(2) ultrasonic testing

Ultrasonic testing of welds.UT is now a primary means of NDE . Of the big five NDE methods, only UT and RT can reveal flaws which are substantially subsurface:Because the propagation of US is essentially a mechanical phenomenon, it is especially suited to determining characteristics of Engg. Materials.The major NDE applications includes:flaw detection thickness measurement, and metallurgical characterization

The principal advantages are:1. Its ability to penetrate to greater

depths.2. Its ability to test from one surface3. Its sensitivity to detect minute flaws4. Its comparative accuracy in

determining flaw size and location5. Its electronic operation for rapid and

automated inspection.The principal disadvantages are:

6. Technicians’ considerable native ability, training, experience,and motivation,

7. Good coupling, surface roughness, geometry

Ultrasonic testing of welds.Selection of Probe angle: Based on grove angle.Probe angle = 90- /2 where is the grove angle.

Common practice of selection of angle:

1. Based on grove angle2. As per AWS 0-30 mm thick 70

degree3. 30-40 mm “” 60 “”4. > 40 mm “”” 45 “”5. Finally as per approved

procedure

Selection of Frequency:1. Depends on job thickness and beam

path2. Surface roughness, & grain size3. Size of discontinuity sought.4. In practice the following is the most

suited frequency range: 2.25 to 4.0 M.Hz5. 2.25 M.Hz is the most ideal as it detects

the discontinuities comparable to calibration standard, without producing very fine indications which are of not interest for acceptance or rejection,greater penetration,good S/N ratio.

The Procedure 1. Study the weld details- process, edge

ppn, backing strip, grove angle, 2. Remove all loose scales, loose paints,

spatter, 3. Scan the entire surface intended for

angle beam probe’s travel up to the weld with normal beam for possible lamination and other gross discontinuities.

4. Study the procedure for UT inspection.

Selection of equipment scanning range:1. Calculate the HSD & FSD and HBP &

FBP HSD (OA) =t tan : FSD(OB) = 2t tan ; HBP(AO’) = t/cos and FBP(BA’O)= 2t/cos

2. Set the range to approx. to 1.25 times of FBP= 1.25 BA’O

B A O

A’ O’

Defect sizing & reference blocksIn UT the relative size of the defect is compared with a known artificial flaw or reflector.There are many methods of sizing in practice, and the common most methods are DGS SCALES, AND a reference block with a SDH hole.DGS STANDS FOR DISTANCE, GAIN, & SIZEASME BPV CODE AND API, AWS THE SIZE IS ESTIMATED WITH SDH

ASME REFERENCE BLOCKS FOR WELD DEFECT SIZING A DAC CURVE IS DRAWN WITH A KNOWN SIZE OF SDH. THE SAME HOLE SERVES AS A REFERENCE REFLECTOR AT DIFFERENT PATH SHOWING REDUCED AMPLITUDE AT INCREASED BEAM PATH.

DAC CURVE.ON A CRT SCREEN

SCANNING TECHNIQUES.

SIN = ID/ODTHE THE LIMITING ANGLE SHALL NOT EXCEED THE CALCULATED VALUE, BUT CAN BE LESS.

Full skip = 2R sin-1{Wt sin /2r}/180 Wt = full path length=2R sin (-)/sinAnd sin = R sin /rR,r outer & inner radii, included angles

r

R

Weld scanning for tubular products, off-shore plat form structures.

ZONEST= TOE,S= SIDE, H=HEEL PROBE IS TRAVERSED ESSENTIALLY

TS

H

SPECIAL TECHNIQUES;TANDEM TECHNIQUE;(ONE BEHIND THE OTHER) to detect vertical cracks.

The distance between the probes =oy-ox = 2 tan (t-d).

y xtd

O

Pitch and catch technique-to detect transverse crack

T

R

TOFD-DIFFRACTION AT THE END OF CRACK TIPS.

