In Situ Microstructure-Sensitive Investigation of Fracture by Coupling
Acoustic Emission with Scanning Electron Microscopy
B. Wisner1, M. Cabal1, P.A. Vanniamparambil2, J. Hochhalter3, W. Leser3, A. Kontsos1
1Department of Mechanical Engineering & Mechanics, Drexel University, USA
2Corning Inc, USA
3NASA Research Center, Langley, VA, USA
[Meso-Scale Testing]
Introduction
Microstructure is known to have a crucial effect on the properties of materials [1]. The
objective of this investigation is to connect micro-damage mechanisms to the observed
acoustic emission (AE) response in order to determine the damage state of the
material. To accomplish this goal, a novel experimental technique combining in situ
Scanning Electron Microscopy (SEM) with AE monitoring was developed. Results of AE
monitoring agree well with findings from a similar meso-scale investigation, while it was
directly (in terms of time and location) correlated with several damage mechanisms in
aluminum alloys such as void coalescence, precipitate fracture and microcracking.
500 kHz Frequency
appears at yield [2]
Load (N)
Above: BSE image of grain structure
Below: EBSD image of grain structure
Inverse Pole
Figure for the
[100] direction
5000
90
4000
80
3000
70
2000
60
1000
50
Amplitude (dB)
100 μm
Methodology
0
40
0
50
100
150
200
250
Time (s)
[Ex-Situ Micro-scale Testing]
Major Observations
100
4000
3000
80
2000
70
60
1000
50
Amplitude (dB)
90
Load (N)
0
40
0
200
400
600
800
Time (s)
Micro-scale Testing]
EMI
signal
100
2000
1500
80
70
1000
60
500
50
0
-200
Amplitdue (dB)
90
Load (N)
[Meso-Scale Testing]: A commercially available
Aluminum 2024-T3 plate was machined to 44 X
9mm specimens with two sharp notches. The
specimen was loaded at 0.5 mm/min while AE data
was monitored.
[Ex-Situ Micro-scale Testing]: Aluminum 2024-T3
Specimens with the same geometry used at the
meso scale were tested in a MTI/Fullam loading
stage under displacement control. Specimens with
two different thicknesses were used to determine
geometric effects. AE signals obtained were
compared to those obtained at the meso-scale.
[In-Situ Micro-structure Testing]: The
specimens were then tested using the
MTI/Fullam loading stage inside the
SEM. AE signals were obtained at the [In-Situ
same time SEM images were taken.
Post mortem evaluation revealed
possible sources of AE signals.
[Low Noise Testing]: Meso-scale
testing was repeated in a reduced noise
environment allowing lower recordings
threshold. The load was halted after the
appearance of the first amplitude above
40dB.
Theoretical & Applied Mechanics Group
Final
signal
• AE signals can be obtained inside the
SEM.
• Signals obtained in the microscope
are identical to those obtained outside
the SEM.
• SEM images can be obtained and
coupled with AE signals to determine
potential sources.
• Low amplitude signals exist prior to
macroscopic peak load, which could
indicate early damage mechanism
activation prior to the appearance of
damage (primarily cracking) at the
macroscale.
[Low Noise Environment Testing]
40
0
200 400 600 800 1000 1200 1400 1600
Time (s)
References
1200
1000
50
800
45
600
40
400
35
200
30
0
0
100
200
Time (s)
300
400
Amplitude (dB)
55
Load (N)
[1] D. L. McDowell, K. Gall, M. F. Horstemeyer,
and J. Fan, "Microstructure-based fatigue
modeling of cast A356-T6 alloy," Engineering
Fracture Mechanics, vol. 70, pp. 49-80, 2003.
[2] P. A. Vanniamparambil, U. Guclu, and A.
Kontsos, "Identification of Crack Initiation in
Aluminum Alloys using Acoustic Emission,"
Experimental Mechanics, 2015.
[3] B. J. Wisner, M. Cabal, P. A.
Vanniamparambil, J. Hochalter, A. Kontsos,
“Identification of Acoustic Emission Sources
using In Situ Microscopy,” Society for the
Advancement of Material and Process
Engineering, Baltimore, MD, 2015.