Nondestructive Testing

Non destructive Testing

Radiography • X-rays or gamma rays are used as probing media. • These are electromagnetic rays capable of penetrating through large thickness of material • Gamma rays obtained from naturally radioactive material • Radium or cobalt 6.

• Gamma radiation are more penetrating than X-rays • However, inferior sensitivity limits its application • No way that source may be regulated for contrast or variable thickness. • Usually requires much longer exposure time than X- ray.

Production of X-rays • When matter is bombarded with rapidly moving stream of electron. • As electron are suddenly stopped by matter, a part of their K.E is converted to energy of radiation X-rays

picture • The essential condition for the production of X-rays are: • A filament (cathode), A target (Anode) • Voltage difference between the Anode and Cathode to regulate the velocity of electron. • A means to control the tube current to control the numbers of electron striking the target.

• A radiograph is a shadow/ picture of material more or less transparent to radiation. • The X-rays darken the region of lower density as compare of high density. • Crack/hole appears as dark area while inclusion appear as light area. • Casting and weld product are inspected • Measure the thickness of the material

Ultra sonic Inspection • Use of sound wave to determine the defects is an ancient method. • Strike the material it radiate certain audible notes • Pitch and damping may be influenced by the presence of internal flaws • Useful for large defects.

Transducer • A device which converts one form of energy to another • Ultrasonic waves are produced by piezoelectric materials. • These materials undergo a change in dimension when subjected to an electric field • This conversion of electrical energy to mechanical energy is known as piezoelectric effect

• When A.C current applied to piezoelectric material • During the first half the material expand • When electric field is reversed it contracts • By varying the frequency of A.C the frequency of mechanical vibration (sound wave) can be varied • Quartz is widely used ultrasonic transducer

Ultrasonic Inspection • Ultrasonic inspection uses a beam of high-frequency sound, in the range of 1 to 5 MHz, • The sound wave is emitted as bursts or pulses of energy vibrations and penetrates in metals/ liquids. • These vibrations travel into the area being inspected with a speed of several thousands feet /second

• They strike or are interrupted by a crack, inclusion, or other discontinuity or by the far side of the material. • When a discontinuity is encountered, some of the sound vibrations are reflected • The larger the discontinuity (crack, porosity, slag inclusion, etc.), • The larger the amount of energy that will be reflected to the transducer.

The transducer converts the returning vibrations into electrical impulses. These are amplified and appear on the screen of a cathode-ray tube (CRT) as indications. The initial pulse on the screen represents the contact face or the testing surfaces The flaw reflects some sound, and the rear surface reflects more sound.

The presentation on the CRT displays the location in the thickness and relative size of the flaw. • The size of the flaw determines the height of the indication • Calibration standard is required to accurately measure defect size.

Methods of ultrasonic testing • • • •

(1) the pulse-echo method; (2) the through-transmission method (3) the pitch-catch method. These three ultrasonic methods use pulses of energy during testing operations.

The pulse-echo method • Piezoelectric transducer mounted on or near the surface of the test material • Used to transmit and receive ultrasonic energy. • The ultrasonic waves are reflected by the opposite face of the material, by discontinuities, layers, voids, or inclusions in the material, The same transducer receive the reflected energy and convert into an electrical signal • .

• The electrical signal is then computer processed for display on a video monitor or TV screen. • The display can show the relative thickness of the material, depth into the material where flaws are located • In aerospace applications, the pulse-echo method is used primarily for the detection of flaws in metals,

The through-transmission method Transmitter is used on one side of the material while a detector is placed on the opposite side. • Scanning of the material using this method will result in the location of defects, flaws, and inclusions • This method is used for nondestructive testing of multi-layered and multi component materials

The pitch-catch method • The ultrasonic energy is transmitted at any angle to the surface of the material • Received as reflected energy returning at the reflected angle, Used primarily for cylindrical tubes and other nonlinear parallel sided surfaces.

limitation of the ultrasonic inspection method • That a permanent record is difficult to obtain during field inspection. • A picture of the CRT and written reports of the inspection results are sometimes difficult to correlate. • Rough surfaces make transducer contact difficult and sometimes impossible. • Calibration standards are usually required to calibrate the instrument

Basic Principles of Eddy Current Inspection • - When alternating current is applied to the conductor, • A magnetic field develops in and around the conductor. • If another electrical conductor is brought into the close proximity to this changing magnetic field, • Eddy current will be induced in this second conductor (Part to be inspected) • The Eddy currents will produce a magnetic field of their own.

• The detection unit will measure this new magnetic field and convert the signal into voltage. • Properties like hardness, alloy composition, chemical purity and heat treatment influence the magnetic field • This change can be measured and calibrated.

• Eddy currents can be used for: • Crack Detection (surface /sub surface), Material Thickness, coating thickness • Materials identification , Case Depth determination , Heat Treatment Monitoring .

Advantages of eddy current inspection Equipment is very portable , Inspection gives immediate results. • Minimum part preparation is required • Test probe does not need to contact the part • Inspects complex shapes and sizes of conductive materials

limitation of eddy current inspection

• • • •

Only conductive materials can be inspected Surface must be accessible to the probe Skill and training required is more extensive than other techniques Reference standards needed for setup Depth of penetration is limited