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The Practical Guide to Ultrasonic Testing In the Real World

©2008 Andrew Cunningham



©2008 Andrew

Cunningham

The Practical Guide to Ultrasonic Testing In the Real World © Andrew Cunningham 2008 All rights reserved. Published by Practical NDT. No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, printed, photocopying, recording, or otherwise, without the prior written permission of the author. Nothing contained in this book is to be construed as a grant of any manufacture, sales or in connection with any method, process, apparatus, product or composition, whether or not covered by letter patent or registered trademark. Printed in Canada by PageMaster Disclaimer The author assumes no responsibility for the safety of persons using the information in this book. This book is for information only.

SAMPLE ONLY To purchase this book please visit IntechNDE.com



©2008 Andrew

Cunningham

Table of Contents Preface ...................................................................................................................................................... Sound ........................................................................................................................................................ Compression Waves ................................................................................................................................ Shear Waves ............................................................................................................................................ Lobes, Cones, and Other Myths .............................................................................................................. Compression and Shear Waves Travel Together...................................................................................

Quick Calculations of the Wavelength (λ) ..............................................................................................

Amplitude and Decibels............................................................................................................................ The Screen Display .................................................................................................................................. The Ultrasonic Flaw Detector................................................................................................................... The Straight Beam Probe and the Dead Zone........................................................................................ Near Zone / Far Zone (Fresnel Zone or Near Field / Far Field) ............................................................. Measuring the Initial Pulse (I.P.) .............................................................................................................. Reflection, Scatter and Acoustic Shadow ............................................................................................... Thickness Measurements ........................................................................................................................ Checking the Probe Prior to Measuring...................................................................................................

Measuring with a 0° Probe....................................................................................................................... Calibrating for Different Alloys ................................................................................................................. Know Your Probes.................................................................................................................................... Using the IIW V1 Block............................................................................................................................. Calibrating to Shear Wave .......................................................................................................................

Scanning with a 0° Probe......................................................................................................................... Sizing Holes in Machined Parts ............................................................................................................... Grid Scan .................................................................................................................................................. Scanning for Internal Pits ......................................................................................................................... Scanning for Pits on a Curved Surface (Vessels and Piping) ................................................................ Scanning for Volumetric Integrity............................................................................................................. Shear Wave Scan..................................................................................................................................... Mode Change, Creeping Wave or Internal Acoustics???....................................................................... Distance-Amplitude Correction (DAC) Curve.......................................................................................... How to Construct a DAC Curve ............................................................................................................... How to use the DAC Curve ...................................................................................................................... Making the Transfer Loss DAC................................................................................................................ Setting Sensitivity ..................................................................................................................................... Plotting System......................................................................................................................................... Locating the Reflector Depth with Simple Mathematics ......................................................................... Finding the Position in Front of the Probe............................................................................................... Finding Depth as a Percentage ............................................................................................................... Sizing Techniques .................................................................................................................................... Sizing for Length with Max Amp .............................................................................................................. Sizing for Length with 6dB Drop .............................................................................................................. Sizing for Length with 20dB Drop ............................................................................................................ Total Drop ................................................................................................................................................. Combination of all Sizing Techniques ..................................................................................................... Cross Sectional Sizing Techniques ......................................................................................................... Finding the True Angle ............................................................................................................................. Cross Sectional Sizing with Maximum Amplitude Technique.................................................................



Interpretation by Cross Sectional Drawing .............................................................................................. Cross Sectional Sizing with 6dB Drop ..................................................................................................... Cross Sectional Sizing with 20dB Drop................................................................................................... Probe Manipulation................................................................................................................................... Echo Interpretation ................................................................................................................................... Cracks ....................................................................................................................................................... Lack of Side Wall Fusion.......................................................................................................................... Lack of Root Fusion.................................................................................................................................. Lack of Interpass Fusion .......................................................................................................................... Slag Inclusion............................................................................................................................................ Isolated Gas Pore ..................................................................................................................................... Porosity Cluster ........................................................................................................................................ Concave Root ("Suck Up") ....................................................................................................................... Bad Pick-Up ("Stop-Start") ....................................................................................................................... Misalignment (High/Low) and, or Mismatch ............................................................................................ Tricks of the Trade.................................................................................................................................... Flanges...................................................................................................................................................... T K Y Joints............................................................................................................................................... End Scans of Welds ................................................................................................................................. Nozzle to Shell Welds and Node Plates .................................................................................................. Sizing "invisible" Flaws............................................................................................................................. Nozzle Testing ..........................................................................................................................................



