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Comparison of Requirements for Sensitivity Verification in Color-Developed Penetrant Testing According to Domestic and International Standards
Release Date:
2021/09/04
Penetrant testing, as one of the five conventional nondestructive testing methods, is widely used in the fields of pressure-containing special equipment and civil nuclear safety equipment.
Penetrant testing, as one of the five conventional nondestructive testing methods, is widely used in pressure-containing special equipment and civil nuclear safety equipment. Among the various penetrant testing techniques, the manually operated solvent-removable dye-penetrant method combined with solvent-suspended developer is the most extensively applied. This paper takes this most common penetrant testing method as an example to examine sensitivity-related issues in its practical application; compares the differences in sensitivity requirements stipulated in the commonly used penetrant testing standards for pressure-containing special equipment and civil nuclear safety equipment in the United States, Europe, and China; systematically reviews and analyzes the advantages and disadvantages of these standards; and puts forward recommendations for reference by penetrant testing personnel.
Usage of Penetrant Testing Standards
"The American Standard"
In the United States, the most widely used penetrant testing standards for pressure-containing special equipment and civil nuclear safety equipment are the ASME (American Society of Mechanical Engineers) series of standards and the ASTM E165 Standard Practice for Liquid Penetrant Examination for General Industry. The ASME standards (taking the 2021 edition as an example) specify only one type of penetrant testing reference block, whose structure is broadly similar to the Type A aluminum alloy reference block described in China’s JB/T 6064-2015 “Nondestructive Testing—General Specification for Penetrant Test Blocks,” as shown in Figure 1. According to the ASME standard, this reference block is primarily used for qualifying penetrant testing procedures under non-standard temperature conditions. Whether in U.S., European, or Chinese standards, the methods and requirements for qualifying penetrant testing techniques are essentially the same.
Figure 1 Schematic diagram of the structure of the Type A aluminum alloy comparison test block
The ASME code does not explicitly specify sensitivity requirements for penetrant testing agents; however, ASTM E165–2018 states that any penetrant testing agent manufactured by a listed supplier may be used, provided it is included in the QPL-AMS-2644–2020 Qualified Products of Inspection Materials, Penetrant list (i.e., no mixing or substitution is permitted). Furthermore, ASTM E165–2018 references ASTM E1220–2016, Standard Practice for Visible Penetrant Testing Using Solvent-Removable Process, which expressly stipulates that, unless otherwise approved by contract or by a Level III personnel, only penetrants that meet the requirements of AMS-2644–2020 Inspection Material, Penetrant shall be used. This standard does not assign sensitivity grades to dye-penetrant testing agents; instead, it distinguishes between agents that are listed on the qualified list and those that are not. The standard mandates that the sensitivity of dye-penetrant testing agents be evaluated using the aluminum alloy comparison test block shown in Figure 1. Specifically, one section of the Type A test block is coated with the reference penetrant, while the other section is coated with the penetrant whose sensitivity is to be verified; the results from the two sections are then compared. A penetrant that yields detection results equal to or better than those obtained with the reference penetrant is deemed acceptable, whereas a penetrant that performs worse is considered unacceptable. The reference penetrant is specified in AMS-2644–2020, and for verifying the sensitivity of solvent-removable dye-penetrant systems, VP-PE, R-2, and D-2 penetrants are designated as the reference agents. The procedure for conducting the sensitivity test is also prescribed in AMS-2644–2020. For solvent-removable dye-penetrant plus solvent-suspended developer systems, the standard specifies a penetration time of 5 minutes, followed by removal of excess penetrant from the test block through sequential wiping: first with a dry cloth, then with a damp cloth moistened with cleaning solvent, and finally with a dry cloth again. After removing the excess penetrant, the test block must be allowed to air-dry at room temperature for 5 minutes, after which a thin, uniform layer of D-2 developer is applied by spraying to develop the indications, with a development time of 5 minutes.
