1 INTRODUCTION Simulation tools support the composite design engineer in the optimisation of the thermoforming process. Key to a good simulation tool is an appropriate material model: complex enough to accurately describe local deformations, but not more complex than needed. Mechanical behaviour of textile composites is quite complex during thermoforming. For instance a woven textile composite shows non-linear tensile stiffness due to biaxial coupling between the two yarn families. Shear stiffness is very low and highly non-linear. Consequently, shear deformation is the major deformation mechanism during forming. The thermoplastic matrix viscous behaviour is dependent on local temperature and shear rate. A vast amount of macro-scale material models have been developed with different degrees of complexity: elastic models [1,2,3], viscous models [4,5,6], elasto-plastic, and visco-elastic models (PAMFORM, [5]). Meso- models of fabric mechanics can produce input data for the macro-models, as demonstrated in [1,6,7]. Relevant experiments and benchmark exercises [8,9] may help to validate these material models and understand their capabilities. It is also useful to study the sensitivity of thermoforming to material parameters and to the effect of non-linearity and interactions as biaxial coupling and shear-tensile coupling. This way material features required to obtain desired precision in local deformation modelling can be deduced. In this work a parameter sensitivity study was undertaken using a visco- elastic material model in PAMFORM. 2 MATERIAL MODEL 2.1 Material model 140 This macro-scale material model is composed of ABSTRACT: Double dome forming simulations are performed on woven textile composites in PAMFORM using a visco-elastic material model, dedicated to reinforced composites. Woven textile composites show very complex behaviour during thermo-forming. Material compliance curves in shear and tension are typically non-linear due to local deformations within the repetitive unit cell of the textile reinforcement, like yarn intertwining, crossover friction, yarn through-the-thickness and lateral compression. Moreover, matrix viscous behaviour is dependent on temperature and local shear rate. The goal of this work is to study the sensitivity of punch force and local fibre deformations on the material parameters. Parameter sensitivity studies can help in determining what the impact is of variability in mechanical test results and the material complexity considered, on the precision of local deformation predictions. Material parameters considered in this work are the non-linearity in uniaxial tensile stiffness, shear stiffness at 200°C and 20°C, bending stiffness (scale factor 0.1 and 0.001 with respect to continuum material), Poisson’s coefficient (0 vs. 0.4) and viscosity. From this simulation case study it can be concluded that shear stiffness and non-linearity in tensile stiffness according to yarn direction have a major impact on local deformations and punch force. In the high shear stiffness model a trade-off has to be made between constant binder force and high ratio of internal energy to hourglass energy. It is felt that viscosity should be studied more in depth, both experimentally - enabling to better define the range of viscosity values - as numerically. Key words: thermoplastic, woven fabric, thermoforming, double-dome stamping, finite element, parameter sensitivity Double dome forming simulation of woven textile composites A. Willems 1 , S.V. Lomov 2 , D. Vandepitte 1 , I. Verpoest 2 1 Mechanical Engineering Department, Katholieke Universiteit Leuven – Kasteelpark Arenberg 41, B-3001 Heverlee, Belgium URL: www.mech.kuleuven.ac.be e-mail: [email protected]2 Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven – Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium URL: www.mtm.kuleuven.ac.be e-mail: Stepan.Lomov@ mtm.kuleuven.be
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Double dome forming simulation of woven textile composites
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1 INTRODUCTION
Simulation tools support the composite design
engineer in the optimisation of the thermoforming
process. Key to a good simulation tool is an
appropriate material model: complex enough to
accurately describe local deformations, but not more
complex than needed. Mechanical behaviour of
textile composites is quite complex during
thermoforming. For instance a woven textile
composite shows non-linear tensile stiffness due to
biaxial coupling between the two yarn families.
Shear stiffness is very low and highly non-linear.
Consequently, shear deformation is the major
deformation mechanism during forming. The
thermoplastic matrix viscous behaviour is dependent
on local temperature and shear rate. A vast amount
of macro-scale material models have been developed
with different degrees of complexity: elastic models
[1,2,3], viscous models [4,5,6], elasto-plastic, and
visco-elastic models (PAMFORM, [5]). Meso-
models of fabric mechanics can produce input data
for the macro-models, as demonstrated in [1,6,7].
Relevant experiments and benchmark exercises [8,9]
may help to validate these material models and
understand their capabilities. It is also useful to
study the sensitivity of thermoforming to material
parameters and to the effect of non-linearity and
interactions as biaxial coupling and shear-tensile
coupling. This way material features required to
obtain desired precision in local deformation
modelling can be deduced. In this work a parameter
sensitivity study was undertaken using a visco-
elastic material model in PAMFORM.
2 MATERIAL MODEL
2.1 Material model 140
This macro-scale material model is composed of
ABSTRACT: Double dome forming simulations are performed on woven textile composites in PAMFORM using a visco-elastic material model, dedicated to reinforced composites. Woven textile composites show very complex behaviour during thermo-forming. Material compliance curves in shear and tension are typically non-linear due to local deformations within the repetitive unit cell of the textile reinforcement, like yarn intertwining, crossover friction, yarn through-the-thickness and lateral compression. Moreover, matrix viscous behaviour is dependent on temperature and local shear rate. The goal of this work is to study the sensitivity of punch force and local fibre deformations on the material parameters. Parameter sensitivity studies can help in determining what the impact is of variability in mechanical test results and the material complexity considered, on the precision of local deformation predictions. Material parameters considered in this work are the non-linearity in uniaxial tensile stiffness, shear stiffness at 200°C and 20°C, bending stiffness (scale factor 0.1 and 0.001 with respect to continuum material), Poisson’s coefficient (0 vs. 0.4) and viscosity. From this simulation case study it can be concluded that shear stiffness and non-linearity in tensile stiffness according to yarn direction have a major impact on local deformations and punch force. In the high shear stiffness model a trade-off has to be made between constant binder force and high ratio of internal energy to hourglass energy. It is felt that viscosity should be studied more in depth, both experimentally - enabling to better define the range of viscosity values - as numerically.