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66 The subsequent sections of this chapter will focus on areas 9.3.1. Density and Dimension Measurements (1), (4), and (5). Specific attention will be given to the glass Several densities of the glass foam material were examined. foam material testing that was conducted and the calibration The higher density samples had higher compressive strengths. of the computer models to match the tests. The development Preliminary screening indicated that the lowest density spec- of the tire models (2) and aircraft model (3) will be reserved imens, at nominally 6 pcf, were most promising for this appli- for Appendix F and Appendix G, respectively. cation. The remainder of the testing discussion is confined to this density of the material. Specimens were obtained in 9.3. Testing Effort cylindrical shapes with 3.65- and 5.625-in. diameters and larger 24 × 18 × 5-in. blocks. The testing effort for the glass foam material involved an extensive battery of mechanical and environmental tests. Table 9-1 depicts the overall test matrix for the glass 9.3.2. Platen Compression Tests foam material. 18.104.22.168. Low Rate All cylinder dimensions specify diameter followed by height. Conditioned specimens were environmental test specimens Platen compression tests were performed for the initial mat- that were compression-tested following freezethaw exposure. erial density screening, enabling a rapid down-selection to the These cylinders were only 2.5 in. tall to maintain the aspect ratio 6 pcf density as the most likely candidate. The platen compres- of prior "short" cylinder specimens. The specimen diameter sion tests further permitted the evaluation of the energy absorp- was constrained by the environmental test apparatus. tion capacity of the material and the effects of loading rate. Table 9-1. Test matrix for glass foam material. Test Properties Detail Number of Tests Characterized Laboratory Tests Hydrostatic · Compressive strength at · Per ASTM D2850 0 psi 1 Triaxial failure (u) · 3.65 x 8" cylinder 5 psi 2 Compression · Shear strength (u) · 0.0592 in./min compression rate Test · Maximum compression of 5 to 10% 10 psi 2 · Confining pressures of 0, 5, 10, 20 psi 2 and 20 psi Total: 7 Parallel Platen · Compressive strength at · Version 1: low speed, tall Fresh Compression failure (u) · Non-standard Capped: 2 Test · Compressive stress · 3.65 x 8" cylinder strain curve · 3 in./min compression rate Uncapped: 3 · Extrapolated: volumetric · Maximum compression of 85% Conditioned energy capacity · Unconfined flat disk specimen · Determined for different compression Uncapped: 2 strain rates · Version 2: low speed, short Capped: 2 · Non-standard Uncapped: 3 · 5.625 x 4" cylinder · 3 in./min compression rate · Maximum compression of 85% · Version 2: high speed, short Capped: 2 · Non-standard Uncapped: 3 · 5.625 x 4" cylinder · 60 in./sec compression rate · Maximum compression of 70% Punch · Combined compression · Non-standard 5 Compression and shear strain · 5" thick material block Test behavior · 1.5" punch diameter · Somewhat similar to · 3 in./min compression rate EMAS material testing · Maximum compression of 70% method Environmental · Durability to freeze · Per ATSM C 666/C 666M-03 2 Chamber Tests thaw cycles (modified) · Effectiveness of sealant · 3.65 x 2.5" cylinder treatments, where · 78 freeze-thaw cycles applicable Pendulum Test Pendulum One- · Dynamic response for · Non-standard 2 Wheel Bogey the material to a · 16.9 mph collision Test moderate-speed tire · 24 x 18 x 5" blocks, glued into overrun characterized stacks
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67 Figure 9-4. Low-rate uncapped platen test of glass foam pre-test (left) and post-test (right). Low-rate platen tests were conducted by placing a cylindrical 22.214.171.124. High Rate specimen between two metal platens. The upper platen was then displaced downward at a fixed rate of 3 in. per minute High-rate compression tests (60 in.-s) were conducted on until the specimen was fully compressed. short cylinders (5.625 × 4 in.) to evaluate the effects of rapid The platen tests were conducted with and without Sor- compression on the material. For a B747 main-gear tire travel- bothane capping material on the top and bottom of the spec- ling at 70 knots, the average vertical compression rate would be imens to determine whether capping was beneficial or not. 40 in./(in.-s). The test produced an average strain rate of 150 in./ For the uncapped specimens, the material crushed in a local- (in.-s), which exceeds the overrun case by a substantial margin. ized failure zone immediately adjacent to the platen. These When comparing the high- and low-rate loading curves, specimens maintained their shape throughout the duration only a minimal difference is observed, which could be due only of the loading up to the final 85% compression state, which to inertial effects of the material. Overall, the high-rate platen aided in capturing the full energy absorption of the material (Figure 9-4). When caps were used, the specimens tended to split along vertical planes, causing a shape change that often led to early terminations of the compression test (Figure 9-5). The un- capped approach was eventually favored due to its superior test repeatability and its ability to capture the full energy capacity of the material. Tall and short cylinder specimens were tested for compar- ison with the hydrostatic triaxial tests, which used tall cylinders (3.65 × 8 in.), and the high-rate platen tests, which used short cylinders (5.625 × 4 in.). The glass foam material exhibited a characteristic crush- able foam load history that rose to a plateau value, where it remained until the material approached full compression (Figure 9-6). Near full compression, the material hardened and the loading increased. The stressstrain curve was integrated to produce the energy absorption curve shown in the figure. Figure 9-5. Mid-test splitting of low-rate capped No measurable rebound occurred after compression. platen test cylinder of glass foam.
