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How the Acoustic Resonance Inspection Process Works

In acoustic resonance inspection the resonances of a part are measured to determine the structural characteristics of that part in a single test. The whole part is examined for both external and internal structural flaws.

The resonances of a part are determined by the mass, structural stiffness, and structural damping of the part. Flaws that are structurally significant, whether arising from material issues (hardness, grain structure, inclusions, residual stress) or from process related issues (cracks, voids, missing material, dimensional issues), will affect one or more of mass, stiffness or damping and this will cause detectable shifts in the resonances of the part.

Once measured, the resonances of the subject part are compared to those from a control set of normal parts and the subject part is determined to be normal (OK) or not normal (NOK).

2. Measure the response – a specialized microphone measures the ‘ringing’ of the part structure in response to the vibration

4. Assess – the SmartTest™ Advanced Resonance Software compares the waveform to agreed acceptance limits to determine if the tested part is normal (OK) or not normal (NOK)

Why ARI Works

For centuries, objects were tested by hitting them with a mallet and listening for a tonal quality difference. The natural resonance of an object gave evidence that the struck object was free from imperfections. Striking of a rail car wheel with a hammer, and listening for the responses, has been used for over 100 years to detect the existence of large cracks.

The SmartTest™ Acoustic Resonance Inspection (ARI) system works with the same principle using a much more sophisticated instrument for making a completely controlled and reproducible test process. Now, Acoustic Resonance Inspection (ARI) based techniques are becoming more commonly and successfully used as an applied nondestructive testing tool, in the manufacture of steel, cast, forged, ceramic, sintered metal powder and other part types.

In a single measurement, ARI-based techniques can test for numerous types of defects including cracks, chips, cold shuts, inclusions, voids, oxides, contaminants, missed processes or operations, as well as variations in dimension, hardness, porosity, nodularity, density, and heat treatment.

Acoustics

Acoustics is the branch of physics that deals with the study of all mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. The study of acoustics revolves around the generation, propagation and reception of mechanical waves and vibrations.

The steps shown in the diagram can be found in any acoustical event or process.  There are many kinds of causes, both natural and artificially induced. There are many kinds of transduction processes that convert energy from some other form into sonic energy, producing a sound wave. The five basic steps are found equally well whether we are talking about an earthquake, a submarine using sonar to locate its foe, or a band playing in a rock concert.

The central stage in the acoustical process is wave propagation. This falls within the domain of physical acoustics. In fluids, sound propagates primarily as a pressure wave. In solids, mechanical waves can take many forms including longitudinal waves, transverse waves and surface waves.

Wave Propagation

All material media undergo a deformation when they are stressed by an external action. This deformation is caused by a certain number of the particles within the medium being displaced from their position of equilibrium.

In elastic media, those particles which are directly disturbed start to vibrate around their position of equilibrium. The vibrations are then transmitted to the particles in the adjacent layers which also start to oscillate. In this way the initial stress, which directly involves only a portion of the medium, propagates throughout the medium.

This atomic movement is constrained by the elastic properties of the carrier, but even the slightest movement of the atoms is passed along through the densely packed molecular structure in the form of mechanical energy.

Modal Analysis

All structures, even structures such as metal stampings, castings, or similar parts that are apparently rigid to the human eye; undergo deformation. They vibrate in special shapes called mode shapes when excited at their resonant frequencies. By understanding the mode shapes, all the possible types of vibration can be predicted.

Modal analysis is defined as the study of the dynamic characteristics of a mechanical structure or system by measuring and predicting the mode shapes and frequencies of a structure. Under normal operating conditions, the structure will vibrate in a complex combination of all the mode shapes. 

The tuning fork shown is a very simple structure that could be represented by very few points. An impact hammer is used to excite the structure at the arrow and the resonant frequency responses are recorded at positions along the legs, using a signal analyzer. The resonant frequencies are the peaks that appear at every point at the same frequency. The amplitude of the peak at each location describes the mode shape for the associated resonant frequency.

Time and Frequency Analysis

Structural vibration can be converted into electrical signals. By analyzing the electrical signals, the nature of the vibration can be understood. Signal analysis is generally divided into time and frequency domains; each domain provides a different view and insight into the nature of the vibration.

Time domain analysis starts by analyzing the signal as a function of time.

The plot of vibration versus time provides information that helps characterize the behavior of the structure. Its behavior can be characterized by measuring:

• The maximum vibration (or peak) level, or
• Finding the period (time between zero crossings), or
• Estimating the decay rate (the amount of time for the envelope to decay to near zero).

Frequency analysis also provides valuable information about structural vibration. Any ‘time history’ signal, for example the microphone signal in an AR inspection system, can be transformed into the frequency domain, giving us the full structural response of the subject part in the frequency domain.

Summary

• All structures undergo a deformation when they are stressed by an external action, for example the impact used to excite the part in an AR inspection system
• The initial stress from the impact, which directly involves only a portion of the structure (the part), propagates throughout the structure according to the very predictable laws of wave propagation
• Modal Analysis is used to determine the resonant frequencies of the structure, i.e. the structural response
• The structural response of a given part to a given excitation is both unique to that part and very repeatable. The same part, struck in the same way, will yield the same structural response
• Flaws that are structurally significant, whether arising from material issues (hardness, grain structure, inclusions, residual stress) or from process related issues (cracks, voids, missing material, dimensional issues), will affect sound wave propagation through the part structure
• The altered wave propagation in the flawed part will lead to shifts in one or more of the resonant frequencies, making it possible to detect the presence of the flaw