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Non-destructive testing is a very broad group of testing processes wherein a material is tested without breaking or getting damaged. Non-destructive testing plays a critical role in assuring that structural components and systems perform their function in a reliable and cost-effective fashion. Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research.
Liquid Penetrant Testing aka LPT or LP Testing is a widely applied NDT method, which is used to check surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). It is probably one of the most widely used NDT techniques. Here are the acceptance criteria for liquid penetrant testing.
Acceptance Criteria for Liquid Penetrant Testing:
All surfaces to be examined should be free from:
1. Relevant Linear Indications:
Indications > 1/16” (1.59mm) shall be considered as Relevant Indications, Relevant indications are rejectable if:
● Evaluated as crater cracks or star cracks and > 5/32” (3.96mm) in length.
● Evaluated as cracks other than crater cracks or star cracks.
● Evaluated as IF and >1” (25.4mm) in total length in a continuous 12” (304.8 mm) weld or >8% of the weld length.
2. Relevant rounded indications greater than 5 mm:
For evaluation, the maximum dimension of rounded indications shall be considered as its size. Indications are rejected as defects if:
● Individual indication > 1/8” (3.17mm)
● Distribution of scattered rounded indications exceeds concentration API 1104 fig. 18 or 19.
● Clustered rounded indications > 1/2” (12.7mm)
● Combined length of clustered indications > 1/2” in any 12” (304.8mm) weld.
● Any individual indication in a cluster > 1/16” (1.59mm).
3. Dye Penetrant Inspection – ANSI/ASME B31.1
* Liner Indication:
- Any crack or liner indication.
* Rounded Indications:
- Indications are dimensions > 3/16” (5.0mm).
- Four or more rounded indications in a line separated by 1/16” (2.0mm) or less edge to edge.
- Ten or more rounded indications in any 6 sq. in. (3870 mm2) of the surface with the major dimension of this area not to exceed 6 in. (150mm).
4. Dye Penetrant Inspection – ANSI/ASME B31.3
* Liner Indications:
- Any cracks or linear indications.
* Rounded Indications:
- Pending verification from ANSI. Until otherwise stipulated by the ANSI B31 Committee, apply B31.1 criteria for rounded indications.
5. Dye Penetrant Inspection – ASME Section VIII Div. 1
- Relevant linear indication >1/16” (1.59mm)
- Relevant rounded indications >3/16: (4.77mm)
- Four or more rounded indications in a line separated by 1/16” (1.59mm) or less (edge to edge)
6. Dye Penetrant Inspection – AWS D1.1
- Refer to the table below from AWS D1.1, for the acceptance criteria for Liquid Penetrant Testing, the acceptance criteria for visual testing also applies to Liquid / Dye Penetrant Testing as per AWS D1.1.
This method is applicable in almost all materials except for materials with a porous surface. It is suitable for both casting/raw material and welding inspection. Remember, Liquid Penetrant Testing is a low-cost and more easily adaptable method for all kinds of industries, but without proper training and certification for specific NDT Methods the LPT test is not reliable.
Conclusion:
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