Quasi-isotropic Layup Research Articles

An experimental study on the effect of bolt-hole clearance and bolt torque on single-lap, countersunk composite joints

An experimental study on the effect of bolt-hole clearance and bolt torque on single-lap, single-bolt, countersunk composite joints are presented. The specimens were manufactured from carbon fiber/epoxy unitapes with quasi-isotropic lay-ups. Bearing stress/bearing strain behavior, bearing strength and joint stiffness were obtained according to ASTM standard D5961. The interaction between bolt-hole clearance and bolt torque on bearing response was evaluated by varying multiple parameters. One focus was to evaluate the surface strain distribution and the out-of-plane deformation of the joints by using a commercial 3D Digital Image Correlation (DIC) system. It is found that there is interaction between bolt-hole clearance and bolt torque on joint stiffness loss and 2% offset bearing strength. Furthermore, bolt-hole clearance intensifies the surface strain concentration and the out-of-plane deformation of the joints. Bolt torque alleviates the surface strain concentration, but has little effect on the out-of-plane deformation.

Meso-scale modeling of hypervelocity impact damage in composite laminates

ObjectivesThis paper presents an approach to numerical modeling of hypervelocity impact on carbon fiber reinforced plastics (CFRP). MethodsThe approach is based on the detailed meso-scale representation of a composite laminate. Material models suitable for explicit modeling of laminate structure, including fiber-reinforced layers and resin-rich regions, are described. Two numerical impact tests with significantly different impact energies were conducted on thermoplastic AS4/PEEK materials with quasi-isotropic layups. Simulations employed both SPH and Finite element methods. ResultsResults of simulations are verified against experimental data available from the literature and demonstrate good correlation with the experiments. ConclusionsDeveloped modeling approach can be used in simulations where post-impact damage progression in composite material is of the main focus.

Influence of fibre misalignment and voids on composite laminate strength

This article studies the effects of manufacturing defects of laminate composite materials on the first ply failure load and ultimate strength of the laminate, which are calculated using finite element modelling with Zinoviev’s damage model. The following types of defects are considered for cross-ply and quasi-isotropic layups: in-plane misalignement and out-of-plane waviness of fibres, misorientation of the layers and void content. The list of laminates studied in this article includes cross-ply and quasi-isotropic layups, loaded in tension and compression in 0°, 90°, 45° and 5° directions. The limits of the first ply failure load and strength degradation for a realistic range of the defect intensities are estimated.

Investigations on adhesively bonded curved CFRP panels with closed debonds subjected to compressive load

This present work aims to investigate the effect of debonds on the structural performance of adhesively bonded curved carbon fiber reinforced plastic (CFRP) panels under compressive loading environment. Representative curved panels are fabricated with a quasi-isotropic lay-up. Two types of specimens are prepared for the studies, one without debonds and the other with debonds of specified size, artificially introduced within the lap region. Pulsed thermography non-destructive technique is employed for the detection of debonds on the specimens. A set of experiments are conducted with displacement, strain and acoustic emission sensors to ascertain the performance of adhesively bonded joints under compression loading. Computational analyses are carried out using commercial finite element code MSC/NASTRAN to reproduce the experimental behavior and to aid in understanding the effect of closed debonds on the bonded joints. The computational model predictions are in good agreement with experimental observations. The effects of location of debond and panel curvatures are studied computationally incorporating debonds of different sizes. It is observed that debond location along the specimen length has significant effect and the buckling load reduces when the panel curvature increases for the same debond size.

Analysis of cylindrical sandwich structures with weak orthotropic core under patch loading

An analysis of cylindrical sandwich structures with weak orthotropic core subjected to patch loading is presented. A high order theory model combined with a novel formulation technique is used to predict the static response of the structure. The face-sheets are considered as thin shells that follow the first order shear deformation theory, whereas the core is considered as a linear elastic medium. The effects of core elastic and shear stiffness, curvature to length ratio, and stacking sequence and orientation on the core’s stresses and displacements are presented. The case of a moving load is explored and performance charts are generated to design and optimize the structure in response to the patch loads. In particular, considering an orthotropic core with quasi-isotropic lay-up in the face-sheets, limit the variation of the transverse core shear stress and provide an efficient structural design configuration.

