A simple model for the prediction of the fatigue delamination growth of impacted composite panels

The fatigue behaviour of composite panels that have been subjected to low‐velocity impact was studied. Impacted specimens were tested under compression–compression fatigue. A delamination propagation model based on the derivation of the strain energy release rate was used. The stress distribution around the initially induced delamination was derived analytically. The shape of the delamination was experimentally monitored by c‐scan imaging and is assumed to be an ellipse. The orientation and aspect ratio of the ellipse were used to calculate the corresponding strain energy‐release rates, which were subsequently used to predict the direction of delamination propagation.     

A mixed-mode damage model for delamination growth applied to a new woven composite

In this paper, delamination initiation and propagation of reinforced fibrous composite laminate, in single and mixed mode, are numerically predicted by taking into account the existence of imperfect interfaces between the plies. The bonding conditions between layers are characterised by jumps in displacements which are proportional to the traction stresses. In order to describe the interface damage an approach based on the indirect use of fracture mechanics considering a softening stress-relative displacements law is presented. The accuracy of the predictions is evaluated in single-mode delamination tests, in the mixed-mode bending test, and in a structural configuration consisting of the debonding of a new woven laminated composite reinforced by particles of date cores for orthopedic use. The interface is regarded as being a whole of several interfacial bonds. Each bond is supposed to be made up of three stiffnesses acting in the three directions.     

A computational model for analysing interactive buckling and delamination growth in composite structures

In this paper, a unified method is presented: (i) to model delaminated stiffened laminated composite shells; (ii) for synthesising accurate multiple post-buckling solution paths under compressive loading; and (iii) for predicting delamination growth. A multi-domain modelling technique is used for modelling the delaminated stiffened shell structures. Error-free geometrically nonlinear element formulations — a 2-noded curved stiffener element (BEAM2) and a 3-noded shell element (SHELL3) — are used for the finite element analysis. An accurate and simple automated solution strategy based on Newton type iterations is used for predicting the general geometrically nonlinear and postbuckling behaviour of structures. A simple method derived from the 3-dimensionalJ-integral is used for computing the pointwise energy release rate at the delamination front in the plate/shell models. Finally, the influence of post-buckling structural behaviour and the delamination growth on each other has been demonstrated.     

A damage mechanics tool for laminate delamination

A simplified method based upon damage mechanics for the delamination analysis of carbon-resin composites is presented. In the neighbourhood of a laminate structure quasi-straight edge, damage is taken concentrated on the interface between layers. The finite element code EDA, acting as a post-processor of an elastic laminate shell computation, allows the onset and propagation forecast of delamination. First numerical simulations of delamination are given and compared with experimental results from literature.     

A digital image analysis to evaluate delamination factor for wind turbine composite laminate blade

Kinetic energy of wind is converted into useful power by the rotation of blades that are fabricated using glass fiber reinforced composite. Bolted connections are used to fasten the spares with composite shells of the blades. In order the effect this fastening, holes of appropriate size are drilled in the composite laminates. Drilling in composite material is a common phenomena. However the damage caused in the material during drilling results in a drastical reduction of the performance of the component. A newer quantitative digital measurement model is generated using Buckingham’s π theorem to characterize the damage level, termed as refined delamination factor (F_DR). Along with F_DR, a comparison of F_D and F_DA is presented. Experimental results are used to validate the accuracy of proposed criteria.     

Cohesive zone modelling of delamination response of a composite laminate with interleaved nylon 6,6 nanofibres

This work simulates numerically Double Cantilever Beam and End Notched Flexure experiments on Carbon Fibre Epoxy Resin specimens that have been performed by some of the authors in a previous work. Specimens have been nanomodified by interleaving plies with a layer of electrospun nanofibres in the delaminated interface. Eight different configurations of nanofibres have been used as interleave, for a total of 9 configurations (8 nanomodified plus the virgin one) to be simulated for both kind of tests to identify the cohesive zone parameters corresponding to the effect of nanofibre diameter, nanolayer thickness and nanofibre orientation on the delamination behaviour of the composite.Results showed that a bilinear damage law is necessary for almost all nanomodified configurations, and presented a clear relationship between nanomat layer parameters and the cohesive energy of the interface.     

A digital image analysis to evaluate delamination factor for wind turbine composite laminate blade

Kinetic energy of wind is converted into useful power by the rotation of blades that are fabricated using glass fiber reinforced composite. Bolted connections are used to fasten the spares with composite shells of the blades. In order the effect this fastening, holes of appropriate size are drilled in the composite laminates. Drilling in composite material is a common phenomena. However the damage caused in the material during drilling results in a drastical reduction of the performance of the component. A newer quantitative digital measurement model is generated using Buckingham’s π theorem to characterize the damage level, termed as refined delamination factor (F_DR). Along with F_DR, a comparison of F_D and F_DA is presented. Experimental results are used to validate the accuracy of proposed criteria.     

