Numerical and experimental analysis of delamination in the T-stiffener integrated composite structure

In this article, two kinds of cohesive zone models (CZMs; exponential and bilinear) are used to evaluate the delamination behaviors of a composite T-stiffener integrated structure. First, based on the analysis of the bilinear CZM using maximum nominal stress damage initiation criterion and power law energy criterion, both the macroscopic mechanical response and the failure process are predicted, which analyzed the influences of the various cohesive zone parameters on the failure load and the damage patterns. Second, both the strength and the fracture characterizations about various T-stiffener integrated composite structures are studied in the experiment, which have a good agreement between the numerical result and the experimental data. Finally, the relationships among the failure load and the thickness of the skin, and the clamp distance are established; also, the energy release rates of the T-stiffeners for the failure process are predicted. These results will play an important role for designing and evaluating the strength and reliability of composite T-stiffener integrated structures.     

Using a new composite expansive material to decrease deformation and fracture of concrete

Using minerals of dolomite, serpentine and magnesite produce a new composite material which provides an expansive stress to decrease deformation and fracture of hydraulic concrete. The compositions of the expansive material are mainly MgO, CaO and C₂S by X-ray techniques. Adding the expansive material to concrete, the shrinkage of concrete may be compensated by the hydration of CaO to Ca(OH)₂ in early ages and the hydration of MgO to Mg(OH)₂ in later ages respectively, so decrease deformation and fracture of concrete.     

Drilling of composite structures

Structural parts made of composites have frequently to be drilled in the aircraft industry. However, little is know about the interacting conditions between the drilling tool and the material, which may be multi-type and multi-size. This study proposes a model which links the axial penetration of the drill bit to the conditions of delamination (crack opening mode I) of the last few plies. Several types of tool/material contact conditions were analyzed and were compared with experimental measurements, and with a model taken from the literature. Our study shows a close correlation between experiment and calculation when the thrust force of the drill is modeled by taking into account the geometrical nature of the contact between the tool and a laminate composite material.     

Optimal design of composite ChamberCore structures

An optimization procedure has been developed to uniquely and efficiently determine the “best” local geometry design of a new composite ChamberCore structure. This procedure is based on minimization of the total mass of a single composite ChamberCore subject to a set of design and stress constraints. The stress constraints are obtained in closed form based on the composite box-beam model for various composite lamination designs and loading conditions. The optimization problem statement is constructed and then solved using the VMCON optimization program, which is an iterative sequential quadratic programming (SQP) technique based on Powell's algorithm. The sensitivity of the solution of the optimal geometry to the values of parameters that characterize the structural durability and the failure mechanism is discussed.     

Design of composite structures including delamination studies

Composite materials are increasing their use in structures used for road transportation vehicles. From car bodies to trucks, these materials are being more popular. This use is obliging designers to take care of topics different to the traditional ones already studied for aerospace applications. Design must be simplified and, what is even more important, commercial codes should be used to continue with the extension of the use of composite materials in non aerospace industry. One of the most important problems arising when dealing with finite element design of composite materials including delamination is the requirement of using huge computer resources to solve the three dimensional stress field in the vicinity of free edges, holes, changes of number of layers and similar discontinuities.

To solve this problem, there are some well known techniques as global-local approaches that can be used for designing with good results. From an engineering point of view, the problem is that they are not usually implemented in commercial finite element codes so that engineers can not use them for everyday calculations.     

Clamping effects on the dynamic characteristics of composite machine tool structures

Substituting composite structures for conventional metallic structures has many advantages for structural dynamic characteristics because composite materials have the higher specific stiffness and damping characteristics than conventional materials. However, the dynamic characteristics such as the fundamental natural frequency and damping of composite structures are influenced much by their joints. In this work, the effects of clamping conditions on the dynamic characteristics of cantilever type composite machine tool structures with clamped joint were investigated to increase the natural frequency and damping of structures. In order to improve the shear property of the clamping part of composite machine tool bar, a new method for the clamping part was developed with metal core or sleeve inserted in the composite body at the clamping part. From the finite element and experimental results, suitable clamping conditions for the maximum dynamic stiffness were obtained for the composite structures with clamped joint.     

