SHEAR STRENGTH MEASUREMENT ON
METAL/POLYMER INTERFACE USING
FRAGMENTATION TEST
S. Charca, O. T. Thomsen
Department of Mechanical and Manufacturing Engineering
Aalborg University, Aalborg Denmark
CompTest 2011, Lausanne
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Overview
 Introduction
 Objectives
 Sample manufacturing and experimental procedure

Results and analysis
 Filament failure mode
 Photoelasticity and isochromatic fringe patterns
 Fragment lengths
 Finite element analysis validation
 Conclusions
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Introduction

The mechanical properties and performance of polymer composites
materials are to a large extent determined by the interface properties.

There are several methods that are currently used to characterize the
interface properties such as single fibre pull-out, micro-tension, microindentation and fragmentation tests.

The single fibre fragmentation test method appears to offer some
advantages compared with other methods (e.g. single fiber pull out and
micro indentation tests) for assessing the fiber-resin interface shear
strength. Moreover it offers the advantage over the other methods that
the number of fragments that can be obtained from one single test
specimen is typically large, thus enabling a complete statistical analysis.

The fragmentation test was proposed initially by Kelly and Tyson (1965)
based on their work on tungsten fibres embedded in a Cu matrix.
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Introduction (cont.)

The low cost and high mechanical properties of the steel filament/cord
compared to the traditional carbon/glass fibers are the main motivation to
the start exploring the potential and reliable application of polymers
reinforced by steel filament/cord for civil engineering, automotive, wind
turbine and others applications

A significant “challenge” in polymers reinforced by steel filament/cord is
the resin-steel interface properties
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Objectives
The objectives of this research include:
 Study the interface properties of single steel filament embedded in
a resin.
 Achieve multiple fragmentations of steel filaments embedded in an
unsaturated polyester matrix.
 Determination of the failure mechanisms.
 Perform a statistical analysis including a data discrimination
process.
 And finally to determine the interface shear strength using the
Kelly and Tyson criterion.
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Sample manufacturing
 Steel filaments:
 Zinc coated ultra high strength steel filament D = 0.1mm
 Sizing: Silane with amino functionality
 Resin: Unsaturated polyester
 Samples were manufactured by casting using treated (sizing) and
non treated filaments
 10 dogbone samples were manufactured for each type of filament 5 samples were made at the Risø DTU National Laboratory for
Sustainable Energy (Denmark) facilities and the rest at the AAU
facilities
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Specimens design
E f  207000
N
mm 2
Em  1151
N
Obtained at 0.05mm/min
2
mm
Fragmentation occurs if: E < ECrit
s
Where: ECrit .  utlm
e ultf
Fiber
sultf
From the ECrit. and rules of mixture.
ECrit  E f V f  Em (1  V f )
Composite
Fiber fragmentation occurs if:
sultm
V f  0.003375
ECrit
Minimum sample cross section for fragmentation test
Dsteel  0.10mm
AT  2.33mm2
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euf
Matrix
eum
Final sample dimensions
30mm
220 mm
20
mm
15
mm
R70 mm
6 mm
In order to fix the filament into the mould in the manufacturing process and
avoid non uniform stress distribution along the filament; filaments were preloaded in tension during the casting and curing process using a 200g weight
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Experimental setup
Fragmentation processes were monitored using the photoelasticity technique, with a
50X magnification stereomicroscope
After samples fails, the specimens were polished until to obtain a mirror surface to
observe and measure the filament fragments
Loading rate: 0.05mm/min
Microscope
and camera
Analyzer
Load cell
Load
Grip
Grip
Sample
Polarizer
Light source
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Filament failure mode
Filament failure in the resin displayed a defined pattern as shown using 50X
magnification
DB
DB
PN
CN
DB ---- Debonding
PN ---- Partially Necking
CN ---- Completely Necking
CN-F ---- Completely Necking &
Fracture
CN
CN-F
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Photoelasticity and isochromatic fringes
Typical stress/strain curve on dogbone fragmentation specimens and the
corresponding polarization image observed during the test @ e~5.33%
50
Fragmentation
AAU_#5_02
45
2
Stress (N/mm )
40
35
30
25
20
15
10
5
0
0
2
4
6
Strain (% )
8
Light areas appears around the filament, which is an indication of apparent
interface debonding
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Microscopic image at ~37N/mm2 and e ~ 5.70%. (Non treated steel filament)
Photoelastic birefringence around the filament fragments at ~37N/mm2 and
e ~ 5.70%
High stress concentration
zones
Matrix is purely subjected to
tension
In the fragmentation experiments high intensity fringe patterns were observed
(light or dark, depending of the polarization angle).
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Fragment length data discrimination
Leff
A1
A2
eave1
eave2
A2<A4<A1<A3
A3
eave3
A4
eave4
e2>e4>e1>e3
Filament fragment representation along the sample
• Dependent on the specimen cross sectional area, distinct differences in the number of
fragments per specimen unit length were observed
• In the zones e2, e4, and e1 the saturation limit was reached and the samples failed
• Longer fragment lengths were observed in zone e3 than in the other zones.
• Accordingly, the fragment lengths in zone 3 have been dismissed from the data processing
The observed fragmentation data shows three different length ranges:
 ~0.5 – 5mm
 ~5 – 8mm
 ~8 – 15mm
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Detailed statistical fitting tests (Kolmogorov-Smirnov and Chi-square) showed that
the fragment length distributions for each specimen fitted with the “extreme
distributions” (Gamma, Gumbel and Weibull).
Histograms show the relative frequencies of occurrence of different fragment
lengths.
Non-treated
filament surface
no. of fragments: 284
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Treated
filament surface
no. of fragments: 329


