19 COMPOSITE JOINING AND JOINTS

Transcription

1 19 COMOSITE JOINING AND JOINTS 19-1

2 ractical structures generally require joints to transfer loads from member to member. The joints may be between similar or dissimilar materials. Joints in composite structures present a greater challenge than for homogeneous, isotopic materials since anisotropic materials do not easily accommodate stress concentrations and have intrinsic weak directions. Composite joints can be mechanical attachments such as bolts and rivets or adhesive attachments using glue. MECHANICAL JOINTS Mechanical joints are used when repeated disassembly and reassembly is required or when surface preparation is not practical. Mechanical joints require that bolt or rivet holes are drilled into the composite that reduced the net cross sectional area of the structure and introduce localized stress concentration. These stress concentrations can cause ply delamination since they will include through thickness tensile and shear stresses (see Chapter 15). Mechanical joints add weight to the structure from the added weight of the bolt or rivet. They also pose a risk for corrosion since the laminate and fastener may comprise dissimilar materials and moisture can be trapped in the crevices inherent in such joints. However mechanical joints can be readily inspected before assembly and while in service. Examples of two typical bolted joints are the single lap joint and double strap joints. These joints and the relevant dimensions are shown in Fig e D W Bolted Single Lap t Axial itch Bolted Double Strap Figure 19-1 Basic types of mechanical joints. 19-

3 The single lap joint is the simplest and most weight efficient but the load results in a moment due to off-set load. The double lap joint eliminate the moment but adds additional weight from the straps and additional bolt. Failure modes in mechanical joints The composite designer must consider four relevant stresses in mechanical joints. The bearing stress, σ is the load, divided by the projected transverse cross sectional area of the b hole σ b = Dt. The shearout stress is determined by the longitudinal shear surfaces, and is given as σ SO = et. The net section stress is σ N = ( W D) t. The transverse splitting stress is a localized stress normal to the applied load. When any of these four stresses reach a critical value the joint will fail with a characteristic mode, shown in Fig.19-. The gross stress defined as σ= Wt is used to rate the effectiveness of the joint. Joint efficiency is the ratio of the gross section stress at failure to the strength of the laminate in the gross section. For metals single fastener joints can have efficiencies as high as 80%. olymer matrix-fiber composite laminates have efficiencies generally less than 50% due to their strength anisotropy and inability to redistribute stress. Net section failures can be prevented by increasing the ratio of a) Net section failure b) shearout c) transverse splitting d) bearing failure Figure 19-. Failure modes in bolted joints the plate width to hole diameter,w D. Generally 6 W D> is sufficient to prevent net section failures. Shearout failures can be eliminated the ratio, edto 3 or greater. Transverse splitting is rare but will occur if there is a high fraction of the fibers in the load direction such as would be the case in a unidirectional composite. Bearing failure is the preferred failure mode since the joined members are not catastrophically separated. Bearing failures are associated with localized hole damage such as local delamination and matrix crazing. Some examples of bearing damage are shown in Fig An extreme case of damage induced by the counter-sunk head fastener, composite plates can be completely pulled out of the fasteners. To promote bearing failures, A 19-3

4 lower stiffness material is placed in the bearing region. lacing ±45º plies in the bearing region is also used to induce bearing failures. Local delamination can be induced near the hole region by using interference fit fasteners. Single Lap Joint Double Lap Joint Figure 19-3 Examples of bearing damage Bolted joint failure analysis Counter-Sunk Head Fastener Chang, Scott and Springer offered the following composite bolt failure analysis. A schematic diagram of the bolt hole specimen and the critical parameters used in the analysis are shown in Fig The bolt shank is loaded uniformly and symmetrically along its length in the X direction. The angle α is measures from the load direction. The failure criterion used in this analysis was originally proposed by Yamada, 1 τ 1 1 σ + = X S c (19.1) where X is the strength of a longitudinal and c S is the shear strength of a symmetric cross-ply. This criterion is a simplified Tsai-Hill criterion with the transverse and biaxial strength terms eliminated. 19-4

