Torsion Testing Wire to ASTM A938 and ISO 7800

Description

ASTM A938 and ISO 7800 address the requirements for simple torsion (twist) testing of metallic wire. Both testing standards require that a wire specimen be twisted about the longitudinal axis until it fails. Upon failure, the number of turns should be recorded. Additionally, some labs want to know the torque required for specimen failure and would also like to see a plot of torque versus the number of turns for the entire test. This plot will show where the wire yielded during the twist test, as well as the failure point and profile.

Challenges

The challenges of testing wire to these standards are:

• Gripping
• Maintaining the rate of turning
• Applying a constant tensile force
• Accommodating various specimen lengths
• Shielding against broken specimen fragments
• Automatic turn counting
• Measuring and plotting torque

Instron Solution

• Gripping - Many common wire diameters can be gripped with standard drill style chucks. Keyless chucks (W-MT01) offer a toothed profile that helps reduce slippage. Keyed chucks (W-MT01-B) provide smaller diameter clamping. Lathe style chucks can be supplied for larger diameters and can also be modified to allow for clamping a wire with a 90 degree bend on the ends.
• Speed of Twisting - The rate of testing can vary and both standards provide recommended ranges that are based on the wire diameter. Instron's Bluehill® or Partner™ Test Software allow the rotational speed to be easily set and stored in the test method so tests are run the same between operators and test systems.
• Axial Tensile Force - To prevent the wire from bending away from the axis of rotation, a small tensile load is required to keep the specimen straight during testing. Our MT1 and MT2 torsion test systems offer an axial preload assembly that provides a constant axial force and can be adjusted for different size specimens.
• Specimen Lengths - Instron's MT torsion testers feature an adjustable tailstock assembly that allows for easy adjustment of the test opening. Guide rails enable the assembly to move freely in the axial direction while a test is being performed. The MT torsion models are available with extended length if longer specimen lengths are required.
• Shielding - A fully interlocked test space enclosure is provided on the MT1 and MT2 torsion testers. Tests cannot be run when the door is opened and the rugged Lexan shield helps protect the operator from broken specimen fragments that may be ejected during failure.
• Total Turns Measurement - The total number of turns to fracture are directly measured by a digital rotary encoder and automatically recorded by the testing software. The real-time measurement data can also be plotted graphically during the test.
• Torque Measurement - Even though these standards do not require measurement of torque, many labs will also benefit from knowing how much torque is required to twist the wire specimen. The MT torsion systems are supplied with precision torque cells that can measure torque accurately down to 0.4% of the cell capacity. A 0.225 Nm (2 lbf-in) torque cell is available for very fine wire requirements. Torque versus Rotation plots are automatically generated during the test and indicate where the wire yields and fractures.