Fiber metal laminates Flashcards
What are Fiber Metal Laminates (FML)?
Fiber Metal Laminates (FML) are hybrid materials made by thin Aluminium sheets and composite layers. The development of FML is due to the need to improve fatigue behaviour of the aerospace structures.
How do FML accomplish the improvement of the fatigue behavior?
1) Bridge effect
The presence of UD fibers orthogonal to the direction of propagation transfers the stresses of damaged metallic layer through the crack and the material can continue to work even in presence of a crack.
2) Crack stopper effect
Due to the presence of composite fibers inside the laminate. The presence of UD fibers orthogonal to the direction of propagation leads to a reduction of the rate of crack propagation.
What is the historic development of FML?
1) 1978 - ARAL
Bonded AI sheets and aramidic fibers into adhesive layers
Problem: Different CTE between layers leads to internal compressive stress of reinforced fibers and breakage due to instability
2) 1987 - GLARE
Bonded aI sheets and glass fibers into adhesive layers. Still used today and has excellent adhesion between fibers and adhesive
3) HULDS (hardened unit-loading devices)
Protection against explosions.
What is the goal of GLARE3?
The goal is to cope with the load increasing on fuselage panels due to high diameter and internal pressure. Other applications of this material are wing skins, stiffeners, ribs, frames, floors, fireproof barriers, cargo barriers..
What is the identification of GLARE?
Example: GLARE 3-2/1 -0.3
- 3 is the “kind of orientation” of UD pre-pregs with respect to the rolling directions of Aluminium sheets
- 2/1 is the number of Aluminium sheets and the number of glass fibers sub-laminates respectively
- 0.3 is the Aluminium sheet thickness (mm)
What are the properties of GLARE 3?
The specific strength and stiffness of GLARE3 is between the ones of the constituent materials. (E/ρ is around 20.60 and σr/ρ is around 0.29)
The properties are:
- Excellent toughness properties (fatigue, damage tolerance, low crack propagation rate)
- Good resistance to corrision, fire exposure (composite materials are flame resistant) and environmental effects
- Specific weight -10% wrt Al
- High formability wrt Al
- Resistance to high velocity impacts
- Easy detection of impact damages (due to the external metal laminate. It is hard to detect impact damages on composite materials)
- Large panels production (splicing concept)
What is the problem of FML and what is the solution?
The problem is the low adhesion between Al sheets and composites
The solution is:
1) Sandblasting
2) Chromic acid anodizing
3) Primer
In order to promote both the mechanical and chemical adhesion
How are the FML materials made?
By autoclave forming technology. We consider the hybrid laminate like a composite laminate and use directly the technologies for the composite (hand-lay-up technique, vacuum bag, deposition of Al sheet and autoclave).
The manufacturing of curved panels by means of Self Forming Technique are a mold. Due to the small thickness, Al stays in position due to the composite/
What cutting techniques are used for FML.
Mostly composite techniques are used.
1) Water-jet
2) Guillotine shear machine
3) Diamond saw
Describe the static tensile test curve.
After the elastic loading, there is a yielding of the Al layers during the loading. After the yielding point, high tangential modulus is showed. At a certain point, the breakage of the fibers occurs with a drop of stress. The stress comes back to Al yielding levels after fibers breakage. Starting from this point we can continue to apply the load and to elongate the material until the final breakage (delamination) at the ultimate loads.
Is there a difference between the yielding point of Al and GLARE3?
The yielding point of the GLARE3 is higher compared to that of Al. The causes are
- Due to the difference in CTE between Al and composite layers which lead to residual stresses (this makes the yielding point lower)
- Ageing of Al sheets during curing
- Presence of bi-axial tensile state of stress due to fibers in both two directions
What do we consider for damage characterization?
For the damage characterization, we consider these:
1) Elastic coefficient of release energy
2) Extension of plastic damaged area
3) Maximum permanent indentation
What is the response of Al to impact?
In a metal panel, the indentation due to an impact has a certain depth and extension. This effect is very visible. The material absorbs energy by yielding (elastic deformations and plastic localized deformations). The area of permanent deformation increases with the energy level. When a certain level of energy is passes, the material fails and it isnt able to absorb more energy -> The impact passes through the plate.
What is the response of composite materials to impact?
In a composite layer, the impact effect isnt clearly visible (in case of a low energy impact) from outside, but inside delamination may occur depending on the direction of the fibers. The delamination increases with the energy level. The energy is absorbed with the breakage of the fibers, of the matrix, creation of cracks inside the fibers and matrix (inelastic dissipations, like interlaminar and intralaminar fractures.) The delamination and fibers inside the single layer can act as a preferable way for the energy to be transmitted and the single layer is intact. Energy is distributed along very large surfaces without leaving the time to produce the mechanicsm of damage locally. The limitation of the amount of energy which can be absorbed.
What is the response of GLARE3 to impact?
There is a collaboration between the constituents. Energy transported by the composite layer arrives to the Al layer which can be yielded. This increases the ability of absorbing energy of the impact thanks to a larger yielding area in the Al sheets. Moreover, the yielding reduces the velocity of propagation of the energy inside the composite layer.
