ultrasonic testing

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 surface

3. Its sensitivity to detect minute flaws

4. Its comparative accuracy in determining

flaw size and location

5. 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 θ = 90- α/2 where α is the grove angle

Common practice of selection of angle:

1. Based on grove angle

2. As per AWS 0-30 mm thick 70

Selection of Frequency:

1. Depends on job thickness and beam path

2. Surface roughness, & grain size

3. Size of discontinuity sought

4. In practice the following is the most suited

frequency range: 2.25 to 4.0 M.Hz

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

Defect sizing & reference blocks

In 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 θ = ID/OD

SHALL NOT EXCEED THE

CALCULATED VALUE, BUT

CAN BE LESS.

Full skip = 2πR sin-1{Wt sin α/2r}/180

Wt = full path length=2R sin (α-β)/sinβ

And sin β= R sin α/r

R,r outer & inner radii, α β γ included angles

r

R

α β γ

Weld scanning for tubular

products, off-shore plat form structures

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 mismatch

2. The orientation of discontinuity w.r.t to the

direction of US beam

3. 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 2X

OFF-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:

TRANSFER LOSS -CONTD.

ON THE JOB SURFACE;

B

Ultrasonic reject criteria API RP2X

INHERENTLY RELATED TO THE

COMPONENTS’S FITNESS FOR PURPOSE- BUT BASED ON THE QUALTIY OF WELD THAT

WELDER SHOULD BE ABLE TO ACHIEVE

PURPOSE QUALITY-BASED UPON

GENERALIZED CONSIDERATION OF FATIGUE, BRITTLE FRCTURE, & TENSILE INSTABILITY MODES OF FAILURE –SEE API RP2A

LEVEL F

SPECIFIC “FITNESS FOR PURPOSE” QUALITYBASED UPON SPECIFIC ANALYSIS OF

FATIGUE, BRITTLE FRACTURE, TENSILE

INSTABILITY AND ANY OTHER POTENTIAL FAILURE MODES