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68 80 48 70 56 70 42 60 48 Energy Absorption [psi/in2] Energy Absorption [psi/in3] 60 36 50 40 50 30 Stress [psi] Stress [psi] 40 32 40 24 30 24 30 18 20 16 20 12 Stress, Average of Test Data Energy Absorbed Per Unit Volume 10 Stress, Average of Test Data 6 10 8 Energy Absorbed Per Unit Volume 0 0 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Strain [in./in.] Strain [in./in.] Figure 9-6. Average load history for glass foam Figure 9-7. Average load history for glass foam material from low-rate platen tests, tall specimen. material from high-rate platen tests, short specimen. tests indicate that there is little to no practical rate effect in the material for the loading regimes of interest (Figure 9.7). The specimens were tall cylinders (3.65 × 8 in.) placed between platens using Sorbothane caps at the top and bottom (Figure 9-8, left). The specimens were fitted with flexible 126.96.36.199. Hydrostatic Triaxial Tests membrane sleeves before immersion in a pressurized vessel of The hydrostatic triaxial tests evaluated the glass foam per- water. While at this hydrostatic pressure, the specimens were formance at different confining pressures to determine if any compressed axially until failure. strength increase or bulking deformation took place. Neither As observed in the platen tests, the use of capping induced was anticipated because the glass foam material behaved as a vertical failure plane formation in the specimen (Figure 9-8, one-dimensional foam with little to no observed lateral bulking right), which typically occurred at a compression displacement (low Poisson ratio). of about 5%. This failure load did show a mild dependence on Sorbothane Cap Figure 9-8. Hydrostatic triaxial compression test of glass foam pre-test (left) and post-test (right).
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69 Figure 9-9. Punch test of glass foam test specimen pre-test (left) and post-test (right). confinement pressure, though no bulking behaviors were similar to the characterization tests performed by the current observed. EMAS manufacturer (34). However, the tests differed with regard to the proportional aspect ratio of the punch, the punch shape, the specimen size, and the loading rate. 9.3.3. Punch Tests The resulting load was due to a combination of the com- Punch tests were performed by pressing a 1.5-in. diameter, pression strength of the material (area under the punch) and smooth-sided punch into a 5-in. thick block of the material the shear strength of the material (circumferential edge of the (Figure 9-9). The glass foam material compressed cleanly, punch). Figure 9-10 illustrates the averaged load history for leaving a smooth-sided hole in the block. These tests were the punch tests. As shown, the effective "pressure" on the 200 100 180 90 160 80 140 70 Energy Absorption (psi/in3) 120 60 Pressure (psi) 100 50 80 40 60 30 40 20 Compression Force Average 20 Energy Absorption 10 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Strain (in./in.) Figure 9-10. Average load history for glass foam material from punch test with stress normalized for cross-sectional area of 1.5-in. diameter punch.
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70 punch was nominally 140 psi, which compares at 2.5 times the nominal 55 psi strength of the material exhibited in the platen tests (Figure 9-6). This increase in apparent strength is due to confinement and shear contributions. In arrestor applications, the shear strength of the material would be rele- vant along the vertical walls of the tire rut. 9.3.4. Pendulum Tests 188.8.131.52. Pendulum Apparatus In addition to the small-scale laboratory tests, a larger-scale one-wheel bogy test was conducted using a large pendulum test apparatus. The pendulum test apparatus featured a heavy 4,400-lb mass that hung from an overhead support frame, giving it a swing arc of 24.5 ft. The mass was hoisted to the desired height and then released; the speed of the mass was controlled by the release height. The pendulum mass was fitted with a strut and wheel assembly, which Figure 9-11 illustrates in an exploded view. The strut was instrumented with three load cells that measured loads at the connections. These three connection loads were resolved into orthogonal vertical and drag loads on the strut and wheel assembly. To reduce the number of variables in the design, a rigid aluminum wheel form was used rather than a pneumatic tire. The diameter and width were 14.8 and 5.5 in., respectively. These proportions were based on a nominal one-third scale B737-800 main-gear tire, which has a diameter and width of 44.5 and 16.5 in., respectively. 184.108.40.206. Glass foam retention box. Figure 9-11. Pendulum test device with one-wheel Below the pendulum assembly, a box was constructed to bogy. retain larger blocks of the glass foam, which each measured 24 × 18 × 5 in. The blocks were arranged in one row, 6 blocks long and 2 blocks in depth, for a total of 12 blocks. The over- all dimensions were 9 ft in length, 2 ft in width, and 10 in. in Figure 9-13 shows the rut created through the material by height. The upper and lower blocks were glued in 6 pairs, the strut and wheel of the pendulum; the wheel cut a clean which were placed in the retention box and held in place by path through the material, leaving the RG-1 material adjacent an upper cap rail around the perimeter. The block pairs were to the rut essentially undamaged. This behavior was consistent not adhered to one another. Figure 9-12 shows the overall with expectations based on the small-scale testing. pendulum apparatus with the glass foam blocks beneath it. 220.127.116.11. Landing Gear Loads 18.104.22.168. Tests Executed The loading history for the pendulum strut showed oscil- For the tests, the pendulum was set to swing such that the lations in both the drag and vertical loads (Figure 9-14). It is wheel penetrated to a depth of nominally one-third diameter, believed that these pulses were caused by the seams between or 5.0 in. One swing height was used to produce an average the block pairs in the bed. As the wheel approached the end of overrun speed of 16.9 mph. Due to the single-use nature of a block, the load decreased; when the wheel began to overrun the material and the limited supply of evaluation samples, the next block, the load increased. Since the blocks were not two tests were conducted for these conditions. Both tests gave glued at the seams, these joints presented a discontinuity in consistent results. the material. The pulsing effect was further amplified by the
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71 Pendulum Mass Retention Box with Glass Foam Blocks Strut and Wheel Assembly Cap Rail Figure 9-12. Overview of pendulum test setup for glass foam. Figure 9-13. Post-test results from pendulum test for glass foam.