L19 Joining

What is potting and when is it necessary in composite sandwich structures?

Potting is the filling of honeycomb cells with resin to strengthen areas for inserts or load introduction. It is necessary when local reinforcement is needed, e.g., for fastener locations in composite sandwich structures.

How can a press fit be achieved?

A press fit is achieved by slightly enlarging the part to be fitted or heating/cooling the components to allow thermal expansion or contraction, resulting in a tight fit after temperature equalization.

What two kind of load transfer principles work for screw fastened joints?

Form closure (for longitudinal loads) and force closure (for transversal loads).

Explain the embedment effect in screw fastened joints. Name three possible countermeasures.

Embedment leads to loss of preload due to settling of contact surfaces. Countermeasures include: applying minimum preload FV,min, re-tightening, using rigid joint partners or high-elastic bolts.

Name two examples of nut types that prevent loosening of screw fastened joints.

Self-locking nuts (e.g. nylon insert nuts) and castle nuts with locking wire.

What are the specialties about threaded bolted (screw) joints on FRP sandwich structures?

They require large support areas, often include metal inserts or sleeves, and must minimize preload loss due to lower stiffness and potential crushing of composite material.

How does a hot and cold riveted joint transfer the load?

Cold rivets rely on interference fit (form/force closure), while hot rivets shrink after cooling, creating a preload that enhances load transfer.

What is the advantage of blind rivets/blindbolts over full rivets/lockbolts?

They can be installed from one side only (single access), which is useful in confined or closed structures.

Explain the difference between bolted joints made with screws, rivets and special aerospace fasteners (e.g. Lockbolts).

Screws rely on torque and friction; rivets are permanent and use deformation; Lockbolts provide high preload and shear strength and don't loosen under vibration.

Name four types of joint partner failure in bolted joints.

Net tension failure, shear-out failure, bearing failure, cleavage tension failure.

What laminate designs are appropriate for bolted joints?

Quasi-isotropic designs or balanced stacking with at least 12.5% of each fiber direction are preferred for better load distribution.

Name a positive and a negative effect of the preload applied to a bolted joint in FRP material.

Positive: increases joint stiffness and reduces fatigue. Negative: may cause crushing or stress concentrations in softer FRP materials.

Why should bolted joints be avoided in FRP? What kind of other joint method is better and why?

Bolted joints can damage FRP due to preload loss and stress concentration. Adhesive bonding is better as it distributes loads over a wider area and avoids drilling.

What are the benefits and drawbacks of welding compared to other joining methods?

Benefits: strong, permanent joints, often lighter than mechanical joints. Drawbacks: high thermal input can distort parts or affect material properties.

What are the basic components of the welding triangle? Give an example for each 'corner'.

Design/Safety (e.g. load conditions), Material (e.g. alloy composition), and Production (e.g. process choice).

Explain the the purpose of the Schäffler diagram.

It predicts microstructure and weldability of steels based on alloy composition, helping avoid defects like cracking.

Describe the spot welding process.

Two electrodes apply pressure and current to create localized heat at the contact point, forming a weld "nugget".

Name an advantage and a drawback of electric arc welding processes.

Advantage: high strength and penetration. Drawback: requires skilled labor and careful control to avoid defects.

What is the difference between TIG and MIG/MAG welding process?

TIG uses a non-consumable electrode and is more precise. MIG/MAG uses a consumable wire electrode and is better for high deposition rates.

Explain the friction welding process. What is the difference between rotational and stir friction welding?

Friction welding uses heat from mechanical friction. Rotational friction involves spinning one part, while stir friction involves a rotating tool that moves along a seam (no melting).

Name two advantages of laser beam welding over other welding processes.

High precision and low heat input, enabling minimal distortion and fast processing.

What will cause trouble when aluminum from the storage is directly joined via material closure methods?

Oxide layers on aluminum surfaces hinder bonding and must be removed before joining.

Why are thermoplastics the only plastics that can be welded?

They can be softened and reformed with heat, unlike thermosets which undergo irreversible curing.

What can you do to increase the radiation absorption (e.g. laser light or microwaves) as well as the total strength of a thermoplastic material at the same time?

Add carbon black fillers or other absorbing additives to increase energy absorption and mechanical strength.

Name three welding processes dedicated to thermoplastics (i.e. that do not work for metals).

Ultrasonic welding, induction welding, laser transmission welding.

What is healing of thermoplastics? Why is it not an adhesive bonding process?

Healing is molecular interdiffusion and re-entanglement at the interface under heat and pressure. It is not adhesive bonding because it forms a cohesive joint without foreign material.

What is the difference between cohesion and adhesion

Cohesion is internal bonding within the adhesive. Adhesion is bonding between adhesive and substrate surface.

What are the three requirements for specific adhesion?

Wetting, chemically active surface, and absence of contamination or moisture.

Why is adhesive bonding considered a special process?

Its quality cannot be fully inspected post-process, and failures are mostly due to surface preparation issues, not material properties.

What are the aims of surface preparation for adhesive joints?

Increase surface energy for wettability, remove contaminants, and activate the surface chemically.

Explain the term 'critical overlap length' in adhesive bonded joints. How can it be influenced (according to the Volkersen model?

It's the minimum overlap needed for effective load transfer. It can be influenced by adhesive shear modulus, overlap length, joint width, and stiffness of adherends.

Starting from the critical overlap length, please sketch the shear stress distribution according to the Volkersen model when we increase (a) the load, (b) the adhesive shear modulus, (c) the joint width, (d) the joint partner stiffness.

(a) Stress increases overall. (b) Higher modulus sharpens peak stress. (c) Wider joint reduces stress concentration. (d) Stiffer partners flatten the stress curve and distribute stress better.

Can an adhesive joint transfer arbitrary high loads? Explain why!

No. The load capacity is limited by the adhesive's strength, bond area, and stress concentrations, especially at edges.

What are chicken rivets? Are they always useful?

Chicken rivets are backup rivets added out of caution. They are not always useful and may cause stress concentrations or fatigue issues.