UT Testing-Section 4 Calibration Methods

Content: Section 4: Calibration Methods

4.1: Calibration Methods 4.2: The Calibrations

4.2.1: Distance Amplitude Correction (DAC) 4.2.2: Finding the probe index

4.2.3: Checking the probe angle

4.2.4: Calibration of shear waves for range V1 Block 4.2.5: Dead Zone

4.2.7: Transfer Correction

4.2.8: Linearity Checks (Time Base/ Equipment Gain/ Vertical Gain) 4.2.9: TCG-Time Correction Gain

4.3: Curvature Correction

4.4: Calibration References & Standards 4.5: Questions & Answers

4.1: Calibration Methods

Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the electronics of the equipment must be

calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a "user calibration" of the equipment. This user calibration is necessary

because most ultrasonic equipment can be reconfigured for use in a large variety of applications. The user must "calibrate" the system, which includes the equipment settings, the transducer, and the test setup, to validate that the desired level of (1) precision and (2) accuracy are achieved. The term

calibration standard is usually only used when an absolute value is measured and in many cases, the standards are traceable back to standards at the

In ultrasonic testing, there is also a need for reference standards. Reference standards are used to establish a general level of consistency in

measurements and to help interpret and quantify the information contained in the received signal. Reference standards are used to validate that the

equipment and the setup provide similar results from one day to the next and that similar results are produced by different systems. Reference standards also help the inspector to estimate the size of flaws. In a pulse-echo type setup, signal strength depends on both the size of the flaw and the distance between the flaw and the transducer. The inspector can use a reference

standard with an artificially induced flaw of known size and at approximately the same distance away for the transducer to produce a signal. By comparing the signal from the reference standard to that received from the actual flaw, the inspector can estimate the flaw size.

This section will discuss some of the more common calibration and reference specimen that are used in ultrasonic inspection. Some of these specimens are shown in the figure above. Be aware that there are other standards available and that specially designed standards may be required for many applications. The information provided here is intended to serve a general

introduction to the standards and not to be instruction on the proper use of the standards.

Introduction to the Common Standards

Calibration and reference standards for ultrasonic testing come in many shapes and sizes. The type of standard used is dependent on the NDE application and the form and shape of the object being evaluated. The

material of the reference standard should be the same as the material being inspected and the artificially induced flaw should closely resemble that of the actual flaw. This second requirement is a major limitation of most standard reference samples. Most use drilled holes and notches that do not closely

represent real flaws. In most cases the artificially induced defects in reference standards are better reflectors of sound energy (due to their flatter and

smoother surfaces) and produce indications that are larger than those that a similar sized flaw would produce. Producing more "realistic" defects is cost prohibitive in most cases and, therefore, the inspector can only make an estimate of the flaw size. Computer programs that allow the inspector to create computer simulated models of the part and flaw may one day lessen this limitation.

The IIW Calibration Block

The IIW Calibration Block

The IIW Calibration Block

Find probe angle

The IIW Phase Array Calibration Block

The IIW 2 Calibration Block

Check focal point Check probe angle Check range

The standard shown in the above figure is commonly known in the US as an IIW type reference block. IIW is an acronym for the International Institute of Welding. It is referred to as an IIW "type" reference block because it was

patterned after the "true" IIW block but does not conform to IIW requirements in IIS/IIW-23-59. "True" IIW blocks are only made out of steel (to be precise, killed, open hearth or electric furnace, low-carbon steel in the normalized condition with a grain size of McQuaid-Ehn #8) where IIW "type" blocks can be commercially obtained in a selection of materials. The dimensions of "true" IIW blocks are in metric units while IIW "type" blocks usually have English

units. IIW "type" blocks may also include additional calibration and references features such as notches, circular groves, and scales that are not specified by IIW. There are two full-sized and a mini versions of the IIW type blocks. The Mini version is about one-half the size of the full-sized block and weighs only about one-fourth as much. The IIW type US-1 block was derived the basic "true" IIW block and is shown below in the figure on the left. The IIW type US-2 block was developed for US Air Force application and is shown below in the center. The Mini version is shown on the right.

