## Effect of interleaving on the impact of a UD carbon-epoxy
*1995* #vem
[online ref](https://doi.org/10.1016/0010-4361(95)91385-i)
[[Effect_of_interleaving_on_the_impact_of_a_UD_composite.pdf|local ref]]

Study of a drop-weight [[Impact-Shock-Collision|impact]] response. The [[Viscoelasticity|VEM]] is poly(ethylene-co-acrylic acid) (PEAA) : high-strain low-modulus.
improvement of [[PCLD]] damping follow Ross-Kerwin-Ungar (RKU) theory for sandwich of isotropic layers, and Ni-Adams (NA) theory for fibrous composite.
for symmetric angle-ply laminate : RKU/NA + something else (attributed to additional shear deformation of the sandwiched layer induced by bending the outer layers).
Interleaving changed the [[Composite Failure Modes|failure modes]] from compressive fracture of base laminate to tensile fracture.

Optimal position needed (too much can lead to adverse compressive properties, and weigh penalties (because lower modulus)). Suggested of locations of premature failure or where there are stress concentrations (where discontinuities).

This paper showed a great increase in energy absorption of the drop-weight [[Impact-Shock-Collision|impact]].

Key point : suppression of propagation of the compressive crack + differences in stress redistribution.


## Experimental optimization of impact absorption of carbon-epoxy laminate
*2008* #perforated
[online ref](https://www.sciencedirect.com/science/article/abs/pii/S0266353808001681)
[[Experimental_optimization_of_impact_absorption_of_carbon-epoxy_laminate.pdf|local ref]]

Aims to optimize the impact energy absorption of carbon-epoxy laminate.
Interleaved with poly(ethylene-terephtalate) (PET), with d=1 mm circular holes at several densities.

*ductility index (**for Charpy impact test**): ratio between the propagation and the initiation energies*

5 basic failure modes in composite (see [[Composite Failure Modes]]) :
(i) fibre fracture (for [aramids](https://www.ipitaka.com/blogs/news/what-are-aramid-fibers), defibrilation)
(ii) resin crazing, micro-cracking and gross fracture
(iii) debonding between fibre and matrix
(iv) fiber pull out from the matrix
(v) delamination of adjacent plies

Elastic modulus practically independent of the level of interlaminar adhesion, while bending strength decreased as the interlaminar [[Fracture Toughness|fracture toughness]] decreased.

While initiation energy increases, the propagation energy decreases with the interlaminar [[Fracture Toughness|fracture toughness]].
->The total fracture energy passes through a maximum for a given interlaminar [[Fracture Toughness|fracture toughness]] (total impact energy increases is 1.8 higher than non interleaved)

## Optimization method of composite laminates with a viscoelastic layer
*2009* #perforated #vem
[[Optimization_of_composite_laminate_with_viscoelastic_layer.pdf|local ref]]

This paper develops a method to transform a multi objective (damping and stiffness)
into a single one, to facilitate optimization.

The [[Viscoelasticity|viscoelastic]] layer is **perforated**, and the sandwich is [[Co-curing|co-cured]], so the resin flows through the VE layer and couples the structure. 
More stiffness, less damping => need optimizing (where holes, which size).

Co-curing means the viscoelastic material within the composite laminate undergo the temperature and pressure cycle needed to cure the composite material.

To make the single-objective, an evaluation function is constructed. This function is a liner weigh sum of two sub objective functions (loss factor and bending stiffness).

The variable in this function is the ratio of the perforation area to the total area. 

The variation of stiffness and damping (here they chose the loss factor) depend a lot on the ratio's value and not just the change (ie 95%->100% see more change than 50%->55%).
So a power function was chosen to approximate the objective function. (confidence of 0.95)

The calculation of the weigh are very interesting (but more complex than I can synthesize properly for now)

Result : < 5% of perforation