Dmrx optical microscope

Spectrally resolved intensity maps were acquired for single stripes in a modified Leica DMRX optical microscope with a × 20 objective where an additional photoport allowed for the collection of the light reflected from the specimen into an optical fibre of 50-μm diameter. Spectra were collected from a spot of 2-μm diameter with a collection cone half angle of 33° and the corresponding maps were obtained by laterally scanning the sample in 2-μm steps while maintaining the specimen in constant focus. Quantitative spectral measurements for the determination of the stripes reflectivity in water were performed by using a × 63 water immersion objective with numerical aperture NA=1.0 allowing for the collection of spectra from spots of ~2-μm diameter over a polar angle range of 0–50°. Optical scattering of the samples was studied using a modified Olympus BXFM microscope, where by the incorporation of a Bertrand lens, scattering of a specimen under quasi-plane wave illumination, achieved via an additional photoport, was analysed by imaging the back focal plane of the objective. A high numerical aperture oil immersion objective (Leica PL APO 100 × /1.4 −0.7 OIL) was used to increase the observable angular range. The corresponding scattering angles for water as a medium above the shell surface were then calculated using Snell’s law. These are the angles displayed in Fig. 4c.

Once the welds had been made in the mockups, each tube-to-tubesheet joint was cut into four pieces as shown in Figure 9. The cuts were identified with the positions 0°, 90°, 180° and 270°. Specimens were extracted from these parts by lubricated cutting with linear precision saw ISOMET 4000 (Buehler USA, Lake Bluff, IL, USA). After that, all cut specimens were hot mounted in 40 mm diameter samples, using XPHB phenolic resin. For optimal macrographical observation and measurement, they were grinded with XPNC SiC abrasive papers to a maximum P2400 grit, and then polished with XALO alumina suspension of 1 and 0.3 µm.
Nital 3% solution (3% HNO ${}_3$ in ethanol) was used to reveal the weld and the HAZ. The measurement of the minimum leakage path ( $MLP$ ) and length of the weld leg ( $A_f$ ) in each sample was carried out with LAS v4.13 software (Leica Microsystems). Macrographs were taken with a Leica DMRX optical microscope equipped with a Leica MC190 HD camera. For each parallelism deviation value tested, four metallographic samples were obtained and measured (one of each quadrant of the orbital weld).