5 X a e L D X 1 X 3 W t Figure 19-4 Schematic of bolt hole used in Chang, Scott and Springer analysis According to this analysis, bolt hole failure will occur when the Yamada failure criterion is met r α measured in any ply on the characteristic curve. The distance to the characteristic curve, ( ) c from the edge of the bolt hole is determined by two length parameters, R0c and R 0t, that are experimentally measured material constants independent of geometry and stress distribution. Theses length parameters are defined schematically in Fig The term R is the X 0c R oc a r c c X 1 D R ot Figure 19-5 Schematic diagram of bolt hole showing characteristic distances characteristic distance in compression and characteristic curve is then R 0t is the characteristic distance in tension. The 19-5

6 D rc( α) = + R0t + ( R0c R0t) cosα (19.) where the range of α is π π α. If at any point on the characteristic curve the Yamada criterion is exceeded, ( r, α) τ ( r, α) σ1 c 1 c + = 1 X Sc (19.3) the joint has failed at that point. The local stresses on the characteristic curve, σ ( r, α), σ ( r, α)and τ ( r, α) must be determined by numerical method. x c y c xy c Converting the local stresses on the characteristic curve to principal material directions then, σ α =σ α θ+σ α θ+ τ α θ θ 1( rc, ) x( rc, )cos y( rc, )sin xy( rc, )cos sin ( ) τ1( rc, α ) = σx( rc, α)cosθsin θ+σy( rc, α)cosθsin θ+τxy( rc, α) cos θ sin θ (19.3) The failure mode of the bolt hole depends on the point on the characteristic curve where the o over-load occurs, as shown in Fig The bearing mode occurs over the range of ± 15 from o o the bottom of the hole. Shear-out occurs over the range from The tension mode failure o o occurs over the range from X R oc Bearing D R ot Shear Out Tension X 1 Figure 19-6 Regions of hole failure modes. ly lay-up and hole diameter relative to the plate width have a significant effect on failure mode type and strength of the joint, as seen in Fig In this figure the maximum joint load, max 19-6

7 0.3 tension max f shear out shear out Figure 19-7 ly layup effects on bonded joints 5 10 W/D bearing bearing + - [0 / 45 / 90 ] + - s [0 / 45 ] [ 0 /90 ] divided by the failure load of the ply without a joint, f is plotted against the ratio of plate width to hole diameter, W D. This ratio is a measure of relative hole diameter. The strongest bolt hole joint occurs for the quasiisotropic lay-up [ 0/ ± 45/90] S. At large relative hole diameters the failure mode is net tension failure. For small relative bolt holes bearing failure is the predominant mode. The cross ply design has the lowest bolt hole strength while the [ 0 / ± 45] S lay-up has the intermediate strength. ADHESIVELY BONDED JOINTS Adhesively bonded joints have high structural efficiency and are used extensively to join composites in advanced aerospace structures. Bonded joints can be between two composite laminates or between a composite laminate and a metal structure. Composite to metal bonds are used often in airframe structures. Adhesively bonded joints can distribute load over a much wider area than mechanical joints. Since no holes are required, the risk of local delamination is practically eliminated. Compared to bolted joints with weight of the joint is significiantly reduced. On the other hand adhesively bonded joints cannot be disassembled without destroying the substrate. Some adhesives are susceptible to degradation by temperature and humidity. The most critical drawback for safety critical structure such as airframes is their inspectability. Critical joints may require ultrasonic inspection over their entire area. Corrosion can be problem in carbon fiber composite to aluminum joints due to galvanic action. In such cases an intermediate insulating layer can be used. Some of the most common types of adhesively bonded joints are shown in Fig In addition to the lap and strap joints that are used in bolted joints additional types are possible with adhesive bonding such as the stepped joints and scarf joints. s s 19-7

