6 JOINING AND ASSEMBLY We have seen previously how to design a laminate to support loads.A second fundamental aspect of the design of a composite piece consists of the design for the attachment of the composite to the rest of the structure.Here we will examine the assembly problems involving riveting,bolting,and bonding: of a composite part to another composite part and of a composite part to a metallic part. 6.1 RIVETING AND BOLTING In all mechanical components,the introduction of holes gives stress con- centration factors.Specifically in composite pieces,the introduction of holes (for molded-in holes or holes made by drilling)induces weakening of the fracture resistance in comparison with the region without holes by a factor of 40 to 60%in tension 15%in compression Example:Figure 6.1 presents the process of degradation before rupture of a glass/epoxy laminate containing a free hole,under uniaxial stress. Causes of hole degradation: Stress concentration factors:The equilibrium diagrams shown in Figure 6.2 demonstrate the increase in stress concentration in the case of a laminate. For the case of slight (and usually neglected)press-fit of the rivet,the stresses shown in these figures are: OM>O in a region where: local rupture<Olaminate rupture 2003 by CRC Press LLC
6 JOINING AND ASSEMBLY We have seen previously how to design a laminate to support loads. A second fundamental aspect of the design of a composite piece consists of the design for the attachment of the composite to the rest of the structure. Here we will examine the assembly problems involving riveting, bolting, and bonding: of a composite part to another composite part and of a composite part to a metallic part. 6.1 RIVETING AND BOLTING In all mechanical components, the introduction of holes gives stress concentration factors. Specifically in composite pieces, the introduction of holes (for molded-in holes or holes made by drilling) induces weakening of the fracture resistance in comparison with the region without holes by a factor of 40 to 60% in tension 15% in compression Example: Figure 6.1 presents the process of degradation before rupture of a glass/epoxy laminate containing a free hole, under uniaxial stress. Causes of hole degradation: Stress concentration factors: The equilibrium diagrams shown in Figure 6.2 demonstrate the increase in stress concentration in the case of a laminate. For the case of slight (and usually neglected) press-fit of the rivet, the stresses shown in these figures are: in a region where: slocal rupture s © 2003 by CRC Press LLC
25% 12.5% 50% 12.5% 9o°ply fracture ±45°ply fracture (resin) (resin) Cracks at o°(resin) Delamination;deformation around the hole Figure 6.1 Cracks in a Laminate with Hole when Load Increases G 01 metal laminated (90) plasticized zone (0月 Figure 6.2 Stress Concentration Factors with the maximum stress o in the laminate given as: where E and E,are the moduli of elasticity in the 0 and 90 directions Gsy is the shear modulus Vy is the Poisson ratio 2003 by CRC Press LLC
with the maximum stress s¢ M in the laminate given as: where Ex and Ey are the moduli of elasticity in the 0∞ and 90∞ directions Gxy is the shear modulus nxy is the Poisson ratio Figure 6.1 Cracks in a Laminate with Hole when Load Increases Figure 6.2 Stress Concentration Factors sM¢ s ¢ 1 2 Ex Ey ----- – nxy Ë ¯ Ê ˆ Ex Gxy + + -------- Ó ˛ Ì ˝ Ï ¸ = ¥ © 2003 by CRC Press LLC
米 weakened zones overstressed zones preferred Figure 6.3 Weakened Zones Due to Presence of Holes Bearing due to lateral pressure:This is the contact pressure between the shaft of the assembly device (rivet or bolt)and the wall of the hole. When this pressure is excessive,it leads to mushrooming and delamination of the laminate.In consequence: The resistance of a hole occupied by the rivet or bolt is weaker than that of an empty hole:decrease on order of 40%). Fracture of fibers during the hole cutting process,or the misalignment of fibers if the hole is made before polymerization:Figure 6.3 illustrates the correlation between the weakened zones consecutive to rupture of fibers and the "overstressed"zones. 6.1.1 Principal Modes of Failure in Bolted Joints for Composite Materials These are represented in Figure 6.4. 6.1.2 Recommended Values Pitch,edge distance,thickness (see Figure 6.5) Orientation of plies:Recommendation for percentages of plies near the holes (see Figure 6.6). 2003 by CRC Press LLC