Buckling of thin-walled composite structures with intermediate stiffeners

The interactive buckling of prismatic, thin-walled composite columns with open sections, reinforced with intermediate stiffeners and with edge reinforcements, has been considered. The columns are assumed to be simply supported. The nonlinear problem has been solved with the Koiter’s asymptotic theory within the first order approximation. The asymptotic theory of the first order nonlinear approximation allows for simultaneous evaluation of the effect of imperfections and interactions of various modes of buckling on the behaviour of thin-walled structures. This evaluation can be only the lower bound estimation of the load carrying capacity. Detailed calculations have been made for several cases of columns.     

Anisogrid composite lattice structures - Development and aerospace applications

The paper is an overview of the recent Russian experience in development and applications of Anisogrid (Anisotropic Grid) composite lattice structures. Anisogrid structures have the form of cylindrical (in general, not circular) or conical shells and consist of a dense system of unidirectional composite helical, circumferential and axial ribs made by continuous filament winding [1] and [2].High weight and cost efficiency of Anisogrid structures is provided by high specific (with respect to density) strength and stiffness of unidirectional ribs used as the basic load-carrying elements of the structure and by automatic winding process resulting in low-cost integral structures. Anisogrid structures proposed about 30 years ago are under serial production in Central Research Institute of Special Machinery (CRISM) which develops lattice interstages, payload attach fittings (adapters) and spacecraft structures for Russian space programs. By now, about 40 successful launches have been undertaken with Anisogrid composite lattice structures.The paper provides the information about fabrication processes, design and analysis methods, mechanical properties of the basic structural elements and application of Anisogrid composite design concept to aerospace structures.     

Damage detection in T-joint composite structures

The use of composite structures in engineering applications has proliferated over the past few decades. This is mainly due to their distinct advantages of high structural performance, high corrosion resistance, and high strength/weight ratio. They are however prone to fibre breakage, matrix cracking and delaminations which are often invisible. Although there are systems to detect such damage, the characterisation of the damage is often much more difficult to achieve. A study is presented of the strain distribution of a GFRP T-joint structure under tensile pull-out loads and the determination of the presence and the extent of disbonds. Finite element analysis (FEA) has been conducted by placing delaminations of different sizes at various locations along the structure. The FEA results are also validated experimentally. The resulting strain distribution from the FEA is pre-processed by a method developed called the damage relativity assessment technique (DRAT). Artificial neural networks (ANNs) were used to determine the extent of damage. A real-time system has been developed which detects the presence, location and extent of damage from the longitudinal strains obtained from a set of sensors placed on the surface of the structure. The system developed is also independent of the magnitude of load acting on the structure.     

Multilevel assessment method for reliable design of composite structures

This work aims at demonstrating the interest of a new methodology for the design and optimization of composite materials and structures. Coupling reliability methods and homogenization techniques allow the consideration of probabilistic design variables at different scales. The main advantage of such an original micromechanics-based approach is to extend the scope of solutions for engineering composite materials to reach or to respect a given reliability level. This approach is illustrated on a civil engineering case including reinforced fiber composites. Modifications of microstructural components properties, manufacturing process, and geometry are investigated to provide new alternatives for design and guidelines for quality control.     

Structural integrity analysis of embedded optical fibres in composite structures

The virtues of having sensors in manufactured goods for increased functionality purposes have been well documented. Benefits include sophisticated structures requiring less maintenance and repair, increased safety and reliability, and avoidance of ‘over design’. Though many schemes of sensing are available, these so-called ‘smart’ products in the near future, will increasingly rely on the optical fibres (OF) principles because of numerous inherent advantages. Optical fibres are small, lightweight, possess geometrical flexibility, Electromagnetic interference (EMI) immunity, operate over a wide range of environmental conditions, and can be configured to respond to many physical parameters.

