Sonic Infrared Research Articles

Physics-based thermal noise effect reduction in sonic IR crack length estimation

ABSTRACT Using heat diffusion models in sonic infrared (IR) inspection technology that captures arbitrary frictional heat generation function along a crack lends itself to estimation of crack length using thermal images alone. This study introduces novel physics-based approach that relies on the governing heat diffusion model linearity and superposition of point heat sources along crack to reduce thermal noise effect in crack length estimation. It utilises thermal images and their timestamps in sonic IR inspection to deliver spatiotemporal average representation of the aforementioned point heat sources to reduce thermal noise effect in crack length estimation. The proposed methodology is tested against two-dimensional and three-dimensional finite element simulations with an arbitrary frictional heat generation function along crack. Virtual random thermal noise levels ranging between 50 and 200 mK, which are higher than IR cameras Noise Equivalent Temperature Difference values, are added to simulated results to reflect realistic and drastic thermal noise levels. Spatiotemporal averaging using combined sets of different thermal images captured at various times in single sonic IR inspection delivered about 16% total uncertainty in the estimation of a crack length at 95% confidence level based on 60 experiments. Similar uncertainty level is obtained using independently reported sonic IR experimental data.

The effect of defect size on the quantitative estimation of defect depth using sonic infrared imaging.

Sonic infrared (SIR) imaging is a hybrid nondestructive evaluation (NDE) method that uses ultrasonic excitation along with thermal imaging to detect defects in materials and structures. SIR NDE uses an ultrasonic pulse in the 15-40 kHz range from a transducer to produce localized heating at the defect while a thermal camera will record surface temperature during the inspection. In a previous article, we presented a model that describes heat diffusion from subsurface defects in a composite material. The model uses certain aspects of the temperature-time curve for defect depth profiling, namely, half-maximum power time, the peak slope time, and the second derivative peak time. In this study, we investigate the effect of defect size on the quantitative estimation of defect depth. The theoretical results are calculated and compared with the experimental data. We demonstrate that the experimental data have good correlation with the theoretical calculation. The peak slope time and the second derivative peak time are less sensitive to changes in defect size than the half-maximum power time in the defect depth assessment.

A simple heat diffusion model to avoid singularity in estimating a crack length using sonic infrared inspection technology

Sonic infrared (IR) is a rapidly developing nondestructive evaluation technique for detecting and estimating the length of fatigue cracks in aircraft components. In this study, a spatial and temporal forward analytical solution of a simple 2D heat diffusion model is developed assuming uniform frictional heat generation along the crack. This model not only captures the temperature rise while applying the excitation load, but also captures the temperature decay after the excitation load stops. The simple forward analytical solution is validated in close comparison with 2D and 3D finite element modeling (FEM) results. With a validated forward model, an inversion algorithm is proposed to estimate the crack length independent of heat source values. The use of predetermined heat source function of uniform heat generation along the crack avoids singularity in estimating a crack length while feeding temperature change along but also around the crack to the inversion algorithm. The proposed algorithm showed high accuracy in predicting a crack length using thermal images generated from 2D and 3D FEM data with and without randomly added virtual temperature uncertainties. Moreover, the temperature-based inversion model as a method of estimating a crack length is tested against experimental sonic IR data to illustrate the potential capabilities of advancing this model.

Sonic IR crack size estimation using 2D heat diffusion model with arbitrary heat source function along the crack

Over the past decade, sonic infrared (IR) has been gaining rapid popularity in fatigue crack detection. Accuracy and robustness of crack size estimation using existing temperature-based image processing techniques are rather limited due to the absence of analytical solutions describing the heat diffusion around the crack. In this study, a 2D heat diffusion model with an arbitrary heat source function along a crack line is developed to make the technique more practical and capture the random nature of heat generation along the crack. The validity of the forward 2D heat diffusion model is established in close comparison with 2D finite element (FE) results. The validated analytical solution is used as a basis for best-fit analysis to retrieve the arbitrary heat source functions along the crack, which are used to estimate crack sizes. Using the 2D FE simulations with and without random temperature uncertainties, the proposed method delivers high accuracy in estimating crack sizes. Moreover, the accuracy in estimating crack sizes is validated using experimental sonic IR data. This makes it a practical approach to unlock the potential capabilities and limitations of sonic IR inspection technology.