Dynamic delamination growth in laminated composite structures

The dynamic growth of a thin strip delamination in a thick base laminate under in-plane loadings has been analysed. A variational principle, coupled with a Griffith-type fracture criterion, is used to formulate the delamination growth problem. Two approximate solutions, including one mode and two modes, respectively, are calculated in this paper. The resulting equations of motion and the dynamic local growth condition at the crack tip turn out to be two and three coupled ordinary differential equations for one-mode and two-mode solutions. A fourth-order Runge-Kutta method is then used to obtain the numerical solutions. The results show that delamination growth will approach a state of arrest for materials with high fracture toughness, and continue all the way without a limit for low fracture toughness materials. The inertial effect is important and should not be ignored in calculation of the arrested delamination length for high fracture toughness materials. For materials with low fracture toughness, the inertial effect is significant and high admissible modes are noticeable. A comparison between the present results and the previously known quasi-dynamic solution is also given.     

Fractographic observations on delamination growth and the subsequent migration through the laminate

This paper describes fractographic observations from the detailed examination of delamination fracture surfaces and offers an interpretation of the key growth mechanisms. Firstly, the relationship between toughness, delamination failure criteria and fracture morphology is presented and the influence of cusp formation and deformation on toughness is discussed. Observations regarding delaminations migrating through the lamina at multidirectional ply interfaces are then discussed. It is demonstrated how this migration process can be avoided in fracture toughness coupons and consequently the toughness of multidirectional ply interfaces can be characterised. The influence of migration on delamination growth from embedded defects in laminates under compression is presented, and these results are extended to demonstrate how migration influences damage growth in structures. The paper concludes by making recommendations for realistic modelling of migration, and suggests how it can be exploited in damage tolerant structural design.     

Anti‐symmetrical curved composite laminate subject to delamination induced by thermal cycling

Composite structures usually undergo to temperature variations in aircraft during landing/taking off and when cruising at high altitude. Under these conditions and in combination with curved structures, they can generate severe thermal stresses that induce delaminations. Considering the importance of studying delamination in these conditions, this research imposed an anti‐symmetrical laminate to cyclic temperature variations of 130 °C and ?70 °C with the objective of inducing varied curvatures and, consequently, crack growth. Different from standardized test procedures, this test setup elastically deformed coupons without external forces and forward experimentally and numerically evaluated the strain energy release rate (SERR) during crack propagation. This procedure enabled the assessment of delamination rate (da/dN) as a function of maximum SERR. The experimental results were compared with numerical results obtained by ABAQUS Finite Element code. Despite large scatter in experimental results, a reasonable correlation between experimental and numerical results was obtained in terms of crack growth rate (da/dN) as a function of the maximum SERR.     

Fatigue life prediction of woven composite laminates with initial delamination

An engineering approach for fatigue life prediction of fibre‐reinforced polymer composite materials is highly desirable for industries due to the complexity in damage mechanisms and their interactions. This paper presents a fatigue‐driven residual strength model considering the effect of initial delamination size and stress ratio. Static and constant amplitude fatigue tests of woven composite specimens with delamination diameters of 0, 4 and 6 mm were carried out to determine the model parameters. Good agreement with experimental results has been achieved when the modified residual strength model has been applied for fatigue life prediction of the woven composite laminate with an initial delamination diameter of 8 mm under constant amplitude load and block fatigue load. It has been demonstrated that the residual strength degradation‐based model can effectively reflect the load sequence effect on fatigue damage and hence provide more accurate fatigue life prediction than the traditional linear damage accumulation models.     

Analytical and numerical calculation of strain energy release rate during delamination growth in a carbon epoxy laminate

Delamination of an infinite laminate subjected to a tension stress is studied. The stress state is studied by a numerical finite element analysis and by an analytical model based on the Love-Kirchhoff plate theory. The global strain energy release rate and its mode I, II and III components are calculated during delamination extension. A correlation is made between these two formulations. The analytical model has the advantage of permitting the analysis of different stacking sequences composed of a large number of plies and determination of their capacity for delamination.     

Embedded delamination growth in composite panels under compressive load

In this paper, crack growth analyses on composite panels containing embedded delaminations has been performed using a geometrically non linear FEM code, based on the total Lagrangian formulation. The code has been improved with an effective virtual crack closure technique to evaluate energy release rate and with penalty method to evaluate contact forces. Validation of the proposed tool has been performed with experimental and numerical data available in the literature for double cantilever beam (DCB) specimens. Finally, the influence of the geometrical parameters of the delamination (size and location along the thickness) on the energy release rate distribution and delamination growth stability in composite panels under compression has been analyzed.     

