Influence of intralaminar cracking on the apparent interlaminar mode I fracture toughness of cross-ply laminates (original) (raw)
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Mode I interlaminar fracture of carbon/epoxy multidirectional laminates
Composites Science and Technology, 2004
An experimental study was conducted on the mode I interlaminar fracture of carbon/epoxy multidirectional specimens with starter delaminations in 0/0 and 0/θ interfaces. The specimens selected for Double Cantilever Beam (DCB) tests had [(0 2 /90) 6 /0 2 //(0 2 /90) 6 /0 2 ] and [(0 2 /90) 6 /0 2 //θ/(0 2 /90) 6 /0 2 ] stacking sequences, where // denotes the position of the starter delamination and θ = 22.5 to 90 degrees. The incorporation of the thick symmetric [(0 2 /90) 6 /0 2 ] block of 20 plies aimed at minimising elastic couplings related problems, such as non-uniform width-wise distributions of the strain energy release rate G I , spurious modes and effects of residual stresses. This was verified in preliminary three-dimensional (3D) Finite Element (FE) analyses, which also showed the applicability of the Corrected Beam Theory data reduction scheme. The DCB specimens were tested according to the procedures defined in the ISO 15024:2001 standard for unidirectionally reinforced specimens. In the tests, intraply damage occurred in the θ-oriented ply soon after initiation from the starter crack, as it is commonly observed in multidirectional specimens. For θ = 22.5 and 45 degrees, a second crack propagated close to a neighbouring interface, while for θ = 67.5 and 90 degrees the starter crack ran inside the θ-oriented ply and proceeded in a wavy propagation trajectory. This resulted in extensive fibre bridging and pronounced R-curves with artificially high final values of the critical strain energy release rate, G Ic. Therefore, only G Ic values of initiation could be considered true interlaminar properties. Moreover, it was found that they were fairly independent of the ply angle θ. An additional FE analysis of the interlaminar stresses ahead of the crack tip was performed to interpret the delaminating interface effect on G Ic values. The results indicated the existence of an interface independent fracture toughness, K Ic , probably resin controlled, thus reinforcing the idea that the measured initiation G Ic were pure interlaminar properties.
Cracking paths at the ply interface in a cross-ply laminate
Composites Part B: Engineering, 2003
It has been observed in both ceramic and polymer-matrix composites that the evolution of transverse matrix cracking shows two distinct stages. When tensile stress is applied in the fiber direction of the 08 ply, cracks are formed in the 908 ply at a load level much lower than the ultimate strength of the laminate. As the load increases, more transverse cracks are initiated. At some critical load, the number of cracks ceases to increase; instead, the existing cracks penetrate into the adjacent 08 layers and eventually fracture the 08 fibers. Catastrophic failure becomes imminent when the 08 fibers are broken. The purpose of this paper is to identify conditions under which transverse cracks are deflected into the ply interfaces at the second stage of damage evolution. To this end, the energy release rates for deflection and penetration are used as a criterion to predict the crack growth paths. It is found that the predicted crack path depends on the geometry, the material properties and the degree of damage. Based on this analysis, a crack path prediction diagram is obtained for crack growth at the ply interface.
Transverse cracks in cross-ply laminates. 1. Stress analysis
Mechanics of Composite Materials, 1997
During service loading of cross.ply laminates, transverse cracks occur in plies. The cracks parallel to the fiber direction are extended over the full thickness of transverse plies and often cross the entire test specimen width. It is widely recognized that the changes of laminate thermomechanical constants, caused by the transverse cracking of composite laminates, can be significant. Theoretical stress analysis in the cross-ply laminates in the vicinity of cracks is performed using numerical (FE) and analytical methods. The effect of transverse cracks on the degradation of elastic properties will be discussed in Part 2 [1]. Approximate analytical micromechanical models based on shear lag predictions, variational analysis, and numerical 219 finite element calculations were verified in their predictive abilities. The three variational models used are based on the principle of minimum complementary energy and have different degrees of accuracy with respect to the stress assumptions used (Hashin's, 2D 0 ~ and 2D 00/90 ~ models). Using FEM, the plane stress and strain state were analyzed. The effect of material properties and layer thickness on the stress distribution in a 90 ~ layer was evaluated by varying the crack spacing. The crack opening displacement (COD), normalized with respect to the far field strain, is proposed as a measure of reduction of the mechanical properties. Since the CODs are rather insensitive to the crack spacing (crack density) in a wide region, they will be used in modeling the stiffness reduction in these laminates [11.
Transverse cracking of cross-ply laminates: Part 2. Progressive widthwise cracking
Composites Part A: Applied Science and Manufacturing, 1996
The purpose of the paper is to investigate the progressive transverse cracking across the width of cross-ply laminates, subjected to uniaxial fatigue tests. First a finite element analysis is implemented for a crack partly spanning the width of an elementary cell. The influence of the crack length across the specimen width is studied. Then, it is shown how the results can be used in the case of a random distribution of transverse cracks observed in fatigue tests.
