact. Wind-borne debris is categorized into two types. One is small hard missile like roof gravel and the other is large soft missile representing the lumber from wood-framed buildings. Laminated architectural glazing (LAG) is the commonly used glazing in buildings where impact resistance is needed. The prefailure stress response of the LAG due to the combined loading due to wind and windborne debris impact is studied. Following the ASTM standards (E1886 and E1996), a steel ball with an impact velocity of 39.62 m/s and a wooden cylinder with an impact velocity 12.19 m/s were chosen to be representative of small and large missiles, respectively. A lateral pressure that corresponds to a wind speed of 58.11 m/s was used to represent wind loading on LAG. The effect of geometric and material properties on the stress response of a rectangular LAG is studied parametrically. Thinner outer ply would result in better prefailure stress pattern than a thicker outer ply, while thicker interlayer generally results in lower stresses in failure critical areas. The contribution of wind loading to the principal stress is between 5–10% of the combined stress with small missile case having higher percentage.

Effects of moisture on matrix cracking in a cryo-cycled cross-ply laminate

This work was motivated by previous cryogenic-cycling experiments using liquid nitrogen and associated analysis of a balanced and symmetric cross ply laminate with polished edges made from IM7 fibers and CYCOM® 5250-4 bismaleimide matrix. Moisture effects were studied both experimentally and analytically in an attempt to explain differences in crack occurrences between inner and outer plies and between otherwise nearly identical experiments. It was shown that moisture locally diffused into a surface layer has a three-dimensional effect on the laminate stress distribution near the edges where cracks initiate. These effects are strongly ply dependent and help explain ply-to-ply variations in crack densities observed in experiments.

Design of Rapid Positioning Multi-Axis and Multi-Blade Saw Machine

We design a rapid positioning multi-axis and multi-blade saw machine in order to meet the processing needs of outer ply of engineered wood flooring and wood shutters in China. The machine uses two sets of saws against cutting in the cutting width, so its saw blade is thin, cutting loss is small and cut-turn percentage is high. Meanwhile, it has the feature of rapid positioning, convenient operation and high precision machining, which provides ideal processing equipment for China sheet production.

Bearing Performance and Design Method of Aluminum Alloy Bolted Connections

Served as the primary form of joints in aluminum structures, the bolted connection is of great necessity to be investigated. The bearing performance of aluminum alloy bolted connections was evaluated by test and finite element (FE) analysis. A total of 20 bolted connections were tested and the varying parameters incorporated screw diameter and end distance. The test results included the ultimate bearing capacities and relationship between applied load and bolt hole deformation. Numerical simulation for the test process was implemented; thereupon reliability and accuracy of the FE models could be validated by good agreement with test results. By virtue of the verified numerical model, elaborated analysis of principle variables including inner and outer plies, end distance, screw diameter, sheet thickness and so on was carried out. Compared to the current overly conservative design rules, a new design method that could make full use of the bearing capacity was proposed. The corresponding design value of bearing strength was also presented with reference to the recommended constructional provisions.

Impact and flexural properties of flax fabrics and Lyocell fiber-reinforced bio-based thermoset

A bio-based thermoset resin was reinforced with flax fabrics and Lyocell fiber. The effect of different weave architectures was studied with four flax fabrics with different architectures: plain, twill (two different types), and dobby. The effect of the outer ply thickness was studied and characterized with flexural and impact testing. Composites manufactured with plain weave reinforcement had the best mechanical properties. The tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength were 280MPa, 32GPa, 250MPa, 25GPa, and 75 kJ/m 2, respectively. Reinforcements with twill-weave architecture did not impart appreciable flexural strength or flexural modulus even when the outer thickness was increased. Plain- and dobby (basket woven style)-weave architectures gave better reinforcing effects and the flexural properties increased with an increase in outer thickness. Water absorption properties of the composites were studied and it was observed that the hybridization with Lyocell fiber reduced the water uptake. Field-emission scanning electron microscopy was used to study the micro-structural properties of the composites.

Assuring life in composite systems

A computational simulation method is presented to assure life in composite systems by using dynamic buckling of smart composite shells as an example. The combined use of composite mechanics, finite element computer codes, and probabilistic analysis enable the effective assessment of the dynamic buckling load of smart composite shells. A universal plot is generated to estimate the dynamic buckling load of composite shells at various load rates and probabilities. The shell structure is also evaluated with smart fibers embedded in the plies right below the outer plies. The results show that, on the average, the use of smart fibers improved the shell buckling resistance by about 10% at different probabilities and delayed the buckling occurrence time. The probabilistic sensitivities results indicate that uncertainties in the fiber volume ratio and ply thickness have major effects on the buckling load. The uncertainties in the electric field strength and smart material volume fraction have moderate effects and thereby in the assured life of the shell.

Stress distribution in outer and inner plies of textile laminates and novel boundary conditions for unit cell analysis

The local stress–strain distribution in a unit cell of a textile laminate depends on the distance of the ply to the surface, the number of plies in the laminate, and the stacking sequence. A conventional meso FE analysis employs boundary conditions for a unit cell of the textile composite based on the assumption of periodicity in the thickness direction. In that case, the stress concentration can be drastically underestimated, especially in outer plies. This paper describes the interaction of plies, local stresses and displacements. To avoid the analysis of the whole laminate and to reduce it to the boundary value problem on one unit cell only, novel boundary conditions are introduced. These conditions are based on the analysis of a single unit cell: they account for the number of the plies in the laminate, distinguish between the outer and inner plies, and reproduce the meso stress–strain state with good precision.