Development of a Versatile Mechanical Testing Device for In Situ Synchrotron Tomography and Diffraction Experiments

It is increasingly the case that tomography instruments are able to accommodate very large samples or in situ sample environments [11]. In some cases, hollow rotation and translation stages even allow equipment or samples to be installed inside the instrument [12]. However, this is not universally the case. Often, the available vertical space between sample stage and the beam is the most important constraint. For example, there is a maximum of 139 mm below the white beam at the TOMCAT beamline, SLS, Switzerland [13], and 144 mm available on the high-resolution tomography station at the ID19 beamline, ESRF France. Furthermore, the total height of the machine is also a source of many space constraints especially in some configurations: on a multi-circle goniometer or when it is necessary to bring an imaging detector very close to the sample to minimize propagation phase contrast. In order to maximize the adaptability of our machine, it was decided to minimize the distance between the sample and the base of the machine, and to develop a compact design where all the mechanical elements of the machine would be located in the space between the beam plane and the mounting surface on the top plate of the beamline sample stage. As the machine is made of thick steel sections, its total mass is 5 kg; if weight was a critical parameter, it could be reduced by replacing steel with aluminium.

Microtomography typically uses specimens with a gauge section of a few millimeters square. The mechanical design was made to test specimens up to 5 kN (for high yield strength steels or titanium alloys) and with a mechanical stroke of 15 mm (for ductile metals and polymers). The overall dimensions and layout of the machine are visible in Fig. 1.

TANGO Controls are a free open-source controls toolkit used in many synchrotron sources (ESRF, DESY, ALBA, SOLEIL, ELETTRA, MAX-IV, etc.) [14, 15]. To enable full integration of the mechanical testing machine with the beamline, the choice has been made to control the tensile device using TANGO and compatible hardware. A mobile bay houses all the control electronics and can be moved between the laboratory and the synchrotron.

The system is autonomous and allows a mechanical test to be reproduced under the same conditions in the laboratory and at the synchrotron. For example, it is possible to program a complex loading method and use it to perform preparatory tests in the laboratory (by recording the force/displacement curve, and adding an extensometer and/or optical image acquisition to perform digital image correlation (DIC) or a shadowing method). At the synchrotron, the Tango server is then connected to the beamline control network in slave mode and all the devices declared in its database can be accessed directly from the network. Then, it is possible to properly synchronize the tensile motion, the recorded force signal and any synchrotron measurements: radiography, tomography or diffraction.

TANGO Controls provides full API in C++, Python and Java. Our implementation uses Python which is quite common on beamlines, and is directly compatible with the Python based SOLEIL scripting language SPyC. Motor movements and load cell data acquisition are managed by a small Python script making use of adequate classes from the xlab library available online [16]. This makes it very simple for the end-user, not necessarily familiar with the inner details of TANGO, to run mechanical tests and to use the same front-end whether in the laboratory or at the synchrotron. For instance, a simple monotonic tensile test with a force/displacement record, could be simply driven by the following lines:

The described hardware/software setup is highly flexible and allows to complete a mechanical test according to specific experimental needs: by adding more sensors or performing more complex loading paths. For instance, for studying crack propagation in steel, an experiment with tensile loading interrupted by several unload/load cycles for measuring the evolution of the compliance (linked to the slope during unload cycles) has been performed (Fig. 3).

As the traction exerted by this machine is asymmetric—i.e., the upper grip is stationary and only the lower grip moves—it may be necessary to correct the vertical position of the specimen in order to maintain the minimum cross section in the center of the field of view throughout the mechanical test. Thanks to the control and acquisition pairing, a correction of half of the traction displacement can easily be applied using the vertical translation motor of the tomograph.

This machine is designed to be versatile so as to offer the possibility of testing different specimen geometries suitable for different materials, because it is possible to design dedicated assemblies for specific samples or loading conditions, and because its flexible control allows complex loading paths. A wide range of different assemblies can be installed to adapt the machine to different configurations (see Fig. 4 for selected examples). The machine has already been used with success in a variety of configurations, including:

PEKK (Poly-Ether-Ketone-Ketone) [20, 21] is a high performance polymer material. This material is important because it has been developed for use as a polymer matrix in future composite materials in highly demanding areas of application such as aerospace and offshore energy, industries requiring replacements for metallic materials and structures. Such composite materials promise weight savings while maintaining excellent mechanical and industrial properties (lightweight, strength at elevated temperature, formability, and recyclability). Unlike most polymers already used in these fields, PEKK is a thermoplastic and not a thermoset material, which makes it possible to use melt-forming processes such as welding, injection-molding or thermoforming, and to consider its recycling.

