BS 131-7:1990 download free.Notched bar tests Part 7: Specification for verification of the test machine used for precision determination of Charpy V-notch impact energies for metals.
o Introduction
In an ideal impact machine the energy in the pendulum will be delivered to the test piece at the instant of maximum velocity of the pendulum and all the energy will be available at the centre of percussion. BS 131-6 seeks to approach more closely to this ideal by specifying tolerances for the constructional parameters of the machine which are stricter than those in BS 131-4, yet still readily attainable.
1 Scope
This Part of BS 131 specifies the way in which the test machine used to carry out precision Charpy V-notch impact tests is verified.
NOTE The titles of the publications referred to in this standard are listed on the inside back cover.
2 Definitions
For the purpose of this Part of BS 131-7 the following definitions apply.
2.1
compensated machine
a machine in which correction for the loss in energy of the pendulum due to windage and friction has been made such that on a free half-cycle of swing of the pendulum from the release position the indicated absorbed energy is zero
2.2
non-compensated machine
a machine in which no correction for loss of energy due to windage and friction has been made
NOTE With a machine of this type the indicated absorbed energy on a free half-cycle of swing of the pendulum will have a small positive value.
2.3
centre of strike
that point on the striking edge of the pendulum at which, in the free hanging position of the pendulum. the vertical plane of the striking edge meets the upper horizontal plane of a test piece of half standard height or equivalent gauge bar resting on the test piece supports
2.4
centre of percussion
that point in a body at which, on striking a blow, the percussive action is the same as if the whole mass of the body were concentrated at that point. When the pendulum delivers a blow along a horizontal line passing through the centre of percussion there is no resulting reaction at the axis of rotation
5.9 Scale graduations
The scales shall be graduated in terms of joules of energy absorbed in fracturing a test piece. However, provision of additional scale graduations, e.g. ft Ibf, degrees, kgfm. is not precluded.
The scale graduation intervals shall not exceed 1.0 J for scale ranges of 150 J or less, or 2.0 J for scale ranges of over 150 .J.
The thickness of the graduation marks and of the indicating pointer shall not exceed 0.2 of the minimum space between graduations.
The minimum length of any graduation mark shall be 2.5 mm.
The indicating pointer shall permit readings free of parallax error to he taken.
The scale and pointer arrangement shall be such that readings can be either directly read or estimated in increments of 0.25 % or less of scale range maximum. This shall also apply on machines provided with digital readout or printout of absorbed energy values.
NOTE For machines equipped with digital indication or printout of absorbed energy, reference in this standard to scales. graduation marks. etc. should be interpreted throughout to mean the equivalent in terms of readings, resolution and capabilities required of such equipment. Where such machines are additionally equipped with scales, the latter should conform in full with the requirements of this standard.
5.10 Windage and friction losses (for non-compensated machines)
NOTE On non-compensated machines, the release position of the pendulum corresponds to that required to provide the nominal (design) energy of the machine and the absorbed energy scale is so calibrated that the pointer indicates zero absorbed energy when the pendulum is elevated to the same height or angle of rise as the release height or angle of fall. Any indicated values obtained during testing using these machines will include the total losses due to windage and the friction of pendulum bearings and indicating mechanism and these readings will thus be higher than the true values to that extent.
The windage and friction losses shall be determined for each range of the machine as follows.
a) Without a test piece on the machine supports, release the pendulum from the starting position. with the slave pointer in operation as during normal testing. Record the value indicated by the slave pointer or by the readout device as applicable.
b) Without resetting the slave pointer and taking care that it is not moved due to any shock in relocating the pendulum, set the pendulum again in its starting position and release it. Record the new indicated position.
c) Repeat the procedure of a) and b) three times and calculate the mean value for the recorded positions.
6 Overall verification
6.1 In addition to verification of details of the machine according to the requirements of clause 5, machines shall be checked under dynamic conditions by means of reference test pieces obtainable from the National Physical Laboratory (NPL) or the Community Bureau of Reference (BCR).
6.2 Dynamic verification shall be carried out at two energy levels using reference test pieces of the highest and lowest energies available. Testing of the reference test pieces shall be performed by the testing laboratory personnel and witnessed by the calibrating laboratory.
NOTE I En cases where the obtained values are outside the specified scatter, advice may be obtained by submitting the broken test pieces to the National Physical Laboratory. NOTE 2 Reference test pieces of intermediate energy levels are also available.
6.3 The five test pieces at each energy level shall be tested in the machine using the method described in
BS 131-6.
6.4 The average vaLue of the five test pieces at each energy level (corrected for windage if necessary) shall correspond to the stated nominal value to within 1.4 J or 5.0 %, whichever is the greater.
7 Certificate of calibration
A certificate of calibration shall be issued for a machine that complies with this Part of BS 131 that shall state the following:
a) location of machine; b) serial number; c) machine type:
d) the date of calibration and a statement that it
complied with this Part of BS 131, when calibrated on that date;
e) the source of the reference specimens and the energy values obtained from them.