Stress redistribution around clusters of broken fibres in a composite

Abstract

A key aspect of the longitudinal tensile failure of composites is the stress redistribution that occurs around broken fibres. Work on this topic has focussed mainly on the stress field surrounding a single broken fibre; however, this is an important limitation as unstable failure in carbon fibre bundles occurs when a cluster of about 16 or more broken fibres is formed.Therefore, we have developed a detailed Finite Element (FE) model to investigate how stress redistribution varies with the number of broken fibres in a cluster. The results show that both the recovery length and stress concentration factor increase significantly with increasing number of broken fibres in a cluster. We have also developed an analytical model, suitable to be included in existing or new fibre bundle models, that captures how the recovery length and stress concentration factor vary with the broken cluster size, and validated its predictions against our FE simulations. Finally, we extended our FE model to predict the survival probability of fibre bundles using Monte Carlo simulations, and found that these predictions were in good agreement with experimental and analytical results on microcomposites.

A computationally-efficient hierarchical scaling law to predict damage accumulation in composite fibre-bundles

“…While this overestimates stress concentration factors predicted through FE analyses [8,11,24,30,41] and neglects the dependence of stress concentrations with size of broken-cluster [30], it is a direct consequence of the hierarchical and self-similar failure propagation mode (see point IV in the previous section), and it can account indirectly for many real effects -e.g. dynamic stress concentrations, ibre misalignments, and effect of large broken clusters -which are neglected in many Monte-Carlo simulation models [5,[16][17][18][19][20][21][22][23][24][25][26][27].…”

“…The present model is analytical and, consequently, calculates the expected stress-strain curves and the evolution of ibre-breaks and clusters in less than one second, even for bundles with a trillion ibres. On the contrary, most stateof-the-art literature models [5,[16][17][18][19][20][21][22][23][24][25][26][27]30] (which require Monte-Carlo and/or Finite Element simulations) are computationally much more demanding, which limits the maximum size of the structures that they can analyse. This is particularly important given the results showing an increasing critical cluster size for larger structures (point d in Section 4.1), which limit the applicability of the WLT to scale-up results from simulations of small bundles;…”

“…These models aim to represent stress ields near single ibre-breaks accurately, although most assume regular packing [5, 17-25, 27, 30]; many models [16,18,25,26] also neglect the increase in the stress-recovery length near broken clusters, which was shown to overestimate bundle strength [30]; moreover, all models [5,[16][17][18][19][20][21][22][23][24][25][26][27]30] consider simpli ied matrix constitutive laws (e.g. neglecting progressive matrix fracture), neglect dynamic stress concentrations, and assume perfectly-aligned ibres (the only exceptions [28,29] considered very small representative volume elements).…”

“…More recently, the increase in computational power enabled several researchers to propose Monte-Carlo simulations for the tensile failure process in UD-FRPs [5,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. These models aim to represent stress ields near single ibre-breaks accurately, although most assume regular packing [5, 17-25, 27, 30]; many models [16,18,25,26] also neglect the increase in the stress-recovery length near broken clusters, which was shown to overestimate bundle strength [30]; moreover, all models [5,[16][17][18][19][20][21][22][23][24][25][26][27]30] consider simpli ied matrix constitutive laws (e.g.…”

The tensile strength test of thermoplastic materials based on poly(butylene terephtalate)

“…The second way is the stretching of certain fibres that have small adhesion to matrix. The reason for this can be defects in boundary fibre/matrix or mistakes during manufacturing of materials [2]. So during planning and manufacturing methods of thermoplastic composites it is very important is consort with a lot of factors and aspects to create materials with the best properties.…”