©2008 Andrew

Cunningham

©2008 Andrew

Cunningham

Preface

During my career, which spans over 30 years, in non-destructive testing and ultrasonics, I have developed many tricks and alternative techniques that, I believe, improve accuracy and results while keeping the job simple and efficient. At the same time, I have also paid attention to whether the data I was seeing corresponded with everything I was taught during my level II training in the UK. I was surprised to find the ultrasonic effects that I saw every day were often never mentioned, and were even contradicted by the accepted texts and tutorials. In this book I have put together some ‘tricks of the trade’, inspection techniques, and beam plot that I thought useful. Parts of this book may sound controversial, but these techniques have been the mainstay of my daily work and have been backed up by many years of experience and experiment. My intent is for this book to be used by the qualified technician so as to improve his skills, the quality of his results and his efficiency in the daily challenges to be met by the NDT industry. Due to some of its controversial content I must caution that it is not recommended for exam preparation. I have endeavoured to make this manual clear, concise and user-friendly. I hope you find this book useful and that it allows you to challenge the more traditional and orthodox techniques we were all taught to enable us to qualify. Please let me know if you disagree or would like to question anything I have written. Your feedback is most welcome. Andrew Cunningham



©2008 Andrew

Cunningham

Quick Calculations of the Wavelength (λλλλ)

The wavelength is determined by the velocity of sound in the material, divided by the frequency (time interval between each cycle) of the probe.

To express this as a formula is λλλλ = = = = v F

Sometimes expressed as λλλλ = = = = c F Examples: To calculate values easily in your head, replace or round up the compression velocity from 5960000mm per second in steel to 6 kilometres per second, and transpose 4,000,000 cycles per second, for example, to 4 MHz. Divide the velocity 6 km per second by the frequency of 4 MHz probe.

λλλλ = Velocity ÷ frequency

λλλλ = 6 (x1, 000,000mm/sec) ÷ 4 (x1, 000,000 cycles/sec)

λλλλ = 6 ÷ 4 mm/cycle

λλλλ = 1½ mm/cycle

Shear wave velocity of 3,240,000mm per second: replace with 3km per second and divide it by probe frequency, for example 4 MHz.

λλλλ = Velocity ÷ frequency

λλλλ = 3 ÷ 4

λλλλ = 3/4 mm

Knowing the wavelength is said to be important. It is also said that the smallest defect that can

be found is ½ λλλλ, 0.1 of a λλλλ , anything larger than grain, twice the grain size to 10 times the grain size (depending on who’s book you read). This may be confusing to the technician, when all the aforementioned are true to a point. The fact is that the smallest defect you can find is irrelevant. The critical statement is, “What is the biggest defect you can miss?” All the mathematical formulae in the world will never compensate for a poor scanning pattern or probe manipulation. Examples will be shown later.



©2008 Andrew

Cunningham

Finding the Position in Front of the Probe

The method I try to use (and advocate to others so as to avoid mathematical errors) is to avoid using mathematics and plotting systems when they are not needed. This is in fact about 90% of the time. Mathematics can be avoided in most situations, apart from when you only have access from one side of the weld, such as when testing tee’s, nozzles and nodes. To determine how far the reflector is in front of the probe without trigonometry, follow these simple steps:

1) Maximize the reflector that you want to position. 2) Mark a line at the front of the probe on the specimen under test. 3) Record the depth at which the signal is maximized (Fig. 73).

Fig. 73

4) Turn the probe around 180°, pull the probe backwards until the signal appears at the same depth regardless of amplitude (Fig. 74).

Fig. 74

5) Mark a line in front of the probe again. 6) Bisect the 2 marks with a dot and the reflector will be directly under the dot.



©2008 Andrew

Cunningham

Fig. 75

As you can see in Figure 75, this method works every time if you have access from both sides regardless of the angle, thickness of the material and surface radius.

Finding Depth as a Percentage

Most of the time, engineers require the defect’s cross sectional dimension together with the wall thickness, to calculate the percentage of the remaining thickness and the joint efficiency. It is possible to plot a reflector, and give its location and cross sectional dimension as a percentage.

Fig. 76 The half skip sound path distance (shown in blue in Fig. 76) is independent of the angle or radius. The position on the screen and beam path travelled is 100% wall thickness, while the 2nd leg shown in red is 100% to 200% This will show on the screen with sufficient gain and is known as Inside Diameter (ID) roll regardless if it is plate or pipe. Some technicians call it “Back Wall”.



©2008 Andrew

Cunningham

Fig. 77

Therefore, if the inner surface reflector (or ID roll) comes up on the screen at 40mm, which is equal to 100% and therefore, the top will be at 80mm and 200%. An indication coming up at 30mm will be 75% down from the top surface. Note: the maths is not pure but it is far more accurate than trying to estimate the angle of a rocking probe. For ease of interpretation, you may adjust the range control so that the ID roll is placed at mid screen. Another method of using the ID roll is to move the ID roll to represent the direct thickness of the test specimen, or multiples there of. For example: The ID roll of a pipe with a 0.450” wall thickness, is to be placed at 4.5, the ID roll from a 20mm thick shell wall is to be placed at 20mm or 40mm or 50mm on the time base

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