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Experimental investigation of the static torque transmission capabilities of the adhesively bonded single lap joints
Journal of
Materials Processing
ELSEVIER
Journal of Materials Processing Technology 48 (1995) 341-347
Technology
Experimental investigation of the static torque transmission capabilities of the adhesively bonded single lap joints Jin Kyung Cboi and Dai Gil Lee
Department of Precision Engineering and Mechatronics Korea Advanced Institute of Science and Technology, Taejon, Korea 305-701 Industrial Summary The carbon fiber epoxy composite materials have been widely used in the structures of aircra~, spacecra~, sport goods and machine elements because of their high specific meduli (E/p), strengths (S/p) and high dampings. Although the carbon fiber epoxy composite material has excellent properties for the structures, the joints for the composite materials often reduce the efficiency of the composite structures because the joints are often the weakest areas in composite structures. In this paper, the effects of the adhesive thickness on the static torque transmission capabilities of the adhesively bonded circular single lap joints have been investigated by the experimental method. Also the dependencies of the joint srengths on the stacking sequences of the composite adherends were tested. Since the circular single lap joint fails catastrophically beyond the static strength, the tubular polygonal adhesively bonded joints such as triangular, tetragonal, pentagonal and hexagonal as well as elliptical joints, which have partial mechanical joint characteristics, were manufactured and tested. From the experimental investigations, it was found that the strength of the circular adhesively bonded joints was much dependent on both the adhesive thicknesses and the stacking sequences of the composite adherends, while the polygonal adhesively bonded joints was less dependent on them.
1. Introduction The filamentary fiber reinforced composite materials have been widely used in the aircraft and spacecraft structuresbecause of their high specific strengths and moduii [I]. Since the carbon fiber epoxy composite material also has good damping [2] and fatigue [3] properties, it has been used in the robot arm [4, 5] and the machine tool spindle [6]. Since the structural efficiency of a composite structure is established, with very few exceptions, by its joints, not by its basic structure, the joint design of the composite material has become an important research area [7]. There are two kinds of joints : mechanical and adhesively bonded. Mechanical joints are created by fastening the substrates with bolts or rivets. The holes for bolts and rivets give an adverse effect on the stress transmission due to fiber breakage. Furthermore, the stress concentration of the comp-
osite material around holes can be larger than that of conventional isotropic materials due to the anisotropy of the composite materials [8]. The adhesively bonded joints can distribute the load over a larger area than the mechanical joints, require no holes, add very little weight to the structure and consequently have superior load transmission capabilities. Therefore, it can have more than 3 times larger joint strength under shear than that of the riveted joints [9]. The adhesively bonded joint reduces the noise and vibration of the structure. However, the adhesively bonded joint requires careful surface preparation of the adherends and its quality is dependent on the skill of the manufacturing personnel. The employment of the adhesive joint is limited by service environment such as humidity and temperature. Also, it is difficult to be disassembled for inspection and repair. Although there are many such difficulties in the realization of reliable adhesively bonded joints,
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 6 6 7 - P
342
J.K. Choi, D.G. Lee / Journal of Materials Processing Technology 48 (1995) 341-347
TABLE 1 Properties of the epoxy adhesive (IPCO 9923) Lap Shear Strength (MPa) Tensile Modulus (GPa) Tensile Strength (MPa) Shear Modulus (GPa) Poisson's Ratio
13.7 (ASTM D-1002-72) 1.3 45 0.46 0.41
TABLE 2 Properties of the unidrectional carbon fiber epoxy composite Tensile Modulus (GPa) Transverse Modulus (GPa) Shear Modulus (GPa) Poisson's Ratio Tensile Strength (GPa) Transverse Strength (MPa)
Shear Strength (MPa) Fiber Content ~Volume Fraction, %) Density (k~m ~) recent developments of good quality adhesives have made the adhesive joint be applicable easily in the composite structures. There are several types of adhesively bonded joints, such as the single lap joint, the double lap joint, the stepped lap joint, and the scarf joint. From these, the single lap joints is most popular, due to its ease of manufacture and its relativelylow cost. However, the single lap joint produces high stresses and rapidly changing stress gradients in the end region of the adhesive layer. The adhesively bonded single lap joints whose cross sections are polygonal shapes can sustain some load aider the adhesive failure, hence, they might be a safer design. In this work, the adhesively bonded circular single lap joints were tested to investigate the dependency of the static torque transmission capabilities on the adhesive thickness in the steelsteel adherends. Also, the polygonal and the elliptical adhesively bonded single lap joints, whose adherends were composed of steel and carbon fiber epoxy composite materials, were manufactured to give partial mechanical characteristics to the adhesively bonded tubular joints. They were statically tested to investigate the dependency of the torque transmission capabilities on the stacking sequences of the composite adherends.
