Non destructive testing II

ULTRASONIC TESTING

Ultrasonic inspection is a non-destructive testing method in which hogh frequency sound waves are introduced into the metal being inspected and the sound emerging out of the test specimen is detected and analyzed.

Ultrasonic waves are mechanical vibrations of the pparticles of the medium in which they travel.most ultrasonic testing is done at frequencies between 0.5 and 25 MHz well above the range of human hearing which is about. 20Hz to 20kHz.The waves are represented by a certain amplitude , frequency , velocity.The velocity is different in different materials.Also different materials offer varying degrees of resistance to the passage of sound through them.This is called acoustic impedance of the materials.Ultrasonic waves are reflected , refracted , scattered or absorbed according to well known laws.Sound waves are usually generated utilizing the piezoelectric effect in some materials.

The general procedure for ultrasonic resting involves applying the piezoelectric crystal to the surface of the test specimen through the use of an intemediate coupling medium , generating and sending the ultrasound pulses into the test specimen , detecting the ultrasound either after transmission through the test specimen or after reflection from interfaces and boundries within the test specimen and finally analyzing and intepreting the recieved sound in terms of internal condition or other properties of the tested medium.

Non destructive testings

Liquid Penetrant Test

This is a method which can be employed for the detection of open to surface discontininuties in any induindual product which is made from a non-porous material.In this method a liquid penetrant is applied to the surface of the product for a certain predetermined time,after which excess penetrant is removed from the surface.The surface is then dried and a developer is applied to it.The penetrant which remains in the discontinuity is absorbed by the developer to indicate the presence as well as the location,size and nature of the discontinuity.

Four stages of liquid penetrant process
a. Penetrant application and seepage into the discontinuity
b. Removal of excess penetrant
c. Application of developer
d. Inspection for the presence of discontinuities


Magnetic particle test

Magnetic particle testing is used for the testing of materials which can be easily magnetised.This method is capable of detecting open to surface and just below surface flaws.In this method the test specimen is first magnetised either by using a permanent magnet or an electric cuurent through or around the specimen.The magnetic field thus introduced into the specimen is composed of magnetic lines of force.Whenever there is a flaw which interrupts the flow of magnetic lines of force,some of these lines must exit and re-enter the specimen.These points of exits and re-entry from opposite magnetic poles and whenever minute magnetic particles are sprinkled onto the surface of the specimen, these particles are attracted by these magnetic poles to create a visual indication approximating the size and shape of the flaw.

a). The effect of defects on the flux flow in a cylindrical specimen

b). The effect of defects on the flux flow ina magnetized ring

Gravity facts

• Objects with mass are attracted to each other, this is known as gravity.
• Gravity keeps Earth and the other planets in our solar system in orbit around the Sun. It also keeps the Moon in orbit around Earth.
• Tides are caused by the rotation of the Earth and the gravitational effects of the Moon and Sun.
• Because Mars has a lower gravity than Earth, a person weighing 200 pounds on Earth would only weigh 76 pounds on Mars.
• It is thought that Isaac Newton’s theories on gravity were inspired by seeing an apple fall from a tree.
• While Newton’s older law of universal gravitation is accurate in most scenarios, modern physics uses Albert Einstein’s general theory of relativity to describe gravity.
• Acceleration of objects to due to the gravity on Earth is around 9.8 m/s2. If you ignore air resistance (drag) then the speed of an object falling to Earth increases by around 9.8 metres per second every second.
• The force of gravity 100 kilometres (62 miles) above Earth is just 3% less than at the Earth’s surface.
• The human body can handle increased g-forces as seen in activities such as dragster races, airplane acrobatics and space training. The highest known acceleration voluntarily experienced by a human is 46.2 g by g-force pioneer John Stapp.
• While formula one racing drivers may feel around 5 g’s under heavy braking, they can experience over 100 g’s if a crash causes them to decelerate extremely quickly over a very short distance.
• Some roller coasters have been known to include g-forces of around 4 to 6 g.
• The higher something is, the greater its gravitational potential energy. Back in the Middle Ages, weapons called trebuchets were used to take advantage of this principle, using mechanical advantage and the gravitational potential energy of a counterweight to hurl rocks and other projectiles at or over walls. In modern times we use the gravitational potential of water to create hydroelectricity.

Ballast tank

A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used asballast to provide stability for a vessel. Using water in a tank allows for easier adjustment of weight than stone or iron ballast as was used in older vessels.

What is an OD box....???what is the purpose....???

oil distribution box is fitted on the intermediate shaft. A set of two aligned pipes axially located in the propeller shaft distributes the servomotor oil from the oil distribution box to the pitch changing servomotor located in the propeller hub.

The movement of the servomotor piston via hub setting mechanism changes the pitch of the blades. The hub internal mechanism is lubricated with oil from the gravity tank. The oil is distributed to the lubricating area of the oil distribution box via an axial hole in the shaft (outside of the two aligned pipes).

The propeller hub is fitted on the&nbsp;back end of the propeller shaft. The propeller shaft is connected with the oil distribution box shaft by a shrink coupling.<BR>In the hydraulic system there are two pumps (including a stand-by one) driven by electrical motors.

A special stop valve in the propeller shaft ensures maintaining a fixed pitch of the propeller when the oil pressure goes down. After stopping the main motor the pitch can be set at any position with the use of a manual pump.