Stress/Strain Measurement. Introduction

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Melvin McKinney
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1 Introduction Proper design of aircraft components is essential to avoid failure Tests need to be conducted to ensure design calculations are correct Beyond design, there is a need to monitor components under manufacture and service conditions in order to ascertain safety Some important measurement techniques: Strain gaging Photoelasticity F14CrashTest NASA_FAA_CrashTest.wmv

2 Structural Failure in Aircraft Boeing 737 belonging to Aloha Airlines in Hawaii

3 Rudder Problem of Airbus 310 The rudder on an Air Transat Airbus 310 plane flying from Cuba to Quebec City nearly fell off. After the incident, Airbus sent precise recommendations on inspection to the owners of about 400 Airbus A-310 and A planes. Wednesday, March 16, 2005

Boeing 787 Wing Test The first Boeing 787 Dreamliner demonstration wing box shown represents two thirds of the airplane's wing span and is full-scale in size.

4 Boeing 787 Wing Test The first Boeing 787 Dreamliner demonstration wing box shown represents two thirds of the airplane's wing span and is full-scale in size. It was tested to demonstrate the structural integrity of the design, gather data required for certification, and validate the repair methods for the materials being used. July 2006

5 Boeing 787 Fuselage The fuselage of the Boeing 787 is composed together of sections known as barrels. A major testing process involves using high-frequency ultrasonic sound waves to scan for imperfections or voids in the skin material. 20 March 2007

6 Strain Gaging The gage is normally A foil between two pieces of paper An etched foil conductor mounted on epoxy or polyimide backing The gage is cemented to the material When material is strained, the wires are lengthened or shortened to change the electrical resistance of the gage The change of resistance can be measured and calibrated to describe the change in axial strain StrainGageSensing

7 Installing a Strain Gage Stress/Strain Measurement

Strain Rosettes The general condition to determine the state of strain is to measure the axial strain in three different directions using a combination of three strain gages The

8 Strain Rosettes The general condition to determine the state of strain is to measure the axial strain in three different directions using a combination of three strain gages The complete state of strain can be determined from The measured values of the gages The gauge orientations Multiple strain gages used to determine the state of strain are known as strain rosettes

9 Strain Measuring Circuit (1) It is possible to measure the resistance change in the strain gage and relate it to the amount of strain developing This is not advisable as the measurement sensitivity is very poor For accurate measurement of strain, Wheatstone bridges often used

10 Strain Measuring Circuit (2) For voltage V supplied to the bridge, potential measured across E is given by E = ( RR 1 3 RR 2 4) ( R + R )( R + R ) V For balanced condition RR = RR If strain gage R1 is loaded to increase resistance to R 1 + ΔR 1 ΔE = V RR ΔR ( R + R ) R Change in voltage is used to correlate to strain

11 Commercial Strain Meter

12 Photoelasticity Some materials exhibit birefringence Birefringence is the decomposition of a ray of light into two rays (the ordinary ray and the extraordinary ray) when the ray passes through the material Photoelasticity involves applying a given stress state to a model and utilising the induced birefringence of the material to examine the stress distribution within the model The instrument used to view the model is called a polariscope

13 Photoelasticity of Model Stress/Strain Measurement

14 Polariscopes Plane Polariscope Circular Polariscope

15 Circular Polariscope - The circular polariscope reveals the isochromatic fringes alone - Isochromatic fringes are lines of constant principal stress difference (σ 11 σ 22 ) - Colored fringes are observed in white light - Dark & light fringes are observed in monochromatic light - The difference in principal stresses is related to the birefringence and the fringe order through the Stress-Optic Law Stress-Optic Law σ 11 σ 22 = Nf / t N relative retardation in terms of complete cycle of retardation f material fringe value t specimen thickness Circular Polariscope

16 Plane Polariscope Plane Polariscope - The plane polariscope reveals the isochromatic & isoclinic fringes - Isoclinic fringes occur whenever either principal stress direction coincides with the axis of polarisation of the polariser. - The two types of fringes can be distinguished by rotating the specimen in a plane polariscope: isoclinic fringes will vary in intensity as they pass through the extinction positions. - Isoclinic fringes provide information about the directions of the principal stresses in the model

Strain Measurement. Prof. Yu Qiao. Department of Structural Engineering, UCSD. Strain Measurement

Strain Measurement. Prof. Yu Qiao. Department of Structural Engineering, UCSD. Strain Measurement Strain Measurement Prof. Yu Qiao Department of Structural Engineering, UCSD Strain Measurement The design of load-carrying components for machines and structures requires information about the distribution