Defect Site Detection in CFRP Composite Materials Using Interferometric X-Ray

Phasecontrast Imaging

Florian BAYER*, Gisela ANTON*, Jürgen DURST*, Wilhelm HAAS**, Thilo MICHEL*, Georg PELZER*, Jens RIEGER*, André RITTER*, Thomas WEBER*

*Erlangen Centre for Astroparticle Physics (ECAP), Universität Erlangen-Nürnberg, Erwin-Rommel-Straße 1, 91058 Erlangen

**Pattern Recognition Lab, Lehrstuhl für Informatik 5, Universität Erlangen-Nürnberg, Martensstraße 3, 91058 Erlangen

Abstract. In recent years X-ray phase-contrast imaging became applicable in the hard X-ray regime through the use of a grating-based Talbot-Lau interferometer and was demonstrated to be a promising technique to gain contrast in different fields in medical imaging as well as in non-destructive testing. In addition to conventional X-ray absorption imaging, through the utilisation of the material’s phase shift influencing the wave front, interferometric phase-contrast and dark-field imaging is capable to yield completely new material contrasts and is able to provide structural information about a specimen at a scale much smaller than the imaging system-based resolution. By defining a contrast measure, the visibility V, a signal intensity loss can be recorded, whose origin is determined in local scattering when an X-ray beam propagates through matter. This contrast information is exploited in the dark-field imaging modality. For the calculation of the dark-field signal, the loss in intensity modulation, represented by the contrast ratio V/V0, is exploited. It depicts a map of local scattering power of the sample, decoupled from any absorption signal, and turned out to offer enormous potential for structure examinations.

Therefore, future X-ray material testing applications could benefit substantially by an effective implementation of this phase information and profit from a detailed look onto interferometer setup based effects on the phase signal.

In this contribution we show first results on interferometric X-ray dark-field imaging, with a focus on the potential utilisation on CFRP samples. Measurements were carried out with a laboratory setup using a Talbot-Lau interferometer, showing a strong dependency of the dark-field signal on the fibre regularity in the examined composite material. This effect can be exploited e.g. for CFRP crack detection or the discovery of imperfections in fibre matrix composition, which represent reasons for a reduced mechanical stability of composite materials, e.g. used as construction components in aerospace.

Introduction

Although X-ray phase-contrast imaging is well introduced in different fields of research for several decades, its practicability in the hard X-ray regime for use in non-destructive testing or medical imaging became increasingly into the focus only since few years. One investigated promising approach is the grating-based Talbot-Lau interferometer [1, 2, 3], which can resolve both the phase and the absorption of an investigated sample.

By defining a contrast measure, the visibility V, a signal intensity loss can be recorded, whose origin is determined in local scattering when an X-ray propagates through

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matter. This contrast information is exploited in the so-called dark-field imaging. It has an enormous potential for structure examinations at a scale much less than imaging system-based resolution [4], its noise behaviour is well understood [5].

In this contribution we present first measurements regarding the future utilisation of this new imaging contrast modality for the investigation of regularly ordered structures, such as Carbon fibre reinforced polymers (CFRP).

Methods

1.1 Talbot-Lau interferometer

The experimental set-up used for this work is based on Talbot-Lau interferometry, adapted from the proposal made by Weitkamp et al. [6]. It is based on an assembly of three gratings, whereas the first grating G0 is a source grating ensuring the coherence requirements of the arrangement formed by the phase grating G1 and an analyser grating G2. The whole interferometer set-up is sketched in figure 1.

Figure 1: Sketch of the Talbot-Lau grating interferometer used for dark-field imaging with a conventional X-ray tube source. Behind a source grating (G0), ensuring spatial coherence, the wave front propagates through an object that causes a phase shift on the impinging wave front. A phase grating G1 generates a well-known periodic intensity pattern at the Talbot-distance dn, which can be sampled by stepping an absorption grating G2 at that distance. A phenomenon which is fundamental for exploiting phase information in a grating interferometer is the Talbot effect, that was first reported by H. F. Talbot in 1836. Its principle is, that periodic structures when coherent illuminated lead to repetitive intensity patterns in certain distances downstream the wave propagation. For a phase grating with period p1, periodically impressing a phase shift of π onto an impinging plane wave front, those intensity patterns show in particular a periodical intensity distribution with a periodicity of p1/2 with locations of maximal interference, the so called Lohmann images [7], in the fractional Talbot-distances dn, which are given by

dn 2n1 p21

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where λ is the wavelength and n a positive integer denoting the Talbot-order.

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1.2 Differential phase-contrast imaging

Any object placed right in front of the phase grating G1 shifts the interference distribution laterally in the detection plane, which is chosen to be in one of the above mentioned Talbot-distances. The distortion of a phase-object is given by a local angular deviation α of the pattern. For X-rays, with wavelengths λ ≤ 10−10 m, this angle is typically in the order of a few microradians. The principle of differential phase-contrast imaging is to detect these deviations by placing an absorption grating with the periodicity of the interference fringes p2 as an analyser right in front of the detection plane. Knowing the angular deviation α, the gradient of the total phase shift Φ through the object can be calculated by

x

2

where x denotes the axis perpendicular to the grating bars.

