Digital Engineering – Composites Analysis: Making New Choices

Posted on 3/2/2009

Source: Digital Engineering

This COMSOL multiphysics simulation illustrates localized heating damage due to lightning striking a composite-material aircraft wing. Image courtesy COMSOL

Although software for composite analysis has been around more than 20 years, you might be tackling your first project that uses this category of materials. Or, your needs in the composite field may have outgrown your current analysis package. Either way, you probably know that dealing with these materials requires capabilities beyond standard modeling and analysis techniques.

The good news is that dozens of companies market software that includes or focuses on using these no-longer-so-exotic materials. The even better news is that these packages work faster with today’s computing platforms, are easier to use than ever, and often connect directly with other vendors’ related offerings.

DE polled the experts to identify the critical issues in evaluating software for composite analysis. They pointed out questions to ask and situations to consider, whether you’re dealing with long-fiber composites, sandwiched layers, or honeycomb structures. The answers are oriented to realworld applications with the goals of durability and manufacturability well in mind. (A resource list at the end of this article includes vendors in composites modeling, analysis, optimization, and manufacturing.)

Firehole Technologies’ Helius:MCT for Abaqus software shows failure analysis details of a composite model structure. Image courtesy Firehole Technologies

This example within SolidWorks describes detailed composite shell ply definitions to enable a manufacturer of bicycle frames to complete a full-scale analysis. Image courtesy SolidWorks

Model Creation
Setting up a composites-based model for finite element analysis (FEA) encompasses the standard steps of defining the geometry, specifying material properties, and meshing the model for analysis. However, working with non-isotropic materials demands that the software lets you define the differing nuances of each layer. Both 3D CAD and analysis packages generally handle this task, but just how they do it, and what else they support, are other points to consider.

• Is the number of possible layers virtually unlimited? Some designs may call for ten layers, others might require thousands. Advances in computing power have meant that analysis time is no longer a gating item, so you shouldn’t encounter restrictions in the modeling.
• Can you import layer data such as strength and stiffness terms from spreadsheets?
• Can you easily insert or delete layers, and at what points in the process?
• What are the options for defining fiber orientation angles? The software should handle angles with respect to a global axis, local axis, normal to the shell surfaces and a plane defined by two intersecting planes, or along an arbitrary curve.
• Can you mix composite elements with noncomposite elements such as beams, springs, and shells made of metallic structures?
• Can you define a layer as a honeycomb, or include rebar material?
• How are glues and adhesives modeled between layers—with a surface-based cohesive contact or as a very thin layer?
• Can the design have physical holes? What is the editor like for updating thickness, angles, and material properties? Does the modeling process correspond
  to different manufacturing processes such as hand layup, filament winding, and compression molding?

Material Properties
When it comes to defining specific materials, the software should either have its own database or the capability to import material properties data from commercial services such as MatWeb and Matereality.

With that taken care of, ask the vendors these questions:

• Does it allow unit-cell-based micromaterial models? Can it also incorporate continuum material models? Can it handle a mix of elastic and inelastic materials?
• Can you designate fiber packing as well as braided or woven configurations?
• Can you introduce random variations in fiber orientation to allow for manufacturing realities?
• Can you enter equivalent stiffnesses as a starting point, especially early in the design phase, then replace those values on a layer-by-layer basis?
• Does the software allow user-defined equations in the property matrix?
• Is any necessary unit-conversion handled automatically?

This screenshot illustrates the lamination sequence modeling in NISA software of a composite aircraft model. Image courtesy Cranes Software

ALGOR is used to analyze a next-gen taxicab frame made of lightweight Alumina composites and carbon fibers. Image courtesy ALGOR

Meshing
When the model is complete, you need options for directing the meshing (finite element definition) process. Do you want to use solid elements or layered shell elements? Some analysts start by modeling the structure as a layered shell element then switch to a detailed solid element mesh to better understand such results as interlaminar shear stresses. Ask whether such a change can be automatically extracted, especially from mid-planes.

HyperSizer imports finite element models and performs compositespecific stiffened and sandwich-panel failure analyses and layup optimization for lighter weight and manufacturable structures by reducing ply drops. Image courtesy Collier Research

General Analysis
The software should offer both implicit and explicit solvers, and include both macro and micromechanical analyses at: 1) the global level, addressing overall deflection, buckling, and natural frequencies of the structure; 2) the ply or layer level, determining interlaminar shear formations and stresses; and 3) the matrix level, often left out, which supplies a detailed stress distribution within a single layer.