Energy absorption rate of composite tube as a function of stacking sequence using finite element method

Abstract Numerical investigations were performed on a series of laminated composites when subjected to dynamic loadings, in order to determine their energy absorption rate and dissipating energy during impact. The details of numerical modeling, contact analysis, material parameters, and failure criteria were explained and discussed. We aimed to determine the energy absorption rate of various quasi-isotropic lay-ups when subjected to the dynamic loadings, and found that energy dissipation rate varied depending on stacking sequence. In this investigation, we studied how the energy absorption rate and the peak impact load change as a result of changing stacking sequence, with all stacking sequences being quasi-isotropic. Using a constant impacting mass and varied impacting speeds, we found that stacking sequence can significantly control the energy transfer rate or energy absorption rate under dynamic load condition.

Formability analyses of uni-directional and textile reinforced thermoplastics

The formability of two different composite materials used in aerospace industry has been investigated for a representative product geometry. The deformations during forming of carbon UD/PEEK and glass 8HS/PPS blanks with a quasi-isotropic lay-up were analysed. The UD/PEEK product showed severe wrinkling in doubly curved areas, whereas the 8HS/PPS product showed better formability in those areas. This can be explained by the relatively high resistance against intra-ply shear for the UD/PEEK material. Moreover, the predictive capability of a finite element based simulation tool was shown. For both materials, the prediction of intra-ply shear and large wrinkles showed good agreement with those observed in the actual product. The smaller wrinkles in the products cannot be accurately represented with the element size used. However, predicted waviness at the corresponding locations could indicate critical areas in the product. The presented modelling approach shows great potential for application in the composite product design process.

Assessing effect of different surface treatments on fracture toughness behaviour of adhesively bonded 8552/AS4 composite joints

Adhesive bonding of aeronautical components made of carbon–fibre reinforced plastics is a popular alternative to mechanical fastening. The continuing research is focused on the optimisation of the surface treatments so as to improve the mechanical properties. In this work, the effect of two atmospheric pressure plasma (APP) treatments before bonding on the fracture toughness behaviour of adhesively bonded joints was experimentally investigated. The laminates were in contact with different ancillary materials during the manufacturing process, thus leading to eight different treatment alternatives. For the investigation, a quasi-isotropic layup was subjected to modes I and II fracture toughness test. To support the understanding of the mechanical behaviour observed, non-destructive testing evaluation as well as failure mode analysis at macroscopic level was carried out. As a result, APP showed promising performances regarding surface preparation, revealing an appreciable dependence of the fracture toughness behaviour on the selected alternatives.

Subcritical damage mechanisms of bolted joints in CFRP composite laminates

A detailed experimental programme is presented that was conducted in order to establish a data base for strength and subcritical damage mechanisms of bolted joints in CFRP composite laminates. Single fastener double-shear tensile tests for various joint geometries were performed for a range of cross-ply and quasi-isotropic lay-ups of HTS40/977-2 CFRP material system. Penetrant enhanced X-ray radiography was used to define the subcritical damage locations which are of great importance when modelling the failure response of the joints. It is suggested that the subcritical damage planes can be modelled using cohesive zone elements (CZEs) in order to develop physically based strength prediction methods for bolted joints in CFRP laminates.

