Public Thesis defense - IMMC

Residual stresses, Geometrical Defects and Surface Hardening: Impacts on Lifetime of Biomedical Implants by Maïté Croonenborghs

The imperative of reliability of biomedical implants is crucial to ensure their long-term functionality, thereby reducing the need for additional, sometimes invasive, surgical interventions. In this context, growing rods used to correct scoliosis in growing children face challenges related to a high fatigue failure rate. Similarly, future generations of stents designed to restore blood flow in arteries must overcome potentially rough surface conditions while ensuring effective deployment without failure.

The primary aim of this thesis is to investigate the relative impact of shot-peening and bending within growing rods on the generation of residual stresses, employing mechanical tests and numerical methods. While shot-peening proves to be beneficial by enhancing compressive stresses, bending has been found detrimental, even if the magnitude of the imposed bending angle itself is not crucial. The multiple surface hardening steps guide towards the selection of metals with significant kinematic hardening capacity. Comparing these numerical results with experimental findings reveals an unexpected fatigue resistance of untreated rods under certain conditions, raising concerns about the shaping processes used by the suppliers.

The second part of the work explored the impact of surface quality. The roughness resulting from shaping processes appears to be the cause of the fatigue resistance reduction in growing rods. Furthermore, geometric irregularities can limit the elongation capacity of structures like in stent struts due to the decrease resistance to plastic localization. This finding, established since the 1960s through closed form models, has been verified using a finite element numerical model and experimental measurements relying on digital image correlation. Additionally, the length of the inhomogeneity has been shown to influence the length of the neck.

Jury members:

• Prof. Pascal Jacques (UCLouvain, Belgium ), supervisor
• Prof. Thomas Pardoen (UCLouvain, Belgium ), supervisor
• Prof. Sandra Soares Frazao (UCLouvain, Belgium ), chairperson
• Prof. Laurent Delannay (UCLouvain, Belgium
• Prof. Greet Kerchofs (UCLouvain, Belgium
• Prof. Xavier Banse (Cliniques universitaires Saint Luc)
• Dr. Pascale Kanouté (ONERA, France)

Pay attention :

The public defense of Maïté Croonenborghs scheduled for Monday February 12 at 4:15 p.m. will also take place in the form of a video conference

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