Rayleigh Wave Acoustic Emission During Crack Propagation in Steel

Michael R. Horne, PhD*

12th International Symposium on NonDestructive Characterization of Materials

June 19-24, 2011

Acknowledgements: Virginia Transportation Research Council; Program (98-1687-04) "Quality Assurance Testing of High Performance Steel Bridge in VA”

Vallen-Systeme GmbH: Wavelet analysis software

*From Dissertation, Virginia Polytechnic Institute and State University, Materials Engineering Science (MESc)Currently with National Institute of Aerospace @ NASA Langley Research Center

mrhorne@cox.net, 757-817-8830

Outline

• Motivation • Objectives• Insight • Approach• Test development• Experimental results• Summary and Conclusions

Motivation

Lindley, T. C. and L. N. McCartney. 1981. "Mechanics and Mechanisms of Fatigue Crack Growth.“ Developments in Fracture Mechanics. G. G. Chell, ed. Vol. 2. Applied Science Publishers (London).

Crack Growth Rate vs. Alternating Stress Intensity

Motivation:

Regime A and B can be the major portion of life for many steels used in infrastructure like bridges.

But:

How does one track crack growth through these regimes without measuring crack length?

Crack extension AE has the best chance of evaluating the state of the crack.

Objectives

Objectives• Investigate AE in near-failure fatigue (regime C) and compare with AE

from earlier in fatigue life.

• Design a test method that distinguishes brittle behavior AE from other AE sources to investigate a hypothesis that development of “brittle behavior” is a mechanism that accelerates crack rates near failure, even in ductile materials.

• Use a sampling of steels, each exhibiting behavior at a different points along the continuum from fully ductile to fully brittle (yield or failure strengths vary from 49 ksi to 300 ksi) to test the above hypothesis.

• Fatigue load in a manner that simulates the dynamic response of a real structure, i.e. bridges.

• Use load profile that resembles light vehicle traffic crossing a bridge interrupted with “overloads” created by large trucks.

Insight

*Miller, G. F., and Pursey, H. (1955). "On the Partition of Energy between Elastic Waves in a Semi-infinite solid." Proceedings of the Royal Society (London), 233(Series A), 55-69.

Energy partitioning: vibrating surface disk*

• 67% Rayleigh surface wave,

• 26% bulk shear wave,

• 7% bulk longitudinal wave.

The most effective way to transport energy awayis the Rayleigh surface wave!

Insight

Vibrating Surface Disk becomes an “Idealized Brittle Crack Extension” (i.e. no void coalescence, ductile ligament tearing, or other types of slip plasticity) with Rayleigh surface waves that carry away majority of the energy along the crack faces.

Insight

Vibrating Surface Disk becomes an “Idealized Brittle Crack Extension” (i.e. no void coalescence, ductile ligament tearing, or other types of slip plasticity) with Rayleigh surface waves that carry away majority of the energy along the crack faces.As crack growth rate accelerates (regime C) strain rate at the tip increases.

Crack propagation can be strain rate dependent.

Hypothesis

Can crack extension in real materials transition from ductile to brittle behavior such that it approaches an “Idealized Brittle Crack Extension”? We test this hypothesis by looking for Rayleigh waves on the crack faces.

Complete response: Seismic Surface Pulse (SSP)

Pekeris, C. L. (1955). "The Seismic Surface Pulse." Proceedings Of the National Academy of Sciences, 41(7), 469-480.

Response to step load on a surface. (

= ct/r, c = shear velocity, r = radius)

Step load on surface generates several modes of wave propagation:

P: bulk longitudinal mode (head wave)

S: bulk shear mode

R: Rayleigh surface wave

Approach

• Modify standard Compact Tensile specimen to use NIST conical high fidelity AE sensors in locations that will distinguish between surface and bulk modes of wave propagation generated at a crack tip.

