Analysis of Composites

Composite materials provide innovative solutions for manufacturers in a wide range of industries, looking for stronger, lighter and more cost-effective materials. At the same time, they pose new modeling and manufacturing challenges because of the nature of the materials. With our extensive experience and state-of-the-art analysis and simulation tools, our engineering team can account for residual stresses, predict performance, analyse reliability and potential failures, optimise construction, and provide accurate information to manufacturing, all before a physical prototype is built.

What is a composite material?

A composite is a material that is built up by multiple layers of different component materials. The main components of a composite material are:

  • Core material
    Honeycomb, wood, PU, PVC, PS, ...
  • Matrix
    This is the bonding material between different layers. The matrix material is generally a resin.
  • Reinforcement fibres
    These fibres are responsible for the anisotropic (i.e. direction dependent) properties and the largest stiffness of the composite material. The fibres can be uni-directional, knitted, sewed or woven.

Advantages of composites

Composite materials have clearly a large number of advantages over more traditional materials:

  • Generally rather low-cost solution
  • Complex geometrical shapes are possible, which are often difficult to achieve with more traditional fabrication techniques
  • High strength and low weight
  • Good resistance against chemicals and can be used, among others for the fabrication of storage tanks for chemical products
  • Corrosion resistant
  • High fatigue strength (e.g. carbon fibres)
  • The material properties of the composite can be tailor-made (material design)
  • Absorb little to no moisture
  • High vibration damping

Disadvantages of composites

Composite materials also have a number of disadvantages:

  • Anisotropic thermal strain behaviour
  • Low interlaminar shear stiffness and strength
  • Long term durability issues (UV, heat, aging, ...)
  • Limited heat and fire resistance of the matrix material
  • Undesirable brittle failure behaviour
  • Recycling of composites is a clear challenge
  • Difficulties in using composites together with conventional joints (bolts, rivets, bonding)
  • Sensitive to the fabrication quality (dust, air pockets, other imperfections)

Special characteristics of composite materials

By carefully choosing the component materials of which a composite is built up, some interesting properties can arise:

  • Composites can be either conductive or not, are non-magnetic and are transparent to radar
  • Composites can have a low thermal conductivity
  • Thermal stability due to a negative thermal expansion coefficient (carbon fibres)

Analysis and assessment of composite materials

Composite analysis with the Finite Element Method

Structures made of composite materials are analysed with the Finite Element Method. The analysis of composite materials differ considerably from the analysis of more conventional materials, because these materials need to be evaluated layer by layer, both for the core material, reinforcement fibres as well as the matrix. The strength and stiffness of a composite material is also strongly direction dependent (anisotropic).

The calculation of the stiffness of a laminate (i.e. a number of stacked material layers) is an important step in the assessment of the composite using the Finite Element Method. The stiffness of the laminate depends on the number of layers and the direction of the individual material layers. On top of that, every layer can have different mechanical properties, different thicknesses and a different fibre direction.

Failure prediction of composites

The simplest approach to predict failure of a composite material is by assuming that the material fails as soon a reference stress value is exceeded in any layer of the material. This approach is called the First Ply Failure mode. Another approach assumes that a composite material fails when the last layer fails. This approach is called the Last Ply Failure mode.

A different failure assessment path is the Tsai-Wu failure criterion. The philosophy of this approach is similar to the Von Mises criterion for isotropic materials. The Tsai-Wu criterion assesses the following:

  • Directional material strength and stiffness (longitudinal, transverse and shear strength)
  • Differentiation between tensile and compressive stresses
  • Directional stress interactions between different layers

Failure criteria

Besides the criteria mentioned above, some other industry standard failure criteria are available to assess the strength of composite materials:

  • Maximum stress
  • Maximum strain
  • Tsai-Hill
  • Tsai-Wu
  • Hashin
  • Puck
  • Cuntze
  • LaRC
  • Hoffman
  • Sandwich

Additonal failure criteria are developed for fatigue analysis of composites:

  • Norris
  • Franklin-Marin
  • Hashin-Rotem
  • Hashin-Sun
  • Modified NU
  • Norris-McKinnon
  • Christensen

Questions about composite material design?

Our engineering team has extensive experience with the design of composite materials and the analysis and assessment of the strength and stiffness of composites. Do you have questions about composite materials or do you have a project involving those materials, please let us know about it via email. We will gladly let you know how we can help you out with your projects!