Inserts
We will do 2 tests with each insert, one in plane tension, 1 out of plane
Current insert design will be a copy of UMNSVP’s grommet design, for a 3/8th inch bolt
Goal is to reach a force of around 1700 lbf for each test but something above 1500 is most likely acceptable as each tab will be bolting into 2 points
In plane tension
Sources
CMH 3
Potting
Purpose: reinforcing sections of core for hardpoints/fasteners
Loads
Light loads
foaming adhesive
high density core
foam in core cells: slows intrusion of water, stabilizes walls for machining, better thermal insulator
higher density core is stronger and stiffer than foam in core cells, also heavier
Heavy loads
synthetic foam: balance of light weight and improved properties
epoxy + chopped fibers
solid laminates
metal inserts
Compressive loads transmitted from face sheet to potted area - attaching member needs to extend past potted area
also prevents insert from slipping under face sheet when loaded
prevents movement from bending moments on bolt
avoid chamfer on outer flange of insert
reduced load transfer, insert can slip under face sheet
this is bad
Configurations
Potting Radius Calculations
Perforated core
b_pmin = 0.93192 * b_i + 0.874 * S_c - 0.66151
b_i = insert radius
S_c = size of core cell
non perforated core
b_pmin = 0.9 * b_i + 0.7 * S_c
more conservative estimate
b_r = b_i + 0.35 * S_c
Process
Aluminum Inner Piece
Matching the thickness of the aluminum insert to the core thickness so it sits flush with the inner surfaces of the composite plies. Slightly under-sizing the diameter by 0.1–0.2 mm can help ensure a tight epoxy bond without inducing stress.
It is not recommended that fasteners be installed using an interference fit in
composite structure because slight delamination occurs as the fastener is pressed in place.
Epoxy Bonding
Epoxy choice: Use a high-strength structural epoxy that is compatible with both aluminum and carbon fiber. Some epoxies also have fillers to improve gap-filling, which is useful if your waterjet cut isn’t perfectly smooth.
Curing: Ensure proper clamping or a fixture to hold the aluminum piece centered while curing, so you don’t get uneven bonding.
Load transfer: The epoxy will transfer shear loads from the bolt through the aluminum piece and into the surrounding composite.
Bonding works better when the mounting surfaces are not glossy- abraid surface before attaching
Gap filling: choose a filled epoxy/potting compound or maybe a filler (microballoons, glass spheres) to avoid voids if the cut/pocket is irregular
Prob do not need
Bonded Washers
Use washers made of aluminum or titanium to avoid galvanic corrosion with the inner aluminum insert
Unsure if this actually matters or not for us
In most cases the adhesive is applied to the surface under the fastener flange
For adjustable inserts, the potting compound is injected through the flange into the void in the panel after the part has been adjusted to the panel thickness
Check link for specific dimensions made by Shur-Lok
Disadvantages of Partial Inserts: source
Extra resin needed to make the part
Quality control of the joint is hard since the bonding strength is determined mainly by cell size of the honeycomb
Fabrication is tougher: extra jigs and steps are needed for leveling and placing the partial insert in the right position
Prep
Sand aluminium and carbon fiber surfaces lightly and clean with alcohol to improve epoxy adhesion.
Insert Geometry and Positioning
Ensure the aluminum insert extends slightly beyond the honeycomb core to provide a solid anchoring point for the bonded washers.
Position the bonded washers as close as possible to the composite skin to maximize load distribution and minimize stress concentrations.
Avoid vibrations/ heavy hammering; structural damage can be induced by heavy shock or vibrations
SDMF:
Basically there are two types of fastener inserts for honeycomb application: the molded-in type and the mechanical type. The molded-in type should always be used if possible because it offers several distinct advantages. Among these is the ability to bond the insert, core and face skins into one rigid unit with the selected potting medium. Another advantage is that molded-in inserts are not particularly sensitive to manufacturing variations within the sandwich structure. Also, bonded parts necessitate a relatively short learning curve for inexperienced installers.
