Study on probability of detection (POD) determination using lock-in thermography for nondestructive inspection (NDI) of CFRP composite materials

doi:10.1016/j.infrared.2015.06.007

Infrared Physics & Technology

Volume 71, July 2015, Pages 448-456

https://doi.org/10.1016/j.infrared.2015.06.007 Get rights and content

Highlights

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A series of CFRP specimens with artificial FBH are inspected using LIT.
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The defect is determined by PDT that is over two times of noise level.
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The POD curves are determined by phase response data and hit/miss data processing.
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The POD analysis is used for estimation of the detection capability and reliability of LIT.

Abstract

In this work, an extension laser beam was used as heating source to evaluate the composite material by active thermographic method. In particular, a series of carbon-fiber-reinforced polymer (CFRP) specimens with various depth and sizes of flat-bottom holes (FBH) were inspected using lock-in thermography (LIT). The quantitative LIT phase was employed to identification the subsurface defect using the phase decision threshold (PDT, a phase deference between defective area and the sound one), and then the detectable and undetectable defects were expressed as 1 and 0 (hit/miss data), respectively. The POD curves of continuous phase response data and the hit/miss data processing were used for estimation the detection capability and reliability of LIT phase image for NDI&E of CFRP composites, and the effects of PDT value and modulation frequency on POD curves were compared.

Introduction

Active infrared thermography has been successfully applied as a nondestructive testing and evaluation (NDT&E) technique in many industrial applications, especially, as composite materials are increasingly used in many fields (i.e. aerospace, aircraft manufacture), active infrared thermography plays a significant role for NDT&E of composite materials (i.e. carbon-fiber-reinforced-polymer, CFRP) [1], [2], [3]. Until now, pulsed thermography (PT) and modulated thermography (i.e. lock-in thermography) have been deeply studied as a particular NDT method to detect subsurface defects of composite materials (i.e. delaminations, impact damage, and cracks, etc.) at a given defect depth (or depth range) [4], [5], [6]. In pulsed thermography, the test material is stimulated by a short duration excited energy pulse (likely, flash lamp, ultrasonic vibration and eddy-current, etc.), and the surface thermal images were collected by an infrared camera. Thermal signals were then processed by advanced processing techniques (i.e. Thermal signal reconstruction, TWR, wavelet Transform, WT, Differential absolute contrast, DAC and Principal component thermography PCT, etc.) [7]. The surface temperature gradients on the specimen help to localize the subsurface defect in material. However, the surface temperature gradients depend not only on subsurface defects, but also on local variations of emissivity as well as non-uniform heating [8]. Pulsed phase thermography, PPT, combines simultaneously the advantages of pulsed thermography and lock-in thermography without sharing their traditional drawbacks [9]. Lock-in thermography (LIT) has been used successfully for NDT&E, and it uses periodic harmonic heat excitation to detect composite materials with subsurface defects by the amplitude and phase image of LIT [10], [11], [12], [13]. Furthermore, LIT phase image has the advantage of being less sensitive to the local variations of illumination or surface emissivity [14], [15].

For a quantitative NDT&E technique or method, the detection reliability of an NDT&E technique is one of the most important aspects for application out of the laboratory. Probability of detection (POD) has been successfully utilized as an accepted quantitative measurement to evaluate the detection capability and reliability of a NDT&E technique. The POD is strongly connected to the topic risk assessment and probabilistic analysis in the evaluation of the performance of components, and it provides the probability for the detection of certain flaw size. The POD delivers the realistic, statistical assessment of the reliability for an NDT&E method, and the knowledge of the POD of a certain defect allows assessing the consequences of this flaw in a probabilistic manner.

Generally, there are two ways to deal with the data, either as a continuous signal response Var, or as a discrete hit/miss response [7]. For the continuous signal response, the POD is obtained from the correlation of variable Var that is related to the defect characterization (i.e. size, area, and aspect ratio). For the discrete response, the data are organized so that the defect is either detectable (hit = 1) or not (miss = 0). In this study, a series of CFRP composite specimens with simulated subsurface defects (flat-bottom holes) were inspected using lock-in thermography. With the objective of POD determination, the focus was put on the lock-in thermographic phase imaging (LITPI) inspection. The continuous phase contrast and the quantitative hit/miss data were obtained by phase decision threshold (PDT) criterion, and then used to calculate the probability of detection (POD) for estimation of the detection capability and reliability of LITPI technique for NDI&E of CFRP composite material. Moreover, the POD determinations as function of aspect ratio r (size/depth) related to PDT value and modulation frequency were performed.

Section snippets

Specimens and experimental setup

In the present work, the CFRP composite samples were manufactured, and the woven carbon fiber reinforcements were 2 dimensional. The CFRP composite density is about 1550 kg/m³, the specific heat is about 1000 J/kg °C, and the thermal conductivities k_x, k_z are about 3.65 W/(m °C) and 0.56 W/(m °C), respectively. Experimental inspections were carried out on a series of CFRP panels with the total of 108 flat-bottom holes (FBHs) with diameter of 2.0 mm ⩽ D ⩽ 13.0 mm at different depth of 0.45 mm ⩽ H ⩽ 2.5 mm, and the

Defect determination

The phase contrast of defect was first obtained, usually used for determination of defect from LITPI with application on NDT &E, and the inspection results were organized in correlation with the phase contrast |ΔPh| vs. aspect ratio r for purpose of implementation of POD analysis.

Fig. 6 shows the phase contrast |ΔPh| as function of modulated frequency for different aspect ratio r (diameter/depth). As can be seen that the phase contrast depended on the aspect ratio of defect, and thereby, a data

Conclusions

A series of CFRP specimens with artificial flat-bottom holes (FBHs) were tested using a laser induced lock-in thermographic phase imaging inspection. Phase decision threshold (PDT) values criterion between defective regions and healthy ones were implanted to determine the defect. The POD analysis of both the continuous phase response data and hit/miss response data were performed based on these results. For the hit/miss data process for POD curve, the POD curves of the log-odds and log-normal

Conflict of interest

The authors declare that there is no conflict of interest.

Acknowledgement

This work was supported by the Chinese National Natural Science Foundation under Contract No. 51173034 and Self-planned Task of State Key Laboratory of Robotics and System (HIT).

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View full text

Study on probability of detection (POD) determination using lock-in thermography for nondestructive inspection (NDI) of CFRP composite materials

Highlights

Abstract

Introduction

Section snippets

Specimens and experimental setup

Defect determination

Conclusions

Conflict of interest

Acknowledgement

NDT&E Int.

Prog. Aerosp. Sci.

Infrared Phys. Technol.

NDT&E Int.

Eng. Fail. Anal.

Compos. A

NDT&E Int.

Infrared Phys. Technol.

NDT&E Int.

Research on the quantitative analysis of subsurface defects for non-destructive testing by lock-in thermography

NDT&E Int.