Other questions to ask about the solver:

• To save time, does it use the same FEA model for implicit and explicit solutions?
• Can it handle metal-matrix, ceramic-matrix, and polymer-matrix composites?
• Do you have the choice of computing equivalent material properties in either the preprocessor or the solver?
• Do material properties change appropriately with a change in geometry shape?
• Does it account for heat-transfer effects? Can it solve for simultaneous multiphysics including conductivity, thermal, and fluidstructure interactions?
• Can it account for initial strains in the different composite layers (e.g., if a layer is preloaded during manufacturing)?

And if those tasks aren’t enough, the software should be able to compute out-of-plane (stress z-axis) interlaminar shear and peel stresses. This capability would be in addition to classical lamination theory (CLT) in-plane stresses and strains.

With so much information contained in every square inch of an analyzed composites design, the ability to visualize the details of material orientation becomes critical. Be sure that you can display visual verification of the orientation angles and normals layer by layer; stress results for core, worst, and specified lamina; strain for composite elements including initial, mechanical, and total strain; and damage energy density.

The new Femap 9.3 Layup Editor allows ply properties to be edited individually or collectively. In addition, plies can be rotated or moved around the layup, and users can easily define symmetry for the laminate as a whole. Image courtesy Siemens PLM Software

Failure Analysis

Composite and laminated materials are subject to failure modes not found in single-material structures. Both delamination (laminate-based failure) and fiber or matrix breakdown (ply-based failure) can degrade performance but the question is, how severely? Will the outer layer wrinkle, crimp, or dimple? Does damage to one layer equate to failure of the entire part? Damage modeling is critical to success.

The software should be able to investigate barely visible impact damage (BVID). Does the software help you define the allowable damage tolerance? Since every designer uses different failure criteria, the package must allow flexibility in the definition, ideally with the option to write your own macros.

If you treat the composite as an equivalent, blended whole, you can examine such values as simple maximum strain and maximum stress, and use quadratic Tsai-Wu, Hill, and Hoffman failure models. Newer criteria account for fiber or matrix failure separately, using such models as Hashin-Fabric, Hashin-Tape, and Puck.

What else should you look at, and how? Some codes provide crush simulation and also use a version of the public-domain VCCT code, a postprocessing and remeshing technique for progressive crack propagation between bonded surfaces. Ask if the failure analysis examines residual strength after first-ply failure, and what approach it takes to do so, e.g., does it use immediate or gradual stiffness reduction for the matrix and fibers? Can you add damage during a simulation (not just calculate it after the simulation)? Can you deactivate elements to simulate material removal or breaking? And can you perform fatigue analysis, a rare capability with composite analysis?

NEi Nastran stress analysis of the radome covering the Southern Astrophysical Research infrared telescope in the Chilean Andes. NEi solved the complete structural analysis. Image courtesy NEi Software

Design Optimization
Typically designers use composite material to minimize weight while meeting performance (strength) requirements. However, the large number of variables involved makes optimization a non-trivial task and can lead to overly conservative designs. Ask about the following points:

• Does the software provide interactive optimization capabilities so you can determine, for a given loading environment, the lightest weight combination of material systems and ply layups?
• How easy is it to reorient layers or change the stacking sequence to compare behavioral differences? Can you quickly change the material type in a layer?
• Can it optimize a complete structural entity such as wings and fuselage of an aircraft?
• Can it handle sandwich structures as well as solid laminate structures?
• Does the package couple with other solvers and pre- and postprocessors as needed?
• Are tolerances in the design parameters taken into account, and how easy are those to vary?
• Is Design of Experiments a fundamental part of the optimization process?
• Can you optimize for an objective such as low vibration, or can you optimize for multiple objectives?

Manufacturing Issues
Simulation provides great insight but you still need to manufacture a real product. Ask about the interface between the analysis software and the various manufacturing simulation packages. Terms such as draping, taping, braiding, flatpatterns and ply drop-offs should become part of your vocabulary. You also need software that will help you incorporate such non-geometric details as fasteners, glues, sealants, and coatings, and predict any expected thermal warping, shrinkage, and springback.

The Next Steps
What do the experts see for the future? Software that allows multi-scale analyses, mesh-independent crack growth computations, and more detailed stress distributions within a single layer, to name a few items on the wish lists. For now, view the many good white papers and tutorials at the company websites, talk to the experts about your particular projects, and let their wealth of experience guide you to the “must haves” for composite analysis success.