Surface inspection methods

A new design of an old concept of thermal surface inspection permits the photothermal camera to be considered as an interesting alternative to NDT methods like penetrant or magnetic testing, particularly when the testing has to be made in a remote or automatic way. 

Surface Crack Detection Using Magnetic Particle Techniques and Penetrant Inspection Method. 

In wide sectors of industry there is a growing need of inspection methods which go without liquid coupling media. The excitation of bulk and surface waves by means of air-coupled ultrasonic probes is therefore an attractive tool for NDE. This is tme e.g. for the rapid scanning of large composite structures in the aerospace industry [1]. In other cases, the use of liquid couplants is prohibitive like the thickness measurement of powder layers. 

A particularly insidious failure mechanism that is commonly found in carbon-steel tubing is under-deposit corrosion. In many cases, corrosion products fomi a scab that can mask the presence of the pitting, making it difficult to quantitatively assess using conventional NDT methods. However, by combining proper cleaning procedures with laser-based inspection methods, the internal surface of the tubing can be accurately characterized and the presence of under-deposit corrosion can be confirmed and quantified. 

In addition, a very simple and low cost method can be used to detect surface flaws. This is the use of penetrants that are typically fluorescent dyes. Usually, a three-step procedure is used. The ceramic part is first soaked in a fluorescent dye. Then the part is dried and cleaned in a very controlled manner to remove the dye from smooth surfaces but not from the surface defects. When the part is examined under ultraviolet li t, the surface defects such as cracks and porosity retain the dye and show up brilliantly. This method is used widely for surface inspection of ceramics and is frequently included as part of a quality assurance certification. Penetrants are effective for nonporous ceramics. With open porosity, the penetrant will enter all the pores of the ceramic, giving fluorescence to the whole ceramic piece, thus preventing detection of surface flaws. Not all penetrants are fluorescent dyes. Radioactive krypton can be used as a penetrant. It is retained in cracks or other defects and can be detected by either a Geiger counter or by carefully wrapping the ceramic piece in photographic film. After development of the film, the location of cracks emd pores can be detected. 

Figure 15.12 Thermographic inspection method, (a) Surface heating (b) surface emission and heat transfer and (c) effect of internal feature on surface emission.

Electromagnetic methods such as eddy current, capacitance, microwaves, and terahertz radiation are not traditional inspection methods for composites, but they can be used in some circumstances [36]. Microwaves (300 MHz—300 GHz, 1000—1 mm) and terahertz (300 GHz—3 THz, 1—0.1 mm) are applicable to fiberglass composite inspection and have been successful at the detection of damage and internal features [37—45]. However, electromagnetic radiation at these wavelengths does not penetrate conductive materials. For CFRP, which is mildly conductive, they are only useful for sensing very near the surface. Capacitance measurements can be used to measure dielectric property changes in composites such as moisture uptake or cure condition. 

The wipe test is used to detect contaminants on visually inaccessible areas as an aid in complementing the above visual inspections. The surface is rubbed lightly with a clean white paper or lint-free cloth that is then examined under white and ultraviolet light. The area should not be rubbed hard enough to remove any oxide film, since this could be confused with actual surface contamination. The item being examined must be recleaned if an unacceptable amount of foreign material is detected by this inspection method. 

Curing time and temperature requirements Inspection methods for coated surfaces Thickness control method Weather resistance Corrosivity Ease of repair... 

The originality of this paper lies in applying a combination of Fourier and wavelets transform to a surface inspection. In this paper, pseudo particle images are processed by Fourier-wavelets transform to extract the defective feature to classify the inferior particles as a first step. Also, IEEE standard image is analyzed with the method. 

Optical inspection methods are used routinely but have serious limitations for certain types of defects. Methods that measure a reflected optical signal often cannot detect defects that do not break the surface and may require that the crystal be chemically etched. Etching will reveal many defects but is usually destructive and thus coupled to the need for expensive monitor wafers. Infrared... 

The aim of inspecting the surface of the substrate after any pretreatment process has been conducted, but prior to bonding, is to ensure that the treatment has been correctly carried out, and hence that good adhesion to the substrate should result. The techniques commonly used attempt to detect the presence of any surface contamination and, if a metallic substrate, to estimate the thickness of the oxide layer. Obviously, modem surface analytical methods such as X-ray photoelectron spectroscopy may readily undertake these tasks, but are not non-destructive with respect to the specimen needed and the size of a typical bonded component, and are difficult to apply on a production line ... 

Kim DM, SutliffE (1978) The contact potential difference (CPD) measurement method for prebond nondestructive surface inspection. SAMPE Quart 9 59-63 Lange YV, Teumin II (1971) Dynamic flexibility of dry point contact. Sov J NDT 7 157—165 Lloyd EA, Brown AF (1978) In Allen KW (ed) Adhesion, 2 edn. Applied Science Publishers, L.ondon, 133 pp Marty NP, Desai N, Andersson J (2004) NDT of kissing bond in aeronautical structures. In Proceedings of the 16th world conference for NDT. Montreal, Canada... 

These surface modifications methods may be combined with a range of chemical add-ons, such as primers, coupling agents, or hydration inhibitors, to stabilize the surface during storage or to further enhance bond durability. A number of useful analytical techniques were identified, ranging from wettability, optical inspection methods, to more complex techniques such as Auger electron spectroscopy and electron spectroscopy for chemical analysis. 

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