  Three-nucleon forces (3NFs), and in particular terms of the Fujita-Miyazawa
type, strongly influence the structure of neutron-rich exotic isotopes.
Ab-initio calculations have shown that chiral two- and three-nucleon
interactions correctly reproduce binding energy systematics and neutron
driplines of oxygen and nearby isotopes. Exploiting the novel self-consistent
Gorkov-Green's function approach, we present the first investigation of Ar, K,
Ca, Sc and Ti isotopic chains. Leading chiral 3N interactions are mandatory to
reproduce the trend of binding energies throughout these chains and to obtain a
good description of two-neutron separation energies. At the same time, nuclei
in this mass region are systematically overbound by about 40 MeV and the $N=20$
magic gap is significantly overestimated. We conclude that ab-initio many-body
calculations of mid-mass isotopic chains challenge modern theories of
elementary nuclear interactions.
