Proportions of composites
materials for aeronautic applications have dramatically increased over the last
decades because of their inherent properties combining high mechanical
properties with low mass. However, the major concern of the traditional 2D
composites is the reduction of properties due to delamination between plies
because of weak through-the-thickness properties.
Three dimensional woven give
the possibility to add a 3rd direction of reinforcement with the
addition of yarns interlacing through the thickness. The main benefit of that
additional direction of reinforcement is a better damage tolerance due to
impact where delamination is the key failure mode.
With the addition of that 3rd
direction of reinforcement, possible combination of interlacement become
extremely numerous. Different patterns commonly observed fall into 2 main
categories:
- The orthogonal 3D woven, which is made of three different type of yarns, the warp (X direction), the weft (Y direction) and the binder (Z direction) which is placed through the thickness. The warp and wefts yarns are straight and perpendicular without interlacement and the binder is weaved in the X direction, interlacing on the top and the bottom surface with an angle of 90 degrees.
- The angle interlock 3D woven, which is made also of perpendicular and straight warp and weft yarns and with a binder weaved with an angle different from 90 degrees. It could also go through all the thickness or from layer to layer;
To support the development of
3D woven and to permit a quick screening of potential weave pattern properties,
numerical tool must be proposed to designers. The last version of Digimat
2018.0 contains a complete workflow to analyze and predict stiffness, failure
and thermo-mechanical behavior of any 3D woven representative volume element
(RVE).
BENEFITS
· Quick screening of various type of 3D weave pattern (orthogonal, angle
interlock, layer-to-layer)
· Integrated workflow for complete performances computation (stiffness,
failure, residual stresses)
DIGIMAT FOR 3D WOVEN AT A
GLANCE
Outstanding features of Digimat 2018 are:
·
A micromechanics approach: Digimat
computes the composite properties from the constituents’ properties and the description
of the microstructure itself. In the context of 3D woven, fiber and resin
material properties, yarn density, yarn section and finally weave pattern
description are needed along the workflow.
·
Performance prediction using homogenization:
Based on the previous inputs, Digimat proposes two homogenization methodologies
to compute the effective properties of the RVE: a mean-field homogenization
algorithm and a full-field resolution. In the context of 3D woven, the complete
analysis of the RVE requires a multi-step homogenization: first yarn properties
are computed, then RVE properties are computed.
·
Pre-defined and customizable weave pattern
definition: Digimat 2018 proposes a user-friendly way to define any types
of weave pattern based on the definition of the number of layers, the number of
weft rows and the interlacement of each warp/binder within each warp section.
·
Integrated workflow: Finally, the
execution of the analysis work is totally integrated in a comprehensive
workflow, with 4 mains steps (figure 1): 1. The weave pattern definition, 2.
The weave pattern geometry creation, 3. The weave pattern mesh generation (in
the case of the full-field approach), and 4. The resolution step
APPLICATIONS
Stiffness Prediction
The computation of the stiffness for three different types of
weave pattern are compared. Input data regarding the weave pattern definition
and test results can be found in [1] and the material properties in [2]. The
carbon fiber used is the HexTow IM7 and the resin is the MTM57. The yarn
density is 12K, except for the binder of the orthogonal and angle interlock
patterns which is 6K.
Prediction of the stiffness using the mean-field homogenization
and the full-field homogenization are given in the table 1.
Table 1:
Stiffness Prediction Using Digimat
Failure assessment
Assessment of the strength of the weave pattern is also possible
in Digimat using the full-field homogenization. In this context, the workflow
is enriched with the failure prediction of the resin and the yarn. The failure
envelop of the yarn is built using a micromechanic model that integrates the
fiber volume fraction in the yarn, the stress limit of the resin alone and the
strength of the fiber in tension and compression. That failure envelop
differentiates the failure in the axial and transverse directions and in the
shear plans of the yarn. During the loading of the RVE, that failure envelop is
used to detect the damage initiation and a progressive failure mechanism is
used to degrade the anisotropic stiffness of the yarn or of the resin (figure
2).
Residual stresses after
cooling
Thermo-mechanical analysis can also be performed to evaluate the
residual stresses during a cooling phase, at the end of a curing cycle for
example. In a first approximation, the behavior of the carbon fiber and the
resin becomes thermo-elastic with the addition of the anisotropic coefficient
of thermal expansion (CTEs). A variation of temperature from curing temperature
to room temperature is applied on the RVE without any mechanical boundary
conditions (free stress state). Residual stresses are expected due to the
mismatch of CTEs between the resin and the yarns and the evaluation of the risk
of apparition of micro-cracks is done.
To include the effect of hydrostatic pressure, a quadratic
stress, combining the Von Mises stress and the hydrostatic stress is introduced
[3] and compare to a classical approach based only on the Von Mises equivalent
stress.
Both criteria can be plotted to localize area prone to
micro-crack apparition. Distribution of von-Mises stress or quadratic stress
can be plotted over all the resin domain to assess risk of failure by comparing
with a stress limit [3] (figure 4).
References
[1] Characterizing the loading direction sensitivity of 3D woven
composites: Effect of z-biender architecture, M; Saleh, A. Yudhanto, P.
Potlari, G. Lubineau, C. Soutis. Composites Part A, Vol. 90 (November 2016)
[2] HexTow IM7 datasheet, Hexcel website.
[3] Thermomechanical analysis of micromechanical formation of
residual stresses and initial matrix failure in CFRP, T. Hobbiebrunken, M.
Hojo, B. Fiedler, M. Tanaka, S. Ochiai, K, Schulte, JSME Internail Journal,
Part A, Vol. 47, 2004.
Comments
Post a Comment