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Abstract: The present paper presents a consistent model of a three dimensional delaminated composite column with a proper consideration of the extensional and bending stiffness coupling and transverse shear effect to determine the axial buckling load. The exact analytical solution of the buckling force is obtained using the linearized stability theory. The three dimensional model allows us to consider a rather general set of delaminations including those that are not necessarilly perpendicular to the symmetry axis of the cross-section or/and have non-symmetrical surfaces. The parametric studies are presented showing the effects of shear, the delamination position, the angle of rotation of the delamination and the ratio of elastic to shear moduli. [Copyright &y& Elsevier]

Abstract: The possibility to accurately analyse the buckling and postbuckling behaviour of plates with delaminations of irregular shapes by utilisation of circular or elliptic delaminations was studied. The behaviour of compressed plates with delaminations of irregular and corresponding smallest enclosing circular and elliptic shapes was predicted by computational analyses and compared in terms of buckling loads and maximum values of the energy release rates found along the delamination boundaries. The study indicates that utilisation of circular delaminations could provide highly inaccurate results. Moreover, for truly accurate analyses of the buckling and postbuckling behaviour of delaminated structures it seems to be inevitable to use as precise representation of the shape of delaminations as possible. [Copyright &y& Elsevier]

Abstract: This study deals with parametric effects on bucking behaviors of laminated composite structures containing an embedded rectangular delamination using the enhanced assumed strain (EAS) three-dimensional element. The three-dimensional finite element (FE) formulation based on the EAS method for composite structures shows excellence from the standpoints of computational efficiency, especially for distorted element shapes. Using the EAS FE formulation developed for this study, the effects of embedded delamination sizes and ply orientations on the elastic buckling behaviors for various aspect ratios and width-to-thickness ratios are studied. The numerical results obtained are in good agreement with those reported by other investigators. Furthermore, the new results reported in this paper show the progression of local buckling and its influence on global buckling and vice versa. Key observation points are discussed and a brief design guideline is given. [Copyright &y& Elsevier]

By introducing new nondimensionalized axial and bending stiffnesses, a generalized analytical analysis using classical engineering solutions is proposed to study the buckling behavior of tri-layer beams having asymmetrical enveloped delaminations. Appropriate boundary and kinematical conditions are used to formulate the systematic homogeneous equations. The present analysis accurately predicts the buckling loads of tri-layer beams having double enveloped delaminations, which is verified with the previously published data for homogeneous beams having single and double delaminations. An excellent agreement between the results is observed. A transition region is observed as the normalized bending stiffness increases. Effective-slenderness ratio is defined and introduced, which gives a measure of local, mixed and global buckling phenomenon. The present analysis provides a useful reference for the tri-layer beams having enveloped delaminations. [Copyright &y& Elsevier]

Abstract: An analytical solution for the free vibrations of beams with two overlapping delaminations in prebuckled states is presented. The delaminated beam is analyzed as seven interconnected Euler–Bernoulli beams. The continuity and boundary conditions are satisfied between adjoining beams. Both the ‘free mode’ and the ‘constrained mode’ assumption in delamination buckling and vibration are used. A parametric study is performed to investigate the influence of the axial compressive load on the natural frequency and the mode shape of the delaminated beam. A monotonic relation between the natural frequency and the compressive load is observed. Comparisons with the analytical results reported for delamination buckling and vibration verify the validity of the present solution. [Copyright &y& Elsevier]

Abstract: The energetic variational analysis of the initial postcritical behavior of homogenized orthotropic columns presented in a previous paper is here revisited in the specific context of sandwich structures. For this purpose, consideration of the transverse deformation in the core is found to be essential. It is shown that imperfection sensitivity can arise for a certain range of stiffness and geometric parameters. The bifurcation is symmetric and Koiter’s 2/3-power law is followed. Finite element simulations of short columns validate analytical predictions of maximum load reduction due to imperfection sensitivity. [Copyright &y& Elsevier]

Abstract: An existing procedure based on the combined use of the Virtual Crack Closure Technique and of a fail release approach for the analysis of delamination growth phenomena in composite structures has been enhanced with a front-tracing algorithm and suitable expressions for the evaluation of the Strain Energy Release Rate when dealing with non-smoothed delamination fronts. The enhanced procedure has been implemented into a commercial finite element software by means of user subroutines and applied to the analysis of a composite stiffened panel with an embedded delamination under compressive load. The effectiveness and robustness of the enhanced procedure have been assessed by comparing literature experimental data and numerical results obtained by using different mesh densities in the damaged area (global/local approach). [Copyright &y& Elsevier]

Abstract: The objective of this paper is to study stability and failure of a composite laminate with a centrally placed cutout of various shapes (i.e., circular, square, diamond, elliptical-vertical and elliptical-horizontal) under combined action of uni-axial compression and in-plane shear loads. The FEM formulation based on the first order shear deformation theory and von Karman’s assumptions has been utilized. Newton–Raphson method is used to solve nonlinear algebraic equations. Failure of a lamina is predicted by the 3-D Tsai–Hill criterion whereas the onset of delamination is predicted by the interlaminar failure criterion. The effects of cutout shape, direction of shear load and composite lay-up on buckling and postbuckling responses, failure loads and failure characteristics of the laminate has been discussed. An efficient utilization of material strength is observed in the case of laminate with circular cutout as compared to the laminate with other shaped cutouts. In addition, it is also concluded that although the buckling strength of the (0/90)4s laminate is lower than that of the (+45/−45/0/90)2s and (45/−45)4s laminates, but its strength is increased in the advanced stage of postbuckling deformation. [Copyright &y& Elsevier]

