Variable mixed-mode delamination in composite laminates under fatigue conditions: testing & analysis (original) (raw)
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A Testing Procedure for Mixed Mode Delamination of Composite Materials
A mixed-mode delamination test procedure was developed combining mode I, double cantilever beam (DCB) and mode II, end-notch fixture (ENF) loading on a split unidirectional laminate. The test is based on the standard D6671 of ASTM international ìStandard test method for mixed mode I ñ mode II interlaminar fracture toughness of unidirectional fibre reinforced polymer matrix compositesî. By loading with a lever, a single applied load simultaneously produces mode I and mode II bending loads on the specimen. The mixed-mode bending test (MMB) was demonstrated using a quasi-isotropic laminate [(-45, 90, 45, 0)4]s made of unidirectional carbon fibre reinforced epoxy. The mode I and mode II components of strain energy release rate GI and GII, respectively were obtained for a wide range of GI/GII ratios. The initial delamination is in the midplane of the laminate.
Simulation of multiple delaminations in composite laminates under mixed-mode deformations
Simulation Modelling Practice and Theory, 2003
A delamination analysis is developed in order to predict the defect evolution when a fiber composite laminate, with outer layers of equal or different thickness, is subjected to a compression load and multiple delaminations. In particular, the presence of two delaminated zones coaxially positioned on the two opposite sides of the laminate is analysed. In order to accurately predict the growth of delamination, when mode I and II are interacting, some fracture criteria have been proposed. Some results will be given to show the influence on the delamination buckling and on the growth phenomenon of the main geometrical and mechanical characteristics of the structure in order to show the pronounced role of the fracture modes on stable or unstable delamination growth.
Multi-directional composite laminates: fatigue delamination propagation in mode I—a comparison
International Journal of Fracture, 2019
Double cantilever beam (DCB) specimens composed of carbon fiber reinforced polymer laminate composites were tested. Two material systems were investigated. One consisted of plies from a woven prepreg alternating with tows in the 0 • /90 •-directions and the +45 • / − 45 •-directions. The second was fabricated by means of a wet-layup process with the same multi-directions as the prepreg. In addition, for the second material system, a unidirectional (UD) fabric ply was added. The delamination for this laminate was between the UD fabric and the woven ply with tows in the +45 • /−45 •-directions. Both fracture resistance Rcurve and fatigue delamination propagation tests were carried out. It is found that the initiation value of the interface energy release rate is substantially lower for the wet-layup; whereas, their steady state values are quite similar. The fatigue delamination propagation tests were performed at various cyclic R-ratios. The delamination propagation rate da/dN was calculated from the experimental data and plotted using a modified Paris equation with different functions of the mode I energy release rate. As expected, the da/dN curves L. Banks-Sills (B) • I.
Engineering Fracture Mechanics, 2017
Due to the lack of fundamental knowledge of the physics behind delamination growth, certification authorities currently require that composite structures in aircraft are designed such that any delamination will not grow. This usually leads to an overdesign of the structure, hampering weight reductions. In real structures, delaminations tend to grow under a mix of modes I and II. Although some studies have tried to assess mixed-mode fatigue delamination, little progress was made in understanding the physics behind the problem. Therefore, this work scrutinizes mixed-mode fatigue delamination growth and examines experimentally the damage mechanisms that lead to fracture. To this aim, mixed-mode delamination fatigue tests were performed at different mode mixities, displacement ratios and maximum displacements. Selected fracture surfaces were analysed after the tests in a Scanning Electron Microscope to gain insight on the damage mechanisms. The physical Strain Energy Release Rate G* was used as the similitude parameter, enabling the characterization of fatigue mixed-mode delamination propagation. The results obtained show no displacement ratio or maximum displacement dependence. Furthermore, the energy dissipated per area of crack created is approximately constant for a given mode mixity. However, the analyses of the fracture surfaces and the correlation of the damage features with energy dissipation indicate that different damage mechanisms that might be activated under different loading parameters cause the resistance to delamination to change under a given loading mode.
The present paper is concerned with an investigation of the mixed-mode delamination of polymeric fibre composite materials. Various test geometries have been used to measure the interlaminar fracture energy, Gc, of both thermoplastic and thermosetting carbon fibre composites when subjected to various ratios of mode I to mode H loadings. In particular, the mixed-mode bending (MMB) delamination test has been studied in detail and the results from this test method compared to those obtained from the fixed-ratio mixed-mode (FRMM) test method. Further, for the FRMM results, two methods of partitioning the measured interlaminar fracture energy, Gc, have been employed: namely, by way of a local singular-field approach and by a global method based on a consideration of the applied energy release rates. It is shown that the latter approach is the more appropriate method. Finally, a general criterion for mixed-mode failure is proposed which assumes that a crack loaded with Gt and G, will have an induced mode I component equal to the failure value, termed Go, such that: Go = Gc[(COS2(~-~0) 4-sin 2 to sin 2(V-Vo)] where Gc is the measured fracture energy, V is the phase angle of the applied loads, Vo is the phase angle which arises from the elastic mismatch across a bimaterial interface (e.g. the fibre~matrix interface) and where to can be regarded as the slope of the fracture surface roughness.
