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HFIM, acronym for high-frequency-impulse-measurement, is a certain kind of measurement technique in acoustics, where structure-borne sound signals are detected and processed with certain emphasis on short-lived signals as they are indicative for crack formation in a solid body, mostly steel. The basic idea is to use mathematical signal processing methods such as Fourier analysis in combination with suitable computer hardware to allow for real-time measurements of acoustic signal amplitudes as well as their distribution in frequency space. The main benefit of this technique is the enhanced signal-to-noise ratio when it comes to the separation of acoustic emission from a certain source and other, unwanted contamination by any kinds of noise. The technique is therefore mostly applied in industrial production processes, e.g. cold forming or machining, where a 100 percent quality control is required or in condition monitoring for e.g. quantifying tool wear.
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Physical basics
High-frequency-impulse measurement is an algorithm for obtaining frequency information of any structure- or air-borne sound source on the basis of discrete signal transformations. This is mostly done using [Fourier series] to quantify the distribution of the energy content of a sound signal in frequency space. On the software side, the tool used for this is the fast Fourier transform (FFT) implementation of this mathematical transformation. This allows, in combination with specific hardware, to directly obtain frequency information so that this is accessible in-line, e.g. during a production process. Contrary to classical, off-line frequency analysis methods, the signal is not unfolded before transformation but is directly fed into the FFT computation. Single events, such as cracks, are hence depicted as extremely short-lived signals covering the entire frequency range (the Fourier transform of a single impulse is a signal covering the entire observed frequency space). Therefore, such single events are easily separable from other noises, even if they are much more energetic.
Applications
Because of its in-line capabilities, HFIM is mostly applied in industrial production processes when it comes to high quality standards e.g. for auto parts that are relevant for crash behavior of a car:
There are also several applications of HFIM devices in materials science laboratories where the exact timing of crack formation is relevant, for instance when determining the plasticity of a new kind of steel.