Micromechanical modeling of short-fiber reinforced composites

Micromechanical modeling of short-fiber reinforced composites
Author: Mueller, Viktor
Publisher: KIT Scientific Publishing
Total Pages: 166
Release: 2016-06-16
Genre: Technology (General)
ISBN: 3731504545

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This work is focused on the prediction of elastic behavior of short-fiber reinforced composites by mean-field homogenization methods, which account for experimentally determined and artificially constructed microstructure data in discrete and averaged form. The predictions are compared with experimental measurements and a full-field voxel-based approach. It is investigated, whether the second-order orientation tensor delivers a sufficient microstructure description for such predictions.

Bioinspired Structures and Design

Bioinspired Structures and Design
Author: Wole Soboyejo
Publisher: Cambridge University Press
Total Pages: 374
Release: 2020-09-17
Genre: Technology & Engineering
ISBN: 1108963447

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Master simple to advanced biomaterials and structures with this essential text. Featuring topics ranging from bionanoengineered materials to bio-inspired structures for spacecraft and bio-inspired robots, and covering issues such as motility, sensing, control and morphology, this highly illustrated text walks the reader through key scientific and practical engineering principles, discussing properties, applications and design. Presenting case studies for the design of materials and structures at the nano, micro, meso and macro-scales, and written by some of the leading experts on the subject, this is the ideal introduction to this emerging field for students in engineering and science as well as researchers.

Understanding and Predicting the Stress Relaxation Behavior of Short-Fiber Composites

Understanding and Predicting the Stress Relaxation Behavior of Short-Fiber Composites
Author: Numaira Obaid
Publisher:
Total Pages:
Release: 2018
Genre:
ISBN:

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The viscoelastic properties of short-fiber composites are complex and not well-understood. Previous experimental work has shown that the viscoelastic properties of short-fiber composites are affected by both elastic fibers and the matrix, which is baffling since elastic fibers do not exhibit any time-dependence of their own. The goal of this study was to understand why and how elastic fibers can alter time-dependent behavior when contained in a composite. In this thesis, conventional shear-lag theory was adapted to include a time-dependent matrix and a novel analytical model was used to predict the tensile relaxation modulus of short-fiber composites. The model highlighted the importance of incorporating both the time-dependent tensile modulus of the matrix as well as its time-dependent shear modulus. Investigations using the model showed that since stress transfer in a short-fiber composite occurs through interfacial shearing, the time-dependent shear modulus of the matrix results in time-varying stress transfer the fiber. Since the stress in the fiber is time-dependent, it exhibits an apparent stress relaxation stemming from the relaxing shear modulus of the matrix. The model predictions were validated using finite-element simulations and experimental data. Comparison to real data confirmed the hypothesis that the time-dependency observed in elastic fibers stemmed from the indirect time-dependency imposed by the time-varying stress transfer from the matrix. The model was also used to determine the effect of various parameters including fiber aspect ratio and fiber volume fraction. For the first time, a critical aspect ratio for viscoelasticity was introduced. This was defined as the aspect ratio at which the contribution to composite stress relaxation by the fiber is maximized. The effect of fiber orientation was also examined, and an analytical model was developed to predict the stress relaxation of composites containing randomly-oriented fibers. It was found that random orientation in the plane would shrink the effect of fibers by one-third of what would be observed in oriented composites. In the last part of the thesis, we investigated the strain rate-dependence of short fiber-reinforced foams. The study highlighted a potential area where knowledge of the stress relaxation behavior of the short-fiber composites could prove useful.

The Virtual Crack Closure Technique: History, Approach and Applications

The Virtual Crack Closure Technique: History, Approach and Applications
Author: Ronald Krueger
Publisher:
Total Pages: 66
Release: 2002
Genre:
ISBN:

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An overview of the virtual crack closure technique is presented. The approach used is discussed, the history summarized, and insight into its applications provided. Equations for two-dimensional quadrilateral elements with linear and quadratic shape functions are given. Formula for applying the technique in conjuction with three-dimensional solid elements as well as plate/shell elements are also provided. Necessary modifications for the use of the method with geometrically nonlinear finite element analysis and corrections required for elements at the crack tip with different lengths and widths are discussed. The problems associated with cracks or delaminations propagating between different materials are mentioned briefly, as well as a strategy to minimize these problems. Due to an increased interest in using a fracture mechanics based approach to assess the damage tolerance of composite structures in the design phase and during certification, the engineering problems selected as examples and given as references focus on the application of the technique to components made of composite materials.