8 Single Lap D o u b l e L a p Double Strap Stepped Lap Double Stepped Lap Single Scarf D o u b l e S c a r f Figure 19-8 Common adhesive joint configurations A variety of polymer adhesives can be used to bond composites. The specific selection depends upon the maximum operating temperature of the structure and the nature of the loads expected at 19-8

9 the joints. Table 19-1 lists some of the more commonly used adhesives. High strength adhesives tend to be brittle, therefore lower strength, flexible adhesives may be preferred if impact loads are expected or high displacements are required. Usually the higher the operating temperature of the adhesive the greater is the cost. Table 19-1 olymer adhesives Type Temp Limit Cure Temp Use Epoxy olyamide 00 RT-00 General Epoxy Amine 400 RT-300 General Epoxy henolic High Temp olyester 300 RT General Silicone 600 RT HT, Flexible olyimide HT Acrylic 300 RT olyester Rubber 400 RT Flexible olyurethane 50 RT Flexible Cyanoacrylates 475 RT Strong, Brittle Strength of adhesively bonded joints The strengths of E-glass to E-glass and E-glass to aluminum single lap bonded joints as a function of lap length are shown in Fig The adhesive is epoxy in both joints. Increasing the lap length to greater than 1 inch has only a small effect on the joint strength. The bond strength of E-glass to aluminum is not as great as E-glass bonded to itself. The effect of joint configuration on the joint strength for various joint lengths of boron-epoxy bonded to aluminum are shown in Fig For this material pair there is a large difference in strength between the single lap and double lap joint strengths. For large joint lengths the stepped lap joint are stronger than the lap joints. Stepped lap joints are often used for bonding multiply laminates where each step corresponds to a ply. The highest bond strength can be achieved with a scarf joint if the bond length is great enough. Adhesive bond strength analysis Elastic solutions to stresses and strains in adhesively bonded joints are available for simple joint configurations such as the single lap joint shown in Fig Applying a load to this joint in the deformations and displacements illustrated in Fig For a single lap joint the loads are off-set from the midplane of the adhesive causing a moment which produces bending in the 19-9

10 E-glass/E-glass E-glass/Aluminum Lap Length, in. Figure 19-9 Joint strength for single lap joints. (E-glass t = 0.04 in., aluminum t = 0.063) 14 Joint Strength, ksi Double Lap Stepped Lap Single Lap Scarf Length of Joint, in. Figure Effect of joint configuration on strength of boron-epoxy to aluminum joints 19-10

11 h t L Figure Geometry of adhesively bonded single lap joint Bending Tension Maximum Shear Figure 19-1 Adhesively bonded single lap joint under load substrate plies. In addition to shear in the adhesive parallel to the plane of the plate, the free edge of adhesive is under tension. This tension can cause the tear and the tear flaw can propagate by mode II crack propagation. The stress distribution of a single lap joint over the joint length, L is shown in Fig The high normal stresses near the free ends of the joint results in bond peeling. Bond peeling can be reduced by tapering the joint as illustrated in Fig eeling can + Shear Stress Stress 0 - Normal Stress L Figure Stress distribution in a loaded single lap joint

12 Figure Tapered adhesively bonded single lap joint be virtually eliminated by using a scarf joint shown in Fig If tapering of the lap joint or use of a scarf joint is not possible, the ends of the joint can be held mechanically to prevent peeling as shown in Fig Figure Adhesively bonded scarf joint Figure Combined mechanical fastened and adhesively bonded single lap joint Adhesively bonded joint design guidelines Good joint design requires that 0º plies are joined together. Bonding 90º to 0º plies or 90º to 90º plies should be avoided. If these joint matching plies cannot be avoided the scarf joints should be used. Joint strength is always improved with increased lap length to thickness ratios, Lt, especially at low Lt ratios. Tapering the substrate plate ends in the overlap reduces the normal stresses that tend to peel the joints apart. Always use equal stiffness substrates for the joined members. If the Young s modulus is different between the two members the thickness should be adjusted such that Et 11 = Et. The ideal adhesive has high shear strength and tensile strength but low shear modulus and Young s modulus. 19-1