Gripping bent specimens is challenging, but there are a few things you do. One, flatten the tabs with a small press prior to inserting into normal grip jaws meant for flat specimens. Two, use a combination of convex and concave grip jaws to eliminate the need to flatten the specimens. Three, use a set of dual side-acting grips that will basically flatten the ends when the grips are closed.

The third solution of using a set of dual-side acting grips requires that the grips have some kind of synchronization to ensure the grip faces close on center. This can be done several ways:

1.    Use traditional rack and pinion-type synchronizer mechanisms. These solutions are not very robust and are susceptible to excessive wear caused by hydraulic flow imbalances between the two pistons and piston load imbalances from bent specimens. Bent specimens are of particular concern because, although the specimen is initially straight, simply flattening the tab in one grip induces a bending of the specimen putting stress on the other grip mechanism.

2.    Hydraulic synchronization uses complex flow controls to ensure the proper amount of hydraulic fluid is ported to each piston so the grip faces close on center. These often require high-resolution position sensors and a flow control valve similar to a servo valve for each piston. This solution is expensive and prone to problems due to the complexity of the system.

3.    Instron’s DuraSync™ dual side-acting grips were designed to deal specifically with the kind of issues experienced when testing pipe sections that have not been flattened prior to installation into the grips.  The self-centering mechanical synchronizer is robust enough to deal with even major load imbalances with minimal wear and the grips incorporate an overload protection mechanism that prevents damage to the grips in unusual situations.  The adjustable clamping force alleviates specimen slippage and grip breaks. The DuraSync grips are also designed so wearable parts are readily accessible so the grips can be serviced in the field when needed.

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