153.0 10.9 5.6 0.3 2.0 56 72 60 1.6
2. Joint specimens and the adhesive The epoxy adhesive used was IPCO 9923 manufactured by the Imperial Polychemicals Corporation(Azusa, California, USA). The epoxy was robber-toughened two-part adhesives that had high shear and peel strength. The mix ratio of the resin and hardener was 1 : 1 by weight and the curing time was 75 minutes at 120°C. Table 1 shows the typical properties of the adhesive. Figures 1 and 2 shows the sizes and shapes of the test specimens. The $45C steel and the carbon fiber epoxy composite materials were used to manufacture the adherends. The teflon bar was tightly inserted inside the outer adherend to remove the fillet of the adhesive that might affect the torque transmission capabilities of the joint. The composite-steel adhesively bonded joint was composed of the outer composite adherend and two inner steel adherends as shown in Figures 3 and 4. The composite adhcrend was made of the carbon fiber epoxy composite material whose unidirectional properties were shown in Table 2. Since there are two adhesively bonded areas in the composite-steel joints, the angle of twist measured in experiments were divided by two to obtain the angle uf twist of one joint.
J.K. Choi, D.G. Lee / Journal of Materials Processing Technology 48 (1995) 341-347
Inner Adherend (Steel)
343
Inner Adherend (Steel)
e 2 o ~ ~ 1 8 . 8
~ 3 0 ~ 1 6 . 8
Outer Adherend (Steel)
Outer Adherend (Composite Materials) I _
L 4o _I so J
Ao.osivo Fig 1. Dimensions of the adherend of the steel-steel circular single lap joints.
oflen
Fig 3. Dimensions of the steel-composite circular single lap joints.
Inner Adherend (Steel)
Inner Adherend (Steel)
• Outer
Adherend (Steel)
T~ng|e Tef.ragon Pentago~ FJex=gon
..o0
TriangleTetreq[onp~taCoaHe=~oAFMJI~ O~terAdberend (CompositeMaterials) I-RS.G7 t-R7,13 FR7.6"3 I-R7.90L-7-70
Adhe|ive
Fig 2. Dimensions of the steel-steel polygonal single lap joints.
Fig 4. Dimensions of the stecl-~mposite polygonal single lap joints.
3. Static tests of the adhesively bonded joints
which was the contradictory result of the previous theory [11]. Since it was found that the adhesive bonding operation was difficultwhen the adhesive thickness was less than O.lmm, the 0.1ram adhesive thickness was recommended for practical bonding operation.
3. 1. Static torque test of the circular single lap joint with steel-steel adherend Since 2p.m arithmetic surface roughness of the adherend was found to be optimum[10], the dependency of the adhesive thickness on the static strength of the adhesively bonded circular steelsteel single lap joint with 2p.m arithmetic surface roughness was tested. Figure 5 shows the dependency of the static torque transmission capabilities of the joint on the adhesive thickness. In Figure 5, the torque transmission capabilities decreased as the adhesive thickness increased,
3. 2. Static torque test of the circular single lap joint with composite-steel adherend Since the mechanical properties of the carbon fiber epoxy composite material which was used as the outer adhercnd material of the composite-steel adhesively bonded joint arc dependent on the stacking sequence, the dependency of the torque
344
J.K. Choi, D.G. Lee / Journal of Materials Processing Technology 48 (1995) 341-347
2.~ o-
210 -
0
0
0
0
0 O0
2° O
170
O O
o E~
O
130
.I
o°%
~ ~
LS*. 1o `
"~ 0.5°.