• Do low K (peak stress intensity) fatigue to grow crack, interrupted by short burst higher K loads at select intervals (“overload”) with the assumption that the overloads are the only fatigue cycles that might generate “brittle” crack extension.

• Monitor entire signal generated during overload.

• Modify standard Compact Tensile specimen to use NIST conical high fidelity AE sensors in locations that will distinguish between surface and bulk modes of wave propagation generated at a crack tip.

• Do low K (peak stress intensity) fatigue to grow crack, interrupted by short burst higher K loads at select intervals (“overload”) with the assumption that the overloads are the only fatigue cycles that might generate “brittle” crack extension.

• Monitor entire signal generated during overload.

Approach

Examine many AE parameters:

Velocity: Different modes of wave propagation have different velocities

Amplitude: Surface wave component has amplitude much greater than any corresponding bulk wave at surface

Time-Frequency response: Wavelet analysis to create time-frequency plots

Shape of the time domain signal: SSP’s have leading edge characteristics different than subsequent ringdown.

Outline

• Motivation • Objectives• Insight• Approach• Test development• Experimental results• Summary and Conclusions

Specimen Design

High fidelity conical NIST sensors:2.50"

1.20"1.20"

2 .00"

0.50"

0.90"

3.10"

0.63"

“Surface” channel: monitor for surface waves

Crack tip

Bulk channel: monitoring bulk waves

Modified compact tensile specimen profile with extended lip to support NIST sensor

Specimen Design

High fidelity conical NIST sensors:2.50"

1.20"1.20"

2 .00"

0.50"

0.90"

3.10"

0.63"

Modified compact tensile specimen profile with extended lip

Crack tip

a = 0.55"

1.44"

1.31"

a = 1.30"

1.63"

1.20"

Distinguish bulk wave signals from surface wave signals (on the surface channel)

“Surface” channel: monitor for surface waves

Bulk channel: monitoring bulk waves

Fatigue Crack Extension and Overload Schedule

0

10

20

30

40

50

60

70

80

90

100

110

120

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Crack Length (in)

Stre

ss In

tens

ity K

(ksi

root

in)

1.6 kip1.8 kip2.0 kip2.3 kip2.5 kip1.0 kip

o1o2

o3-o5o6

o7o8

o9

o10

o11

o12

o13

o14

Low K fatigue crack extension (hydraulic load frame)

AE not collected

“High” K Overloads monitored for AE

(modified creep frame)

Overload Test Configuration and Data Acquisition

View of weight pan end

• Modified dead-load creep frame:– Low mechanical and RF noise – Attempted to simulate real loading

conditions (truck on bridge) i.e. dynamic response of specimen is allowed

• Uninterrupted data acquisition:– Digital analogue of a tape recorder

capturing everything

Unloading cable link

Loading weights

Pneumatic cylinder lift

“Overload” Load Traces

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

Time (sec)

App

lied

Load

(kip

s)

50 lb60 lb70 lb80 lb90 lb

• Load threshold triggered data capture• DAC card memory: approximately 0.4 second “AE” captured

(10 Msamples/sec, 4 million data points per channel)

Outline

• Motivation • Objectives• Insight• Approach• Test development• Experimental results• Summary and Conclusions

Surface Channel (“Calibration specimen”)

Brittle fracture of fully hardened A2 steel (tensile strength > 300 ksi):

200 mV Amplitude

Note magnitude of the higher frequency

content

SSP profile

Bulk Channel (“Calibration specimen”)

50 mV Amplitude

Brittle fracture of fully hardened A2 steel (tensile strength > 300 ksi):

< 1/2 magnitude of surface channel high

frequency content

Partially ductile crack extension: Surface Channel

Not the failure overload of an A2 steel not fully brittle (Yield strength =250 ksi)

Partially ductile crack extension: Bulk Channel

Not the failure overload of an A2 steel not fully brittle (Yield strength =250 ksi)

Fully Ductile Failure: Surface Channel

1018 steel (yield strength = 65 ksi): final failure by “plastic hinge effect”

Fully Ductile Failure: Bulk Channel

1018 steel (yield strength = 65 ksi): final failure by “plastic hinge effect”

If one ignores the very brittle failures and very soft loading pin hole plasticity events: less than 100 events could be identified as seismic surface pulses (SSP).