DIFFRACTION-BENDING OF WAVES OVER AN OBSTACLE (SAY THE CRACK) WITH IN THE MEDIUM. TOFD TO LOCATE CRACK TIPS.

SIZING THE DEFECTS FOR ACCEPTANCE/REJECTION.

ACCEPTANCE CODES SUCH AS ASME, API, AWS,DIN, BS SPECIFY THE SIZE OF EACH DISCONTINUITY CONSIDERING THE TYPE OF LOADING/STRESS DIRECTION ETC.SO THERE IS NO COMMON ACCEPTANCE CODE FOR ALL THE ENGG. PRODUCTS.

IN UT ACTUAL SIZE CAN NEVER BE ESTIMATED. BUT THE ESTIMATED SIZE IS SMALLER THAN THE ACTUAL SIZE.BECAUSE: 1. DEFECTS’ REFLECTING CHARACTERISTICS DO NOT MATCH WITH THE REFERENCE ARTIFICIAL REFLECTORS.THE PERCENTAGE OF REFLECTED INTENSITY IS HIGHER FOR REFERENCE REFLECTOR – SMOOTH MACHINE MADE SURFACE, STEEL-AIR INTERFACE,NORMAL INCIDENCE OF US WAVES.DEFECT-ROUGH IRREGULAR SURFACE, NOT STEEL AIR INTERFACE ,PERFECT NORMAL INCIDENCE CAN NEVER BE ACHIEVED BECAUSE OF ORIENTATION.

Interpretation of test results;The percentage of energy reflected back

to the transducer depends on:1. The acoustic impedance mismatch2. The orientation of discontinuity w.r.t to

the direction of US beam3. The surface profile of the

discontinuity-smooth and plane, irregular or curved

4. The size of the discontinuity.

Ultrasonic testing of off-shore platform structures: RP 2XOFF-SHORE STRUCTURES REQUIRE EXTENSIVE USE OF T,K,Y TUBULAR MEMBER INTERSECTIONS. US IS THE BEST MTHOD TO INSPECT THE WELDED AREA.RT IS THE MOST SENSITIVE TO THREE DIMENSIONAL DISONTINUITIES SUCH AS INCLUSIONS, POROSITY. CRACKS, LOF, ARE LESS RELIABLY DETECTED ESPECIALLY WHEN ORIENTED A FEW DEGREES ASKEW OF THE RADIATION BEAM.AS THE THICKNESS OF THE WELD INCREASES, THE QUALITYOF THE DISCONTINUITY IMAGE DECREASES DUE TO RADIATION QUALITY, SCATTERING EFFECTS, ETC.,TKY TRANSPARENCY SHEETS.

UT ON TUBULAR PRODUCTS:API RP2-X

WELD PROFILES:

RP-2X TRANSFER LOSS CORRECTION.

REFERENCE BLOCK

00

A BC D

TRANSFER LOSS -CONTD.ON THE JOB SURFACE;

A A ON REFERENCE BLKBB ON JOBTHE DIFF. IN dB is transferLoss in fig.

A

ABB

Ultrasonic reject criteria API RP2X

LEVEL A – WORKMANSHIP QUALITY-NOT INHERENTLY RELATED TO THE COMPONENTS’S FITNESS FOR PURPOSE- BUT BASED ON THE QUALTIY OF WELD THAT WELDER SHOULD BE ABLE TO ACHIEVE.LEVEL C- EXPERIENCED BASED FITNESS FOR PURPOSE QUALITY-BASED UPON GENERALIZED CONSIDERATION OF FATIGUE, BRITTLE FRCTURE, & TENSILE INSTABILITY MODES OF FAILURE –SEE API RP2A

CONTD…LEVEL F

SPECIFIC “FITNESS FOR PURPOSE” QUALITYBASED UPON SPECIFIC ANALYSIS OF FATIGUE, BRITTLE FRACTURE, TENSILE INSTABILITY AND ANY OTHER POTENTIAL FAILURE MODES.