2 90
I •
0 o
O~
Fiber Angle (degree)
50
i
0.01
i
i
i iiii
0.05 Adhesive
I
i
0.1
0.2
Thickness
i
t
i iiii
0.5
I
1
(ram)
Fig 5. Static torque transmission capabilities of the adhesive circular steel-steel single lap joints w,r.t. the adhesive thickness when the arithmetical average surface roughness was 2p,m transmission capabilities of the joints on the stacking angle [i-(Z]nT was tested by varying the stacking angle o~of the composite outer adherends. The inner adherend was made of steel. In order to compare the torque transmission capabilities, the size of the composite adherend was designed to be the same as that of the outer steel adherends. The adhesive thickness and the arithmetic surface roughness of the adherend were 0.1ram and 2~n, respectively. Even though the control of the surface roughness of the composite adherend was not easy, the arithmetic surface roughness was managed to be 2~m by abrading with sand papers. The stacking angles of the composite adherend were increased from [~.5]nT to [+45]nT with the interval of :~.5°. Figure 6 shows the static torque transmiss-
Fig 7. Maximum static twist angle of adhesive with respect to the stacking sequence [:L-(Z]nT of the composite adherends. ion capabilities of the adhesively bonded single lap joint whose outer adherend was made of the carbon fiber epoxy composite material and whose inner adherend was made of steel. In Figure 6, the torque transmission capabilities increased as the stacking angle increased until [+95]nT and then decreased as the stacking angle increased further. Figure 7 shows the angle of twist of the joint in which the angle of twist decreased as the stacking angle increased. Figure 8 shows the fractography of the adhesive of the composite-steel adhesively bonded circular joint.
l°° f ~ 6o
~
,o
o
20
Fig 8. Photograph of the fracture shape of the circular adherends. Angle (degree)
Fig 6. Static torque transmission capabilities with respect to the stacking sequence [t-C~]nT of the composite adherends.
ZK. Choi, D.G. Lee I Journal o f Materials Processing Technology 48 (1995) 341-347 3. For the adhesively bonded single lap joints whose outer adherend was made of the carbon fiber epoxy composite material and whose inner adherend was made of steel, the largest torque transmission capabilities of the joints with the triangular and tetragonal cross sections were obtained when the stacking sequence of the composite material was [:i:15]nT. For the circular cross sections, the largest torque transmission capability was obtained when the stacking sequence was [-+20]nT. For the pentagonal and hexagonal cross sections, the torque transmission capabilities were independent of the stacking sequences.
