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. Fiber sultf Composite sultm Matrix ECrit euf eum Obtained at 0.05mm/min Fragmentation occurs if: E < ECrit Where: From the ECrit. and rules of mixture. Fiber fragmentation occurs if: Minimum sample cross section for fragmentation test Specimens design

8. 30mm 220 mm 20 mm 15 mm R70 mm 6 mm 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. DB ---- Debonding PN ---- Partially Necking CN ---- Completely Necking CN-F ---- Completely Necking & Fracture DB DB PN CN CN CN-F Filament failure mode • Filament failure in the resin displayed a defined pattern as shown using 50X magnification

11. Fragmentation Photoelasticity and isochromatic fringes • Typical stress/strain curve on dogbone fragmentation specimens and the corresponding polarization image observed during the test @ e~5.33% 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. Leff A1 A2 A3 A4 eave1 eave2 eave3 eave4 A2<A4<A1<A3 e2>e4>e1>e3 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?