Joining of Advanced Composites

ByVimal Kumar Thummalapalli

Why Joining of Composites is important

• Assembly account for 50% of manufacturing cost for composites.

• The Achilles heels of composite structures are the joints.

• “You can’t just go to a handbook and look up composites to get the right tools, right machine and processing procedures for joining the composites” like conventional metals, Polymers and ceramics.

Objectives of Composite Joints:

• It should retain good mechanical properties at extreme conditions.

• It should provide an conducting path to the commercial composite structure.

• It should reduce the weight of the composite material.

• It should be easily to repair .• The structure should be easily to inspect like

NDE and quality control.

Types of Joining of Composites:

• Adhesive bonding• Mechanical Joining• Combined Adhesive and Mechanical Joining• Co-Curing Process• Stitching

Advantages of Mechanical Bonding

• These mechanical bonds are easily to repair and easily to inspect while quality control.

• The mechanical joints are simple in construction so production is faster.

• These are not effected by the environment and temperature except they under go corrosion.

• These parts do not require a neat room and environment like adhesive joints.

• We can get a good joint by adding extra composite material ,metallic reinforcements at the composite join but it will increase the overall weight of the composite structure.

• Also these joints have some disadvantages.

Drilling of Composites

• In the process of drilling matrix could melt from too much heat, while the carbon fibers don’t cut well because they fracture instead of shearing smoothly.

• A drill cutting through a layered composite structure is likely to push the layers ahead of it, producing unacceptable delimitation on the exit side.

• diamond coating of 12-micron coating delivers the best value in terms of cost vs. tool life.

• Optimum speed of drilling will give good Quality.

• The drilling process often relies on some kind of backside support.

• Plain weave fabrics were used at the top and bottom layer of composite so damage can be reduced.

Bolts and Screws • Commonly used materials are : Ti, Al and Cd coated Stainless Steel, Cold worked

Stainless Steel (A286), Inconel 718, Ni-Co-Cr Alloy MP35N and MP 159.• bolts and nuts may exert too much compressive force and the materials could be

deformed. • If the Composites are not strong in shear then screws may not hold huge weights.• In a big complex composite structure bolts and nuts are the stress concentration

points.• Long term effects are: Fatigue, Solvents and water penetration through the hole.• Composite joints with bolts, hole have poor performance in tension (20% - 50%).• Some people were trying use fasteners used from the composites so that weight

can be reduced.• Carbon can cause galvanic corrosion with Al and Steel so Titanium (Ti-6Al-4V) is

usually best for carbon. Glass , Kevlar are non conductive so these will not corrode metal fasteners.

• If any gaps are present near the fastener and composite shimming process is done.

Finishing and Failure

Adhesive bonding

• A wide variety of materials is available when adhesives are used to bond materials together.

• The choice of which adhesive is best is usually dictated by the type of composite to be bonded, the application of the bonded composite, the service environment, and cost.

• The most common polymers in the structural adhesives class are: epoxies, polyurethanes, acrylics, cyanoacrylates, silicones, and phenolics.

Selection of a proper adhesive:

• Thermal Conductivity of adhesive.• Chemical Compatibility.• Viscosity.• Temperature resistance.• Mechanical strength of adhesive.

Fundamentals of Adhesives:• All substances known to us consist of atoms

and molecules with their cohesion being based on electric forces.

• Inside the material these forces are equally distributed between the atoms and/or the molecules.

• On the surface of the material atoms and molecules no longer have equal “neighbors” ,therefore They are capable of binding other substances, for example, dust particles or water drops.

• These sites are useful in the adhesion process.• Such force actions are developed in a similar

way by polymer molecules, which then fuse to form a strong bond with those of the adhered surface

Production of Bonded Joints

Surface Treatment:• Plasma Treatment• Solvent Cleaning in the

presence of heat• Cleaning the surface with

heated Distilled water• Mechanically trimming small

layer by abrasion or by shaving the surface.(Also Grinding, brushing, sanding or blasting)

Viscosity

• Depending on the adhesive viscosity and the wettability of a surface, liquid drops applied to a surface take on different forms.

• The contact angle α smaller the value, the better the wetting. Good wetting is talked of, if the values of α are below 30 °.

• Also bond line thickness will increase if viscosity is more.

• So viscosity plays meager role in the designing of adhesive.

Effect of Adhesive thickness

Effect of Bond Length:

Effect of Surface roughness

• Rough surface will get good adhesion . As high surface area is available for the bonding.

