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Impact echo


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Impact echo

  1. 1. Impact-EchoA Non-destructive Testing Technique
  2. 2. Agenda• Introduction to Impact Echo• Theory• Instrumentation• Purpose• Operation• Interpretation• Strengths/Weaknesses• Commercial Products• References
  3. 3. Introduction• Non-destructive test• Looks for flaws/imperfections in the material• Can estimate thickness to within 3%• Primarily used for concrete & masonry• Approx. 25 years old o Developed from ultrasonic pulse echo (1940’s)• Similar idea to chain dragging
  4. 4. Theory – Stress (Sound or pressure) wavesFigure 1: A mechanical impact creates stress waves through a material. These(reflected)waves can be measured to gage depth of a discontinuity. Certainwaves will dominate based on the location of the discontinuity. Wave velocitiesmust be known to determine depth of flaw.
  5. 5. Instrumentation• Steel ball (4-15mm) impacts  Source: (2-10 m/s) the concrete, creating stress waves (<80kHz & λ=5cm-6m). o Transmitter  Pulse echo o Can be spring loaded o Start from large impactor small• Transducer measures surface displacements o Placed adjacent to impactor o Measures primarily P-waves o Piezoelectric
  6. 6. Purpose• Plain, reinforced and post-tensioned concrete• Can be used to sense: – Cracks – Delamination – Voids – Honeycombing – Debonding
  7. 7. Operation• Impact produces stress waves in the material; reflected waves from voids are detected by the transducer• These reflected waves set up a resonance condition having a distinctive frequency• Waveform is transformed into spectra (FFT)• Should have an idea of what to look for o Size of flaw• Operate parallel to regular occurring grooves
  8. 8. InterpretationPeaks are associated with a certain “characteristic frequency”, specifyinga distance to the discontinuity.
  9. 9. Strengths and Weaknesses• Only need access to one surface • Data can be difficult to interpret; especially on thick plates or on layered materials (overlays, soil) o Layered- needs special attention• Internal flaws can be detected • Small voids can be missed o limited by size of wavelength• Can determine depth of the internal flaws • Complicated geometries poses difficulties• Easily repeatable • Flaws beneath sensed flaw must be evaluated from the opposite side• Can construct a map of • Flaw detection length constraints: discontinuities o Lmin = d/4, L>d/3 • Requires adequate frequency resolution
  10. 10. EquipmentBasic Test Set Up Portable Impact Echo (PIES) Test Set Up
  11. 11. TransducersCylindrical Transducer Pistol Grip Transducer Dual-Head Transducer
  12. 12. Use of Each Transducer• Cylindrical – For testing in narrow and confined spaces• Pistol Grip – is easier to use and well suited to flat surfaces.• Dual Head – Used for independent measurements of wave speed which can be used to determine depth and thickness
  13. 13. ASTM C 1383 ProceduresFigure 5: Two-step procedure for measuring plate thickness:Procedure A is used to determine the P-wave speed and ProcedureB is used to determine the thickness frequency.
  14. 14. Suppliers and Costs• Impact Echo Instruments – System “A” - $12,500 – System “B” - $11,500 – System “C” - $9,500• Olson Instruments – IE 1 - $5,000 – IE 2 - $10,000 – IE T - $13,000• Qualitest USA – PIES System - $16,995 *does not include prize of laptop computer*
  15. 15. Test Standards• ASTM Standard C1383-98a for measuring the p-wave speed and thickness of concrete plates using the impact echo method• ACI 228.2R-98 Nondestructive Test Methods for Evaluation of Concrete Structures