non destructive testing
Introduction to NDT and Penetrant Testing (PT) G.Jothinathan Scientific Officer Gr.I Department of Metallurgical and Materials Engineering I I T M, Chennai 600 036 Email – gjndt@yahoo.com Introduction To NDT Non Destructive Testing (NDT) is a noninvasive method and the testing is carried out without impairing further usefulness of the material, component and structure The component can be put into use after the Testing. The signals that are employed do not alter the properties permanently . Method Signal Remarks UT US waves - elastic waves Elastic deformation RT X-rays and gamma rays No damage during the course of testing Radiation damage!!! MT Magnetic field Remnent field- Demagnetisation - interesting and highly paying field - interdisciplinary of metallurgy, physics and chemistry NDT accounts for 1/3 of the cost of an aircraft (without raw material and fabrication cost !!) Definition of terms NDT NDI Used interchangeably- detection and reporting NDE The scope is broader –serviceability is ascertained Also includes material characterisation Material anomalies-interruption ,break,disruption Discontinuity- can meet the service requirements Defect/flaw - renders unsuitable for service Anamoly whether Discontinuity or defect/flaw 1.Nature of the imperfection –linear-volumetric 2.Material –structural or high strength 3.Criticality of use – storage vessel or boiler boiler in thermal power plant boiler in nuclear power plant Linear -crack – length > 3 times width Volumetric - porosity and inclusion Applications of NDT The main application of NDT is flaw detection and evaluation. It is also used for geometric dimension measurement. material characterization, bond integrity testing , condition monitoring etc Material characterisation: 1. E and μ determination 2. Grain size evaluation 3. Proportion of microstructural phases 4. Extent of deformation 5. Nodularity of nodular cast iron etc 6. Fracture toughness determination Purpose of NDT Fracture mechanics approach – damage tolerance approach (propagates the idea of “live with discontinuities”) A material property, fracture toughness (K Ic ) is defined and this characterizes the material behaviour in presence of discontinuities which the earlier strength of materials design approach has not taken into account. KIc can be experimentally determined. The stress intensity factor (K I ) the stress distribution around a discontinuity is given by K I = G σ √πa where G is geometric constant, σ is the working stress and ’ a’ is flaw size. Failure occurs when K I is equal to or greater than K Ic To determine K I,, flaw size information needed and is provided by NDT personnel. Life extension and life predication methods also require information on the flaw size. APPLIED STRESS YIELD TENSILE STRENGTH (a) The Strength of materials approach APPLIED STRESS FLAW SIZE FRACTURE TOUGHNESS (b) The Fracture Mechanics approach Comparison of classical design philosophy with fracture mechanics approach NDT methods 1.Visual testing (VT) 2.Liquid Penetrant Testing (PT) 3.Magnetic particle Testing (MT) 4.Radiographic Testing (RT) 5.Ultrasonic Testing (UT) 6.Eddy Current Testing (ET) - highly sensitive 7.Acoustic Emission Testing (AET) monitors activity 8.Thermal Infrared imaging (TIFR) 9.Leak testing (LT) Surface NDT Surface &internal Visual Testing (VT) This ancient and original method of examination by human eyes is still widely employed to find gross discontinuities, surface irregularities, roughness and corrosion products on the surface. Many gadgets like lenses, cameras are used. For surfaces inaccessible to the human eyes as in the case of inside surfaces of pipes and boilers, boroscopes and flexible fibroscopes are employed. These are called endoscopes (internal vision). They are nothing but lens arrangement to transfer the image or the optical fibers arrangement for transferring the image. VT of welds(contd) • Mainly for noncritical welds • Before, during and after welding • Before -Surface roughness and cleanliness (oil) • During - Electrode size and welders performance • After -Dimensional accuracy of weldments Conformity of welds to size and contour requirements VT of welds (contd) • Acceptability of weld appearance with regard to: • Presence of surface discontinuities such as: • surface roughness weld spatter • cleanliness underfill pores • Undercuts overlaps • Cracks spatter • Establishing definite procedure to ensure uniformity and accuracy [...]