IIW Blocks- US-1

IIW type blocks are used to calibrate instruments for both angle beam and

normal incident inspections. Some of their uses include setting metal-distance and sensitivity settings, determining the sound exit point and refracted angle of angle beam transducers, and evaluating depth resolution of normal beam inspection setups. Instructions on using the IIW type blocks can be found in the annex of American Society for Testing and Materials Standard E164, Standard Practice for Ultrasonic Contact Examination of Weldments.

ROMPAS Calibration Block

AWS Shear Wave Distance/Sensitivity Calibration (DSC) Block DSC Block, Mini block, Rompas Block are all mini blocks.

A block that closely resembles the miniature angle-beam block and is used in a similar way is the DSC AWS Block. This block is used to determine the

beam exit point and refracted angle of angle-beam transducers and to calibrate distance and set the sensitivity for both normal and angle beam inspection setups. Instructions on using the DSC block can be found in the annex of American Society for Testing and Materials Standard E164,

A block that closely resembles the miniature angle-beam block and is used in a similar way is the DSC AWS Block. This block is used to determine the

beam exit point and refracted angle of angle-beam transducers and to calibrate distance and set the sensitivity for both normal and angle beam inspection setups. Instructions on using the DSC block can be found in the annex of American Society for Testing and Materials Standard E164,

Calibration Range Using DSC AWS Block

The DC AWS Block is a metal path distance and beam exit point calibration standard that conforms to the requirements of the American Welding Society (AWS) and the American Association of State Highway and Transportation Officials (AASHTO). Instructions on using the DC block can be found in the annex of American Society for Testing and Materials Standard E164,

AWS Resolution Calibration (RC) Block

The RC Block is used to determine the resolution of angle beam transducers per the requirements of AWS and AASHTO. Engraved Index markers are provided for 45, 60, and 70 degree refracted angle beams.

The RC Block is used to determine the resolution of angle beam transducers per the requirements of AWS and AASHTO. Engraved Index markers are provided for 45, 60, and 70 degree refracted angle beams.

30 FBH Resolution Reference Block

The 30 FBH resolution reference block is used to evaluate the near-surface resolution and flaw size/depth sensitivity of a normal-beam setup. The block contains number 3 (3/64"), 5 (5/64"), and 8 (8/64") ASTM flat bottom holes at ten metal-distances ranging from 0.050 inch (1.27 mm) to 1.250 inch (31.75 mm).

Miniature Resolution Block

The miniature resolution block is used to evaluate the near-surface resolution and sensitivity of a normal-beam setup It can be used to calibrate

high-resolution thickness gages over the range of 0.015 inches (0.381 mm) to 0.125 inches (3.175 mm).

Step and Tapered Calibration Wedges

Step and tapered calibration wedges come in a large variety of sizes and

configurations. Step wedges are typically manufactured with four or five steps but custom wedge can be obtained with any number of steps. Tapered

Distance/Sensitivity (DS) Block

The DS test block is a calibration standard used to check the horizontal linearity and the dB accuracy per requirements of AWS and AASHTO.

Area Amplitude Blocks provide standards for discontinuities of different size at the same depth

Distance Amplitude Blocks provide standards for discontinuities of same size at the different depth

The ASTM basic set of Area/Distance Amplitude Blocks consists of ten, two inches diameter blocks

The ASTM basic set of Area/Distance Amplitude Blocks consisits of ten, two inches diameter blocks

Distance/Area-Amplitude Blocks

Distance/area amplitude correction blocks typically are purchased as a ten-block set, as shown above. Aluminum sets are manufactured per the

requirements of ASTM E127 and steel sets per ASTM E428. Sets can also be purchased in titanium. Each block contains a single flat-bottomed, plugged hole. The hole sizes and metal path distances are as follows:

• 3/64" at 3"

• 5/64" at 1/8", 1/4", 1/2", 3/4", 11/2", 3", and 6" • 8/64" at 3" and 6"

Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and Type 304 Corrosion Resistant Steel. Aluminum blocks are fabricated per the requirements of ASTM E127, Standard Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Standard Reference Blocks. Steel blocks are

fabricated per the requirements of ASTM E428, Standard Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic Inspection.

