COMPOSITES JOINTS

M.THIRUMALAIMUTHUKUMARANASSISTANT PROFESSOR

DEPARTMENT OF MECHANICAL ENGINEERINGDr.N.G.P INSTITUTE OF TECHNOLOGY

COMPOSITE JOINTS - INTRODUCTION• High stiffness and strengths can be attained for

composite laminates. However, these characteristics are quite different from those of ordinary materials to which we often need to fasten composite laminates.

• Often, the full strength and stiffness characteristics of

the laminate cannot be transferred through the joint without a significant weight penalty. Thus, the topic of joints or other fastening devices is critical to the successful use of composite materials.

• The two major classes of laminate joints are bonded joints , bolted joints and bonded – bolted joints.

• Joints involving composite materials are often bonded because of the natural presence of resin in the composite.

• Several characteristics of fiber-reinforced composite materials render them more susceptible to joint problems than conventional metals.

• These characteristics are weakness in in-plane shear, transverse tension, inter laminar shear, and bearing strength relative to the primary assets of a lamina, the strength and stiffness in the fiber direction.

BONDED JOINTS

SINGLE-LAP JOINT DOUBLE-LAP JOINT

STEPPED-LAP JOINT SCARF JOINT

• Goland and Reissner studied the stresses in bonded single-lap joints for two important limiting cases:(1) a bond layer so thin that it has no contribution to the joint flexibility (inverse of stiffness), and (2) a bond layer so thick that it is the primary contributor to the joint flexibility .

• They considered the shearing and normal stresses in the bond layer as well as those in the joined plates. For fiber-reinforced composite materials, the thick-bond-layer approach of Goland and Reissner is more appropriate than the thin-bond-layer approach because of the presence of epoxy resin in the composite material and the effective thickness of the bond relative to the joined pieces.

• The fundamental design problem in bonded joints is to get enough bond area in shear to carry the load through the joint. Bond area in tension is of little value because of the typically low strength of bonding materials compared to the far higher strength of the metals or composite materials being joined. The contrast between the two types of bonding area is shown in below fig.

Fig : Good and Bad Load Transfer in Bonded Joints

• The extension of these concepts to many types of bonded joints is illustrated in Fig below along with their types of failure. There, an increase in adhered thickness does not always lead to a stronger joint.

Fig - Types of Bonded Joints and Their Failures

BOLTED JOINTS

SINGLE-LAP JOINT DOUBLE-LAP JOINT

REINFORCED-EDGE JOINT SHIMMED JOINT

• The principal failure modes of bolted joints are

• (1) bearing failure of the material as in the elongated bolt hole of Figure.

• (2) tension failure of the material in the reduced cross section through the bolt hole.

• (3) shear-out or cleavage failure of the material (actually transverse tension failure of the material).

• (4) bolt failures (mainly shear failures). Of course, combinations of these failures do occur.

Fig: Bolted Joint Failures

• One of the ways to increase the bearing strength of a joint is to use metal inserts as in the shimmed joint of Figure.

• Another way is to thicken a section of the composite laminate as in the reinforced-edge joint in Figure.

• Net-tension failures can be avoided or delayed by increased joint flexibility to spread the load transfer over several lines of bolts.

• Composite materials are generally more brittle than conventional metals, so loads are not easily redistributed around a stress concentration such as a bolt hole.

• Simultaneously, shear-lag effects caused by discontinuous fibers lead to difficult design problems around bolt holes.

• A possible solution is to put a relatively ductile composite

material such as S-glass- epoxy in a strip of several times the bolt diameter in line with the bolt rows. This approach is called the softening-strip.

Bonded-Bolted Joints

Figure : Bonded-Bolted Double-Lap Joint

• Bonded-bolted joints generally have better performance than either bonded or bolted joints. The bonding results in reduction of the usual tendency of a bolted joint to shear out.

• The bolting decreases the likelihood of a bonded joint debonding in an interfacial shear mode.

• The usual mode of failure for a bonded-bolted joint is either a tension failure through a section including a fastener or an interlaminar shear failure in the composite material or a combination of both.

• Bonded-bolted joints have good load distribution and are generally designed so that the bolts take all the load.

• Then, the bolts would take all the load after the bond breaks (because the bolts do not receive load until the bond slips). The bond provides a change in failure mode and a sizable margin against fatigue failure

• An example of a complex bonded-bolted joint used in the box beam of a folding aircraft wing is shown in Figure. There, a basic structure of graphite-epoxy and boron-epoxy layers over honeycomb is attached to an aluminum forging. The honeycomb is gradually replaced by graphite-epoxy as the joint is approached from the wing direction.

• The titanium sheaves are bonded to the graphite-epoxy with a film adhesive. The graphite-epoxy is bonded to the aluminum forging with a paste adhesive. The entire joint is then bolted together.

ENVIRONMENTAL EFFECTS • Composite materials must survive in the environment to which

they are subjected at least as well as the conventional materials they replace.

• Some of the harmful environments encountered include exposure to humidity, water immersion, salt spray, jet fuel, hydraulic fluid, stack gas (includes sulfur dioxide), fire, lightning, and gunfire as well as the combined effects of the space environment.

• Humidity or water immersion can lead to degraded stiffnesses and strengths. However, after dehydration, the original properties are recovered.

• Some of the same but irreversible effects are found for salt spray (although it is somewhat corrosive), jet fuel, hydraulic fluid, and stack gas. Fire is, of course, an extreme environment, and its damage is obvious.

• Aircraft are subjected to lightning strikes, so must be protected and certainly not built of materials that are particularly susceptible to lightning damage. Lightning tests were performed on aluminum, fiberglass, boron-epoxy, and graphite-epoxy.

• The specific degradation mechanisms for environmental exposure A. Loss of strength of the reinforcing fibers by a stress-corrosion mechanism. B. Degradation of the fiber-matrix interface resulting in loss of adhesion and interfacial bond strength. C. Permeability of the matrix material to corrosive agents such as water vapor which affects both A and B above. D. Normal viscoelastic dependence of matrix modulus and strength on time and temperature. E. Combined action of temperature and moisture accelerated degradation."

• Composite materials in space, such as in orbiting space stations, are subject to an environment of hard vacuum, thermal cycling because of passing in and out of the sun's rays, as well as ultraviolet, electron, and proton radiation.

• The effects of those environmental factors include:

(1) vacuum causes out gassing and migration of low-molecular-weight components from matrix materials such as polymers.(2) thermal cycling can cause significant dimensional changes in such dimension-sensitive instruments as space telescopes. (3) radiation causes damage because of competing effects of polymer chain-cutting and enhanced cross-linking.

• The latter effects change the melting point, hardness, strength, and stiffness as well as the dimensional stability of polymers.