81 research outputs found
Study of the Fracturing Behavior of Thermoset Polymer Nanocomposites via Cohesive Zone Modeling
This work proposes an investigation of the fracturing behavior of polymer
nanocomposites. Towards this end, the study leverages the analysis of a large
bulk of fracture tests from the literature with the goal of critically
investigating the effects of the nonlinear Fracture Process Zone (FPZ). It is
shown that for most of the fracture tests the effects of the nonlinear FPZ are
not negligible, leading to significant deviations from Linear Elastic Fracture
Mechanics (LEFM) sometimes exceeding 150% depending on the specimen size and
nanofiller content. To get a deeper understanding of the characteristics of the
FPZ, fracture tests on geometrically-scaled Single Edge Notch Bending (SENB)
specimens are analyzed leveraging a cohesive zone model. It is found that the
FPZ cannot be neglected and a bi-linear cohesive crack law generally provides
the best match of experimental data.Comment: arXiv admin note: substantial text overlap with arXiv:1707.0925
Enhancing the Electrical and Thermal Conductivities of Polymer Composites via Curvilinear Fibers: An Analytical Study
The new generation of manufacturing technologies such as e.g. additive
manufacturing and automated fiber placement has enabled the development of
material systems with desired functional and mechanical properties via
particular designs of inhomogeneities and their mesostructural arrangement.
Among these systems, particularly interesting are materials exhibiting
\textbf{Curvilinear Transverse Isotropy} (CTI) in which the inhomogeneities
take the form of continuous fibers following curvilinear paths designed to e.g.
optimize the electric and thermal conductivity, and the mechanical performance
of the system. In this context, the present work proposes a general framework
for the exact, closed-form solution of electrostatic problems in materials
featuring curvilinear transverse isotropy. First, the general equations for the
fiber paths that optimize the electric conductivity are derived leveraging a
proper conformal coordinate system. Then, the continuity equation for the
curvilinear, transversely isotropic system is derived in terms of electrostatic
potential. A general exact, closed-form expression for the electrostatic
potential and electric field is derived and validated by Finite Element
Analysis. Finally, potential avenues for the development of materials with
superior electric conductivity and damage sensing capabilities are discussed
A Study on the Fracturing Behavior of Thermoset Polymer Nanocomposites
This work proposes an investigation on the fracturing behavior of polymer
nanocomposites. Towards this end, the study leverages on the analysis of a
large bulk of fracture tests from the literature with the goal of critically
investigating the effects of the nonlinear Fracture Process Zone (FPZ). It is
shown that for most of the fracture tests, the effects of the nonlinear FPZ are
not negligible, leading to significant deviations from Linear Elastic Fracture
Mechanics (LEFM). As the data indicate, this aspect needs to be taken into
serious consideration since the use of LEFM to estimate mode I fracture energy,
which is common practice in the literature, can lead to an error as high as
157% depending on the specimen size and nanofiller content.Comment: arXiv admin note: text overlap with arXiv:1702.0582
Spectral Stiffness Microplane Model for Quasibrittle Textile Composites
The present contribution proposes a general constitutive model to simulate
the orthotropic stiffness, pre-peak nonlinearity, failure envelopes, and the
post-peak softening and fracture of textile composites. Following the
microplane model framework, the constitutive laws are formulated in terms of
stress and strain vectors acting on planes of several orientations within the
material meso-structure. The model exploits the spectral decomposition of the
orthotropic stiffness tensor to define orthogonal strain modes at the
microplane level. These are associated to the various constituents at the
mesoscale and to the material response to different types of deformation.
Strain-dependent constitutive equations are used to relate the microplane
eigenstresses and eigenstrains while a variational principle is applied to
relate the microplane stresses at the mesoscale to the continuum tensor at the
macroscale. Thanks to these features, the resulting spectral stiffness
microplane formulation can easily capture various physical inelastic phenomena
typical of fiber and textile composites such as: matrix microcracking,
micro-delamination, crack bridging, pullout, and debonding. The application of
the model to a twill 22 shows that it can realistically predict its
uniaxial as well as multi-axial behavior. Furthermore, the model shows
excellent agreement with experiments on the axial crushing of composite tubes,
this capability making it a valuable design tool for crashworthiness
applications. The formulation is computationally efficient, easy to calibrate
and adaptable to other kinds of composite architectures of great current
interest such as 2D and 3D braids or 3D woven textiles
Fracturing Behavior and Size Effect of Discontinuous Fiber Composite Structures with Different Platelet Sizes
This study investigates the mode I intra-laminar fracture and size effect in
Discontinuous Fiber Composites (DFCs). Towards this goal, the results of
fracture tests on geometrically-scaled Single Edge Notch Tension (SENT)
specimens are presented and critically discussed for three platelet sizes.
The results clearly show a decrease in nominal strength as the specimen size
increases. This effect becomes more important as the structure size increases.
It is found that, when the specimen is sufficiently large, the structural
strength scales according to Linear Elastic Fracture Mechanics (LEFM) and the
failure occurs in a very brittle way. In contrast, small specimens exhibit a
more pronounced pseudo-ductility with a limited scaling effect and a
significant deviation from LEFM.
To characterize the fracture energy and the effective length of the fracture
process zone, an approach combining equivalent fracture mechanics and
stochastic finite element modeling is proposed. The model accounts for the
complex random mesostructure of the material by modeling the platelets
explicitly. Thanks to this theoretical framework, the mode I fracture energy of
DFCs is estimated for the first time and it is shown to depend significantly on
the platelet size. In particular, the fracture energy is shown to increase
linearly with the platelet size in the range investigated in this work.Comment: 42 pages, 16 figure
Strength and Cohesive Behavior of Thermoset Polymers at the Microscale: A Size-Effect Study
This study investigated, experimentally and numerically, the fracturing
behavior of thermoset polymer structures featuring cracks and sharp u-notches.
