Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L1/00—Measuring force or stress, in general
  • G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
  • G01L1/248—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet using infrared

Definitions

• the object of the present invention is to provide a method and apparatus for measuring static stress in a body.
• the present invention provides in one form a method for indicating static stress in a predetermined part of an object comprising the steps of: a) applying a controlled quantity of electromagnetic energy to a predetermined surface part of the object; b) measuring the resulting increase in temperature in the predetermined part of the object; and c) determining from the temperature increases an indication of the local stress in the predetermined part.
• the invention relies upon the dependence of the specific heat of an object under inspection on local stress.
• the specific heat from the temperature increase or observing variations in the specific heat across the surface of an object it is possible to obtain an indication of the static stress at a point on the surface or a map of the variation of static stress across the surface.
• the stress-dependence of the specific heat is such that a very sensitive measuring technique is required.
• the quantity of heat applied to a surface part must be accurately known and the temperature sensor desirably should be capable of measuring temperature changes down to 0.001°C.
• the temperature is measured by detection of changes in thermal radiation emitted by the surface area of the predetermined part of the object.
• the surface area of the object is scanned such that the predetermined part whose increase in temperature is measured is variable.
• the energy is applied cyclically to the predetermined part of the object.
• the source of energy is a pulsed laser operating in the optical waveband so that surface reflections of the optical energy incident on the object do not corrupt the measurements of temperature increases.
• the invention also provides apparatus for indicating static stress in a predetermined part of an object comprising means for applying a controlled quantity of energy to a predetermined part of the surface of the object and means for measuring increases in temperature of the predetermined part of the object with energy applied to the object.
• the means for applying the energy operates cyclically any may be a pulsed laser operating at an optical frequency.
• Preferably signal processing means is included which receives a first output signal from the temperature measuring means and correlates this with a second signal proportional to the cyclical variation of the energy applying means thereby enabling rejection of any temperature change resulting from other random energy sources and produces an output indicating tensile or compressive stresses at the predetermined part.
• an infra red detector unit 18 which is arranged to receive infra red radiation emitted from the surface of the object 10 after focussing by a germanium lens 19.
• a single cadmium mercury telluride detector or a lead tin telluride detector may be used for the infra red detector unit 18.
• the beam splitter 15 is placed in the path of the infra red radiation from the object 10 such that the field of view of the detector unit 18 is subject to the same raster scan action by the scanning mirrors 16, 17 as the beam from the pulsed laser source 12.
• the laser beam is very narrow it is arranged to be directed off-axis at these mirrors while ensuring that the same spot on the item 10 under inspection is scanned by the laser and the infra red detector. This ensures that the infra red detector does not respond to any infra red emissions from the mirrors 16 and 17 resulting from heating by the laser beam.
• a signal 23 from the laser energy source representing the pulsed energy waveform is connected to a second input of the correlator 22.
• the correlation product of the received temperature waveform 21 and the transmitted pulsed energy waveform 23 from the correlator 22 is connected to a computer unit 24.
• the temperature pattern resulting from the scanning movement represents the pattern of the local static stress level over the object and may be displayed on a TV monitor 25 or recorded on a chart recorder 26 or magnetic disc store 27 for subsequent analysis.
• the computer 24 may be provided with data from a keyboard such that the output signal from the correlator can be biassed to zero at the temperature change produced in unstressed material so that the display can readily distinguish between static stresses of tension or compression.
• the received data may be further processed by the computer prior to display or storage to provide desired information in any readily comprehended manner.
• the present invention approximates to these conditions by using pulses of energy from an external source.
• the pulse frequency must be selected to be low enough for energy to be absorbed by the material and to provide the temperature change, and high enough for the measurement conditions to approximate to adiabatic conditions. Careful attention must also be paid to the surface finish of an object under investigation to ensure uniform absorption of light and emission of infra red radiation over the whole of the object.
• a laser source it is not necessarily limited thereto.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Investigating Or Analyzing Materials Using Thermal Means (AREA)
• Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)

Priority Applications (2)

Applications Claiming Priority (2)

Publications (1)

Family

ID=10566527

Family Applications (1)

Country Status (7)

Cited By (3)

Families Citing this family (9)

Citations (4)

Family Cites Families (11)

• 1984
  • 1984-09-11 GB GB848422873A patent/GB8422873D0/en active Pending
• 1985
  • 1985-09-09 JP JP60504020A patent/JPS62500468A/ja active Granted
  • 1985-09-09 EP EP85904469A patent/EP0192722B1/en not_active Expired
  • 1985-09-09 US US06/865,726 patent/US4798477A/en not_active Expired - Fee Related
  • 1985-09-09 AU AU48010/85A patent/AU584919B2/en not_active Ceased
  • 1985-09-09 WO PCT/GB1985/000403 patent/WO1986001893A1/en not_active Ceased
  • 1985-09-09 DE DE8585904469T patent/DE3568637D1/de not_active Expired

Patent Citations (4)

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