NASA Technical Reports Server (NTRS) 20150004111: Correlation of Electrical Resistance to CMC Stress-Strain and Fracture Behavior Under High Heat-Flux Thermal and Stress Gradients

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by  NASA Technical Reports Server (NTRS)

Correlation of electrical resistance to CMC stress-strain
and fracture behavior under high heat-flux thermal and
stress gradients
Matthew Appleby* †, Gregory Morscher*, Dongming Zhu†
*The University of Akron, Akron OH
† NASA Glenn Research Center, Cleveland OH
39th International Conference and Exposition on
Advanced Ceramics and Composites. Daytona Beach, FL
January 29, 2015
Introduction
(cid:190) Electrical Resistance (ER) monitoring has been shown as a tool for detecting
room temperature tensile damage accumulation in woven melt-infiltrated (MI)
SiC/SiC CMCs.
(cid:190) However, under stress gradients and complex thermo-mechanical loading,
mechanisms controlling contributions to ER change are still not well
understood.
(cid:190) In this study, experiments were chosen to explore the capabilities of ER as a
relevant non-destructive evaluation (NDE) technique for testing under high-
temperature thermal gradient conditions, with stress concentrations.
(cid:190) To overcome the disadvantage of far-field in-situ ER measurement, notched
samples provided a highly localized strain field in gage section for ER
correlation
(cid:190) Acoustic Emission (AE) measurements and Digital Image Correlation (DIC)
were also performed to correlate with ER results
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Experimental Material
(cid:190) SiC/SiC CMC material (Hyper-Therm HTC)
(cid:190) 8 plies, balanced 5 harness satin 2D woven 0°/90°, SiC/BN/SiC
(cid:190) Hi-Nicalon Type-S fiber reinforced
(cid:190) Produced by molten silicon melt infiltration (MI)
(cid:190) Machined into 6 in. tensile bars
w = 12.74 mm w = 12.82 mm
(10.77 mm) (10.25 mm)
t = 2.27mm
t = 2.17mm
f = 0.141
f = 0.147 High Temperature
Room Temperature 0
0
DOUBLE NOTCH
SINGLE NOTCH
1.22mm 1.05mm
1.97mm 1.29mm
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Experimental Technique
(cid:190) Specimens are loaded in uni-
axial tension rig
(cid:190) Digital Image Correlation (DIC) is
used to determine localized
strain fields
(cid:190) Nominal strain measurements
are taken from using a 25.4 mm
extensometer with a ±0.5 mm
travel
(cid:190) High temperature testing:
(cid:190) Face of specimen gage-section
heated by a 3.5kW CO high
2
heat-flux laser
(cid:190) Asymmetrical heating by laser
generates thermal gradients
(thru thickness and longitudinal)
(cid:190) Thermal gradients can be
increased by the addition of
active back side air-cooling
(cid:190) Front and back temperatures of
the heated region are monitored
by optical pyrometers
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NDE Measurement
Gripped
(cid:190) Electrical Resistance (ER)
region
measured by four-point probe
method
(cid:190) In order to avoid high
temperature exposure during
laser heating, ER leads for in-
Voltage
situ measurement are Constant difference
current to outer measured by Modal AE
attached within the gripped
electrodes inner
Sensors
areas electrodes
(cid:190) Acoustic Emission (AE)
sensors are attached ±40 mm
from center
(cid:190) Post-test ER “inspection” is
performed by taking discrete
Gripped
measurements along the region
specimen length
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NDE Principles
Modal Acoustic Emission Monitoring
(cid:190) Fracture energy of solids released as elastic waves
which are detected by the use of wide-band sensors
in order to quantify stress-dependent cracking
Modal AE
initiation and accumulation. Sensors
AE event
(cid:190) Location of AE events estimated by the difference in
arrival times of AE signals
(cid:1874)
(cid:1876) (cid:3404) (cid:1872) (cid:3398) (cid:1872)
(cid:3029)(cid:3042)(cid:3047)(cid:3047)(cid:3042)(cid:3040) (cid:3047)(cid:3042)(cid:3043)
(cid:884)
Fiber/Matrix
Electrical Resistance Measurement Debond
Region
(cid:190) Damage in the form of matrix cracks and associated
fiber debonding/sliding increase the overall electrical
Matrix Fiber
resistance of the composite specimen
(cid:190) Matrix cracking of MI SiC/SiC is especially sensitive
due to the highly conductive matrix formed from
excess silicon deposits left from processing
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Room Temperature – Single Notch
(cid:190) Post-test microscopy revealed
high crack densities is the vicinity
of the notch, as would be
expected from locally increased
stress.
8mm
(cid:190) However, due to a large density
of porosity, another localized
Fracture plane
strain field was produced, leading
to the ultimate failure location
Crack densities were therefore calculated for the
notched region and the far-field regions
-12.7mm < x < -5mm ± 5mm 5mm < x < 12.7mm
Crack Density, 2.1 2.5 1.70
mm-1
(far-field/notch) (0.85) 1 (0.68)
1 mm
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Damage Location
(cid:190) Damage location determined from AE events in good agreement with DIC
strain-field mapping of gage section at peak load
σ = 324 MPa
Notch
Ultimate Failure Plane
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Damage Growth: Notch vs Far-field
(cid:190) Acoustic energy has been
shown to be directly related to
transverse matrix cracking in
CMCs
(cid:190) AE events separated by each
region
(cid:190) The cumulative energy was
normalized in all regions by
the total energy recorded in
the notched region (± 5mm
from mid-plane of specimen)
(cid:190) The area around the notch
shows increasing damage at
lower stress and higher total
energy released; which is in
good agreement with AE/DIC
location analysis
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In-situ electrical behavior
(cid:190) While the elastic region shows
little increase, ER begins to
increase rapidly with damage
onset.
(cid:190) The electrical resistance of
the entire specimen increases
over 500% with an increase in
nominal strain of the gage
section of 0.5% at failure.
(cid:190) ER is monitored over the
entire specimen showing both
the contribution to localized
and far-field damage
(including all matrix crack and
fiber sliding)
(cid:190) However, as previously noted,
the damage accumulation is
Inner leads
being dominated by the stress 25.4mm gage (measurement Outer leads
distance = 121mm)
concentration region
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