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HYDRAULIC TENSILE TESTER

BCA acquired a tensile tester built by Paul Thorne to whom we are grateful for advice and practical help. The device was based on a manually pumped hydraulic ram capable of generating a force of up to 40kN in both tensile and compression mode over a length of 1 metre. The rig came with a pressure gauge and non return value so peak values could be captured. Knowing the ram's dimensions, it is possible to convert the pressure into force.

After some exploratory work it was clear that we needed to incorporate both a pressure transducer and an electronic device for measuring displacement as well as extend the rig so that we could test rope samples which would extend to well over 1 metre.  We also felt some safety screens were appropriate so the rig could be used with samples other than rope.  (A enquiry had been made several years ago for a facility to test maillons following the breaking of a few at unexpectedly low forces whilst testing anchors.)  Paul came to our aid and the detail of his work can beread here .

So our refurbished and extended rig now also boasts an electric hydraulic pump (fig 1).  After some initial work on the strength of digging buckets and rock nets (to be published) we turned to ropes.  Our first foray was to look at the behaviour of a simple loop of rope.  We recorded the outputs from a load cell, the pressure transducer and the displacement device (fig 2).  The noise on the displacement device was due to a poor selection of voltage and the range on our analogue to digital convertor.  That has been easily side stepped.  But plotting load cell against pressure transducer gave us a concern (fig 3) in that whilst we had expected the unloading period C to D when the pressure is ‘instantaneously’ removed from the system to not replicate the loading period A to B, we had expected the leak period B to C to follow the behaviour of the loading period A to B.  For the time being we shall keep the load cell in place to provide a reliable indication whilst we figure out what is going on and see if the behaviour of the pressure transducer can be improved.  One neat feature is that the pump is sized to provide us with linear displacement at a speed of 180 millimetres per minute.  (The pump has a multi pulley gearing system so we can also rapidly slew the ram into position before starting the test.)  That compares with the requirement in the standard for a speed of between 50 and 150 mm/s.

Our next step is to look at how samples behave when tested using this ‘static’ rig to break them and compare the results with similar work done on the dynamic rig to break samples in a single drop.

Fig 1

Rig

Fig 2

Wave1

 

Fig 3

Wave2