/'
345
\
(e)
(a)
(d)
Co)
(e) Fig 9. Crack shapes of the surfaces of the polygonal outer adherends when the stacking sequence was [:J:30]nT. (a) Triangular (Io) Tetragonal (c) Pentagonal (d) Hexagonal (e) Elliptical
346
J.K~ Choi, D.G. Lee / Journal of Matedals Processing Technology 48 (1995) 341-347
TABLE 3 Torque transmission capabilities of the adhesively bonded polygonal single lap joints (Arithmetic average surface roughness: 2lain, Bonding thickness: lmm)
Triangular Tetragonal Pentagonal Hexagonal Elliptical Circular
[+151nT 108-123 104-117 69-76 70-87 86-97 62-71
I~-30]nT 64-82 63-72 72-77 87-98 152-157 79-83
[unit: Nm] [:~45]nT 42-61 52-60 75-83 76-87 22-26 16-20
TABLE 4 Torque transmission capabilities of the polygonal single lap joints when the composite adherends were failed after adhesive fracture [unit: Nm] Stacking angle [+15]n T [:V.30]nT [+45]n T Torque 23 -41 62 -82 75 - 120 3. 3. Static torque test of the polygonal composite-steel single lap joint Since the polygonal joint can sustain some torque after adhesive failure, in this experiment, the polygonal single lap joints whose adherends were made of the carbon fiber epoxy composite materials and the steel, were tested to investigate the effects of the cross sectional shapes and the stacking sequence on the torque transmission capabilities. The adhesive joining areas of the polygonal joints were designed to be the same as the circular joint to compare the torque transmission capabilities. To reduce the number of experiments, only the stacking sequences [+15]nT, [:V.30]nT and [~-45]nT of the outer composite adherends were tested. Table 3 shows that the torque transmission capabilities of the triangular and the tetragonal joints increased as the stacking angle of the composite adherends decreased, while those of the pentagonal and the hexagonal joints were independent of the stacking sequences. For the elliptical joint, the maximum torque transmission capability was obtained when the stacking sequence was [_+_30]nT. Figure 9 shows the fracture initiation shapes of the polygonal joints and Figure 10 shows the progressive failure of the adherend of the triangular cross section after the first adhesive failure. Figure 11 shows the torque versus twist
angle of the triangular joint after the first adhesive failure. Table 4 shows the secondary torque transmission capabilities of the joints after the first adhesive failure. The secondary torque transmission capabilities of the joint atter the first adhesive failure were found that they were not dependent on the cross sectional shapes but dependent on the stacking sequences of the outer composite adherends. 4. Conclusions From the static experiments of the adhesively bonded single lap joints such as the circular, triangular, tetragonal, pentagonal, hexagonal and elliptical joints, the following conclusions were made: 1. The static torque transmission capability of the adhesively bonded circular single lap joint increased as the adhesive thickness decreased. 2. The adhesively bonded circular single lap joint whose outer adherend was the carbon fiber epoxy composite material and whose inner adherend was steel had the largest static torque transmission capability when the stacking sequence of the composite material was [:V.25]nT. The angle of twist before fracture decreased as the stacking angle increased.
J.K~ Choi, D.G. Lee / Journal of Materials Processing Technology 48 (1995) 341-347
2.
3. 4.
Fig 10. Photograph of the failed shape of the triangular outer adherend under static torque.
5.
100
80-
6.
8o- 1 40-
7.
200
8.
0
' '
' 1'~ ' 1~ ' ~'o'
24
9.
Twist Angle (degree) Fig 11. Variation of the torque with respect to the twist angle of the triangular adhesive joint. (Stacking sequence : [~_30]nT)
10.
Acknowledgments This research was financially supported by the Korean Agency for Defense Development.
References 1.
T. J. Reinhart(Ed.), Engineered Material Vol. 1, Composites, ASM International, 1897, pp. 816-822.
handbook,
11.
