Bridging stress

Fig. 6.18 Schematic representation showing how a reduction in the bridging force by interface wear leads to matrix crack growth. The fibers carry the applied load across the matrix cracks, reduing the crack opening displacement and the net stress intensity (Klip) at the tip of matrix cracks. If the interfacial shear stress t decreases during fatigue, then the bridging stress p(x) decreases, leading to a reduction in Kp. This reduction increases Kfy, which can cause the further extension of a matrix crack.

Brunner A.J., Blaclman, B.R.K., Williams, J.G. Deducing Bridging Stresses and Damage from Gic Test on Fibre Composites (at press). Proceedings 3 " ESIS TC4 Conference on Fracture of Polymers, Composites and Adhesives, Elsevier. 

DEDUCING BRIDGING STRESSES AND DAMAGE FROM G,c TESTS ON FIBRE COMPOSITES... 

Composite, delamination, bridging stress, damage, R-curve. 

This paper will analyse these variations in , and A with a view to rationalising the observations and then examine the A values to determine if they may be used to characterise the damage process via a damage factor and a bridging stress. Such parameters are additional items of information which may be deduced from the usual test which may prove useful in understanding damage in composites caused by interlaminar crack growth. 

Deducing Bridging Stresses and Damage from Gic Tests on Fibre Composites... 

Fig. 5. Bridging stress aand damage factor cj) for the uniaxial GF/PMMA material.

Fig. 7. Bridging stress aand damage factor tj) for the cross-ply CF/epoxy [0/90]6s material.

B = constant that depends on the bridging stress profile (approximately 6 for aliunina-SiC whisker composites)... 

In relation (4.27), Cq refers to the penny-shaped flaw size at the moment of crack initiation. Kq is an intensity factor chosen as Kq = 2 MPa.m. The bridging-zone length, Db, prior to critical fracture, has a bridging stress, p, assumed to be constant. For the derivation of Eq. (4.27), the reader should consult the work of Chien-Wei Li et al. [34]. The estimated p for the FG, medium and... 

Fig. 4.70 a Critical bridging stress decreases with grain width b Variation of bridging zone length with average grain width [34]. With kind permission of John Wiley and Sons... 

C.K.Y. Leung and V.C. Li, Effect of fiber inclination on crack bridging stress in brittle fiber reinforced brittle matrix , J. Mech. Phys. Solids. 40,1992,1333-1362. 

Figure 4.20 Bridging stress-crack opening curve,for fibre bridging over a crack (after Leung [47]).

Strength criterion to enable strain hardening with a modest amount of fibres. Eqs 12.9-12.12 outline the quantification of this criterion, which requires that the first crack strength will not exceed the maximum bridging stress of the crack in the characteristic stress-crack opening function. 

An essential element required to meet these criteria is to control the bond strength weak bond will result in low bridging strength and will compromise the first criterion, while bond which is too high will lead to a maximum bridging stress at small crack opening and will compromise the energy criterion. 

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