Mode-I interlaminar fracture of carbon/epoxy cross-ply composites
Composites Science and Technology, 2002
Mode-I double-cantilever beam (DCB) tests were performed on carbon/epoxy [0 /90 ] 12 specimens. The starter crack was created at mid-thickness, between the 0 and 90 mid-layers. During the tests, however, the crack also propagated along the neighbouring 0 / 90 interface and within the 90 mid-layer. Nevertheless, the test results were apparently consistent with the assumptions of the corrected beam theory (CBT) that was used to obtain the interlaminar critical strain energy release rate, G Ic. The measured values were higher than those of unidirectional [0 ] 24 specimens, especially the final propagation values. A finite-element analysis confirmed the applicability of the CBT for interlaminar propagation along the two 0 /90 interfaces. The results also indicated that the intralaminar G Ic is significantly smaller than the interlaminar G Ic. This will prevent pure interlaminar propagation in multi-directional specimens with high interlaminar fracture toughness.
Stiffness degradation in cross-ply laminates damaged by transverse cracking and splitting
Composites Part A-applied Science and Manufacturing, 2000
In contrast to the few existing theoretical models (Highsmith and Reifsnider, ASTM STP 1986;907:233-251; Hashin, Trans ASME J Appl on the consideration of a repeated laminate element defined by the intersecting pairs of transverse cracks and splits, the new approach for evaluating the stiffness degradation in [0 m /90 n ] s laminates due to matrix cracking both in the 90Њ (transverse cracking) and 0Њ (splitting) plies employs the Equivalent Constraint Model (Fan and Zhang, Composites Science and Technology 1993;47:291-298). It also uses an improved 2-D shear lag analysis (Zhang et al., Composites 1992;23(5):291-298; 1992;23(5):299-304) for determination of stress field in the cracked or split lamina and In-situ Damage Effective Functions for description of stiffness degradation. Reduced stiffness properties of the damaged lamina are found to depend explicitly upon the crack density of that lamina and implicitly upon the crack density of the neighbouring lamina. Theoretical predictions for carbon and glass fibre reinforced plastic cross-ply laminates with matrix cracking in the 90Њ ply revealed significant reduction in the Poisson's ratio and shear modulus due to additional damage (splitting) in the 0Њ ply. ᭧
Analysis of damage mode transition in a cross-ply laminate under uniaxial loading
Composite Structures, 2002
The aim of this work is to study the transition from one damage mode to another. The three damage mechanisms usually observed in 0 m =90 n s long ®bre cross-ply laminates with organic matrix are transverse cracking, which appears ®rst, longitudinal cracking and/or delamination. The proposed analytical model is meant to describe the development of transverse cracks and to predict the order of initiation of subsequent damage modes; it relies on the strain energy release rate associated with each damage mechanism.
International Journal of Engineering Research and Technology (IJERT), 2012
https://www.ijert.org/fracture-behaviour-of-frp-cross-ply-laminate-with-embedded-delamination-subjected-to-transverse-load https://www.ijert.org/research/fracture-behaviour-of-frp-cross-ply-laminate-with-embedded-delamination-subjected-to-transverse-load-IJERTV1IS8362.pdf One of the most important damage mechanisms in composite materials is the delamination between plies of the laminate. In industrial applications, composite plates are sensitive to impact and delamination occurs. Many composite components have curved shapes, tapered thickness and plies with different orientations, which make the delamination grow depending on the extent of the crack. It is therefore important to analyze the delamination characteristics of composite structures. The main objective of the present investigation is the characterization of the delamination growth in four layered cross ply (0/90/90/0) fibre reinforced composite laminates along all sides of the delamination. The analysis has been carried out using Virtual Crack Closure Technique (VCCT) in combination with Finite Element Methods (FEM) with the help of commercially available Finite Element Software, ANSYS. Nomenclature E 1 = Young"s modulus of the lamina in the fibre direction E 2 = E 3 = Young"s modulus of the lamina in the transverse direction of the fibre G 12 = G 13 = Shear modulus in the longitudinal plane of the fibre G 23 = Shear modulus in the transverse plane of the fibre ν 12 = ν 13 = Poisson"s ratio in the longitudinal plane of the fibre ν 23 = Poisson"s ratio in the transverse plane of the fibre G = Strain energy release rate
Coupled stress and energy criterion for multiple matrix cracking in cross-ply composite laminates
International Journal of Solids and Structures, 2018
Transverse cracking, i.e. matrix cracking in the off-axis plies of the laminate, is widely recognized as the first damage mode to appear in continuous fibre-reinforced composite laminates subjected to in-plane loading. Since transverse cracking has a great influence on the subsequent damage steps such as delaminations or oblique cracks, it is important to be able to predict its onset and growth accurately. In this paper, it is proposed to use a combination of the Coupled Criterion of Finite Fracture Mechanics (FFM) and the Equivalent Constraint Model (ECM) to predict the evolution of crack density with increasing applied load. Two formulations-a discrete formulation and a continuous formulation-are developed for the energy criterion within the Coupled Criterion. Some dependences between the two formulations are proved, which justifies the good agreement found by the models based on continuous formulations presented by other authors despite the inherent discrete nature of the phenomenon. Dependence of the failure load predicted by the Coupled Criterion on the layer thickness ratio and brittleness number (a structural parameter that characterizes a combination of stiffness, strength, fracture toughness and the thickness of the cracked ply of the laminate) is examined and discussed for carbon/epoxy and glass/epoxy laminates. Finally, comparison against experimental results shows a good agreement.