Role of in flax fibre internal pores as stress concentrator
Topic(s) : Special Sessions
Co-authors :
Delphine QUEREILHAC (FRANCE), Emmanuel DE LUYCKER (FRANCE), Sofiane GUESSASMA (FRANCE), Marwa ABIDA , Timm WEITKAMP , Alain BOURMAUD (FRANCE), Pierre OUAGNE (FRANCE)Abstract :
INTRODUCTION
The mechanical potential of natural fibres such as the European bast fibres (flax, hemp or nettle) has been the subject of many publications and reviews. The internal biochemical composition as well as the internal organisation of the different components into a composite structure was also intensively studied and reviewed [1]. In this review, it is explained that the flax fibres are mainly constituted by crystalline cellulose organised in microfibrils placed with an angle varying between 5 to 10° to the longitudinal axis. If one considers this, the mechanical behaviour of the flax fibre should be quite close to the one of the crystalline cellulose with minor decreases due to the orientation of the microfibrils and the lumen porosity. The role of lumen was also studied by Richely et al as well [2], but the discrepancy between what can be theoretically expected and the real tensile properties of flax or hemp fibres maybe quite important, with a decrease of properties that is at the best of about 1/3 of the crystalline cellulose potential. Recently, different types of fibre internal pores situated within structural defects (kink-bands) were evidenced using synchrotron phase contrast X-ray tomography [3]. Beyond quantifying them and describing their spatial organisation, it is proposed in here to study their role as stress concentrator using finite element (FE) analysis.
RESULTS
Different pore elements were considered and their role as stress concentrator analysed using FE-analysis. It appears that stresses are concentrated at the vicinity of pores and particularly around the kink-band pores. Stress concentration up-to a 7 times factor was evaluated in critical zones at the periphery of kink-band internal pores. Such high stress concentrations within the fibres are of course zones of weakness of the fibre and this of course explains why the tensile strength of flax fibres are lower than the one of a defect less fibre. Other types of pores including lumen and surface defects were also considered and the respective role of each of them will be presented in relation to their respective severity towards the decrease in tensile properties of flax elementary fibres from an ideal structure.
The mechanical potential of natural fibres such as the European bast fibres (flax, hemp or nettle) has been the subject of many publications and reviews. The internal biochemical composition as well as the internal organisation of the different components into a composite structure was also intensively studied and reviewed [1]. In this review, it is explained that the flax fibres are mainly constituted by crystalline cellulose organised in microfibrils placed with an angle varying between 5 to 10° to the longitudinal axis. If one considers this, the mechanical behaviour of the flax fibre should be quite close to the one of the crystalline cellulose with minor decreases due to the orientation of the microfibrils and the lumen porosity. The role of lumen was also studied by Richely et al as well [2], but the discrepancy between what can be theoretically expected and the real tensile properties of flax or hemp fibres maybe quite important, with a decrease of properties that is at the best of about 1/3 of the crystalline cellulose potential. Recently, different types of fibre internal pores situated within structural defects (kink-bands) were evidenced using synchrotron phase contrast X-ray tomography [3]. Beyond quantifying them and describing their spatial organisation, it is proposed in here to study their role as stress concentrator using finite element (FE) analysis.
RESULTS
Different pore elements were considered and their role as stress concentrator analysed using FE-analysis. It appears that stresses are concentrated at the vicinity of pores and particularly around the kink-band pores. Stress concentration up-to a 7 times factor was evaluated in critical zones at the periphery of kink-band internal pores. Such high stress concentrations within the fibres are of course zones of weakness of the fibre and this of course explains why the tensile strength of flax fibres are lower than the one of a defect less fibre. Other types of pores including lumen and surface defects were also considered and the respective role of each of them will be presented in relation to their respective severity towards the decrease in tensile properties of flax elementary fibres from an ideal structure.