Strength scaling in fiber composites

Abstract

A research program has been initiated to study and isolate the factors responsible for scale effects in the tensile strength of graphite/epoxy composite laminates.Four lay-ups, (±30° n/9002n)s, (±45° n/O° n/90° n)s, (90° nlO° n/90° n/O° n)s, and (±45° n/±45 ° n)s, have been chosen with appropriate stacking sequences so as to highlight individual and interacting failure modes. Four scale sizes have been selected for investigation including full scale size, 3/4, 2/4, and 1/4, with n equal to 4, 3,2, and 1, respectively. The full scale specimen size was 32 plies thick as compared to 24, 16, and 8 plies for the 3/4, 2/4, and 1/4 specimen sizes respectively.Results were obtained in the form of tensile strength, Stress/strain curves and damage development. Problems associated with strength degradation with increasing specimen size have been isolated and discussed. Inconsistencies associated with strain measurements have also been identified. Enhanced X-ray radiography was employed for damage evaluation, following step loading.It has been shown that fiber dominated lay-ups were less sensitive to scaling effects compared to the matrix dominated lay-ups. Further, it has been shown that fabrication induced damage was partly responsible for the observed behavior.Extrapolation to the full scale strength was attempted by means of three basic methods: a Weibull statistics based model, a fracture mechanics based model, and a combination model involving the previous two models in conjunction with a failure criterion. The predictive performance of each one of these models has been assessed and their applicability to the present problem has been discussed.

Consideration of concurrent transverse cracking and induced delamination propagation using a generalized micro‐meso approach and experimental validation

“…Now, using the strains of damaged lamina, the Young's modulus and shear modulus of the lamina with transverse cracks can be defined. These two damage parameters (d 22 , d ), which are the indication of transverse cracking and induced delamination, are used in the compliance matrix of lamina as presented in (11). This compliance matrix is employed in the continuum damage FEM code.…”

“…Effects of various damages on material degradation of composite laminates have been studied extensively using experimental and analytical approaches. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Micromechanics approaches including shear-lag method, 1-3 variational principles, 4,5 stress transfer mechanics 6,7 and mesomechanics approaches based on the concept of continuum damage mechanics (CDM) 8,9 have been proposed to predict the degradation of material properties in composite laminates due to transverse cracking and induced delamination. Many researchers also studied the effects of different damage mechanisms on composite laminates behaviour using experimental approach.…”

“…E-mail: Hosseini@aut.ac.ir investigated the effects of transverse cracking on stiffness degradation by experiments for [0/90] s cross-ply graphite/epoxy (T300/5208) composite laminates. Kellas and Morton 11 performed experiments on different [±45 n /0 n /90 n ] s stacking sequence for n = 1-4. They observed large strength/size effect, and a large reduction in strain to failure for laminates with larger values of n. Herakovich 12 performed experimental investigations to study the damage mechanisms of various graphite/epoxy composite laminates with different lay-up configurations.…”

Size effects on the bending strength of fiber-reinforced polymer matrix composites

“…Considerably few researchers used the Weibull theory to evaluate the size effect on composite materials and their work has shown a large range of Weibull parameter values. For instance, under tensile loading condition, the Weibull parameters ranged from 7.22 to 156, for various composite material types and layups, 15,16 and for flexural loading the Weibull parameters values ranged from 25 to 29. 4,17 The weakness of the Weibull model as presented in Equation ( 7), is that it does not consider the effect of each dimension (width, length, thickness), separately on the ultimate strength, also it does not weigh the effect of each of these dimensions, on the ultimate strength.…”

Scaling Effects in the Low Velocity Impact Response of Fiber-Metal Laminates

“…Recently, a number of studies have been undertaken in order to understand scaling effects in composite structures [4][5][6]. Wisnom [6] observed a size dependency when testing a carbon fiber-reinforced epoxy under four-point bending and in buckling.…”