Influence of the Processes Parameters on the Properties of The ...

Chapter 3.
Analytical Methods and Designs <strong>of</strong> Experiments
Small influence <strong>of</strong> <strong>the</strong> surface quality: cost reduction, time reduction.
Small samples: more samples per volume; testing <strong>of</strong> fragments possible.
Large effectively loaded volume.
Low variation: higher precision <strong>of</strong> <strong>the</strong> single values, required number <strong>of</strong> samples is lower, higher
reproducibility <strong>of</strong> <strong>the</strong> measured values.
The relationship between applied forces and yield loads is given by:
σ t = - 2.F/.D.t (3.14)
where σ t = splitting (brazilian) tensile strength (MPa), F = load at (splitting) failure (N),
t = average specimen thickness (mm), and D = diameter (mm) [Yu et al., 2006].
Figure 3.18 shows <strong>the</strong> principle <strong>of</strong> Brazilian test (A) and <strong>the</strong> modelling by <strong>the</strong> finite elements
method (B). Figure 3.22 (C) is representative <strong>of</strong> stress concentrations and (D) <strong>of</strong> a typical fracture pattern.
Figure 3.18: (A): Principle, (B): Load geometry, (C): Simulation and (D): Cleavage <strong>of</strong> a Brazilian disk test.
[Rasch et al., 2005]
6.1.2 Compression <strong>of</strong> Porous Materials
In bone and tissue engineering applications, porous scaffolds depending upon applications must
have sufficient mechanical strength to restrain <strong>the</strong>ir initial structures after implantation in vivo. The ASTM
terminology for porous materials is classified into three groups: interconnecting pores (open pores), nonconnecting
pores (closed pores) or a combination <strong>of</strong> both, <strong>the</strong> scaffold falling in each group has specific
properties [Hutmacher et al., 2008]. When pores are open, <strong>the</strong> foam material is usually drawn into struts
forming <strong>the</strong> pore edges through open faces forming a low density solid. When <strong>the</strong> pores are closed, a
network <strong>of</strong> interconnected plates produces a high density solid. The closed pores are sealed <strong>of</strong>f from <strong>the</strong>
neighbouring pores. The interconnecting pores are critical parameter in designing a tissue engineering
scaffold. The interconnecting pores should be large enough to support cell migration and proliferation in <strong>the</strong>
initial stages [Hutmacher et al., 2008]. A large interconnection means a low density solid, and <strong>the</strong>refore low
mechanical structure.
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