Bearing due to lateral pressure: This is the contact pressure between the shaft of the assembly device (rivet or bolt) and the wall of the hole. When this pressure is excessive, it leads to mushrooming and delamination of the laminate. In consequence: The resistance of a hole occupied by the rivet or bolt is weaker than that of an empty hole: decrease on order of 40%). Fracture of fibers during the hole cutting process, or the misalignment of fibers if the hole is made before polymerization: Figure 6.3 illustrates the correlation between the weakened zones consecutive to rupture of fibers and the “overstressed” zones. 6.1.1 Principal Modes of Failure in Bolted Joints for Composite Materials These are represented in Figure 6.4. 6.1.2 Recommended Values Pitch, edge distance, thickness (see Figure 6.5) Orientation of plies: Recommendation for percentages of plies near the holes (see Figure 6.6). Figure 6.3 Weakened Zones Due to Presence of Holes © 2003 by CRC Press LLC
tensile fracture shear fracture bearing failure (insufficient number (necessary to reinforce (insufficient thickness) ofo°plies) ±45°plies) tensile and normal fracture fracture of bolt bolt lifting Figure 6.4 Different Types of Bolt Joint Failures Condition of nonbearing pressure:In Figure 6.7,F and T designate the normal and shear loads respectively,that are applied on the assembly over a width equal to one pitch distance. The equivalent bearing pressure which leads to the crushing of the wall of the hole of diameter is F/(x e).It must remain smaller than an admissible maximum,as: F admistble bearing pressure carbon:admistble bearing 500 MPa glass:Tadmistble bearng =300 MPa 2003 by CRC Press LLC
Condition of nonbearing pressure: In Figure 6.7, F and T designate the normal and shear loads respectively, that are applied on the assembly over a width equal to one pitch distance. The equivalent bearing pressure which leads to the crushing of the wall of the hole of diameter Ø, is F/(Ø ¥ e). It must remain smaller than an admissible maximum, as: Figure 6.4 Different Types of Bolt Joint Failures F fe -----£sadmissible bearing pressure carbon: sadmissible bearing = 500 MPa glass: tadmissible bearing = 300 MPa © 2003 by CRC Press LLC
d O 4ds pitch s6d foot O foot≥6d pitch 909 45 Oe2 d2 O reinforcement at t45 recommended pitch safety factor 22 pitch 2 Figure 6.5 Recommended Pitch,Edge Distance,and Thickness 210% 15to32% 25to60% 15t032% Figure 6.6 Recommended Orientation pitch Figure 6.7 Normal and Shear Loads on Assembly Evaluation of the admissible stresses:The principle of calculation con- sists of magnifying the stresses that are given by elementary considerations, by means of the empirical coefficients of magnification': TWhen one takes into account the aging of the piece,an additional 10%is applied to the maximum stresses. 2003 by CRC Press LLC
Evaluation of the admissible stresses: The principle of calculation consists of magnifying the stresses that are given by elementary considerations, by means of the empirical coefficients of magnification1 : Figure 6.5 Recommended Pitch, Edge Distance, and Thickness Figure 6.6 Recommended Orientation Figure 6.7 Normal and Shear Loads on Assembly 1 When one takes into account the aging of the piece, an additional 10% is applied to the maximum stresses. © 2003 by CRC Press LLC
Due to the presence of the hole and Due to pressure of contact or bearing on the wall of the hole (rivet,bolt). With the notations of Figure 6.7,one has: F 0.2 tension:a=0.6 compression:a 0.8 1T Tmagnified= 0.73 One must also verify that these stresses are admissible (that is,they do not lead to the fracture of the ply)by using the method of verification of fracture described in Paragraph 5.3.2. 6.1.3 Riveting The relative specifics and recommendations for riveting the composite parts can be presented as follows: Do not hit the rivets as this can lead to poor resistance to impact of the laminates. ■Pay attention to the risk of“bolt lifting”of the bolt heads due to small thickness of the laminates. Note the necessity to assure the galvanic compatibility between the rivet and the laminates to be assembled. Riveting accompanied by bonding of the surfaces to be assembled provides a gain in the mechanical resistance on the order of 20 to 30%. On the other hand,the disassembly of the joint becomes impossible,and the weight is increased. Characteristics of rivets for composites are shown in Figure 6.8. 6.1.4 Bolting Examine a current example that requires a bolted joint. Example:Junction of a panel by bolted joint(simple case):Consider a sandwich panel fixed to a support component that is subjected to simple loadings that can be represented by a shear load and a bending moment (see Figure 6.9). One expects an attachment using bolt.As shown in the schematics of Figure 6.10,even if the bolt is not tightened,it is able to act to equilibrate the bending moment.However,action of the shear load will separate the facings. A more complete caseon the fixation of the panel is examined in the application in Paragraph 18.1.6. 2003 by CRC Press LLC
