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9-3 HOW TO CALCULATE STRAINS
This section describes how to measure strains and stress.
9-3-1 Method of Calculation Applicable to Strain Gage
1)
Calculating the maximum strain
m
and the maximum stress
m
.
Measure the respective heights of the calibration signal and the maximum strain from the base line of
zero by using a scale.
Suppose that a measurement of
c
is h
c
[mm], and that of
m
is h
m
[mm]. The maximum strain
m
can be
calculated using the equation below.
m
is in the same direction as +
c
. Therefore it is “elongation.”
When once the strain has been obtained, stress at that point (provided that it is of the monoaxial
stress system) can be found using the equation below.
However, E is Young’s modulus of the measuring point and the gage axis is considered as attached
according to stress axis direction.
Therefore, the maximum stress
m
is obtained by the following equation.
2)
Calculating strain ´
m
and stress ´
m
Strain ´
m
is in the direction opposite to +
c
. Therefore it is “construction.”
Suppose that the height of ´
m
is h´
m
[mm], the amount of ´
m
can be obtained using the equation below.
Stress ´
m
(provided that it is of the monoaxial stress system) is calculated using the equation below.
3)
Time from the start point of the strain waveform to the maximum strain
To calculate T
1
(i.e. time from the start point to the maximum strain), use the time maker. Measure 10 or
so intervals between time makers, and obtain their average, then t
c
[mm] against time T
C
. Suppose that a
measurement of T
1
is t
1
[mm]. T
1
can be calculated using the equation below.
Suppose that a strain waveform as shown at right was
obtained. The diagram shows a positive (+) calibration
signal value recorded at the top of the waveform.
Suppose that the direction of this calibration signal
waveform shows the direction of “elongation.”
The time factor of the strain waveform can be obtained
referring to Tc (i.e. an interval between one time-maker
and another).
Various values can be obtained as follows from the right
figure.
Time Maker
Zero Baseline
Calibration Wavaform
(CAL)
Waveform of Measuring Variable
