Synchrotron computed tomography

Synchrotron radiation is produced in a different way to that previously described for microfocus CT, and necessitates the use of much larger facilities; for example the circumference of the storage ring at the European Synchrotron Radiation Facility (ESRF) is 844 m. Synchrotron radiation benefits from having a bright and parallel beam and the beamline at ESRF used in this project (ID19) is specially designed to ensure that the beam is also highly coherent for CT imaging.

A schematic diagram showing the basic design of a synchrotron facility is presented in Figure 3-16. Electrons are produced in an electron gun and initially accelerated in a linear accelerator. They are then transmitted into a circular accelerator (the booster synchrotron) where they are accelerated to the required energy (typically 6 GeV). The electrons are then injected into the storage ring in discrete bursts. Electrons travel in straight lines and so magnets are required to ‘bend’ the electrons around the storage ring. There are three types of magnets in the beamline (ESRF, 2007):

1. Focusing magnets – These are found in straight sections of the storage ring and are used to ensure the electron beam remains focused.

2. Bending magnets – These are used to direct the electron beam around the storage ring. It is the change in direction that causes synchrotron radiation to be produced and so the beam lines start at these magnets.

3. Undulators – These are used by some beamlines to produce a higher intensity beam. The undulator forces the electrons to follow a ‘wavy’ trajectory. The radiation caused by this movement constructively interferes hence increasing the intensity of the beam.

An advantage of the ID19 setup is that the X-ray beam may be either polychromatic (as in the microfocus CT system) or monochromatic. Using a monochromatic X-ray beam prevents beam hardening from occurring (Salvo et al., 2003). Due to the very small cross-sectional area of the electron beam used to create the X-ray beam, the X- ray beam is parallel (i.e. diverges very little) (Cloetens et al., 2001) and so the magnification effect that occurs in microfocus CT systems does not occur in

synchrotron systems. As a result, the X-ray beam may also be used for phase contrast imaging in addition to the conventional absorption imaging, which is an important tool in identifying the edges of defects.

When the X-ray beam is transmitted through the sample a phase change is imposed in addition to the attenuation of the beam. At interfaces (phase-phase or phase-air) there is a sharp difference in the phase retardation. Phase contrast forms when these

neighbouring sections of the beam interfere as they propagate away from the sample. On beamline ID19 phase contrast imaging is easily performed. In practise all that is required is that the X-ray detector is placed a certain distance, approximately a tenth of a metre, from the sample (Cloetens et al,. 2001); this is in the near-field Fresnel region.

Synchrotron CT was performed on samples cut from the short crack fatigue test pieces. These samples encompassed a fatigue crack and were 1x1x12 mm in size. Samples were also taken from long fatigue crack growth test piece for the LVD25 alloy at 350˚C. Several tests were performed, one was stopped at a low crack growth rate (~3x10-6 mm/cycle) and another were stopped at a high crack growth rate (just before final failure, ~ 2x10-4 mm/cycle). The samples were of size 1x1x12 mm and 2x2x12 mm and were sectioned so that they would encompass the crack tip region.

In this work a monochromatic X-ray beam with an energy of 20 keV was used. The X-rays were captured using a scintillator coupled to a fast readout low noise

(FRELON) 14 bit dynamic range CCD camera with an array of 2048x2048 pixels. A pixel size of 0.7 µm and 1.4 µm was obtained in the 1x1x12 mm and 2x2x12 mm samples respectively. An exposure time of 1 second was used and 1500 images were taken over a 180˚ rotation. The detector was placed 60 mm behind samples and so some absorption and phase contrast imaging was expected. Flat field and dark-current corrections were performed to account for error/inconsistencies in the beamline setup. A detailed description of the ID19 optics setup can be found at the ESRF website (ESRF, 2007)

The images were reconstructed using the filtered back projection method (Kak and Slaney, 1988). The software used for this is in-house software produced at ESRF. The reconstructed volumes are 32 bit files and these were reduced to 8 bit files to

minimise the file size. However, each scan still produces 8GB files even after this process. The images were analysed in VG Studio Max and ImageJ software.

Alloy Si _(wt.%) Cu _(wt.%) Ni _(wt.%) Mg _(wt.%) Fe _(wt.%) Mn _(wt.%) Ti _(wt.%) Zr _(wt.%) V _(wt.%) P _(ppm) Sr _(ppm) LVD25 12.45 3.93 2.78 0.67 0.44 0.03 0.01 0.05 0.04 55 0 LVD26 unmod 6.90 3.89 3.00 0.62 0.22 0.03 0.01 0.05 0.04 45-50 0 LVD26 mod 6.90 3.89 3.00 0.62 0.22 0.03 0.01 0.05 0.04 45-50 150- 155 LVD27 0.67 3.91 2.99 0.80 0.21 0.05 0.01 0.01 0.01 45 0

Stage grinding grinding 2nd Polish 3rd Polish 4th Polish 5th Polish Abrasive 600 Grit 1200 Grit 6µm Diamond 3µm Diamond 1µm Diamond OPS Force (lbf) 17 17 20 17 17 5 Speed (rpm) 300 300 150 150 150 150

Time (min) Till plane 3 4 3 3 ~15

Table 3-2 Metallography polishing route.

Alloy Temperature theoretical stress (%YS)

strain (%) extrapolated

stress (MPa) stress (%YS) extrapolated

LVD25 RT 213 0.76 172 158

LVD26 unmod RT 210 1 160 151

LVD26 mod RT 215 1.1 170 159

LVD27 RT 175 0.54 132 105

Table 3-3 Stresses and strains at which short crack tests were performed.

Figure 3-1 A photograph of a piston supplied by Federal Mogul and a sectioned piston crown placed on it.

Figure 3-2 Analysis on the effect of the number of points used for the point fraction analysis.

Figure 3-5 Tensile test piece specimen geometry conforming to ASTM E8M standard.

Figure 3-8 a) typical force displacement curve for an indent and b) is a schematic showing the displacement during loading and unloading (after Pharr et al. 1992).

Figure 3-9 Schematic diagram of sink-in and pile-up.

Figure 3-11 A schematic diagram of the fracture surface of a long fatigue crack growth test sample, the dashed lines show where the sample was sectioned.

Figure 3-13 Schematic diagram of back projection.