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“Damage Detection in Composite Materials and Quantitative

Structural Integrity Assessment Using Modal Acoustic Emission”

Michael R. Gorman

Digital Wave13760 E Arapahoe Rd

Centennial, CO 80112

M. Gorman, Digital Wave, ASNT ISIW February 2, 2017


Does what we measure feed into engineering analysis?

Waveform Analysis To Identify SourceSources Sought = Types of Damage

Structural integrity assessment through modeling using finite elements, fracture mechanics, and stress analysis.

Wideband Detection

Wave Prediction Absolute Calibration

Modal AE and Modeling

Sound consists of waves with many frequencies


Each frequency is a sine wave.


Sine waves add to form a complex wave.


The figure shows the addition of two sine waves to form the complex wave in red.

In music, each note is a different frequency.


MAE detects and analyzes all the frequencies in the sound waves so you know what is being played.


Graphic Equalizer changes the sound we hear by suppressing or enhancing certain frequencies

Narrowband – Analogy with Music

• Now imagine listening to music through a graphic equalizer with one band full on and the others full off. It might be hard to recognized the music being played. The sound could be “tinny” or “bassy” but it would not sound right. This is what using narrowband detection is like in AE. You know that “something” is playing but you have a hard time interpreting what it is.

Conventional AE detects only part of the sound.


AE utilizes a resonant transducer, typically 150 kHz, and narrowband filtering like 100-300 kHz.The AE signal is treated as a damped sine wave for analysis purposes.

AE “parameters” counts, etc. are correlated to sources through experience.

Modal Acoustic Emission (MAE)

• A broadband (typically 20 – 2000 kHz) transducer is used to detect the AE wave. The wave is digitally recorded and displayed on a computer screen.

• Dynamic elasticity theory is used to predict the waveforms for various types of sources in different materials and geometries. This is called the forward problem.


QUANTITATIVE ACOUSTIC EMISSION SOURCECHARACTEZRISATION DURING LOW TEMPERATURE

CLEAVAGE AND INTERGRANULAR FRACTUREH. N. G. WADLEY, C R SCRUBY and G. SHRIMPTON

AERE, Harwell. Oxfordshire OX1 1 ORA, U.K.Received 26 June 1980


Inverse Problem Example

Forward Problem Recent Publications

• Application of Normal Mode Expansion to AE Waves in Finite Plates. Gorman, M and Prosser, W. 2, 1996, Journal of Applied Mechanics, Vol. 63, pp. 555-557.

• Application of Mindlin plate theory to analysis of acoustic emission waveforms in finite plates. Huang, W. US : Springer, 1998. Review of Progress in Quantitative Nondestructive Evaluation. pp. 493-500.

• On the far-field structure of waves generated by a pencil lead break on a thin plate. Gary, J and Hamstad, M. 1994, Journal of Acoustic Emission, Vol. 12, pp. 157-170.

• Finite element and plate theory modeling of acoustic emission waveforms. Prosser, W, et al., 3, 1999, Journal of Nondestructive Evaluation, Vol. 18, pp. 83-90.

• On Lamb modes as a function of acoustic emission source rise time. Hamstad, M. 2010, Journal of Acoustic Emission, Vol. 28, pp. 41-58.

• Simulation of Acoustic Emission in planar carbon fiber reinforced plastic specimens. Sause, M and Horn, S. s.l. : Springer, 2010, Journal of Nondestructive Evaluation, Vol. 29, pp. 123-142.

• A modal acoustic emission signal classification scheme derived from finite element simulation. Burks, B and Kumosa, M. 1, s.l. : Sage, 2014, International Journal of Damage Mechanics, Vol. 23, pp. 43-62.


Waves and Sources


Water waves.

Step function source.

Pulse on a string with one end fixed.

Reflection of pulse on a string with one end fixed.

Waves in Strings and Rods


Transverse wave on a string.

𝜕2𝑦

𝜕𝑥2=

1

𝑐2𝜕2𝑦

𝜕𝑡2

𝑐 = ൗ𝑇 𝜌

y = Asin(x – ct).

𝜕2𝑢

𝜕𝑥2=

1

𝑐02

𝜕2𝑢

𝜕𝑡2 EQM for extensional wave in a rod.

EQM – equation of motion This is a wave equation.

Sine wave solution

velocity

_______________________________________________________________

𝑈 𝑥, 𝑡 =

𝑛=1

∞

(𝐴𝑛sin(𝜔𝑛𝑡) + 𝐵𝑛 cos(𝜔𝑛𝑡)) 𝑐𝑜𝑠𝑛𝜋𝑥

𝑙Normal mode solution for free-free rod

T is tension and ρ is linear density.

Beam Bending


𝐸 𝐼𝜕4𝑦

𝜕𝑡2+ 𝜌𝐴

𝜕2𝑦

𝜕𝑡2= 0

EQM for a beam. (Bernoulli-Euler)

Shear and bending moments in a beam

𝑐 = 𝜔𝑎 =𝐸𝐼

𝜌𝐴

1/4

2𝜋𝑓

Dispersion of Fourier components.

