Shaping processes are predominant to produce parts meeting specific requirements. Manufacturers need to optimize their production processes to meet the high demand for new products of greater value in terms of accessibility, quality, productivity and profitability.

Currently, the optimization of machining operations allowing to obtain, in addition to the geometry, the expected characteristics of the manufactured parts (roughness, residual stresses, metallurgical characteristics, etc.), is still done by trial and error while these properties have a considerable impact on the parts durability. For each new tool/workpiece material pair, instrumented machining tests must be carried out in order to determine the most suitable tool geometries and coatings, and the process parameters capable of ensuring the quality of the machined surfaces while reducing production costs.

Numerical simulation of machining is becoming an essential tool to access thermal fields, metallurgical characteristics and residual stresses, the durability of parts being conditioned by such quantities. Despite its potential in process optimization, numerical simulation of machining is not currently reliable. The scientific barriers to overcome concern the constitutive laws of engineered materials, the tribology of the processes, the development of fast and accurate numerical simulation and experimental validation through local strain and temperature field measurements.

This proposal is part of a larger project of the research team whose objective is to increase the predictive nature of machining simulations, thanks to an inverse identification of the material behavior and friction laws via:

- Measurements of macroscopic (cutting and feed forces, geometric characteristics of the chips, tool/chip and tool/material contact length) and mesoscopic quantities (strain and temperature fields in the main shearing areas of the machined materials) recorded during machining.

- Post mortem analysis on the machined surface (microstructure, residual stress measurements) and on the cutting tool (changes in its geometry with respect to wear).

The I2M got equipped with an experimental bench and optical system, including high magnification optics, lighting means, digital image processing facilities, etc…, allowing in-situ access to a large numbers of macroscopic and mesoscopic quantities, essential for the validation of numerical models.

The improvement of the system, developed during ENABLE project for measurement of local strain and temperature fields, is the main purpose of this post-doctoral research project. However, the experimental measurements cannot be disconnected from the numerical simulations. Indeed, the choice of the method for digital images processing is of highly importance because resulting strain and strain rate fields are to be compared with simulated one. Therefore, both numerical and experimental aspects need to be simultaneously addressed.

The work carried out during P. LIMJE's thesis (to be defended in December 2022), has improved the parameters identification of the TANH material behavior law, developed in the I2M laboratory, and thus extend its validity to a wider range of cutting conditions. Increasing the predictive capability of numerical simulation of machining by integrating both the latest advances in the optimization of the TANH behavior law (Limje et al., 2021) and recent developments in friction laws, is also one of the objectives of this post-doctoral research.

Scientific supervisors :

- Madalina CALAMAZ, MCF, I2M

- Olivier CAHUC, PU, I2M

During previous work at I2M laboratory (R. Laheurte Univ. de Bordeaux thesis, 2004), it was shown that in a situation of severe friction between two metal surfaces (i.e., the cutting face of a machining tool and the chip), some new phenomena appear and have been measured. Indeed, a Moment vector is measured and identified in addition to the Forces vector usually considered. The identification of this friction torsor, which is not considered in the usual friction laws used in numerical process simulation codes, makes it possible to improve the results obtained from these codes through the implementation of more relevant friction laws.

The work proposed in this Post Doctorate must make it possible to carry out new tests on a new friction bench equipped with numerous measurement devices that the Laboratory has recently been equipped with, as well as to implement in the finite elements of open simulation codes all the necessary elements to implement new conditions and contact laws in line with real phenomena.

The work will be carried out according to the following sequence.

The modelling of the aforesaid friction bench is fairly challenging. Indeed, the following aspects need to be properly addressed: large and localized deformations, elasto-visco-plasticity at large strains and accurate computations of contact pressures with advanced contact algorithms and acceptable computational times.

As previously stated, results obtained at I2M laboratory call for innovative friction and behavior laws: one must model couple stresses (i.e., stresses generating local torques) to accurately represent the experiments. This is not possible with classical continua. Cosserat continua, on the other hand, introduce microrotations of a material point associated with couple stresses. This is why these continua were extensively investigated during the ENABLE project led by Professor Cahuc. A machining-dedicated hardening law (the so-called TANH law) was successfully coupled with a Cosserat medium, the formulation of Cosserat elasto-visco-plasticity at large strains was achieved and the implementation in a HPC open-source finite element code (Fenics) was realized. Yet, friction between Cosserat media remains to be implemented and studied in transient dynamics. This represents quite a challenge since it requires to modify contact algorithms.

