Crack-induced Computational Model for Acoustic Emission
Jefferson Cuadra and Antonios Kontsos
Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, PA 19104
FRACTURE TEST
INTRODUCTION
Structural Health Monitoring (SHM) applications are currently
transitioning from periodic inspections to advanced conditionbased evaluations to minimize maintenance costs in addition to
reduce retrofit and service time interruption.
Theoretical & Applied Mechanics Group
DYNAMIC SIMULATION
A fracture toughness experiment was performed following ASTM
1820 to extract all accessible parameters. The mechanical test was
coupled with Digital Image Correlation (DIC) and AE.
(ii)
(iii)
(i)
Pure Mode I
Figure 4: (i) Compact tension (CT) Mode I boundary conditions (ii) optical Image of
fractured specimen, and (iii) DIC surface strain field results
EXTENDED FINITE ELEMENT METHOD
Figure 1: Next generation SHM condition-based schematic for aerospace structures [1]
The capabilities of Acoustic Emission (AE) institute it as a unique
SHM tool, attributes include: (i) damage-induced phenomenon, (ii)
real-time detection, (iii) identifies and characterizes nucleating and
evolving damage.
Research Motivation:
XFEM is a tool to model both crack initiation
and propagation. Incorporates the crack as
an enrichment part into FEM basis functions.
Quantifies the magnitude of the discontinuity
(displacement jump across crack faces)
using a Heaviside function.
M
Despite the advantages of AE and recent technological
improvements, there are still issues and challenges regarding AE
as a tool for SHM. These include: (i) insufficient quantitative
evaluation and validation of recorded signals and (ii) difficulty
interpreting the AE source utilizing inversion methods.
H(x)=sign(ξ)
ξ (=
x) min x − xΓ sign(n ⋅ ( x − xΓ ))
aI Jump Discontinuity
Nodes in elements cut
b ( el )
+F
t ( el )
+F
c ( el )
θ
θ
θ
θ

fα ( x) =  r sin , r cos , r sin θ sin , r cos θ cos 
2
2
2
2

For isotropic material
(ii) Analyze and decompose AE generation and wave propagation
Extract Modeling
Parameters
Static FEM
Critical Damage
Initiation
Transient
Dynamic FEM
N Λ Nodes from elements at crack tip
A forward AE model is
implemented. XFEM is used to
identify crack initiation and
studied the dynamic response
(i.e. wave propagation)
ACOUSTIC EMISSION
The release of transient elastic stress waves that are due to the
sudden redistribution of energy by localized sources.
Crack
Twinning
Matrix Failure
RESULTS
STATIC SIMULATION
 crack initiation and propagation
 fiber fracture
 delamination
 dislocation motion/glide
 twinning
 yielding
 hardening
 phase transformations
Signal
Load
Fatigue Crack
20 μm
400 µm
Figure 2: Deformation and failure mechanisms
associated with AE
CONCLUSIONS
• The forward AE model successfully linked the experimental parameters to a
computational wave propagation simulation
• A shift of the peak frequency is shown as a function of distance and time from the
crack source providing information about the sensor type selection and location
• Scattering role of geometrical parameters in the computational models, such as the
pin/holes in simulated compact tension specimens identify possible sensor locations
• The effect of plasticity on simulated travelling waves ahead of the crack tip was
investigated and revealed nonlinear interaction as second harmonics
REFERENCES
[1] W.J. Staszewski et al. / Comp. Sc. and Tech. 69 (2009) 1678–1685
[2] Boller, C. / I. J. of Systems Science 31.11 (2000): 1333-1349.
[3] Belytschko, T et al. Modelling and Simulation in Materials Science
and Engineering, 2009. 17(4): p. 043001-043001
ACKNOWLEDGEMENTS
Flaw
crack path
100 μm 10 μm
Figure 9: Stress free and plasticized velocity waveforms due to a 500 kHz toneburst
bIα Nodal DOF (crack tip enrichment )
APPROACH
(i) Construct computational models for fracture-induced sources
BACKGROUND
uˆ = F
Figure 5: Mesh independent
XFEM crack


4

h
α
= ∑ N ( x) uˆ + 
u (x)
H ( x)aI + ∑ fα ( x)bI 
I ∈N
α
1
=

I ∈N Γ


I ∈N Λ


An integrated computational method is developed that attempts to
address difficulties in interpreting experimental AE recordings by
implementing a forward approach. The objectives are:
(iii) Identify and characterize key AE parameters
( el )
Conventional Nodal
shape functions
Figure 6: Stressed continuum
body with XFEM crack
Project Objectives:
u + K
(el) ˆ
Figure 8: Dynamic response due to numerical AE source using fracture model. Time and
frequency domain analysis.
Figure 3: AE phenomenon
Figure 7: (a) Stress contour at critical state, (b) crack profile with overlay
displacement contour