DEBOND FRACTURE CHARACTERIZATION IN SANDWICH COMPOSITES UNDER ARCTIC LOW TEMPERATURE CONDITIONS

Transcription

1 DEBOND FRACTURE CHARACTERIZATION IN SANDWICH COMPOSITES UNDER ARCTIC LOW TEMPERATURE CONDITIONS Arash Farshidi 1, Christian Berggreen 1 1 Department o Mechanical Engineering, Technical University o Denmark Nils Koppels Allé, Building 404, DK-800 Kgs. Lyngby, Denmark ara@mek.dtu.dk, cbe@mek.dtu.dk, web page: Keywords: Sandwich composites, mixed-mode racture, MMB, oam core, low temperature ABSTRACT Mixed-mode I/II racture characterization and measurements o low temperature racture properties or typical naval type sandwich material conigurations has been carried out using the mixed mode bending (MMB) test ixture inside a state-o-the-art climatic chamber acility. The developed test methodology is utilized to measure mixed mode racture toughnesses as well as crack propagation speeds (da/dn) vs. cyclic energy release rate data, investigating and comparing ambient and low temperature ace/core racture behaviour at dierent mode-mixities in Navy type sandwich composites. Sandwich specimens were manuactured using H100 PVC oam cores and E-glass/Epoxy ace sheets. All specimens were pre-cracked in order to deine a sharp crack ront. 1 INTRODUCTION Sandwich structures are considered as key enablers or uture and present lightweight structural applications in naval ships because o their superior stiness/weight and strength/weight ratios compared with traditional metallic as well as monolithic structures made rom composite materials. To permit high-perormance manuacture o naval vessels, it is inevitable that the structural design o the vessel will necessitate large unstiened panels that are susceptible to large deormation under repeated cyclic loading, caused by a combination o in-service loading rom a variety o sea states as well as accidental loads. Failure oten occurs as a consequence o either manuacturing laws or in-service loads experienced by the structure, such as general overload and impact events, eg. hull bottom slamming, which exacerbate the eect o the ongoing atigue loading. Figure 1: Royal Danish Navy patrol vessel in ice illed waters near Greenland

2 Naval vessels are expected to operate in a variety o climatic conditions, including arctic regions, see Figure 1. Thereore, the ability or the Navy to operate saely in arctic polar regions under the presence o ice, highlights the need or urther research aimed at the inluence o low temperature operational conditions on debond racture growth in sandwich composites. EXPERIMENT PROCEDURE Sandwich composite specimens has been manuactured using Divinycell H100 cross-linked PVC oam cores bonded to quasi-isotropic ace sheets made rom 850 g/m E-glass/Epoxy non-crimp multi-axial (0/45/90/-45) Devold AMT DBLT-850. The mechanical properties o the ace sheets and core have been listed in Table 1. Properties H100 Cell size [mm] 0.45 Density [kg/m 3 ] 100 Tensile modulus [MPa] 130 Tensile strength [MPa] 3.5 Compressive modulus [MPa] 135 Compressive strength [MPa].0 Shear modulus [MPa] 035 Shear strength [MPa] 1.6 Shear strain [%] 40 Fracture toughness G c [J/m ] 310 Face DBLT-850 (0/45/90/-45) Young s modulus (E x) [GPa] 18.6 Poisson s ratio (v xy) Shear modulus (G xy) [GPa] 6.1 Table 1: Mechanical properties o H100 PVC oam core and iber glass sheets [1] Figure 1: Navy type sandwich composite MMB specimen The MMB sandwich specimens used in this experiment have been cut out o the test panel which has been manuactured using a resin inusion process. The artiicial debond starter crack has been created by placing a thin Telon ilm insert at the upper ace/core interace beore resin injection. However, it has been observed or sandwich composite specimens with H100 core that a crack can propagate either in the core, in the ace sheet or in between the ace and core depending on the mode-mixity at the crack tip and the racture toughnesses o three crack positions respectively [1]. The specimens have been cut in a rectangular shape with

