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Non-Destructive Methods of Geophysical Engineering


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Non-Destructive Methods of Geophysical Engineering

  1. 1. Nondestructive Testing Kerry Hall Department of Civil and Environmental Engineering Adapted from L. J. Struble and J. S. Popovics Motivation US infrastructure is deteriorating: 2009 ASCE Report card forAmerican infrastructure gave an overall grade of “D” – estimated $2.2trillion investment needed for improvements Infrastructure agencies are shifting efforts from building newstructures to assessing and rehabilitating existing structures Minneapolis I-35 bridge collapse
  2. 2. Momentum and Collisions Rebound Hammer Test Elastic collisions, both momentum and kinetic energy are conserved Inelastic collision, momentum is conserved, but energy is absorbed Object impacts solid, energy absorbed, thus V2<V1 adapted from J.S. Popovics Low strength material absorbs more energy, thus lower rebound height
  3. 3. Rebound Hammer - ASTM C 805 Rebound Hammer - ASTM C 805 Sometimes called the Measures surface hardness Schmidt Hammer or Swiss Related to Modulus of Elasticity Hammer Light weight, portable, hand Affected by varied conditions operated form material and type of finish Spring loaded steel hammer moisture content impacts plunger – hammer aggregate type and proportion rebound is measured surface smoothness A test is the average rebound temperature number of ten determinations direction of impact made in a small area. depth of carbonation of the surfacePenetrating Probe – ASTM C 803 Penetrating Probe – ASTM C 803 Drives steel probe or pin into concrete A steel probe driven Measures toughness of the concrete, into the concrete the ability to resist fracturing. surface using a powder Related to the tensile strength actuated gun Affected by varied conditions Hazardous – wear Nature of formed surface protective equipment Coarse aggregate type, size and hardness Moisture contentPenetrating Probe – ASTM C 803 Correlation with strength Portable and hand A correlation of the nondestructive test operated parameter to strength for each type of concrete May require a license to be tested is necessary to determine in-place to operate strength. Three probes shot into A statistical evaluation of the correlated data is the concrete and the necessary average penetration ACI 228.1R, In-Place Methods to Estimate determined. Concrete Strength presents methods for correlation and statistical analysis.
  4. 4. What are waves? Mechanical body waves in Propagation of a disturbance through a medium; solids: P-waves mass is not transported in propagation direction also: Longitudinal (L-) Waves, Compression Waves The time dependant disturbance is 1.5 T usually expressed in harmonic form 1 The period (T) is the time required for ExcitationDis place me nt 0.5 <T. Voigt > 0 wave motion to complete a round trip -0.5 (measured in seconds) -1 The frequency (f) is the inverse of T phase delay -1.5 (measured in 1/seconds or “Hertz”) In 0 20 40 60 80 100 audible sound, frequency is interpreted Time as the pitch Direction of Travel Direction of Particle Motion Frequency-wavelength relation for all harmonic waves Wave Velocity: Governing Parameters Propagation velocity V V =λ f Wavelength λ in Young’s Modulus E in units of distance units of distance E(1 − ν ) per time ω= 2πf is “circular frequency” and vP = Poisson’s Ratio v k = 1/λ is “wave number” ρ (1 + ν )(1 − 2ν ) Density ρ Mechanical body waves in Guided wave modes solids: S-waves Shear Waves also: Transverse (T-) Waves, <T. Voigt > Excitation <> Rayleigh surface wave travels along free surface, slightly slower than S-wave Direction of Travel Direction of Particle Motion Wave Velocity in solids: a propagating “resonance”, must be set up over distance or time Governing Parameters propagation G Shear Modulus G E vS = v BAR = ρ Density ρ ρ no propagation in liquids or gases ! VP > VS in all known solids 1-D bar wave travels along long cylinder or prism, slightly slower than P-wave Reflection and refraction – Guided waves in plates Impact echo frequency normal incidence When an incident wave Lamb wave are set up in large plates encounters the boundary with another material, part of the Multiple (infinite) modes of incident energy is reflected, and propagation, with varying motion character and propagation velocity the rest is transmitted (refracted) Can be visualized as a propagating pr/po is the reflection coefficient resonance (r); pt/po is the transmission Increasing frequency or plate thickness coefficient (t) r is maximized and t minimized when Z1>>Z2 or Z1<<Z2. <Krautkramer and Krautkramer> Acoustic impedance : Z = V ρ <N. Ryden>
