Ultimate strength of composite laminates with free-edge delamination

The present paper deals with the free edge effect of composite laminates by using a generalized quasi-three dimensional analysis and experimental verification of an analysis performed for laminates with Teflon inserted on interfaces to simulate initial free-edge delamination. We performed tensile tests for laminates [30₂/?30₂/90]_s carbon-epoxy laminates with free-edge delamination under uniaxial tension. The experiment reveals that extensional stiffness of the laminate decreases by the initiation of the delamination, and that strength of the laminate without delamination is smaller than that of the laminates with delamination. Generalized quasi-three dimensional finite element technique, which employs energy release rate and maximum stress criteria, is developed to estimate behavior of the laminate after initial delamination. The numerical result by use of this technique predicts the ultimate strength of the laminates with sufficient accuracy according as the comparison with an experimental stress-strain curve. In the experiment conducted both for the laminate with initial delamination and the laminate without initial delamination, an unexpected results were obtained that is the ultimate load of the laminate without initial delamination was lower than that of the laminate with initial delamination. We presented clear explanation on the phenomenon occurred and developed the method to predict the nonlinear behavior of the laminate with or without initial delamination.     

Investigation on delamination morphology during drilling composite laminates

Drilling-induced delamination of composite materials is a key factor that affects the quality of subsequent machining. To investigate the developing process of delamination, experiments with different drilling depth are conducted. In order to observe the delamination of different cross-sections in radial direction of the hole, the grinding method is adopted. Three-dimensional morphology of delamination at the exit of hole is obtained. The regularity of delamination with the change of drilling depth is analyzed, and the existence of “hidden delamination zone” is obtained finally. Due to the rebound effect of hole diameter and the inverted cone of drill guide section, the “hidden delamination zone” will be generated under the condition that the edge of delamination area is compressed tightly again. The critical thrust force of delamination is also studied, and it is proved to be correct.     

Post-buckling behavior of carbon fiber epoxy composite plates

In this study, the pre-buckling and post-buckling behaviors of layered composite plates which were made of woven carbon fiber fabric with a circular hole in the middle were investigated experimentally and numerically. Firstly, load-displacement graphs of composite plates with different hole diameters were experimentally obtained under compressive load. Then the numerical load-displacement graphs of the plates were found with the ANSYS package program which used the finite element method. As a result, after linear buckling experimental and numerical results were found to be compatible with each other. In addition, damage behavior of plates after buckling with the aid of Tsai-Wu damage criterion was obtained similar to experimental results. The increase in hole diameter did not change the load-displacement behavior characteristics of the plates after buckling. However, it has reduced maximum damage load and maximum failure displacement. The stress at the perimeter of the hole increased significantly with the increase of the vertical displacement with immediately after the buckling but later was not significantly affected by this increase.

     

The fatigue behavior and delamination properties in fiber reinforced aramid laminates

The fuselage-wing intersection suffers from the cyclic bending moment of variable amplitude. Therefore, the influence of cyclic bending moment on the delamination and the fatigue crack propagation behavior in AFRP/A1 laminate of fuselage-wing was investigated in this study. The cyclic bending moment fatigue test in AFRP/A1 laminate was performed with five levels of bending moment. The shape and size of the delamination zone formed along the fatigue crack between aluminum sheet and aramid fiber-adhesive layer were measured by an ultrasonic C-scan. The relationships between da/dN and K, between the cyclic bending moment and the delamination zone size, and between the fiber bridging behavior and the delamination zone were studied. As results, fiber failures were not observed in the delamination zone in this study ; the fiber bridging modification factor increases and the fatigue crack growth rate decrease ; and the shape of delamination zone is semi-elliptic with the contour decreasing nonlinearly toward the crack tip.     

