Basic Materials Properties for Materials Selection - Medical Device Design: Innovation from Concept to Market - page 338

APPENDIX E

Basic Materials Properties for
Materials Selection

E.1 Density
Density is a measure of the amount of matter (mass M ) compressed into an enclosed volume
( V ). Hence, more matter compressed into a space means high density, less matter means low
density. It is commonly assumed that the density of a material is homogeneous (the same
throughout the whole volume). Its symbol is, commonly, the Greek character rho, ρ ; its units
are normally in kg/m 3 , though equivalent variants do exist.

For a specific body

mass
volume
M
V

density =

(E.1)

kg
m

ρ =

3

Table E.1: Common Material Densities

Density (kg/m 3 ) Lb/in 3 Material

Steels 7500-8080 0.271-0.292

Aluminum alloy c3500 c0.126

Titanium 4500 0.163

Nylon 900-1120 0.0325-0.0405

PEEK 250-300 0.00903-0.0108

E.2 Stress and Strain
Designers are concerned about how materials fail. Consider a rod under tension by a force F
( Figure E.1 ).

It is assumed that the force is evenly distributed across the whole area. This distribution is a ratio
of the force ( F ) and the rod's cross-sectional area ( A ). This ratio is called stress and is given by the
Greek character sigma, σ . Its units are commonly given as N/m 2 or Pa (both are the same).