ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2024 1 7 Table 3 gives a summary of ultrasound and magnetic residual stress measurement methods. MAGNETIC METHODS Magnetic methods for measuring residual stress primarily rely on the change of the material’s magnetic properties when residual or applied stress is present. The measurement of residual stress is done by analyzing changes in magnetic parameters like permeability, Barkhausen noise, or magnetic-field strength when a magnetic field is applied to the material. Magnetic methods are considered nondestructive and are particularly useful for inspecting steel components. Magneto-elastic methods are rapid, nondestructive, portable, and applicable for in-situ measurement at industrial plants and on their components. Disadvantages of the technique are the high sensitivity to various internal micro-structural features, and it is applicable only to ferro-magnetic materials[20]. MEASUREMENT PLANNING AND EXECUTION CONSIDERATIONS The following considerations should be taken when planning residual stress measurements: Measurement objectives. Failure investigation; model calibration or validation; component life or design; process optimization; and process quality control. Allowable component damage. Nondestructive methods; semi-destructive methods; and destructive methods. Semi-destructive methods are only locally destructive, i.e., performed in an area of the component that is discarded or machined off as the final product is finished. Component geometry and dimension. Large parts or small parts; can samples or parts fit within the measurement instrumentation? Curvature in the measurement locations; curved surfaces are not ideal for diffraction methods due to additional alignment TABLE 2 — MECHANICAL-BASED METHODS FOR RESIDUAL STRESS MEASUREMENT Mechanical methods Method Hole drilling Contour Slitting Deep-hole drilling What is measured? Radial deformation at multiple angles near perimeter of drilled hole on specimen surface Deformation normal to plane cut through specimen over entire cut surface Deformation perpendicular to slit at one (or more) locations Radial deformation inside a drilled reference hole through the specimen at multiple depths and angles Typical area analyzed 1 mm 250 µm (perpendicular to cut) × 1 mm (in-plane) 250 µm (perpendicular to slit) × 10 mm (along slit) 12.7 mm Smallest area achievable 0.5 mm (with standard tooling) 100 µm (perpendicular to cut) × 250 µm (in-plane) 100 µm (perpendicular to slit) × 5 mm (along slit) 8 mm Minimum lateral resolution ~20 µm ~50 µm ~20 µm ~20 µm Resolution in material thickness direction ~20 µm ~50 µm ~20 µm ~20 µm Overall depth achievable destructively 1-2 mm Full part thickness (>5 cm) Full part thickness (>5 cm) Full part thickness (>5 cm) Stress types Type I Type I Type I Type I Stress state measurable Biaxial Uniaxial Uniaxial Biaxial Measurable materials Metals, plastics, ceramics, composites Generally limited to electrically conductive materials Metals, plastics, ceramics, composites Metals, plastics, ceramics, composites Uncertainty ±10 MPa ±10 MPa ±5 MPa ±10 MPa Precision ±5 MPa ±5 MPa ±5 MPa ±5 MPa
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