Plate Waves


𝜕2𝑢

𝜕𝑥2+1 − 𝑣

2

𝜕2𝑢

𝜕𝑦2+1 + 𝑣

2

𝜕2𝑣

𝜕𝑥𝜕𝑦=

1 − 𝑣2 𝜌

𝐸

𝜕2𝑢

𝜕𝑡2

𝜕2𝑣

𝜕𝑦2+1 − 𝑣

2

𝜕2𝑣

𝜕𝑥2+1 + 𝑣

2

𝜕2𝑢

𝜕𝑥𝜕𝑦=

1 − 𝑣2 𝜌

𝐸

𝜕2𝑣

𝜕𝑡2

𝐷𝜕4𝜔

𝜕𝑥4+ 2

𝜕4𝜔

𝜕𝑥2𝜕𝑦2+𝜕4𝜔

𝜕𝑦4= −𝜌ℎ

𝜕2𝜔

𝜕𝑡2

_______________________________________________________________________

𝜎𝑖𝑗 = 2𝐺휀𝑖𝑗 + 𝜆휀𝑘𝑘𝛿𝑖𝑗

𝜇𝜕

𝜕𝑥𝑗

𝜕𝑢𝑖𝜕𝑥𝑗

+ 𝜇 + 𝜆𝜕휀𝑘𝑘𝜕𝑥𝑖

= 𝜌 ሷ𝑢𝑖

3-D Hooke’s Law Isotropic Case

EQM – Isotropic material

Solutions are called Lamb modes.𝜇 ത𝛻2 ത𝑢 + 𝜇 + 𝜆 ത𝛻 ത𝛻 ∙ ത𝑢 = 𝜌 ሷത𝑢

Predicted HOPT Waveform


1 µsec impulse on edge of 1.5 mm thick aluminum plate, z=0.12 mm.

Comparison of UT wave propagation with MAE waves

Waves propagate in what is called the medium.

Dimensions of the medium relative to wavelength:

Bulk Medium: λ < a, b, c

Plate Medium: λ < b, c but λ ≈ a


a

b

c

P (L) or S wave

R

Plate Flexural Wave

Bulk

CL>CE>CS>CR>CF

Plate Extensional Wave

Identifying Fracture Sources

• Different sources create distinct waves. For example, rubbing on a vessel surface produces waves with low frequencies, while fiber breakage produces waves with very high frequencies. Some examples of waveforms are given in the next few slides to illustrate.

• Forward problem calculations are used as guidance to source identification, along with other types of calculations and structural testing. Over the past 25 years Digital Wave has identified and catalogued over a dozen types of sound waves found in MAE testing of composites and metals.


FRP Matrix Crack Waveform/FFT

Vo

lts

Time (µs)

Frequency (kHz)

E F

Vo

lts


Flow Noise Waveform/FFT

Vo

lts

Vo

lts

Frequency (kHz)

Time (µs)


FRP Debonding Waveform/FFT

Vo

lts

Vo

lts

Frequency (kHz)

Time (µs)


CFRP Fiber Rupture WaveformV

olt

sV

olt

s

Time (µs)

Frequency (kHz)M. Gorman, Digital Wave, ASNT ISIW

February 2, 2017

Development of Modal AE Testing Protocol for US Navy SCBA Life Extension

Development of Modal AE Testing Protocol for US Navy SCBA Life Extension


Requalification Hydrotest Insufficient

• It is acknowledged by many in the compressed gas industry that hydrotesting is wholly inadequate for COPVs. For example, many COPVs were intentionally damaged, some quite severely, yet they passed the proof test.


Modulus did not vary over 24,000 cycles


Cycling Chamber


Burst Chamber


Burst Test


• The formula that shows how to connect the number of ruptured fibers to the energy in the fiber rupture waveform can now be found in ASME/NBIC/ISO codes and standards.

• The effect of damage was modeled and a failure criterion established to meet minimum burst pressure with the same factor of safety used for new vessels.

Fiber Rupture Magnitude


Overall Results

• All EOL bottles passed proposed MAE A/R criteria.• All 10K and 24K passed MAE A/R.• All ISO dropped passed MAE A/R .• ISO notched COPVs failed MAE A/R. Two bottles

burst >1000 psi below min burst.• All damaged vessels survived hydrostatic test

pressure (proof pressure) twice.• USDOT approved MAE test for life extension.• Navy doesn’t have to throw away perfectly good

vessels.


Life Extended Cylinders


MAE Accepted by Government, ASME, National Board and ISO

• USDOT SP 16343

• ASME Sec X Mandatory Appendix 8

• NBIC NB10-0601

• ISO 19016 (final draft stage)


In Design, Manufacturing and Repair Testing MAE Can Provide Validation of Assumptions

• Design – many tests are done on parts and subscale components – Do they behave the same way in the actual structure? MAE can provide critical scaling validation.

• Manufacturing – are larger than expected flaws and defects (porosity, dry zones, etc.) present?

• Repair – Is the repair behaving correctly? Is there still a stress concentration?

• MAE techniques applied during development and manufacturing testing can answer the above questions and validate the design assumptions.


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