ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2024 15 synchrotron-based methods can be performed in a nondestructive manner up to a few centimeters deep into metals. The maximum achievable penetration depth depends on the material type and geometry of the test article. Synchrotron-based methods also have the advantage of being able to probe relatively small volumes, on the order of tens of micrometers depending on material type, component, and diffraction geometry. Neutron diffraction methods can generally penetrate deeper than synchrotron-based methods; however, interrogated volume is on the order of millimeters[1-13]. Here, it is important to note that with recent developments in laboratory-based high energy sources and detectors, laboratory diffraction-based methods could become nondestructive to greater depths than conventional methods. Table 1 gives a summary of diffraction-based residual stress measurement methods and measurement features used by each specific technique. TABLE 1 — DIFFRACTION-BASED AND PIEZO-SPECTROSCOPIC/RAMAN METHODS Diffraction based Piezospectroscopic Method Laboratory x-ray diffraction Synchrotron x-ray diffraction Neutron diffraction Raman What is measured? Bragg diffraction peaks position Bragg diffraction peaks position Bragg diffraction peaks position Raman shift Gauge Atomic planes spacing Atomic planes spacing Atomic planes spacing Atomic spacing Typical volume analyzed 1-2 mm Ø × 5-10 µm Few tens of micrometers 1 mm × 1 mm × 1 mm ~1 µm Ø × 1 µm Smallest volume analyzed 100 µm Ø × 5-10 µm ~10 µm × 10 µm × 5 mm (energy dispersive) ~100 µm × 100 µm × 100 µm (angle dispersive) Sub-grain volumes with specialized optics ~300 µm × 300 µm × 300 µm ~1 µm Ø × 1 µm Depth resolution ~50 µm* ~50 µm* ~50 µm* <1 µm Spatial lateral resolution ~50 µm* ~50 µm*, 1 µm with specialized optics/techniques ~50 µm* ~1 µm Overall depth achievable destructively Up to 1-2 cm using layer removal and corrections Several centimeters Tens of centimeters - Certain materials can become radioactive in which case the part/ sample is quarantined till cleared Method only used surface-wise Overall depth achievable nondestructively 5-25 µm using conventional or grazing angle diffraction methods Material dependent, 1-5 cm in energy dispersive set-ups and 0.5 cm in angle dispersive set-ups ~10 cm material dependent ~1 µm Stress types Type I, II, III Type I, II, III Type I, II, III Type I, II, III Stress state measurable Uniaxial, biaxial, triaxial Uniaxial, biaxial, triaxial Uniaxial, biaxial, triaxial Uniaxial, biaxial Measurable materials Crystalline materials: Metals, plastics, ceramics Crystalline materials: Metals, plastics, ceramics Crystalline materials: Metals, plastics, ceramics Plastics, ceramics Accuracy ±10 MPa ±10 × 10-6 strain < 100 × 10-6 strain ±50 MPa Precision ±5 MPa ±10 × 10-6 strain ~100-150 × 10-6 strain ±50 MPa * Stage and instrumentation dependent
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