Joints
The type of joint that will be tested will be a T-joint because with overlay in composite frame design all joints could be made T-joints which will be stronger than L-joints.
It will also be important to test bonding of panels paralell to each other, end to end.
Things we should determine:
Goal stress that joints should withstand
Fillet radius for bond
Necessity of extra composite or metal supports
End-to-End Bonding:
Our end-to-end bonding will likely be double-strap joints, but tapered strap joints could be worth looking into depending on the thickness of adherends we’re willing to use because of the ability it has to minimize the affets of shear and peel stresses.
Because the adhesive layer is extremely prone to imperfections and inconsistencies, it’s important that the adherend fails before the bond because the properties of the adherend will be more consistently predictable. (Using relatively thin adherends shoud allow us to avoid this issue)
The ratio of in-plane shear moduli of the panel(p) and adherend(a) [p:a] should be equal to the inverse of the max allowable thickness ratio of panel(p) to adherend(a) [a:p]. (This will likely not pose as a prolem as the panels will be significantly stiffer than our adherend and the adherends hould be relatively thin)
Tentative Plan:
The most important part of the joints will be the bond, and the bond will fail either by bending or tension so those will be the main cases that we test.
Gluing
Adhesive bond may be destroyed under high mechanical shear, bending or shear loads
Welding
Useful for polymeric materials (repeating monomers in structure)
Hot air and welding wire
Inner layer of composite and wire heated/ joined
Bevel ends of material prior to attachment
Overlap on joint surface later taken off
Mechanical connection
Pick a material that avoids corrosion
Screws
Consider threading
Diameter of bolt< diameter of washer
Consider all loads (wind, vibrations, wtv)
Riveting
Cushioning pads can help distribute loads
Failures usually bc of stress concentrations
Used in airplanes tho
Bolted joints
Profiles
Adhesives are preferred over mechanical fastening because they spread loads over large areas with minimal fiber damage and fewer high-stress regions.
Types of stresses in adhesive joints:
(a) compression, (b) tension, (c) shear, (d) peel, and (e) cleavage.
Compression
Least likely manner to fail
Tension
Comparable to shear but offset loads have a significant negative effect
Thicken adherends if possible
Deflection leads to nonuniform stress
Should have physical restraints to ensure axial loading
Shear
Optimal bc it provides a even stress to the whole area
Peel
Adherends must be flexible
High stress is applied to the boundary line of the joint
Avoid if possible
Cleavage
Stress concentrated on one end (offset tensile force/ moment)
Large area needed if planning for this type of stress
General rules:
Maximize bonded area
Stress adhesive in direction of max strength
Avoid brittle adhesives
Epoxy is good
Use highly elastic adhesives
Don’t concentrate stresses
Impact requires elasticity
Strength is calculated with the following:
Mechanical properties of the adherend and the adhesive
Residual internal stresses
Degree of interfacial contact (where they touch microscopically)
Joint geometry
Analyses:
Nonuniform material characteristics
Stress concentrations or localized partial failures
Creep and plastic yielding
Methods of Stress Analysis
Theory of Volkersen
Only use with stiff adhesives and no bend on loading the joint
Tearing or peeling stresses ignored
Delta= GL^2/(Etd)
G is the shear modulus of the adhesive
E is the Young’s modulus of the adherend
d is the thickness of the adherend
L is the length of the overlap
t is the thickness of the adherend
Predicts that the shear stresses in the adhesive layer reach a maximum at each end of the overlap, when the bonded plates are in pure tension.
In this system, joint strength reaches a limiting value with increasing overlap length. Limiting strength is actually obtained because the adherends are loaded to their ultimate strength.
Joint design:
Strength affected by environmental conditions, age, temperature of cure, composition and size of adherends, and the thickness of the adhesive layer
Flexibility of the adhesive has a pronounced effect on the stress distribution
Greatest gain in strength is obtained by increasing the joint width
Strength of the lap joint is dependent on the yield strength of the adherend
Constraints
Should expect a total torsional strain of 1500-1600 ft-lbs/deg throughout the frame
Joining sandwich structures, especially sandwich T-joints, is complex due to the difficulty of maintaining continuous fiber reinforcement at right angles
Continuous fiber reinforcement enables efficient load transfer and significantly increases joint strength
Some designs incorporate foam fillets, such as pads or triangular inserts combined with overlaminates
Experimental studies show that failure behavior is influenced by
Surface conditions (contamination, abrasion, plasma treatment)
Fillets and bondline thickness
Surface ply angle and stacking sequence
Environmental conditions
Structural adhesives include several types, each suited for specific applications
Epoxies: High strength and temperature resistance
Cyanoacrylates: Fast bonding for plastics and rubber but poor moisture and heat resistance
An
aerobics: Best for bonding cylindrical shapes
Acrylics: Fast-curing, versatile, and tolerate less prepared surfaces
Polyurethanes: Flexible at low temperatures and fatigue-resistant
Silicones: Ideal for low-stress sealing, with high flexibility and heat resistance
High-temperature adhesives: Include phenolics, polyimides, and bismaleimides.
