NON DESTRUCTIVE AND DESTRUCTIVE TEST

 NON-DESTRUCTIVE TESTING

It is used to describe a method of testing  without affecting the material physically or chemically. 

Non-destructive testing can involve the following methods: 

1. Visual examination 

2.  Radiography test 

3.  Magnetic particles testing 

4.  Dye penetrant 

5.  Ultrasonic test 

6.  Eddy current test

1. VISUAL INSPECTION 

- It start with a visual examination for leaks, wear, cracks, bulges, loosening, and similar signs of trouble. 

- A borescope or fiberscope may be used in hard-to-see vessel areas, such as in tubing of the boiler.

- The boroscope is a slender, handheld tube consisting of a series of lenses.

-  Fibrescope , which consists of bundle of lights and image transmitting fibres within a long flexible sheath having an eye piece lens at one end. 

- Camera can be attached to the eyepiece to obtain a permanent record. 

- Defects found with such instruments include corrosion, blockage of fluid and gas passages, dents, bulges etc.

2. Radiographic Examination

- Radiography  uses X-rays and gamma-rays to produce a radiograph of a specimen, showing any changes in thickness, defects (internal and external).

- The radiation passing through the test object exposes the media, causing an end effect of having darker areas where more radiation has passed through the part and lighter areas where less radiation has penetrated.

- It  can detect porosity, inclusions, cracks, and voids in weld interiors.

- For thinner or less dense materials such as aluminum, electrically generated  x-rays is  used, and for thicker or denser materials, gamma radiation is generally used.

3. MAGNETIC PARTICLE TESTING

- It is a inspection method that provides detection of linear flaws located at or near the surface of ferromagnetic materials. It is viewed primarily as a surface examination method.

- It is a very effective method for location of surface breaking and slight sub-surface defects such as cracking, pores, cold lap, lack of sidewall fusion in welds.

-The magnetic field can be applied with a permanent magnet or an electromagnet. 

- When the magnetic field encounters a discontinuity transverse to the direction of the magnetic field, the flux lines produce a magnetic flux leakage field .

- When very fine colored ferromagnetic particles are applied to the surface of the part the particles will be drawn into the discontinuity, reducing the air gap and producing a visible indication on the surface of the part. 

4. DYE LIQUID PENETRANT INSPECTION 

- The basic principle of liquid penetrant testing is that when a very low viscosity  (highly fluid) liquid (penetrant) is applied to the surface of a part, it will penetrate into  voids open to the surface.

- Once the excess penetrant is removed, the penetrant trapped in those voids will flow back out, creating an indication. 

- The surface being tested is clean and free of any foreign materials or liquids that might block the penetrant from entering voids or fissures open to the surface of the part.

- To perform test -:

a) First apply penetrant on surface and wait for few minutes.

b) Then the part is carefully cleaned to remove excess penetrant from the surface.

c) Then light coating of developer is  applied to the surface and given time ("developer dwell time") to allow the penetrant from any voids or fissures to seep up into the developer, creating a visible indication.

d) Then the part is inspected thoroughly.

5. ULTRASONIC TEST 

- In this test  we apply ultrasonic waves through an object or material. 

- These high frequency sound waves are transmitted into materials to characterise the material or for flaw detecting.

- This beam travels through the object with insignificant loss, except when it is intercepted and reflected by a discontinuity. 

The ultrasonic contact pulse reflection technique -

- This system uses a transducer that changes electrical energy into mechanical energy. 

-The transducer is excited by a high-frequency voltage, which causes a crystal to vibrate mechanically. 

- The crystal probe becomes the source of ultrasonic mechanical vibration. 

-These vibrations are transmitted into the test piece through a coupling fluid, usually a film of oil, called a couplant. 

- When the pulse of ultrasonic waves strikes a discontinuity in the test 

piece, it is reflected back to its point of origin.

- Transducer now serves as a receiver for the reflected energy. The initial signal or main bang, the returned echoes from the discontinuities, and the echo of the rear surface of the test piece are all displayed by a trace on the screen of a cathode-ray oscilloscope.

- It has Greater accuracy than other nondestructive methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces.

6.Eddy Current Test 

- The  principle of eddy-current testing is the measurement of impedance of electron flow in the part being tested. 

- Weak electric currents or eddy currents are induced by a probe containing inducing and sensing coils.

- If there is any defect as pit, crack or thinning will affect the flow of the eddy currents. 

- This change in the flow of the eddy currents will be detected by the sensing coil and displayed on a strip chart display.

- It allows detection of external or internal pits, thickness reduction, cracks, voids, inclusions, dezincification and carbonization.

 DESTRUCTIVE TEST

 

In destructive testing it changes the dimensions or physical and structural integrity of the specimen in order to understand a specimen performance or material behavior under different loads. 

1. TENSILE TEST

- For this standard test pieces are prepared from samples of material; these are then placed in a tensile test machine and loaded to the required values. This enables both the tensile strength end the percentage elongation of the material to be determined.

-This test method is used to determine yield strength, ultimate tensile strength, ductility, strain hardening characteristics, Young's modulus and Poisson's ratio.

2. BEND TEST 

- The bend test is a simple and inexpensive qualitative test that can be used to evaluate both the ductility and soundness of a material. 

- In this test a prepared test piece is bent cold either by hydraulic or other pressure, or by repeated hammer blows.

3.IMPACT TESTING

- It  is performed to determine the impact resistance or toughness of materials by calculating the amount of energy absorbed during fracture.

- Absorbed energy is a measure of the material's notch toughness.

 

- There are two main forms of impact test, the Izod and the Charpy test.

- Both involve striking a standard specimen with a controlled weight pendulum travelling at a set speed. The amount of energy absorbed in fracturing the test piece is measured and this gives an indication of the notch toughness of the test material.

- These tests show that metals can be classified as being either 'brittle' or 'ductile'. 

- A brittle metal will absorb a small amount of energy when impact tested, a tough ductile metal a large amount of energy.

4. HARDNESS TEST

- It is a test to determine the resistance a material exhibits to permanent deformation by penetration of another harder material.

- It evaluate a material’s properties, such as strength, ductility and wear resistance.

- A hardness test is typically performed by pressing a specifically dimensioned and loaded object (indenter) into the surface of the material you are testing.

- The hardness is determined by measuring the depth of indenter penetration.

5. FATIGUE TEST

- It is defined as the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points .

- Fatigue cracks generally start at changes in section or notches where the stress is raised locally.

- The sharper the notch the shorter the fatigue life .