Area-Amplitude Blocks

Area-amplitude blocks are also usually purchased in an eight-block set and look very similar to Distance/Area-Amplitude Blocks. However,

area-amplitude blocks have a constant 3-inch metal path distance and the hole sizes are varied from 1/64" to 8/64" in 1/64" steps. The blocks are used to determine the relationship between flaw size and signal amplitude by

comparing signal responses for the different sized holes. Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and Type 304 Corrosion Resistant Steel. Aluminum blocks are fabricated per the requirements of

ASTM E127, Standard Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Standard Reference Blocks. Steel blocks are fabricated per the

requirements of ASTM E428, Standard Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic Inspection.

Distance-Amplitude #3, #5, #8 FBH Blocks

Distance-amplitude blocks also very similar to the distance/area-amplitude blocks pictured above. Nineteen block sets with flat-bottom holes of a single size and varying metal path distances are also commercially available. Sets have either a #3 (3/64") FBH, a #5 (5/64") FBH, or a #8 (8/64") FBH. The

metal path distances are 1/16", 1/8", 1/4", 3/8", 1/2", 5/8", 3/4", 7/8", 1", 1-1/4", 1-3/4", 2-1/4", 2-3/4", 3-14", 3-3/4", 4-1/4", 4-3/4", 5-1/4", and 5-3/4". The

relationship between the metal path distance and the signal amplitude is determined by comparing signals from same size flaws at different depth.

Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and Type 304 Corrosion Resistant Steel. Aluminum blocks are fabricated per the requirements of ASTM E127, Standard Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Standard Reference Blocks. Steel blocks are

fabricated per the requirements of ASTM E428, Standard Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic Inspection.

Key Words:

Distance Amplitude Blocks

DSC Distance sensitivity calibration

DC Distance calibration

SC Sensitivity calibration

Q56: On the area-amplitude ultrasonic standard test blocks, the flat-bottomed

holes in the blocks are:

A. All of the same diameter

B. Different in diameter, increasing by 1/64 inch increments from the No. 1 block to the No. 8 block

C. Largest in the No. 1 block and smallest in the No. 8 block

Q: A primary purpose of a reference standard is:

A. To provide a guide for adjusting instrument controls to reveal discontinuities that are considered harmful to the end use of the product.

B. To give the technician a tool for determining exact discontinuity size C. To provide assurance that all discontinuities smaller than a certain

specified reference reflector are capable of being directed by the test. D. To provide a standard reflector which exactly simulates natural

4.2: The Calibrations

4.2.1: Distance Amplitude Correction (DAC)

Distance Amplitude Correction (DAC): Acoustic signals from the same

reflecting surface will have different amplitudes at different distances from the transducer. Distance amplitude correction (DAC) provides a means of

establishing a graphic ‘reference level sensitivity’ as a function of sweep

distance on the A-scan display. The use of DAC allows signals reflected from similar discontinuities to be evaluated where signal attenuation as a function of depth has been correlated. Most often DAC will allow for loss in amplitude over material depth (time), graphically on the A-scan display but can also be done electronically by certain instruments. Because near field length and

beam spread vary according to transducer size and frequency, and materials vary in attenuation and velocity, a DAC curve must be established for each different situation. DAC may be employed in both longitudinal and shear modes of operation as well as either contact or immersion inspection techniques.

