Damage analysis of multi-layered composite structures

Delamination and transverse crack growth prediction for laminated composite plates and shells

In aerospace and many types of civil infrastructure applications, the use of adhesive-bonded composite joints have been constantly increasing. In this study, fracture behaviour of adhesively bonded single lap joint (SLJ) with pre-existing damage at the interface of free edge of the top adherend and adhesive along the width of the joint is studied by strain energy release rate (SERR) approach using virtual crack closure technique (VCCT). Adherends are four layered laminated fiber reinforced plastic (FRP) composite plates and each lamina of adherend having either +45° or -45° fiber angle. The distributions of strain energy release rates along the delamination front are predicted by evaluating 3D local displacement fields and reaction forces required to close them, along the delamination length and width. The present investigation reveals that strain energy release rates are not constant along the delamination front, insisting on three dimensional modeling of problem. It is observed that location of damage propagation and mode of failure responsible for damage propagation varies with lay-up sequence of +45° or -45° ply oriented laminas within the top and bottom adherends. Total strain energy release rate (G) values are very high in the SLJs having (+45/-45)s FRP laminated adherends and in the SLJ with (-45/+45)s FRP laminated adherends, when compared with SLJs having (+45/-45)asand/or (-45/+45)as FRP laminated adherends. Key words: Delamination length, strain energy release rate (SERR), virtual crack closure technique (VCCT), suffix s and as, fiber angle.

Mixed-mode fracture of adhesively-bonded pultruded composite lap joints

Multilayer composite structures have the characteristics of high strength, high stiffness, and good corrosion resistance, and are increasingly used in underwater equipment. The stability analysis of multilayer composite structures is one of the key issues in engineering design. As an important analysis method, the analytical calculation method can achieve rapid design of structural parameters. This paper proposes an analytical method for stability calculation of multilayer composite structures based on residual pressure recursion. Through numerical experiments, the effectiveness of the method is verified, providing an option for stability analysis and engineering design of multilayer composite structures.

Modelling the interaction between matrix cracks and delamination damage in scaled quasi-isotropic specimens

The use of advanced composite materials for cryogenic fuel tanks reduces the mass of the structure but microscopic damage can cause fuel leakage. The amount of leakage depends on the connectivity of the damage and crack opening displacements. In this paper, the damage growth and the opening near the intersection of transverse matrix cracks (TMC) and a delamination are discussed. Three-dimensional finite element analysis (FEA) was used to analyze cross-ply and angle-ply graphite/epoxy laminates. The strain energy release rates (SERR) along the delamination front were calculated based on the virtual crack closure technique. The strain energy release rate has a very large variation along the delamination front and is sensitive to a variety of geometric and load parameters. There is an extreme gradient of the strain energy release rate near the intersection of the delamination front and the TMC. Based on the SERR results, likely delamination shape and size for cross- and angle-ply laminates were determined. Prediction of the likely shape and size will help one to understand the conduit for leakage. The opening near the intersection of the TMC and the delamination was calculated. The effect of ply thickness on the SERR and the opening is also discussed.

Modeling of facesheet crack growth in titanium–graphite hybrid laminates, Part I

The deflection of a matrix crack near 0°/90° interface in a cross-ply laminate was studied numerically. In the finite element (FE) model, an initial matrix crack was introduced in the 90° layers away from the 0°/90° interface. The initial matrix crack could be initiated either at the middle of 90° layer or at one side of 0°/90° interface. The 0° layers and a part of the initial matrix crack were modeled using homogenized layer properties to simplify the model. The nonuniformly distributed fibers were modeled explicitly close to the 0°/90° interface in order to study the influence of this nonuniformity on the crack deflection process. The Energy Release Rate (ERR) of debond crack tip was calculated using Virtual Crack Closure Technique (VCCT) to study the debond growth. Maximum principal stress was then adopted to access the debond crack kinking qualitatively. It’s found that when a macro-size matrix crack forms and propagate towards ply interface, the subsequent debonding and debond cracking process in nearby intact fiber shows some distinct differences compared to the same processes at single isolated fiber without considering the interaction with nearby debonded fiber and existing matrix crack. Meanwhile, present analysis shows clear influence of microstructures on the crack deflection process by affecting the fiber/matrix debonding and debond kinking processes.