347
E.F. Crawley and D. G. Mohr, Experimental Measurement of Material Damping in Free Fall with Tuneable Excitation, AIAA/ASME/ASCE /AHS, Structures,
Structural Dynamics and Materials Conference, Paper No. 83-0858-CP. P.K. Mallick, Fiber-Reinforced Composites, Marcel Dekker, Inc., 1988, pp. 214-248. D . G . Lee, K. S. Kim and Y. K. Kwak, Manufacturing of a SCARA-Type DirectDrive Robot with Graphite Epoxy Composite Materials, Robotica, Vol. 9, 1991, pp. 219229. D.G. Lee, K. S. Jeong, K. S. Kim and Y. K. Kwak, Development of the Anthropomorphic Robot with Carbon Fiber Epoxy Composite Materials, Composite Structures, 1993, pp.313-324. D . G . Lee, H. C. Sin and N. P. Sub, Manufacturing of a Graphite Epoxy Composite Spindle for a Machine Tool, Annals of the CIRP, Vol. 27(1), 1985, pp. 365-369. T . J . Reinhart(Ed.), Engineered Materials Handbook, Vol. 1, Composites, ASM International, 1988, pp. 479-495. S.G. Lekhnitskii, Theory of an Anisotropic Elastic Body, Holden-Day, Inc., 1963, pp.169-174. J . R . Vinson and R. L. Sierakowski, The Behavior of Structure Composed of Composite Materials, MARTINUS NIJHOFF PUBLISHERS, 1987, chap. 8. D . G . Lee, K. S. Kim and Y. T. Ira, An Experimental Study of Fatigue Strength for Adhesively Bonded Tubular Single Lap Joints, Journal of Adhesion, Vol. 35, 1991, pp.39-53. R. D. Adams and N. A. Peppiatt, Stress Analysis of Adhesive Bonded Tubular Lap Joints, Journal of Adhesion, Vol. 9, 1977, pp. 1-18.
Materials Processing
ELSEVIER
Journal of Materials Processing Technology 48 (1995) 341-347
Technology
Experimental investigation of the static torque transmission capabilities of the adhesively bonded single lap joints Jin Kyung Cboi and Dai Gil Lee
Department of Precision Engineering and Mechatronics Korea Advanced Institute of Science and Technology, Taejon, Korea 305-701 Industrial Summary The carbon fiber epoxy composite materials have been widely used in the structures of aircra~, spacecra~, sport goods and machine elements because of their high specific meduli (E/p), strengths (S/p) and high dampings. Although the carbon fiber epoxy composite material has excellent properties for the structures, the joints for the composite materials often reduce the efficiency of the composite structures because the joints are often the weakest areas in composite structures. In this paper, the effects of the adhesive thickness on the static torque transmission capabilities of the adhesively bonded circular single lap joints have been investigated by the experimental method. Also the dependencies of the joint srengths on the stacking sequences of the composite adherends were tested. Since the circular single lap joint fails catastrophically beyond the static strength, the tubular polygonal adhesively bonded joints such as triangular, tetragonal, pentagonal and hexagonal as well as elliptical joints, which have partial mechanical joint characteristics, were manufactured and tested. From the experimental investigations, it was found that the strength of the circular adhesively bonded joints was much dependent on both the adhesive thicknesses and the stacking sequences of the composite adherends, while the polygonal adhesively bonded joints was less dependent on them.
1. Introduction The filamentary fiber reinforced composite materials have been widely used in the aircraft and spacecraft structuresbecause of their high specific strengths and moduii [I]. Since the carbon fiber epoxy composite material also has good damping [2] and fatigue [3] properties, it has been used in the robot arm [4, 5] and the machine tool spindle [6]. Since the structural efficiency of a composite structure is established, with very few exceptions, by its joints, not by its basic structure, the joint design of the composite material has become an important research area [7]. There are two kinds of joints : mechanical and adhesively bonded. Mechanical joints are created by fastening the substrates with bolts or rivets. The holes for bolts and rivets give an adverse effect on the stress transmission due to fiber breakage. Furthermore, the stress concentration of the comp-
