Research Papers

Epoxy Adhesives Modified With Nano- and Microparticles for In Situ Timber Bonding: Fracture Toughness Characteristics

[+] Author and Article Information
Z. Ahmad

 Faculty of Civil Engineering, University Technology Mara Malaysia, 40450 Shah Alam, Selangor, Malaysia e-mail: zakiahah@hotmail.com

M. P. Ansell

Department of Mechanical Engineering, Materials Research Centre,  University of Bath, Bath BA2 7AY, UK e-mail: m.p.ansell@bath.ac.uk

D. Smedley

 Rotafix (Northern) Limited, Rotafix House, Abercraf, Swansea SA9 1UR, UK e-mail: daverotafix@aol.com

J. Eng. Mater. Technol 133(3), 031006 (Jul 18, 2011) (9 pages) doi:10.1115/1.4003776 History: Received May 02, 2010; Revised January 23, 2011; Published July 18, 2011; Online July 18, 2011

Adhesives used for bonded-in steel or composite pultruded rods and plate to make connections in timber structures are commonly room temperature cure adhesives. The room temperature cure, applied without pressure, thixotropic, and shear thinning characteristics of the adhesives, is for ease of application when repairs and reinforcement are being made in situ in the field. The room temperature cure adhesive may not fully cross-link and this may cause brittleness. Therefore to improve the toughness properties of such adhesives, nanoparticles can be added. This paper reports the experimental investigation carried out on the fracture toughness of three thixotropic and room temperature cured epoxy-based adhesives formulated specifically for in situ timber bonding, namely, CB10TSS (standard adhesive), Albipox is CB10TSS with the addition of nanodispersed carboxyl-terminated butadiene acrylonitrile (CTBN), and Timberset is an adhesive formulation containing ceramic microparticles. The fracture toughness behavior of the adhesives was investigated using the Charpy impact test on unnotched and notched specimens conditioned at 20C/65%RH to evaluate notch sensitivity, and a single-edge notched beam (SENB) test was performed to evaluate the stress intensity factor KIC. The fracture surfaces were investigated using scanning electron microscopy. Under high impact rate, toughness was in the order of CB10TSS, Albipox, and Timberset. CB10TSS and Albipox were found to be ductile in the unnotched state and brittle when notched. Timberset was brittle in both unnotched and notched states. Under low strain rate (SENB) conditions the addition of CTBN significantly improved the fracture toughness of Albipox compared with CB10TSS and Timberset. Examination of the topography of the fractured surface revealed marked changes in crack propagation due to the addition of nano- or microfillers accounting for the variation in toughness properties.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

Schematic of tensile specimens

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Figure 2

Stress-strain curve

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Figure 3

(a) Geometry and dimensions of the Charpy impact specimen and (b) notched sample on the Charpy impact test equipment

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Figure 4

Single-edge-notch bending specimen: (a) schematic and (b) test setup

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Figure 5

Typical examples of stress-strain diagram for epoxy-based adhesive stressed in tension

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Figure 6

Construction for the determination of plastic deformation

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Figure 7

Effect of composition on plastic strain and strain to failure

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Figure 8

TEM micrographs: (a) almost uniformly distributed nanosilica fume in CB10TSS (×40k), (b) CB10TSS at higher magnification (×100k), (c) sharp boundaries between agglomerations of nanoparticles and the matrix in Albipox adhesive (×25k), and (d) SEM micrograph of heavily filled Timberset adhesive (×110)

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Figure 9

SEM micrographs of the fracture surface of an unnotched Charpy CB10TSS impact specimen: (a) schematic of the area taken for Figs.  1010; (b) different zones of crack propagation—the arrow shows the direction of crack propagation, transition zone between the area of crack initiation A, and smooth zone B ×25; (c) zone B at high magnification ×450; and (d) an abrupt jump (marked C) between smooth B and rough zone D, ×27

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Figure 10

Scanning electron micrographs of the fracture surface of Albipox unnotched specimens: (a) different zones of crack propagation—the arrow shows the direction of crack propagation, transition zone between the crack initiation point E, and smooth zone F and direction of crack propagation adjacent to the crack initiation, at ×27; (b) zone F at higher magnification, ×500; and (c) transition zone between smooth (F) and rough (H) zones, ×27

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Figure 11

Scanning electron micrographs of the fracture surface of Timberset unnotched specimens: (a) arrow head shows direction of crack propagation, smooth zone (marked J), and final propagation zone (marked K), ×27; and (b) smooth zone (marked J), ×200

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Figure 12

SEM microscopy of the fracture surface for CB10TSS (Charpy notched specimen): (a) with initiation zone L, propagation zone M, and rough zone N at ×30; (b) rough zone M at higher magnification ×450; and (c) the transition between M and N, ×80

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Figure 13

SEM microscopy of the fracture surface for Albipox (Charpy notched specimen): (a) initiation P and smooth fracture Q zones at ×30; (b) the transition between smooth Q and rough R regions, ×45; (c) zone Q at high magnification ×400; and (d) transition between smooth deformation zone Q and brittle fracture surface zone R, ×450

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Figure 14

SEM microscopy of the fracture surface for Timberset (Charpy notched specimen): (a) initiation S, smooth zone T, and rough zone U at ×27; and (b) the transition ridge showing the jump between smooth T and rough zone U, ×130

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Figure 15

SEM micrographs of the fracture surface of the SENB specimen for CB10TSS: (a) cut edge V, smooth zone W, and rough zone X, ×30; (b) shear deformation W in second zone, ×130; and (c) transition zone between smooth W and rough X zones, ×300

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Figure 16

SEM micrographs of the fracture surface of the SENB specimen for Albipox: (a) the initiation cut Y and smooth zone Z, ×100; (b) shear deformation in Z, ×600; and (c) transition zone between smooth Z and rough AA zones, ×300

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Figure 17

SEM micrographs of the fracture surface of the SENB specimen for Timberset: (a) starter notch BB and cracks around the filler particles in the smooth zone CC, ×130; (b) fracture surface at the crack tip CC, ×160; and (c) debonding of fillers in the rough zone DD, ×500




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