Delamination of composite plates under tensile load

Conclusions Among the two configurations discussed, the beam has a rather theoretical importance, as an extension and generalization of the cracked beam problem. Unlike this, embedded delaminations can be found often in real constructions. The fact that the maximum value of SERR vs. fibre orientation angle curve for the circular embedded delamination is surpassed by that for the split beam only by three times and by that for the split beam under mode II loading approximately six times means that an embedded delamination can lead to difficulties when the plate is tensed (or compressed — even in an underbuckled state — within the linear approach the non-opening modes of stress concentration will just change their signs when the load changes its sign), especially under a cyclic loading. Of course, the shell model used is fairly rough and does not take in account several features or real delaminations extension and does not allow one to evaluate properly the stress field near the crack front; thus, more analytical and experimental studies would be needed, to establish the crack development process. The 3D analysis is necessary when initiating delaminations are studied, for the beam/plate assumptions can no longer be regarded as necessary since the delamination linear dimensions are comparable to or less than the thickness of the sublaminate.Published in Mekhanika Kompozitnykh Materialov, Vol. 31, No. 1, pp. 45–50, January–February, 1995.     

Wave propagation in composite multilayered structures with delaminations

Using the Rayleigh–Ritz method, the natural frequencies of layered cylindrical shells with delaminations are derived. The results are compared with FE computations and with data available in the literature. It is demonstrated that the transverse shear effects can play a significant role in a correct and accurate evaluation of the natural frequencies of such shells. Numerical investigations into the problem of radial excitations of cylinders with delaminations have also been performed. From the results obtained, it follows that their optimal excitation frequency is related to the natural frequency of a local region containing the delamination.     

Local Delaminations Induced by Interaction Between Intralaminar Cracking and Specimen Edge in Quasi-Isotropic CF/EP NCF Composites in Fatigue Loadings

Mechanics of Composite Materials - Experimental results are presented on the onset and propagation of local delaminations caused by the interaction between specimen edges and intralaminar cracks in...     

Localization of a delamination and estimation of its length in a composite laminate beam by the VSHM and pattern recognition methods

The purpose of this study was to investigate the delamination damage in laminate composite beams in order to adapt the vibration-based structural health monitoring (VSHM) method for laminated structures. The analysis was concentrated on the vibration characteristics of laminated specimens, in particular, on the first several natural frequencies of a composite laminate beam with a delamination damage. The core of this work is an experimental investigation into the vibration response of a composite laminate beam and its changes caused by delaminations of different sizes and different location in the beam. The aim was to determine how the first six harmonic frequencies are changed by a delamination, and the results show that they can be successfully used to clarify the presence, location, and dimensions of delaminations in a composite beam. A pattern recognition analysis was used to locate the damage, while its detection and evaluation were performed by using changes in the harmonic frequencies. A finite-element analysis was carried out, and the variations in the natural frequencies due to delamination are found to be in good agreement with experimental results.     

Bending Stiffness of Damaged Cross-Ply Laminates

Mechanics of Composite Materials - The bending stiffness of carbon/epoxy and glass/epoxy cross-ply laminates with intralaminar cracks in the surface 90° plies and local delaminations were...     

The applicability of a model of shear delaminations

For the purpose of ascertaining the limits of applicability of a previously proposed model [1] of non-propagating shear delaminations for analysing a three-layer rod with delamination-type defects, the numerical solution of the corresponding threedimensional problem in the theory of elasticity is constructed and analysed using Version 6.1 of the ANSYS (Analysis of Systems) finite-element software package. The limits of applicability of the proposed delamination model are estimated.     

Two-parameter model of the failure of adhesive joints

Conclusions The proposed two-parameter model of adhesive failure makes it possible to study the limiting states of a composites with delaminations by constructing criterional relations of the type (4.5), (4.6). Within the framework of the above-stated hypotheses and results, the process of the adhesive failure of composites with delaminations can be represented as follows. The application of an external load results in the formation of zones of interfacial shear prefracture near the ends of the delaminations. The dimensions of these zones depend both on the geometry of the delaminations and on the external load. With a further increase in the load, microscopic interfacial defects accumulate in the region S_es ⁽⁰⁾, which leads to an increase in the size of this region in accordance with a relation of the type (4.3). Avalanche shear failure occurs when the external loads reach the limiting values established in accordance with criterion (1.2).The results obtained here are based on the assumption that the strength conditions for the constituent phases are satisfied. It should be noted that the proposed approach can be used to construct a model which will embrace the process of adhesive-cohesive failure of composites. To do this, it is necessary to also employ the corresponding strength hypotheses of the phases, which in turn increases the number of phenomenological constants (such as the critical stress intensity factors K_Ic, the yield point _y, the fracture stress f, etc.) In the last case, it is possible to qualitatively describe a range of composite failure processes with allowance for the interaction of brittle microcracks within the phases and delaminations of different types. It should also be noted that the results obtained here -particularly the model of failure of adhesive joints with allowance for finiteness of the interfacial stresses — justify the use of the methods of plate and shell theory [8] and other methods in the analysis of laminated composites.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 1034–1039, November–December, 1988.     