osite material around holes can be larger than that of conventional isotropic materials due to the anisotropy of the composite materials [8]. The adhesively bonded joints can distribute the load over a larger area than the mechanical joints, require no holes, add very little weight to the structure and consequently have superior load transmission capabilities. Therefore, it can have more than 3 times larger joint strength under shear than that of the riveted joints [9]. The adhesively bonded joint reduces the noise and vibration of the structure. However, the adhesively bonded joint requires careful surface preparation of the adherends and its quality is dependent on the skill of the manufacturing personnel. The employment of the adhesive joint is limited by service environment such as humidity and temperature. Also, it is difficult to be disassembled for inspection and repair. Although there are many such difficulties in the realization of reliable adhesively bonded joints,
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 6 6 7 - P
342
J.K. Choi, D.G. Lee / Journal of Materials Processing Technology 48 (1995) 341-347
TABLE 1 Properties of the epoxy adhesive (IPCO 9923) Lap Shear Strength (MPa) Tensile Modulus (GPa) Tensile Strength (MPa) Shear Modulus (GPa) Poisson's Ratio
13.7 (ASTM D-1002-72) 1.3 45 0.46 0.41
TABLE 2 Properties of the unidrectional carbon fiber epoxy composite Tensile Modulus (GPa) Transverse Modulus (GPa) Shear Modulus (GPa) Poisson's Ratio Tensile Strength (GPa) Transverse Strength (MPa)
Shear Strength (MPa) Fiber Content ~Volume Fraction, %) Density (k~m ~) recent developments of good quality adhesives have made the adhesive joint be applicable easily in the composite structures. There are several types of adhesively bonded joints, such as the single lap joint, the double lap joint, the stepped lap joint, and the scarf joint. From these, the single lap joints is most popular, due to its ease of manufacture and its relativelylow cost. However, the single lap joint produces high stresses and rapidly changing stress gradients in the end region of the adhesive layer. The adhesively bonded single lap joints whose cross sections are polygonal shapes can sustain some load aider the adhesive failure, hence, they might be a safer design. In this work, the adhesively bonded circular single lap joints were tested to investigate the dependency of the static torque transmission capabilities on the adhesive thickness in the steelsteel adherends. Also, the polygonal and the elliptical adhesively bonded single lap joints, whose adherends were composed of steel and carbon fiber epoxy composite materials, were manufactured to give partial mechanical characteristics to the adhesively bonded tubular joints. They were statically tested to investigate the dependency of the torque transmission capabilities on the stacking sequences of the composite adherends.
153.0 10.9 5.6 0.3 2.0 56 72 60 1.6
2. Joint specimens and the adhesive The epoxy adhesive used was IPCO 9923 manufactured by the Imperial Polychemicals Corporation(Azusa, California, USA). The epoxy was robber-toughened two-part adhesives that had high shear and peel strength. The mix ratio of the resin and hardener was 1 : 1 by weight and the curing time was 75 minutes at 120°C. Table 1 shows the typical properties of the adhesive. Figures 1 and 2 shows the sizes and shapes of the test specimens. The $45C steel and the carbon fiber epoxy composite materials were used to manufacture the adherends. The teflon bar was tightly inserted inside the outer adherend to remove the fillet of the adhesive that might affect the torque transmission capabilities of the joint. The composite-steel adhesively bonded joint was composed of the outer composite adherend and two inner steel adherends as shown in Figures 3 and 4. The composite adhcrend was made of the carbon fiber epoxy composite material whose unidirectional properties were shown in Table 2. Since there are two adhesively bonded areas in the composite-steel joints, the angle of twist measured in experiments were divided by two to obtain the angle uf twist of one joint.
J.K. Choi, D.G. Lee / Journal of Materials Processing Technology 48 (1995) 341-347
Inner Adherend (Steel)
343
Inner Adherend (Steel)
e 2 o ~ ~ 1 8 . 8
~ 3 0 ~ 1 6 . 8
Outer Adherend (Steel)
Outer Adherend (Composite Materials) I _
L 4o _I so J
Ao.osivo Fig 1. Dimensions of the adherend of the steel-steel circular single lap joints.
oflen
Fig 3. Dimensions of the steel-composite circular single lap joints.
Inner Adherend (Steel)
Inner Adherend (Steel)
• Outer
Adherend (Steel)
T~ng|e Tef.ragon Pentago~ FJex=gon
..o0
TriangleTetreq[onp~taCoaHe=~oAFMJI~ O~terAdberend (CompositeMaterials) I-RS.G7 t-R7,13 FR7.6"3 I-R7.90L-7-70
Adhe|ive
Fig 2. Dimensions of the steel-steel polygonal single lap joints.