Experimental and computational investigation of progressive damage accumulation in CFRP composites

This manuscript presents an experimental and computational study of progressive damage accumulation in carbon fiber reinforced polymers. A series of monotonic tension experiments with acoustic emission, X-ray radiography, and X-ray computed tomography were performed on a quasi-isotropic layup to ascertain the initiation, progression, and modes of damage accumulation under tensile loading. A multiscale computational model was used to aid in understanding the progressive damage accumulation. The eigendeformation based reduced order homogenization modeling approach was used due to its computational efficiency and its ability to incorporate distinct damage modes. The combined experimental and computational investigation provided a detailed picture of the progression of damage accumulation in the CFRP composite. The multiscale model predictions were in good agreement with experimental observations.

Comparative study on the behavior of woven-ply reinforced thermoplastic or thermosetting laminates under severe environmental conditions

This work aims at determining whether thermoplastic-based composites can be used in secondary aircraft structures to replace thermosetting-based composites or not. In order to answer this question, the mechanical behaviors of carbon fiber fabric reinforced thermoplastic (PPS or PEEK) and thermosetting (epoxy) laminates subjected to different stress states under severe environmental conditions (120°C after hygrothermal aging) have been compared. In addition to usual mechanical tests (tensile, open hole tensile), single-bolt double lap joint and single-bolt single lap joint tests were also performed. Severe conditions help enhance the ductile behavior of the epoxy matrix, but degrade the fiber/matrix interface, resulting in lower stiffness and strength of laminates with a quasi-isotropic lay-up. In thermoplastic-based laminates, the degree of retention of mechanical properties is quite high even for PPS-based laminates when T>Tg. In laminates with a [45]7 lay-up, severe conditions adversely affect the mechanical properties of the three composite systems. However, the combination of matrix ductile behavior, and the strain gradient near the hole, lead to an extensive plastic deformation along the ±45° oriented fibers bundles in notched A-P laminates. It results in decreasing significantly the hole-sensitivity of C/PPS and C/Epoxy under severe conditions. In bolted joints, a severe environment has a limited impact on the bearing strength of epoxy-based laminates. In the case of thermoplastic-based laminates, it increases the strength of double lap joints, but is detrimental to the strength of single lap joints.

Stacking Sequence and Geometrical Effects on Low-Velocity Impact Behaviors of GLARE 5 (3/2) Fiber–Metal Laminates

Impact responses and damage induced by a drop-weight on GLARE 5 (3/2) fiber–metal laminates (FMLs) with various lay-up configurations were studied experimentally. The effect of specimen geometry was also considered. The damage characteristics were evaluated using both nondestructive ultrasonic and mechanical-sectioning techniques. Only the contour of the entire damage area could be obtained using ultrasonic C-scan, whereas more details of the damage were provided through the mechanical-sectioning technique. The impact properties of FMLs were affected by laminate stacking sequence. The first severe failure induced by low-velocity impact occurred as debonding between aluminum and the adjacent fiber–epoxy layer at the nonimpact side. It was followed by a visible crack in the outer aluminum layer at the nonimpact face. GLARE 5 made of unidirectional fibers had the least impact resistance; followed by cross-ply and angle-ply configurations; while the quasi-isotropic lay-up showed the best in resistance to impact. By introducing circular, rather than square, geometry as the outer perimeter of the specimens, the damage patterns as well as impact behaviors changed due to the relative anisotropy of the specimen with respect to the specimen clamping.

Construction of constant fatigue life diagram for a carbon fiber composite

Using the constant amplitude fatigue data at various stress ratios, the constant fatigue life (CFL) diagram was constructed for the CFC material which is useful in prediction of fatigue life under variable amplitude fatigue loads. A quasi-isotropic lay-up sequenced carbon fiber-epoxy composite (CFC) laminate was fabricated by resin infusion technique. The tensile and compression tests were carried out to determine the static strength of the material. About 175 mm long, constant rectangular cross-sectioned fatigue test specimens were cut and prepared from the laminate. The stress-controlled, constant-amplitude fatigue tests were conducted in a 100 kN servo-hydraulic test machine, at room temperature and in lab air atmosphere. All the fatigue tests were performed with a sinusoidal waveform and a frequency of 1–3 Hz. The fatigue tests were conducted at four different stress ratio, R = σmin/σmax, Viz., 0.7,0.5 (tension-tension), −1.0 (tension-compression), and 4.0 (compression-compression). Anti-buckling guide was employed during the fatigue tests which involved compressive load cycles. The CFL Diagrams are material specific with predictive capability for any designed levels from selective experimental data.