The material is used in its semi-crystalline form. The crystalline phase is crucial for maintaining mechanical strength with increasing temperature: The maximum continuous operational temperature is 260^∘C and up to 300 ^∘C for a short duration. However, the glass transition temperature, corresponding to the shift in mechanical behavior of the amorphous phase from rigid to rubber-like, is about 160 ^∘C. The microstructure consists of thin crystalline lamellae arranged radially around a nucleation point (spheroidal structure) and surrounded by an amorphous phase [20, 22]. The density difference between the two phases is a very slight number and the stacking scale of these lamellae (≈ 10 nm) is several orders of magnitude below the resolution observable in micro-tomography (≈ 1 μm).

The samples used in these experimental series were machined from 2-mm-thick injected-molded macroscopic ISO 1BA specimens provided as experimental material by Arkema. Therefore, flat specimen geometries with pin mounting were chosen as shown on Fig. 5. Different geometries of lateral notches create triaxial stress states in the center part of the specimen where the tomography is recorded. The notches with a 1-mm or 10-mm radius were machined by milling. Sharper notches have been obtained by cutting using a pair of razor blades symmetrically mounted in a jig, and actuated using a laboratory mechanical testing machine. All geometries depicted in Fig. 5 give an initially square central section of 2 mm × 2 mm. In total, 7 specimens were tested: three specimens with notch radii of 1 mm, two with radii of 10 mm, and two with razor blade notches.

These measurements were made on the PSICHE beamline of the SOLEIL synchrotron, France. This is a hard X-ray beamline using an in-vacuum wiggler source. Tomographic measurements were made in pink beam mode. The beam spectrum was defined using filters including silver foil to exploit the absorption edge at 25.2 keV. High-energy photons are rejected by using an x-ray mirror as a low-pass filter.

The radiographic projection of the sample is converted to visible light using a scintillator. A visible light optic forms an image on a camera, giving an effective pixel size of 1.3 μm for a field of view 2.6 mm wide by 1.3 mm in height [12]. As the absorption contrast is low for a polymeric material at this energy, propagation-based in-line phase contrast and Paganin filtering of the radiographs [23] were used to enhance contrast. The sensor head was placed 65 mm from the observed area in order to get a good compromise in terms of phase contrast.

In addition, the Wide-Angle X-ray Diffraction (WAXS) results presented in part “Diffraction” were obtained in monochromatic mode. A monochromator, with two Si (111) crystals in Bragg-Bragg geometry, has been adjusted to have a beam with an energy of 25 keV [24]. This mode can also be used to perform tomography (although at a much slower rate).

A tensile test was carried out with a continuous increasing traction displacement at a crosshead speed of 0.4 μm s^− 1. Since the elongation before breaking is much greater than the height of the field of view, the automated height correction described in part “Control” was applied. As the vertical translation of the tomograph cannot achieve continuous movement at such a low speed, this correction is performed in jerks, carefully avoiding acquisition times thanks to the acquisition/control pairing.

Tomography acquisitions consist of the recording of a thousand radiographs equally distributed over a half turn and require a total of 6 seconds. Here, only half of the detector height was used in order to speed up the acquisition. It is also possible to take a reference image of the beam, for further flat-field correction of radiographs, by sweeping the sample away and calculating the average over a small time series of images. To move the PMMA outer tube completely out of sight, it is necessary to move sideways by 15 mm, which makes the operation relatively time-consuming: 20 seconds are necessary to remove the entire tube from the field of view, image the beam, and reposition the sample.

A Python script, running in a beamline control terminal, automatically executes the progress of a mechanical test:

Tomograms can be recorded at constant time intervals or, in the present case of highly non-linear material response, the acquisition frequency can be controlled directly by the user. This allows for more data acquisition in the vicinity of an interesting transition and by triggering the taking of a reference image when the beam has slightly drifted and the previous one becomes inadequate. Mechanical tensile tests presented here typically lasted 20 min with 25 to 50 tomography images recorded.

During a tensile test with the diffraction frame presented in part “Versatility”, several WAXS patterns were recorded throughout the deforming process showing that the crystalline phase undergoes microstructural changes during the mechanical test. In particular, in Fig. 9, the spectra show a different peak shift depending on the directions: axial in the loading direction or transverse perpendicular to the axial direction. The shape of the peaks widens. These changes in the WAXS patterns were attributed to a heterogeneous strain state within the semi-crystalline microstructure [27, 28]. Future works aim at analyzing these simultaneous strains at the scale of the crystallites (XRD) and the void evolution (as obtained by X-ray tomography). The multi-modal characterization will allow a better understanding of the distribution of the stress within the semi-crystalline microstructure.