The Indian Engineering College, Vadakangulam-627116
B.E.AERONAUTICAL ENG, AE 54 EXPERIMENTAL STRESS ANALYSIS
Two marks questions and answers
1. Define Measurement:
The measurement of a given quantity is essentially an act or result of comparison between a quantity
whose magnitude (amount) is unknown, with a similar quantity whose magnitude (amount) is known, the
later quantity being called a standard.
2. What are the basic requirements for measurement?
(i) The standard used for comparison purposes must be accurately defined & should be commonly
acceptable.
(ii) The standard must be of the same character as the measure and (ie, the unknown quantity or the
quantity under measurement)
(iii) The apparatus used and the method accepted for the purposes of comparison must be provable.
3. What are the methods of measurements?
(i) Direct Method: In these methods, MEASURAND is directly compared against a STANDARD.
(ii) Indirect Method: Measuring systems are used in indirect methods for measurement purposes
4. What are the uses of measuring instruments?
Measurements involve the use of ‘instruments’ as a physical means of determining quantities (or)
variables. In simple cases, an instrument consists of a single unit which gives an output reading or signal
according to the unknown variable (MEASURAND) applied to it.
5. What is dimensional measurement?
Dimensional Measurements are characterized as determination of size of an object. These are the
measurements of dimensions of an object.
6. Explain the measurements of length and displacement
A displacement means the movements of a point with respect to another point. Generally length (or
change in length) means it referring a distance between two reference points. Thus measurement of
length includes both the dimensional measurement and the displacement measurement.
7. What are the ‘STANDARD’S for the measurement of an angle?
The primary standards of angle may be taken either as angle subtended by a circle at its centre ie, 360° or
as angle between two straight line intersecting in a manner such that all four angles so formed are equal,
ie, each angle is 90°
8. How we can measure the Area of survey plats?
Measurement of area of regular and standard geometrical figures can be obtained from the dimensions
using standard mathematical relationships.
There are many engineering applications which require the measurement of plane area. Graphical
determination of the area of survey plots form maps, the integration of function to determine the area
enclosed by a curve analysis of experimental data may require the use of measurement of plane area.
9. Give any two methods for measure an unknown force
An unknown force may be measured by the following methods,
1. Balancing the unknown force against the known gravitational force either directl(or) indirectly using a
system of levers. 2. Transferring the unknown force to a fluid pressure and then meaning the resulting
fluid pressure. Hydraulic and Pneumatic load cells are used for transferring the force into pressure.
10. How we can measure the temperature changes?
Temperature is not measured directly, but is measured through indirect means; change of temperature of
a substance causes a variety of effects. These effects may be physical, chemical, electrical (or) optical
and they may be used for the measurement of temperature through use of proper temperature sensing
devices.
11. Tell some thing about‘static characteristics’ and ‘static calibration’ in measurements:
‘Static characteristics’ of a measurement system are in general those that must be considered when the
system or instrument is used to measure a condition not varying with time. All the static performance
characteristics are obtained in one form or another by a process called’ static calibration’
12. What is accuracy and tell about point accuracy?
Accuracy is the closeness with which an instrument reading approaches the true value of the quantity
being measured. Thus accuracy of a measurement means conformity to truth. ‘Point Accuracy’ is the
accuracy of the instrument only at one point on it scale.
13. Give the Type of errors in measurements
(i) Gross errors, (ii) Random errors, (iii) systematic errors - Instrumental errors, Environmental errors,
Observational errors
14. Differentiate Gross error and Instrumental Error
Gross errors mainly cover human mistakes in reading instruments and recording and calculating
measurement results.
Instrumental Error occurs due to the wrong adjustments of a measuring devices while it construction or
misalignments of its pats.
15. Differentiate environmental and systematic errors
Environmental errors causes due to the surrounding condition of instruments, but systematic errors causes
because of the fault of the arrangement of measuring system during the whole measuring process.
16.. What absolute static error? Give example
Absolute static error is the difference between measured values of quantity and the
true value of quantity.ie, Eo=X(m)-X(t), Example: an error of 2mm is negligible when the length being
measured is of the order of (1000 mm) 1 m but the same error of 12mm may be considerable when the
measurement of 10mm length.
17. What is relative static error?
It is the ratio between absolute error to the true value of quantity, ie, Er= Eo/Xt
18. What is static correction?
Static correction is the negative value of absolute static error ie, Cs= -Eo
19. A meter reads 127.50V and the true value of the voltage is 127.43V.
Determine (a) static error and (b) static correction for this instrument.
