Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Threaded fastener stress
Patent
1998-03-20
2000-09-12
Ballato, Josie
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Threaded fastener stress
324635, G01N 300, G01N 2998, G01H 1300, G01L 125
Patent
active
061160946
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method for determining the axial load on an elongate member by means of ultrasonic waves.
In particular, the invention concerns a load determining method in which longitudinally as well as transversely directed ultrasonic waves are introduced at one end of an elongate member and the time of flight for these waves is individually measured.
Such a method is described in U.S. Pat. No. 4,602,511. This known method is based on the fact that the propagation speed dependency on stress is different between longitudinal waves and transverse waves.
A problem concerned with the above technique is that the time of flight varies with changing temperature in the object being inspected. The time of flight varies due to the fact that the propagation speed and the length of the object vary with the temperature. This results in an undesireable temperature related deviation in the calculated load acting on the object.
This is a problem in particular in assembling screw joints, because the temperature of the screw may differ to a large extent in the production environment. It is also very difficult to measure the temperature on each screw in mass production, and the tightening process itself generates heat in the screw.
A recent study verifies that there is a difference between longitudinal and transverse waves as regards the time of flight dependency on the temperature in the object being inspected. It has been found that the time of flight of transverse waves is more affected by temperature changes than longitudinal waves, whereas, oppositely, the time of flight for longitudinal waves is more dependent on the load magnitude than the time of flight for transverse waves.
This means that measured changes in flight time for the longitudinal and transverse waves provide information from which changes in temperature and load can be calculated.
The invention comprises a method for determining the actual load on an elongate member by measuring during a first, preliminary zero-load inspection stage the time of flight for longitudinal and transverse ultrasonic waves introduced into the elongate member for determining the zero-load length of the elongate member by measuring the time of flight (TOF) for both types of waves measuring during a second inspection stage the time of flight (TOF) for both type of waves under the actual load and temperature condition of the elongate member, comparing the time of flight (TOF) for the introduced waves measured at the zero-load condition with the time of flight (TOF) measured at the actual load and temperature condition, and calculating the actual load magnitude under compensation for the difference in temperature between the actual load and zero-load conditions.
In a practical application of the method according to the invention the transverse and longitudinal waves are introduced into the elongate member by means of an ultrasonic transducer of the type which is permanently attached to an end surface of the elongate member. Such a transducer is described in U.S. Pat. No. 5,205,176.
For carrying out the method according to the invention, there is needed the following physical data of the elongate member: measured, should be set to about room temperature. T.sub.0. to a change in temperature. to a change in temperature. to a change in tensile load. due to a change in tensile load.
The above mentioned data must be collected for each application as they depend on both the material properties and the geometry (area) of for instance a screw in a screw joint application. case.
In the initial stage of the tightening of a screw joint a zero-load measurement is made. From that information it is possible to determine the length (and the temperature) of the screw as follows:
The zero-load TOF (t.sub.S0 and t.sub.L0) must be equal to the zero-load TOF at T.sub.0 with a temperature correction factor added: ##EQU1## .DELTA.T is then extracted from (1): ##EQU2## (3) in (2) gives: ##EQU3## l.sub.0 is then extracted from (4): ##EQU4##
If the actual temperature is needed it can be obtain
REFERENCES:
patent: 3975948 (1976-08-01), Makino et al.
patent: 4602511 (1986-07-01), Holt
patent: 4846001 (1989-07-01), Kibblewhite
patent: 5205176 (1993-04-01), Kibblewhite
patent: 5625146 (1997-04-01), Hull
patent: 5719503 (1998-02-01), Burnett
Atlas Copco Controls AB
Ballato Josie
Nguyen Jim
LandOfFree
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