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89 1. Coefficients of Static Friction a standard friction def- inition based on the contact force between particle/ particle and particle/surface. 2. Coefficients of Rolling Friction defines the resistance of the particles to tumbling/rolling past one another or over a surface, which could be due to shape irregularities (jagged- ness) or other material properties. 3. Coefficients of Restitution defines the degree of rebound Figure 10-4. Angle of repose test for engineered that particles have following a collision. aggregate material. 4. Particle Bond Strengths defines a bonding force between particles simulating glue or adhesion-type forces. Here this parameter was used to define the bonding of particles The higher angle of the wet samples results from increased wet together for simulating a turf layer. particle adhesion. This physical test eventually helped to calibrate the coeffi- These parameters drove the selection of suitable physical tests cient of rolling friction in the DEM model. of both the engineered aggregate and reinforced turf materials. Naturally, this collection of tests differed substantially from 10.3.3. Hydrostatic Triaxial Tests those of the crushable material candidates. Standard hydrostatic triaxial compression tests (Figure 10-5) were performed on the engineered aggregate at various con- 10.3. Testing Effort fining pressures in accordance with ASTM D2850. Cylindrical 10.3.1. Density and Dimension samples of the aggregate were enclosed in a flexible membrane Measurements and subjected to various hydrostatic confinement pressures. The specimens were then compressed uniaxially until the spec- The density and dimensions of the engineered aggregate imen bulged and deformed out of its original cylindrical shape. were measured for several hundred randomly selected aggre- The aggregate behaved much like a soil sample, which was gate particles. The particle size was found to follow a bounded in accordance with expectations. Values obtained from these normal distribution, with a diameter mean and standard tests included the elastic modulus of the material, the inter- deviation of 0.348 and 0.047 in., respectively. The particles nal angle of friction, and the effective particle-to-particle were essentially round, with an average max/min diameter cohesion. These tests further illustrated that the behavior of ratio of 1.24. the material was consistent with essentially non-deforming The density of the actual material, on a per-particle basis, particles and standard aggregate/soil behavior, thereby con- was 94.9 pcf, while the effective density of the continuum firming that DEM modeling of the material was appropriate. aggregate was 52.1 pcf. This leads to nominally 45% void space in the aggregate bed. The high void percentage and sphericity of the particles 10.3.4. Pendulum Tests lead to good drainage through the bed and effectively limit Test Apparatus the settling of the material over time. The density is about three times greater than that of typical EMAS cellular cement In addition to the small-scale laboratory tests, a larger-scale (18 pcf). The higher density leads to an increased sensitivity one-wheel bogy test was conducted using a large pendulum to speed in an arresting application; higher speeds will lead to test apparatus. The pendulum test apparatus featured a heavy higher loads on the landing gear. 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 10.3.2. Angle of Repose controlled by the release height. Angle of repose measurements (Figure 10-4) were performed The pendulum mass was fitted with a strut and wheel by overturning and raising a 5-gal bucket of the engineered assembly, which Figure 10-6 illustrates in an exploded view. aggregate material, allowing the material to spill and settle into The strut was instrumented with three load cells that meas- a pile. The sides of the resulting heap settled to the aggregate's ured loads at the connections. These three connection loads natural angle of repose. The test was repeated several times for were resolved into orthogonal vertical and drag loads on the the dry material, and again for wet material, which had been strut and wheel assembly. fully soaked and drained. The dry and wet angle of repose To reduce the number of variables in the design, a rigid measurements averaged to 18.7 and 19.4 degrees, respectively. aluminum wheel form was used rather than a pneumatic tire.

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90 Figure 10-5. Hydrostatic test specimen for engineered aggregate pre-test (left) and post-test (right), 10 psi confining pressure. The diameter and width were 14.8 and 5.5 in., respectively. After cutting the initial rut, the pendulum continued to These proportions were based on a nominally one-third scale swing back and forth until eventually coming to rest. Load B737-800 main-gear tire, which has a diameter and width of measurements were only considered for the initial pass 44.5 and 16.5 in., respectively. Although this wheel size was through the material. The final resting position and resulting smaller than full-scale, it was substantially larger than the typ- rut are illustrated in Figure 10-9. ical particle size; this ensured the continuum-type behavior Figure 10-10 shows the average drag and vertical forces of the aggregate that was necessary for characterization. during the pendulum swing for the different permutations of Below the pendulum assembly, a bed of aggregate was con- the test matrix. The error markers atop the bars indicate a structed with a length of 22 ft, a width of 8 ft, and a depth of 90% confidence interval, as an indication of the spread in the 22.5 in. Figure 10-7 shows the overall pendulum apparatus test results for the three repetitions of each case. The drag with the aggregate bed beneath it. force is defined as the rearward load on the strut, which would decelerate the aircraft (Figure 7.9). The vertical force is defined as the upward load, which would support the air- Test Matrix craft from sinking further into the aggregate. For the test series, the pendulum was set to swing such that Overall, both the speed and the moisture conditions affect the wheel penetrated to a depth of one-half diameter, or the outcome, but speed has the greatest effect. Reducing the 7.4 in. Two swing heights were used to produce speeds of 17.2 speed from 17.2 to 8.5 mph created a 65% reduction in the and 8.5 mph. For the high speed tests, both wet and dry beds drag force. Wetting the aggregate increased the drag force by were used. The wet bed was well soaked with water and 26%. allowed to freely drain prior to every test. Three repetitions These findings were consistent with initial observations and were conducted for each permutation tested, as shown in the assumptions regarding the aggregate as an arrestor medium. test matrix of Table 10-1. The imparted landing gear load is highly dependent on the speed of the aircraft overrun and somewhat dependent on the moisture content of the aggregate. Test Results The results of these tests were subsequently used to help Figure 10-8 gives an action sequence of photos for the pen- calibrate the coefficients of static friction and restitution for dulum strut cutting a rut through the material. the aggregate material.

