1. A method of detecting crack propagation in a wall of a metallurgical furnace having two or more layers, by a detection unit, the detection unit comprising a signal generating unit, at least one sensor unit, a processor, a memory, and an I/O interface, the method comprising: transmitting, by the signal generating unit of the detection unit, a stress signal into the wall, at one or more locations on the wall of the metallurgical furnace; receiving, by the processor of the detection unit through the at least one sensor unit, a reflected stress signal from each of the one or more locations based on the corresponding stress signal transmitted at each of the one or more locations; extracting, by the processor of the detection unit, one or more dominant frequency parameters (fa) from the corresponding reflected stress signal, from each of the one or more locations, each of the one or more dominant frequency parameters (fa) corresponding to a respective one of the two or more layers; analyzing, by the processor of the detection unit, a phase from each of the one or more dominant frequency parameters (fa) for a corresponding location of the one or more locations, wherein analyzing the phase comprises: determining, by the processor of the detection unit, one or more coefficients (Y) for each of the one or more dominant frequency parameters (fa) based on the reflected stress signal and a reference signal; identifying, by the processor of the detection unit, a dominant phase (O) for each of the one or more dominant frequency parameters (fa) based on the corresponding one or more coefficients (Y), selecting, by the processor of the detection unit, a frequency relevant to a thickness parameter (ft) of a layer of the two or more layers from the one or more dominant frequency parameters (fa) for the corresponding location on the wall, based on the dominant phase (O), and detecting, by the processor of the detection unit, the crack propagation in the wall of the metallurgical furnace based on analyzing the frequency relevant to the thickness parameter (ft) at each of the one or more locations; and responsive to detecting the crack propagation in the wall of the metallurgical furnace, providing, by the processor of the detection unit via the I/O interface, an alert indicating the crack propagation in the wall.