3D Printing FFF: Nozzle Diameter, Layer Height effects on part strength

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Hi ! I found a previous publication of the paper I cited in https://steemit.com/printing3d/@boucaron/3d-printing-fff-pla-temperature-print-speed-cooling-and-scheduling-effects-on-part-strength.
It covers the effects on the nozzle diameter and the layer height on the part strength, it is a complement to the previous publication.

Paper: Strength of PLA Components Fabricated with Fused Deposition Technology Using a Desktop 3D Printer as a Function of Geometrical Parameters of the Process
Authors: Vladimir Kuznetsov , Alexey Solonin, Oleg Urzhumtcev , Azamat Tavitov , Richard Schilling

Link: http://www.mdpi.com/2073-4360/10/3/313
Link: https://www.preprints.org/manuscript/201803.0036/v1

In a nutshell, given a fixed print speed and fixed temperature, to obtain strong parts :

Smaller layer height improves the strength (up to a point)
Bigger nozzle diameter improves the strength, it enables a wider range for the choice of layer heights while still having a good strength, it allows to produce faster. It is a drawback for very detailed parts or organic structures (the part orientation can help in some cases).

Why ?
The layer height has a direct influence on the geometry of the voids between layers. There is a ratio between the layer height and the nozzle diameter (50% of the nozzle diameter) where above a given threshold those layers do not weld enough to each other and it slightly reduces the surface of contact between the layers, generating cracks that weaken slightly the parts.

Like the previous cited paper, there are nice photos of the mesostructure where you understand quickly what is happening.

The part under test is the following, it contains only a shell (no top/bottom and no infill):

The mesostructure is really interesting, you can see that when there are small layers the crack propagates through the voids, while when there are thicker layers the crack propagates on the layer.

The following graph shows the strength according to the layer height and the nozzle diameter:

This graph shows a normalized layer height to nozzle diameter, basically if your layer height goes above 50% of the diameter you are going on the weak side.