Analysis of fiber-matrix interface debonding under normal loading using in-situ microscopy and cruciform-shaped single glass fiber composite specimens

One of the important microscale properties that determine a composite's performance is the fiber-matrix interface strength. Research has primarily been focused on the interface strength under shear loading, while studies on interface strength under normal loading are limited. Nevertheless, normal loading is very relevant, for example to understand failure mechanisms in 90 degrees plies. Therefore, in this work, we modified a single fiber composite test to a cruciform shape with a thinned middle section to test glass fiber matrix interface properties under normal loading conditions. These specimens allow testing the interface up till the point of failure of the polymer matrix, thus improving over regular cruciform specimens. An in-house designed tensile stage enabled the use of in -situ transmission and reflection microscopy. Insights into the debonding process were obtained over a large fiber length (+/- 10 mm) via the use of a step-and-shoot method. The methodology is applied to a glass fiber with an epoxy compatible and incompatible sizing. It shows that the debonding process can be divided into two partially independent debonding processes (initiation and propagation). The obtained data enabled the construction of a descriptive function for the total debond length of a fiber-matrix interface. This showed that besides a high interface strength, a homogeneous application of the sizing should be aimed to decrease the chances of initiating debonding at lower stresses.