Composite Design with FEA
Design process of composite structures and structure optimization regarding mechanical properties and weight comprises three main steps:
- Preliminary Analysis
- Finite Elements Analysis
- Post-processing Analysis
The composite design process at CompDesE is based on Finite Elements Analysis. The computation of the structure is done using HyperWorks® software from Altair Engineering, Inc., a market leader in Structural Analysis.
Preliminary Analysis is the first step in structural analysis of composite structure. It comprises four steps:
- Requirement Analysis
- Concept Development
- Selection of Structure and Materials
- Selection of Composite Reinforcement Structure
In this opening step of analysis, requirements given for the designed composite structure are analyzed. Considered are applied loads, environmental conditions as working temperature range and temperature gradients, humidity and an exposition on UV-light. Other requirements that are considered during requirements analysis are structure durability, standardization, joining with other structures. From economic point of view, manufacturing and operational costs are considered too.
It is based on the in previous step defined requirements, concept geometry of the structure is modelled. This first structure optimization of the part focus on topology and topography optimization regarding stiffness or mass, respectively. An example of the concept development is presented on the figure below. Starting from left, importing of CAD geometry and finishing right with refine a concept in CAD.
Selection of structure and materials
The design process of composite structures involves tasks that require special attention, and which are not typically covered by general finite element simulation tools. An example is a huge number of available material systems. The preliminary design phase helps to limit the choice of design parameters to a manageable degree early enough in the process. Function of the part is a core of the design process.
Second important characteristics is a composite material (we are focusing as on laminate structures, based on Classical Laminate Theory CLT). Several constitutive materials build a one material. Shape is an idealized basic shape. The modelling process based on the selection of structure and materials is then conducted in the next step, modelling of composite.
This selection is solved using ESAComp software. Selected are material combinations and a structural concept. Structures to be chosen are such as solid laminate, sandwich structure, or stiffened panel. Furthermore, through laminate lay-up design will be specified by using the directional properties of composite material. After this step, structure and material data are exported to pre-processing tool, see figure.
Selection of composite reinforcement structure
Design of composites requires the definition of a high number of variables. Most of them are described by parameters of textile reinforcement like:
- Fiber orientation in the ply (for example 0/90; 20-80),
- Number of plies in one layer (2D and 3D structures),
- Ply thickness depending on linear mass density of fiber (TEX/Denier) or amount of spreading,
- Geometrical form of reinforcement and draping ability on core (2D sheet or 3D structure),
- Locally placed reinforcements.
CompDesE apply following textile structures in the analysis: Non-crimp Fabrics (NCF's), Woven Fabrics, Braids, Spacer Fabrics and derivative technologies like Automated Fiber Placement or Filament Winding, which is applied for circular closed profiles. We tempt to implement stitching technology to assemble reinforcement plies regarding achieve desired composite structure properties.
- Non-crimp fabrics (NCF's)
- UD Tapes
- woven fabrics
- filament winding architectures
- automated fiber placement textile architectures
- 3D woven fabrics
- 3D braided structures
- spacer fabrics
- assembly by stitching
After selection of reinforcement, adequate Matrix Material is selected. All this data (Mechanical properties, Orthotropic Material Properties, Volume Fractions and Spacing/Orientation of constituents) are selected in this step. They serve as input data in the next Composite Design step: Finite Elements Analysis.
Finite Elements Analysis
Finite Elements Analysis (FEA) for composites is based on Classical Lamination Theory and divided into following steps:
- Unit Cell Model Definition
- Modeling of Composite Laminate
- Optimization of Composite Lay-up in respect to goal
Unit cell model definition
Using Multiscale Designer a multiscale material models for continuous (unidirectional and woven) and chopped (short and long) reinforced product forms made from carbon, glass, or Kevlar with polymer (thermoplastic and thermoset) matrices are developed. The fully nonlinear multiscale material models are subsequently used in linear or nonlinear macro analysis to resolve micro-scale fields (stresses and strains within each phase material of the unit cell).
Modelling of Composite Laminate
After definition of unit cell model and laminate parameters, the model considering ply angles, numbers of plies, and stacking sequence is created in FEA software. As a result, we obtain a visualization of the reinforcement directly on structure geometry. Figure underneath shows the results of modelling of composite part using Altair® HyperMesh® pre-processor.
Optimization of composite lay-up
Composite lay-up defined in the previous step will be optimized in design step . The purpose is to increase performance and reduce weight by utilizing optimization methods to find the best combination of variables for a given application. Two approaches of optimization can be followed: minimization of weight and maximization of strength.
The composite optimization design process has a range of benefits including:
- Cuts development time and cost by providing high performance designs in the initial stages of the product development process
- Reduces product design time by eliminating the “trial and error” process of typical design iterations
- Automates calculation of the number of plies needed for each ply fiber orientation
- Automates composite laminate stacking sequence determination
- Automates incorporation of manufacturing constraints and Ply Book Rules for certified designs
Phase I: Free-size optimization
Description: Topology or freeform optimization is conducted to define the concept geometry. Manufacturing constraints are important for composite design. One of them is the number of consecutive plies (typical limitation is 3 to 4 consecutive plies).
Action: Free-size optimization is applied to composite model consisting of several 'super-plies' with each representing one available fiber orientation.
Result: Overall concept of material distribution throughout the structure.
Phase II: Size optimization
Description: The conventional discrete size optimization is performed for the interpreted FEA model to define the optimal number of plies for each patch.
Action: The ply shape of the composite component is studied to identify ideal ply drop-off zones. The thickness of each ply shape is then analyzed to remove any unrequired material from the ply stack.
Result: The thickness distribution of each fiber orientation is interpreted into ply-patches (ply-boundles) according to thickness contour.
Phase III: Stacking
Description: Design process zooms in on the stacking details of individual plies. The stacking sequence is optimized until all manufacturing as well as behavior constrains are satisfied.
Manufacturing constrains are:
- limit on consecutive plies of the same orientation,
- paring of +/- angles,
- pre-defined cover lay-ups,
- pre-defined core lay-ups.
Action: The ply order within the stack is optimized to find the ideal order that multi-directional plies should be laid up.
Result: Composite design that contains all the ply tailoring and stacking details for manufacturing.
Post-processing consits of visualisation and control of the simulation results, creating graphs and design informations. Using HyperView tool, it is possible to visualize data interactively as well as capture and standardize post-processing activities. When working with changing parameters (HyperStudy) it is possible to check for correlations between many designs variables. According to post-processing the best design