AOE 3054
Measuring static displacement
and deformation in structures
• Preparation for experiments 2 and 5.
• Relevant reading:
– Lab manual, experiment 2.
– Notes on Maxwell’s Reciprocal Theorem (on
website)
Dr. Eric Johnson
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Guidelines and tips
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Measuring Static Displacement and Deformation in Structures: Techniques and Applications, Study notes of Aerospace Engineering
The methods used to measure static displacement and deformation in structures, focusing on the use of strain gages and electrical resistance changes. The principles behind these techniques, the configuration of metal foil strain gages, and the impact of temperature on measured strains. The document also mentions the importance of testing structures for military and commercial aircraft and the regulations that govern their certification.
Typology: Study notes
Pre 2010
1 / 20
This page cannot be seen from the preview
Don't miss anything!
Related documents
Partial preview of the text
Download Measuring Static Displacement and Deformation in Structures: Techniques and Applications and more Study notes Aerospace Engineering in PDF only on Docsity!
AOE 3054
Measuring static displacement
and deformation in structures
• Preparation for experiments 2 and 5.• Relevant reading:
- Lab manual, experiment 2.– Notes on Maxwell’s Reciprocal Theorem (on
website)
Dr. Eric Johnson
Why Test Structures?
It is…
For military airplanes: Naval Air Systems Command (NAVAIR) of theU.S. Navy or Aeronautical Systems Division (ASD) of the U. S. AirForce.
For commercial transports: Federal Aviation Administration (FAA), U.S. Department of Transportation.
Federal Aviation Regulations (FAR), Part 25, “AirworthinessStandards: Transport Category Airplanes,” spells out the design andtest requirements that must be met before a type certificate is issued.
Airworthiness is defined as “that quality by which an aircraft makes itscontribution to the safety of flight.”
- To…
Displacement Measurements
Displacement Measurements
Moving contact on a slide-wire converts linear displacement or angulardisplacement into an electrical signal. (Holman, p.157)
Magnetic core moves freely inside three coils. An alternating voltage isimpressed on the primary coil and the output voltage of the device asmeasured by two secondary coils is determined by the magneticcoupling between the core and secondary coils, which in turn dependson the position of the core. (Holman, p. 158)
Holman J P, 1989, Experimental Methods for Engineers, 5th Edition,McGraw Hill, New York.
Principle of Electrical Strain Gage
1. Lord Kelvin (1856) established by experiment that
Change of Wire Resistance Due To
Strain
The differential of
R
ρ
L / A
is
Divide this equation by
R
to get
The area change for a circular cross section is related to the changein diameter by
dA =
π
D
( dD
), and the diameter change is due to the
Poisson effect
dD
νε
) D
, where
ν
is Poisson’s ratio. Hence, the
change in area per unit area isNote that the axial normal strain
ε
is defined as
Configuration of Metal Foil Strain
Gages (Micro-Measurements)
The Wheatstone Bridge as a Direct
Readout Device
The output voltage
e
0
is determined by bridge resistances
So
e
0
=0 when
(called the ‘bridge balance equation’)
Quarter Bridge Circuit – One Active
Strain Gage
Quarter Bridge Circuit – Two Lead
Wire Arrangement
Balance equation:Effective gage factor:The increase in resistance caused bythe lead wires in strain gage leg of thebridge results in a decrease in voltage at the S- junction. The gage factor frommanufacturer is
F
= (
∆
R
G
/ R
G
)/
ε
. The percentage change due to lead wire
resistance, or gage factor desensitization, is
% (^77). 1
100
'^100
2
1
2
1
−
−
−
L
L
G
L
L
R
R
R
R
R
F
F
F
for
R
G
=
Ω
, and 20ft of
AWG#30 lead wire.
Effect of Temperature on Measured
Strain – Thermal Output
Once an installed strain gage is connected to a strain indicator
and the instrument balanced, a subsequent change intemperature of the gage …
. N.B.; There is no thermal output strain at a fixed
temperature if
Causes Of Thermal Output From An Installed Strain
Gage When Temperature Changes
Thermal output caused by temperature change ispotentially the most serious error source in the practice ofstatic strain measurement with strain gages.
Methods to compensate for thermal output
Self-Temperature-Compensated
(S-T-C) Strain Gages
- • An installed S-T-C gage minimizes the thermal output strain over a widetemperature range if it is bonded to the intended substrate; i.e., there is littlethermal mismatch.