Fiber metal laminates Flashcards
(7 cards)
What are fiber metal laminates?
These materials are designed to improve fatigue behavior and response. After the comet disaster fatigue was seen as a critical point, requiring material innovation. To do so, two effects are implemented; the bridge effect and the crack stopper effect.
What is are the technological problems behind GLARE?
Low adhesion between Al sheets and composite sheets. As such, we perform surface treatments on the Al sheets. These include sandblasting (peaning would be better in terms of surface but it is too aggressive on 0.3mm thickness sheets), chromic acid anodizing (alternatively, sulfuric or phosphoric acid see use), and priming with EC-3924B by 3M. Autoclave forming technology is also frequently utilized. Autoclave forming involves laying up the different constituent layers of the laminate (metal and fiber) and then curing them under elevated pressure and temperature inside an autoclave. This process ensures proper bonding and consolidation of the laminate, mitigating risk of defects. The part is created around a mold, generating either a flat or curved geometry.
In the autoclave forming of GLARE 3, temperature is gradually increased and decreased to carefully manage the epoxy resin’s curing behavior, ensuring proper resin flow, minimizing thermal stresses from mismatched expansion between aluminum and glass fibers, and preventing voids or delamination. This controlled thermal profile allows volatiles to escape and avoids premature resin gelation. In contrast, pressure is typically applied more suddenly, but may be intentionally delayed until the resin reaches a higher temperature - often around 130 °C - to prevent excessive resin squeeze-out, allow better air evacuation, and align with the resin’s gel point for optimal consolidation without compromising the fiber-matrix interface.
Cutting is then performed to give shape to the final part. Three main alternatives are used; water jet, guillotine shear machine, and diamond saw. Of these, only the diamond saw causes no delamination, but it can generate significant thermal loads and inclusion of Al burr due to overheating of glass layers. Guillotine is instead used for high operative speeds and low costs, even if it results in delamination and fiber fraying; and water-jet is used to avoid thermal loads whilst having complex shapes and high operative speeds, but delamination and costs are problems. Drilling has similar issues, so the helix angle and feed rate must be carefully considered. Drill choice, feed rate and rotation speed are crucial. Upper delamination is generally caused by drill helix angle issues, whereas lower delamination or permanent deformation may be more likely to be caused by feed rate issues.
How does GLARE 3 fail?
See figure in doc.
Considering typical mechanical properties, we can see that the yield modulus of GLARE 3 has a substantial increase over aluminum on its own (0,0042 to 0,0048 increase, almost 15%). This difference is due to three things: the different CTE between aluminum and composite, generating a residual stress, decreasing the yield modulus slightly; ageing of Al sheets during curing, increasing the yield modulus slightly; and presence of bi-axial tensile states of stress due to fibers in two different directions, greatly increasing the yield modulus.
What are the important parameters to characterize for impact environmental effects?
There are three main things to characterize: elastic coefficient of release of energy, extension of plastic damaged area, and maximum permanent indentation.
The elastic coefficient of release of energy tends to drop with projectile velocity for aluminum, whereas it increases with composites. Particularly, in aluminum we get elastic deformations and plastic localized deformations, whereas composites have inelastic dissipation. Glare has a mix between the two.
Maximum permanent deformation and damaged area see a similar behaviour. Permanent deformation is lower (as part of the energy is spent to damage the composite) but plastic zone is larger, as the fibers of composite layers carry stress waves farther than aluminum.
What is the SPLICE junction technique?
Splicing is based on SFT (Self Forming Technique): the joint is done during manufacturing of laminates. In general, it takes disrupted Al layers but continuous longer fiber layers. This allows for production of panels larger than the ones made of Al and reduces production time and costs. This ensures the desired weight reduction and mechanical properties, but can cause some issues at the discontinuities in the Al layers, and where the matrix (composite layers) is exposed to external agents.
Internal doublers are extra reinforcing layers embedded within the laminate structure at or near a joint or repair zone. In GLARE or other FMLs, an internal doubler consists of additional layers of aluminum and/or prepreg glass fiber that are co-cured or secondarily bonded to the laminate. They serve to reinforce areas of high stress, such as near fasteners or cut-outs, to compensate for material removed (e.g., due to damage or during scarf repair), and to maintain laminate continuity without creating an abrupt change in stiffness.
An overlap refers to the region where two laminate sections physically overlap each other, often with tapered or stepped ends to avoid a sudden thickness increase. The aluminum and fiber layers are individually stepped or scarfed so that they gradually transition into the overlapping laminate. The overlap zone is typically bonded and sometimes mechanically fastened. Overlaps are carefully designed to avoid stress concentration and to ensure load transfer through both metal and fiber paths. They function to allow for smooth structural load transition between two panels and to maintain FML fatigue resistance by preventing abrupt geometrical discontinuities.
A double lap joint is a mechanical or bonded joint configuration where a central laminate panel is sandwiched between two overlapping panels on either side. Each side of the joint has a lap, hence “double lap.” These joints are more robust than single-lap joints, as they reduce peel and bending stresses. The outer layers may be full GLARE laminates or custom doublers. The joint allows for symmetrical load transfer, minimizing eccentricity and reducing fatigue-critical stress concentrations. They are used where high joint efficiency is required (e.g., fuselage skin joints), and may be used in repair zones, especially when symmetry and strength restoration are priorities.
It is worth noting that all of these techniques serve to compensate for some of the performance loss that comes from using SPLICE over regular GLARE3.
These joint techniques also all have their own unique failure modes. Double lap, overlap and doubler, respectively:
In double lap joints the breaking is usually sudden and cohesive, at the interface between laminates. For overlap, crack nucleation occurs from the extremity of the superposition and is transferred to the fibers, increasing delamination and propagating the crack.
See figures in doc.
How are GLARE 3 MFLs repaired?
Cut out the damaged zone, size a glare 3 repair doubler in dependence of damage geometry, bond it. This reduces time for repairing with respect to a standard classical procedure, reduces weight (no rivets and smaller patches), and has higher fatigue tolerance.
See figures in doc.
