Understanding the Minimum Defect Size in Ultrasonic Testing

Table of Content

• Acceptance Criteria for the Ultrasonic Testing
• Factors Affecting the Minimum Defect Size
• Practical Considerations

Ultrasonic testing equipment typically consists of a transducer, pulser-receiver, and display unit. The transducer converts electrical energy into mechanical vibrations, which generate ultrasonic waves. In UT, the minimum defect size refers to the smallest imperfection that the equipment and procedures are capable of detecting reliably. This parameter is critical as it determines the effectiveness of UT in identifying defects that could compromise the structural integrity or functionality of a component. The minimum Defect Size Acceptance Criteria for Ultrasonic Testing varies depending on several factors.

Various international standards and regulatory bodies provide Guidelines for Ultrasonic Testing and specify acceptance criteria for defect sizes. These standards ensure consistency and reliability in defect detection across different industries, such as aerospace, automotive, manufacturing, and oil and gas. Additionally, industry standards help ensure that Ultrasonic testing procedures are performed consistently and accurately, leading to reliable results. Adhering to these standards also helps in ensuring the safety and quality of products and structures in various industries

Acceptance Criteria for the Ultrasonic Testing

1. ASME Section VIII Division 1 – Pressure Vessel Construction – Ultrasonic Testing (UT) Inspection Services

These Standards shall apply unless other standards are specified for specific applications within this Division. Imperfections that produce a response greater than 20% of the reference level shall be investigated to the extent that the operator can determine the shape, identity, and location of all such imperfections and evaluate them in terms of the acceptance standards given in (a) and (b) below.

Indications characterized as cracks, lack of fusion, or incomplete penetration are unacceptable regardless of whether other imperfections are unacceptable. If the indications exceed the reference level amplitude and have lengths which exceed:

• 1/4 in. (6 mm) for t up to 3/4 in. (19 mm)
• 1/3t for t from 3/4 in. to 21/4 in. (19 mm to 57 mm); (3) 3/4 in. (19 mm) for t over 21/4 in. (57mm)

where t is the thickness of the weld excluding any allowable reinforcement. For a butt weld joining two members having different thicknesses at the weld, t is the thinner of these two thicknesses. If a full penetration weld includes a fillet weld, the thickness of the throat of the fillet shall be included in t.

2. ASME B 16.34 – Valves – Ultrasonic Testing

Straight Beam Examination: Indications which are equal to or exceed that obtained from a 6.4 mm (0.25 in.) diameter flat-bottomed hole in a calibration test piece of thickness equal to the defect depth are unacceptable.

Angle Beam Examination: Indications which are equal to or exceed those obtained from a 60 deg V-notch, 25 mm (1.0 in.) long, and having a depth not greater than 5% of the nominal wall thickness in a test piece are unacceptable.

3. ASME B 31.1 – Power Piping – Ultrasonic Testing

Welds that are shown by ultrasonic examination to have discontinuities that produce an indication greater than 20% of the reference level shall be investigated to the extent that ultrasonic examination personnel can determine their shape, identity, and location so that they may evaluate each discontinuity for acceptance in accordance with (B.1) and (B.2) below.

(B.1) Discontinuities evaluated as cracks, lack of fusion, or incomplete penetration are unacceptable regardless of length.

(B.2) Other discontinuities are unacceptable if the indication exceeds the reference level and their length exceeds the following:

• (B.2.1) ¼” (6.0 mm) for t up to ¾” (19.0 mm).
• (B.2.2) 1/3 t for t from ¾” (19.0 mm) to 2 ¼ “ (57.0 mm).
• (B.2.3) ¾”. (19.0 mm) for t over 2¼” (57.0 mm)

Where t is the thickness of the weld being examined. If the weld joins two members having different thicknesses at the weld, t is the thinner of these two thicknesses.

4. ASME B31.3 Process Piping – Ultrasonic Testing

Inspection of Weld Joints carried as per ASME Section V, Article 4

Pipe and Tubing

• Pipe and tubing, required or selected in accordance with Table K305.1.2 to undergo ultrasonic examination, shall pass a 100% examination for longitudinal defects in accordance with ASTM E213 Ultrasonic Testing of Metal Pipe and Tubing. Longitudinal (axial) reference notches shall be introduced on the outer and inner surfaces of the calibration (reference) standard in accordance with Fig. 3(c) of ASTM E213 to a depth not greater than the larger of 0.1 mm (0.004 in.) or 4% of specimen thickness and a length not more than 10 times the notch depth.
• Acceptance Criteria. Any indication greater than that produced by the calibration notch representation
• Welds over 6mm thick can be ultrasonically tested in reference to the procedure given in ASME

Sec V article 4.