... penetrant and before application developer Inspection and Interpretation Fig.1 Principle of Penetrant Testing Principle Of PT Properties of penetrant The entire penetrant testing is based on the ability of the liquid to penetrate into discontinuities and later ability to come out The required properties are 1 Wettability 2 Capillarity The ability of the liquid to wet the solid surface or spread over the solid... the volume of the penetrant indication will get reduced (The volume of penetrant is already very small) In any NDT method the Signal to Noise ratio is important and this should be as high as possible S- Volume of penetrant that has entered into the discontinuity N- Unremoved excess surface penetrant in the discontinuity It is obvious that in PT to achieve a high S/N ratio, the excess penetrant on the... completely as possible and the penetrant is the discontinuity should not be lost by overwashing Penetrant removal methods Dissolve and remove (SR) 1 Disperse (emulsify) and remove by water PE 2 Simply water wash if penetrant contains built in Emulsifier (WW) In each case, care need be exercised so as to completely remove the excess penetrant on the surface completely and at the same time the penetrant in the... indication Blue-black to yellowish green: white to red 3 To spread the penetrant so as to increase the apparent size of the indication 4 To mask some confusing indications Mechanism of development 3.6.3 Mechanism of developing action : Natural seepage is assisted and the absoptive developers blot out more penetrant from the discontinuity The thickness of the penetrant layer is increased to the levels above... surface penetrant , in developing step, the penetrant from the discontinuity is brought to the surface so as to form an visible indication 3.6.1 Developer functions The developer functions are 1 .To assist the natural seepage of the penetrant in the discontinuity and extract or blot out the penetrant so as to form an visual indication (mechanism of visual indication) 2 To provide a contrasting base which enhances... Visible dye penetrant indications are inspected under white light and fluorescent dye penetrant indications are viewed under blach light (ultraviolet light?) Electromagnetic radiation spectrum Visible and ultraviolet spectrum Visible light spectrum - 380 nm to 770 nm or ( V to R ) or 3800 Ao to 7000 Ao Ultraviolet spectrum - 10 nm to 380 nm or 100 -3800 Ao Black light spectrum - 320 nm to 380 nm or... swabbing and pouring The only requirement is that a thin layer of penetrant should be present for the specified time (dwell time) on the surface to be inspected Dwell time : The total time the penetrant is contact with the test surface including the time required for application and for drain Dwell time = application time + drain time Normally 5-30 mts Depends on the size and nature of discontinuity and. .. the penetrant thereby providing an indication in a contrasting background of white colour of developer 1.Preparation of part 2 Application of penetrant 3 Removal of excess surface penetrant 4 Development 5 Inspection and Evaluation 6 Post Cleaning Clean surface of Component Penetrant application Removal of excess surface penetrant Application of developer After removal of excess surface penetrant and. .. is directly applied to the weld to overcome this difficulty Of course fine cracks may be missed 3 In the case of TIG and MIG where better surface is obtained, it is better to go solvent wiping method 4 Inspection with multiple pass welds with penetrants requires that each weld bead is inspected prior to the next pass to provide the next the best reliability Cleaning after each testing Hints for weld... the material and surface condition of the material etc Removal of excess penetrant The excess surface penetrant on the surface is carefully removed without affecting the penetrant that is residing in the discontinuity This is an important step as the unremoved excess surface penetrant will affect subsequently by affecting the contrast of the indication (excessive background) and if any penetrant in . Chennai 600 036 Email – gjndt@yahoo.com Introduction To NDT Non Destructive Testing (NDT) is a noninvasive method and the testing is carried out without. Introduction to NDT and Penetrant Testing (PT) G.Jothinathan Scientific Officer Gr.I Department of Metallurgical and Materials Engineering I