Due to the presence of the hole and Due to pressure of contact or bearing on the wall of the hole (rivet, bolt). With the notations of Figure 6.7, one has: One must also verify that these stresses are admissible (that is, they do not lead to the fracture of the ply) by using the method of verification of fracture described in Paragraph 5.3.2. 6.1.3 Riveting The relative specifics and recommendations for riveting the composite parts can be presented as follows: Do not hit the rivets as this can lead to poor resistance to impact of the laminates. Pay attention to the risk of “bolt lifting” of the bolt heads due to small thickness of the laminates. Note the necessity to assure the galvanic compatibility between the rivet and the laminates to be assembled. Riveting accompanied by bonding of the surfaces to be assembled provides a gain in the mechanical resistance on the order of 20 to 30%. On the other hand, the disassembly of the joint becomes impossible, and the weight is increased. Characteristics of rivets for composites are shown in Figure 6.8. 6.1.4 Bolting Examine a current example that requires a bolted joint. Example: Junction of a panel by bolted joint (simple case)2 : Consider a sandwich panel fixed to a support component that is subjected to simple loadings that can be represented by a shear load and a bending moment (see Figure 6.9). One expects an attachment using bolt. As shown in the schematics of Figure 6.10, even if the bolt is not tightened, it is able to act to equilibrate the bending moment. However, action of the shear load will separate the facings. 2 A more complete case on the fixation of the panel is examined in the application in Paragraph 18.1.6. smagnified 1 a --- F S -- 0.2 F fe + ----- Ë ¯ Ê ˆ = tension: a = 0.6 compression: a = 0.8 tmagnified 1 0.7 -------T S = -- © 2003 by CRC Press LLC
titanium diameter: alloy d=3.2:4:4.76:6mm colombium (cold welded on titanium) ·cone angle: 130°≤0≤156 aluminum alloy materials: copper-nickel titanium alloy (TA6V) these rivets are ductile mechanical strength: Trupture (rivet)#400 MPa Inconel or stainless steel or monel (shear fracture) overlay nvet Figure 6.8 Different Types of Riveting 7 shear stress resultant bending moment resultant sandwich panel support Figure 6.9 Junction of a Panel using Bolted Joint 2003 by CRC Press LLC
Figure 6.8 Different Types of Riveting Figure 6.9 Junction of a Panel using Bolted Joint © 2003 by CRC Press LLC
junction place support of bending support of shear without moment by bolt bolt tightening shear (neglect friction) Figure 6.10 Local behavior without Bolt Tightening tension shear work in bolt thread support of shear with bolt tightening (neglect friction) Figure 6.11 Bolt tightening Reduces the Possibility of Damage It is the tightening of the bolt that will lead to a distribution of contact pressure between the support component and the facings.The sum of the forces due to this contact pressure will balance out the shear load,while suppressing the risk of separating the facings (see Figure 6.11). The tightening of the bolt is therefore indispensable.However,the laminated facings being fragile cannot admit high contact pressures that are localized under the bolt head and under the nut.This leads to the insertion of metallic washers as shown in Figure 6.12. The bolting accompanied by bonding of the surfaces provides a gain in mechanical resistance on the order of 20 to 30%.On the other hand,the joint cannot be disassembled,and there is an increase in weight. 6.2 BONDING Remember briefly that this assembly technique consists of the adhesion by molec- ular attraction between two parties to be bonded and an adhesive that must be able to transfer the loads.One can cite the principal advantages of this mode of joining: 2003 by CRC Press LLC
