Determining the Interlaminar Shear Strength (ILSS)

The interlaminar shear strength (ILSS) characteristic describes the shear strength between the laminate planes.

Purpose of the Test

The interlaminar shear strength of laminates with a brittle matrix, for example, those made of epoxy resin, is typically determined using a short beam shear test (SBS). It makes use of the fact that shear stresses always occur in a flexure test. If the span is small in comparison to the specimen thickness, the shear stresses that occur in comparison to the normal stresses generated by the bending moment are very large. In this way, shear stress can be generated in brittle matrix materials, which enables measurement of the shear strength.

The shear strength of the matrix material or the quality of the fiber-matrix bonding can be characterized depending on the break type.

This test method is relatively simple to employ and requires a simple tool that has good alignment options and precise centering of the die. For this reason, the method is often used in quality control and is suitable for comparing materials.

However, the method determines only in-plane shear properties, since compressive stress peaks usually arise near the upper anvil and can influence the measured results.

Testing of Standard Laminates

  • In materials development, standard laminates with a thickness of 2 mm are tested. Depending on the standard used, the span is 8 or 10 mm and must be set to an accuracy of up to ±0.1 mm. EN 2563 defines a very small tolerance of ±0.02 mm for the flexure tup.
  • Zwick's test tools simplify the tasks of specimen centering and setting the span, as well as precise parallel alignment of the supports. 

Testing of Laminates Made of Finished Structures 

Laminates made of profiles, plates, or components often have other thicknesses. In these cases, it is important that the relationship of the span to the laminate thickness is observed. The span must be able to be set accurately for each test series with minimum effort and the centering should have to be adjusted only once. Zwick's ILSS test fixture is equipped with a counter lead screw for this purpose. The supports can be set to the required span in observance of the mid-point. Defined gage lengths on the sides of the supports make it possible to accurately measure the span.