We use 3D printing to manufacture lattices with uniform and graded relative density, made
from a composite parent material comprising a nylon matrix reinforced by short carbon fibres.
The elastic-plastic compressive response of these solids is measured up to their densification
regime. Data from experiments on the lattices with uniform relative density is used to deduce
the dependence of their elastic-plastic homogenised constitutive response on their relative
density, in the range 0.2-0.8. This data is used to calibrate Finite Element (FE) simulations of
the compressive response of Functionally Graded Lattices (FGLs), which are found in good
agreement with the corresponding measurements, capturing the salient features of the measured
stress versus strain responses. This exercise is repeated for two lattice topologies (body-centred
cubic and Schwarz-P). The phenomenological constitutive models produced in this study can
be used in topology optimisation to maximise the performance of 3D printed FGLs components
in terms of stiffness, strength or energy absorption.