SSP events did occur early in fatigue life but not for all specimens, so for these tests there is no correlation between fatigue life and SSP creation.

Overload AE Summary

Fatigue Summary

Fatigue Crack Extension Summary

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

0 200 400 600 800 1000 1200 1400

Kilocycle

Cra

ck le

ngth

(in)

OverloadsA36A5881018HPS70A514A2,r50A2,r60

Overload 1

Overload 2

Overload 3-5

Overload 6

Overload 7

Overload 8

Overload 9

Overload 10-

Total: 3.8 M cycles for all 7 specimens

Fatigue Crack Extension Summary

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

0 200 400 600 800 1000 1200 1400

Kilocycle

Cra

ck le

ngth

(in)

OverloadsA36A5881018HPS70A514A2,r50A2,r60

Overload 1

Overload 2

Overload 3-5

Overload 6

Overload 7

Overload 8

Overload 9

Overload 10-

The number of SSP AE per cycle (overload and crack propagation)

is less than 0.01%

Where is all the AE?

It is a real possibility that even though we think “overloads” are the bad actors, the AE we monitor does not come from those events.

Fatigue Summary

Total: 3.8 M cycles for all 7 specimens

Outline

• Motivation • Objectives• Insight• Approach• Test development• Experimental results• Summary and Conclusions

Summary

• Near-failure fatigue AE behavior was investigated with a comprehensive approach to:– specimen design, – test development, – data analysisto distinguish brittle behavior AE from other AE sources to investigate

the hypothesis that brittle behavior could be a mechanism that accelerates crack rates near failure even from ductile materials.

• Blunt notch w/overload results: Crack tip plastic zone formation does not create a seismic surface pulse, hence, does not create false positive. Hence all sources in the plastic zone such as inclusion fracture cannot generate an SSP

• Seismic surface pulses were created during overloads that produced failure or near failure, but also at overloads earlier in fatigue life:

– Non-catastrophic cracking of brittle material in fatigue (near-failure)– Ductile failure.

Conclusions

Hypothesis• Can crack extension in real materials transition from ductile to brittle

behavior such that it approaches an Idealized Brittle Crack Extension (i.e. generate SSP’s and Rayleigh waves on crack faces)? Yes

• Can it be used to track crack growth? Inconclusive

• The fact that I could identify experimental signal content that resemble an SSP generated by a possibly fictitious “idealized brittle crack extension” using a test methodology designed to distinguish between SSP’s and other sources of AE indicates that this methodology may be useful to investigate the mechanisms of fracture and subsequent AE that can occur approaching failure under various conditions.

“Change your tools to get new results”

Future Work

• Tests more conducive to catastrophic failure:– cold temperatures (ductile-brittle transition), – corrosive environments (stress corrosion cracking) – or otherwise detrimental (hydrogen embrittlement).

• Redesign specimens:– more isolation between the surface and the bulk modes of AE – reduced potential for plastic hinge failure.

1. A conceptual model that illustrates the possibility of surface modes of wave propagation being generated from crack extension events.

2. An investigation into many aspects of AE monitoring in finite objects3. A unique testing paradigm: specimen design, experimental method,

and data analysis.4. Criteria to determine if an AE event is the type we are looking for.5. Plastic zone formation during fatigue, a secondary AE source, does

not seem to produce surface modes of wave propagation.6. Non-catastrophic crack extension deformation in some steels does

seem to produce surface modes of wave propagation that have only been shown, by others, to exist during catastrophic fracture.

7. Experimental results support the conceptual model presented in the first contribution.

Contributions