Finite element modeling (FEM) software and Saflex® interlayer modulus properties

EN 16612 Glass in building standards

Many modern glass design standards allow the use of numerical modeling models for glass design, in parallel with or as alternatives to glass design charts and analytical formulas. One such standard is EN 16612 Glass in building — Determination of the lateral load resistance of glass panes by calculation (2019). This standard specifies that the method used for determining stress and deflection must be an engineering formula or method appropriate to load distribution, glass shape, and support conditions. It is important to select the appropriate standard(s) for the glass you are designing, as practices vary widely across countries.

For the interlayer properties specifically, EN 16612 requires that an appropriate finite element method consider the viscoelastic properties of the interlayer material. Eastman provides the viscoelastic properties of Saflex® and Vanceva® interlayers in its product technical sheets in a table with shear (G) and Young’s (E) relaxation modulus. The most current product technical sheets are available to download at www.saflex.com/technical-documents

Material models are available for advanced users as well. The modulus data in our product technical sheets have been validated by third-party data and are confirmed by measurements on laminated glass specimens for selected load scenarios. The use of actual interlayer properties often provides design benefits over the use of default generic properties for polyvinyl butyral (PVB) as a material category. An example is given below in the table for load scenario of three minutes at 22°C.
 

 picture_10.png
 
For Saflex Clear PVB, the software default for PVB is a good approximation, but for Saflex Structural PVB, it is a gross underestimate. Therefore, it can be advantageous for the glass designer to use the actual material properties rather than generic default properties.
 

How to use actual Saflex interlayer modulus properties in FEM

The following steps provide a practical guide on how to use actual Saflex interlayer modulus properties in FEM glass design software:
 
  1. Review if, for the project you are designing, FEM methods and actual interlayer properties can be used for glass design, according to applicable regulations and standards.
  2. Identify the temperature and duration of relevance for the load scenario to be calculated.
  • Some standards provide tables, some only duration guidance for kmod  .
  1. Identify which parameters are required to model the interlayer.
  • Typically, these are Young’s modulus (E), Poisson’s ratio (ν – Greek nu), coefficient of thermal expansion (αT) and density (ρ)  
  1. Take the relevant data from the Saflex product technical sheet; this involves looking up the modulus value for the load scenario to be considered.
  • If the modulus for a particular load scenario is not available, ask your Eastman representative, interpolate, or evaluate the available material models for the product under consideration.
  • The other properties are listed in the Saflex product technical sheet as well.
  1. Create a custom material with the associated properties.
  • As an example, in SJ MEPLA software go to Settings. Select the “Material” tab. Create a new item. Name the interlayer; in this case Saflex Structural DG. Identify the load scenario that you want to calculate, e.g., five min (live) load at 30°C. Look up the relevant value of E in the table of the product technical sheet; in this case it is 7.1 MPa.
  1. Go back to the definition of the layer stack and add your custom Saflex material for the modeling of the laminated glass behavior.
 
Using the Saflex interlayer properties in this way will allow you to achieve a more accurate calculation of glass stresses and deflections than using generic PVB properties. The use of Saflex Structural PVB may allow you to use a thinner glass package or achieve a size that cannot be responsibly realized using conventional PVB.
Related Products: