Three ways to carryout tensile test:
1. CRE: Rate of increase of specimen length is uniform with time (the load measuring mechanism moves a negligible distance).
2. CRL: Rate of increase of the load is uniform with time and rate of extension is dependent on the load-elongation characteristics of the specimen.
3. CRT: Pulling one clamp at a uniform rate and the load is applied through the other clamp. Which moves appreciably to actuate a load measuring mechanism so that the rate of increase of either load or elongation is usually not constant.
a)CRE V/S CRL:
With CRE principle, maximum load reaches before 3 sec., and rest of the time the specimen remains at higher load (initially very high rate of loading).
For CRL, initial extension was very low and afterwards very high extension with small time (very high extension rate)
So, nature of the curves of same specimen under two different principles will be different.If specimen length increase in a CRE machine, the rate of loading will decrease.
b) Stelometer (CRL):
Capable of measuring strength as well as elongation of fibre bundle.
Works with Pendulum lever principle .
The loading of the specimen is carried out by a pendulum system, which is mounted in such a way that it rotates about its C.G.
It eliminates the inertia effects associated with normal pendulum principle.
The beam and pendulum start in a vertical position but the C.G. of beam is such that when it is released the whole assembly rotates.
The speed of rotation is controlled by adjusting the dashpot.
c)Loading by spring (both CRL and CRE):
The instrument is used for single fibre or fine yarns
(i) To test with CRL condition:
Motor M1 runs continuously
As motor M1 starts, the H1 moves upwards at a constant speed.
The spring extends and load is applied on the specimen at a constant rate.
The extension of specimen will cause the leaf spring to touch upper contact C2, which starts the motor M2 and H2 moves down for a short period. This cycle continues until the specimen breaks;
G1 and G2 are the sample grips
The upper grip G1 is connected to a leaf spring, which has a restricted movement between the electrical contacts C1 ad C2.
