Thesis Open Access

Thermomechanical Response of Advanced Materials under Quasi Instantaneous Heating

Federico carra

Thesis supervisor(s)

Lorenzo Peroni

The study of the thermomechanical response of materials to a time‑dependent heat load is of paramount importance in the design of a variety of components widely adopted in the industry and in research laboratories. Three regimes can be identified in the thermomechanical problem, depending on the heating rate: quasi‑static, slow‑transient and quasi‑instantaneous heating. This PhD thesis focuses on the latter scenario, where the heat deposition rate is high enough to lead to the origination of stress waves, propagating from the locally-heated zone to the surrounding of the structure, and superposing with the quasi-static stress field.

In the first part of the thesis, the dynamic response of materials to quasi‑instantaneous heating is evaluated as a function of the stress waves generated. At low thermal energies, stress waves remain below the yield stress of the material, in the elastic regime. When the amplitude of the wave surpasses the yield stress of the material, plasticity takes place and the signal is dispersed into an elastic wave travelling at the speed of sound, and plastic waves at lower velocity. Finally, the shock regime can be attained only at critical levels of energy and pressure induced by the fast heating. This scenario features a sharp discontinuity in temperature, pressure and density, requiring the adoption of finite element codes for the solution of the thermomechanical problem. The hydrostatic response of shocked materials depends on the equation of state (EOS), while the deviatoric contribution to the stress tensor is controlled by the strength model. Failure models govern fracture mechanisms due to void coalescence, spallation and micro‑spallation. Examples of the main categories of EOS, strength and failure models, are given in this thesis. A new method to explore unusual regions of the EOS, based on intense isochoric heating driven by particle beams, is also introduced.

In the second part of the thesis, the several phenomena induced by a quasi‑instantaneous heating, due to particle beam impact on the matter, are explored in detail. Such phenomena involve changes of phase, cylindrical pressure waves at the elastic, plastic and shock regime, as well as spallation and micro‑spallation fracture. To explore each of these mechanisms, numerical studies by means of implicit and explicit finite element codes are presented and combined, when available, with analytical methods and experimental tests performed in particle accelerator facilities.

In the final part of the thesis, the studies performed are applied to the design and engineering of CERN HL‑LHC accelerator components known as collimators. These components, closely interacting with the beam particles, are potentially submitted to accidental impacts, whose consequences on the collimator and on the overall machine must be minimized. With this goal, new composites were developed at CERN in recent years to replace the carbon‑fibre‑reinforced carbon (CFC) currently adopted in the present LHC, combining the good thermal and electrical properties of metals with the high thermal stability of carbon allotropes such as graphite and diamond. The most promising ones are Copper‑Diamond (CuCD) and Molybdenum‑Graphite (MoGr); these materials were fully characterized in order to derive EOS and constitutive models necessary for the study of their response under intense isochoric heating. To prove the accuracy of such models, and to experimentally verify the collimator resistance under the direct impact of proton beams involving energy densities typical of the HL‑LHC design scenarios, a test was devised and performed in 2015 at the CERN HiRadMat facility. Three collimator jaws, in CFC, MoGr and CuCD, were extensively instrumented, and submitted to proton impacts at increasing intensities. Experimental results of the tests and comparisons with the numerical predictions are presented.

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