1,720,971 research outputs found
The effect of the through-the-thickness compressive stress on mode II interlaminar fracture toughness
Get PDFThe effect of through-the-thickness compressive stress on the mode II interlaminar fracture toughness is investigated experimentally and replicated numerically. The modified Transverse Crack Tensile specimen recently proposed by the authors is used, together with an experimental device designed to apply a constant transverse compressive stress on the surface of the specimen. Experiments are conducted using IM7/8552 specimens for different compressive stresses, ranging from 0 to 100 MPa, covering all the practical applications commonly encountered in the aeronautical industry (e.g., tightened filled holes or bolted joints). It is shown that the mode II interlaminar fracture toughness increases with the applied compressive through-the-thickness stress. Finally, experiments are replicated using appropriate numerical models based on cohesive elements that take into account frictional effects. A good agreement between numerical predictions and experiments is found.</p
The effect of through-thickness compressive stress on mode II interlaminar fracture toughness
Get PDFThe effect of through-thickness compressive stress on mode II interlaminar fracture toughness is investigated experimentally and replicated numerically. The modified Transverse Crack Tensile specimen recently proposed by the authors is used, together with an experimental device designed to apply a constant transverse compressive stress on the surface of the specimen. Experiments are conducted using IM7/8552 specimens for different compressive stresses, ranging from 0 to 100 MPa, covering all the practical applications commonly encountered in the aeronautical industry (e.g., tightened filled holes or bolted joints). It is shown that mode II interlaminar fracture toughness increases with the applied compressive through-thickness stress. Finally, experiments are replicated using appropriate numerical models based on cohesive elements that take into account frictional effects. A good agreement between numerical predictions and experiments is found.</p
An Efficient Approach to the Modeling of Compressive Transverse Cracking in Composite Laminates
No full textA wedge-shaped failure mechanism occurs when a composite ply in a laminate is loaded under transverse compression. Because of this, a typical failure model for tensile load cases is not straightforward applicable for compressive load cases. In this paper, a method is described to approximate the compressive behavior using a vertical crack plane. Rather than adopting the crack topology, a transformation is applied prior to the evaluation of the cohesive law. The method is applied on predefined interface elements in a 2D model, as well as on cohesive segments in the extended finite element method in a 3D model. It is found that the compressive failure of composite laminates including the wedge effect can be approximated accurately with this method using a vertical crack plane. In 3D, a variable fracture plane angle is easily taken into account with the method, which means that the whole range of failure modes, from tensile, through shear-dominated to compressive failure can be covered in a single approach.Structural Mechanics / Computational MechanicsStructural EngineeringCivil Engineering and Geoscience
Computational modeling of failure in composites under fatigue loading conditions
No full textApplied Mechanic
Multiscale modeling of strain rate effects in FRP laminated composites
No full textFiber reinforced polymer composites are increasingly used in impactresistant devices, automotives, and aircraft structures due to their high strengthtoweight ratios and their potential for impact energy absorption. Dynamic impact loading causes complex deformation and failure phenomena in composite laminates. Moreover, the high loading rates in impact scenarios give rise to a significant change in mechanical properties (e.g. elastic modulus, strength, fracture energy) and failure characteristics (e.g. failure mechanisms, energy dissipation) of polymer composites. In other words, both mechanical deformation and failure are strainrate dependent. The contributing mechanisms can be roughly classified as viscous material behavior, changes in failure mechanism, inertia effects and thermome chanical effects. These effects involve multiple length and time scales. In experiments it is difficult to isolate single mechanisms contributing to the overall ratedependency. Therefore, it is difficult to quantify the contribution of each mechanism at different scales. The aim of this thesis is to establish a multiscale numerical framework in which three of the contributing mechanisms, i.e. the viscous material behavior, changes in fracture mechanisms and inertia effects, can be investigated at different scales. The research in this thesis is divided into four parts, one related to the macroscale, where the composite material is treated as homogeneous, and three on exploring possibilities to include microscale information, taking into account the microstructure of fibers and matrix.Applied Mechanic
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Extending the Thick Level Set Approach: Plasticity, Parallel Computing and Cohesive Cracks
