As stated in the introduction, this Handbook concentrates on instrumented inspection techniques.
While visual inspection, tap testing, and liquid penetrant testing are performed on composites, they were not the focus of the ACP Rapid Inspection and Characterization task. Disbonds, voids, AFP defects and fiber tow failures are typically buried within the composite, making visual inspection of limited use. Visual inspection is also extremely slow and tedious, not lending itself to automation techniques, which would reduce the time to design and develop validated composite components. Where visual inspection is suitable, it is noted in Section 4 of this Handbook where appropriate methods to detect a given defect type are discussed.
Tap testing has been demonstrated successfully on composite components, but like visual inspection does not lend itself to rapid inspection techniques like automation and is therefore not
Although commonly applied to metallic materials, liquid penetrant testing methods have applications in NDT and NDE of composite materials as well. Liquid penetrant testing can be implemented as an aid to visual inspection and can enhance the visual assessment of damage to the composite matrix or gel-coat from impact. Radiopaque liquid penetrants have proven useful when applied to composite materials prior to X-ray radiography to improve the contrast of inspection [ref. 1], particularly in the assessment of the depth and extent of impact damage [ref.
2]. Liquid penetrant testing is often a useful NDE tool for inspection of metallic components used in composite systems. One such example is the use of liquid penetrant to inspect thin-walled metallic liners used in COPVs [ref. 3]. Liquid penetrant testing has also proven a useful technique to monitor the performance of erosion and oxidation protective coatings in polymer-matrix and ceramic-matrix composites used in aircraft engines [ref. 4]. Given its relative low costs and ease of implementation, liquid penetrant testing remains a useful NDE tool for engineers and technicians working with carbon reinforced composite materials.
The principle of liquid penetrant testing relies on specially formulated liquid dyes designed to infiltrate surface-breaking discontinuities. After application of liquid dye penetrant to the part, discontinuities are filled with liquid through capillary forces. Excess liquid is then removed from the surface of the part leaving only the liquid trapped within the discontinuity. A developer is then added to the surface of the part, which draws in the liquid out of the discontinuities and provides a contrast background to aid in the examination of indications. Parts are visually inspected and indications are interpreted based on the base material, process, and associated suspected defects.
Rigorous pre-cleaning and proper surface preparation and conditions are critical to the success of a liquid penetrant test. Often parts are also rigorously post-cleaned in order to remove any residual dye and developer. Table 5.5-1 provides an overview of the liquid penetrant process with advantages and limitations of the liquid penetrant testing technique.
Table 5.5-1. Overview of liquid penetrant testing process with identified advantages and limitations.7 Liquid Penetrant Process Advantages of Liquid Penetrant
Testing
Limitations of Liquid Penetrant Testing
7 American Society of Nondestructive Testing, Tracy, N. A., Moore, P. O. (1999) Nondestructive Testing Handbook Volume 2, Liquid Penetrant Testing. Columbus, Ohio: American Society of Nondestructive Testing
Can be implemented on complex part geometries
Sensitive to small surface discontinuities
Can be applied to a wide range of materials
Use of relatively inexpensive, non-sophisticated equipment
Sensitivity can be adjusted through material selection and process control and allows for focus on specific defects of interest for a given material, process, or service condition
Technique is direct visual, not sensor based and does not require measurement of a signal output in comparison to a reference standard
Will only reveal discontinuities open to the surface
Coatings and contaminants must be removed from the surface
Residual or unremoved organic surface contamination can affect inspection
Mechanically disturbed surfaces (e.g., machined, peened, buffed, brushed, etc.) smear the surface of most metals and can mask defects. Etching of the surface is often required for materials with mechanically disturbed surfaces to ensure a reliable test
A reliable test and inspection requires a well-controlled testing, the requirements of ASTM-E1417, Standard Practice for Liquid Penetrant Examination, are commonly employed and often required per NASA standards and requirements [ref. 2].
ASTM-E1417 standardizes a detailed classification system of liquid penetrant materials and processes, personnel qualifications, process and equipment controls, and quality control provisions, thus ensuring a well-controlled and reliable liquid penetrant testing process [ref. 3].
Liquid penetrant testing is a useful NDE technique often applied to monolithic materials and is particularly useful in the identification of weld defects. Figure 5.5-1 shows a fluorescent penetrant inspection of a weld showing micro-cracking in the heat-affected zone of the weld.
Figure 5.5-1. Fluorescent liquid penetrant indications of micro-cracking in weld heat-affected zone.8 References
1. SAE International: Composite Repair NDT/NDI Handbook, SAE Recommended Practice ARP 5089, 2011.
2. Rummel, W. D.; Tedrow, T.; and Brinkerhoff, H. D.: Enhanced X-ray Stereoscopic NDE of Composite Materials, AFWAL-TR-80-3053, 1980.
3. ASTM International: Standard Guide for Nondestructive Testing of Thin-Walled Metallic Liners in Filament-Wound Pressure Vessels used in Aerospace Applications, ASTM-E2982, 2014.
4. National Aeronautics and Space Administration: Erosion Coatings Developed To Increase the Life and Durability of Composites, NASA Glenn Research Center Research and Technology Report, pg. 45, 2004.
5. Tracy, N. A.; and Moore, P. O.: Nondestructive Testing Handbook Volume 2, Liquid Penetrant Testing. Columbus, Ohio: American Society of Nondestructive Testing, 1999.
6. National Aeronautics and Space Administration: Nondestructive Evaluation Requirements for Fracture-Critical Metallic Components, NASA-STD-5009A, 2018.
7. ASTM International: Standard Practice for Liquid Penetrant Inspection, ASTM-E1417, 2016.
8. Smith, Nathan A.: Liquid Penetrant Testing at Goddard Space Flight Center, Greenbelt, Maryland: National Aeronautics and Space Administration Goddard Space Flight Center, 2019.