Lars Pilegaard Hansen

Fatigue analysis and testing of adhesive joints

Abstract The fatigue resistance of a single-lap aluminium adhesive joint to cyclic loading in combined shear and bending mode is investigated by nonlinear finite element analysis and crack propagation experiments. The epoxy adhesive is modelled by an elasto-plastic overlay material model. The initial cycles build up a residual stress state, leading to nearly linear material behaviour in the following cycles. Fatigue crack propagation is modelled by removing adhesive elements. Two series of experiments with one-sided cyclic load were carried out. The crack length was monitored by measuring the bending compliance around the end of the overlap with clip-gauges. The crack length is determined as a simple linear function of the measured compliance. The experiments show nearly constant rate crack growth until failure, with no appreciable crack initiation period. The rate of crack growth is proportional to the stress level to the power m = 6.2. Fatigue life results are given in the form of SN curves for adhesive thickness of 0.1 and 0.3 mm. There is no systematic influence of the thickness of the adhesive on the fatigue life. This supports the use of a crack propagation and fatigue life criterion formulated in terms of the energy release rate.

Fracture mechanical Markov chain crack growth model

Abstract On the basis of the B -model developed in [J. L. Bogdanoff and F. Kozin, Probabilistic Models of Cumulative Damage. John Wiley, New York (1985)] a new numerical model incorporating the physical knowledge of fatigue crack propagation is developed. The model is based on the assumption that the crack propagation process can be described by a discrete space Markov theory. The model is applicable to deterministic as well as to random loading. Once the model parameters for a given material have been determined, the results can be used for any structure as soon as the geometrical function is known.

Human damping and its capacity to control floor vibrations

This paper addresses a damping mechanism not often considered although typically present. The damping mechanism in question is that originating from stationary humans (standing or sitting) on floors. Floors may encounter vertical vibrations due to actions of humans in motion. The vibrations hereby generated can be a problem because stationary humans are excellent vibration sensors and may perceive vibrations as being discomforting. This paper demonstrates that in addition to acting as receivers/perceivers, stationary humans can also add significant damping to the floor which they occupy; and thus assist in reducing the discomforting vibrations. In the analyses the stationary crowd of people are modelled as an auxiliary (spring-mass-damper) system attached to the floor. Experimental results reported in this paper show that this modelling approach is reasonable even though a rigid mass assumption is often used. The latter model does not account for a human damping mechanism. Implications of its presence are evaluated for a set of floors. These evaluations also encompass the scenario that a tuned mass damper (TMD) is fitted to the floor so as to mitigate excessive floor resonant vibrations. The effectiveness of such TMD is shown to reduce substantially during the presence of stationary humans on the floor.

Fracture Energy of Plain Concrete Beams at Different Rates of Loading

A simple experimental investigation is performed to measure how different fracture parameters of concrete are dependent on the displacement rate. One of the simplest structural geometries plain concrete beams with a notch in the midsection subjected to three point bending is used as test specimen. The beams are tested in a closed loop servo controlled materials testing system. The experimental results for these preliminary tests show that the bending tensile strength is increasing with the displacement rate and that the fracture energy is constant at lower displacement rates and then increasing at higher displacement rates. Further tests will be carried out.