Fig 4. Dimensions of the stecl-~mposite polygonal single lap joints.
3. Static tests of the adhesively bonded joints
which was the contradictory result of the previous theory [11]. Since it was found that the adhesive bonding operation was difficultwhen the adhesive thickness was less than O.lmm, the 0.1ram adhesive thickness was recommended for practical bonding operation.
3. 1. Static torque test of the circular single lap joint with steel-steel adherend Since 2p.m arithmetic surface roughness of the adherend was found to be optimum[10], the dependency of the adhesive thickness on the static strength of the adhesively bonded circular steelsteel single lap joint with 2p.m arithmetic surface roughness was tested. Figure 5 shows the dependency of the static torque transmission capabilities of the joint on the adhesive thickness. In Figure 5, the torque transmission capabilities decreased as the adhesive thickness increased,
3. 2. Static torque test of the circular single lap joint with composite-steel adherend Since the mechanical properties of the carbon fiber epoxy composite material which was used as the outer adhercnd material of the composite-steel adhesively bonded joint arc dependent on the stacking sequence, the dependency of the torque
344
J.K. Choi, D.G. Lee / Journal of Materials Processing Technology 48 (1995) 341-347
2.~ o-
210 -
0
0
0
0
0 O0
2° O
170
O O
o E~
O
130
.I
o°%
~ ~
LS*. 1o `
"~ 0.5°.
2 90
I •
0 o
O~
Fiber Angle (degree)
50
i
0.01
i
i
i iiii
0.05 Adhesive
I
i
0.1
0.2
Thickness
i
t
i iiii
0.5
I
1
(ram)
Fig 5. Static torque transmission capabilities of the adhesive circular steel-steel single lap joints w,r.t. the adhesive thickness when the arithmetical average surface roughness was 2p,m transmission capabilities of the joints on the stacking angle [i-(Z]nT was tested by varying the stacking angle o~of the composite outer adherends. The inner adherend was made of steel. In order to compare the torque transmission capabilities, the size of the composite adherend was designed to be the same as that of the outer steel adherends. The adhesive thickness and the arithmetic surface roughness of the adherend were 0.1ram and 2~n, respectively. Even though the control of the surface roughness of the composite adherend was not easy, the arithmetic surface roughness was managed to be 2~m by abrading with sand papers. The stacking angles of the composite adherend were increased from [~.5]nT to [+45]nT with the interval of :~.5°. Figure 6 shows the static torque transmiss-
Fig 7. Maximum static twist angle of adhesive with respect to the stacking sequence [:L-(Z]nT of the composite adherends. ion capabilities of the adhesively bonded single lap joint whose outer adherend was made of the carbon fiber epoxy composite material and whose inner adherend was made of steel. In Figure 6, the torque transmission capabilities increased as the stacking angle increased until [+95]nT and then decreased as the stacking angle increased further. Figure 7 shows the angle of twist of the joint in which the angle of twist decreased as the stacking angle increased. Figure 8 shows the fractography of the adhesive of the composite-steel adhesively bonded circular joint.
l°° f ~ 6o
~
,o
o
20
Fig 8. Photograph of the fracture shape of the circular adherends. Angle (degree)
Fig 6. Static torque transmission capabilities with respect to the stacking sequence [t-C~]nT of the composite adherends.
ZK. Choi, D.G. Lee I Journal o f Materials Processing Technology 48 (1995) 341-347 3. For the adhesively bonded single lap joints whose outer adherend was made of the carbon fiber epoxy composite material and whose inner adherend was made of steel, the largest torque transmission capabilities of the joints with the triangular and tetragonal cross sections were obtained when the stacking sequence of the composite material was [:i:15]nT. For the circular cross sections, the largest torque transmission capability was obtained when the stacking sequence was [-+20]nT. For the pentagonal and hexagonal cross sections, the torque transmission capabilities were independent of the stacking sequences.
