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 < slaminate rupture s ¢ M > 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