The structural integrity of a novel composite adhesively bonded flap-track beam

The structural integrity of a flap-track beam (FTB) made from non-crimp fabric composites and assembled using mechanical fasteners and adhesively bonded joints is investigated using full-scale experiments and numerical modeling. The FTB, being an aircraft structural component, has been selected as the verification structure and demonstrator of a novel material-driven design concept aiming to progressively replace mechanical fasteners by adhesively bonded joints and introduce fabric composites of enhanced through-thickness properties in aero-structures. For the sake of investigation, the FTB has been subjected to a combination of static and fatigue loading conditions taken from realistic load-scenarios. Due to size and complexity of the structure and testing demands, a special test-rig was developed for the occasion. During loading, the response of the structure has being fully monitored using a network of sensors along with an optical measurement system. After loading, the health of the structure was monitored using non-destructive inspection. Complementary to mechanical testing, a 3D detailed FE model was developed for virtual experimenting. On the basis of the model, progressive damage modeling has been implemented to simulate initiation and progression of composite failure as well as debonding. Both experiments and model have shown that the composite FTB is capable to effectively carry the load which the original fastened metallic FTB has been designed to carry. Localized damage that has no significant effect on the integrity of the structure has been detected at fastener areas and bonded joints containing initial defects. Between experiments and model a satisfactory agreement has been achieved considering the uncertainties in experimental procedure and numerical errors of the model.     

Failure analysis in postbuckled composite T-sections

Blade-stiffened skin designs made of composite materials have the potential to produce highly efficient structures, when the large strength reserves in the postbuckling range are utilised. This paper investigates the failure under postbuckling deformations of T-section specimens cut from a blade-stiffened panel, by comparing experimental results to finite element models. In the experimental work, T-section specimens with a particular lay-up and geometry were tested to failure in antisymmetric and symmetric loading rigs. These loading rigs simulate deformations on skin-stiffener interfaces during panel postbuckling. For the numerical analysis, two-dimensional models of the interface cross-section were used with a strength-based criterion that monitored failure within each ply. The use of a zero-thickness layer of cohesive elements has also been investigated in order to simulate the delamination behaviour. The numerical predictions are compared to the experimental results in terms of the failure load, specimen stiffness and specimen behaviour. The analysis approach is shown to be capable of predicting the critical damage locations and initiation loads for both antisymmetric and symmetric loadings. The successful prediction of failure in skin-stiffener interfaces can be linked to a global-local approach for efficient analysis of large, fuselage-representative composite structures.     

Failure load evaluation and prediction of hybrid composite double lap joints

Hybrid joints are a combination of adhesive bonding and mechanical fastening and are known to combine the advantages of both joint types. In this paper, we evaluate and compare the strengths of mechanical joints, adhesive joints and hybrid composite joints. We manufactured and tested 10 hybrid joint specimens with different width-to-diameter (w/d) ratios, edge-to-diameter (e/d) ratios and adherend thicknesses. Additionally, the strengths of the hybrid joints were predicted using the Failure Area Index (FAI) method and the damage zone method, and we compared our theoretical predictions with our experimental results. From these data, we were able to predict hybrid joint strengths to within 23.0%.     

Structural analysis of composite laminates using a mixed hybrid shell element

The structural analysis of thin composite structures requires robust and effective shell elements. In this paper the variational formulation is based on a Hu–Washizu functional with independent displacements, stress resultants and shell strains. For the independent shell strains an additional interpolation part is introduced. This yields an improved convergence behaviour especially for laminated shells with coupled membrane and bending stiffness. The developed mixed hybrid shell element possesses the correct rank and fulfills the in–plane and bending patch test. The formulation is tested by several nonlinear examples including bifurcation and post–buckling response. The essential feature of the new element is the robustness in nonlinear computations with large rigid body motions. It allows very large load steps in comparison to standard displacement models.     

Charpy impact damage behaviour of single and multi-delaminated hybrid composite beam structures

The ply delamination which is known as a principle mode of failure of layered composites due to separation along the interfaces of the layers is one of the main concerns in designing of composite material structures. In this regard, the effect of hybrid laminate lay-up with different delamination positions in composite beam was investigated. The Charpy impact test was chosen to study the energy absorbing capability of delaminated composite beam. Hybrid composite beams were fabricated from combination of glass/epoxy and carbon/epoxy composites. It was shown that composite beams with closer position of delamination to impacted surface are able to absorb more energy in comparison with other delamination positions in hybrid and non-hybrid ones. The Charpy impact test of delaminated composite beams was also simulated by finite element software LS-DYNA and the results were verified with the relevant experimental results.     

Modal analysis of cracked, unidirectional composite beam

In this paper, a model and an algorithm for creation of the characteristic matrices of a composite beam with a single transverse fatigue crack are presented. The element developed has been applied in analysing the influence of the crack parameters (position and relative depth) and the material parameters (relative volume and fibre angle) on changes in the first four transverse natural frequencies of the composite beam made from unidirectional composite material.     

