  This work presents numerical techniques to enforce continuity constraints on
multi-patch surfaces for three distinct problem classes. The first involves
structural analysis of thin shells that are described by general Kirchhoff-Love
kinematics. Their governing equation is a vector-valued, fourth-order,
nonlinear, partial differential equation (PDE) that requires at least
$C^1$-continuity within a displacement-based finite element formulation. The
second class are surface phase separations modeled by a phase field. Their
governing equation is the Cahn-Hilliard equation - a scalar, fourth-order,
nonlinear PDE - that can be coupled to the thin shell PDE. The third class are
brittle fracture processes modeled by a phase field approach. In this work,
these are described by a scalar, fourth-order, nonlinear PDE that is similar to
the Cahn-Hilliard equation and is also coupled to the thin shell PDE. Using a
direct finite element discretization, the two phase field equations also
require at least a $C^1$-continuous formulation. Isogeometric surface
discretizations - often composed of multiple patches - thus require constraints
that enforce the $C^1$-continuity of displacement and phase field. For this,
two numerical strategies are presented: For this, two numerical strategies are
presented: A Lagrange multiplier formulation and a penalty method. The
curvilinear shell model including the geometrical constraints is taken from
Duong et al. (2017) and it is extended to model the coupled phase field
problems on thin shells of Zimmermann et al. (2019) and Paul et al. (2020) on
multi-patches. Their accuracy and convergence are illustrated by several
numerical examples considering deforming shells, phase separations on evolving
surfaces, and dynamic brittle fracture of thin shells.
