Experimental Overview

4.4.1. Materials and Manufacturing Procedures

The experimental portion of this study primarily utilized 26-ply IM7/977-3 and 18-ply T800S/3900-2B test specimens. These were chosen based on the results in Davidson and Sediles (2011), and produced specimens of both materials that were nominally 3.3 mm thick. A limited number of 32-ply IM7/977-3 specimens were also fabricated and used for exploratory testing. All test specimens were fabricated at the SU-CML using an autoclave and the manufacturer’s recommended cure cycle.

As described above, specimens were fabricated with and without EDs, and with a variety of preimplanted insert lengths. All specimens that contained EDs were tested with a delamination length of 32 mm. These were generally fabricated following the procedure described in Davidson and Sediles (2011). Here, a 12.7 μm Teflon template is cut and placed at the mid-plane of a 330 mm long (fiber direction) x 305 mm wide plate during manufacture. The template is such that it will produce 10 specimens containing starter delaminations and EDs. Following plate

manufacture, a sequential c-scanning and cutting procedure is performed to obtain the specimens, each of which is nominally 25 mm wide with β = 1/16. For NE specimens,

manufacture is simpler and only requires that a rectangular 12.7 μm Teflon insert of the desired length be placed at the mid-plane during plate fabrication. These plates produced 20 NE

specimens with B = 25 mm. A Teflon template was also created such the 5 ED and 10 NE specimens could be obtained from the same plate. As described subsequently, this allowed toughness results from ED and NE specimens from the same plate to be compared, thereby eliminating any possible effect of plate-to-plate property variations.

After final cutting, all specimens are c-scanned to locate the distance from their

delaminated end to the end of the Teflon insert. An alignment jig is then used to bond rectangular (non-machined) low-carbon steel load tabs to the specimen at a location that will result in the desired delamination length. Four semi-circular cut-outs are then machined through the tabs and specimen to create the configuration of Figure 4.2. Specimens are compressed through their thickness to prevent delamination growth during machining. Following the above, specimens are again c-scanned to obtain a pre-test scan that contains the tab, and therefore to which post-test c- scans can most accurately be compared.

Figure 4.5. Test fixture setup. (a) SST front view, (b) side view, (c) STB back view.

4.4.2. Fixture Design

Photographs of the SST and STB fixtures are presented in Figure 4.5. Figure 4.5a presents a front view of the SST fixture containing a specimen. A small amount of the (pink) adhesive used to bond the load tabs is evident just outside of the gripping arrangement. The tabbed specimen is sandwiched in-between two load block/grip assemblies. These are integral units, each of which contains two load pins and a backing plate that presses against the outer surface of the load tab. The upper assembly connects to the load cell and remains stationary

Platen Load pin

Specimen Lower load

block/grip Displacement transducer Upper load block/grip (b) End support assembly Specimen Center roller/ support assembly Upper load block/grip (c) Platen Load pin Specimen Lower load block/grip Upper load block/grip (a) y x z y x z y x z

during testing. The lower assembly is mounted to a platen that connects to the actuator and is displaced downwards during the test. As shown in the figure, this lower load block/grip assembly is bolted to the platen through slots. This allows it to slide during specimen installation and thereby to accommodate specimens of different thicknesses.

Figure 4.5b shows the view looking from the left side of Figure 4.5a, and is identical for both the SST and STB tests. A displacement transducer is evident in this figure and is utilized to measure the movement of the delaminated region outside of the grips for both tests. The

displacements measured by this transducer were found to be essentially the same as those from the actuator, indicating the desired zero slope boundary conditions were maintained for all specimens tested. Thus, in what follows, all displacement measurements are as obtained from the actuator.

Figure 4.5c shows the back view of the STB fixture. The upper load block/grip assembly and its attachment to the load cell adapter are visible in the right of the figure. The lower load block/grip assembly is behind this and attaches to the platen. As is evident from a comparison of this figure to Figure 4.5a, the STB fixture is obtained by bolting a center roller/support assembly and an end support assembly to the configuration shown in Figure 4.5a. Both of these assemblies bolt to the platen through slotted holes in order to allow specimens with different thicknesses to be tested. The top of the center roller/support assembly is a separate piece that is bolted onto the two sides of the center roller support and which contains the center edge support, comprised of a captive 6.4 mm diameter steel rod. The edge support at the end of the specimen is also a 6.4 mm diameter rod and is visible in Figure 4.5c near the lower left corner of the specimen. As

described previously, when the actuator is lowered, the lower block/grip assembly, the center roller/support assembly and the end support assembly all displace downwards uniformly.

4.4.3. Test Procedures

The procedures to install tabbed specimens into the grips are similar for all configurations and were developed to maximize reproducibility. A specimen is initially placed into the upper grip. The load tabs are aligned with the tab cut-outs and the tab is lightly pressed against the backing plate. The lower (platen-mounted) grip is then slid into place until its load pins mate with the cut-outs and the backing plate contacts the tab. Here, we endeavor to ensure that no gaps are observable between either load tab and its associated backing plate and that there is little or no through-thickness compression acting on the delaminated region. For SST tests, this

completes the procedure. For STB tests, the center double rollers are then adjusted to contact the specimen and are bolted into place. Next, the specimen is preloaded to approximately 1000 N, at which time the center edge support is aligned to be perpendicular to the specimen, but not yet secured. This alignment is performed under load to eliminate any movement of the specimen. The specimen is then unloaded and the support tightened into place. The height of the edge roller at the end of the specimen is then adjusted via shims to contact the lower edge, and the end roller that contacts the specimen along its width direction is slid and bolted into place such that light contact is maintained.

All tests were performed under displacement control at a loading rate of 2.0 mm/min for loading and 3.8 mm/min for unloading. After testing, c-scans were again performed to assess whether the delamination front advanced in its entirety and, in those instances where growth did not occur along the entire delamination front, to ascertain where advance occurred. Post-test destructive assessments were also conducted to measure the preimplanted Teflon insert size and therefore to validate or correct the pre-test delamination length measurement and, for specimens with edge delaminations, to measure the actual edge delamination width.

4.4.4. Material Property Determination

As discussed in Davidson and Sediles (2011), Johnston et al. (2014), and subsequently, the accuracy of the data reduction approach will be quite sensitive to the accuracy in material property determination; in particular, to the values of E11 and G12. Therefore, similar to the

approach used in Davidson and Sediles (2011), two specimens were cut from the non- delaminated portions of each plate that was manufactured. These specimens were used to determine E11 in accordance with ASTM D3039 (2014) and using the procedure given in

Appendix A. Relatively tight distributions were obtained, with coefficients of variation (CVs) on the order of 2% for specimens taken from all plates manufactured from a given material. Testing to determine G12 was conducted according to ASTM D5379 (2012) and using the

procedure given in Appendix B. Here, v-notched beam specimens were cut from an additional plate fabricated specifically for the purpose of determining G12. Shear modulus testing produced

distributions with CVs on the order of 6% for each material. The experimentally determined values for these two properties correspond to those listed in Table 4.1 for both IM7/977-3 and T800S/3900-2B, and were used in the reduction of all data.