This paper will report on the suitability of embedding OF in commonly used carbon-fibre composites. These panels will be designed, manufactured and tested for the effects of typical fibre-optic geometrical and physical parameters such as types of fibre coating polymers, fibre diameter and fibre distribution. Corroboration of these test results with finite element (FE) results will be shown. Based on tensile and compression tests on OF-embedded composites, it is shown that significant deterioration on strength is observed beyond a certain OF density level. This paper will focus on the macroscopic effect of having optical fibres in composites from a structural integrity point of view. To this end, an exposition on the theoretical considerations using continuum mechanics and energy principles is provided.     

Geometrically nonlinear composite beam structures: optimal design

This paper applies the formulation and the finite element analysis and sensitivity analysis model developed in a companion paper to the optimal design of various geometrically nonlinear composite laminate beam structures. The element design sensitivities are imbedded in the finite element code and the global sensitivities are interfacing the analysis with a nonlinear programming optimizer. Laminate thickness and lamina orientations are considered as design variables. Mass, stiffness and deflection are used as objectives or combined as multiobjective functions. The nonlinear critical load or the Tsai-Hill stress failure index, are used as constraints. Postcritical domain behavior allows the advantageous substitution of the critical load constraint by a displacement constraint.     

Progressive damage modeling for large deformation loading of composite structures

A nonlinear constitutive model for large deformation loading at different strain rate condition was developed to represent tensile progressive damage of the nonlinear large deformation rate dependent behavior of polymer-based composite materials. The material was characterized by using off-axis composite specimens at different strain rates. A new failure criterion was proposed for the analysis of different loading directions and strain rates. Based on a method of combining the nonlinear constitutive theory and the proposed failure criterion for different strain rates, the progressive damage behavior of large deformation composites was represented. The strength of the material was also successfully represented with a single material constant.     

Scale effects on the response of composite structures under impact loading

For several years, composite materials have taken a significant part in the realization of structures designed for transport (aeronautical, nautical, automotive…). In order to qualify the behavior of such structures, preliminary validation tests have to be done. These specific tests are often very expensive and difficult to set up, especially when the structure dimensions are large (fuselages of aircraft, ship hulls…). An alternative way is then to employ small-scale models.The use of these reduced scale structures requires the identification of similitude models allowing the extrapolation of the small-scale model behavior to the real structure. Although largely developed in the case of homogeneous materials, such similitude techniques are not clearly identified for composite materials taking into account the damage evolution during an impact.The purpose of this article is firstly to present existing similitude techniques making it possible to predict the composite structure behaviour from the knowledge of small-scale model response. Secondly, experiments were done on two scale of samples carried out by stratification of unidirectional carbon/epoxy plies. These results were finally compared with the analytical predictions of similitude laws currently used.The aim of this paper is to contribute to similitude laws development applied to composite structures. These laws permit to extrapolate the small-scale model behavior to the real scale one. Existing approaches have been established following two different methods. They are summarized in this paper and applied to impact loadings on two laminated plate scales. In order to complete data collected by “conventional” instrumentation (force transducer, displacement sensor, accelerometer…), optical device such as an high-velocity CCD camera, associated with optical techniques for the monitoring of markers, were used. These techniques make possible to compare displacement lines corresponding to each scale. It is shown that existing similitude laws, used for elastic materials, do not allow to simulate the behavior of the real scale when this one is damaged.     

Static behavior of composite beams using various refined shear deformation theories

Static behavior of composite beams with arbitrary lay-ups using various refined shear deformation theories is presented. The developed theories, which do not require shear correction factor, account for parabolical variation of shear strains and consequently shear stresses through the depth of the beam. In addition, they have strong similarity with Euler–Bernoulli beam theory in some aspects such as governing equations, boundary conditions, and stress resultant expressions. A two-noded C¹ finite element with six degree-of-freedom per node which accounts for shear deformation effects and all coupling coming from the material anisotropy is developed to solve the problem. Numerical results are performed for symmetric and anti-symmetric cross-ply composite beams under the uniformly distributed load and concentrated load. The effects of fiber angle and lay-ups on the shear deformation parameter and extension-bending-shear-torsion response are investigated.     