Investigation into laminate design of open carbon–fibre/epoxy sections by quasi–static and dynamic crushing

The effect laminate design on the crush performance of carbon–fibre/epoxy “DLR” crush elements has been experimentally investigated. A quasi-isotropic lay-up was found to result in the highest Specific Energy Absorption (SEA) for Four-Harness (4HS) reinforced laminates; however a hybrid of unidirectional weave and 4HS fabric produced the highest SEA of 114 kJ/kg. Interleaving with thin thermoplastic films increased the steady state crushing force, however the increase in laminate density associated with the addition of the film caused no improvement and in some cases a reduction in SEA depending on material and lay-up. Dynamic crush testing of selected laminate designs resulted in a reduction in SEA of between 6% and 15% compared to the quasi–static case.

Woven ply thermoplastic laminates under severe conditions: Notched laminates and bolted joints

This work is aimed at investigating the influence of environmental conditions on the mechanical behavior of carbon fiber fabric reinforced PPS laminates. Changes in the strength and in the failure mechanisms of notched laminates and bolted joints were investigated for different environmental conditions: room temperature/dry – 120°C/hygrothermally aged. These conditions help enhance the ductile behavior of the PPS matrix but degrade the fiber/matrix interface. However, the degree of retention of mechanical properties is quite high for a quasi-isotropic lay-up and hole sensitivity is slightly decreased by environmental conditions. For bolted joint tests, severe conditions increase the strength of double-lap joints (+11%) but are detrimental to the strength of single-lap joints (−26%).

The Tensile Fatigue Behavior of a Glass-fiber Reinforced Plastic Composite Using a Hybrid-toughened Epoxy Matrix

A thermosetting epoxy-polymer was modified by incorporating 9 wt% of carboxyl-terminated butadiene-acrylonitrile rubber microparticles and 10 wt% of silica nanoparticles. The tensile fatigue behavior at a stress ratio, R = 0.1 for both the neat-epoxy polymer (i.e., unmodified) and the hybrid-epoxy polymer was first investigated. The fatigue life of the hybrid-epoxy polymer was about six to ten times higher than that of the neat-epoxy polymer. Secondly, the neat- and the hybrid-epoxy resins were infused into a quasi-isotropic lay-up, E-glass fiber fabric via a ‘Resin Infusion under Flexible Tooling’ set-up to fabricate glass-fiber reinforced plastic (GFRP) composite panels. The tensile fatigue tests at a stress ratio, R = 0.1 were performed on both of these GFRP composites during which the matrix cracking and stiffness degradation were routinely monitored. The fatigue life of the GFRP composite increased by about six to ten times due to employing the hybrid-epoxy matrix, compared to employing the neat-epoxy matrix. Suppressed matrix cracking and a reduced crack propagation rate were observed in the hybrid-epoxy matrix, which resulted from the various toughening micromechanisms induced by the presence of both the rubber microparticles and silica nanoparticles. These factors were considered to contribute towards the enhanced fatigue life which was observed for the GFRP composite employing the hybrid-epoxy matrix.

