  We study a class of Landau-de Gennes energy functionals with a sextic bulk
energy density in a three-dimensional domain. We examine the asymptotic
behavior of uniformly bounded minimizers in two distinct scenarios: one where
their energy remains uniformly bounded, and another where it logarithmically
diverges as a function of the elastic constant. In the first case, we show that
up to a subsequence, the minimizers converge to a locally minimizing harmonic
map in both the $ H_{\loc}^1 $ and $ C_{\loc}^j, j\in \Z_+ $ norms within
compact subsets that are distant from the singularities of the limit. For the
second case, we establish the existence of a closed set denoted as $
\cS_{\op{line}} $. This set has finite length and consists of finite segments
of lines locally such that the energy of minimizers are locally uniformly
bounded away from it. This work solves an open question raised by Canevari
(ARMA, 223 (2017), 591-676), specifically concerning point and line defects in
the Landau-de Gennes model with sextic potentials.