3 35 mm width and 15 mm length, span length o 160 mm (L), core thicknesses o 10 and 0 mm, and ace sheet thickness o mm. In the resin inusion process, the ace sheets have been impregnated with resin and the oam cells on the top and bottom surace o the oam core at ace/core interace, which were partially open due to the oam core cutting, have been illed by resin orming a resin-rich interace layer with approximately thickness o a single oam cell (i.e mm). The resin-rich layer oten proves to be tougher than the core, and racture may occur in the core, just underneath the resin-rich layer, depending on the mode-mixity at the crack tip and toughness o the interace []. Also a pre-cracking technique has been used to ensure a realistic and sharp crack tip. Figure : MMB test ixture and the climatic chamber The climatic chamber and the MMB test ixture are shown in Fig.. It has been proved that the MMB test can be considered a superposition o the mode II CSB (Cracked Sandwich Beam) and mode I DCB (Double Cantilever Beam) and the load P, can be partitioned into CSB and DCB using a load partitioning parameter α [3]. The MMB compliance and energy release rate depend on the crack length, a, ace and core thicknesses, mechanical properties o the sandwich constituents, geometry o the specimen and loading conditions controlled by the lever arm distance, c. Analytical expressions or the MMB compliance, C, and energy release rate, G, have been derived previously [3]: [ c c L ]( c c L DCB upper DCB lower ) ( c C C C L ) CCSB (1) L L L L L c c c L ( ) [ a a ] 3 L L L Eh P G b c L c c L 1 a c L a 1 1 ( )[ ] ( ) ( [ ]) L L L hcgxz B L 8 Ddebonded Dint act ( D ) A Where decomposed compliance components are: () C a 1 a [ ] 3( D ) A DCB _ lower b hcgxz B C [ a 3a 3 a ] (4) E h b DCB _ upper 3 (3)

4 C CSB 3 3 L L a 1 1 [ ] (5) 6bD h bg 1b D D int act c xz debonded int act The load partitioning actor and elastic oundation modulus parameter can be expressed as: 3 a 1 a 1 3 B k G h Gxzhc D [ A ] 3 3 a 1 a 1 a 1 a B k G h Gxzhc 3 E h k G h D A 1 (6) 4 hbe (7) 6bE Where k is the shear correction actor (k=1.) and also A, B and D are the extensional, coupling and bending stinesses and D debonded and D intact are the lexural stinesses o the debonded region and intact region o the cracked beam respectively. Further details are provided in [3]. Quasi-static tests at -0C with dierent mode-mixities have been carried out in order to characterize the debond racture o oam core sandwich composite at typical arctic conditions. To this end, several specimens have been loaded in displacement control mode inside a climatic chamber to achieve the critical load, P c, in accordance with the MMB standard (i.e. ASTM D6671) where it has been deined as the load at which the compliance has increased by 5%, or the maximum load, depending on which occurs irst along the load displacement curve [4]. Substituting the measured P c in Eq. () leads to the critical energy release rate (i.e. racture toughness). The mode-mixity at the crack tip has been controlled by lever arm position, c, and tests have been done at several c values in order to evaluate the eect o temperature on racture toughness, G c, at dierent mode-mixities. c 9 CONCLUSIONS Mixed-mode I/II racture characterization o low temperature racture properties or typical naval type sandwich material conigurations has been carried out using the mixed mode bending (MMB) test ixture inside a state-o-the-art climatic chamber acility. It has been observed that the measured racture toughnesses or dierent mode mixities decrease at -0C compared to similar measurements at ambient temperatures. ACKNOWLEDGEMENTS Financial support rom the US Navy Oice o Naval Research, Grant N , and the interest and encouragement o the Grant Monitor, Dr. Y.D.S. Rajapakse, are grateully acknowledged. REFERENCES [1] M. Manca, A. Quispitupa, Ch. Berggreen, L. A. Carlsson, "Face/core debond atigue crack growth characterization using the sandwich mixed mode bending specimen," Elsevier, pp , 01.

5 [] A. Quispitupa, Ch. Berggreen, L. A. Carlsson, "Face/core interace characterization o mixed mode bending sandwich specimens," atigue racture engineering material structure, pp , 011. [3] A. Quispitupa, Ch. Berggreen, L. A. Carlsson, "On the analysis o a mixed mode bending (MMB) sandwich specimen or debond racture characterization," Engineering Fracture Mechanic, pp , 009. [4] ASTM D6671/D6671M-13, Standard test method or mixed mode I-mode II interlaminar racture toughness o unidirectional iber reinorced polimer matrix composites, 013.