Mechanical Behavior of Mode I Delamination of a Laminated Composite Material
2019
The objective of our work is to analyze numerically in three dimensions by the finite element method of the effect of a mechanical load on the delamination of unidirectional and multidirectional stratified composites in order to determine the energy release rate G in Mode I and the Von Mises equivalent stress distribution along the damaged area under the influence of several parameters such as applied load and delamination size. The results obtained in this study show that unidirectional composite laminates have better mechanical strength on the loading line than multi-directional composite laminates.
Mixed mode dynamic delamination in fiber reinforced composites
Composites Part B: Engineering, 2009
An investigation on dynamic delamination problems under steady-state crack growth is proposed. Through the thickness delamination phenomena in unidirectional composite laminates are analyzed in the context of the interface methodology, based on the combination of shear deformable beams and interface elements. Analytical solutions of the relevant governing equations are proposed and closedform expressions for simple cases, involving pure mode I and mode II components, are provided. Moreover, analytical expressions for mixed mode loading conditions regarding the energy release rate modal components are given in terms of beam stress resultant discontinuities, emphasizing the presence of new terms, absent in the static case, which arise from the inertial description of the composite structure. Comparisons between analytical and numerical results show the accuracy of the proposed formulation. Some applications are developed to point out the influence of the crack front speed, the shear deformability, the inertial contributions on the energy release rate and the corresponding mode partition. Special attention is devoted to analyze the maximum speeds of the moving crack. In particular, a parametric study in terms of the main characteristic geometric parameters of the laminate is proposed to show the main features of the crack tip behavior.
Theoretical and Applied Fracture Mechanics, 2019
This paper demonstrates how the critical strain energy density in the delamination tip vicinity may be used to explain the physics of delamination growth under mixed mode I/II. A theory previously proposed to physically relate mode I and mode II delamination growth is further extended towards describing the onset of mixed mode I/II delamination. Subsequently, data from the literature is used to demonstrate that this new concept of the critical strain energy density approach indeed explains, based on the physics of the problem, the strain energy release rate level at which crack onset occurs. This critical strain energy density for the onset of delamination appears to be independent of the opening mode. This means that, in order to characterize the fracture behaviour of a laminate, fracture tests at only one loading mode are necessary. Because the load level at which the physical delamination onset occurs at the microscopic level is much lower than the traditional engineering definition of macroscopic onset, further work must reveal the relationship between the macroscopically visible delamination onset, and the microscopic onset.
A REVIEW STUDY OF DELAMINATION IN COMPOSITE LAMINATED PLATES
A REVIEW STUDY OF DELAMINATION IN COMPOSITE LAMINATED PLATES, 2019
Failure analysis of laminated composite structures has attracted a great deal of interest in recent years due to the increased application of composite materials in a wide range of high-performance structures. Intensive experimental and theoretical studies of failure analysis and prediction are being reviewed. Delamination, the separation of two adjacent plies in composite laminates, represents one of the most critical failure modes in composite laminates. In fact, it is an essential issue in the evaluation of composite laminates for durability and damage tolerance. Thus, broken fibers, delaminated regions, cracks in the matrix material, as well as holes, foreign inclusions and small voids constitute material and structural imperfections that can exist in composite structures. Imperfections have always existed and their effect on the structural response of a system has been very significant in many cases. These imperfections can be classified into two broad categories: initial geometrical imperfections and material or constructional imperfections.
Fatigue <html_ent glyph="@amp;" ascii="&"/> Fracture of Engineering Materials and Structures, 2003
Delamination is one of the most frequent failure modes in laminated composites. Its importance is crucial, because a delamination can occur in the interior of a panel without any noticeable damage on the surface, drastically reducing its strength and stiffness. A study has to be made on critical dimensions of delaminations and their shape, through the calculation of the strain energy release rate (SERR), G. This study was performed numerically, for a given geometry, with varying loads and shapes of delamination, in pure and mixed-mode propagation. All numerical values were obtained with threedimensional finite element (FE) analyses from a commercial package. The use of three-dimensional analyses in simple geometries helps establish the basis for the more complex ones, and the correspondence with the usual analytical or numerical bidimensional plane-strain analysis. The conclusions were (a) G is not constant along the crack tip, even for mode I propagation and straight crack tip; (b) the mean value of G obtained from a three-dimensional analysis equals the value obtained in bi-dimensional plane-strain analysis; (c) in mixed-mode propagation, the method exhibits a good correlation with experimental results and (d) the shape and mode partitioning of the SERR depend not only on the loading, but also on the shape of the crack front.