The rise of high-temperature composite matrices has driven the need for compatible, heat-stable adhesives
Epoxy adhesives are most common for epoxy-based composites due to chemical compatibility.
Also consider substrate type, curing and application method, operating environment, cost and service stresses
Test to run
Lap Shear Test
Bond two flat plates with adhesive in an overlapping area
Pull the ends in tension until failure
Measure maximum load / bonded area; shear strength
Double Cantilever Beam (DCB) Test
Bond two rigid beams with adhesive in between
Apply a force at the ends to open the joint like a hinge
Measure crack growth vs. applied load; fracture toughness
Wedge Test
Insert a wedge into the adhesive bond
Measure crack propagation or separation
Used to see resistance to solvents or debonding
Notched Beam Shear (Iosipescu)
Cut a beam with a central notch in the adhesive
Apply opposing forces at notches; see shear stress concentrated in adhesive
Notched Plate Shear (Arcan)
Adhesive is sandwiched between plates with a notch
Apply load at a specific angle; combination of shear and tension
Torsion / Butt Torsion (Napkin Ring)
Form adhesive into a cylinder or ring
Twist it torsionally until it fails
Gives shear strength in torsion
Thick-Adherend Shear Test (TAST)
Bond thick, rigid plates with adhesive in overlap
Pull ends in tension so stress is mainly shear
Minimizes peel stress; good for design data
Single-Lap Tension Test
Bond two plates in an overlap
Pull ends in tension
Measures failure load
Often affected by peel stresses and rotation
Moisture can degrade adhesive and composite joints
Plasticization of the polymer
Hydration
Microcracking of the polymer
Fiber–matrix weakening
Degradation effects can be measured using constitutive tests and fracture tests
T-Joints!
T-joints are structural elements that connect opposing surfaces, providing the load path between flat or curved panels.
Composites have a low interlaminar strength
Through-thickness reinforcements used to enhance the interlaminar capability
Subjected to out-of-plane loads
High interlaminar stresses
Progressive failures due to delamination between the skin and the T-joint flanges
Testing (a) resin fillets, (b) bonding angles, (c) bonding angles with fillets, and (d) bonding ties
Testing plan:
Experiments were performed on an Instron 5965 testing machine with a 5 kN load cell
Tests were conducted at 24–25 °C and 37–38% relative humidity
The custom-built test rig had a 340 mm span between clamp points, similar to the setup by Li et al. (2006)
Also a great article for calculations
30 mm clamps held the 50 mm wide T-joints in place
The vertical bulkhead experienced uniaxial tension at a displacement rate of 2 mm/min
Data collection began at a 1 N preload, recorded at 100 Hz
Test conditions followed ISO 527-4:2019, a standard for tensile testing of marine composite materials
Honeycomb-cored samples display a steeper slope with a lower yield strain than foam-cored samples
Bonding angles with standard tabbing (50 mm + 25 mm/ply); abrupt failure of resin fillets is now replaced with a progressive failure
Bonding angles with fillets (10mm) results in a notable increase in yield strength for honeycomb
Conclusion:
For honeycomb-cored T-joints, bonding ties provided no structural benefit over resin fillets, showing identical peel strength within experimental uncertainty
Bonding angles increased peel strength by 50.56%, and bonding angles with fillets improved it by 89.70% compared to bonding ties
Bonding angles with fillets are identified as the optimal configuration, while bonding ties are discouraged due to lower strength, higher complexity, greater labor and material costs, and added weight
When epoxy fills the honeycomb cells in contact with the base plate, peel strength matches that of foam cores, within the limits of experimental uncertainty
Tooling and assembly equipment (jigs and fixtures) are essential for high-quality, reproducible adhesive joints
Locate and hold components during adhesive application, assembly, and cure.
Jigs and fixtures control critical joint factors
Bond-line thickness
Joint alignment
Fillet profile
These factors affect mechanical performance, appearance, and assembly time
Types of assembly aids:
Internal agents: glass beads, wires, shims
External agents: clamps, presses, plates
Combination systems: riv-bonding (adhesive plus rivets)
Most adhesives must be liquid or semi-liquid before curing to ensure full surface wetting and intimate physical and chemical bonding
Once the adhesive reaches this strength, the structure can support itself, allowing removal of tooling
Pressure during adhesive curing
Hold adherends together
Promote surface wetting
Compensates for poor fit or tolerances
Excessive pressure can squeeze out the adhesive, reducing bond strength.
