SHEAR STRENGTH MEASUREMENT ON METAL/POLYMER INTERFACE USING FRAGMENTATION TEST

1
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

2
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

3
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,
  micro-indentation 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.

4
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

5
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.

6
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

7

• Specimens design

Obtained at 0.05mm/min
Fragmentation occurs if E lt ECrit
Where
From the ECrit. and rules of mixture.
Fiber fragmentation occurs if
Minimum sample cross section for fragmentation
test
8
Final sample dimensions

• In order to fix the filament into the mould in
  the manufacturing process and avoid non uniform
  stress distribution along the filament filaments
  were pre-loaded in tension during the casting and
  curing process using a 200g weight

9
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
10
Filament failure mode

• Filament failure in the resin displayed a defined
  pattern as shown using 50X magnification

11
Photoelasticity and isochromatic fringes

• Typical stress/strain curve on dogbone
  fragmentation specimens and the corresponding
  polarization image observed during the test _at_
  e5.33

Light areas appears around the filament, which is
an indication of apparent interface debonding
12
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).
13
Fragment length data discrimination
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

14
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
Treated filament surface no. of fragments 329
15

• 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

Non-treated filament surface
Treated filament surface
16
FEA modeling

• ANSYS 12.1
• Assumption Material is linear elastic
• Element type 2D plane183 (Axi-symmetric 32000
  elements)
• Perfect interface bonding assumed
• Thermal analogy for resin shrinkage

Filament under study

• sult 3016 N/mm2 (Steel)
• Calculated critical fragment length for filament
  failure using FEA
• LcFEA 1.65mm
• Experimental average fragment length
• LcExp 1.70mm

17

• 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

18
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

19

• Questions?