• On rough surface we can’t get uniform wetting as resin can’t penetrate into the small pores.

• We can find some air gaps at the interface of the adhesive bond in the case of the rough surface .

Failure of the adhesive joint

• Cohesive fracture

• Interfacial fracture

• Mixed Failure• Failure of

adherent

Disadvantage of Adhesive bonding:• The heat resistance of the adhesive layer is limited. Depending on the

basic material of the adhesive, temperatures for continuous stress range between approximately 120 and 300 °C.

• Adhesive layers and their boundary layers towards the adherents’ surfaces may be damaged by environmental impacts, such as humidity, which results in a reduction of strength.

• In the production of bonded joints, the time required for the relevant reaction kinetics of curing is more .

• The growing demand for recyclability of industrial products calls for respective design-engineering measures.

• The availability of nondestructive test methods is rather limited.• Quality control , Inspecting, Repairing of complex adhesive joints is

big problem.

Combination of Mechanical and Adhesive Joining

• Application of both a mechanical joining device and an adhesive are required for better mechanical strength.

• Here fast curing adhesives may be used as a method of rapid assembly before the permanent mechanical joints are installed.

• As the complexity of structure and its application increased we can’t use both methods.

Stitching of Composites:• Through-the-thickness stitching is a

method that increases trans laminar strength while at the same time preventing crack propagation.

• Results showed that the fracture toughness increased 30 fold when compared to the unstitched laminate

• Stitching will damage the fibers and insert some holes in the composite so it will decrease the in plane properties of composites.

• Stitching leads to better control on the handling while processing of composites.

• Stitching is a cost-effective method for joining stacked fabric plies along their edges to make the perform .

• Without stitching or some other type of binding, stacked plies often slip during handling that can cause fiber distortions and resin-rich regions in the composite.

Ref: Master Thesis “MIXED MODE FRACTURE TOUGHNESS OF STITCHED LAMINATED COMPOSITES” By T.Rys ,Department of Aerospace Engineering, University of Florida 2004.

Types of stitches used:

• Lock stitch• Modified Lock stitch• Chain stitch

Ref: Master Thesis “MIXED MODE FRACTURE TOUGHNESS OF STITCHED LAMINATED COMPOSITES” By T.Rys ,Department of Aerospace Engineering, University of Florida 2004.

Defects in Composites due to Stitching

Mechanical properties of Stitched Composites:

Mechanical properties of Stitched Composites:

Mechanical properties of Stitched Composites:

Co-Cured composite Joints:

• This method is very good for joining small composite parts.

• The size of the composite part increases we can’t use this method as size of autoclave is limited.

• Very limited chance for the repairing of the composite.

• We can’t control the resin rich spaces in the co-cured composites

Conclusions:

• “You can’t just go to a handbook and look up ‘composites’ to get the right tools, speeds and feeds,” and processing procedures.

• Helicopter manufacturers were among the earliest adopters of advanced composites. While commercial aircraft are only now reaching 50 percent composites by weight, some helicopters have long been 90 percent composites by weight. So composite manufactures should adapt their skills.

• The choice of joining method is complex and should be made with careful consideration of the application purpose, the use environment, and the fabrication process.

References:• http://www.mmsonline.com/articles/how-to-machine-composites-part-4----drilling-composites• http://www.mmsonline.com/articles/how-to-machine-composites-part-1----understanding-compo

sites• Journal of Materials Processing Technology 140 (2003) 335–339• http://strong.groups.et.byu.net/pages/articles/articles/joining2.pdf

• Master Thesis “MIXED MODE FRACTURE TOUGHNESS OF STITCHED LAMINATED COMPOSITES” By T.Rys ,Department of Aerospace Engineering, University of Florida 2004.

• 3D fiber reinforced polymer composites By Liyong Tong, Adrian P. Mouritz, Michael K. Banniste• Manufacturing processes for advanced composites By Flake C. Campbell• Joining of composite-matrix materials By Mel M.Schwartz• Applied Adhesive Bonding: A Practical Guide for Flawless Results By Gerd Habenicht• The Effect of Joint Configuration on Joint Strength. H. S. ROH and C. T. SUN ASC 21 Annual Meeting

Proceedings.• www.wikipedia.org• http://www.compositesworld.com/articles/joining• Composites Part A: Applied Science and Manufacturing Vol:34, Issue:4, May 2003, Pages 403-410• Composites Science and Technology , Volume:58, Issue:8 , 1998, Pages 645-651