A method for inspecting joined material interfaces, such as butt fusion pipe joints of plastic pipes, using ultrasonic waveforms to discriminate between acceptable and unacceptable interfaces. The waveforms (amplitude vs time) having been transmitted to a material interface of interest are divided into a number of time zones and the amplitudes of the waveforms within the time zones compared against waveform amplitudes obtained from known acceptable joined material interfaces to determine the quality of the material interface of interest.