Evaluation of water degradation of vinylester and epoxy matrix composites by single fiber and composite tests

Abstract

Degradation of the mechanical properties of vinylester and epoxy matrix composites exposed to water has been approached by monitoring the strengths of glass and carbon fibers and resins. In addition, the fiber/matrix (F/M) interface strengths and debond lengths of single-fiber composites were determined and test results were compared to test results of macroscopic composite specimens. The single-fiber tensile test results indicate a substantial loss of the tensile strength of glass fibers and the fragmentation tests reveal loss of F/M shear strength and substantial debonding for both glass and carbon fiber composites after water exposure. The transverse strengths of the composites are also degraded to large extents. The tests results identify water degradation of the F/M interface as a major strength limiting mechanism.

Appendix: analysis of wicking

Consider a polymer matrix composite. At dry conditions, the weight of the composite, W _c,dry, is given by:

$$ W_{{{\text{c,dry}}}} = W_{{\text{m}}} + W_{{\text{f}}} , $$

(A1)

where W _f and W _m are the weights of the fibers and matrix, respectively, in the composite,

$$ W_{{\text{f}}} = \rho _{{\text{f}}} V_{{\text{c}}} v_{{\text{f}}} , $$

(A2a)

$$ W_{{\text{m}}} = \rho _{{\text{m}}} V_{{\text{c}}} v_{{\text{m}}} , $$

(A2b)

where ρ _f and ρ _m, ν _f, and ν _m are the fiber and matrix densities and volume fractions, respectively, and V _c is the volume of the composite.

At the maximum moisture content, the weight of the composite W _c,max, is given by:

$$ W_{{{\text{c,max}}}} = W_{{\text{m}}} + W_{{\text{f}}} + W_{{{\text{w,abs}}}} , $$

(A3)

where W _w,abs is the weight of water absorbed by the composite,

$$ W_{{{\text{w,abs}}}} = W_{{{\text{c,dry}}}} M_{{\text{c}}} , $$

(A4)

and M _c is the composite maximum moisture content. Notice that the composite can absorb water through the matrix and interface,

$$ W_{{{\text{w,abs}}}} = W_ {\rm wm} + W_{\rm wi},$$

(A5)

where W _wm and W _wi are the weights of the water absorbed by the matrix and interface, respectively. The weight of the water absorbed by the matrix is given by:

$$ W_{\rm wm} = \rho _{\rm w} V_{\rm c} v_{\rm m} M_{\rm m} $$

(A6)

where ρ _w is the density of water and M _m is the maximum moisture content for a neat resin specimen.

Therefore, the weight of water absorbed by the interface (wicking) is given by:

$$ W_{{{\text{wi}}}} = W_{{{\text{c,dry}}}} M_{{\text{c}}} - \rho _{{\text{w}}} V_{{\text{c}}} v_{{\text{m}}} M_{{\text{m}}} . $$

(A7)

Finally, the weight fractions of the water absorbed through the matrix, w _wm, and by wicking, w _wi, are given by:

$$ W_{{{\text{wm}}}} = \frac{{\rho _{{\text{w}}} v_{{\text{m}}} }}{{\left( {\rho _{{\text{f}}} v_{{\text{f}}} + \rho _{{\text{m}}} v_{{\text{m}}} } \right)}}\frac{{M_{{\text{m}}} }}{{M_{{\text{c}}} }}, $$

(A8a)

$$ W_{{{\text{wi}}}} = 1 - \frac{{\rho _{{\text{w}}} v_{{\text{m}}} }}{{\left( {\rho _{{\text{f}}} v_{{\text{f}}} + \rho _{{\text{m}}} v_{{\text{m}}} } \right)}}\frac{{M_{{\text{m}}} }}{{M_{{\text{c}}} }}. $$

(A8b)