A new self-aligning pipe compression jig has been developed by Technical & Scientific Equipment, distributors of Lloyd Instruments’ materials testing machines in Australia.
The jig is designed for use with the Lloyd Instruments plus range of materials testers in order to evaluate the resistance to collapse of smooth-walled or corrugated pipes manufactured from PVC, PP and HDPE. These are used for fresh, drainage and waste water transportation in buried locations.
The new fixture can accommodate plastic pipes from 20 to 300mm diameter and allows the sample to be aligned along its axis while a compression test up is carried out on each of three samples at 120˚ separation from each other.
The results are used to automatically calculate the stiffness of the pipe in 3 directions at 120° separation in accordance with Australian/New Zealand standard AS/NZS 1562.22:1997 Methods of test for plastic pipes and fittings Method 22: Method for determination of pipe stiffness.
The test is run automatically using Nexygen materials test and analysis software and uses an extrapolation of the linear part of the curve back to zero force, as a way of defining the corrected zero point. Testing is possible at forces up to 10kN.
The test procedure involves cutting a 3 x 300mm lengths of pipe, measuring the inside diameter, and marking 3 positions at 120 degrees from one another.
The sample is then placed between the platens of the jig and located centrally using the alignment rods, which are placed through the pipe to align it on the central axis, by locating into a special slot at either end of the jig. The self-aligning mechanism uses different stainless steel rods for pipe diameters of less than or greater than 50mm.
During the test, the sample is compressed by 5% of its original diameter at a speed selected according to the pipe diameter and after correcting for the initial contact. The second sample is then tested at 120°, and the third at 240° from the first. A special calculation of the pipe stiffness is made using the average of the 3 test results.
The new jig overcomes many problems associated with manual methods of testing, which required the operator to hold the sample near the centre line of the plates until the plates contact the sample.
Manual positioning is potentially dangerous since if the operator lets go too soon the pipe rolls, which runs the risk of testing with the sample off-axis, or with the potential for damaging or reducing the accuracy of the load cell.