Micromechanical Modeling of Short-fiber Reinforced Composites

Micromechanical Modeling of Short-fiber Reinforced Composites
Author: Viktor Müller
Publisher:
Total Pages: 160
Release: 2020-10-09
Genre: Technology & Engineering
ISBN: 9781013279881

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This work is focused on the prediction of elastic behavior of short-fiber reinforced composites by mean-field homogenization methods, which account for experimentally determined and artificially constructed microstructure data in discrete and averaged form. The predictions are compared with experimental measurements and a full-field voxel-based approach. It is investigated, whether the second-order orientation tensor delivers a sufficient microstructure description for such predictions. This work was published by Saint Philip Street Press pursuant to a Creative Commons license permitting commercial use. All rights not granted by the work's license are retained by the author or authors.

Tension Stiffening in Reinforced High Performance Fiber Reinforced Cement Based Composites

Tension Stiffening in Reinforced High Performance Fiber Reinforced Cement Based Composites
Author: Daniel Mauricio Moreno Luna
Publisher:
Total Pages:
Release: 2014
Genre:
ISBN:

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Cement-based composites, such as concrete, are extensively used in a variety of structural applications. However, concrete exhibits a brittle tensile behavior that could lead to reduced durability and structural performance in the long term. The use of discontinuous fibers to reduce the brittleness of the concrete, and improve its post-cracking tensile behavior, has been a focus of structural materials research since the 1960's. Cement-based materials reinforced with short discontinuous fibers are known as Fiber Reinforced Composites (FRC). High Performance Fiber Reinforced Cement-based Composites (HPFRCC) are a special type of FRC materials that exhibit tensile strain-hardening behavior under varied types of loading conditions such as direct tension or bending. The use of HPFRCC materials in structural applications has shown to improve not only durability and long term performance, but also has proven to enhance inelastic load-deformation behavior, ductility, energy dissipation and shear capacity. The use of HPFRCC materials can also result in a potential reduction of steel reinforcement required for both flexure and shear relative to traditional reinforced concrete structures. The interaction between the mild steel and the ductile HPFRCC matrix in tension was investigated in contrast to that of normal weight concrete. The measured responses demonstrated both the tension stiffening effects of HPFRCC materials as well as the early strain hardening and fracture of the reinforcing bar relative to that in a normal weight concrete observed through full specimen response up to fracturing of the reinforcement. All of the HPFRCC specimens tested exhibited multiple cracking in uniaxial tension. Splitting cracks observed in the concrete at low specimen strain levels and in HyFRC and SC-HyFRC specimens at higher specimen strain levels contributed to the spreading of strain along the reinforcing bar in those specimens, resulting in a larger displacement capacity relative to the ECC specimens, which did not exhibit splitting cracks. Early strain hardening is hypothesized to be the reason for the additional strength observed in specimens subjected to flexure where the interaction between the steel and the HPFRCC matrix plays an important role in the load-displacement response. A modified approach for estimating the flexural capacity of a section of reinforced HPFRCC using experimental tension stiffening data was proposed and demonstrated to improve the accuracy of flexural capacity predictions. Two-dimensional finite element modeling approaches using a total strain based constitutive model were investigated. The numerical simulations demonstrated the relevance of using standard characterization tests to define the tensile and compressive stress-strain curves for the material constitutive model. The simulations capture the initial and post cracking stiffness, load at first cracking, load and strain at localization and deformation capacity observed in the experiments. Multiple cracking was observed in the numerical simulations for the ECC and HyFRC. The models were able to simulate the cracking progression and localization of strains at primary and secondary cracks for the ECC and the HyFRC. The numerical simulations that used the splitting bond-slip model captured the distribution of the strains in the steel better than perfect bond and pull-out bond-slip models as the slip in the interface allowed for a less localized failure of the specimens, especially in the ECC models. The models were also able to accurately capture the early hardening behavior observed in the experiments. A methodology to estimate the flexural strength of HPFRCC structural components by using numerical simulation of tension stiffening has been proposed and validated on a high performance fiber reinforced concrete (HPFRC) infill panel and ECC and HyFRC beams. This methodology serves as an extension of the methodology proposed using experimental tension stiffening results. In the absence of additional experiments, numerical simulation is proposed. A good level of accuracy has been found between the predicted and actual flexural capacities of the investigated components. The proposed methodology is based on the current assumptions from planar analysis used in the calculation of flexural strength in reinforced concrete components.

