SCREW_HISCC - Hydrogen-induced stress corrosion cracking of self-tapping timber screws

Since about 20 years, self-tapping timber screws have been frequently applied in modern timber engineering. The reasons for their success are their simple and economic installation without pre-drilling, as well as their flexibility in terms of geometry, realising the use for various different purposes. To enable the application without pre-drilling, especially in timber members with high density (hardwood), the steel strength of self-tapping screws exceeds 1,000 MPa in major cases. In practise, this is achieved by screw hardening, which takes place after the geometrical forming. Initial examinations made in the past show that this combination of a hardened, high-strength carbon steel, given (bending) tensile stresses in the fastener and a protective zinc-coating may provoke hydrogen induced stress corrosion cracking (HISCC) of the timber screws. Since this phenomenon leads to a sudden, brittle failure without prior warning at load levels far below the yield strength it has to be avoided as far as possible. By means of an interdisciplinary cooperation of the fields of timber engineering and material science, the project “SCREW_HISCC” shall expand the related, insufficient state-of-knowledge for timber engineered structures by far. This shall be done in the frame of three working packages: the first step is about the design of a suitable test and measurement set-up, which is subsequently applied for the quantification of influencing parameters by means of executing a comprehensive test programme. Thereby, the whole process chain “self-tapping screw” shall be considered. In the final project step, we want to develop guidelines for a secure production process as well as for the practical application of self-tapping timber screws. The overall aim of the project is to guarantee the reliability of self-tapping screws as a loadbearing building product.