European Standard
In Europe, the most widely used standard for penetrant testing in the field of pressure-containing special equipment is ISO 3452-1:2013 Non-destructive testing—Penetrant testing—Part 1: General principles. The French standard RCC-M, “Rules for the Design and Construction of Mechanical Equipment in the Nuclear Island of Pressurized Water Reactor Nuclear Power Plants,” does not specify particular penetrant testing methods; instead, it refers to the provisions of other standards. Both the 2000+2002 addendum and the 2007 edition cite the requirements of NF EN 571-1:1997 Non-destructive testing—Penetrant testing—Part 1: General principles; whereas the 2016 edition cites the requirements of ISO 3452-1:2013. According to ISO 3452-1:2013, the sensitivity grade of penetrant testing agents shall be Type 1 as specified in ISO 3452-3:2013 Non-destructive testing—Penetrant testing—Part 3: Reference test blocks.
The measurement is performed using a reference test block whose structure is broadly similar to Type 1 reference test block specified in China’s national standard GB/T 18851.3-2008, “Nondestructive Testing—Penetrant Testing—Part 3: Reference Test Blocks,” as shown in Figure 2. The international standard ISO 3452-2:2013, “Nondestructive Testing—Penetrant Testing—Part 2: Examination of Penetrant Materials,” specifies the detailed procedures for sensitivity testing.
Figure 2 Schematic diagram of the Type 1 reference test block structure
Standard ISO 3452-2:2013 specifies the use of two Type 1 reference test blocks with nickel–chromium plating thicknesses of 30 μm and 50 μm to verify the sensitivity of solvent‑removable dye‑penetrant testing agents; the reference penetrant is also specified by this standard. When the test object is a solvent‑removable dye‑penetrant testing agent with Level 1 sensitivity, the reference penetrant shall be designated as VP‑1PE, R‑2, or D‑2; when the test object is a solvent‑removable
When performing dye-penetrant testing with a specific penetrant, the reference penetrant models to be used are VP-2PE, R-2, and D-2. Apply the reference penetrant (a Level 3–sensitivity fluorescent penetrant) to both test blocks and record the number of indications corresponding to transverse cracks whose extension reaches at least 80% of the block width; use this number as the baseline. After thoroughly cleaning the test blocks, apply the penetrant whose sensitivity is to be verified to the same two test blocks and again record the number of crack indications under the same conditions, then compare this result with the baseline. If the Type 1 reference block with a nickel–chromium coating thickness of 50 μm detects 100% of the discontinuities, and the Type 1 reference block with a nickel–chromium coating thickness of 30 μm detects no less than 75% of the discontinuities, then the sensitivity level of the penetrant being verified is Level 2. If the Type 1 reference block with a nickel–chromium coating thickness of 50 μm detects no less than 90% of the discontinuities, and there is no limitation on the detection rate for the Type 1 reference block with a nickel–chromium coating thickness of 30 μm, then the sensitivity level of the penetrant being verified is Level 1. The procedure for sensitivity testing is specified in ISO 3452-2:2013. For solvent‑removable dye‑penetrant suspension‑type developers, the penetration time is 5 minutes; any excess penetrant remaining on the test block must be wiped off with a damp cloth moistened with cleaning solvent. Following removal of the penetrant, the test block should be allowed to dry at room temperature for 5 minutes, after which a thin, uniform developer layer should be applied using a D‑2 type developer, with a development time of 5 minutes.
"Chinese Standards"
In China, the most widely used penetrant testing standards in the fields of civil nuclear safety equipment and pressure-containing special equipment are NB/T 20003.4-2010 “Nondestructive Testing of Mechanical Equipment in the Nuclear Island of Nuclear Power Plants—Part 4: Penetrant Testing,” NB/T 20328.4-2015 “Another Specification for Nondestructive Testing of Mechanical Equipment in the Nuclear Island of Nuclear Power Plants—Part 4: Penetrant Testing,” and NB/T 47013.5-2015 “Nondestructive Testing of Pressure Equipment—Part 5: Penetrant Testing.” The first two standards are primarily applied in the civil nuclear safety equipment sector, whereas the latter has a broader scope of application. Standards NB/T 20003.4-2010 and NB/T 20328.4-2015 stipulate that the sensitivity level of dye-penetrant testing agents shall be at least Grade 2 as specified in GB/T 18851.2-2008 “Nondestructive Testing—Penetrant Testing—Part 2: Verification of Penetrant Materials.” GB/T 18851.2-2008 specifies the use of Type 1 reference test blocks from GB/T 18851.3-2008 to verify the sensitivity of penetrant testing agents. The sensitivity requirements for dye-penetrant testing agents in NB/T 20003.4-2010 and NB/T 20328.4-2015 are consistent with those set forth in the RCC-M 2000+2002 Addendum and the 2007 edition. In contrast, NB/T 47013.5-2015 does not explicitly specify sensitivity requirements for dye-penetrant testing agents. All three standards—NB/T 20003.4-2010, NB/T 20328.4-2015, and NB/T 47013.5-2015—provide methods for verifying both the sensitivity of the penetrant testing system and the correctness of the operational procedures. These methods employ the three-point Type B test block shown in Figure 3 (where l denotes the test block thickness, which is 3–4 mm). Prior to, during, and after the inspection, inspectors may, if they deem it necessary, use this method to verify sensitivity.