The compressive behavior of CFRP laminates with delamination of different shapes using numerical and experimental approaches

Composite laminates often produce delamination due to a series of factors during the manufacture and service process. In order to research the effect of containing oblique elliptical and circular initial delamination damage on the compressive strength of composite laminates, numerical and experimental methods are used in this paper. Finite element models (FEMs) and the progressive damage subroutine USDFLD are developed to predict the damage initiation and extension behavior of the intralaminar of the laminate. Interlaminar damage is predicted based on cohesive zone models (CZM). At the same time, four compression tests containing different initial delamination damage are performed. The results show that experimental measurements of compression tests concur with the numerical predictions and validate the FEMs. The strength and stiffness of the specimens gradually decrease with the increase of the initial delamination area. It reveals that, for the compression process, the delamination damage extends from the edge of the initial damage to the surroundings. The bearing capacity of the composite laminates is mainly determined by the material properties of the fibers and matrix, rather than the initial delamination.

     

Energy release rate of postbuckled laminates with a through-width delamination

The strain energy release rate is calculated for buckled one-dimensional delamination (through-width delamination) in composite laminates subjected to in-plane compression. A crack closure method based on plate finite elements is used in this analysis. For some laminates containing a one-dimensional delamination in cylindrical bending, closed form solutions are available. The present finite element solutions show excellent agreement with the analytical solutions. The strain energy release rate for various types of laminates is also calculated using the present finite element method. The results show that the strain energy release rate strongly depends on the type of laminate.     

Post-buckling behavior of poroelastic columns

The post-buckling behavior of poroelastic columns subjected to axial compressive forces is investigated. The fluid-saturated poroelastic columns are permeable in the longitudinal direction and impermeable in the transverse directions as a result of the microgeometry of the material. The time-dependent behavior of the columns is governed by three coupled equations, obtained using the large deflection theory. These equations are transformed into a single one, enabling the analytical derivation of the initial and the final responses. It is shown that unlike the quasi-static response obtained by using the small deflection theory, the long time response derived here is bounded. The imperfection sensitivity of these columns is also investigated.     

Mechanics of crack propagation in delamination wear

A mathematical model of subsurface crack propagation in sliding contact is developed. It is shown that linear elastic fracture mechanics may be applied to such problems, even for elastoplastic solids. An equation to predict wear rates is derived, which should apply in delamination wear to materials in which crack nucleation is easy. Results of calculations of stress intensity factors for various subsurface cracks and coefficients of friction are presented. The numerical calculations were done only for coefficients of friction greater than 0.5.The calculations show that there is a characteristic crack propagation depth below the surface, which increases with increasing coefficient of friction. In addition, the change in stress intensity factor and hence the crack propagation rate, increases with increasing coefficient of friction. A comparison between these calculations and approximate crack growth rates in sliding wear shows that the results agree reasonably well.The model is used to explain the phenomenon that increased hardness sometimes increases wear. A possible explanation for seizure in geometrically constrained systems is advanced. The model may be used to predict wear rates from first principles in the near future.     

Interlaminar stress analysis of general composite laminates

In this study, based on the reduced from of elasticity displacement field for a long laminate, an analytical method is established to exactly obtain the interlaminar stresses near the free edges of generally laminated composite plates under the extension and bending. The constant parameters, which describe the global deformation of a laminate, are properly computed by means of the improved first-order shear deformation theory. Reddy's layerwise theory is subsequently utilized for analytical and numerical examinations of the boundary layer stresses within arbitrary laminated composite plates. A variety of numerical results are obtained for the interlaminar normal and shear stresses along the interfaces and through the thickness of laminates near the free edges. Finally the effects of end conditions of laminates on the boundary-layer stress are examined.     

Numerical study on failure behavior of open-hole composite laminates based on LaRC criterion and extended finite element method

Journal of Mechanical Science and Technology - A new numerical model combining LaRC failure criterion and extended finite element method (XFEM) is created to describe the failure behavior in...     