Effect of Stacking Sequence on Open-Hole Tensile Strength of Composite Laminates

Theeffectofaplystackingsequenceontheopen-holetensilestrengthofcompositelaminateshasbeeninvestigated both numerically and experimentally. A finite element technique has been developed that includes the subcritical damage within and between the plies. It is thus able to capture, in some detail, the effects of variation in stacking sequence. All possible permutations of 0, 90, and 45deg plies in a quasi-isotropic layup have been analyzed. Variations in strength have been observed andexplained in terms of subcritical damage development. Two stacking sequences showing different behavior were chosen for scaling in the thickness direction by increasing the ply thickness. Analysis of the first predicted a significant decrease in strength and a change in failure mode from fiberdominated to delamination-dominated failure with increasing thickness. The second predicted no such decrease in strength or change of failure mode. Both of these stacking sequences were then tested experimentally at both thicknesses, and excellent agreement with the numerical models was obtained, both in terms of damage mode and failure stress.

Influence of Fillers in Stiffener Core and Structural Parameters on Compaction of T-Stiffened Skins in Autoclave Process

Using carbon fibre/bismaleimide unidirectional prepregs, T-stiffened skins were fabricated by integral moulding technology of co-bonding in autoclave. The research was focused on comparing the effects of different filler content and structural parameters (curvature and ply sequence considered) on the compaction of T-stiffened skins. The measurement thickness and observation of micrographs were used to characterise the consolidation and compaction of T-stiffeners, and the experimental results indicated that filler content and structural parameters were both significant factors. A non-uniform thickness distribution and thickening at the corner section of T-stiffeners were observed. For T-stiffeners with quasi-isotropic lay-up, more uniform thickness distribution was obtained under the conditions of rolling unidirectional prepregs as fillers into stiffener core and increasing curvature radius, which could help to compact T-stiffeners and eliminate rich resin defect. The results provided in this study will contribute to understanding of the fracture mechanism of T-stiffened skins.

Characterization of a composite structure obtained by RFI using HexFIT ® semi-products

The HexFIT ™ is a semi-product used for large structures manufactured by the RFI process. In this work, large plate made of a quasi-isotropic lay-up is considered. The presented study aims at mechanically characterizing and identifying the process impact for such a structure. NDT (US & IR) are first used to highlight material heterogeneities and thickness variations. This approach helps us to define a sampling protocol in order to select tensile coupons taking into account thickness and width. Multi-instrumented tensile tests (strain field measurement, gauges and extensometer) are undertaken on specimens showing different widths, but almost same thicknesses. Comparison between the different strain measurements leads to qualify specific sensors for future characterizations.

Measuring the notched compressive strength of composite laminates: Specimen size effects

Large fibre reinforced composite structures can give much lower strengths than small test specimens, so a proper understanding of scaling is vital for their safe and efficient use. Small size (scale) specimens are commonly tested to justify allowable stresses, but could be dangerous if results are extrapolated without accounting for scaling effects. On the other hand large factors are sometimes applied to compensate for uncertainties, resulting in overweight designs. The most important variables of scaling effects on the strength of composites with open holes have been identified from experimental tests as notch size, ply and laminate thickness. In this study, these have been scaled both independently and simultaneously over a large range of combinations. The specimens are fabricated from commercially available (Hexcel Composites Ltd.) carbon/epoxy pre-impregnated tapes 0.125 mm thick (IM7/8552). The material is laid up by hand in unidirectional [0 4] ns with n = 2, 3, 4, and 8 (i.e., 2, 3, 4 and 8 mm thick) and multidirectional laminates; two generic quasi-isotropic lay-ups, one fabricated with blocked plies [45 n /90 n /−45 n /0 n ] s and the other with distributed layers [45/90/−45/0] ns with n = 2, 4 and 8 are examined. It is shown that the critical failure mechanism in these laminates is in the form of fibre microbuckling or kinking. The unnotched compressive strength in unidirectional specimens thicker than 2 mm is found to be limited by the stress concentration developed at the end tabs and manufacturing induced defects in the form of ply waviness, fibre misalignment and voids rather than specimen size (scaling). In the open hole specimens, for both lay-ups, the strength reduction observed is due to hole size effect rather than specimen thickness or volume increase. The open hole (notched) compressive strength results obtained compare favourably to predictions by a linear softening cohesive zone fracture model developed in earlier work by the second author.