Failure analysis of composite laminates under large deflection

This paper presents the results of the application of a procedure, developed by Reddy & Reddy [ Composites Science and Technology 1992, 44, 227–255], for the evaluation of the first ply-failure load in multilayered composite plates. The procedure, which is based on the use of the finite element method (FEM) and which is suitable for the analysis of generally loaded plates, uses the non-linear von Karman formulation and, therefore, allows comparison of the failure loads in both the linear and the geometrically non-linear behaviour. Nevertheless, the use of the Newton-Raphson technique in searching the non-linear equilibrium points restricts its application to the case of plates without limit-point behaviour. The displacement model adopted in the FEM formulation is the traditional firstorder Reissner-Mindlin plate model that takes the shear deformation effect into consideration. Concerning the failure criteria, the analysis is based on a tensor polynomial criterion to which all other polynomial and independent criteria are brought back as particular cases. The study refers to the failure analysis of thin and thick plates under a uniformly distributed transverse load. Furthermore, a comparison of the failure criteria when the shear stresses are evaluated by means of the constitutive equations and by means of the local equilibrium equations is carried out. Finally, adopting a very simple degradation model of the mechanical properties to account for the stiffness decrease consequent to the failure, the qualitative behaviour of plates after the first non-catastrophic failure is also presented.     

Failure analysis of adhesively bonded joints in composite materials

The present paper is concerned with a phenomenological model to perform the failure analysis of composite adhesive single lap joints with arbitrary glued area. The theory is conceived for joints composed by highly resistant elastic adherends bonded with brittle–elastic adhesives. It is shown that, under certain conditions, the rupture forces (in the case of monotonic loading) and lifetimes (in the case of cyclic loading) of two joints with different glued areas can be correlated using a shape factor. Results from experimental static and fatigue testing of joints with carbon/epoxy laminates bonded with epoxy adhesive and different bonding areas are compared with model prediction showing a good agreement.     

Geometrically nonlinear composite beam structures: design sensitivity analysis

The purpose of this paper is to develop a finite element model for optimal design of composite laminated thin-walled beam structures, with geometrically nonlinear behavior, including post-critical behavior. A continuation paper will be presented with design optimization applications of this model. The structural deformation is described by an updated Lagrangean formulation. The structural response is determined by a displacement controlled continuation method. A two-node Hermitean beam element is used. The beams are made from an assembly of flat-layered laminated composite panels. Beam cross-section mass and stiffness property matrices are presented.

Design sensitivities are imbedded into the finite element modeling and assembled in order to perform the structural design sensitivity analysis. The adjoint structure method is used. The lamina orientation and the laminate thickness are selected as the design variables. Displacement, failure index, critical load and natural frequency are considered as performance measures. The critical load constraint calculated as the limit point of the nonlinear response is also considered, but a new method is proposed, replacing it by a displacement constraint.     

Thermal buckling and postbuckling analysis of a laminated composite beam with embedded SMA actuators

In this paper, the thermal buckling and postbuckling behaviours of a composite beam with embedded shape memory alloy (SMA) wires are investigated analytically. For the purpose of enhancing the critical buckling temperature and reducing the lateral deflection for the thermal buckling, the characteristics of thermal buckling are investigated through the use of the shape recovery force associated with SMA wire actuators. The results of both thermal buckling and postbuckling behaviours present quantitatively how the shape recovery force affects the thermal buckling behaviour. The analytical results show that the shape recovery force reduces the thermal expansion of the composite laminated beam, which results in both an increment of the critical buckling temperature and also a reduction of the lateral deflection of postbuckling behaviours. A new formula is also proposed to describe the critical buckling temperature of the laminated composite beam with embedded SMA wire actuators.     

Elastic analysis of hybrid bonded joints and bonded composite repairs

The authors extend the closed-form bonded joint linear elastic analysis method of Delale et al. [Delale F, Erdogan F, Aydinoglu MN. Stresses in adhesively bonded joints: a closed-form solution. J Compos Mater 1981;15:249–71] and Bigwood and Crocrombie [Bigwood DA, Crocombe AD. Elastic analysis and engineering design formulae for bonded joints. Int J Adhes Adhes 1989;9(4):229–42] to include the composite deformation mechanisms and the thermal residual strains that arise in hybrid metal-composite joints such as those presented by bonded composite repairs applied to metallic aircraft structures. The analytical predictions for the adhesive stresses and the compliance are compared to the results of a linear elastic finite element model that has itself been validated by comparison with experimental results. The results are applied to the problem of coupled linear extension and bending of a bonded composite repair applied to a cracked aluminum substrate. The resulting stress intensity factor and crack-opening displacement in the repaired plate are compared to the results of a three-dimensional finite element analysis, and also exhibit excellent results. Throughout the text, observations are made regarding the practical application of the results to failure prediction in hybrid joints, whereby the authors demonstrate the need for consistency in the analytical methods used to determine the fatigue and failure of composites from the coupon level to the analysis of the final structural details.     