Summary of results of the fragmentation test after data discrimination
The apparent interface shear strengths were calculated using the Kelly and
Tyson relation considering the critical fragment length
b (mm)
L (mm)
15
210
Dia (mm)
sult (N/mm2)
0.1
3016
Sample
t
(mm)
smax
(N/mm )
E
(N/mm2)
emax
(%)
AAU_#8_01
AAU_#8_02
AAU_#8_03
RISO_#8_02
RISO_#8_03
RISO_#8_04
6.70
6.20
6.25
7.15
6.60
6.60
40.45
37.94
39.71
42.94
44.03
44.71
1156
1287
1251
996
1238
1325
5.86
5.08
5.93
6.04
5.31
5.29
2
Non-treated filament surface
AAU_#5_01
AAU_#5_02
RISO_#5_01
RISO_#5_04
6.05
6.80
6.00
6.50
42.21
43.59
43.59
42.72
1281
1048
1399
1728
5.14
8.75
5.24
4.95
Treated filament surface
4
lc  l ave
3
~e @1st
Frag
(%)
4.96
4.91
5.10
5.29
5.24
5.24
Ave.
Frag.
Length
(mm)
1.758
1.965
1.724
1.509
1.791
1.471
1.703
5.00
5.33
5.12
Ave.
1.199
1.379
1.093
1.197
1.217

Lc
( mm)

(N/mm2)
PN
DB
CN-F
DB
PN
CN-F
2.34
2.62
2.30
2.01
2.39
1.96
Ave.
SD
64.32
57.55
65.61
74.95
63.15
76.91
67.08
7.41
DB
CN
DB
DB
1.60
1.84
1.46
1.60
Ave.
SD
94.35
82.04
103.45
94.47
93.58
8.79
SD
(mm)
Number of
Fragments
D. M. F.
1.187
1.198
1.056
0.785
0.882
0.675
84
51
88
55
10
17
0.803
0.656
0.702
0.653
110
136
75
30
sfd
2lc
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FEA modeling


Sym
ANSYS 12.1
Assumption: Material is linear elastic
Element type: 2D plane183 (Axisymmetric 32000 elements)
Perfect interface bonding assumed
Thermal analogy for resin shrinkage



8000
7000
2
Fiber axial stress (N/mm )
Filament under
study
L = 1.0 mm
L = 1.6 mm
L = 2.0 mm
L = 4.0 mm
L = 8.0 mm
L = 20.0 mm
6000
5000

sult = 3016 N/mm2 (Steel)

Calculated critical fragment
length for filament failure using
FEA:
LcFEA = 1.65mm

Experimental average
fragment length:
LcExp = 1.70mm
4000
3000
2000
1000
0
0
0.2
0.4
0.6
0.8
1
x/(L/2)
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Conclusions







Fragmentation tests were successfully implemented with single steel filaments
embedded in polyester resin.
The fragmentation process start with debonding, followed by necking
(yielding) and finally fracture of the steel filaments.
Filament fragmentation starts to develop at specimen longitudinal strains
exceeding ~4.90%.
Fragmentation length distributions fit the “extreme distributions” (Gamma,
Gumbel and Weibull).
The apparent interface shear strengths derived using the Kelly and Tyson
equation are very large.
The experimentally observed critical fragment length was confirmed using
Finite Element Analysis
Apparent improvement of the interface shear strength was observed for
samples manufactured using surface treated steel filaments
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Acknowledgement
 The research reported was sponsored by the Danish National Advanced
Technology Foundation. The financial support is gratefully acknowledged.
 The authors wish to thank Dr. Jakob I. Bech, Dr. Hans Lilholt, Mr. Tom L.
Andersen, Dr. R.T. Durai Prabhakaran and other colleagues at Risø
National Laboratory for Sustainable Energy, Technical University of
Denmark, for inspiring discussions
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Questions?
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