Abstract: This paper evaluates the humidity effect on the elastic properties and failure mechanism of two types of layered corrugated paperboard structures composed of entirely recycled material and virgin radiata pine (Pinus radiata D. Don) kraft and mixed fibre linerboards and fluting medium. Board samples and their component paper sheets were tested at 50% and 90% constant relative humidity (RH) under compression load and images of failure zones were obtained by serial sectioning and confocal microscopy. The compressive strength and modulus of elasticity of all components were found to be highest in the machine direction at 50% RH and lowest in the cross-machine direction at 90% RH. Virgin paper sheet components performed better in compressive performance than recycled components in both directions and at both relative humidities. In both types of corrugated board microscopic examination showed that failure under compression at 50% and 90% RH occurred along the adhesive line as well as by failure of inter-fibre bonds within the components. The combination of failure mechanisms resulted in local buckling of the boards. The elastic performance of recycled corrugated board was found to be inferior to that of virgin corrugated board at both RH conditions. [Copyright &y& Elsevier]

Abstract: In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites. [Copyright &y& Elsevier]

Abstract: This manuscript investigates compression-after-impact failure in woven fiber-reinforced composites. Compression failure of composite structures previously damaged by an impact event is due to the propagation of impact-induced damage mechanisms such as interlaminar debonding, constituent (i.e., matrix and fiber) microcracking, sublaminate buckling, as well as the interactions between these mechanisms. The failure mechanisms within each ply are idealized based on a reduced order multiscale computational model, in which, the damage propagation in the matrix and fibers upon compression is explicitly modeled. Delamination along the ply interfaces is idealized using a cohesive surface model. The initial impact-induced damage within the microconstituents and interfaces are inferred from experimental observations. A suite of numerical simulations is conducted to understand the sublaminate buckling, propagation of delamination and constituent damage upon compression loading. The numerical investigations suggest extensive propagation of delamination with mode transition preceding sublaminate buckling. Initiation and propagation of matrix and fiber cracking, observed upon sublaminate buckling, is the cause of ultimate shear failure. [ABSTRACT FROM AUTHOR]

In past few decades, the interest in using pultruded fiber reinforced polymeric (PFRP) composites in construction applications has grown rapidly. Several research studies were conducted and focused on the performance of PFRP beams, columns and frame structures. The results of the majority of previous studies highlighted a major problem associated with the deficiency of the off-the-shelf, unidirectional open-web pultruded profiles. In this regards, a common conclusion was drawn by many researchers; that is: the inherent structural deficiency of commercially produced unidirectional PFRP profiles, especially at the flange/web(s) junction(s) that lacks fiber continuity. The lack of fiber continuity creates a “resin-rich” zones at the junctions that were shown to be responsible for rapid degradation of both axial and rotational stiffness as well as the strength of the majority of PFRP profiles. Another related problem is the use of incorrect framing connection details, currently being used by industry. Such connection details mimic those associated with steel structures. This approach ignores both the anisotropic and the viscoelastic nature of composites as well as the aforementioned inherent junction deficiency that, in most cases, lead to a greater risk with regard to the safety, reliability and economic aspects of such structures. This paper presents a summary of an experimental study aimed at evaluating both axial and rotational stiffnesses and strengths of web–flange junctions, which may affect stiffness, buckling, post-buckling, torsional and overall strength of PFRP structures. In particular, three sizes of commercially-produced unidirectional pultruded H-profiles and two sizes of L-profiles were evaluated under both service and ultimate loads. Using full-scale experimental data, P–δ and M–θ relations and idealized expressions for each pultruded profile were developed that can be used for accurate modeling and for establishing design limit-states for PFRP structures. In addition, two special test fixtures were designed, fabricated and validated that can be adopted by ASTM/ISO standards for characterizing such critical mechanical properties that are essential for reliable design of pultruded composite structures. Conclusions and design recommendations are also presented. [ABSTRACT FROM AUTHOR]

Abstract: A fuselage representative carbon fibre-reinforced multi-stiffener panel is analysed under compressive loading. An intact and pre-damaged configuration is loaded into the postbuckling region and further on until collapse occurs. An analysis tool is applied that includes an approach for predicting interlaminar damage initiation and degradation models for capturing interlaminar damage growth as well as in-plane damage mechanisms. Analysis of the intact panel configuration predicts collapse due to fibre fracture in the stiffeners close to the panel clamps, which agrees well with the results from experimental testing. The pre-damaged configuration was proposed containing Teflon-coated layers to generate the initial debonds in the skin-stiffener interface. The outcome of the simulation of this configuration shows that crack growth is not predicted to occur, which agrees with the observations of the experiment. A parametric study is conducted to investigate the effect of the skin-stiffener debond parameters such as length, width and location on crack growth and the collapse behaviour of the panel. It is found that the sensitivity of the panel design to the damage parameters is highly dependent on the postbuckling mode shape or displacement pattern, and particularly the extent to which this influences the conditions at the crack front. More broadly, the analysis tool is shown to be capable of capturing the critical damage mechanisms leading to structural collapse of stiffened composite structures in the postbuckling region. [Copyright &y& Elsevier]

Abstract: A shear loaded, stringer reinforced composite panel is analyzed to evaluate the fidelity of computational fracture mechanics analyses of complex structures. Shear loading causes the panel to buckle. The resulting out-of-plane deformations initiate skin/stringer separation at the location of an embedded defect. The panel and surrounding load fixture were modeled with shell elements. A small section of the stringer foot, web and noodle as well as the panel skin near the delamination front were modeled with a local 3D solid model. Across the width of the stringer foot, the mixed-mode strain energy release rates were calculated using the virtual crack closure technique. A failure index was calculated by correlating the results with a mixed-mode failure criterion of the graphite/epoxy material. The objective was to study the effect of the fidelity of the local 3D finite element model on the computed mixed-mode strain energy release rates and the failure index. [Copyright &y& Elsevier]