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The Mechanical Properties Of Sandwich Structures Based On Composite Column Cores

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    Hassan J

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Presented at the 20th International Conference on Composite Materials (ICCM20) Copenhagen, Denmark (2015) Topic Presented: The mechanical properties of sandwich structures based on composite column cores

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    KHALIFA UNIVERSITY Aerospace Research and Innovation Center (ARIC) The Mechanical Properties of Sandwich Structures Based on Composite Column Cores Hassan Jishi Supervisors: Wesley J. Cantwell, Rehan Umer, Khalifa University
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    KHALIFA Latvia Lithuania Denmark rlands Germany Poland Czech Rep Slovakia Austria — Hungary Belarus Ukraine Moldova Romania Croatia Serbia Italy, Bulgaria— GTyrrhenian Sea Greece Mediterranean Sea Tumsia Plack Sea 4TÅrkQyatee Georgia Casptah'Sea Azerbåijan Turkm United Arab Emiråtésx..,--, Khalifa University is undergoing a major expansion. It's current Abu Dhabi campus will quadruple by the end of 2015. Lebanon Iraq Israel' Jordan Egypt Saudi!Arabia
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    Introduction Sandwich panels provide excellent mechanical properties at minimal weight. In recent years, there has been a growing interest in manufacturing and characterizing the properties of metallic lattice structures Structures with metallic truss cores can be stiff, strong and lightweight as state-of the art structural panels (honeycomb cores) in addition to multi-functional capability. Attention is focused on developing composite lattice structures that should, in principle, out-perform their metallic counterparts 35mm 20mm (a) Aluminum octet truss structure (b) copper 3D kagome structure* Wadely Research Group: htt ://www.vir research/wadle /celluar-mat ria s.
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    Introduction Ashby diagram of Strength vs. Density chart for engineering materials Unattainable material space CFRP lattice structures (theoretical limit) • Diamond Composites TiaUoys Steels CFO C,FRP 102 10t 100 attices MMA Ni alloys Metals Ceramics Foams Flexible 10-2 to-2 5/19/2016 Rigid Cork Natural materials Metal foams foams 10-1 Polymers and elastomers 101 102 Density (Mg / m3)
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    Lost Mold Procedure: Materials Removable Core Material: A high quality wax block, or Dissolvable material (Slab of Salt) Reinforcing fabric sheet and Tows: Could be carbon, glass, natural fibres, etc. unidirectional fabric / carbon - UT-C300 Resin System: Epoxy (Prime 20LV) with a fast hardener Weight mix ratio: Geltime: 100 Resin : 26 Hardener 30 minutes @ 250C 5
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    Lost Mold Procedure: Preparation Process The lost mold is prepared by drilling an array of holes reflecting the final configuration of the lattice structure. (a) (b) (c) (d) (a) Vertical column, (b) Pyramidal truss, (c) 3D Kagome lattice structure and (d) octet structure. Fibres were then fed through the array of holes (this can be via either a manual or an automated process). 6
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    Lost Mold Procedure: Preparation Process Two weaving patterns were adopted and these are shown schematically Fibre tows (a) Skin Core (b) The fibre volume fraction within an individual hole was varied by increasing/decreasing the number of fibre tows during the threading process.
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    1. 2. 3. 5 4 3 2 1 Lost Mold Procedure: VARTM Process Distribution medium, was used to facilitate the flow of resin. The edges of the mold were sealed using a vacuum bagging material and a sealant tape. The mold was then infused with resin under vacuum. Resin flow direction 6 7 8 Vacuum pump esi tra Release coated mold Sealant tape Vacuum bagging film Wax block (5) Peal Ply Skin Flow Media (S) Core Resin reservoir 8
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    Lost Mold Procedure: Mold Removal Process The mold was then removed, either by dissolving it (salt) or by heating in an oven (wax). A schematic diagram of the entire process : Vertical drill Circular holes Lost wax mould Carbon fiber tows *XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXA- Infusion process (VARTM) Wax removed by melting in an oven Skin Unit cell 9
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    Lost Mold Procedure This is the first time that a lattice structure of this complexity has been made from composite materials. (b) (a) (d) (D (a) Various views of a structure based on three octet unit cells, (b) Lattice wheel structure, (c) Vertical column core, (d) Pyramidal truss core (e) Single skin pyramidal truss core, and (f) Natural fibre.
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    Vertical Column Core: Manufacturing The composite column cores were manufactured using a lost mold/core procedure. Individual and a Series of vertical columns were manufactured The two weaving patterns were adopted. p = 16.3% 000000 000000 000000 000000 000000 000000 2mm (6x6) p = 26.7% 000000 000000 000000 000000 000000 000000 2.5mm (6x6) p = 30.4% 00000 00000 00000 00000 00000 00000 3mm {6x5) p = 34.90/0 0000 0000 0000 0000 0000 Amm (5x4) 11
