  Efficient optimization of topology and raster angle has shown unprecedented
enhancements in the mechanical properties of 3D printed materials. Topology
optimization helps reduce the waste of raw material in the fabrication of 3D
printed parts, thus decreasing production costs associated with manufacturing
lighter structures. Fiber orientation plays an important role in increasing the
stiffness of a structure. This paper develops and tests a new method for
handling stress constraints in topology and fiber orientation optimization of
3D printed orthotropic structures. The stress constraints are coupled with an
objective function that maximizes stiffness. This is accomplished by using the
modified solid isotropic material with penalization method with the method of
moving asymptotes as the mathematical optimizer. Each element has a fictitious
density and an angle as the main design variables. To reduce the number of
stress constraints and thus the computational cost, a new clustering strategy
is employed in which the highest stresses in the principal material coordinates
are grouped separately into two clusters using an adjusted $P$-norm. A detailed
description of the formulation and sensitivity analysis is discussed. While we
present an analysis of 2D structures in the numerical examples section, the
method can also be used for 3D structures, as the formulation is generic. Our
results show that this method can produce efficient structures suitable for 3D
printing while thresholding the stresses.
