Keywords: finite element modelling, dynamic, threaded faster, loosening, finite element.

Threaded fasteners are still produced in large quantities to be used for fastening in engineering structures. It is well known that when they are subjected to dynamic loads due to impact or vibration they can become loosened, resulting in failure of the functioning of the structures or even in catastrophic accidents. The cost of manufacturing and maintenance efforts to overcome fastener loosening under the effects of dynamic loads is quite high. The mechanism of interaction between friction and vibration has not been well understood, mainly because of a great many parameters involved and the nonlinearity of the problem, in spite of research efforts over the last fifty-five years.

This paper looks at effects of vibration on the friction between two threaded surfaces subject to preloading by finite element modelling (FEM). By assuming uniform geometrical, contact surface properties and loading characteristics of the mated threaded surfaces, a simple two dimensional FEM model of the threaded surface system was created: a block and an inclined plane system. The surface-to- surface contact between the incline and block was modelled by using ANSYS 5.7 CONTACT12 elements, each has two nodes that are coincident, one connected to the block and the other to the incline surface. Elastic Coulomb dry friction model was used to represent the friction between the two surfaces. The block is under gravity loading, preloading and contact forces due to friction.

The structure is subjected to transient impact forces in both axial and transverse directions, and harmonic vibration. By monitoring the relative displacements of the coincident nodes of these contact elements, effects of dynamic loading on the contact surfaces can be found, namely whether the two surfaces are still in contact, or some sliding or even separation has taken place. The values of the gap status indicated that there are many instances of definite separation intertwined with coming into contact again. Instances of separation are coupled with sliding, mainly down the incline plane, indicating overall thread loosening. For the same transient pulse the effect of impact in transverse direction is more prominent than the same signal in axial direction.

In harmonic vibration, similar results were observed. As the acceleration is increased, the displacements increase as expected, the tendency to move up the incline plane also increases resulting in tightening of the thread also increases.

The simple two-dimensional FEM model is able to show prominent features of the effect of dynamic forces on the friction between the mated threaded surfaces. The results confirmed that loss of friction leading to fastener loosening could happen during harmonic vibration or impact loading. These results will be validated against experimental results before incorporating more realistic operating conditions of threaded fasteners and extending to a more complex three-dimensional model. The FEM simulation will help investigating many contentious findings that have accumulated over the last fifty-five years of research in this area.

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