Inverse Identification of Delamination Regions in Layered CFRP-Composites

In this paper we are concerned with the complete non-destructive reconstruction (shape, depth and topology) of delaminations in one damaged interface in layered composites using the Cauchy data on the outer boundary. The proposed method combines an iterative algorithm for solution of mixed ill-posed elliptical boundary value problems and a parameter estimation in sense of the ill-posed optimization problems. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Circular Delaminations - the Effect of Thermal Strains on the Strain Energy Release Rate

The analysis presented focuses on the extent to which the thermal loading resulting from a difference between the curing and service temperatures and from variations in the thermal-expansion coefficients from layer to layer can affect the growth of delaminations. Square laminate plates containing circular delaminations are examined. The reinforcement combinations for the delaminated layer and the core plate considered are [0°]/[90°], [90°]/[0°], [0°]/[±45°] _s , and [±45°] _s /[0°]. The analysis is carried out using the FEM. The strain energy release rate components are calculated using the modified crack-closure integral method. The results obtained show that, for all the reinforcement combinations studied, with the exception of [±45°] _s /[0°], a temperature drop considerably reduces all the strain energy release rate components, but only minor changes in their proportion occur. In the case of the [±45°] _s /[0°] plates, all these components increase, and noticeable changes in their proportion are also observed.     

Flexural Vibrations of Shafts with Delaminations

The purpose of this theoretical work is to present a general model of laminated rotating shaft with circumferential delaminations. The shaft is treated as a thin‐walled composite cylindrical shell. The delamination of constant width is parallel to the shell reference surface and it covers the entire circumference. The edge delamination is modeled by changing the effective reduced stiffnesses of debonded parts. The stabilizing effect of external damping and destabilizing effect of internal damping are taken into account in the dynamic stability analysis. The influence of the relative delamination length and configuration on the critical angular velocity of shaft is shown.     

Analysis of critical strains and loads in layered composites

The present work is devoted to investigation of the compressive fracture for piecewise homogeneous materials with specific kinds of delaminations (defects with connected edges). The critical buckling strains and loads at which ply instability in laminated composites occurs under uniaxial and biaxial compression were evaluated. Different types of layers (linear elastic, elastic-plastic, hyper-elastic incompressible) were investigated. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interlaminar fracture of multidirectional glass/epoxy laminates under mixed-mode I + II loading

The mixed-mode I + II interlaminar fracture of multidirectional glass/epoxy laminates is investigated. Mixed-mode bending (MMB) tests were performed on specimens with delaminations in 0/θ-type interfaces, with θ varying from 0 to 90°. Preliminary three-dimensional finite-element analyses validated the beam theory model (BTM) used for analysing experimental data. The compliances measured are in a good agreement with BTM predictions. The total critical energy release rate G_c varies linearly with the mode II ratio G_II/G, although some discrepancies are observed in the high-mode II results for the 0/45 and 0/90 specimens. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 3, pp. 349–366, May–June, 2007.     

Theory of fiber-matrix delamination in sphero-fibrous plastics

A method for calculating the deformed state and the effective characteristics of sphero-fibrous composites with a three-dimensional structure and local delaminations at the fiber-matrix interface is proposed. It is assumed that the interfacial cracks take place along the whole length of fibers within the representative volume. The effect of the interface crack dimension on the variations in shear moduli of the 3Dm composite is investigated in detail. It is shown that small cracks have practically no influence on the integral characteristics of the composites.Blagonravov Machine Building Institute, Russian Academy of Sciences, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 644–650, September–October, 1997.     

Numerical elastic-plastic simulation of interlaminar stresses in a notched angle-ply thermoplastic composite laminate

A thermoplastic angle-ply AS4/PEEK laminate with a hole is considered. The interlaminar stresses along the hole edge at different interfaces under uniaxial extension are investigated. According to the symmetries of the structure and loading, a suitable finite-element model is developed. Utilizing a three-dimensional elastic-plastic finite-element procedure elaborated previously, a finite-element modeling of the interlaminar stresses in a thick angle-ply composite laminate is carried out. Based on the interlaminar stresses obtained, the dangerous locations of delamination initiation are predicted. The results obtained indicate that there is some relationship between the dangerous locations and fiber orientations in the adjacent layers, and it maybe inferred that the critical locations are near the regions where the hole edge is tangent to the fiber orientation in the layers adjacent to the interface. The interlaminar stresses at the same interfaces are not sensible to distances from the midplane of the laminate. Very high interlaminar tensile stresses are found to exist on the hole edge at the +25°/+25° or –25°/–25° interfaces, and delaminations can initiate there first. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 45, No. 3, pp. 427-440, May-June, 2009.     