It is the tightening of the bolt that will lead to a distribution of contact pressure between the support component and the facings. The sum of the forces due to this contact pressure will balance out the shear load, while suppressing the risk of separating the facings (see Figure 6.11). The tightening of the bolt is therefore indispensable. However, the laminated facings being fragile cannot admit high contact pressures that are localized under the bolt head and under the nut. This leads to the insertion of metallic washers as shown in Figure 6.12. The bolting accompanied by bonding of the surfaces provides a gain in mechanical resistance on the order of 20 to 30%. On the other hand, the joint cannot be disassembled, and there is an increase in weight. 6.2 BONDING Remember briefly that this assembly technique consists of the adhesion by molecular attraction between two parties to be bonded and an adhesive that must be able to transfer the loads. One can cite the principal advantages of this mode of joining: Figure 6.10 Local behavior without Bolt Tightening Figure 6.11 Bolt tightening Reduces the Possibility of Damage © 2003 by CRC Press LLC
metal good bad titanium plate bonded on the laminate bonded assembly titanium-carbon a few mm (i.e:2.5 mm) Figure 6.12 Configuration for Bolted Joints 00o0 8g cunng 000 initial molecules macromolecules reticulated or not-reticulated Figure 6.13 Curing of Adhesive distribution of stresses over an important surface possibility to optimize the geometry and dimensions of bonding light weight of the assembly insulation and sealing properties of adhesive 6.2.1 Adhesives Used The adhesives used include: ■epoxies ■polyesters ■ polyurethanes ■methacrylates In all cases,the mechanism of curing is shown schematically in Figure 6.13 The adhesives are resistand simultaneously to ■high temperatures(>l8o°C) ■humidity a number of chemical agents 2003 by CRC Press LLC
distribution of stresses over an important surface possibility to optimize the geometry and dimensions of bonding light weight of the assembly insulation and sealing properties of adhesive 6.2.1 Adhesives Used The adhesives used include: epoxies polyesters polyurethanes methacrylates In all cases, the mechanism of curing is shown schematically in Figure 6.13. The adhesives are resistand simultaneously to high temperatures (>180∞C) humidity a number of chemical agents Figure 6.12 Configuration for Bolted Joints Figure 6.13 Curing of Adhesive © 2003 by CRC Press LLC
metal high stresses adhesive high laminated stresses Figure 6.14 Stresses in Bolted Joint decohesion fracture of adhesive fracture at adhesive/part interface (initialized at 1 and 2) decohesion fracture in one of the assembled pieces Figure 6.15 Fracture Modes in a Bonded Joint The pieces to be assembled have to be surface treated.This consists of three steps: ■degreasing ■surface cleaning protection of cleaned surface The case of metal-laminate bond: The differences in physical properties of the constituents requires that the adhesive must compensate for the differences in ■thermal dilatations elongation under stress The schematic in Figure 6.14 indicates in an exaggerated manner the deformed configuration of a double bonded joint.This shows the role of the adhesive and the gradual transmission of the load from the central piece to the external support. Fracture of a bonded assembly can take different forms,as indicated in Figure 6.15. 6.2.2 Geometry of the Bonded Joints One must,as much as possible,envisage the joint geometries that allow the following specifications: the adhesive joint must work in shear in its plane tensile stresses in the joint must be avoided Consequently,the transmission of the loads will be dependent on the geometries, as shown in Figure 6.16.A double sided joint with increasing thickness is shown in Figure 6.17. Transmission of couples is shown in Figure 6.18. 2003 by CRC Press LLC
The pieces to be assembled have to be surface treated. This consists of three steps: degreasing surface cleaning protection of cleaned surface The case of metal–laminate bond: The differences in physical properties of the constituents requires that the adhesive must compensate for the differences in thermal dilatations elongation under stress The schematic in Figure 6.14 indicates in an exaggerated manner the deformed configuration of a double bonded joint. This shows the role of the adhesive and the gradual transmission of the load from the central piece to the external support. Fracture of a bonded assembly can take different forms, as indicated in Figure 6.15. 6.2.2 Geometry of the Bonded Joints One must, as much as possible, envisage the joint geometries that allow the following specifications: the adhesive joint must work in shear in its plane tensile stresses in the joint must be avoided Consequently, the transmission of the loads will be dependent on the geometries, as shown in Figure 6.16. A double sided joint with increasing thickness is shown in Figure 6.17. Transmission of couples is shown in Figure 6.18. Figure 6.14 Stresses in Bolted Joint Figure 6.15 Fracture Modes in a Bonded Joint © 2003 by CRC Press LLC