Indications shall be sized using the applicable technique(s) provided in the written procedure for the examination method. Indications shall be evaluated for acceptance as follows:

• All indications characterized as cracks, lack of fusion, or incomplete penetration are unacceptable regardless of
• Indications exceeding 1/8 in. (3 mm) in length are considered relevant and are unacceptable when their lengths exceed
• 1/8 in. (3 mm) for t up to 3/8 in. (10 mm).
• 1/3t for t from 3/8 in. to 21/4 in. (10 mm to 57 mm).
• (3) 3/4 in. (19 mm) for t over 21/4 in. (57 mm),

where “t” is the thickness of the weld excluding any allowable reinforcement. For a butt weld joining two members having different thicknesses at the weld, t is the thinner of these two thicknesses. If a full penetration weld includes a fillet weld, the thickness of the throat of the fillet shall be included in t.

5. API 1104 Welding of Pipelines & Related Facilities – Ultrasonic Testing – Acceptance level

> Acceptance Standards 9.6.2.1 Indications determined to be cracks (C) shall be considered

> Linear surface (LS) indications (other than cracks) interpreted to be open to the D. or O.D. surface shall be considered defects should any of the following conditions exist:

1. The aggregate length of LS indications in any continuous 12”(300-mm) length of weld exceeds 1”. (25 mm).
2. The aggregate length of LS Indications exceeds 8% of the weld

> Linear buried (LB) indications (other than cracks) interpreted to be subsurface within the weld and not I.D. or O.D. surface-connected shall be considered defects should any of the following conditions exist:

1. The aggregate length of LB indications in any continuous 12”(300-mm) length of weld exceeds 2” (50 mm).
2. The aggregate length of LB indications exceeds 8% of the weld

> Transverse (T) indications (other than cracks) shall be considered volumetric and evaluated using the criteria for volumetric indications. The letter T shall be used to designate all reported transverse indications.

> Volumetric cluster (VC) indications shall be considered defects when the maximum dimension of VC indications exceeds 1/2 “ (13 mm).

> Volumetric individual (VI) indications shall be considered defects when the maximum dimension of VI indications exceeds 1/4 “ (6 mm) in both width and

> Volumetric root (VR) indications interpreted to be open to the D. surface shall be considered defects should any of the following conditions exist:

1. The maximum dimension of VR indications exceeds 1/4“ (6mm).
2. The total length of VR indications exceeds 1/2 “ (13 mm) in any continuous 12” (300 mm)

> Any accumulation of relevant indications (AR) shall be considered a defect when any of the following conditions exist

1. The aggregate length of indications above evaluation level exceeds 2 “(50 mm) in any 12” (300mm) length of
2. The aggregate length of indications above the evaluation level exceeds 8% of the weld.  

Factors Affecting the Minimum Defect Size

Factors Affecting Minimum Defect Size include the material being tested, the type of defect being sought, the frequency and amplitude of the ultrasonic waves used, and the skill and experience of the operator. These factors must be carefully considered to ensure accurate and reliable defect detection in ultrasonic testing. Several factors influence the minimum defect size accepted in ultrasonic testing:

1. Material Properties: The type of material being tested, its density, and its acoustic properties influence the propagation of ultrasonic waves and the detectability of defects.

2. Frequency and Wavelength: The frequency of the ultrasonic waves and their corresponding wavelengths determine the resolution and penetration depth of the testing. Higher frequencies provide better resolution but may have limited penetration, while lower frequencies offer greater penetration but lower resolution.

3. Equipment Sensitivity: The sensitivity of UT equipment, including the transducer and signal processing capabilities, plays a significant role in determining the minimum detectable defect size.

Practical Considerations

Despite advancements in UT technology, accurately measuring the size of defects remains challenging. Factors such as surface roughness, material thickness, and the orientation of defects can affect the accuracy of defect sizing. Advanced techniques, such as phased array ultrasonics and time-of-flight diffraction (TOFD), have improved defect sizing capabilities but require specialised training and equipment.

Additionally, the technician conducting the inspection's skill and experience can have an impact on how to interpret the UT results. It is important for companies to invest in proper training and certification programmes to ensure accurate defect detection and sizing across various industries.