May only work with pre- manufactured inserts
Mechanical inserts may work better for our uses?
Ideally, all loads entering the sandwich panels are carried first through skins. The core member acts to stabilize and transmit transverse stress across the panel.
Maintain proper load distribution
Want a relatively low load per fastener by using an adequate number of the proper type and size fasteners
Making the hole/filling
Use the proper drill such as special spade or diamond abrasive impregnated core drills. It may be necessary to back up the entry and exit area of the panel to prevent splintering and delamination.
Unsure how waterjets would fit in with this method
Watch out for fibers absorbing liquid when becoming exposed
Place holes outside primary load paths. Put vents in areas that see low stresses (near neutral axis, non-structural tabs, or sacrificial regions)
Keep holes as small and few as possible. Minimize total cutout area.
May want to reinforce vents
Chamfer/finish hole edges and seal. Smooth edges (no frayed fibers)
Keep holes away from ply drops and cut edges
Test specimens with identical holes under representative loads (tension, fatigue, bending) before committing
Run Ansys with them in
Keep hole diameter small relative to flange width
Roughness on the sides of the honeycomb from the process of making the hole should be beneficial for the insert overall
Being able to calculate the amount of space that is created by breaking the honeycomb cells will help determine the correct amount of epoxy to use
Epoxy weight for a 3/16 inch diameter insert in a ½ to ¾ inch sandwich will range from 5 to 20 grams
Test for adhesive strength, viscosity, and tendency to shrink
Try to include a vent or overflow so air isn’t trapped
The potting epoxy viscosity is low enough to flow into the void and wet the core walls
Potting dimensions are important for load capacity
Double cell walls= stronger
Full potting:
The maximum possible potting height is identical to the core height, c. This is fundamentally the case if the insert height is in the range between core height c
As long as the core height is less than or equal to the minimum necessary potting height, the potting is considered as ‘full potting’ where the potting height is equal to core height
Strength of the insert:
The most important parameters related to strength are
Insert overall diameter di
Insert overall height hi
Tension loading
In tension loading of a fastener, the bonding of core to face skin plays a major role
Try to minimize unsupported middle areas
More calculations needed for molded- in inserts (if using)
Supposedly the most important test: Source
Shear loading
Pay attention to face skin thickness and bearing area
Want complete distribution of the potting medium
May need to undercut surrounding cells in order to provide a sufficiently large bearing area for stability
Edge Distance
Min of ‘2 diameters’ away from edge
Test if you want a closer distance to the edge
Make sure to run multiple trials
Core strength
The core properties are more important for inserts subjected to normally‐acting tensile or compression loads
Shear modulus
Shear strength
Tensile/ compressive loads: the load‐carrying capability is determined by the ability of the core to take the axially‐induced load transmitted from the insert via the potting compound
Tensile strength, perpendicular to the sandwich plan
Tensile rupture
Compressive strength, perpendicular to the sandwich plane
Math for Aluminum:
Data sources for core:
an effective core shear modulus, Gc = Gw/3
Gw shear modulus in W‐direction
Core to face sheet bond: usually not a problem (pg 95)
Failure modes:
Tests:
While this setup was used to test a Gyroid core with a threaded partial insert, they do a great job of explaining their overall process and show some results with a honeycomb core along with their failures.
Possible failure: The picture below shows the failed specimen of the insert after the pull-out static test. The first interfacial delamination occurred at the adhesive bonding layer between the bottom surface of the insert and the composite face. After the delamination, the composite face and the sandwich structure themselves sustained the applied pull-out load until the local fiber breakage of the upper composite face occurred.
Insert pull-out tests shows always that a core shear failure occurs first before the potted cells fail under tensile rupture.
Under shear-out loading, the potted cells together with the upper skin fail in shear with the insert position within the potted area having a significant influence on the results.
