Standard Test Method for Shear Strength of Plastics by Punch Tool

 

 

Theory:

 

The shear strength of plastics by a punch tool is one of the easier to set up shear tests. Plastic film or sheet manufacturers will often investigate the shear strength as it relates to compressive loads on their products. This test can also be used to test polycarbonates which are used as lens material in sunglasses. 

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Geometry:

Samples can either be square or circular. An 11mm diameter hole is punched out of the center of the specimen. 5 specimens are usually tested at a time in order to create a more uniform test result. 

 

Solution:

The punch tool will make a 1 inch hole in the sample by shearing it against two pieces of metal. This apparatus is similar to a puncture fixture. The shear strength is given by the load divided by the surface area of the sheared surface. Thicknesses of .25 to .5 inch are common, and technicians will often stack multiple sheets on top of each other to reach a desired thickness. 

 

Analysis:

The shear strength is calulcated by dividing the max shearing load by the total area of sheared edge. Total area is calculated using total thickness of the specimens and also the circumference give by pi*diameter(1"). The units are give in pounds per square inch or by megapascals.