Figure 3 Schematic diagram of the three-point Type B test block configuration
Standard NB/T 47013.5-2015 specifies in detail the length-to-diameter ratio of the crack region on the three-point B-type test block and, based on the distinguishability of the visible crack region, defines three sensitivity levels (A, B, and C). Standards NB/T 20003.4-2010 and NB/T 20328.4-2015, on the other hand, directly reference the relevant provisions of Standard JB/T 6064-2015 and require that all three crack regions be clearly visible (which corresponds to the C-level sensitivity specified in Standard NB/T 47013.5-2015).
Issues Related to the Sensitivity of Penetrant Testing Agents
Penetrant Testing Agent Sensitivity
Although the standard RCC-M 2000+2002 Addendum and the 2007 edition, as well as NB/T 20003.4-2010 and NB/T 20328.4-2015, specify sensitivity levels for penetrant testing agents, they do not clearly address whether, when a newly purchased aerosol-can–packaged dye‑penetrant is supplied with certification demonstrating compliance with the required sensitivity level, the user is still obligated to verify that agent’s sensitivity level using a Type 1 reference test block. According to the method described in AMS-2644-2020, the sensitivity of a penetrant determined by using an aluminum alloy comparison test block, or according to the methods specified in GB/T 18851.2-2008 or ISO 3452-2-2013 and measured with a Type 1 reference test block as defined in GB/T 18851.3-2008 or ISO 3452-3-2013, primarily reflects the sensitivity of the penetrant itself; it does not represent the maximum sensitivity attainable by that particular penetrant, nor does it reflect the sensitivity achievable under the penetrant testing procedure actually employed by the user (since both the test method and the developer are prescribed by the standards, rather than being determined by the user’s chosen procedure and developer).
Process Control Inspection of Penetrant Testing Agents
The structure of Type 2 reference test blocks as specified in ISO 3452-3:2013 and GB/T 18851.3-2008 is shown in Figure 4.
Spray-can penetrant testing agents are characterized by “spray, use, and discard,” as they are sealed under positive pressure and are neither recovered nor reused. Under normal conditions, the penetrant in the spray can remains uncontaminated. Standards such as AMS-2644-2020, ISO 3452-2:2013, GB/T 18851.2-2008, and T/CAMAC 0005-2020 “Nondestructive Testing in Civil Aviation—Penetrant Testing” do not require in-process control inspections of spray-can penetrant testing agents during use.
2.3 Verification of the Sensitivity of the Penetrant Testing System and the Correctness of the Operating Procedures
Verification of the sensitivity of penetrant testing systems and the correctness of their operating procedures is fundamentally different from verification of penetrant sensitivity.
1. Verification of the sensitivity of the penetrant testing system and the correctness of the operating procedures shall be performed using a penetrant whose sensitivity is to be verified, including the developer.
2. Verification of the sensitivity of a penetrant testing system does not require comparison with a reference penetrant; the sensitivity level can be directly determined by the number of crack indications revealed by the penetrant under test.
3. The procedures for verifying the sensitivity of the penetrant testing system and the correctness of the testing procedure are identical to those used by personnel performing penetrant testing. Standards NB/T 20003.4-2010, NB/T 20328.4-2015, and NB/T 47013.5-2015 specify the use of the three-point Type B test block outlined in JB/T 6064-2015 to verify both the sensitivity of the penetrant testing system and the correctness of the testing procedure.