Adhesion failures between the insert and the potting, between the core and the face sheet or tear-out failures (potting rupture around the insert) are rare in aerospace applications.
Testing plan:
Maximum Torque values should be controlled – especially with large fastener sizes. This reference (NASA/TM-2006-241323, 2006) shows that fasteners of ¼ diameter and below can be installed using the recommended torque/clamp up without damaging the laminate
Method:
Create a clean hole through the face sheets and core that matches your insert outer geometry (slightly undersized if you’re bonding)
This does not mean forcing it in; it means that after cleaning and abrading, the epoxy fills that small clearance and forms a uniform adhesive layer around the insert
Remove core material around the hole to create a cavity for potting — this is usually done by digging out a small region of core (about 2× the insert diameter)
Abrasively prep the surfaces of the aluminum insert and the face sheets — rough, matte surfaces promote epoxy adhesion.
Clean with isopropyl alcohol or acetone to remove any grease or dust
Tape off the bottom face sheet if you don’t want resin leaking out.
Place the insert in position (through the top face sheet) using a fixture or clamp to hold it perfectly centered while the epoxy cures
Make sure there’s a small clearance gap (0.1–0.2 mm) for adhesive to flow between insert and face sheets.
Convention:
Upper flange is pierced by two holes, one for the injection of the potting resin and one for venting purposes
Cell opening: honeycomb cells let epoxy flow into cell walls. To get a reliable bond either:
Remove several cell layers and create a defined potting pocket (recommended)
Pre-wet cell walls with low-viscosity resin before potting
Injecting Potting Compound:
Drill a small fill hole through the face sheet or insert flange
Insert a syringe or small nozzle and inject the potting compound into the cavity — keep going until it starts to flow out a vent hole if made
Use a syringe/dispensing gun
This ensures full coverage and eliminates voids
Wipe away excess and seal any vent holes after curing
For honeycomb, go slowly so resin wicks into cells instead of trapping air
Follow epoxy manufacturer’s cure schedule
Maintain the fixture or clamp so the insert stays centered during cure
After curing, remove masking and clean off squeeze-out
Check for voids or bubbles
tapping test
Verify the insert sits flush with both inner surfaces
Optionally apply a thin adhesive fillet around the insert flange for secondary sealing
Why bonded washers help:
Spreads bearing from bolt head to face sheet, lowering localized compression on face/core.
Provides a sealing flange for potting/epoxy injection and reduces exposed metal to moisture.
Makes torque distribution more predictable (washer gives consistent contact area)
Bond washer: either
Apply adhesive under washer and seat washer on outer face while insert is held (preferred to get a single bonded system)
After potting and cure, bond washer to outside with the same epoxy. If bonding after potting, ensure potting and face are fully cured and abraded/cleaned before washer adhesive.
Aluminum insert & aluminum washer: degrease before including; lightly abrase (scotch-brite or 180–240 grit) and solvent clean with chromate/adhesion promoter as specified by epoxy vendor
Use a thin sealant bead around the washer if additional moisture protection is desired
Common Mistakes
Not venting trapped air
Letting the potting overflow onto bonding surfaces meant for other attachments
Using potting that’s too viscous to flow into the core cells
Formula (cylindrical annulus pocket):
Volume (mm³) = π × (Rp² − Ri²) × h
Rp = pocket radius (mm), Ri = insert radius (mm), h = pocket height (mm).
Convert to grams: multiply volume in mm³ by 0.001 → cm³, then × epoxy density (g/cm³). Use density ≈ 1.2 g/cm³ as a working number.
Example (pocket OD = 2× insert OD = 9.525 mm, so pocket radius R_p = 4.7625 mm; insert radius R_i = 2.38125 mm; h = 12.7 mm (½″ core)):
Volume ≈ π × (4.7625² − 2.38125²) × 12.7 ≈ 678.7 mm³ = 0.6787 cm³
Mass ≈ 0.6787 × 1.2 ≈ 0.81 g of epoxy.