Three or four-point bend testing has long been a go-to method for the composites engineer to gather relevant design data for their material or component. Depending on the span used, and with some alterations to the diameter of the loading rollers, the same test set-up can be utilised to determine both the interlaminar shear and the flexural properties of a composite material.
Although interlaminar shear testing doesn’t provide a pure shear state (where there are no additional compressive or tensile forces), it remains very popular in the industry. This popularity is due to the test providing useful information on the interlaminar shear behaviour of the laminate, while also being a cheap and easy test to undertake. However, because of this imperfect shear state, ASTM has removed the word shear from the title of ASTM D2344, simply referring to it as short beam strength.
Whilst we all refer to the relevant standards to tell us what spans and radii to use for these tests, what actually determines whether the test will produce a delamination or a fibre breaking failure? Additionally, at what point will the span become so narrow that we observe an invalid failure due to crushing of the specimen under the rollers? By answering these questions and understanding the parameters that determine these transitions, we can react to the failure modes being exhibited during testing; ensuring that we adjust the test to get valid results.
The answers to these questions are determined by the shear strength of the fibre matrix interface. The shear stresses in a beam during 3-point bending are the same regardless of specimen length. Therefore, if a flexural failure mode is desired, then an increase in the bending moment during testing will ensure that flexural failure is achieved prior to reaching the shear strength of the composite.
Experiments carried out on carbon fibre specimens with different matrices have identified that shear failure is most likely to occur is in the span-to-thickness range of 4:1 and 9:1. Below 4:1 there is a high likelihood of crushing, or combined failures. 10:1 or above and you would expect a flexural failure.
With this information in mind, it is possible, within an R&D environment, to tailor the span to thickness ratio to suit the desired failure mode. If crushing failures are occurring, it is recommended to increase the span to thickness ratio to promote a shear failure. Similarly, if a shear failure is observed when flexural failure is desired, a further increase in the span-to-thickness ratio will promote a flexural failure.
An interesting inconsistency in this discussion is the differing instructions given by the ISO and ASTM standards for Interlaminar Shear tests. ASTM has chosen a span-to-thickness ratio of 4:1 for short-beam testing, while the equivalent ISO standard states a 5:1 ratio should be used. Given the discussion above, there seems to be wisdom in ISO choosing 5:1 as the span ratio. Not only does this span reduce the likelihood of achieving a crushing or mixed mode failure, but the specimens are longer, easier to work with and it becomes that little bit easier to determine the failure mode.