No full textDeveloping accurate and robust numerical approaches that are capable of modeling fracture in solids has been a challenging undertaking in the computational mechanics community for decades. Models based on a continuous formulation or on a discontinuous one have been proposed by numerous authors, expanding upon abilities and disadvantages of these approaches. However, models attempting to bridge these two approaches have been less often encountered in the literature.Over the last ten years, a new approach for modeling fracture in solids has been developed, coined the Thick Level Set (TLS) method, in which the damage evolution is linked to the movement of a damage front described with the level set method. This model offers an automatic transition from damage to fracture and deals with merging and branching cracks as well as crack initiation in a easy and robust manner. Furthermore, the TLS in its new (second) version, coined the TLSV2, is able to model explicitly the displacement discontinuity at the position of a crack.These TLS features are very beneficial for the modeling of cusp crack patterns in resin-rich regions of fiber reinforced polymer composites under mode II loading. In this process, plasticity might occur prior to fracture, which begins with a series of inclined cracks that eventually merge to form what is at higher scale of observation understood as a single crack. When the crack reaches one of the boundaries of these resin-rich regions, the localized deformation in these parts is a sliding one, which is expected to be traction-free.This in situ process has been reported as one of the reasons for the differences in terms of fracture energy between mode I and mode II crack growth since forming cusps requires the emergence of more crack surface than forming a single straight crack. Therefore, in order to simulate this fracture process under realistic boundary conditions, a model based on the TLS can be embedded in a ‘macroscopic’ setup, such as three-point bend end-notched flexure. A monolithic scheme with extreme refinement in a zone of interest is the most straightforward approach; however, for a specimen with realistic dimensions, this can be computationally unfeasible due to the computational resources needed to solve the large systems of equations involved in such problem. An ability to comprehensively model this microscopic process could help to achieve a better understanding of the mechanism behind the observed dependence of the fracture energy on the mode of fracture, which may in turn improve macroscale simulations.This work focuses on extending the TLS method in order to profit from its full capabilities to deal with simulations of failure in solids under quasi-static loading conditions. For this purpose, several original numerical and theoretical components are proposed for reaching qualitative agreement with experimental observations of cusp formation in polymer matrix. In this context, the primary application of this thesis relies on the experimental observations at the microscopic level of such process. However, it is worth mentioning that the numerical tools developed in this thesis are not limited to the problem of cusps; in fact, they can be either used or easily extended to simulate other problems, for instance crack growth through the microstructure of cementitious materials with different aggregates.First, the TLS is combined with plasticity in order to deal with ductile fracture since polymers may behave plastically prior to failure, particularly when loaded in shear. To accommodate for plasticity, several changes to the TLS framework are introduced. A strength-based criterion for initiation of damage based on the ultimate yield surface of such plasticity model is proposed. A mapping operator for transferring plastic history is included if the integration scheme in a finite element changes due to the evolution of the level set field. Furthermore, a new loading scheme is devised in order to take into account permanent strain.Next, a generalized framework for the TLSV2 is introduced. The TLSV2 couples continuous and discontinuous approaches within a single framework, where the continuum part allows for handling crack initiation, branching and merging, whereas the discontinuous part brings the capability to handle discrete cracks with large crack opening or sliding without heavily distorted elements, as well as the possibility to model stiffness recovery upon contact.Two major issues with the TLSV2 method that have not been dealt with since its inception are addressed in this thesis, and solutions are proposed. Firstly, the method depends on identifying the location of the skeleton curve of the level set field, on which the discontinuity in the displacement field is evaluated. The problem of locating the skeleton curve can be a complicated task, even more so because topological events may emerge as the analysis progresses, such as crack branching. The skeleton curve is determined through a combination of ball-shrinking and graph-based algorithms and then mapped onto the finite element mesh. Secondly, the cohesive forces and displacement discontinuity of the TLSV2 are modeled using the phantom node method. Furthermore, a new approach to compute the non-local crack driving force is introduced, and model calibration is discussed. The degree of stiffness recovery under compression that is still needed for the continuum part is investigated.The TLS can be a computationally demanding approach. Therefore, a domain decomposition strategy is introduced in order to obtain a parallel implementation of the TLS method. To handle the numerical components specific to the TLS analysis steps involving level set update, equilibrium solution, and damage front advance, a parallel strategy is introduced for each of them. The most demanding task in terms of computational cost, i.e., solving the linearized system of equations from the equilibrium problem, is performed with a parallel iterative method profiting from the adopted domain decomposition method. A communication strategy is provided to deal with enriched nodes and new nodes necessary for the phantom node method belonging to shared regions of subdomains. Collective communication strategies are also proposed to deal with operations related to the level set update, damage front advance, and skeleton curve.Numerical experiments demonstrate the accuracy and efficiency of the proposed framework in handling simulations of failure analysis with complex crack patterns in a sequential and parallel context
