Continuous Damage Fiber Bundle Model for Strongly Disordered Materials

We present an extension of the continuous damage fiber bundle model to describe the gradual degradation of highly heterogeneous materials under an increasing external load. Breaking of a fiber in the model is preceded by a sequence of partial failure events occurring at random threshold values. In order to capture the subsequent propagation and arrest of cracks, furthermore, the disorder of the number of degradation steps of material constituents, the failure thresholds of single fibers are sorted into ascending order and their total number is a Poissonian distributed random variable over the fibers. Analytical and numerical calculations showed that the failure process of the system is governed by extreme value statistics, which has a substantial effect on the macroscopic constitutive behaviour and on the microscopic bursting activity as well.Comment: 10 pages, 13 figure

Drive-in torque for self-tapping screws into timber

Self-tapping screws have been widely used in timber constructions nowadays. Current practice considers self tapping screws perform best in connecting two members when they are fully threaded, however the drive-in resistance caused by the friction between woods and screws can potentially damage the screw and reduce the effectiveness of its applications. The relationship between their thread configuration and the drive-in torque force has not been investigated, and how would knots in the member affect the drive-in force remains in question. This study conducted a series of tests aiming to demonstrate the influence of thread configuration on the drive-in torque of screws. Two types of self-tapping screws and three different thread configurations were studied. The drive-in torque for partially threaded screws was found to be significantly slower than that of the fully threaded ones. The results showed knots can significantly influence the positioning of screw and increase the drive-in torque. The application of pre-drilled hole was found to be an effective way to minimise the influence of knots. This article points out that with appropriate consideration of thread configuration, partially threaded self-tapping screws can not only achieve the same efficiency with fully-threaded ones, they will also benefit from reduced drive-in torque force

Time-variant reliability of timber beams according to Eurocodes considering long-term deflections

In order to achieve a consistent level of failure probability, structural design codes are optimized using probabilistic methods. This optimization process traditionally focuses on the ultimate limit states (ULS). However, in the design of timber structures the performance of the structural members is often governed by the serviceability limit state (SLS) associated with different load levels than applied in the ULS. The probability of serviceability failure is strongly dependent on the loading sequence and the time-dependent response of timber; therefore, a time-variant probabilistic model is recommended to estimate them properly. This study aims to investigate the time-dependent reliability for long-term deflections of timber office and residential floor beams according to the specifications of the Eurocodes. A simple creep model is used to calculate the deflections and Monte Carlo simulation is carried out to determine the reliability index. It was found that the creep factor and the suggested deflection limits given in Eurocode 5 might not be appropriate to achieve the expected target reliabilities. To obtain a more consistent reliability, more suitable values for the mentioned parameters were suggested. However, the primary aim was to present a framework to determine appropriate deflection limits for structural codes