Thursday, 16 February 2012

Preconditioning or “Scragging” for Compression Testing of Samples

In compression testing, “scragging” describes the process of preconditioning or exercising an object or material with elastic properties. Components such as springs, and objects such as tennis balls are scragged so that a representative measurement of their resistance to load can be obtained.

Since very few objects are single-use only, measurement of an object’s resistance to compression during operationally typical conditions is necessary. Scragging induces in a material or object physical conditions that approximate the state of the material or object during intended usage. For example, scragging a tennis ball would provide simulation of the conditioning experienced by the ball during play. Without prior scragging, a tennis ball – even fresh from its pressurized container – would display atypically low resistance in a compression test. Tennis players will precondition a ball by bouncing it several times prior to serve. Bouncing warms the ball, which raises its pressure, providing it with increased elastic performance, ideally resulting in a more powerful serve.

To the layman, such preconditioning processes might be simply recognised as “warming up”, which is a fairly accurate description. Depending on their temperature, elastic materials can exhibit differing resistance to force, so scragging a ball prior to compression testing allows for a more realistic reading of the ball’s elastic properties.

So what governs the preconditioning procedure? To what limits is a material or object to be scragged? Spring scragging will depend on the requirements of the manufacturer, who might stipulate a scrag count of several hundreds. Tennis balls might be scragged only a dozen times, but manufacturers will usually submit their samples to a regulations body that will perform scragging and testing to ensure the balls perform to official standards.

When scragging has been completed, a test of the sample’s resistance to compressive force will be taken. Scragging and testing are normally performed on the same machine. A motorised force testing stand fitted with load cell, self-levelling plate, digital gauge, and compression plates is a typical configuration. UK-based Mecmesin are long-established makers of such equipment. The sample is placed between the compression plates and the test is run manually, or through a computer using software such as Mecmesin’s “Emperor”, or via a programmable controller.

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