Natural Fiber Composites

Natural Fiber Composites
Author: R.D.S.G. Campilho
Publisher: CRC Press
Total Pages: 354
Release: 2015-11-05
Genre: Technology & Engineering
ISBN: 1482239019

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This book brings value to anyone working with or designing natural fiber composite structures. It helps readers understand the value these materials can add to projects, how to choose the best materials and treatments, how to safely design and fabricate products made of natural fiber composites, and how to test them for safety. It covers the characterization of natural fibers, matrices and respective composites, and how to enhance their performance and processing as well as testing and degradation issues.

Nonupdating Methods for the Characterization of Nonlinear Stress-strain and Delamination Fracture Properties of Fiber Reinforced Composites

Nonupdating Methods for the Characterization of Nonlinear Stress-strain and Delamination Fracture Properties of Fiber Reinforced Composites
Author: Bastiaan Corstiaan Wouter van der Vossen
Publisher:
Total Pages: 143
Release: 2021
Genre: Composites (Materials)
ISBN:

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Efforts to develop strength and life prediction analysis tools for aircraft composite structures have shown a great need to understand the complexity and interaction of failure modes. Many failure models are highly sensitive to transverse nonlinear shear stress-strain and delamination fracture properties, which are notoriously hard to measure. Without proper input parameters, life predictions analyses will not be successful. The objective of this work was to accurately characterize these mechanical properties of carbon fiber reinforced polymer composites through advanced experimental methods. Full-field noncontact deformation measurements using Digital Image Correlation are used to quantify all surface strains components, enabling the design of experiments subject to complex loading. The presented splined-based optimization techniques link measured strains to applied loading, converging to accurate material properties without ad hoc assumptions on the material model. These methods improve on first-order closed-form and iterative numerical solutions due to flexibility and accuracy without the computational expense. Applications of the optimization model on the Short Beam Shear and Thick Adherend shear tests proved the usefulness of these methods. Full three-dimensional characterization of selected carbon fiber-reinforced polymer composites is presented with a discussion of the applicability of simplifying assumptions on the material model. This experimental program showed that the assumption of transverse anisotropy must be verified, as it may be inaccurate depending on the material. Furthermore, large shear strain response of IM7/8552 UD specimens has been measured by asymmetric three-point bending test. Applicability of the common D3518 ±45 degree off-axis tensile test was also deliberated. Shear stress-strain response measured from D3518 specimens disagrees with the Short Beam Shear and Small Plate Twist test results after about 5000 microstrain. This conclusion was shown to be influenced by the specimens' stacking sequence and microdamage which develops well before damage can be spotted in X-ray Computed Tomography reconstructions. High-magnification deformation measurements are applied to directly measure cohesive laws in precracked Double Cantilever Beam and Thick Adherend shear specimens. Supported by non-destructive in situ crack front measurements by Computed Tomography, the loaded crack tip displacements are directly related to the J-integral. The derived traction-separation law is verified by the excellent agreement of the global response between cohesive model Finite Element Analysis and measurements. This study was the first application of the direct measurement of mode II traction-separation law in Thick Adherend shear specimens.