Ultrasonics - Civil Engineering (CE) PDF Download

Ultrasonics
Sound waves whose frequency is higher than 20000 Hz is called Ultrasonic waves. Non -Destructive testing: The method by which the internal flaw of the given material can be detected without damaging the material is called Non destructive testing. Out of various methods used, Ultrasonic methods are more commonly used in Non-destructive testing.

Ultrasonic method for Non-Destructive testing
If the material has an internal flaw then the material is non-homogeneous. When the ultrasonic waves are passed through this material, some part of the incident waves are reflected back at the region of change in density and the remaining part is transmitted. The ratio of the reflected and transmitted intensities of ultrasonic waves are given by the expression

where,I₂ and I₂ are the intensities of reflected and transmitted ultrasonic waves respectively.
ρ₂ and ρ₁ are the density of medium 2 and 1 respectively.
C₂ and C₁ are the Ultrasonic wave velocities in medium 2 and 1 respectively.
If C₁ ∼= C₂ then, the expression becomes

Thus, the intensity of the reflected wave is dependent on the difference between the densities of the mediums for a given incident wave intensity.

Principle:
When the ultrasonic waves travel from one medium to other of different density then, some part of incident waves gets reflected. By noting the time elapsed between the instant of transmission and reflected wave we can find out the location of change in density. If the change in density is unexpected at that location then it indicates the presence of an internal flaw. The reflected wave can can tell us about the location of an internal flaw. The reflected wave is echo. Hence this testing is called pulse-echo method.

Working procedure: 

The testing set up consist of the pulse generator, piezoelectric crystal, signal amplifier and CRO (Cathode Ray Oscilloscope). The experimental setup is as shown.

The pulse generator will generate a pulse of signal whose frequency is same as the Ultrasonic frequency. The pulse generator is connected to CRO (time base) which detects a pip ‘P’ the incident pulse.

This signal is fed to a transducer (Piezoelectric crystal) which converts the electrical signal to mechanical vibration. The piezoelectric crystal is in contact with the material which is being tested. If there are any air gaps between the contact of piezoelectric crystal and the material then the energy is dissipated. To avoid this dissipation of energy a thin film of oil is placed between the piezoelectric crystal and the material.

The vibrations are passed through the material and they are reflected wherever there is change in density or discontinuity. There is a discontinuity at the boundary hence the wave gets reflected at B. Point A has internal damage which reflects some amount of light. After the pip P the Q and R are due to reflection at the region of change in density at the location of internal flaw and at boundary. The distance MAN is less than MBN hence we get pip Q before R. By noting down the spacing of Q and R from P we can calculate the location of the internal flaw.

Experimental determination of elastic constants in solids
 Determining the velocity of ultrasonic waves in solids:
In solids sound waves can exists as longitudinal and transverse waves hence there are two velocities
(i) Longitudinal velocity(C_L)
(ii) Transverse velocity(C_S)

By cutting the piezoelectric crystal in a suitable manner we can either generate longitudinal or a transverse waves. Therefore, we should find both the Longitudinal Velocity (C_L) and the Transverse velocity (C_S) for ultrasonic waves in case of solids.

If the piezoelectric crystal is cut in such a way that it generates longitudinal waves. This longitudinal wave which is incident gets reflected back at the boundary due to change in density. By knowing the dimension of the specimen we can know the distance d travelled from the point of incidence to the boundary where it gets reflected. The reflected wave travels a distance d before it is received by the piezoelectric crystal. If t is is the time taken for the transmitted wave to be received, then in time t the total distance travelled is 2d from the transmitting end to the receiving end. Therefore, the velocity CL = 2d/t . Similarly the shear velocity C_S can be obtained if the piezoelectric crystal is cut in such a way so as to give out transverse waves.

Determination of elastic constants in Solids:
In case of transverse waves, the velocity is 

From equation 9.1 and 9.2 we get

From equation 9.1 we get,

Thus by knowing C_s and C_L we can find q, η and σ where
q=Young’s modulus
η=Rigidity modulus
σ = Poissons ratio = lateral strain/longitudinal strain

Experimental determination of elastic constants in liquids

Velocity of Ultrasonic waves in liquids:
To find the velocity of ultrasonic waves in liquids we set up an experiment as shown. Here, we have a water tight flask which is transparent and filled completely with water. Inside the flask at the bottom we keep a quartz crystal between two plates. . The plates are connected to a function generator (oscillator). When the frequency of the oscillator matches with the natural frequency of the quartz crystal it gives resonant vibration. These vibrations travel through the liquid

The waves are reflected back by a reflector at the top of the flask. The reflected waves superimpose with the incident waves and they form stationary waves. The density of water increase at the nodes and decreases at the antinodes. Due to increase in density at the nodes they become opaque (doesn’t allow light) where as antinodes allow light to pass through them. Thus, the liquid in the flask behaves like a diffraction grating with slits closely spaced. Hence, diffraction takes place such that

where 1/N =d=grating constant(distance between 2 lines on the grating)
In this the grating constant d is the distance between two nodes which is λ_u/2 , where λ_u and λ_L are wavelengths of ultrasonic waves and sodium light respectively.

and distance between two nodes d is given by hence

 Therefor
λ_u = 2nλL/sin θ 
and the velocity of ultrasonic waves is

Thus we have determined the velocity of ultrasonic waves.

Determination of elastic constants in liquids:
In liquids the velocity of ultrasonic waves are given by 
∴ the bulk modulus   thus knowing the density ρ and the velocity of the ultrasonic waves in the liquid we can find the bulk modulus of that liquid.