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.

Scaling Effects in Sublaminate-Level Scaled Composite Laminates

“…2 In agreement with the behavior of many brittle materials, a strength degradation was observed with increasing specimen size. The strength degradation with increasing specimen size was more pronounced in the matrix-dominated layups where off-axis plies carry a significant amount of the load.…”

“…ATb c E 2 E c (ct c -a 2 )°* = ifr+U (2) where E c , b c \ and a c are the stiffness, thickness, and thermal expansion coefficient, respectively, of the constraining plies. The parameters E 2 ,t, and a 2 are the stiffness, thickness, and thermal expansion coefficient, respectively, of the 90-deg plies; and AT" is the difference between operating and stress-free temperatures.…”

Defect controlled transverse compressive strength of polyethylene fiber laminates

“…While cross-ply CFRPs loaded in through thickness compression also fail by fiber tensile rupture (Gan et al, 2014;Kim et al, 2010), the dependence of this strength upon sample thickness has not been ascertained. Studies by Jackson et al (1992) and Kellas and Morton (1992) and a review by Sutherland et al (1999) show a decrease in the tensile and flexural strength with increasing sample size. Weakestlink' statistical models (i.e.…”

Measuring the notched compressive strength of composite laminates: Specimen size effects

“…More detailed study that examined the behaviour of the unnotched UD and MD specimens of the same composite system has appeared in [15]. 4. Mistype has been corrected and scale is giving in Fig.4.…”

“…Although there is a significant, but inconclusive, amount of evidence that there is a size effect in composites under tensile and flexural load [2][3][4][5][6][7], scaling of composite strength is not well documented or understood. Most of the research to date has looked at the size effect on the unnotched rather than notched strength under unidirectional tensile [4][5][6][7] or compressive loading [8][9][10]. The results show the importance of ply thickness and the interactions between delamination and matrix cracking with the fibre direction failure modes.…”