PSI - Issue 28

E. Solfiti et al. / Procedia Structural Integrity 28 (2020) 2228 – 2234

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E. Solfiti et al. / Structural Integrity Procedia 00 (2020) 000–000

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perature. Some recent works moved the investigation in static compression and recovery loading at room temperature (Toda et al. (2013); Kobayashi et al. (2012)), also observing how the deformation units behave upon the external load application. The use of Sigraflex ® in the LHC external dumps, can be considered as a unique application and lays out of the commercial ones listed above. This means that an extension of the investigation upon a wider range of tem perature and strain-rates is requested, with the purpose of predicting the behavior and improving the suitability in the current and future TDE design conditions. Indeed the High Luminosity Large Hadron Collider (HL-LHC), that is the future update of LHC whose first run is planned for 2027, is being designed to achieve a larger beam intensity (Apol linari et al. (2017)). In this collaboration between CERN and NTNU, a first exploratory investigation is performed on Sigraflex ® specimens regarding their static tensile properties at room temperature in the in-plane direction, in order to approach the material, to understand the requested experimental techniques and therefore establish a starting point for a wider testing campaign.

External dumps

Sigrafine HLM

R

Vessel - Uranus 45 (Stainless steel 318 LN, Ø = 700 mm, h = 12 mm)

Magnet deflecting the beam

Titanium window

Particle beam extracted from LHC circular path

Sigrafine R7300

R

3260 mm 1650 Sigraflex sheets R

R Sigrafine HLM R

Titanium window

Sigrafine R7300

(a)

(b)

Fig. 1: (a) Large Hadron Collider facility overview (CERN website) and external beam dumps or TDE blocks (picture reworked from Schmidt et al. (2006)) and (b) TDE graphitic core (section view).

2. Materials and methods

Despite its softness, the procedure to obtain flexible graphite specimens should be carefully chosen in order to get a su ffi cient surface refinement. A rolled foil of Sigraflex ® L20010C, whose properties are reported in Table 1, was provided by SGL Carbon. The waterjet CNC machine WJS NCH 30 was chosen for the cutting procedure and the following parameters were found to give the better results: jet pressure = 3800 bar, beam diameter = 0.3 mm and sand GRIT = 230. The Sigraflex ® foils as delivered and the specimen geometry are shown in Fig. 3(a) and (b). No polishing or further machining was worked out on the specimens.

Size

1000 × 1000 × 2 mm

1 g / cm 3 > 4 MPa ≤ 0.15 %

Density

Tensile strength

Ash content

Elongation at break Specific heat (20 ◦ C)

> 1 %

0.7 J / (Kg K) Coe ff . of thermal expansion (20 - 1000 ◦ C) 1 × 10 − 6 K − 1 (in - plane), 30 × 10 − 6 K − 1 (out - of - plane) Thermal stability - 250 to + 3000 ◦ C (up to + 400 ◦ C in presence of oxygen)

Table 1: Sigraflex ® L20010C foils properties as reported in SGL Carbon website.

The tensile static tests were performed in a MTS Exceed ® Universal testing machine with 5 kN load cell. A batch of four specimen couples was chosen upon four di ff erent crosshead displacement rates as reported in Fig. 3(a).