DAC- Distance Amplitude Correction DGS- Distance Gain Size

A distance amplitude correction curve is constructed from the peak amplitude responses from reflectors of equal area at different distances in the same

material. A-scan echoes are displayed at their non-electronically

compensated height and the peak amplitude of each signal is marked on the flaw detector screen or, preferably, on a transparent plastic sheet attached to the screen. Reference standards which incorporate side drilled holes (SDH), flat bottom holes (FBH), or notches whereby the reflectors are located at

varying depths are commonly used. It is important to recognize that

regardless of the type of reflector used, the size and shape of the reflector must be constant. Commercially available reference standards for

constructing DAC include ASTM Distance/Area Amplitude and ASTM E1158 Distance Amplitude blocks, NAVSHIPS Test block, and ASME Basic

The following applet shows a test block with a side drilled hole. The

transducer was chosen so that the signal in the shortest pulse-echo path is in the far-field. The transducer may be moved finding signals at depth ratios of 1, 3, 5, and 7. Red points are "drawn" at the peaks of the signals and are used to form the distance amplitude correction curve drawn in blue. Start by

pressing the green "Test now!" button. After determining the amplitudes for various path lengths (4), press "Draw DAC" and then press the green "Test now!" button.

DAC Java

Developing a Distance Amplitude Correction (DAC) Curve

Distance Amplitude Correction (DAC) provides a means of establishing a graphic ‘reference level sensitivity’ as a function of sweep distance on the A-scan display. The use of DAC allows signals reflected from similar

discontinuities to be evaluated where signal attenuation as a function of depth may be correlated. In establishing the DAC curve, all A-scan echoes are

displayed at their non-electronically compensated height.

Construction of a DAC involves the use of reference standards which incorporate side drilled holes (SDH), flat bottom holes (FBH), or notches whereby the reflectors are located at varying depths. It is important to

recognize regardless of the type of reflector that is used in constructing the DAC, the size and shape of the reflector must be constant over the sound path distance. Commercially available reference standards for constructing DAC include ASTM Distance/Area Amplitude and ASTM E1158 Distance Amplitude blocks, NAVSHIPS Test block, and ASME Basic Calibration Blocks.

Sequence for constructing a DAC curve when performing a straight beam contact inspection on 1 ¾” thick material.

1.) Using a suitable reference standard, calibrate the sweep for a distance appropriate for the material to be inspected, i.e.. using a 1” thick standard, calibrate the sweep for 2” of material travel.

Back Wall Echo

2.) This example represents the use a 1 3/4” thick reference standard with 1/8” side drilled holes located at 1/4 T and 3/4 T respectively. ‘T’ being equal to the block thickness.

3.) Position the transducer over the 1/4T hole and peak the signal to

approximately 80% FSH (Full screen height), mark the peak of the echo on the display using a suitable marker, and record the gain setting.

4.) With no further adjustments to the gain control, position the transducer over the 3/4T hole and peak the signal, mark the peak of the echo on the display.

5.) To complete the DAC curve connect the dots with a smooth line. The

Gain Control for FSH: It should be remember that the dB is a means of

comparing signals. All UT sets are different and a FSH with a gain controls of 36dB in one UT set and be at FSH at another UT set with a gain control

reading of 26dB.

The gain controls allow us to set sensitivity and form the basis of Ultrasonic Sizing Techniques.

Birring NDT Series, Ultrasonic Distance Amplitude Correction - DAC

Alta Vista UT Calibration DAC Curve

Exit Point

Q16: Notches are frequently used as a reference reflector for: A. Distance amplitude calibration for shear wave

B. Area amplitude calibration

C. Thickness calibration for plate

Calibration of shear waves for range V1 Block

Test block 1 for calibrating the time base (depth scale) of a flaw detector for vertical probes

(longitudinal waves) for angle probes (transverse waves), for determining the probe index and beam angle of angle probes, and for checking the short term

consistency of the sensitivity of vertical probes

Shear Wave Distance Calibration IIW Block & DSC Blocks

Exit Point /Range/Probe Angle calibration using IIW Block (Repeat-Code1)

