Wire and cord used for the reinforcement of motor vehicle tires is one of the most demanding applications for any steel product. As thin as a human hair, the wire needs to reinforce the tire to help withstand complex in-service loading. However, the wire often fails through impact, fatigue and fracture.
One major challenge associated with these types of fine wire is the thinness and hardness that makes gripping difficult. Without the appropriate gripping approach, stress concentrations at the jaw face edge will produce premature failure or jaw breaks. Additionally, because the material has a small amount of strain during the test, we find that using a traditional position measurement transducer is not sufficient for accurate strain data. Lastly, if using contacting extensometry it may introduce errors, including those caused by slippage of knife-edges or stress concentration points.
We suggest using our ElectroPuls™ and 8800 servohydraulic systems to investigate the fatigue and fracture properties of wires used in the reinforcement of tires. For the finest of wires, it is even more important to select an appropriate low-force load cell and suitable low-mass grips for a dynamic test to succeed.
In this particular case, we used an ElectroPuls E1000 test instrument equipped with a low-force Dynacell™ load cell, and miniature fine wire collet grips that grip the wire without introducing stress concentrations. Additionally, the optical digital encoder on the ElectroPuls system was used to monitor position data. Our tests were successfully run between 0.5 N and 1.5 N at test frequencies up to 80 Hz.
Additionally, specimens for fracture testing were produced from thicker sections of the wire and were notched by electrical discharge machining. The specimen was then fatigued at frequencies in excess of 20 Hz, while optical microscopy was used to measure crack growth.