A new characteristics matrix method along with the formulas based on conductivity, which can be used to calculate the optical properties of an ultra-thin conductive composite multilayer dielectric structure, is derived for the first time as faras we know based on the electromagnetic boundary conditions Maxwell's equations required. It can be used to calculate the reflection, transmission, and absorption of light, provided that the conductivity of the conductive body is known, also it can overcom the shortcoming of the traditional transfer matrix method, i.e.it is necessary to know the permittivity and permeability of the material. By using the proposed method, the optical behavior of graphene and composite multilayer structures can be obtained in the THz frequency range.

Delamination is one of the most commonly observed failure modes in laminated composites. The existence of delamination in a structure can significantly reduce the stiffness and strength of the structure. The simulations of delamination are performed by two different methods: Virtual Crack closure Technique (VCCT) and Cohesive Zone Method (CZM).VCCT is a fracture mechanics approach which is widely used to compute energy release rates. CZM is a progressive event governed by progressive stiffness reduction of the interface between two separating faces which uses bilinear material behavior for interface delamination and fracture energies based debonding to analyze delamination of unidirectional Double Cantilever Beam (DCB) specimen. The proposed methods are validated with the benchmark results. The load-displacement response predicted by CZM agreed well with the benchmark results. The other approach, VCCT, also successfully simulated the load-displacement response curve but this method overestimated the critical load. Parametric study is carried out for a range of height of the beam and load-displacement response is studied.

Incompatible extended layerwise method for laminated composite shells

In this paper, a 3-dimension finite element with dummy nodes for calculating and outputting the stress energy release (SERR) at the crack tip was built up based on virtual crack closure technique (VCCT), it is presented to demonstrate the virtual crack closure technique has high accuracy and good feasibility. The calculation results curve and the analytical solution curve are in good agreement. The results show that the proposed interface elements can be used to calculate to get accurate results by finite element analysis. It can give us some new ideas for Hydraulic structure crack research.

Open-ended rectangular waveguides are widely used for microwave and millimeter-wave nondestructive testing (NDT) applications, such as detecting disbond and delamination in multilayered composite structures, thickness evaluation of dielectric sheets and coatings on metal substrates, etc. When inspecting a complex multilayered composite structure that is made of generally lossy dielectric layers with arbitrary thicknesses and backing, the dielectric properties of a particular layer may be of particular interest (e.g., radome inspection). The same is also true when one is interested in the thickness, or, more importantly, thickness variation, of a particular layer within such complex structures. An essential tool for closely estimating the complex permittivity and/or thickness is an accurate forward electromagnetic model for simulating the reflection coefficient at the aperture of the probing open-ended waveguide. To this end, this paper provides a full-wave accurate forward model for calculating the reflection coefficient from a generally lossy multilayered composite structure possessing an arbitrary number of layers and respective thicknesses while accounting for the influence of higher order modes. This model is subsequently validated through comparisons with a commercial numerical tool and actual measurements. Furthermore, a measurement model is provided, which results in an iterative inverse technique for estimating the complex permittivity and thickness of a dielectric layer. Subsequently, this technique is applied to the measured reflection coefficients of several structures. To evaluate the accuracy of this technique, an analysis on its sensitivity to various sources of errors, and, most importantly, the effect of a finite flange size, is also demonstrated by using the simulated data. Finally, the potential of this model to accurately estimate the thickness of an individual layer, which represents a thin disbond, in a multilayered composite structure is presented.

Efficient phase shift migration for ultrasonic full-matrix imaging of multilayer composite structures

This paper aims at experimental and numerical research of delamination process in carbon/epoxy composite laminates with different fiber orientation angles in stacking sequence exhibiting the bending–twisting elastic couplings. Experimental specimens were subjected to the double cantilever beam (DCB) tests according to the ASTM D5528 regulations. Values of the mode I strain energy release rates were calculated by using three different data reduction schemes: the modified beam theory, the compliance calibration method and the modified compliance calibration. Determination of delamination initiation point was conducted in twofold way: by visual observation of crack tip using high resolution camera and by utilization of the acoustic emission technique. Numerical analyss were prepared in Abaqus/CAE Software environment by using the virtual crack closure technique (VCCT). The numerical beam model consisted of SC8R continuum shell elements. Obtained outcomes revealed that extensive fiber bridging phenomenon occurring during delamination process pronouncedly affected propagation values of strain energy release rate (GIprop) and numerically obtained load–displacement curves. Nevertheless, in initial stage of delamination, results obtained by using the VCCT were in agreement with experimental data. The greatest value of the mode I fracture toughness equal 0.56 N/mm was obtained for the BT45 laminate.