P_1.2.1 screw_long_time

Time-related Development of the Load and Creeping Behaviour of Axially Stressed, Self-drilling Woodscrews

Wood exhibits a distinct dependence of firmness characteristics concerning the factors moisture and load impact duration. The influence of the moisture on the individual specific parameters has more or less been examined and its application made known. For building components such as bending beams or pin-jointed bars, the effect of the load impact duration has, from its first observation by the Comte de Buffon (in 1741), been examined in a large number of research studies (e.g. by L. W. Wood in 1943, and later by B. Madsen in 1970), and has also been processed for application in the form of the MADISON-CURVE, for instance.

Also for joining technique tools that are strained in terms of shearing, such as dowels and nails, examinations have been carried out concerning their behaviour among others at TU Delft by J. W. G. van de Kuilen in the years 1991-1999. As far as rod-like joining techniques strained in this way are concerned, the dependence of the firmness on the load impact duration and load intensity has been confirmed.

Here, the project picks up the thread of Project P06 II and the examinations carried out in its course, dealing with the question of behaviour of axially strained screws over a period of time of two years. The strongly increasing application of woodscrews following the production of self-drilling fully threaded screws over the last few years has made it possible to explore the characteristics of screws in relation to a time-dependent development of the load and creeping behaviour. In the course of the pre-experimental state for the series of experiments leading up to the DoL Project, data concerning the basic parameters of the screw – such as the extraction strength f1,k and stiffness Kser – will be obtained and evaluated together with other individual projects in the field of screws.

The procedure of the experiment for examining the duration of load effect will be realised by means of a lever system. Each lever is fitted with two screws of 8 mm diameter and a screwed in thread length of 100 mm; subsequently, the time under varying load intensities is reported right up to the point of breakage. Additionally, the creeping deformation of the screws is observed in order to thus examine also the creeping behaviour of the screws in principle. Parallel to the experiments carried out using failing screws, a pure creeping series under low load intensities (approx. 60 % and 30 % short-term stress, respectively) is carried out. Finally, towards the end of the experiments, those creep series and levers where no breakages occurred are examined via ramp loading or short-term tests as to their residual support potential.

The evaluation of experimental results will be carried out according to the various damage models, and if possible breaking-mechanical approach (after L. F. Nielsen) shall be used for a description of the time-dependent development of stiffnesses under axial strain. If there is sufficient data available, elaborating the principles of a creep model for extraction is to be carried out.