Effect of tool wear on delamination in drilling composite materials

Among all machining operations, drilling using twist drill is the most frequently applied for secondary machining of composite materials owing to the need for structure joining. Delamination is mostly considered as the principal failure model in drilling of composite materials. Drill wear is a serious concern in hole-making industry, as it is necessary to prevent damage of cutting tools, machine tools and workpieces. The industrial experience shows the worn drill causes more delamination. This paper presents a comprehensive analysis of delamination caused by the drill wear for twist drill in drilling carbon fiber-reinforced composite materials. The critical thrust force at the onset of delamination for worn drill is predicted and compared with that of ideal drill. The experimental results demonstrate that though the critical thrust force is higher with increasing wear ratio, the delamination becomes more liable to occur because the actual thrust force increases to larger extent, as the thrust factor (Z) illustrates. Compared to sharp drill, the worn twist drill allows for lower feed rate below which the delamination damage can be avoided.     

The delamination theory of wear and the wear of a composite surface

The delamination theory of wear postulates that there is a “nonworkhardening” soft surface layer which deforms continuously due to the instability of dislocations, and that the low speed sliding wear of metals is caused by the subsurface crack nucleation and propagation nearly parallel to the surface. A corollary of the theory is that when hard metal surfaces are plated with a soft metal to reduce the coefficient of friction and the wear rate, the soft metal layer must be thinner than a critical thickness so as to prevent the accumulation of dislocations in the plated layer and the formation of the delaminated layer. This corollary was investigated by plating annealed AISI 1018 steel with cadmium. The wear rate of the steel specimen plated with 0.1 μm cadmium on both of the contacting surfaces was three orders of magnitude smaller than the unplated specimen when they were tested in argon. In the case of thicker coatings, the cadmium layer wears by the delamination process which occurs within the plated layer. The very thin cadmium plate is also effective in reducing wear in inert oil, but not effective in an oxidizing atmosphere. The coefficient of friction of the 0.1 μm Cd plated steel was less than the unplated steel under all test conditions.     

Some observations on the interlaminar strength of composite laminates

Definition of the influence of interlaminar stresses on the failure characteristics of composite laminates may require the development of novel experimental characterization procedures. The so-called free edge problem in laminate elasticity offers a cnovenient mechanism to accomplish this purpose because of the high interlaminar stresses in the neighbourhood of a free boundary. The detailed design of a laminate specimen which can exhibit catastrophic delamination induced by interlaminar tension is presented, along with a preliminary failure hypothesis to characterize this mode of rupture and the associated experimental demonstration of the phenomena.     

Modelling of cross-ply piezoelectric composite laminates in cylindrical bending with interfacial shear slip

An approximate theory for cross-ply piezoelectric composite laminates in cylindrical bending with interfacial shear slip is developed. This theory uses only 4 displacement and potential variables, the number of which is independent of the number of layers involved. The displacement and electric potential fields are depicted by the displacement and electric potential distribution functions through thickness, respectively. The two functions are formulated according to particular solutions to the three-dimensional elasticity equilibrium equations and electrostatics charge equation. In this shear slip modelling interfacial opening is neglected. The interfacial bonding conditions are characterised by a linear slip law and an electrically permeable assumption. A corresponding finite element is also developed to deal with piezoelectric laminates with local shear slip. The accuracy and effectiveness of the present theory are demonstrated in numerical examples.     

Mechanical behavior and numerical analysis of corrugated wire mesh laminates

The objective is to show a possibility of corrugated wire mesh laminate (CWML) structure for bone application. CWML is a part of open-cell structures with low density and high strength built with bonded mesh layers. Specimens of CWML made of 316 stainless steel woven meshes with 0.22 mm wire diameter and 0.95 mm mesh aperture, bonded by transit liquid phase (TLP) at low temperatures, were fabricated and tested under quasi-static conditions to determine their compressive behavior with varying numbers of layers of the sample. The finite element software was used to model the CWML and studied their response to mechanical loading. Then, the numerical model was confirmed by the tested sample. Consequently, CWML specimens were reasonably matched with the human tibia bone ranged over apparent density from 0.05 to 0.08 g/cm³ in Young’s modulus and from 0.05 to 0.11 g/cm³ in compressive yield strength. The CWML model can have the potential for bone application.     

