  We consider the scattering of in-plane waves that interact with an edge of a
structured {penetrable (inertial)} line defect contained in a triangular
lattice, composed of periodically placed masses interconnected by massless
elastic rods. The steady state problem for time-harmonic excitation is
converted into a vector Wiener-Hopf equation using Fourier transform. The
matrix Wiener-Hopf kernel of this equation describes all dynamic phenomena
engaged in the scattering process, which includes instances where localised
interfacial waves can emerge along structured defect. This information is
exploited to identify the dependency of the existence of these waves on the
incident wave parameters and properties of the inertial defect. The symmetry in
the structure of scattering medium allows us to convert the vectorial problem
into a pair of scalar Wiener-Hopf equations posed along the lattice row
containing the defect. The solution embodies the exact representation of
scattered field, in terms of a contour integral in the complex plane, that
includes the contributions of evanescent and propagating waves. The solution
reveals that in the remote lattice, the reflected and transmitted components of
incident field are {accompanied by dynamic modes from three distinct symmetry
classes in addition to localised interfacial waves}. These classes correspond
to tensile modes acting transverse to the defected lattice row, shear modes
that act parallel to this row, and wave modes represented as a mixture of these
two responses. Benchmark finite element calculations are provided to validate
results against the obtained semi-analytical solution, which involve numerical
computations of the contour integrals. Graphical illustrations demonstrate
special dynamic responses encountered during the wave scattering process,
including dynamic anisotropy, negative reflection and negative refraction.