Sol: (a) static error (E0)=Xm-Xt =+0.07V, (b) static correction (Cs)= -Eo= -0.07V
20. a thermometer reads95.45°c and the static correction given in the correction curve is - 0.08 0c
Determine the true value of the temperature
Sol:Cs= -(Xm-Xt)ie, Xt=Cs+Xm = -0.08+95.45=95.37°C
21. An inclined Limb manometer is used for measurement of flow rate reads
0.161 x 10- ³m³/s. The true value of flow rate is 0.159 x 10-³ m³/s. Determine (a) static error (b) static
correction.
Sol: Eo=Xm ±Xt = (0.61-0.15)x10-³= 0.002 10-³m³/s
22. What do you mean by limiting errors?
The limits of the deviations from the specified values of measurement data are defined as `Limiting
errors' (or) `Guarantee error'
23. Determine relative limiting error:
The relative error (fractional error) is defined as the ratio of the error to the specified magnitude
(nominal magnitude) of a quantity.
Relative limiting error(Er)=δA/As=limiting error/a normal value
24. What is an uncertainty in a measurement?
Un certainty of measurement is a parameter associated with the result of measurement, that
characterizes the dispersion of values that could be reasonably attributed to the measurand.
25. Define static sensitivity
the static sensitivity of an instrument or an instrumentation system is the ratio of the
magnitude of the output quantity to the magnitude of input quantity being measured
26. What is known as deflection factor?
Deflection factor or inverse sensitivity is the reciprocal of the static sensitivity.
27. Define scale range:
The scale range of an instrument is defined as the difference between the largest and the smallest reading
of the instruments.
28. What is the frequency range in measurements?
Frequency range is defined as frequency over which measurements can be performed with a specified
degree of accuracy.
29. What is extensometer?
Extensometer is an instrument used top measure minute deformation of material while it is subjected to
a stress.
30. Classify the extensometers depending upon the magnification systems.
1. Mechanical Extensometers, 2. Optical extensometers, 3. Acoustical (Vibrating wire) extensometers ,
4. Electrical extensometers
5. Pneumatic extensometers.
31. What are the Basic Requirements of extensometer?
1. Very high magnification:The magnification required is usually greater than 1000: 1
2. Sensitivity: The relation between input and output should not be affected by the
reversal in the direction of input and this requires that the movement should not have
any friction. 3. Low input force: The input force required to cause displacement should be extremely
small thus there is no defamation of the component due to the process of measurement.
32. Classify the mechanical extensometers depending upon the manner of obtaining the
magnification.
1. Wedge magnification , 2. Screw magnification , 3. Compound magnification, 4. Lever magnification
33. Give the disadvantages of a simple mechanical lever magnification
For strain measurements on inner surfaces of small structures, the clearance may be in sufficient for the
gauge and all its auxiliary equipment.
34. Give the magnification and gauge length of porter- Lipp strain gauge.
Gauge length is 25 mm, Magnification is 300 times
35. Give the advantages of porter- Lipp gauge
1. Very compact , 2. Light weight
36. Give the magnification capacity & gaugelength of Huggen-Berger tensometer
The magnification may vary from 300 to 2000 depending upon the model. The gauge length varies from
12.5 to 25 mm.
37. What is the measurement capacity of C.E. John extensometer?
C.E. Johnson extensometer can measure strain up to 5 micro strains over a gauge length of 50 mm.
38. Give the minimum strain value that scratch gauge can be sense.
The minimum strain that a strain gauge can sense is above 100 micro strains.
39. Give the advantages of scratch gauge
1. Compact in size, 2. It can be attached to almost any surface with clamps or screws & adhesive
bonding, 3. It can measure scratch under all types of loading (static, shock, fatigue), 4. It can be used to
record stress in all types of environments.
40. Tell something about the magnification capacity of Marten's extensometer
This instrument is extremely sensitive and by using telescope, regarding may be estimated to 1/10 mm
which corresponding to a length charge of approximately
0.0002m.
41. What is the sensitivity of Tuckerman gauge?
The sensitivity of tucker man gauge is 2 micro strains
42. For the following reading find the deformation sensitivity & strain sensitivity
for Tuckerman optical gauge. Base length = 25 mm, d= l/20, magnification
factor = 5.
Sol: Ds=d/m = 1/(20x5)= 0.01mm, Ss= Ds/L = 0.01/25 = 4 x 10-4 strains
43. Give the formula to obtain fundamental frequency for a vibrating wire
F = (1/2L)Γ(P/m)=(1/2L)Γ{[E(δL/L)/m]xA}, Where A =Cross sectional Area of vibrating wire,
E=Young' s moduls of wire material, L=Length of Vibrating wire, M=Mass for unit length, P=tensioning
force in the wire, δL=incremention length of the vibrating wire.