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    Micrograph Analysis Typical cross-sections of individual columns following manufacture An examination of the figure indicates that the vertical columns have been successfully infused with resin during the manufacturing process. osga 2mm (a) Vf -0.51 5/19/2016 2.5mm (b) Vf -0.28 3mm (c) Vf -0.28 4mm (d) Vf -0.28 12
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    Compression Tests and Results Experiment: Compression strengths were measured by loading the specimens between circular steel plattens at a crosshead displacement rate of 2 mm/min. Typically, four repeat tests were conducted on each core structure. FE model: A series of finite element models have been created using the ANSYS FE package. The finite element model was created using three-dimensional Solidl 85 and a second model using Beam188 [2]. Hashin failure criteria was used to predict the onset of failure. Analytical model: Euler critical buckling stress: 712 Ex Ocritical SR 2 + I. 2 IT 2 Where SR = the slenderness ratio, Ex is the modulus in the fiber direction and Gxz is the shear modulus.
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    Compression Tests on Individual Columns Typical stress-strain traces following compression tests on individual struts with diameters of 2.5 and 3.0 mm. 300 250 200 150 100 50 0.1 Config. (A) Config. (B) 5/19/2016 (B) 0.3 300 250 200 150 100 50 11 0.2 0.05 0.15 0.1 0.2 Strain [mm/mm] 0.25 (B) 0.3 Strain [mm/mm] Config. (A): failure occurred as a result of interfacial failure between the horizontal tows and the inner skin. Fracture of this interface leads to lateral movement. Config. (B): anchoring the fibers to the skins leads to a significant increase in strength and much greater energy absorption 14
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    Compression Tests on Individual Columns Variation of specific compression strength with slenderness ratio for single columns rn rn 400 300 200 100 9. 0.42 0.28 Vf 0.14 20 30 - Euler theory • Experiment O FE model 80 40 50 60 70 Slenderness Ratio (SR) 90 Compression strength increases with decreasing SR and/or increasing Vf Euler model accurately predicts the strength of the samples based on intermediate and high values off SR. The model breaks down for the 4 mm diameter samples based on fiber volume fractions of 0.28 and 0.42. Here, failure was associated with crushing at one end of the column rather than buckling. 15
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    Compression Tests on the Composite Truss Cores Typical stress-strain curves for truss cores based on 3 and 4 mm diameter columns. 70 60 50 40 30 10 iuu, (B) 0.25 80 70 60 50 40 30 20 10 0.05 0.3 0.05 (B) 0.25 0.15 0.1 0.2 Strain [mm/mm] 0.15 0.1 0.2 Strain [mm/mm] As was the case for the simple columns, threading the fibers through the skins serves to greatly enhance the mechanical properties of the core. Loading Configuration A (i) results in skin-core interfacial failure, causing the columns to splay outwards as the crosshead displacement increases. Configuration B (ii) of 3mm samples failed as a result of some initial buckling failure. Crushing at the truss extremities was the predominant mode of failure in the 4 mm 'B samples. 0.3 16
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    Compression Tests on the Composite Truss Cores Specific compression strength vs. slenderness ratio for the truss cores. v f = 0.28. Compression data for both configurations are divided by the relative density. Included in the figure are the predictions associated with Euler buckling theory and the finite element analyses. The Euler model breaks down at lower values of SR, again due to the fact that failure occurs in a crushing mode, rather than buckling. 350 300 250 200 150 100 50 0 - Euler theory Experiment (Config. A) Experiment (Config. B) O FE model 20 30 50 40 60 Slenderness Ratio 70 80 5/19/2016 90 17
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    Compression Tests on Pyramidal Structures 80 70 60 50 40 30 0 20 10 Vf=O.06 Pyramidal core 5/19/2016 Vf=O.12 Vf=O.12 Vf=O.06 Modified pyramidal core Fiber tows were not anchored to the skins (Configuration A) Fiber tows did not connect through the skins and were not inter-twined at the mid nodes. 18
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    Conclusions A novel lost-mold manufacturing technique has been used to produce composite truss structures Weaving the fibers through the core and across the facesheets greatly improves the lateral stability of the struts, resulting in significant improvements in strength and energy-absorbing capability. Truss cores based on intermediate and high slenderness ratios failed in a buckling mode at low stresses. Larger diameter structures, based on higher fiber volume fractions failed in a crushing mode, offering specific energy absorption values above 70 kJ/kg. It has also been shown that it is possible to predict the mechanical response of the individual columns and the associated core using both Euler buckling theory and finite element techniques. 5/19/2016 19
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    Thank you

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