  5. 5. Reflection and refraction, mode Beam divergenceconversion wave encounters the boundary with When an obliquely incident The principles of wave interference and superposition control the directivity of the generated pressure field. A given transducer mayanother material, reflection and refraction become dependant on ϑi primarily generate P-wave energy in some directivity field, although (Snell’s law). Conversion to other wave modes also occurs. some S-wave and Rayleigh wave energy, may also generated in solid media The beam divergence angle α of a given transducer can be estimated: α 1.2λ sin α = <> D <Gibson 2005> <>Beam divergence: point source Scatteringof waves The reflection of ultrasonic energy away from the original direction of Point sources of waves have poor directivity and generate P-waves, S- propagation; caused by reflection, refraction and mode conversion from internal waves and Rayleigh waves inclusions. Causes signal loss, signal dispersion and scattering “noise” Solid material Detected back- <Richard et al. 1970> scattered signalSnapshot of wave fields (stress) in material owing to transient point load <Oelze 2007> at some time “t” after wave excitation Implications: transducer contactAbsorption and attenuationWave absorption is the conversion Wave attenuation is the overall loss (coupling) To eliminate significant wave reflection at theof ultrasonic wave energy to other of wave energy with propagation, transducer-test material interface, must use aforms of energy (heat). A caused by substance to displace air and ensure goodsignificant source of wave energy contact: oil, gel, grease, solidloss for asphalt concrete * beam divergence (geometric) * scattering Problematic for rough or uneven surfaces * absorption <> Dry point contact transducers obviate the need for couplant material. Each point transducer needs vertical pressure to ensure wave energy transfer <> <> <M. Schickert and MSIA Spectrum>
  6. 6. Implications: Detection of Implications: lateral defect defects resolution Ultrasonic waves show large reflection at interfaces between high (concrete) and low The ability to resolve side by side reflectors is improved by reducing α (air-filled defects) acoustic impedance time voltage General Rule: D Echo height size of defects (but shape Ultrasound can resolve dependant!) defects of size x if x is Simplified A-scan the same size or larger than the wavelength λ of wave pulse. αvoltage Solution: use small λ (large f) 1.2λ sin α = <> D time Solution: use small λ (large f) Application: Ultrasonic pulse UPV application: concrete strength? velocity (UPV) Effects Parameter on UPV on concrete strength w/cm ratio Measurement of very first wave arrival (P-wave) through a age specific wave path. Requires good coupling to surface moisture content Standard method in many Agg type and content n/a useful for countries (ASTM C597) relative n/a Proximity of steel measurements Frequencies between 20kHz to within Presence of defects 100 kHz typically used a single structure However, UPV cannot be used to measure in place <Naik, Carino and Popovics 2005> strength absolutely in most cases! UPV application: defect UPV applications: Modulus detection? determination 60 concrete specimens υ d = 0.20 50 E u measured from UPV, GPa 40 void crack Paste Specimens 30 υd = 0.25 Limestone River Gravel 20 Air-Entrained High Strength 10 PC, w/c = 0.34 PC, w/c = 0.45 Loss of transmission or Loss of transmission or Little to no effect 0 apparent lower velocity apparent lower velocity 0 10 20 30 40 50 60 E d measured from resonant frequency, GPa Ed is directly related to VP by wave theory. However, measurements obtained Defects cause wave path to deviate, thus lowering the apparent velocity from wave velocity (UPV) do not agree with those obtained by vibration! in most cases. However, UPV cannot be used to fully characterize Wave propagation over predicts Ed for concrete samples, assuming median defects (shape, depth, location, etc.) values of Poisson’s ratio
