Measuring and testing – Specimen stress or strain – or testing by stress or strain...
Reexamination Certificate
1999-02-17
2003-01-21
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
C073S767000, C073S073000, C073S769000
Reexamination Certificate
active
06508132
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to apparatus for the dynamic testing of materials and structures and more particularly to a load cell for use in such apparatus.
Such apparatus is well known and
FIG. 1
shows a typical arrangement where a specimen
10
to be tested is mounted in a framework
11
by means of two grips
12
and
13
. One of the grips, in this case the grip
13
, is connected to a suitable load applying device such as an hydraulic actuator for moving the grip while the other grip, in this case the grip
12
, is connected to a load cell for monitoring the forces placed on the specimen by the load applying device. When testing dynamically, utilising frequencies over say 10 Hz, the force indicated by the load cell
15
equals the force in the specimen
10
and the inertia force generated by the grip between it and the load cell, in this case the grip
12
. In other words, this can be expressed as:
F
cell
=F
specimen
+ma
Where F
cell
is the force seen at the load cell, F
specimen
is the force seen at the specimen, “m” is the mass of the grip or fixture between the specimen and the cell and “a” is the acceleration of the grip or fixture. The inertia force “ma” is of course an unwanted error signal and needs to be removed in order to measure specimen forces accurately.
Historically, the error caused by the inertia force has either been unappreciated or else ignored. However, the error magnitude can easily amount to 10% of the actual force and considerably more if mechanical resonances in the system enhance the local grip acceleration. As a result, fatigue data accuracy is potentially compromised. This can have serious safety consequences when the parts undergoing fatigue testing in service are aircraft or automotive components such as critical fasteners.
Further on uncompensated systems, inertia force errors at high frequencies, circa 100 Hz, can cause load control loops to become prematurely unstable, particularly when testing compliant specimens.
Previously it has been proposed to try and correct the inertia force error by using a separate accelerometer fixed to the grip between the specimen and the cell in order to provide a proportional signal which is used to correct the load cell output signal. While this system was an improvement on the original uncompensated system, it still did not totally correct for the inertia force error.
SUMMARY OF THE PRESENT INVENTION
It is an object of the invention to provide accurate correction of the error caused by the grip inertia in an apparatus for dynamically testing objects.
In the present invention, this is achieved by placing an accelerometer within the load cell on the same axis as the force applied to the object under test.
Preferably, the signal from the accelerometer is fed with a signal derived from the load cell down a single cable and then combined with the load cell signal at the testing machine structural rig controller and digitised.
In addition to providing an accurate result, dynamic calibration of the apparatus can be automated enabling rapid optimisation of the apparatus to a new set-up where the grip mass may have changed. Also, the frequency range over which valid testing can be carried out is increased, enabling fatigue tests to be completed much more quickly. As an example, the time to apply ten million cycles (typical for high cycle fatigue testing) at 20 Hz is about 6 days non-stop whereas at 60 Hz it reduces to 2 days. This offers major cost savings to users of existing fatigue testing machines by upgrading their load cells and controller.
Furthermore, inertia compensation enables a much tighter load control loop giving greater bandwidth and more accurate control.
Finally, the invention provides good force measurement and control when the load cell is mounted to the moving end of the actuator piston, where accelerations are significant, such as in many bio-mechanics and structural rig applications.
REFERENCES:
patent: 4047144 (1977-09-01), Wong
patent: 4478086 (1984-10-01), Gram
patent: 5739411 (1998-04-01), Lee et al.
patent: 5798462 (1998-08-01), Briefer et al.
patent: 6114965 (2000-09-01), Schoch
Bahra Chanchal Singh
Hayford Paul Derek
Lohr Raymond David
Allen Andre
Fuller Benjamin R.
Gals Mark R.
Instron Corporation
Jarosik Gary R.
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