Interpreting the single fiber fragmentation test with numerical simulations
No full textCharacterization of the mechanical properties of the fiber/matrix interface is a challenge that needs to be addressed to enable accurate micromechanical modeling of failure in composite materials. In this paper a numerical investigation is presented into one of the tests that has been proposed for measuring these interfacial properties. A new cohesive zone model with friction is presented, as well as an original numerical framework for modeling of embedded fibers. The research generates new insight into the meaning of the single fiber fragmentation test, confirming the applicability of shear lag theory also in presence of multiple cracks, and emphasizing the relevance of matrix plasticity for the development of friction in the test. Although the frictional stress that can be obtained from the test should not be confused with the cohesive strength of the fiber/matrix interface, measurements of fracture process zone length can give indirect information on this cohesive strength.Accepted Author ManuscriptApplied Mechanic
Computational modeling of failure in composite laminates
No full textThere is no state of the art computational model that is good enough for predictive simulation of the complete failure process in laminates. Already on the single ply level controversy exists. Much work has been done in recent years in the development of continuum models, but these fail to predict the correct failure mechanism in cases where matrix cracking in off-axis plies is part of the global mechanism. The way forward is to model matrix cracks as true discontinuities in the displacement field, in which case the orientation of the cracks can easily be controlled. A mesh-independent representation with partition of unity based methods or, similarly, the phantom node method is to be preferred, because with these methods cracks can initiate and grow at arbitrary locations in the model, wherever the stress field gives rise to it. This requires an initially rigid mixed mode cohesive law. Unfortunately, straightforward formulation of such a law leads to a singularity; the traction is not uniquely defined for zero crack opening and zero damage. Robustness of the simulation requires that this singularity is removed from the description.Two methods exist that do this. Firstly, it is possible to relate the cohesive traction not only to the displacement jump, but also to the stress in the surrounding material. Secondly, one can start from a cohesive law with a finite initial stiffness and then apply a shift to the origin to mimic initially rigid behavior. With both methods a cohesive law is derived that satisfies the Benzeggagh-Kenane relation for mixed mode fracture energy. The constitutive model for the single ply is completed with a damage/plasticity law for shear nonlinearity and a continuum damage model for fiber failure. The ply model can be used as a building block for analysis of complex failure mechanisms in laminates. For full-laminate analysis, an additional failure process is possible, namely delamination. The proposed laminate model consists of a single layer of elements for each unidirectional ply, interconnected with interface elements. These interface elements are equipped with a cohesive law for delamination. Notably, the interaction between the phantom node method in the plies and the interface elements is accurate without updating the interface elements, particularly when a nodal integration scheme is used. A complicating aspect of laminate analysis, is that matrix cracking may occur in a distributed fashion because of mutual constraint between the plies. It is possible to model many cracks with the phantom node method. However, the number of cracks must be limited with a minimum crack spacing parameter to keep the problem well posed and to allow for coalescence of cracks. Because laminate failure is a highly nonlinear process, a carefully designed solution algorithm is indispensable to bring simulations to a successful end. Sharp snapbacks may occur because the stiff fibers may unload suddenly when failure progresses through the laminate. In order to follow the equilibrium path through these snapbacks, an arclength method is needed. The dissipation-based arclength method is used for this purpose, because it is robust and generic. The formulation is extended for cases with both permanent deformations and damage. Adaptive time stepping is crucial and in some cases a modified Newton-Raphson scheme is to be preferred. The discontinuities that are represented matrix cracks, which may be numerous, are inserted during the computation. This is handled after equilibrium has been found. After crack propagation, equilibrium is sought again before the next time step is entered, but multiple crack segments may be introduced at once. The framework is validated against experimental observations for several laminate cases. Subcritical damage consisting of delamination and matrix cracks is generally captured well. Different failure mechanisms can be described and the appropriate one is `chosen' automatically because the different processes are incorporated realistically. The fiber failure model lacks a representation statistical strength distribution of the fibers and is therefore not predictive in brittle cases where the strength in fiber direction is a key parameter. This work has resulted in a robust integrated framework for computational modeling of failure in composite laminates. The numerical results are objective with respect to discretization. Different failure processes and their interaction are represented such that the simulations not only provide insight in when the laminate fails, but also in how it fails.Structural MechanicsCivil Engineering and Geoscience
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