department of mechanical and manufacturing engineering shear strength measurement on metal/polymer...

Department of Mechanical and Manufacturing Engineering

SHEAR STRENGTH MEASUREMENT ON METAL/POLYMER INTERFACE USING FRAGMENTATION TEST

S. Charca, O. T. Thomsen


Aalborg University, Aalborg Denmark

CompTest 2011, Lausanne


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


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.


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

Introduction (cont.)


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.


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


Specimens design

From the ECrit. and rules of mixture.

Where:

)1( fmffCrit VEVEE

2207000

mm

NE f 2

1151mm

NEm

003375.0fVFiber fragmentation occurs if:

mmDsteel 10.0 233.2 mmAT

Minimum sample cross section for fragmentation test

Fiber

Matrix

uf um

Composite

ultm

ultf

ECrit

Obtained at 0.05mm/min

Fragmentation occurs if: E < ECrit

ultf

utlmCritE

.


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

220 mm30mm

R70 mm

20 mm

15 mm

6 mm

Final sample dimensions


Experimental setup

Light source

Polarizer

Analyzer

Microscope and camera

Sample

Load cellLoad

Grip Grip

Light source

Polarizer

Analyzer

Microscope and camera

Sample

Load cellLoad

Grip Grip

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


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

Filament failure mode

DB DB

PN

CN CN

CN-F

DB ---- DebondingPN ---- Partially NeckingCN ---- Completely

NeckingCN-F ---- Completely

Necking & Fracture


0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8

Strain (%)

Str

es

s (

N/m

m2 )

AAU_#5_02Fragmentation

Photoelasticity and isochromatic fringes

Typical stress/strain curve on dogbone fragmentation specimens and the corresponding polarization image observed during the test @ ~5.33%

Light areas appears around the filament, which is an indication of apparent interface debonding


Photoelastic birefringence around the filament fragments at ~37N/mm2 and ~ 5.70%

Microscopic image at ~37N/mm2 and ~ 5.70%. (Non treated steel filament)

In the fragmentation experiments high intensity fringe patterns were observed (light or dark, depending of the polarization angle).

High stress concentration zones

Matrix is purely subjected to tension


Fragment length data discrimination

ave1 ave2 ave3 ave4

Leff

A2<A4<A1<A3

• Dependent on the specimen cross sectional area, distinct differences in the number of fragments per specimen unit length were observed• In the zones 2, 4, and 1 the saturation limit was reached and the samples failed• Longer fragment lengths were observed in zone 3 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

Filament fragment representation along the sample


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-treatedfilament surface no. of fragments:

284

Treated filament surfaceno. 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

c

f

l

d

2

avec ll3

4

b (mm) 15 Dia (mm) 0.1L (mm) 210

ult (N/mm2) 3016

Samplet

(mm)

max

(N/mm2)

E

(N/mm2)

max

(%)

~ @1st Frag(%)

Frag. Length(mm)

SD(mm)

Number ofFragments

D. M. F.Lc

( mm)

(N/mm2)

AAU_#8_01 6.70 40.45 1156 5.86 4.96 1.758 1.187 84 PN 2.34 64.32AAU_#8_02 6.20 37.94 1287 5.08 4.91 1.965 1.198 51 DB 2.62 57.55AAU_#8_03 6.25 39.71 1251 5.93 5.10 1.724 1.056 88 CN-F 2.30 65.61RISO_#8_02 7.15 42.94 996 6.04 5.29 1.509 0.785 55 DB 2.01 74.95RISO_#8_03 6.60 44.03 1238 5.31 5.24 1.791 0.882 10 PN 2.39 63.15RISO_#8_04 6.60 44.71 1325 5.29 5.24 1.471 0.675 17 CN-F 1.96 76.91

Ave. 1.703 Ave. 67.08SD 7.41

AAU_#5_01 6.05 42.21 1281 5.14 5.00 1.199 0.803 110 DB 1.60 94.35AAU_#5_02 6.80 43.59 1048 8.75 5.33 1.379 0.656 136 CN 1.84 82.04RISO_#5_01 6.00 43.59 1399 5.24 5.12 1.093 0.702 75 DB 1.46 103.45RISO_#5_04 6.50 42.72 1728 4.95 - 1.197 0.653 30 DB 1.60 94.47

Ave. 1.217 Ave. 93.58SD 8.79

Non-treated filament surface

Treated filament surface


0

1000

2000

3000

4000

5000

6000

7000

8000

0 0.2 0.4 0.6 0.8 1

x/(L/2)

Fib

er a

xial

str

ess

(N/m

m2)

L = 1.0 mmL = 1.6 mmL = 2.0 mmL = 4.0 mmL = 8.0 mmL = 20.0 mm

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

FEA modeling

ult = 3016 N/mm2 (Steel)

Calculated critical fragment length for filament failure using FEA:LcFEA = 1.65mm

Experimental average fragment length: LcExp = 1.70mm

Filament under study

Sym


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


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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