The analysis of skin-to-stiffener debonding in composite aerospace structures

A comprehensive finite element (FE) analytical tool to predict the effect of defects and damage in composite structures was developed for rapid and accurate damage assessment. The structures under consideration were curved, T-stiffened, multi-rib, composite panels representative of those widely used in aerospace primary structures. The damage assessment focussed on skin-to-stiffener debonding, a common defect that can critically reduce the performance of composite structures with integral or secondary bonded stiffeners. The analytical tool was validated using experimental data obtained from the structural test of a large stiffened panel that contained an artificial skin-to-stiffener debond. Excellent agreement between FE analysis and test results was obtained. The onset of crack growth predictions also compared well with the test observation. Since the general damage tolerance philosophy in composite structures follows the “no-growth” principle, the critical parameters were established based the onset of crack growth determined using fracture mechanics calculations. Parametric studies were conducted using the analytical tool in order to understand the structural behaviour in the postbuckling range and to determine the critical parameters. Parameters considered included debond size, debond location, debond type, multiple debonds and laminate lay-up.     

Modal coupled instabilities of thin-walled composite plate and shell structures

The problem of buckling and initial post-buckling equilibrium paths of thin-walled structures built of plate and/or shell elements subjected to compression and bending has been solved. Plate and shell elements can be made of multi-layer orthotropic materials. A method of the modal solution to the coupled buckling problem within the first-order approximation of Koiter’s asymptotic theory, using the transition matrix method, has been presented. In the solution obtained, the effect of cross-sectional distortions and a shear lag phenomenon is included. The calculations are carried out for a few thin-walled structures.     

An experimental study of the insert joint strength of composite sandwich structures

This paper addresses an experimental study of the pull-out and shear failure loads of composite sandwich insert joints. For specimen fabrication, a Nomex honeycomb core and a carbon–epoxy composite were used. The film-type adhesive FM73 was used for core and face co-cured bonding. Specimen sizes are 120 × 120 mm² for the pull-out test and 60 × 120 mm² for the shear loading test. About 80 specimens of 16 different types depending on the core height and density, face thickness, insert clearance, and loading direction were tested. The results show that while the insert joint failure loads for pull-out loading are affected by the core height and density, they are also greatly influenced by the face thickness. The joints with 0.11 mm clearance between the insert flange and the composite face, which is filled with potting material, showed larger failure loads compared to tightly fitted insert joints. In the shear loading, the failure loads of joints were dominated by the face thickness, while core properties such as the core height and direction had little effect on the failure load.     

General model for constitutive relationships of concrete and its composite structures

This paper presents a unified mathematical model that can be used to construct various constitutive relationships for concrete materials and its composite interfaces. The unified model has a single expression and a maximum of six variables/parameters, whereas in most cases, three to four parameters are sufficient. The model is simple, continuous, and easy to be integrated and differentiated. Its parameters can be assigned clear physical meanings, and used to control variations of different parts of the curve separately. Its greater versatility and flexibility enables possible applications in almost all kinds of constitutive relationships for concrete and its composite structures. For example, it is demonstrated to be suitable for stress-strain relationships of plain and confined concrete, bond-slip relationships of steel bar-to-concrete interfaces, externally-bonded fiber-reinforced polymer (FRP) interfaces, near surface mounted FRP interfaces, etc. Furthermore, a simple equation for the bond-slip relationship of steel reinforcement-to-concrete interface is derived using the unified model to replace the four-segment CEB-FIP code model.     

Thermal characteristics of composite sandwich structures for machine tool moving body applications

The thermal deformation affects the accuracy of a precision machine tool. There are various heat sources in machine tools such as motors, spindle units, and friction in LM-guide systems. In this work, the thermal characteristics of composite sandwich structures for machine tool parts were investigated both by FEM analysis and experiment. Then the machine tool slide of a high speed CNC milling machine was designed and manufactured with composite sandwich structures combined with a welded steel structure––a hybrid machine tool structure. In addition, the reliability of adhesive joints between the composite sandwich and the steel structure was investigated in terms of adhesive joint strength and thermal stress induced by heat generation of linear motors.