The choice of carbon fiber composite manufacturing method has been discussed in other articles and Mazumdar has described the manufacture of composite in details.
Obviously, cost is a controlling factor and to compete against Al, the lowest cost form of carbon fiber must be used in conjunction with an automated high speed lay-up process. These criteria have been exemplified by Mils and co-workers, working at Cranfield University, who have achieved these aims by developing a machine that can achieve a prepreg output of about 40kg/houre.
Bonding and joining:
There are number of useful publications which discuss structural joints and adhesives in details.
As a rule, adhesive bonding should only be used if it can be undertaken in the controlled conditions of a manufacturing plant and with skilled labor. To ensure a good bond it is necessary to:
- Maximize the bond area.
- Aim for compression and shear loads in the adhesive joint.
- Attempt to avoid peel and tensile stresses.
Surface cleaning is critical and depending on the choice of adhesive, can include solvent cleaning, application of a primer, surface abrasion and peel of the ply embedded in the laminate.
Adhesive bonded joints are not weakened by drill holes, as there is no local stress concentration, and can accommodate tolerances in the component part and act as a sealant with very good fatigue resistance.
Moussiax and lugering have outlined factors that may help in selecting the type of adhesive:
- Toughened epoxies—have highest mechanical and best temperature performance with good resilience, but require good surface roughening and are best cured with external heating.
- Two-component polyurethanes—are easy to apply and provided the environmental conditions are not too critical, they have good toughness and very good durability in European climate and in wet conditions.
- Cyano-acrylates—have the fastest cycle time using moisture in the air for cure, but are not in common use. They have the ability to bond to most surfaces with little preparation and are used mainly for GRP assemblies.
- Two-pack methacrylate—have a fast cure and are strong and flexible, but the onset of cure is sudden, reaches peak exotherm in about 3 min and heating will speed up the onset of cure.
Bonded joints are limited by the shear strength of the adhesive. The highest stress is at the edges and for a single bonded joint, can be 3-10 times the stress in the remaining 90% of the joint. When joining metal to composite, machining grooves in either part to within 5 mm of the edge will limit the effects of edge stress.