44. Give the classifications of Electrical strain gauges
(a) inductance (or) magnetic strain gauges, (b) Electrical resistance strain gauges,(c) The capacitance
strain gauges.
45. How strain can be measured by using Electrical strain gauges?
Electrical strain gauge is a device in which a change in length produces a change in some electrical
characteristics of the gauges.
46. Give the working principle of inductance strain gauges?
An electrical inductance gauge is a device in which the mechanical quantity to be measured produced a
change in the magnetic field, and hence in the impedance of a current carrying coil.
47. Give the working principle of variable air gap gauges
In this types of gauges the reluctance of the magnetic field is varied by changing the air gap.
48. Give the working principle of movable core solenoid gauges
In this type of gauges the reluctance of the magnetic circuit is varied by changing the position of the iron
core in the coil.
49. Give the working principle of eddy- current gauges
In this types of gauges the losses in the magnetic circuit are varied by changing the thickness or position
of the high- loss element inserted in the magnetic field.
50. Give the formula for the impedance of a coil to the passage of alternating current.
Z=Γ(2πfl)² +R², Where Z= impedance in ohms, f= frequency in herzs, L= inductance of the coil in
henrys, R= resistance component in ohms
51. How the displacement of material structures can be measured by using electrical resistance strain
gauges?
In the electrical resistance strain gauges the displacement (or) strain is measured as a function of the
resistance change produce by the displacement in the gauging circuit.
52. What is known as bonded strain gauges?
A thin paper sheet or metal sheet with strain gauge wire is bonded with an adhesive material to the
structure under strictly.
53. What is the use of gauge factor in strain gauge measurements?
Gauge factor is used to determine the sensitivity of strain gauges.
54. Give the requirements for strain gauges?
Gauge factor should be high, Wires used in a strain gauge should have high resistance.
The wires should have a linear relationship between strain & resistance of the wire.
55. What is the resistance of material?
Resistance is the property of a material that restricts the flow of charges through the material.
56. What is known as foil strain gauges?
The foil strain gauge has metal foil photo- etched in a grid pattern of the electric insulator of the thin
resin and gauge leads attached.
57. Give the formula for electrical capacity between parallel plates in capacitance strain gauges
C=[8.86x10-3KA(N-1)]/H, Where c- capacitance in picofoarads, K- dielectric constant of the medium
between the two plates A- area of one side of the plate(mm2), N- No of plates, H- distance between plates
in mm
58. What is the basic principle of unbonded metallic strain gauges
the principle of unbonded metallic gauges is based on change in electrical resistance of a metallic wire
due to change in tension of the wire.
59. What is photo-etching?
Photo etching is the act of producing grid configuration on metal foil with the help of photo effect.
60. What is known as strip gauges?
The number of strain gauges are arranged in a particular manner is called strip gauges.
61. What is known as strain rosettes?
An arrangement of strain gauges in particular orientation or angle is known as strain rosettes
62. Give the quantities required for a good gauge material?
High gauge factor, High resistance, Low temperature sensitivity, High electrical stability,
Good corrosion resistance, High yield point stability
63. Give some arrangements of strain gauges to obtain strain rosettes
1. Two gauge rosette, 2. Rectangular rosette-a) Three element,b) Four element , 3. Delta (or)
Equiangular rosette, 4. T. Delta rosette
64. Give the formula for strain measured by a strain gauge in particular angles εθ= ½ (εx+εy)+½ (εxεy)cos2θ+½γxy sin2θ, Where εθ is the strain value in θ direction
65. Give the relation on between principl stress and principal strains
Where σ1 and σ2 are principal stresses, ε1 and ε2 are principal strains
66.What are the methods are available for computing the strain rosette datas?
1. Analytical Solutions,2. Graphical Solutions, 3. Semi ± graphical (or) vectorial layout method, 4.
Nomographic Solutions, 5. Geometrical Computers.
67. Give the advantages of strain Rosette analysis.
(i)Extreme simplicity and speed of application. (ii)Possibility of allowing for transverse effects. (iii)No
requirements for additionalequipments. (iv) The possibility of training relativity unskilled persons to use
the method.
68.Give the type of strain gauge circuits.
1. Wheat stone bridge circuits for static strain measurements. 2. Potentiometer circuits for dynamic strain
measurements.
69. Define sensitivity of potentiometer:
Sensitivity of the potentiometer circuit is defined as the ratio of the output voltage divided by the strain.
70. Define Light
Light is usually defined as the radiation that can affect the human eye.
71. Define White light:
Light from a source that emits a continuous spectrum with equal energy for every wave length is called
white light.