Bond-line thickness control is critical
Too thin
Weak joint, bubbles, poor wetting
Too thick
Bulk adhesive dominates, inefficient load transfer
Optimal BLT
Maximized load transfer, minimized creep
BLT ranges vary by adhesive type:
Epoxy: 50–350 µm
Acrylic: 100–500 µm
Polyurethane: 500–5000 µm
Internal or external assembly aids (glass beads, shims, tooling) are needed to maintain BLT while applying the correct pressure
Weakest part of the overall T-joint structure is the overlaminate section
Curved part of the circular overlaminate = most critical section to 3 point bending test
The CTE (Cohesive Traction Energy) method is effective for analyzing T-joint disbond problems in thin structures
For thick structures with proportions fitting Classical Laminate Plate Theory (CLPT) assumptions, the CTE method can be applied directly, eliminating solid elements and MPCs
VCCT calcs from NASA
Fracture mechanics approach uses energy (toughness) as a failure criterion, predicting crack growth if the mixed-mode energy release rate exceeds a critical value.
Linear elastic fracture mechanics (LEFM) assumes an existing crack and linear elasticity, but is limited in practice because:
Well-fabricated joints may lack macroscopic cracks
Laminated structures can develop large-scale plasticity in adherends
Stress singularity approach (fracture mechanics without a pre-existing crack) uses a generalized stress-intensity factor to predict fracture initiation in bonded joints
Proposed that fracture starts at interface corners when the generalized stress-intensity factor reaches a critical value
Size
Roughly equal to the thickness of the skin plies being reinforced, or slightly thicker if the joint is heavily loaded
Should distribute stresses effectively
The overlaminate thickness is typically chosen based on the desired strength and stiffness of the joint
Consider honeycomb- bridge the core cells without causing excessive stress concentrations
Most in industry extend 3–6 times the core thickness onto each panel from the intersection
Conclusion: Increasing overlap length generally improves joint strength almost linearly. Adherend thickness has less influence, except in thin (3 mm) adherends with 40 mm overlap, which showed lower strength.
Adherend thickness is the dominant factor; thicker adherends roughly double performance compared to thinner ones.
Failure mechanisms:
Tensile: Cohesive failure in the adhesive or mixed failure involving both adhesive and plies (notably for short/thin overlaps).
Bending: Failure occurs in the plies near the adhesive layer.
At least the same length as the bonding angle, often 1–1.5× the bonding angle length
Cover the full width of the bonding angle and ideally extend slightly beyond it if possible
Try 1-2 ply to start
Filler Gap
Between 50 µm and 350 µm for epoxy?
Maybe filler material to control thickness: glass beads, wires, or shims
The gap should extend slightly beyond the bonding angle on either side
Roughly 1–2 times the width of the bonding angle ply, to avoid stress concentrations at the edges.
Ensure uniform gap along the joint
Any tapering or inconsistent spacing can cause voids or premature failure.
Bonding Angle
1–3 plies of carbon fiber are used for bonding angles usually
See if we can get away with 3 or 4
Proposed: [0°/ +45° / -45° / 0°]
Match or slightly exceed the thickness of the surface laminate
Go 3-6 times up/down core thickness:
Around 2.25 inches (.76 x 3= 2.22); whole sheet should be 5 in ( around 2x for both side)
Tested setup :P
Jigging:
Base plate (flat tooling plate; maybe wood?, traditionally aluminum)- Holds the horizontal panel (the “skin”). Needs a smooth, flat surface
Vertical stop / back plate- Holds the stiffener leg upright at exactly 90°
Clamping pads or fingers- Apply even pressure to keep the adhesive line closed without squeeze-out
Shims / spacers- Set the filler gap thickness (e.g. 0.2–0.5 mm)
Release film / tape- Prevents sticking to the jig
Side stops- Keep stiffener leg from sliding along the bondline
Prep Panels
Clean bonding surfaces with acetone or isopropanol
Apply adhesive film or paste uniformly along the T-joint base
If you’re adding a filler (epoxy noodle or adhesive fillet), form it now
Place Skin
Lay the horizontal panel flat on the jig, or dowel pins so it can’t move.
Position the Stiffener
Use L-shaped tooling blocks or a machined back plate to hold the stiffener upright
Insert nylon or PTFE shims between the stiffener and the skin to maintain the filler gap
Ensure the stiffener leg is flush along the entire bondline
Clamp
Use clamping fingers or bars
Apply pressure evenly (not just at the ends).
Pressure should be just enough to close the adhesive line and extrude a small amount of adhesive (indicating good wetting), but not so high that it starves the joint.
Typical methods:
Spring clamps for small specimens.
Toggle clamps or screw-down plates for larger parts
Procedure:
Material and Adhesive Selection
Probably epoxy; make sure it is compatible with honeycomb
Surface Preparation
Clean surfaces: remove dust, oils, and contaminants
Roughen or abrade lightly to improve mechanical bonding
Ensure surfaces are dry, especially for moisture-sensitive adhesives
Maintain an optimal BLT (50–350 µm for epoxy)
Use internal spacers (glass beads, wires) or external jigs/shims to keep the adhesive layer uniform
Joint Geometry and Fillets
Add fillets or bonded angles
Increases peel strength
Reduces stress concentration
Assembly and Tooling
Maintain alignment
Hold adherends at the correct angle
Cure correctly