Blog

I was tensile testing a batch of pins this afternoon and as I watched the load cell count up the pounds to fracture, I was reminded why you should never reuse pins that have previously been in service.
 

Typically, the load cell shows a linear increase as the crosshead applies strain to the sample. This stress vs. strain relation is constant up to a material’s yield point. At that point, the linear relation breaks down and the material enters the final plastic deformation phase. This phase is just like it sounds – the pin is being stretched like taffy and it won’t recover.
Watching the load cell, you can see the force flat-line while the crosshead is still moving – this is the moment before failure. If you were to stop the test right at this point, the plastic deformation would appear as a thinned-down neck in what was once a straight bar – see the photo. The atomic rearrangements at this point get pretty hectic, as dislocations start skipping through the crystal lattice and hanging up on the grain boundaries!
Similarly if you were using this pin in a breakaway connector: you can see the pin has deformed if it’s been loaded near its maximum capacity. But sometimes, the plastic deformation phase is brief and the pin breaks suddenly. It breaks at the expected load but there’s no plateau on the load cell and the necking is hard to spot.
A visual inspection won’t necessarily show a previously stressed pin and once a pin has entered the plastic deformation phase, it will pick up where it left off and continue to fail in short order. It’s better to start a pull with a new breakaway pin than risk the job.