The stability of delamination growth in compressively loaded composite plates

The stability of growth of internal delaminations in composite plates subjected to compressive loading is investigated. Due to the compressive loading, these structures can undergo buckling of the delaminated layer and subsequently growth of the delamination. The study does not impose any restrictive assumptions regarding the delamination thickness and plate length (as opposed to the usual thin film assumptions). The growth characteristics of the delamination under monotonic compressive loading are obtained on the basis of a combined delamination buckling/postbuckling and fracture mechanics model. The postbuckling solution is derived through a perturbation procedure, which is based on an asymptotic expansion of the load and deformation quantities in terms of the distortion parameter of the delaminated layer, the latter being considered a compressive elastica. The closed form solutions for the energy release rate at the delamination tip versus applied compressive strain during the initial postbuckling phase are used to define the combinations of delamination length and applied strain that lead to unstable growth. This would practically cause either contained jump growth or complete (catastrophic) growth of the delamination. Estimates for the lower and upper bounds of the jump distance (unstable growth) are provided. Moreover, a study of the influence of the mode dependence of interface toughness on the conditions of initiation and extent of delamination growth is performed.     

A damage model for simulation of mixed-mode delamination growth

An interface element capable of modelling delamination progression under mixed-mode loading is presented. The kinematics of the element are based on the concept of regularised displacement discontinuity. This concept allows the interfacial constitutive equations to be formulated in terms of the traction vector of the interface and the corresponding displacement discontinuity. The decohesion within the interface, corresponding to delamination progression, is accomplished by assigning a non-associative perfectly plastic material model including isotropic damage to the interface element. All parameters of the model can be determined from experimental material data. Damage initiation is calibrated against the interlaminar fracture stresses whereas the evolution of damage is calibrated against the mixed-mode fracture toughness. The interface element has been implemented in a finite element code and results for simulations of standard fracture toughness tests are shown. The results display the applicability of the proposed model and the calibration procedure.     

Modelling delamination onset and growth in pin loaded composite laminates

A three-dimensional (3D) finite element (FE) model is created with cohesive zone elements (CZE) to simulate a mechanically fastened [0°/90°]_s pin-loaded joint in a composite laminate. The model incorporates fully integrated solid elements in the pin-loaded area to accurately capture the high stress gradients. Contact based cohesive elements with a bilinear traction–separation law are inserted between the layers to capture the onset and growth of delamination. The stress distribution around the pin-loaded hole was verified with the widely used cosine stress distribution model. Results from the FE model show that delamination damage initiated at the point of maximum average shear stress at the 0°/90° interface. The delaminated area develops an elliptical shape which grows in a non-self similar manner with increasing pin displacement. It is concluded that a progressive damage model should be included to provide a full understanding of the failure sequence, work that the authors are currently engaged with.     

Finite element simulation of delamination growth in composite materials using LS-DYNA

In this paper, a modified adaptive cohesive element is presented. The new elements are developed and implemented in LS-DYNA, as a user defined material subroutine (UMAT), to stabilize the finite element simulations of delamination propagation in composite laminates under transverse loads. In this model, a pre-softening zone is proposed ahead of the existing softening zone. In this pre-softening zone, the initial stiffness and the interface strength are gradually decreased. The onset displacement corresponding to the onset damage is not changed in the proposed model. In addition, the critical energy release rate of the materials is kept constant. Moreover, the constitutive equation of the new cohesive model is developed to be dependent on the opening velocity of the displacement jump. The traction based model includes a cohesive zone viscosity parameter (η) to vary the degree of rate dependence and to adjust the maximum traction. The numerical simulation results of DCB in Mode-I is presented to illustrate the validity of the new model. It is shown that the proposed model brings stable simulations, overcoming the numerical instability and can be widely used in quasi-static, dynamic and impact problems.     

A numerical model for delamination growth simulation in non-crimp fabric composites

In this paper, a novel finite-element tool, for the simulation of delamination growth in non-crimp fabric (NCF) composite materials, is presented. The proposed finite-element tool is based on the stiffness averaging method (SAM), on the modified virtual crack closure technique (MVCCT) and on the penalty method (PM); all these methods have been implemented in the research oriented B2000 finite-element code. The stiffness averaging method allows taking into account the effects of the processing variables, which characterize the representative volume element (RVE) of the non-crimp fiber composites (NCF) on their mechanical performances; while the modified virtual crack closure technique is used to determine the strain energy release rate (SERR) for the delamination growth. Already available experimental data on Mode I fracture toughness, obtained by using double cantilever beam (DCB) tests have been employed for validation purpose of numerical procedure. The modeling of DCB tests, considering different geometrical cases, has been performed by means of non-linear analyses. Excellent results in terms of deformed shapes and load–displacement curve, compared with experimental data, are reported to support the validity and the accuracy of the presented computational procedure. Moreover, the ability of the developed tool to take account for the NCF performances variability with processing parameters along with the delamination growth has been assessed and critically discussed.