  This paper is concerned with inverse crack scattering problems for
time-harmonic acoustic waves. We prove that a piecewise linear crack with the
sound-soft boundary condition in two dimensions can be uniquely determined by
the far-field data corresponding to a single incident plane wave or point
source. We propose two non-iterative methods for imaging the location and shape
of a crack. The first one is a contrast sampling method, while the second one
is a variant of the classical factorization method but only with one incoming
wave. Newton's iteration method is then employed for getting a more precise
reconstruction result. Numerical examples are presented to show the
effectiveness of the proposed hybrid method.