Mechanical properties of hybrid kenaf/glass reinforced epoxy composite for passenger car bumper beam

It is estimated that the annual world car production rate will reach 76 million vehicles per year by 2020. New regulations such as the EU End of Life Vehicles (ELV) regulations are forcing car manufacturers to consider the environmental impact of their production and possibly shift from the use of synthetic materials to the use of agro-based materials. Poor mechanical properties and certain manufacturing limitations currently limit the use of agro-based materials to non-structural and semi-structural automotive components. The hybridization of natural fiber with glass fiber provides a method to improve the mechanical properties over natural fibers alone. This research is focused on a hybrid of kenaf/glass fiber to enhance the desired mechanical properties for car bumper beams as automotive structural components with modified sheet molding compound (SMC). A specimen without any modifier is tested and compared with a typical bumper beam material called glass mat thermoplastic (GMT). The results indicate that some mechanical properties such as tensile strength, Young’s modulus, flexural strength and flexural modulus are similar to GMT, but impact strength is still low, and shows the potential for utilization of hybrid natural fiber in some car structural components such as bumper beams.     

A statistical analysis of the tensile failure and hybrid effect of an intraply hybrid composite

The random critical-core probability model is adapted to model a unidirectionally arrayed hybrid sheet with alternating low elongation fibers and high elongation fibers. Utilizing this model in conjunction with the results of the stress analysis by Fukuda and Chou, both the first failure and ultimate failure of the hybrid are analysed. The concept of the hybrid effect for strains has been clarified and quantitative evaluation of the part of the hybrid effect which is not of thermal origin has been done. Numerical results are compared with available experimental data. A reasonable agreement is found between the analytical predictions and the experimental results.     

Failure analysis of corroded pipelines reinforced with composite repair systems

The present paper is concerned with the analysis of corroded thin-walled metallic pipes reinforced with polymer based composite repair systems. The goal is to propose a simple methodology to predict the failure pressure of a reinforced pipeline with arbitrary geometry of the corroded region and considering any composite repair system. Hydrostatic tests performed in different laboratories were used to validate the proposed methodology, showing that a simple expression allows estimating a lower bound for the failure pressure.     

Failure analysis of multilevel metallized lsi using optical beam induced current

In order to analyse failures caused in multilevel metallized LSI devices. we developed the OBIC (optical beam induced current) observation technique using an infrared laser incident upon a backside of the chip which appears from a plastic package by being lapped and polished. We analysed the electrical rejects in the reliability stressing of surface mount plastic packages with this technique. The latch-up phenomenon in 4Mbit dynamic RAM with retrograded-well fabricated by a high energy (MeV) ion-implantation was also examined. The high latch-up immunity of the device with retrograded-well is confirmed by this analysis.     

附加约束阻尼层的复合材料梁单元建模分析

复合材料空心圆截面梁是桁架和刚架结构中大量采用的常用构件，而实践证明约束阻尼层能有效改善复合材料空心圆截面梁的动力学特性，但传统的约束阻尼层结构有限元计算方法需要大量的单元，这给大型复杂结构的计算带来了巨大的困难。本文采用Timoshenko梁假定。建立了一类附加约束阻尼层复合材料空心圆截面梁弯曲的数学模型。应用Hamilton原理。采用三节点高次梁单元对构件进行离散化。建立了附加约束阻尼层复合材料空心圆截面梁的梁单元。同传统的锥壳单元相比，该方法极大地减少了计算时间。用实验验证了本文计算结果的正确性。同时也分析了约束层厚度对损耗因子的影响。     

Analytical layerwise solution of nonlinear thermal instability of SMA hybrid composite beam under nonuniform temperature condition

Abstract

Thermal buckling and post-buckling behavior of composite beam reinforced with shape memory alloy (SMA) wires under nonuniform temperature distribution is explored. Thermo-mechanical behavior of SMA wires is formulated by using the one-dimensional Brinson SMA model. Considering von Karman strain–displacement relation, corresponding nonlinear governing equations are obtained and solved analytically. Heat conduction equation is employed and through-the-thickness temperature distribution is obtained by discretization scheme of layerwise method. Influence of SMA-wire positioning across the thickness, temperature distribution, SMA wire pre-straining level and volume fraction of SMAs upon the thermal buckling and post buckling of reinforced beam are examined and discussed in detail.     

Damage analysis for structural integrity and durability of composite materials

Composite structures for mechanical and aerospace applications are designed to retain structural integrity and remain durable for the intended service life. Since the early 1970s important advances have been made in characterizing and modelling the underlying mechanical behaviour and developing tools and methodologies for predicting fracture and fatigue of composite materials. This paper presents a review of the concepts and analyses related to this area, and illustrates these by a few examples. The topics discussed are composite material strength in tension, compression and shear, damage and its progression in monotonic and cyclic loading, fatigue life prediction and damage induced changes in visco‐elastic response.