4.2.5: Dead Zone

Determine the dead zone by finding the hole echo which is easily identifiable from the probe noise at the shortest range

Dead Zone

Determine the dead zone by finding the hole echo which is easily identifiable from the probe noise at the shortest range

20 dB Profile

20 dB Profile

4.2.7: Transfer Correction

Methods of compensating for transfer and attenuation loss differences for 0attenuation 000compression probes and for shear wave compression probes. These are based on obtaining similar echo responses on both the calibration block and on the component.

For 0degree probes backwall echoes are used to probes establish transfer and attenuation correction.

For shear wave probes two identical probes are used in “pitch-catch” in order to obtain what are effectively backwall echoes.

either method cannot be used if the either component does not have a convenient parallel section.

Example:

0 degree Probe Calibration

Example:

0 degree Probe Calibration

TRANSFER & ATTENUATION CORRECTION: 0 degree Probes

If the results are plotted on log -linear paper they will form straight parallel lines provided that there is no attenuation difference if an attenuation difference

occurs then the resultant lines will no longer be parallel.

Transfer and Attenuation Correction: Shear Probe

The principle for obtaining transfer correction for shear wave probes is the same as it was for compression probes except that backwall echoes are replaced by pitch --catch responses.

4.2.8.1: Linearity of time base General

This check may be carried out using a standard calibration block eg A2, and a compressional wave probe. The linearity should be checked over a range at least equal to that which is to be used in subsequent testing.

Method

a) Place the probe on the 25mm thickness of the A2 block and adjust the controls to display ten BWEs.

b) Adjust the controls so that the first and last BWEs coincide with the scale marks at 1 and 10.

c) Increase the gain to bring successive backwall echoes to 80% FSH. The leading edge of each echo should line up with the appropriate reticules line.

d) Record any deviations at approximately half screen height. Deviations should be expressed as a percentage of the range between the first and last echoes displayed (ie 225mm).

Tolerance

Unless otherwise specified by the testing standard, a tolerance of ±2% is considered acceptable.

Frequency of checking

Ultrasonic Testing - Horizontal Linearity (Calibration)

4.2.8.2: Linearity of Equipment Gains General

This is a check on both the linearity of the amplifier within the set and the calibrated gain control. It can be carried out on any calibration block

containing a side-drilled hole and should be the probe to be used in subsequent testing. Reject/suppression controls shall be switched off.

Method

 Position the probe on a calibration block to obtain a reflected signal from a small reflector eg 1.5mm hole in the A2 block.

 Adjust the gain to set this signal to 80% FSH and note the gain setting (dB). - Increase the gain by 2dB and record the amplitude of the signal.

- Remove the 2dB and return the signal to 80% FSH. - Reduce the gain by 6dB and record signal amplitude.

- Reduce the gain by a further 12dB (18 intotal) and record signal amplitude. - Reduce the gain by a further 6dB (24 in total) and record signal amplitude.

Tolerance

Frequency of checking

5.2.8.3-1: Linearity of vertical display to EN12668-1

Procedure: Test the ultrasonic instrument screen linearity by altering the

amplitude of a reference input using an external calibrated attenuator and observing the change in the signal height on the ultrasonic instrument

screen. Report the gain setting at the beginning of the test. Check the linearity at prescribed intervals from 0 dB to - 26 dB of full screen height. Repeat the test for centre frequencies for of each filter as measured in 9.5.2. Using the same set-up shown in Figure 6 set the external calibrated attenuator

to 2 dB and adjust the input signal and the gain of the ultrasonic instrument so the signal is 80 % of full screen height. Without changing the gain of the ultrasonic instrument switch the external calibrated attenuator to the values given in the Table 4. For each setting measure the amplitude of the signal on the ultrasonic instrument screen.

