Local approach to fracture on a semi-crystalline thermoplastic : the polyoxymethylene

Thermo-mechanical modelling of a screwing-threading process

Figure 1: Tapped zone observed on cryo-fractured surface by scanning electron microscope





The first investigations (cf. figure 1) showed that the process involved complex thermo-mechanical loading for which all the deformation components of the polymeric material were involved: elastic, viscous, plastic and volumetric. The study was performed within the framework of the local approach to fracture, that required fine examinations of the micro-mechanisms of deformation and damage within the polymer.





Results of the study

The experimental database were composed of measurable variables obtained from mechanical tests together with a comprehensive X-ray tomographic inspections. This latter non destructive technique revealing 3D images of the deformed microstructure allowed for characterization of the morphology and the distribution of voids within the polymer in the vicinity of the crest.

To account for void volume fraction, a model from the mechanics of porous media was utilized: the Gurson-Tvergaard-Needleman model, modified for polymeric materials purposes. The present model allowed for a construction of a failure assessment diagram that would permit to optimize the screwing rotation speed of the screwing-tapping process (cf. figure 2).

                            Figure 2 : Realistic prediction of damage's state in root of thread

An alternative solution consisting of an assembly composed of polymeric screw and polymeric nut was additionally investigated. Thanks to the local approach of fracture criterion based on the critical porosity, the weakest point of the assembly as well as its mechanical residual strength were estimated.


Jonathan Ricard obtained a professionnal master degree in 2007 at the University of Lorient UBS. Following a end-of-year internship in the area of automotive crash test simulation, he's hired by the company as a computation engineer.

Desiring to develop strong skills in terms of numerical modelling for structural design, he chooses to follow a specialized training in 2008, the MINES ParisTech Advanced Master's Degree "Material Behaviour and Structural Design". During this training he gets excited about the mechanical caracterization of materials together with more generally the applied research. He then turns to a PhD thesis where he will work three and a half years in the MINES ParisTech - Centre of materials's laboratory and the PSA Peugeot Citroën group.

After defending his PhD thesis in october 2013, he now wishes to work in the industry on projects to reduce the CO2 emmisions by lightening the structure thanks to the optimization of its geometry or the use of low density materials such as light alloys, the composite materials or the polymeric materials. In the future, his ambition is to become project manager R & D, notably because the management of a technicians team is certainly a great human experience.