Philippe van Bogaert,

bogimac nv-sa

Brusselsesteenweg 107

BE-1850 Grimbergen, Belgium

To validate alternative steel and man-fibre eventually hybrid materials and constructions of tire reinforcement cords, fabric, strip and tape and their adhesion to the rubber, it is important to consider their fatigue behavior in real usage conditions instead of their static performance. Comparing alternatives only by static tensile strength and peal tests could result in biased optimizations, because improvement based on static behavior could affect in a negative way the fatigue resistance that is required for the tire application. It is thereby important to understand that crack initiation and growth as observed under fatigue conditions are based on completely other mechanisms than the commonly only considered static failures.

Fatigue test should therefore be done under similar conditions and life cycling as the final tire usage. Thanks to modern FEA methodology a very high level of confidence is achieved today on the understanding of occurring stresses and strains. Luckily simplified models can help define easily the test geometry and sample construction for those simulation conditions.

An overview is therefore given about the modern test equipment that is used by manufacturers of tires, of cord reinforcement, of wires and fibres, of rubber adhesion systems, of rubber fillers but currently also of new rubbers.

Specific equipment is dedicated to the steel based reinforcements : traverse contact loading to evaluate the internal tension level of bead wire, Hunter-like high speed reverse bending of steel wire and cord, clamping systems for static tensile testing. The importance of sample preparation is demonstrated by the use of a new wire spiral straightening system, that helps reduce the deviation on the results of the Hunter tests.

Modern generic Bend-over-Sheave material fatigue testing equipment, used for both steel and man-fiber reinforced rubber, is able to validate in an early stage : dynamic flexibility in single and reverse bending, dynamic adhesion of the cord in the rubber matrix as well as dynamic compression of the cord. Depending on the aimed characteristic, samples are made in single or dual layer, and they are run with tire inflation rubber pressure at high speed tire rolling frequency with sample temperatures up to 160°C . Early Warning and Adaptive Control functions enable the observation of the first degradations in the rubber matrix system, thereby easying the understanding of the reasons of the failure mechanisms.

On the Bend-over-Sheave test, especially in dual layer, the upmost attention has to be given to the sample preparation. A new concept of cord tensioning and positioning frame integrated in a plunger mold system is presented, that positions and tensions the reinforcements precisely during the vulcanisation process.