Extract from: BS EN 12668-1:2010 Non-destructive testing- Characterization and verification of ultrasonic examination equipment Part 1: Instruments

Figure 6 —

G

4.2.8.3-2: Linearity of vertical display to ASTM E317-01 Vertical Limit and Linearity:

Significance—Vertical limit and linearity have significance when echo signal amplitudes are to be determined from the display screen or corresponding output signals, and are to be used for evaluation of discontinuities or

acceptance criteria. A specified minimum trace deflection and linearity limit may

be required to achieve the desired amplitude accuracy. For other situations they may not be important, for example, go/no-go examinations with flaw alarms or evaluation by comparison with a reference level using calibrated gain controls.

This practice describes both the two-signal ratio technique (Method A) and the input/output attenuator technique (Method B).

Extract from: ASTM E317-01 Standard Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Examination Instruments and Systems without the Use of Electronic Measurement Instruments

Note: Method A: two-signal ratio technique collecting 2 signal from the reflectors of same size at different depth.

Method A:

6.3.2.1 Apparatus—A test block is required that produces two non interfering signals having an amplitude ratio of 2 to 1. These are compared over the

usable screen height as the instrument gain is changed. The two amplitudes will be referred to as HA and HB (HA > HB). The two signals may occur in either screen order and do not have to be successive if part of a multiple-echo pattern. Unless otherwise specified in the requesting document, any test block that will produce such signals at the nominal test settings specified can be used. For many commonly used search units and test conditions, the test block shown in Fig. 1 will usually be satisfactory when the beam is

directed along the H dimension toward the two holes. The method is

applicable to either contact or immersion tests; however, if a choice exists, the latter may be preferable for ease of set-up and coupling

4.2.9: Time Correction Gain (TCG)

Please read:

Q61: The vertical linear range of a test instrument may be determined by

obtaining ultrasonic responses from: A. a set of distance amplitude blocks

B. steel ball located at several different water path distances

C. a set of area amplitude blocks

Q29: Test sensitivity correction for a metal distance and discontinuity area

responses are accomplished by using: A. An area amplitude set of blocks

B. An area amplitude and a distance amplitude set of blocks

C. A distance amplitude set of blocks D. Steel balls of varying diameters.

4.3: Curvature Correction

Curvature in the surface of a component will have an effect on the shape of the ultrasonic beam. The image to the right shows the beam from a focused immersion probe being

projected on to the surface of a

component. Lighter colors represent areas of greater beam intensity. It can be seen that concave surfaces work to focus the beam and convex surfaces work to defocus the

beam. Similar effects are also seen with contact transducers. When using the

amplitude of the ultrasonic signal to size flaws or for another purpose, it is necessary to

correct for surface curvature when it is

encountered. The "correction" value is the change in amplitude needed to bring signals from a curved surface measurement to the flat surface or DAC value.

Convex surfaces work to defocus the beam

Diverge if the surface is convex.

Concave surface contour-Focusing effects

Concave surfaces work to focus the beam

Concave surface contour-Focusing effects

convex surfaces work to defocus the beam

Convex surfaces work to defocus the beam- When sound travels from a liquid through a metal, it will converge if the surface is concave or diverge if the surface is convex.

Q: In an immersion method, the incident sound path enter the specimen

interface with convex geometry, the sound path on entry into the specimen, the convex surface works to

a) De-focus the sound b) Focus the sound

c) Has no effect on the focusing or de-focusing the sound d) Reflected totally all the incident sound.

Q: In transmitting sound energy into a part shown below in a immersion

testing, the sound beam will be:

a) Diverge

b) Converge c) Straight into d) Will not enter

A curvature correction curve can be generated experimentally in a manner similar to that used to generate a DAC curve, This simply requires a

component with a representative reflector at various distances below the curved surface. Since any change in the radius will change the focus of the sound beam, it may be necessary to develop reference standards with a range of surface curvatures.