To broaden the measurable sensitivity range, a five-point Type B test block as shown in Figure 5 may also be used (from top to bottom, the “+” symbols denote crack regions 1, 2, 3, 4, and 5, respectively). The three crack regions on the three-point Type B test block roughly correspond to crack regions 2, 3, and 4 on the five-point Type B test block.
Figure 4: Physical specimen of Type 2 reference test block and its structural schematic
According to these two standards, the test block is used for process control inspection of penetrant testing systems; however, process control inspection is not applicable to spray-can penetrant testing systems.
Figure 5 Schematic Diagram of the Five-Point Type B Test Block Configuration
Although the Type 2 reference test block specified in ISO 3452-3:2013 and GB/T 18851.3-2008 also comprises five crack indications, their maximum dimensions range from 3.0 to 5.5 mm, which is narrower than the 1.0–6.0 mm range of the five-point Type B test block. The smallest crack indication on the Type 2 reference test block roughly corresponds to the third crack indication on the five-point Type B test block (i.e., the second crack indication on the three-point Type B test block). Compared with the five-point Type B test block, the Type 2 reference test block offers a narrower sensitivity range and a lower maximum detectable sensitivity. Even when all five crack indications on the Type 2 reference test block are clearly visible, the achievable sensitivity level is only that of Class B as specified in NB/T 47013.5-2015; it thus fails to meet the requirements for penetrant testing system sensitivity set forth in NB/T 20003.4-2010 and NB/T 20328.4-2015. If verification of a higher sensitivity class is required, the PSM-5 test block shown in Figure 6 may be used. This test block is widely employed in process-control inspections for penetrant testing in the civil aviation sector, with its smallest crack indication having a major axis length of 0.38 to 0.79 mm.
Figure 6: Appearance of the PSM-5 Test Block
Whether it is Type B test blocks, Type 2 reference test blocks, or PSM-5 test blocks, they all have the following drawbacks:
1. Cracks on the test specimen are naturally formed under compressive loading applied to the back surface of the specimen. During service, stress inputs such as impact, drop, bending loads, or additional pressure on the back surface can cause these cracks to propagate, increase in size, and even give rise to new cracks.
2. Cracks on the test block have extremely sharp tips, which continuously accumulate penetrant during use, causing the indications for these fine cracks to become increasingly faint or even disappear altogether as the number of inspections increases. This issue can be addressed by ultrasonic cleaning of the test block, but the results are not always satisfactory; ultimately, the test block must still be replaced.
3. The surface roughness of the crack zone on the test block generally does not exceed 2.5 μm, which is superior to almost all surfaces to be inspected. This results in penetrant being very easily removed from the smooth surface of the test block; if the same overpenetrant removal procedure used for the workpiece under inspection is applied to the test block, there is a high likelihood of overcleaning the test block.
4. Cracks on the test blocks form naturally under pressure; therefore, the width and depth of the cracks cannot be precisely controlled, and no two test blocks are exactly alike. This may result in penetration testing agents with the same system sensitivity yielding different sensitivity ratings when applied to different test blocks.
Conclusion
The sensitivity of penetrant testing agents to a certain extent limits the upper bound of sensitivity that penetrant testing can achieve. To ensure that the sensitivity of penetrant testing meets the required standards, the minimum sensitivity grade for penetrant testing agents should be specified during implementation. Furthermore, the sensitivity grade of penetrant testing agents shall be self-declared by the supplier, accompanied by a sensitivity certification report issued by an accredited third-party laboratory as supporting documentation. It is recommended that the relevant authorities, following the model of standard QPL-AMS-2644-2020, establish a national catalog of certified and qualified penetrant testing materials to facilitate the procurement of penetrant testing agents that meet the required sensitivity grades. Verification of the system sensitivity of penetrant testing agents and the correctness of the application procedures is suitable for promotion within the penetrant testing industry; however, using Type B test blocks, Type 2 reference test blocks, or PSM-5 test blocks for such verification yields only a single measure of sensitivity, which does not correspond directly to actual defect detection capability and should therefore be regarded merely as a reference.
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