/'
345
\
(e)
(a)
(d)
Co)
(e) Fig 9. Crack shapes of the surfaces of the polygonal outer adherends when the stacking sequence was [:J:30]nT. (a) Triangular (Io) Tetragonal (c) Pentagonal (d) Hexagonal (e) Elliptical
346
J.K~ Choi, D.G. Lee / Journal of Matedals Processing Technology 48 (1995) 341-347
TABLE 3 Torque transmission capabilities of the adhesively bonded polygonal single lap joints (Arithmetic average surface roughness: 2lain, Bonding thickness: lmm)
Triangular Tetragonal Pentagonal Hexagonal Elliptical Circular
[+151nT 108-123 104-117 69-76 70-87 86-97 62-71
I~-30]nT 64-82 63-72 72-77 87-98 152-157 79-83
[unit: Nm] [:~45]nT 42-61 52-60 75-83 76-87 22-26 16-20
TABLE 4 Torque transmission capabilities of the polygonal single lap joints when the composite adherends were failed after adhesive fracture [unit: Nm] Stacking angle [+15]n T [:V.30]nT [+45]n T Torque 23 -41 62 -82 75 - 120 3. 3. Static torque test of the polygonal composite-steel single lap joint Since the polygonal joint can sustain some torque after adhesive failure, in this experiment, the polygonal single lap joints whose adherends were made of the carbon fiber epoxy composite materials and the steel, were tested to investigate the effects of the cross sectional shapes and the stacking sequence on the torque transmission capabilities. The adhesive joining areas of the polygonal joints were designed to be the same as the circular joint to compare the torque transmission capabilities. To reduce the number of experiments, only the stacking sequences [+15]nT, [:V.30]nT and [~-45]nT of the outer composite adherends were tested. Table 3 shows that the torque transmission capabilities of the triangular and the tetragonal joints increased as the stacking angle of the composite adherends decreased, while those of the pentagonal and the hexagonal joints were independent of the stacking sequences. For the elliptical joint, the maximum torque transmission capability was obtained when the stacking sequence was [_+_30]nT. Figure 9 shows the fracture initiation shapes of the polygonal joints and Figure 10 shows the progressive failure of the adherend of the triangular cross section after the first adhesive failure. Figure 11 shows the torque versus twist
angle of the triangular joint after the first adhesive failure. Table 4 shows the secondary torque transmission capabilities of the joints after the first adhesive failure. The secondary torque transmission capabilities of the joint atter the first adhesive failure were found that they were not dependent on the cross sectional shapes but dependent on the stacking sequences of the outer composite adherends. 4. Conclusions From the static experiments of the adhesively bonded single lap joints such as the circular, triangular, tetragonal, pentagonal, hexagonal and elliptical joints, the following conclusions were made: 1. The static torque transmission capability of the adhesively bonded circular single lap joint increased as the adhesive thickness decreased. 2. The adhesively bonded circular single lap joint whose outer adherend was the carbon fiber epoxy composite material and whose inner adherend was steel had the largest static torque transmission capability when the stacking sequence of the composite material was [:V.25]nT. The angle of twist before fracture decreased as the stacking angle increased.
J.K~ Choi, D.G. Lee / Journal of Materials Processing Technology 48 (1995) 341-347
2.
3. 4.
Fig 10. Photograph of the failed shape of the triangular outer adherend under static torque.
5.
100
80-
6.
8o- 1 40-
7.
200
8.
0
' '
' 1'~ ' 1~ ' ~'o'
24
9.
Twist Angle (degree) Fig 11. Variation of the torque with respect to the twist angle of the triangular adhesive joint. (Stacking sequence : [~_30]nT)
10.
Acknowledgments This research was financially supported by the Korean Agency for Defense Development.
References 1.
T. J. Reinhart(Ed.), Engineered Material Vol. 1, Composites, ASM International, 1897, pp. 816-822.
handbook,
11.
347
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