In the current contribution, we present a recent experimental development that allows one to carry-out coupled electrical-mechanical investigations of polymer composite materials upon mechanical stretching. Our approach enables one to characterize the evolution of the electrical properties of the samples in the elastic and in the plastic regime of mechanical deformation. Two orthogonal directions are accessible for the electrical investigations: parallel and perpendicular to the direction of the mechanical stretching. For the perpendicular direction, a non-conventional approach to measure conductivity values without employing a direct contact between the electrodes and the samples under investigation has been developed and implemented. The method was based on establishing the mathematical relation between the value of the conductivity and the frequency position of the Maxwell-Wagner-Sillars interfacial polarization peak observed in dielectric spectra of composite materials. For composite materials based on polyethylene filled with carbon black, a pronounced anisotropy in the evolution of conductivity upon mechanical stretching is observed: the conductivity varies by 5-6 orders of magnitude along the parallel direction and only 1-2 decades along the perpendicular direction. This behavior is attributed to a morphological anisotropy induced by stretching.