Two different models come out to deal with the referred damage modes. Such interaction is fundamental to be considered in a failure model prediction because one mode may initiate the other and they may intensify each other. The interaction between these two modes of damage constitutes a complex damage mechanism that has not been addressed at a realistic level. Matrix crack and delamination are intrinsically associated in composite materials, namely under bending loads. DE MOURA, in Delamination Behaviour of Composites, 2008 11.4 Conclusions Interaction of matrix cracking and delamination When composites have complex fiber architectures, as in 2D woven composites, or the 3D architectures discussed in Chapter 3, quantification of matrix crack density is extremely difficult indeed. For example, even in simple lay-ups (such as (±45/90) s), matrix cracks in the 90° plies can initiate short cracks in adjacent ±45° plies in such cases, characterizing the crack density accurately-if this is required-then becomes complicated.

Quantification of matrix cracking damage can be difficult except in the simplest cases. However, where cracks are discontinuous, an equivalent measurement is often useful, that is, the cracked area per unit volume of the cracked plies. For cracks that extend across the width of a flat planar specimen, a simple measure used is the number of cracks per unit length of specimen, often called D, the crack density this is also the reciprocal of the average crack spacing, that is, 1/(2s). To characterize the extent of matrix cracking damage and its relationship to changes in composite properties, a quantification of the cracks is necessary. Consequently, if the characterization of crack initiation in the absence of free edges is desirable, it is preferable to use specimens without free edges (e.g., the tubular specimens, which are described in Chapter 4). They often initiate at the free edges of a test specimen where the stress state differs to that away from the edge.

Matrix cracks normally extend across the thickness of the ply (or plies with a common orientation, e.g., a group of 90° plies) but do not necessarily extend across the width of the coupon, or component. Zangenberg, in Modeling Damage, Fatigue and Failure of Composite Materials, 2016 1.5.3.2 Observation and quantification of matrix cracks

Macdown matrix crash