Effect of driving frequency on non-linear coupling between ultrasound transducer and target under inspection in Sonic Infrared Imaging

Sonic Infrared (IR) Imaging, also referred as vibrothermography, is a novel Nondestructive Evaluation (NDE) technology to find cracks through infrared imaging of vibration-induced crack heating. The vibration source plays an important role in the detection of cracks. In this paper, the effect of driving frequency on the ultrasound vibration to the thermal imaging is presented. The research is organized by using different frequency system and coupling materials on the same aluminum bar sample. The analysis is conducted by combination of the vibration waveforms with the IR images and signals. Correlation analysis between the acoustic energy and the thermal energy in the crack is discussed as well.

Developing signal processing method for recognizing defects in metal samples based on heat diffusion properties in sonic infrared image sequences

In sonic infrared (SonicIR) imaging, heat is generated in defect areas during the sonic pulse; the heat appears bright in SonicIR images as the indication of a defect. However, in practical applications of SonicIR, there are lots of disturbing bright areas in infrared images, such as heat reflection and paint problem. When crack size is small, the generated heat appears not bright enough to be recognizable. Based on heat diffusion properties in the one-dimensional temporal and two-dimensional spatial domain, a method is developed to automatically recognize defect signals from SonicIR image sequences. The algorithm is verified with the SonicIR image sequences of 100 metal plates which may have different thickness, materials, or crack sizes.

Study of the effect of crack closure in Sonic Infrared Imaging

Sonic infrared (SIR) imaging is a novel hybrid technology that employs ultrasonic excitation and infrared imaging to detect defects in materials. There are several parameters can affect the detectability of a defect in Sonic IR. One is the closure status of a defect, say a crack in a material. We have studied the relationship between the heating in a crack and crack closure. The crack closure was realized by applying a variety of clamping forces on the crack. In this paper, we provide quantitative results of the study on infrared signal levels at different status of crack closure. The temperature profile of a fixed point at the crack during the excitation period was presented. The thermal energy transformed from the kinetic movement was analysed over a fixed region for all clamping cases. Meanwhile, the crack energy at two fixed points crossing the crack was also studied.

Finite element modeling of the heating of cracks during sonic infrared imaging

We describe a finite element model that calculates the energy dissipation to be expected in a crack during sonic infrared (SIR) imaging. These calculations confirm the experimental result that the presence of acoustic chaos enhances the heating and thereby enhances the probability of crack detection using SIR. Although thermoplastic contributions are allowed in the calculation, under the assumed conditions of sonic excitation, which mimic those of typical SIR experiments, the heating appears to be dominated by frictional effects.

Response of sub-surface fatigue damage under sonic load – a computational study

Graphite/epoxy composite samples with a center hole were subjected to fatigue loading, and were tested for damage using the sonic infrared (sonic IR) imaging technique. The technique revealed the presence of semi-elliptical delaminations near the circular notch. In the sonic IR technique, a short pulse of sound infuses into the material and results in frictional heating at the delamination surfaces. The heating at the damage locations reaches the surface of the sample by conduction, and is captured by an IR camera. In the present analysis, the finite element technique was employed to model a notched sample with two semi-elliptical delaminations. The delaminations were modeled using a surface-to-surface contact algorithm, and the frictional energy developed during the infusion of sound pulse was graphed. A separate finite element model was developed for transient heat transfer analysis by utilizing the calculated frictional energy as a heat source. Variation in surface temperature within the delamination regions was plotted with time, and was compared with the temperature–time plot obtained from the sonic IR technique.

Acoustic chaos and sonic infrared imaging

We describe a quasichaotic mechanism for the generation of complex vibrations from the application of a 450 ms, 40 kHz excitation coupled nonlinearly to a sample under inspection by means of sonic infrared (SIR) imaging. Monitoring the sample vibration by means of a laser vibrometer, we find that the vibration switches from the fundamental and harmonics to several series of frequencies that are rational fractions of the fundamental driving frequency. The transition to this quasichaotic behavior is important in obtaining high-quality SIR images.