Defect detection in fiberglass reinforced epoxy composite pipes reproducing field inspection conditions

Abstract

Many works that adopt active thermography as a non-destructive inspection method use very sophisticated equipment for test piece thermal excitation and data acquisition, which demand highly prepared operators, causing their application in industrial environments very costly. This paper aims at detecting defects in adhesive joints of fiberglass reinforced epoxi pipes by means of the Fourier transform phase angle analysis of the time evolution pixel amplitude, using simple and low-cost equipment. Results show that is possible to detect defects in adhesive joints, studying the phase angle behavior, even in regions, where the thermograms show no defects. IntroductionInfrared thermography is a well known technique that provides the capability to see the unseen. As a nondestructive method of inspection, it can reveal surface, subsurface and internal structural defects over a wide variety of materials. The technique is based on the reflectiveness of thermal waves by the surface of the materials [1,2].The thermographic technique is showing a good performance on the inspection of composite materials, including adhesive layers [3,4] and has been used by numerous sectors of the industry reaching exciting levels of detection and characterization of a large variety of materials and devices, justifying the great number of studies that have been made recently [4].The active thermography is the kind of infrared inspection that uses the heat of external sources as excitation that can be analysed on permanent or transient state of heat flow. The lockin thermography uses the information of a unique frequency to analyse the amplitude and phase angles relating these data to location and size of the defects. The pulsed thermography analyses the decay on time of a pulse heat excitation without selecting a specific frequency [1,2].The Pulsed Phase Thermography is an active method that allows the analysis of a wide band of frequencies of heat excitation giving information about the magnitude and phase angles of the temperature decay of the objects inspectioned. The behavior of the phase angle can indicate the presence of defects even inside the material´s structure [1,2], not detectable by conventional termography. Pulsed Phase ThermographyPulsed phase thermography allows the analysis of a series of frequencies by means of a single excitation, and it can be performed in a fast way [2]. However, the equipment used in all active techniques still represents some difficulty for a fast and reliable application in the industry environment, because of the high level of qualification necessary for the operators to set and use the equipment, as well as the high costs involved.The information that can be extracted from the pulsed thermography pass through math algorithms to be analyzed on a frequency basis less affected by the noise of the thermal excitation allowing the observation of phase angles over a wide spectrum of frequencies and can be used to compose the phase images to visualize defects non-detectable on the original thermogram...

Non-destructive testing of jute–polypropylene composite using frequency-modulated thermal wave imaging

“…There are two approaches that could be successfully exploited for this purpose, namely PT and LT. [17][18][19] Beside these two established techniques, an indigenously developed method, known as FMTWI 20 may also be employed. In FMTWI, the material surface heating is neither a pulse nor a single frequency, but a linear upchirp whose bandwidth spans over the frequencies necessary to produce thermal diffusion lengths to cover the entire range of defect depths in question.…”

Pulsed Phase Thermography analysis on cylindrical geometry

“…Despite this fact, many issues arise when this technique is used in specific applications, especially about the analysis and interpretation of thermographic sequence, or its Fourier transform. In particular, industrial pipelines are an important field of application of NDT techniques, and various specific studies on this subject can be found in the open literature [2][3][4].…”

“…Despite this fact, many issues arise when this technique is used in specific applications, especially about the analysis and interpretation of thermographic sequence, or its Fourier transform. In particular, industrial pipelines are an important field of application of NDT techniques, and various specific studies on this subject can be found in the open literature [2][3][4].In this paper, some preliminary experiments have been carried out to evaluate how a variation of specimen orientation affects the test, taking into account directional emissivity, out-of-focus effects and uneven thermal excitation; then this knowledge is applied in a test of a hollow emicylindrical (half pipe) polymethylmethacrylate sample, analyzed with the depth retrieval procedure developed in our laboratory. …”