1.3 Interferometric dark-field imaging

The intensity pattern imprinted by the phase grating G1 in an ideal case generates a rectangular modulation on the originally plane wave front in the level of the absorption grating G2. The period of G2 accords to the period of the Talbot intensity pattern. This is why G2 acts as an absorption mask for these patterns. Therefore they can get stepped with G2 in steps smaller than the grating period. As a result, the intensity values at different positions of the grating G2 within one period of the modulation pattern imprinted by G1 can be recorded by the pixelated detector attached close behind G2 in a stepping method, which in the real case of not absolutely perfect gratings leads to a cosine-like function. Figure 2 points out this procedure. By measuring both the intensity modulation with and, in a reference image, without object, object influences in absorption and lateral phase shift can be obtained. As a quantitative description, the visibility parameter V is defined as the contrast

V I max Imin

I max Imin

The visibility has dependencies on the setup geometry and the absorption efficiency of grating G2 as well as on the beam coherence. The dark-field signal is determined by the ratio V/V0 of the visibilities V with object and V0 without object and depicts a map of the local scattering power of the sample, decoupled from any absorption signal.

Figure 2: Principle of the visibility retrieval: Through stepping of the analyser grating G2 in steps smaller than the grating periodicity, the modulation of the incident wave front can be determined. A reference stepping without object depicts the periodic Talbot pattern imprinted by the phase grating G1 (red curve). Stepping with object leads to a modulation carrying information about the sample (blue curve), which can be ascertained through the drop in modulation amplitude and phase shift. Latter information is essential for dark-field imaging.

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1.4 Experimental set-up

The Talbot-Lau interferometer was installed with gratings manufactured by the Karlsruhe Institute of Technology (KIT) using the so called LIGA procedure [8]. The phase grating G1 was designed as a π−shifting nickel grating for a photon energy of 25 keV. Therefore the height of the phase grating is h1 = 8.7 μm. The gratings G0 and G2 are absorption gratings with gold bars with a height of 150 μm for G0 and 110 μm for G2, respectively. The periods are 23.95 μm for G0 and 2.4 μm for G2. As X-ray source a commercial available rotating anode X-ray tube was used. The interferometer was set up in the third Talbot-order, which results, considering the image magnification, in a Talbot-distance of 15.87 cm between phase grating G1 and analyser grating G2, and a distance of 161.19 cm between source grating G0 and G1. Directly behind G2, the intensity modulation is detected using a Varian flatpanel detector. Figure 3 shows an overview of the laboratory setup.

Figure 3: Photograph of the laboratory setup. In the backmost end the X-ray tube can be seen, whereas in the front end the flatpanel detector is apparent. In between the setup of gratings and object stage is installed.

Results

The Talbot-Lau interferometer was used to ascertain the dark-field signal of different samples of Carbon fibre reinforced polymers (CFRP), especially Carbon fibre reinforced carbon (CFC) with fibre diameters of 6 - 8 micron. First measurements show a clear dark-field signal, which differs in its information content from the well-known absorption image and provides an additional contrast signal.

As the reduction of the object visibility, and therefore the dark-field signal, is strongly directional, the adjustment of the objects’ fibre orientation to the orientation of the grating bars is a prominent observable in this modality, which can be exploited for structure examinations in non-destructive testing, whereas conventional absorption radiography cannot provide such information. Figure 4 shows a comparison of the absorption and dark-field image of a CFC-sample: In the lower part, a defect site is apparent, constituted by a differing fibre orientation. This site could result in reduced material solidity and therefore stiffness properties of a component part.

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Figure 4: Comparison of the conventional absorption (upper image) with the new dark-field image (lower image) of a Carbon fibre reinforced Carbon (CFC) sample. In the lower part of the object (marked with arrows), an area with differing fibre orientation is clearly apparent in the dark-field imaging modality. The red line denotes the pixel row which the lineplot (Figure 5) shows.

Figure 5: Lineplot comparing absorption (blue curve) and inverted dark-field (1-DF) signal (red curve) of a horizontal line in the upper area of the CFC sample (see Figure 4). As it is clearly visible, the dark-field provides higher signal contrast, depending on fibre layer orientation in the plane. Furthermore, those irregularities in orientated composite materials can be caused by crack damages. Because of the high sensitivity of the dark-field signal on the fibre orientation, already slight divergences, due to small cracks induced for example by dull shocks onto the workpiece, can be detected by this contrast modality.

Conclusion

Grating-based phase-contrast imaging opens a window to further contrast generation in non-destructive testing. Additional to absorption, it simultaneously provides differential phase-contrast and dark-field information. Especially the dark-field, calculated out of the contrast ratio V/V0, provides data of specimen structures below pixel resolution, which are of a special interest in material structure examinations focussing fibre orientation and porosity tasks for construction materials in aerospace.

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References

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