Case Study: Residual Stress Measurement

Transcription

1 Case Study: Residual Stress Measurement

2 Life Prediction/Prognostics 15 Alternating Stress [MPa] service load natural life time with opposite residual stress intact (no residual stress) increased life time Fatigue Life [cycles] endurance limit Residual stresses have numerous origins that are highly variable. Residual stresses relax at service temperatures.

3 Load stress (LS) versus residual stress (RS) external loads ( L) x 3 x 3 ΔF lim i ΔA ΔA = Ti τ 33 τ32 T = τ n i ij j ΔA n ΔF A ΔF3 ΔA ΔF 2 ΔF 1 x 2 τ 31 τ 13 τ τ11 12 τ 23 τ 21 τ 22 x 2 τ ij = Cijkl εkl LS T da = L A A x 1 x 1 RS T da = A stress measurement direct semi-direct indirect relationship LS M τ A = T da A M M (1 +ε) M M (1 +ηε+... ) selectivity excellent high low

4 Potential NDE Techniques direct semi-direct indirect stiffness hole drilling magnetic vibration analysis contour mapping ultrasonic etc. x-ray or neutron diffraction eddy current T v τ ρ T = τ A etc. stress relief hole strain rosette thermoelectric etc. H Amplitude B noise Time

5 Sources of Residual Stress material-related processing-related service-related multi-phase microstructure composites inclusions etc. casting quenching cold working cutting joining case hardening surface treatments etc. mechanical thermal chemical etc.

6 Surface-Enhancement Techniques Shot Peening (SP) Laser Shock Peening (LSP) Low-Plasticity Burnishing (LPB) Residual Stress [MPa] Ti-6Al-4V -6 SP Almen 4A SP Almen 12A -8 LSP LPB Depth [mm] Cold Work [%] Ti-6Al-4V SP Almen 4A SP Almen 12A LSP LPB Depth [mm]

7 X-ray Bragg Diffraction incident beam diffracted beam θ B θ B D(1) = a incident beam diffracted beam θ B (ε) Θ(ε p ) cold work 2Dsinθ B = nλ a Dhkl ( ) = (cubic) h + k + l a 2 1 Å, λ.5 5Å Main Challenge: low penetration depth ( 1-1 µm) ΔD 3 4 ε= 1 1 D

8 XRD Measurements on Shot-Peened Waspaloy Specimens before (solid circles) and after full stress relaxation (empty circles) at 9 C, 24 hrs Residual Stress [MPa] Almen 4A Almen 8A Almen 12A Almen 16A Depth [mm] Cold Work [%] Almen 4A Almen 8A Almen 12A Almen 16A Depth [mm] Almen strip peening intensity electrochemical etching

9 Synchrotron Radiation and Neutron Diffraction incident beam 2θ B s.5 mm diffracted beam Residual Stress [MPa] shot-peened turbine alloy (Ezeilo et al., 1992) x and y directions z direction Distance from Surface [mm] synchrotron (Univ. Dortmund) isotope reactor (ORNL) Main Advantage and Disadvantage: good penetration depth ( 1-5 cm) availability of source

10 Indirect NDE Methods for Near-Surface Residual Stress Assessment interferometer laser ultrasonic transducer SAW eddy current probe coil magnetic field eddy currents magnetometer thermoelectric heat nozzles thermoelectric current fluxgate

11 Nonlinear Acoustoelastic Effect parabolic potential function Potential Energy [a. u.] parabolic potential well typical Elastic Stiffness [a. u.] unstrained typical 1 2 Normalized Lattice Distance Normalized Lattice Distance (Strain) Relative Velocity Change [%] Al-224 T351.3 parallel polarization normal polarization Uniaxial Strain [%]

12 Surface Acoustic Wave Dispersion surface wave Normalized SAW Velocity d << stiff coating layer compliant coating layer λ d λ uncoated d >> λ Normalized Frequency Relative Velocity Change [%] Al 224 Almen 6A 225 C 2 C 15 C intact Frequency [MHz] Relative Velocity Change [%] a Al 224 Almen 8A 3 C 25 C 225 C 2 C 15 C intact Frequency [MHz]

13 Indirect NDE Methods for Near-Surface Residual Stress Assessment interferometer laser ultrasonic transducer SAW eddy current probe coil magnetic field eddy currents magnetometer thermoelectric heat nozzles thermoelectric current fluxgate

14 Piezoresistive Measurements in Different Metals.4.2 parallel normal Ti-6Al-4V.4.2 parallel normal Al parallel normal Al 775 Δσ / σ Δσ / σ Δσ / σ τ ua / E τ ua / E τ ua / E.4.2 parallel normal Waspaloy.4.2 parallel normal IN parallel normal Copper Δσ / σ Δσ / σ Δσ / σ τ ua / E τ ua / E τ ua / E

15 Thermal Stress Relaxation in Waspaloy Almen 8A, repeated 24-hour heat treatments at increasing temperatures Apparent Conductivity Change [% ] intact 3 C 35 C 4 C 45 C 5 C 55 C 6 C 65 C 7 C 75 C 8 C 85 C 9 C Frequency [MHz] The excess apparent conductivity gradually vanishes during thermal relaxation!

16 Inversion of Measured AECC in Low-Plasticity Burnished Waspaloy eddy current 2 XRD 2 AECC Change [%] Cold Work [%] Residual Stress [MPa] Frequency [MHz] Depth [mm] XRD eddy current Depth [mm]

17 Indirect NDE Methods for Near-Surface Residual Stress Assessment interferometer laser ultrasonic transducer SAW eddy current probe coil magnetic field eddy currents magnetometer thermoelectric heat nozzles thermoelectric current fluxgate

18 Magnetic Signatures Produced by Semi-Spherical Inclusions and Cavities C11 copper, T.6 C/cm, 2 mm lift-off distance, 3" 3 milled plastic zone pressed.25 -diameter tin inclusion, 33 nt p.375 -diameter cavity, 18 nt p before annealing.375 -diameter tin inclusion, 14 nt p T T = T after annealing

19 Noncontacting Thermoelectric Inspection in Shot-Peened C11 Copper fluxgate gradiometer thermoelectric current T Flux Density [5 nt/div] Almen 4A Position [2 mm/div] Flux Density [5 nt/div] Almen 8A Position [2 mm/div] Flux Density [5 nt/div] Almen 12A Position [2 mm/div] before stress release after partial stress release (3 min, 315 C) Magnetic Signature [nt] before relaxation relaxation at 235 ºC relaxation at 275 ºC relaxation at 315 C 2nd relaxation at 315 C 3rd relaxation at 46 C recrystallization at 6 C Almen Intensity (A)

20 Conclusion Essentially the same destructive method works in every material. Very different nondestructive methods are needed in different materials.