September AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 1 8 2 8 strength design margins, and fre- quency of inspection. 2. Identify the material to be inspect- ed plus the geometry of interest and any preferential flaw location if applicable. 3. Produce an actual and/or analytical representation of the component to be inspected with a range of flaws, defects, or anomalies, preferably including examples near the mini- mum target size. 4. Examine the produced sample(s) and either justify the detectabili- ty to a referee who is competent in NDE to corroborate claims of detectability or demonstrate suc- cessful detection of flaws of interest through blind test samples. 5. If flaw sizing is also of interest (e.g., wall thickness, crack length, crack width, or flaw volume), measure- ment accuracy is usually deter- mined by a regression analysis of estimated or measured versus true flaw sizes. Errors are typically char- acterized as the root mean square error of the NDE flaw size estimate. NDE is a mature engineering sci- ence where inspection methods have been developed based on a number of well understood physics principles. A noncomprehensive overview of these methods and their applications, limita- tions, and advantages is presented in Table 2. ~AM&P TABLE 2 — EXAMPLES OF NDE METHODS AND TEST CATEGORIES Method Principle of operation Application Limitations Advantages Materials Visual test — direct viewing, borescopes, video, magnifying glass, speckle metrology Visual observation of test object surface to evaluate dimensions, color, and presence of surface discontinuities indicative of defects Postmanufacturing inspections, in- service inspections for dimensional anomalies, color variation, cracks, and pits Only sensitive to surface flaws Low cost, intuitive; may be enhanced with other methods, laser reference lines, video, and magnifying lenses All Liquid penetrant test Liquid penetrant fluid preferentially collects in crevices and attracts dyes that accentuate flaw visibility Locating fabrication discontinuities, stress cracks Will not find subsurface defects Ease of application; improvement over simple visual inspection Most metals and composites with nonporous surfaces Magnetic particle test Magnetic particles are attracted to breaks in magnetic lines of force Near-surface flaws are sensitive to magnetization; includes blow holes, laps, and cracks Not applicable to nonmagnetic metals or materials Can detect flaws up to 0.25 in. below surface under good conditions Iron and steel, nickel, cobalt Barkhausen noise test Changes in magnetic flux from stress applied to magnetic material Online monitoring of magnetic material Not suitable for nonmagnetic material Online monitoring; can sense stress without cracking Magnetic metals, steel, nickel, iron, chrome Penetrating radiation test (RT), computed tomography (CT), digital x-ray, neutron RT, x-ray diffraction Penetrating rays (x-ray, gamma, or neutron) passing through or reflecting from test object cast shadows or patterns on film or digital imaging plates Manufacturing, weld inspection, finding objects in closed containments, metrology of enclosed objects, thickness Hazardous radiation operation, not sensitive to defects less than 1–2% thickness of total metal; complex shapes difficult to analyze Permits visual analysis of buried defects or components in assembly; also possible to measure near- surface strain Metals, foods, films, nonmetals, composites, assemblies Acoustic emission (AE) test Multiple distributed sensors detect and triangulate AE stress wave source in response to mechanical or thermal stress Corrosion, stress corrosion cracking, weld cracking, creep and fatigue cracking Sensitive to noise and vibration; identifies location of defect rather than type Allows entire volume of structure to be inspected nonintrusively in a single loading operation Aircraft, bridges, welds, metal forming, composite and metal pressure vessels, piping