Strain-rate, temperature and size effects on the mechanical behavior of fiber bundles

TL;DR

This paper investigates how strain-rate, temperature, and size influence the mechanical behavior of fiber bundles, highlighting the significant role of thermal activation and cautioning against traditional strength distribution estimation methods under certain conditions.

Contribution

It introduces a fiber-bundle model with thermal activation via kinetic Monte-Carlo simulations, revealing effects of temperature, strain-rate, and size on fiber bundle strength and behavior.

Findings

Decreased bundle strength at lower strain-rates and higher temperatures.

Classical strength distribution estimation can underestimate fiber strength parameters.

Size effects influence the mechanical response of fiber bundles.

Abstract

The mechanical characteristics of fibers (of various materials), as well as of fiber bundles, are of primary importance for the design and the mechanical behavior of textiles, or of fibrous and composite materials. These characteristics are classically determined from strain-rate controlled tensile testing, generally assuming a negligible role of thermal activation on damage and fracturing processes. Under this assumption, the distribution of individual fiber strengths can be deduced from a downscaling of the macroscopic mechanical behavior at the bundle scale. There are however many experimental evidences of strain-rate and temperature effects on the mechanical behavior of individual fibers or bundles, which can also creep under constant applied load. This indicates a strong role of thermal activation on these processes. Here, these effects are analyzed from a fiber-bundle model with equal-load-sharing, in which thermal activation of fiber breakings is introduced from a kinetic Monte-Carlo algorithm adapted for time-varying stresses. This allows to rationalize these rate or temperature effects, such as a decrease of bundle strength, strain at peak stress, and apparent Young's modulus with decreasing strain-rate and/or increasing temperature. This also shows that the classical downscaling procedure used to estimate the distribution of individual fiber strengths from the mechanical behavior at the bundle scale should be considered with caution. If mechanical testing of the bundle is performed under conditions favoring the role of thermal activation (e.g. low applied strain-rate), this procedure can strongly underestimate the intrinsic (athermal) Weibull's parameters of the fiber strengths distribution. The same model is used as well to explore size (number of fibers) effects on bundle mechanical response.