Figure 4.1 presents a schematic representation of the mode III STB test shown in two orientations. As indicated in Figure 4.1, the loading on the specimen was introduced via load blocks, load pins, and load tabs at the delaminated end, along with a center double roller and edge support, and an end roller and edge support. Each load block and load pin assembly is an integral unit and contains two load pins that are arranged diagonally. One of the load pins acts on one edge of the specimen near the delamination tip (evident in the lower load block in Figure 4.1a), and the other acts on the opposite edge closer to the delaminated end (evident in the upper load block in Figure 4.1a).
Figure 4.1b represents the simplified fixture used in this work. Here, the back (negative z) load block and load pin assembly shown in Figure 4.1b is directly threaded into the load cell, which is located in the top portion of a uniaxial load frame. The front (positive z) load block and pin assembly that is shown in Figure 4.1b attaches to a platen that is threaded into the actuator. The z-direction location of this front load block is adjustable. This is achieved via a slotted connection to the platen and allows specimens of various thicknesses to be accommodated. The center and end roller and support assemblies also attach to the platen and are adjustable in both the specimen’s width and thickness directions. Downward, or tensile direction movement of the actuator in Figure 4.1b corresponds to the loading shown in Figure 4.1a. Note that in the original STB (Figure 4.1a), only one load block was forced to translate and all other components were fixed, whereas in the implementation herein, that load block is fixed and all other components are forced to translate. However, it is clear that the two approaches produce the same loading on the specimen.
Figure 4.2. Specimen with load tabs.
Following Davidson and Sediles (2011), all specimens used in this work were 25 mm wide. The load tabs were also identical to those used in this earlier work, and appear as shown in the photograph in Figure 4.2. The load tabs are 38 mm long x 25 mm wide x 6.3 mm thick. They are bonded to the specimen using Hysol EA 9309.3 NA adhesive, which is a two-part epoxy containing 0.13 mm diameter glass beads to enforce a consistent and uniform thickness bond line. An 8 mm diameter threaded hole is in the center of the load tab. A bolt may be run through the center of the load block to attach to the load tab. This feature was instituted in the original STB specimens to accommodate the mode I loading and is retained here for possible use. The load pins mate to machined semicircular holes in the load tabs and extend through the tabs to nearly the plane of the delamination.
The nomenclature for the STB test is presented in Figure 4.3a. Here and subsequently, the term “STB” will be used to denote “mode III STB” as applied in a uniaxial load frame. The delamination in Figure 4.3a consists of a Teflon insert at the specimen’s mid-plane that spans the full width of the specimen. All specimens considered are unidirectional with their fibers oriented in the x-direction. The delamination length, a, is defined from the midpoint between load pins to the delamination tip. This datum point for delamination length is also used to define the half- span length, L, as shown. The specimen’s width is denoted by B and the thickness by 2h.
Load tab
Delamination Specimen
Machined holes
Figure 4.3b shows a plan view of a specimen containing preimplanted edge delaminations (EDs). The EDs extend a distance βB into the specimen, such that the width of the delamination front that advances during the test is W = B(1-2β). In the absence of EDs, the traction free boundary conditions on the edge surfaces of the specimen require that the mode III ERR, GIII,
goes to zero. This further requires that significant mode II stresses and an accompanying mode II ERR, GII, arise near the free edges to enforce equilibrium. That is, there is an intrinsic coupling
of mode II and mode III at a free edge under anti-plane shear loading (Bažant and Estenssoro, 1979; Nakamura and Parks, 1989; Dhondt et al., 2001; Buchholz et al., 2004). Davidson and Sediles (2011) addressed this in the original STB by introducing EDs. They studied the effect of the non-dimensional edge delamination length, β, on the ERR distributions and showed that choosing β = 1/16 provides an essentially uniform distribution of GIII across the specimen’s
width and produces reasonably small local and average values of GII. Further, mode III fracture
tests resulted in simultaneous advance of the delamination across the full width for these types of specimens. For this reason, specimens containing preimplanted EDs were also considered herein. However, the EDs significantly complicate specimen fabrication, and it is unclear whether the intrinsic coupling of the mode II and III components near the free edge of a specimen without EDs affect a region that is sufficiently large to prevent obtaining accurate values of GIIIc. This is
one issue addressed in the study that follows. To this end, specimens with and without EDs represent the two permutations of the STB test to be evaluated. All specimens containing EDs appeared as shown in Figure 4.1a and Figure 4.3b, and used β = 1/16. This was achieved by using an appropriate template for the preimplanted Teflon insert during specimen manufacture. Specimens without EDs were similar, but there was no preimplanted Teflon insert to the right of the delamination tip, as in the specimen of Figure 4.3a.
Figure 4.3. (a) Nomenclature and (b) edge delamination geometry.
The geometries described above are attractive because the work performed by Davidson and Sediles (2011) on the original STB can be directly transferred. However, a simpler test would be to eliminate the fixture mid-span and end supports. This test configuration can readily be visualized using Figure 4.1b. This results in a fixture that is similar to the MSCB test, in the sense that it consists solely of a split beam geometry with a shear load and restoring torque applied to the delaminated regions (Sharif et al., 1995; Cicci et al., 1995; Trakas and Kortschot, 1997; Szekrényes, 2009; 2011). However, the method of load introduction is different and perhaps somewhat simpler. Also, FE results indicate that the restoring torque that is produced by the load blocks in order to maintain the zero slope condition is less than the torque that is applied in the MSCB (Davidson and Sediles, 2011). In what follows, this configuration will be referred to as a split shear torsion (SST) test. In addition to its increased simplicity, the SST geometry allows thinner specimens to be tested in comparison to the MSCB, and more flexibility in the choice of delamination length, a, in comparison to the STB. As in the case of the STB, SST geometries will be considered with and without EDs. Thus, the four baseline configurations
Load pin End roller Delamination Center roller 2h B L L a y x z (a)
Geometry of insert at mid-plane βB βB y x (b) W
evaluated consist of STB tests with edge delaminations (ED) and without edge delaminations (NE), subsequently referred to as STB ED and STB NE tests respectively, and SST tests with and without edge delaminations, referred to as SST ED and SST NE tests.