Modeling and detection of delamination in a composite beam: A polyspectral approach

This work considers the difficult problem of detecting delamination in a composite beam structure based on a polyspectral analysis of the structure's vibrational response. First, a low-dimensional model of the structure is presented that captures the delamination-induced nonlinearity and shows how it influences the beam's dynamic response. A Volterra series solution is then used to approximate the nonlinear beam response. The associated Volterra kernels are derived and then substituted into previously obtained expressions for the power spectral, auto-bispectral and auto-trispectral density functions for multi-degree-of-freedom systems subject to Gaussian distributed, random inputs. The influence of the delamination on the polyspectra is then explored with respect to both delamination size and depth. Specifically, it is shown how the dominant peaks of the polyspectra relate directly to these delamination parameters. The limits of polyspectral approaches to delamination detection are presented.     

Investigation of the effect of temperature and layup on the press forming of polyvinyl chloride-based composite laminates and fiber metal laminates

Semi-crystalline thermoplastic-based composite laminates and fiber metal laminates have a narrow forming temperature window, which limits formability of these products. The intention of this study was investigation of non-melting amorphous polyvinyl chloride as a proper matrix to increase the formability and forming temperature window of these products. For this, [45/?45] and [0/90] layups of polyvinyl chloride-based composite laminates and fiber metal laminates were produced using the film-stacking procedure and later press formed into channel sections at six temperatures in the range of 80 to 200 °C. The effects of the layups and forming temperatures on the forming loads and spring back of the formed profiles were measured, and their effects on the fiber buckling, wrinkling, and delamination of the profiles were evaluated using optical microscope images. The effects of layups and forming temperatures on the deformation mechanisms were also analyzed using the grid strain analysis method. Of the fiber metal laminates, 160 °C was found as the minimum forming temperature, and for the composite laminates, 120 and 160 °C were found as the minimum proper forming temperatures of [45/?45] and [0/90] layups, respectively. Finally, the forming temperature windows and formability of polyvinyl chloride matrix composite laminates and fiber metal laminates were found higher than semi-crystalline matrices.     

复合材料层合板首末层失效强度的预测方法研究

建立复合材料的三维有限元模型,该模型能有效计算自由边界区的应力以及层间应力,与二维模型相比提高了求解精度;采用Tsai-Wu张量理论作为单元失效的判断依据,引入安全系数计算首层失效,然后使用增量法求解复合材料最终失效强度,使得求解速度加快的同时又不影响求解精度;使用正交实验方法研究不同刚度缩减系数( SCR)对首末层失效强度的影响,发现在进行渐进失效分析时,有限元模型是否合理应该根据首层失效强度与实验值进行比较,不能只考虑最终失效强度与实验值的差异.     

Post-buckling behaviour of plates with discontinuous change of thickness

A theoretical study of the compressional behaviour of plates with discontinuous change of thickness across the width is presented. The analysis considers each portion with a different thickness as an individual plate. The deflections of the plate in the loaded direction are assumed to be sinusoidal, and polynomial functions are used to describe the deflected shape across the plate. By matching the boundary conditions of each portion, the basic equations are solved together using the semi-energy approach. The effect of initial imperfections are also examined. Typical results from an experimental investigation are presented to verify the validity of the analysis.     

An experimental study to demonstrate the superior response characteristics of mechanisms constructed with composite laminates

A detailed experimental investigation is presented into the dynamic flexural responses of slider-crank and four-bar linkages constructed from steel, aluminum and two graphite-epoxy laminates. This is the first time that an experimental investigation has demonstrated that composite linked mechanisms have superior response characteristics to comparable mechanisms manufactured in the commercial metals, and also that this dynamic behavior is governed by the stiffness-to-density ratio of the link material.