Fatigue damage modeling of fibrous soft tissues

Ultimate tendon failure is often caused by fatigue loading. Recent interventions revealed a three-phase progression of histological changes during cyclic loading of the tendon. It starts from localized kinked fiber deformations, continues with additional fiber delaminations and finally leads to fiber angulations and discontinuities [5, 6]. In the present contribution, we propose a physically motivated constitutive model able to describe fatigue evolution in tendon subject to cyclic loading. The damage of the collagen fibers is elucidated by a successive permanent opening of tropocollagen molecules [7], which represent the basic building blocks of collagen fibrils. The fibril strain increase is triggered by a time-force depending rupture of glycosaminoglycan sidechains of adjacent collagen fibrils. The so obtained model is in line with recent experimental findings available in literature. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modelling of Wave-Based SHM Systems Using the Spectral Element Method

In nondestructive testing, the use of ultrasonic elastic waves has proven as one of the most successful techniques to detect structural damage such as cracks or delaminations. Especially, Structural Health Monitoring (SHM) is characterized by permanently installed actuators and sensors. As the capability of most approaches strongly depends on adequate choice of parameters like excitation signals and actuator/sensor positions, there is a growing interest in efficient and accurate simulation tools to be able to perform virtual SHM-system design. With respect to high frequency excitation an efficient simulation method is required. This contribution presents the formulation of spectral elements for flat shells based on the first order shear deformation theory (FSDT). A load-dependent geometrical stiffness matrix is included to take static preloads into account. The spectral element method generates an optimally concentrated mass matrix leading to significant savings of memory and to a reduction of complexity of the time integration algorithm. Dispersion curves of the FSDT are compared to those of 3D elasticity theory to determine the useful application range. Numerical and experimental examples for the propagation of waves including the influence of static prestress and the interaction with delamination are presented. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Compressive stability of multilayered delaminations in composites

The behavior of defects of the near-surface crack type was investigated in laminated composites. Multilayered delaminations in compressed elements were examined on the assumption that cylindrical bending of the delamination is accompanied by shear within the framework of a model similar to the Timoshenko model. The effect of the mechanical properties and thickness and number of layers in the delaminated section on the critical buckling strain and strain corresponding to the beginning of growth of the delamination was investigated. It was shown that when the delaminated section contains several bearing layers and the rigidity of the composite with respect to interlaminar shear is small, consideration of the shear in the delaminated section can lead to a significant change in the critical compressive strains and/or critical size of the delamination.Translated from Mekhanika Kompozitnykh Materialov. Vol. 33, No. 3, pp. 312–320, May–June, 1997.     

Evaluation of self-activating temperature influence on cracks initiation in GRP laminates

The paper deals with problems of fatigue processes of GRP laminates under the influence of cyclic loads. An important aspect which takes place in laminate fatigue with polymer matrix is self-activating temperature generated by friction. As it has been shown in previous authors' research, the phenomenon has significant influence on laminate behavior, also, when self-activating temperature comes to glass-transition temperature, which begins transition from elastic to viscoelastic material model. After passing over of this temperature the stiffness decreasing is supervised, that can lead to more quick propagation of faults (especially delaminations) and decreasing of life cycle of composite laminate element. The aim of the presented paper is simulation research of fatigue processes with taking into consideration self-activating temperature and rheology of material. In the research four cases are analyzed, in which layers rotation, self-activating temperature increasing and changes of rheological model were taken into consideration. As the research shows, the self-activating temperature has significant influence on fatigue processes, because it can cause transition of the rheological model of the material. In the future research an experimental verification of the model is planned. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simulation of Wave Propagation in Complex Structures with the Spectral Element Method

In nondestructive testing, the use of ultrasonic elastic waves has proven as one of the most successful principles to detect structural damage like cracks, delaminations etc. Especially, Structural Health Monitoring (SHM) is characterized by permanently installed embedded or surface–mounted actuators and sensors (e.g. piezoelectric patches). The capability of most approaches strongly depends on adequate choice of parameters like excitation signals and actuator/sensor positions. For this reason there is a growing interest in efficient and accurate simulation tools to shorten time and cost of the necessary pretests. With respect to high frequency excitation a computationally efficient method is required. This contribution presents the theoretical background of the spectral element method including the electro–mechanical coupling of piezo elements. The spectral element method generates a diagonal mass matrix leading to significant savings of memory and to a crucial reduction of complexity of the time integration algorithm. Both in–plane and out–of–plane waves can be handled. Numerical examples for the propagation of waves in stiffened structures are presented. The effect of improper placement of actuators/sensors is shown. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

多值函数在复变函数中的应用

通过讨论初等多值函数的单值解析分枝问题,重点研究了初等多值函数在复变函数中的具体应用.