M. Morbidini

A study of the vibro-acoustic modulation technique for the detection of cracks in metals

One implementation of the vibro-modulation technique involves monitoring the amplitude modulation of an ultrasonic vibration field transmitted through a cracked specimen undergoing an additional low frequency structural vibration. If the specimen is undamaged and appropriately supported, the two vibration fields do not interact. This phenomenon could be used as the basis for a nondestructive testing technique. In this paper, the sensitivity of the technique is investigated systematically on a set of mild steel beams with cracks of different sizes and shapes. A damage index was measured for each crack. The correlation obtained between the crack size and the strength of the modulation is fairly poor. The technique proved extremely sensitive to the initial state of opening and closing of the crack and to the setup due to the modulating effects of contacts between the specimens and the supports. A simple model is proposed which explains the main features observed and approximately predicts the level of sideband obtained experimentally.

Detection of impact damage in CFRP composites by thermosonics

This study investigated methods of reducing the electrical power required to detect defects by thermosonics, or vibro-thermography, and thus to reduce surface damage that can occur at the exciter attachment point during testing. The surface temperature rise over excited defects has been modelled to determine the heating requirements to visualise damage. A long pulse, low power excitation method has been found to produce satisfactory impact damage images whilst eliminating damage at the exciter attachment point. Thermosonics has been found to detect impact damage in composites more reliably than optically stimulated thermal NDE methods.

A calibration procedure for sonic infrared nondestructive evaluation

Sonic infrared is potentially a very attractive nondestructive evaluation technique offering the possibility of rapid testing of complex components. However, at present it is difficult to be sure that sufficient excitation has been applied so that a null (no defect present) result can be trusted. This paper presents a calibration method to improve the reliability of the technique. The method uses a measurement of the vibration of the component during the test, the vibration signal being processed to give a “heating index” which is a measure of the ability of the vibration field to generate heat at any defects of interest that are present. The calculation of the heating index and the rationale for its formulation are described. The method is then applied on two sets of beamlike specimens with cracks of different sizes. The maximum temperature rise in successive tests on a given specimen is shown to correlate well with the maximum heating index, so validating the method. The threshold heating index required...

The detectability of cracks using sonic IR

This paper proposes a methodology to study the detectability of fatigue cracks in metals using sonic IR (also known as thermosonics). The method relies on the validation of simple finite-element thermal models of the cracks and specimens in which the thermal loads have been defined by means of a priori measurement of the additional damping introduced in the specimens by each crack. This estimate of crack damping is used in conjunction with a local measurement of the vibration strain during ultrasonic excitation to retrieve the power released at the crack; these functions are then input to the thermal model of the specimens to find the resulting temperature rises (sonic IR signals). The method was validated on mild steel beams with two-dimensional cracks obtained in the low-cycle fatigue regime as well as nickel-based superalloy beams with three-dimensional “thumbnail” cracks generated in the high-cycle fatigue regime. The equivalent 40kHz strain necessary to obtain a desired temperature rise was calculate...

Vibro‐Acoustic Modulation NDE Technique. Part 2: Experimental Study

The crack detection capability of the Vibro‐Acoustic Modulation NDE technique is experimentally investigated over a broad range of ultrasonic frequencies. The generation of low frequency modes of the samples is performed using harmonic excitation applied by a shaker or by tapping. For both excitation methods the effect of different strain levels on the sideband activity is analyzed. The implementation of practical and reliable supports for testing the specimens is also considered in this study, as well as examples of results on industrial components.

A new approach for the prediction of the thermosonic signal from vibration records

In this study a set of cracked steel beams was tested using the thermosonic method of NDT. Each beam was instrumented with one strain gage for the measurement of the vibrations excited. The heat released at the crack was predicted from the measured vibration and an experimental estimate of the additional damping introduced in the specimens by each crack. Hence the local surface temperature rise could be predicted as a function of time during the excitation and compared with the infrared camera measurements.

Advances in Thermosonics for Detecting Impact Damage in CFRP Composites

This study investigated methods of reducing the electrical power required to detect defects by thermonoics, or vibro‐thermography, and thus to reduce surface damage that can occur at the exciter attachment point during testing. The surface temperature rise over excited defects has been modelled to determine the heating requirements to visualise damage. A long pulse, low power excitation method has been found to produce satisfactory impact damage images whilst eliminating damage at the exciter attachment point.

Vibro-acoustic modulation NDE technique. Part 1: Theoretical study

Vibro‐modulation involves monitoring the amplitude modulation of an ultrasonic vibration field transmitted through a cracked specimen undergoing an additional low frequency structural vibration. If the specimen is undamaged the two vibration fields do not interact. This technique is a potential non‐destructive testing method. A model is presented which accounts for this phenomenon quantitatively. Using receptance analysis and an Finite Element model, it is shown that the effect is strongly dependent on the carrier ultrasonic frequency and the system damping.

The relationship between crack damping, vibration amplitude and temperature rise in thermosonics NDT

In this study a set of cracked steel beams was tested using the thermosonic method of NDT. Each beam was instrumented with a strain gage for the measurement of the vibrations excited. The heat released at the crack was calculated from the vibration records and an experimental estimate of the crack damping. Thus, the local surface temperature rise could be predicted and compared with the infrared camera measurements. The relationship between crack size and level of vibration required for reliable crack detection was discussed.

Comparative Sensitivity Study of Vibro-Acoustic Modulation and Damping as NDT Techniques

The sensitivities of the conventional damping test and a particular implementation of the emerging vibro‐acoustic modulation (VAM) NDT technique have been compared on a set of mild steel beams cracked in the laboratory. The comparison showed very similar performances on the specimens used. Both techniques work best for lightly damped specimens and in setups such that the influence of the support can be minimized, but they also show a poor sizing performance. A preliminary explanation for this latter point is given.