Study on Reliability Assessment System of Composite Materials (1 st Report)

Establishment of Failure Model and Examination on Reliability Level

Abstract

A general judgement method to evaluate material properties of advanced composites such as carbon, aramid and hybrid fiber reinforced plastics has not well been established on account of a much broader range of properties and a much stronger anisotropy than conventional glass fiber reinforced plastics. The number of factors to affect a scatter of properties is generally larger in composite materials, which are composed of much different phases of constituents, than in conventional monolithic materials : a variation in defects mixed in and dispersion state of constituents during a composition process tends to make the amount of scatter in mechanical properties of composites larger in addition to a variation in constituent material properties themselves.
Furthermore, as the failure process of fiber reinforced composites is a very complicated accumulation process of damage due to random failure of fibers, matrix and interface, which leads to a catastrophic fracture, it should be necessary to introduce a reliability assessment system to evaluate effectively a decrease in strength due to cumulative damage and defects taking every aspect of variation into consideration in order to understand thoroughly the statistical nature of strength properties of composite materials.
For this purpose, the present paper aims at establishing a general assessment system to predict a descrease in reliabilty of composite materials due to cumulative damage, with a main system to simulate a stochastic failure process of composite materials considering the effect of a scatter in strength of fibers and matrix and defects mixed in during a fabrication process, accompanied with a subsystem of systematic statistical analysis.
In this report, a new failure simulation model of composite materials has been introduced considering the effect of matrix shear failure as well as fiber breaks based on a shear-lag theory in which a failure can occur randomly not only in fiber elements but also in matrix elements : a stochastic tensile failure process has been simulated by means of a Monte Carlo method based on a repeated increment scheme using a finite difference technique. Then a Weibull analysis technique has been applied to determine design allowable properties and the results have been described as 'A' and 'B' allowable levels for unidirectional carbon and glass fiber reinforced plastics.