Downloaded from https://www.everyspec.com MIL-STD-1312B 60.3 Measurement of static strains. ‘I’heinstrumentation used to be.the same as that to be used to record the dynamic strains. record the static strains shall a . ‘Toachieve high resolution and accuracy, a null-reading method may be emp~oyed in which the output from the transducer can be adjusted to be zero at any particular load. It is then necessary only to detect zero output of the transducer at any load.
When using the electrical resistance strain gage transducer, a convenient method is to have the strain gages in the adjustable bridge circuit fed from a direct current (de) supply and to observe the output on an oscilloscope which ia switched rapidly between the bridge output and ground. When the switching produces no change, the bridge is in balance and the adjustment made to produce a balance relates to the applied load. The oacillnscope must be sensitive tn load changes of O.2 percent of maximum load when the strain gage bridge is in a nearly balanced condition.
It is necessary to ensure that the output of the bridge circuit and any amplifier between the bridge and the oscilloscope ia linear with amplitude and independent of frequency throughout the frequency range.over which the force will be cycled (see Figure 3 for typical equipment). b. If a null methnd ia not used, the oacillnscope used must be capable of neaauring loads with sn accuracy of 0.2 percent of the maximum load. Use of a pen-type recorder is discouraged due tn the inertia of tbe writing pen; however, light beam oscillographs are acceptable. c. A preferred method is to use a dual-channel oscilloscope where one channel records the output of the bridge and the other channel is grounded.
The bridge i: then balanced when the null ia nbserved, that is, the two s i gna l s a r e coincident. 60.4 Static calibration. Tbe dynamomete~ shall be statically calibrated using dead weighca, a static testing machine, or a fatigue machine using a loading range that complies with ASTM E4. Calibration of the’dynamometer ia tn be performed at a minimum of five loada, consisting of a low load not lnwer than 10 percent nf the dynamometer range, the highest lnad of 2 the dynamometer range, and a minimum of.three additional loads spaced at approximately equal intervals through the dynamometer range. The dynamometer shall be mounted in the testing machine in such a way as to minimize bending.
The sequence for the calibration procedure is aa follows: 12