A two-step plan is proposed to achieve the aforesaid tasks. The demonstration that usual friction/behaviour laws fail to represent experiments will first be carried out. This demonstration can be made thanks to a classical commercial code (Abaqus). Improvements brought by Cosserat media will then be investigated. Several roads can be taken according to the post-doctoral student experience: i) implementation of Cosserat media in a commercial code (Abaqus) or ii) using the previously developed FEniCS code as a starting point. If modifying contact algorithms in Abaqus and FEniCS proves overly impractical, an in-house code developed by one of the advisors will be employed.

As we have seen previously, one of the the scientific barriers to overcome concern the tribology of the processes, the development of fast and accurate numerical simulation and experimental validation through local strain and temperature field measurements.

The objective of the experimental part of this proposal is to increase the predictive nature of friction simulations, thanks to an identification of the parameters of the new friction laws via:

Measurements of macroscopic (forces, moments, tool/chip and tool/material contact surface) and mesoscopic quantities (strain and temperature fields in the main shearing areas of the machined materials) recorded during machining.

Post-mortem analysis on the machined surface (microstructure, residual stress measurements) and on the cutting tool (changes in its geometry with respect to wear).

The I2M got equipped with an experimental bench and optical system, including high magnification optics, lighting means, digital image processing facilities, etc…, allowing in-situ access to a large numbers of macroscopic and mesoscopic quantities, essential for the validation of numerical models.

The improvement of the system, developed during ENABLE project for measurement of local strain and temperature fields, is an important purpose of this post-doctoral research project. However, the experimental measurements cannot be disconnected from the numerical simulations. Indeed, the choice of the method for digital images processing is of highly importance because resulting strain and strain rate fields are to be compared with simulated one. Therefore, both numerical and experimental aspects need to be simultaneously addressed.

Scientific supervisors

- Simon ESSONGUE-BOUSSOUGOU, MCF, I2M

- Olivier CAHUC, PU, I2M

• In order to take into account the various manufacturing steps, the aim is to develop a global model of a forming process for a mechanical part. This model will integrate the environmental impact dimension of the whole process.
• The ultimate goal is to control the whole manufacturing process by integrating the history of the part made (in a matter point of view), technical and environmental performance. Optimization of the execution scheme (choice of processes and operating parameters) can lead to the production of parts with equivalent technical performances but minimizing (controlling) the environmental impacts. Energy and materials consumptions, the follow-up of solid waste, liquid effluents and GHGs (linked to consumption of energy and non-processed raw material) can already be considered.
• For example in aeronautic, some critical parts are forged, machined and then subjected to a shot blasting or heat treatment process. Models exist separately to represent each process and their environmental impact but not the interactions and especially the effect of one process on its next.
• This approach requires very transversal skills combining mechanics, materials, processes and analysis of environmental impacts. Which tools, models and data of the two fields of expertise (processes and environmental impacts) are available and compatible to carry out these studies, or which gaps are to be filled, while maintaining a minimum level of the models and data necessary for which responses can be simulated and information returned can be used to choose alternative process and operating parameters.
• Over of the last decades the computational capacity has evolved in a considerable way, the advances of numerical simulation models of manufacturing processes have accompanied this progress and evolved at the same time. Optimizations based on real time machinery and product flow big data acquisition are the main characteristic in the fourth industrial revolution era.  
• Current scientific locks are related to the following questions:
  • Process point of view:
    • What is the influence of a process on its next (and possibly the cumulation of previous processes)?
    • How to optimize the parameters of the previous process to support the next one?
    • How to model and analyze the environmental impact of manufacturing processes with a representation and models derived from physic laws?
• Environmental Assessment Perspective:
  • Which model (environmental impacts oriented) allows representing the process and its environmental impacts?
  • What is the relevance and availability of data in environmental databases of manufactured processes, and standardization of measurement methods?
  • How to take into account the environmental impacts of processes (adaptation of calculation methods) using a Life Cycle Assessment approach that is a product approach?
• Numerical simulations help to reduce the number of experimental tests and to optimize independent and the global manufacturing chain process. They are also a tool to study the nature of the phenomena and changes that occur during plastic deformation and manufacturing processes.
• The current work focuses on the problem of the processes interaction from a physical point of view, in order to follow the evolution of the material. When a different hardening rule is used, other state variables are required, which indeed will have a major influence for a multiple operations global process simulation.
• In order to optimize a global manufacturing process it is necessary identify the variables that characterize the state of the each point of the material. This action will allow assuring the traceability of the process.
• The aim is to study the influence of the hardening rule adopted in the formulation of the behavior law to evaluate its influence on the global manufacturing production. The main long-term goal of the scientific field of the activity is to model the behavior of the workpiece all along the manufacturing process. Taking into account that the actual strategy to control processes interaction is to eliminate undesirable internal stresses by annealing, gaining a prediction capacity would allow not only to control the processes parameters avoiding highly energetic operations but also a new tool to redesign the overall manufacturing process.