Multisensory experimental determination of the in-situ strength of embedded CFRP plies

• Applicant: Professor Dr.-Ing. Oliver Völkerink
• Subject Area: Lightweight Construction, Textile Technology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558129216

The strength of unidirectional fibre-reinforced plastic composites (FRP) is highly directional. While the strengths in the fibre direction are high, they are usually less than 5% of the strength in the fibre direction at perpendicular to the fibre, as in this case it is mainly the matrix material that is stressed. For this reason, the strengths are determined both in the fibre direction and across the fibre direction on test specimens with unidirectional (UD) fibre orientations. As early as the end of the 1970s, various authors observed that the transverse tensile strength of plies embedded between plies of a different orientation is significantly higher than the UD strength. A group led by Parvizi first discovered this effect on [0/90n]s test specimens made of glass fibre reinforced plastic (GRP) and later also on those made of carbon fibre reinforced plastic (CFRP). Due to the embedding of the 90° layer between the 0° layers, cracks in the 90° layer only occur well above the experimentally determined UD strength of the 90° layer. This phenomenon of increased transverse tensile strength of the 90° layer due to the surrounding layers is also referred to as in-situ strength. In-situ strengths are therefore required in order to optimally utilise the mechanical properties of FRP and thus enable a lightweight construction-compliant design.However, there is no established measurement method for recording the failure of the embedded 90° layer due to microcracks, which defines the in-situ strength. In addition, only a few papers are known on the experimental determination of in-situ shear strengths, as most publications refer to transverse tensile strength. An uninterrupted test method that not only observes the cracking at the edges of the specimen but also records the strain information within the specimen as well as the acoustic emissions of the cracking is missing. The aim of the research project is to provide a new uninterrupted multisensed test method for the reliable recording of in-situ tensile and shear strengths and to compare the generated data set with a data set generated according to the state of the art.

• DFG Programme: Research Grants