Implication for Mode III SST Testing

For conventional SST laminates layups, where the orientation of the plies is aligned with the direction of “macroscopic delamination advance,” the first fracture events that occur will consist of the initiation and growth of cracks at an inclination of approximately 45° to the delaminated plane. With increasing load, these cracks will increase in number, grow, and branch along the plane of the preexisting planar delamination. What is typically taken as the

these processes, rather than to a simple planar advance of the delamination along its original plane. Thus, similar to what has been observed in other materials, delamination advance in laminated polymeric matrix composites occurs via segmentation of the delamination front into multiple crack fronts. This produces a relatively rough surface with a “sawtooth” (Pons and Karma, 2010) profile that the geologic fracture literature refers to as echelon cracking (Pollard et al., 1982; Roering, 1968) and the literature on fracture of homogeneous materials refers to as being comprised of “lances” (Knauss, 1970; Sommer, 1969), “river lines” (Hull, 1995) and/or “facets” (Goldstein and Osipenko, 2012; Mróz and Mróz, 2010; Pons and Karma, 2010;

Greenhalgh, 2009; Lazarus et al., 2001). The above growth behaviors bear some resemblance to what occurs during mode II delamination toughness testing of unidirectional specimens, where a “mode II delamination” also consists of a linking and coalescence of mode I events. The

difference is that the plies bounding the delamination in a unidirectional mode II specimen constrain the microcracking to the interlaminar region (O’Brien, 1998), whereas in conventional SST specimens the bounding plies allow the microcracks to develop into intralaminar transverse cracks.

In terms of application to delamination toughness testing, the fracture surface evolution in SST specimens is clearly quite different than that assumed to occur. All current mechanistic models used to extract a mode III delamination toughness (including the assumptions used in a compliance calibration method of data reduction) are therefore invalidated, and the critical values of ERR that are obtained are inaccurate. Thus, a true delamination toughness cannot be extracted with this method or any method that assumes an uncracked matrix. In this light, the observed variations in apparent GIIIc with delamination length in SST tests are perhaps not

calculated ERR may be reasonably accurate prior to the onset of transverse cracking, and it is possible that the apparent delamination toughness may still reflect a measure of the energy expended in the overall fracture and growth processes. The fact that it has not been possible to correlate the apparent toughness with the development of different fracture surfaces may be due to either (1) errors in extracting toughness from SST tests due to the data reduction method used, or (2) a missing piece to the understanding of fracture mechanisms in SST testing.

5.8. Conclusions

This chapter described a study to determine the manner in which the progression of damage and development of fracture surfaces occurs in the mode III split shear torsion

delamination toughness test. To this end, specimens from two different carbon/epoxy materials were tested in an SST fixture using an improved test geometry compared to that introduced in Chapter 4. A series of tests examining damage progression found that the first inelastic event consists of the initiation of near-tip matrix cracks within the resin rich region between plies. These cracks were inclined to the direction of loading and were perpendicular to the direction of maximum tensile stress. With increasing load, they were observed to extend into the neighboring plies above and below the plane of the delamination as well into the uncracked region ahead of the original delamination front. A network of crack branches that were essentially parallel to the delamination front was also observed to develop. All of these processes occurred prior to any observations or indications of planar delamination advance, which helps explain the apparent delamination toughness dependency initially described in Chapter 4. Thus, what has heretofore been referred to as “mode III advance” in laminated composites actually reflects an intrinsically coupled process of near-tip matrix crack formation and growth that occurs prior to any advance

of the planar macrocrack, and is quite similar to processes previously identified in homogeneous and geologic materials.

A series of tests examining fracture surface evolution produced clear differences that caused the fracture surfaces that develop in composite laminates to differ from those in

homogeneous materials. The first is that composite laminates contain energetically preferential fracture paths along interlaminar interfaces. The second is that the laminate’s fibers constrain the fracture surface from twisting as the planar delamination advances. The third is that the amount by which transverse cracks can extend in a laminate is determined by the proximity of the preimplanted insert to the laminate’s free surfaces.

As a direct consequence of the events determined in damage progression testing, ERR predictions that are based on any mechanistic model or set of assumptions that considers an uncracked matrix will no longer be accurate once any appreciable amount of near-tip damage has occurred. However, it has not been possible to correlate the damage observations presented in this chapter to the measured variations in GIIIc(a). It therefore appears the dependency of

apparent toughness on geometry has not been fully explained and could be due to errors in the toughness data reduction technique or a lack of full understanding of the mechanisms of fracture in these specimens. This will be addressed in Chapter 6.