72. Define monochromatic light:
Light of a single wave length is called monochromatic light. Example: sodium vapour lamp, Hg arc lamp.
73. What is wave front?
The focus of points on different radial lines from the source exhibiting the same disturbance at a given
instant of time, in a surface known as a wave front.
74. Define Ray?
A line normal to the wave front, indicating the direction of propagation of the waves is called a ray.
75. What are longitudinal waves?
The waves in which vibration are along the direction of their travel is known as longitudinal waves.
76. What are transverse waves?
Transverse waves are waves in which vibrations are perpendicular to the direction of wave travels.
77. What is polarized light?
The light having vibration only along a single straight line perpendicular to the direction of propagation
of light is said to be polarized.
78. What is plane of polarization?
The plane containing the direction of propogation of light, but containing no vibrations is called the plane
of polarization.
79. What are the methods are available to obtain plane polarized light?
(i) Refraction and Reflection for glass plate.(ii) By using Nichol Prism.
80. What are the disadvantages are available in Nichol prism when we use that to obtain plane polarized
light?
1. Costly, 2. Intensity is Poor.
81. What is the basic principle for photo elasticity?
When polarized light enters a loaded transparent component, it is split into two beams both beams travel
along the same path, but each vibrates along a principal direction and travels at a speed proportional to
the associated principal stress.
82. What is Refractive index?
The ratio of the velocity of light In air to the velocity in the medium is called the refractive index of the
medium and is denoted by `n' .
83. What is wave length?
The wave length of any given frequency in the distance traveled during one complete vibration.
84. Define Amplitude?
Amplitude is a magnitude of the disturbance, the intensity of light may be taken as directly proportional
to the square of the amplitude of the vibration.
85. Define phase
The phase of vibration at any instant defines the stage of the cycle reached at that instant.
86. Give the general equation of motion of a transverse light wave propagating in Z-direction.
E=acos(θ-wt), Where E=magnitude of light vector, t = amplitude of the wave.
87. What do you mean by Polariscope?
Polariscope is an optical instrument that utilizes the properties of polarized light in its operation.
88. Give the types of polariscopes used in experimental stress analysis?
(i) Plane Polariscope, (ii) Circular Polariscope
89. Give the four different possible setups in circular polariscope?
Set up
Polariser-Analuser
QWPs
Field
1
Crossed
Parallel
Bright
2
Crossed
Crossed
Dark
3
Parallel
Crossed
Bright
4
Parallel
Parallel
Dark
90. What is the standard setup in circular polariscope?
The crossed ± crossed setup is called the standard setup of the circular polariscope.
91. State Stress optic law:
The relative change in index of refraction is proportional to the difference of principal stresses.
92. What is known as isotropic point in a polariscope setup?
In a particular point in a stressed model where σ1 = σ2 that fringe order is zero and permanent block dots
appear at these points such are called isotropic points.
93. What is known as singular points in a polariscope setup?
σ1 = σ2 =0 then also the fringe order is zero at these points and permanent block dots appear such are
called singular points.
94. What are the properties of photo elastic materials?
1. Transparent to light used in the Polariscope. 2. Easily machinable by conventional means. 3. It should
be force form residual stresses 4.They should have both mechanical and optical isotropy and
homogeneity.
95. Give the most commonly used methods for compensation techniques
1. Babinet compensation method, 2. Babinet soleil compensation method, 3. Tension or compression stop
method. 4. Tardy method of compensation, 5. Senarment method of compensation, 6. Photometric
method.
96. What are the techniques used to determine the stresses at the inner layers of the body in 3D photo
elasticity?
1. Locking in the stresses in the model , 2. Multilayer reflection technique.
97. Define Non - destructive testing
Non ±destructive testing is a technique for revealing flaws and defects in a material or device without
damaging as destroying the test sample.
98. Give some advantages of NDT:
1. Can be done directly on production items without regards to part cost or quantity available. 2. Can be
done on 100% of production of on representative samples.
99. Give the limitations of NDT:
1. Results often must be interpreted by a skilled. Experienced technician.
2. In absence of contact with each other, different observers may disagree on meaning & significance of
test results.
100. What is neutron Radiography?
Radiographic inspection using a stream of neutrons known as neutron Radiography.
101. Give the advantages of Radiographic inspection.
The ability to detect internal flows, An ability to detect significant variations in composition, Permanent
recording of raw inspection datas.
102. Give some advantages of Brittle coating method.
Provides nearly whole field area, Is non destructive if the coating is sensitive enough, Is simple to analyze
103. Give some advantages by using fiber optic sensors.
Light weight (very small in size), Low power requirement, Resistant to electro magnetic interference,
High sensitivity