  7. 7. Ultrasonic Pulse Velocity – Ultrasonic pulse velocityASTM C 597 v >v >v solids liquids gases Calculate Young’s modulus Young’ More sophisticated from UPV (theoretical): electronic equipment 1 −ν E Portable, may require V= electrical power (1 +ν )(1 − 2ν ) ρ Usually requires two Calculate strength from or more people for Young’s modulus (empirical): Young’ testing E = 0.04 3 ρ 1.5 σ where V is velocity (m/s) E is Young’s m odulus (MPa) ν is Poisson’s Ratio ρ is bulk density (kg/m 3) σ is compressive strength (MPa) Serway and FaughnDynamic (vibration) methods Resonance Frequency AnalysisNDE Technique - shallow Impact-echo Impact-echo (ASTM C 1383) Analysis Reflection from slab bottomPhenomena The resonant Propagating waves frequency (at the generated by impact peak) is related to event. Multiply-reflected distance to reflector waves are detected by (d) and wave velocity surface sensor. (V): f = V/(2 d) Reflected waves set up a resonance Thus, condition having a characteristic frequency d = V/(2 f) Reflection from delamination
  8. 8. Impact-echo Impact-echo Analysis (cont’d) Advantages Relatively simple test to perform; commercially available test equipment. Effective for detecting • Strong wave reflectors delamination and slab depth. more readily detected. Disadvantages Operator experience needed for data interpretation. • Reflections Not as effective slabs over very stiff subgrade. Not from embedded rebar effective for rebar detection. and at the interface of a Application slab and a stiff subgrade are weak. Slab depth and delamination detection for most slab systems.NDE Technique - shallow GPR Ground Penetrating RADAR (ASTM D 4748) AnalysisPhenomena Many time domain signals stacked together to Wave pulses are reflected form an imageantenna at interfaces having Scan direction air: εr = 1 a difference in electrical properties (εr ) concrete: εr = 6 to 11 Slab Reflected pulses (time depth and amplitude) are soil: εr = 2 to 10 monitored in the time domain signal (water: εr = 80; metal εr = infinite)GPR Analysis (cont’d) GPR Large wave reflection from metallic objects and moist areas. Less reflection from slab-subgrade interfaces and air-filled cracks • Physical contact Slab surface between antenna and Scan direction slab not needed antenna • Allows for rapid non-contact scanning Rebar reflections
  9. 9. GPRAdvantages NDT of Steel Very rapid data collection (non-contact technique). Sensitive to presence of embedded rebar and moisture. Liquid Dye Penetrant Eddy CurrentsDisadvantages Very involved data interpretation; operator experience Ultrasound needed. Testing limited to 750mm depth. Not sensitive to delaminations. Not effective beyond congested reinforcement. X-rayApplication Rapid scanning of slabs for depth or rebar location.Defects in Steel Liquid (Dye) Penetrants Observe visually Enhance with penetrating dye Clean surface and apply penetrant Allow liquid to penetrate then remove excess from surface Apply developer (draws penetrant out of defects) No indication of crack depth No indication of subsurface defects Not for porous/rough materialsLiquid (Dye) Penetrants Eddy currents Magnetic fields setup electrical currents in a conductive material (eddies) They in turn generate a secondary magnetic field that counteracts the first This change in the field can be detected by original coil or a pick-up coil
  10. 10. Eddy currents Ultrasonic Wave Reflection incident reflected wave wave θi θr Medium 1 Medium 2 θt transmitted wave Reflected angle equals the incident angle Amplitude of reflected wave depends on the properties of the two media If media have large differences in stiffness and density, most energy is reflected (flaws!) If media have similar stiffness and density, most energy is transmittedAngle-Beam TransducerAngle- Ultrasound: Steel vs. ConcreteInspection Ultrasonic pulse echo not effective in concrete Why? Wave frequency 1-10 MHz 1- Angle beams allow lateral Aggregate scattering! detection of flaws in and f = V/λ around welded areas If aggregate size (D) is 1” and we need λ > D 1” Vertical cracks are not V = 4000m/s f < 150 kHz detectable by normal beam f was in MHz for steel! incidence Low frequency pulse echo is problematic Reduce extra echoes with angle beam Difficult to manufacture transducers Low f leads to large beam divergence (poor lateral Hole Crack Hole resolution) Crack, no Crack Crack detection no detection Transducer face must be very largeX-ray Radiography NDT Lab Today Concrete tests Bright is low x-ray x- Schmidt rebound hammer: Surface hardness, intensity due to strength high absorption Ultrasonic Pulse Velocity: thickness, strength, modulus Dark is high x-ray x- Ultrasonic Resonance Frequency: modulus intensity due to low density Steel Tests Dye Penetration: surface defects Ultrasonic wave reflection: thickness, defects X-ray: surface/internal defects Eddy Currents: surface defects