In situ observations of microscale damage evolution in unidirectional natural fibre composites

Abstract

Synchrotron X-ray tomographic microscopy (XTM) has been used to observe in situ damage evolution in unidirectional flax fibre yarn/polypropylene composites loaded in uniaxial tension at stress levels between 20% and 95% of the ultimate failure stress. XTM allows for 3D visualization of the internal damage state at each stress level. The overall aim of the study is to gain a better understanding of the damage mechanisms in natural fibre composites. This is necessary if they are to be optimized to fulfil their promising potential. Three dominating damage mechanisms have been identified: (i) interface splitting cracks typically seen at the interfaces of bundles of unseparated fibres, (ii) matrix shear cracks, and (iii) fibre failures typically seen at fibre defects. Based on the findings in the present study, well separated fibres with a low number of defects are recommended for composite reinforcements.

Flax/polypropylene composites for lightened structures: Multiscale analysis of process and fibre parameters

“…Bourmaud et al [10] have shown how flax fibre bundles can affect the mechanical properties of composites. Rask et al [11] studied the damage of processes for unidirectional PP/flax (UD) by using X-ray diffraction; they concluded that fibres that are well separated are recommended for composite reinforcements. The influence of the dispersion of single fibres on the mechanical properties of flax/polypropylene composites has also been shown [8].…”

A probabilistic model of the tensile strength of a UD flax–fabric‐reinforced polymer composite

“…Whereas the tensile strength of flax fibers closely agrees with the modified Weibull distribution [37] and the stress transfer to flax fibers appears to be dominated by mechanical interlocking and friction [44,51] thus meeting two of the assumptions of the numerical simulations in [42,43,45], there is appreciable scatter in interfiber distances and the arrangement of flax fibers in a yarn is far from regular [9,32]. Markedly different stress concentrations at a fiber break in unidirectional composites with random fiber packings compared with those for ordered fiber packings have been reported in [59] highlighting the need for strength models with more realistic fiber arrangement.…”

“…In situ observations, by using the synchrotron X‐ray tomographic microscopy, of microscale damage evolution in a UD flax fiber yarn/PP composite provide a tentative explanation for the detrimental effect of the presence of technical fibers on the composite strength. Fiber failures, leading to the ultimate fracture of the composite, were observed to initiate at about 90% of the composite failure stress at the locations of localized structural irregularities of fibers, that is, kink bands.…”

“…The fibers contain cell wall defects in the form of kink bands (also referred to as dislocations or nodes), that is, localized misalignments of cellulose microfibrils extending through the cross section of a fiber , which originate during their growth and processing . Fiber failure in tension is observed to initiate in the kink bands both of free‐standing fibers and fibers in a composite . It has been argued that more complete separation of the reinforcement into elementary fibers would result in an increased tensile strength of a composite, as long as the separation technique does not induce additional fiber defects .…”

“…Fiber failure in tension is observed to initiate in the kink bands both of free‐standing fibers and fibers in a composite . It has been argued that more complete separation of the reinforcement into elementary fibers would result in an increased tensile strength of a composite, as long as the separation technique does not induce additional fiber defects .…”