However, computer modeling can also be used to generate a close

approximation of the curvature correction value. Work by Ying and Baudry (ASME 62-WA175, 1962) and by Birchak and Serabian (Mat. Eval. 36(1), 1978) derived methods for determining "correction factors" to account for

change in signal amplitude as a function of the radius of curvature of convex, cylindrical components.

An alternative model for contact and immersion probe inspection was more recently by researchers at the Center for NDE at Iowa State University. This mathematical model further predicts transducer radiation patterns using the Gauss-Hermite model, which has been used extensively for simulation of immersion mode inspections.

The resulting model allows computationally efficient prediction of the full ultrasonic fields in the component for

1. any frequency, including broadband measurements. 2. both circular and rectangular crystal shapes.

3. general component surface curvature

4. both normal and oblique incidence (e.g., angle beam wedges) transducers. When coupled with analytical models for defect scattering amplitudes, the

model can be used to predict actual flaw waveforms. The image shown above was generated with this model.

The plot to the right shows an example curvature correction curve and DAC curve. This curvature correction curve was generated for the application of detecting a #4 flat bottom hole under a curved surface as shown in the

sketch and photograph. An immersion techniques was used generate a

shear wave since the reflective surface of the target flaw was not parallel with the surface. The DAC curve drops monotonically since the water path

ensures that the near field of the sound beam is always outside the part. The correction factor starts out negative because of the focusing effect of the

curved surface. At greater depths, the correction factor is positive due to the increased beam spread beyond the focal zone caused by the surface

https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/CalibrationMeth/table/table.htm

A table of correction values and the DAC and curvature correction curves for different size radiuses can be found at the following link.

4.4: Calibration References & Standards

Standards are documented agreements containing technical specifications or other precise criteria to be used consistently as rules, guidelines, or

definitions of characteristics, in order to ensure that materials, products, processes, and services are fit for their purpose.

For example, the format of the credit cards, phone cards, and "smart" cards that have become commonplace is derived from an ISO International

Standard. Adhering to the standard, which defines such features as an

An important source of practice codes, standards, and recommendations for NDT is given in the

Annual Book of the American Society of Testing and Materials, ASTM. Volume 03.03, Nondestructive Testing

is revised annually, covering acoustic emission, eddy current, leak testing, liquid penetrant, magnetic particle, radiography, thermography, and

ultrasonics.

There are many efforts on the part of the National Institute of Standards and Technology (NIST) and other standards organizations, both national and international, to work through technical issues and harmonize national and international standards.

Reference Reflectors:

Spherical reflectors are often used in immersion techniques for assessing

sound fields.

1. Omni direction

2. Sphere directivity patterns reduce reflectance as compare with plane reflector

3. Sphere of any materials could be used, however steel balls are often preferred.

Reference Reflectors are used as a basis for establishing system performance and sensitivity.

4.5: Questions & Answers

Exercises

Q80: The 50 mm diameter hole in an IIW block is used to:

(a) Determine the beam index point (b) Check resolution

(c) Calibrate angle beam distance

(d) Check beam angle

Q81: The 100 mm radius in an IIW block is used to: (a) Calibrate sensitivity level

(b) Check resolution

(c) Calibrate angle beam distance (d) Check beam angle

Q6: The Notches are frequently used for reference reflectors for: A. Distance amplitude calibration for shear wave

B. Area amplitude calibration C. Thickness calibration of plate

D. Determine of near surface resolution

Q17: Notches provide good reference discontinuities when UT examination is conducted to primarily detect defects such as:

A. Porosity in rolled plate

B. Inadequate penetration at the root of weld

C. Weld porosity D. Internal inclusion

4.6: Video Time

Birring NDT Series, UT of Welds Part 1 of 2 - CALIBRATION

Birring NDT Series, Ultrasonic Testing # 4, Angle Beam Shear Wave UT as per AWS D1.1