It is shown that, even for cases in which the sharpness of the notch would
suggest otherwise, the failure behavior of cracked and pre-notched specimens is
substantially different, the failure loads of the former configuration being
about three times lower than the latter one. To capture this interesting
behavior a two-scale cohesive model is proposed. The model is in excellent
agreement with the experimental data and its predictions allow to conclude that
(a) residual plastic stresses cannot explain the very high failure loads of
notched structures; (b) the strength of the polymer at the microscale can be
from six to ten times larger than the values measured from conventional tests
whereas the fracture energy at the microscale can be about forty times lower;
(c) the pre-notched specimens investigated in this work failed when the stress
at the tip reached the microscale strength whereas the cracked specimens failed
when the energy release rate reached the total fracture energy of the material.
The foregoing considerations are of utmost importance for the design of
microelectronic devices or polymer matrix composites for which the main damage
mechanisms are governed by the strength and cohesive behavior at the
microscale.Comment: 35 pages, 12 figures, 2 table
Micro-Computed Tomography Analysis of Damage in Notched Composite Laminates Under Multi-Axial Fatigue
The broad application of polymer composites in engineering demands the deep
understanding of the main damage mechanisms under realistic loading conditions
and the development of proper physics-based models. Towards this goal, this
study presents a comprehensive characterization of the main damage mechanisms
in a selection of notched composite structures under multiaxial fatigue
loading. Thanks to a synergistic combination of X-ray micro-computed tomography
(-CT) and Digital Image Correlation (DIC), the main failure modes are
identified while the crack volume associated to each mechanism is
characterized. This study provides unprecedented quantitative data for the
development and validation of computational models to capture the fatigue
behavior of polymer composite structures.Comment: 21 pages including cover page, 15 figure
Mode I and II Interlaminar Fracture in Laminated Composites: A Size Effect Study
This work investigates the mode I and II interlaminar fracturing behavior of
laminated composites and the related size effects. Fracture tests on
geometrically scaled Double Cantilever Beam (DCB) and End Notch Flexure (ENF)
specimens were conducted to understand the nonlinear effects of the cohesive
stresses in the Fracture Process Zone (FPZ). The results show a significant
difference between the mode I and mode II fracturing behaviors. It is shown
that, while the strength of the DCB specimens scales according to the Linear
Elastic Fracture Mechanics (LEFM), this is not the case for the ENF specimens.
Small specimens exhibit a pronounced pseudo-ductility with limited size effect
and a significant deviation from LEFM, whereas larger specimens behave in a
more brittle way, with the size effect on nominal strength closer to that
predicted by LEFM. This behavior, due to the significant size of the Fracture
Process Zone (FPZ) compared to the specimen size, needs to be taken into
serious consideration. It is shown that, for the specimen sizes investigated in
this work, neglecting the non-linear effects of the FPZ can lead to an
underestimation of the fracture energy by as much as 55%, with an error
decreasing for increasing specimen sizes. Both the mode I and II test data can
be captured very accurately by Ba\v{z}ant's type II Size Effect Law (SEL)
Direct Testing of Gradual PostPeak Softening of Notched Specimens of Fiber Composites Stabilized by Enhanced Stiffness and Mass
Static and dynamic analysis of the fracture tests of fiber composites in
hydraulically servo-controlled testing machines currently in use shows that
their grips are much too soft and light for observing the postpeak softening.
Based on static and dynamic analysis of the test setup, far stiffer and heavier
grips are proposed. Tests of compact-tension fracture specimens of woven
carbon-epoxy laminates prove this theoretical conclusion. Sufficiently stiff
grips allow observation of a stable postpeak, even under load-point
displacement control. Dynamic stability analysis further indicates that stable
postpeak can be observed under CMOD control provided that a large mass is
rigidly attached to the current soft grips. The fracture energy deduced from
the area under the measured complete load-deflection curve with stable postpeak
agrees closely with the fracture energy deduced from the size effect tests of
the same composite. Previous suspicions of dynamic snapback in the testing of
composites are dispelled. So is the previous view that fracture mechanics was
inapplicable to the fiber-polymer composites
Experimental and Numerical Investigation of Intra-Laminar Energy Dissipation and Size Effect in Two-Dimensional Textile Composites
Design of large composite structures requires understanding the scaling of
their mechanical properties, an aspect often overlooked in the literature on
composites. This contribution analyzes, experimentally and numerically, the
intra-laminar size effect of textile composite structures. Test results of
geometrically similar Single Edge Notched specimens made of 8 layers of 0
degree epoxy/carbon twill 2 by 2 laminates are reported. Results show that the
nominal strength decreases with increasing specimen size and that the
experimental data can be fitted well by Bazant's size effect law, allowing an
accurate identification of the intra-laminar fracture energy of the material.
The importance of an accurate estimation of Gf in situations where
intra-laminar fracturing is the main energy dissipation mechanism is clarified
by studying numerically its effect on crashworthiness of composite tubes.
Simulations demonstrate that, for the analyzed geometry, a decrease of the
fracture energy to 50% of the measured value corresponds to an almost 42%
decrease in plateau crushing load. Further, assuming a vertical stress drop
after the peak, a typical assumption of strength-based constitutive laws
implemented in most commercial Finite Element codes, results in an strength
underestimation of the order of 70%. The main conclusion of this study is that
measuring accurately fracture energy and modeling correctly the fracturing
behavior of textile composites, including their quasi-brittleness, is key. This
can be accomplished neither by strength- or strain-based approaches, which
neglect size effect, nor by LEFM which does not account for the finiteness of
the Fracture Process Zone
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