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91 For tire overruns, it was assumed that the material would essentially act as a confining membrane layer atop the aggre- gate. It was further assumed that the through-thickness homogeneities could be neglected in the DEM models, where the turf layer would be represented using a homogeneous layer with equivalent membrane properties. This layer of par- ticles would use defined particle bonds to create a membrane- type behavior. With that end in mind, a simple tensile test was selected as the best all-around means of determining membrane per- formance. It produced results that could be transferred to bond strength values in the DEM modeling code. While the tests sought to characterize the material prop- erties for the turf layer, this was undertaken with fairly modest expectations for exactness. In actual application, the use of a turf layer presents several factors that could be difficult to control. The strength of the layer would be dependent on: Type of soil base used (sandy, clayey, etc.), Amount of reinforcement per unit area, Type of grass used (regionally dependent), Age/maturity of the grass, and Seasonal rainfall levels. Therefore, the tests sought only to establish a nominal strength level for the turf layer, which in practice could either be stronger or weaker for different arrestor beds, at different times of the year, and at different ages. Tests Conducted Figure 10-6. Pendulum test device with one-wheel bogy. The turf specimens began as 4-ft square samples, which were 4.6 in. thick with a nominal areal density of 30 psf. From these squares, test specimens were cut out in dog-bone shapes, which narrowed to an 8-in. wide neck that was 8 in. long 10.3.5. Tensile Turf Tests (Figure 10-11). The total specimen length was 14.25 in. These specimens were clamped in a special jig and attached to a ten- Overview and Assumptions sile testing device. The specimens were then pulled apart axi- The reinforced turf material used a sandy soil mixed with ally, and the load history was recorded. Three specimens were small swatches of reinforcing material. Grass seed was planted tested overall, with a typical failure strength of 120 lbf at in this soil base and grown to maturity. The resulting turf elongations of 4 in. layer had changing properties through its thickness, with grass on top and sandy soil on the bottom. 10.3.6. Environmental Tests Characterizing the reinforced turf required non-standard testing procedures. Several different test methodologies were Environmental tests were considered for the engineered considered, including punch tests, tensile tests, and tearing aggregate and turf materials. However, after consideration of tests. At the small scale, the soil and reinforcement swatch the nature of both materials, as well as the body of existing mixture presented problems of heterogeneous properties, research information, it was determined that such testing which eliminated some of the punch test concepts from con- would not be included in the research effort. A simple set of sideration. To ensure a generally homogeneous behavior, the environmental tests was not useful, and a comprehensive set specimen size needed to be fairly large. of tests was not affordable within the current effort.

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92 Figure 10-7. Overview pictures of engineered aggregate pendulum test setup. Neither material degenerates under environmental condi- Perhaps the most relevant issue is that of ice crust forma- tions in a manner analogous to crushable materials, such as tion on an arrestor bed during winter conditions. Rainfall dur- the currently used cellular cement. Both the turf and aggre- ing freezing conditions could lead to water penetration and gate are durable to freezethaw, rain, and heat. Obviously, freezing in the upper layer of the aggregate and turf. However, turf can degrade over time during periods of drought, which freezing at deeper levels may be self-limiting due to insulating would lead to impacts on strength as discussed in Section properties of the aggregate bed. Prior research by Rogers dis- 10.3.5. However, characterizing such impacts would require cusses cold-weather testing information that demonstrates the a very substantial body of tests using multiple hydration con- potential for aggregate arresting beds to freeze over and become ditions, soil types, and grass types. ineffective in harsh winter environments (13). In the event of such freezing, we can deduce several areas Table 10-1. Pendulum test matrix of performance impact. First, the particles would effectively for engineered aggregate. be adhered to one another, which would create a solid block Number of Tests Conducted of material that an overrunning tire would likely break apart. After breakage of the ice bonds, however, the void spaces High Speed Low Speed between particles would still be filled with ice and water, Dry Aggregate 3 3 increasing the rolling friction of the particles and decreasing Wet Aggregate 3 - their effective flow around the tire. This would likely result in

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93 Figure 10-8. Pendulum wheel passing through engineered aggregate material (upper left is first frame, lower right is last frame). increased drag and/or vertical loads on the tire. Overall pen- and freezethaw conditions. Further, this would need to be etration of the tire could prove similar to that in a normal followed for the two different drainage and waterproofing bed, or it could be diminished. However, predictability of the approaches to the arrestor bed. In the former, the ice crust total performance impact is difficult since the degree of such could penetrate below the turf layer; in the latter, it should be crusting would be variable. confined to the turf alone. To assess such effects on performance, a larger test bed Larger testing as described above was beyond the scope would have to be constructed and subjected to artificial rain of the current effort. However, it would be advisable if the 1,200 1,000 800 HS Dry 600 HS Wet 400 LS Dry 200 0 Drag Avg Force (lbf) Vertical Avg Force (lbf) Figure 10-10. Plot comparing strut forces for Figure 10-9. Post-test view of rut in engineered high-speed dry, high-speed wet, and low-speed dry aggregate created by pendulum wheel. conditions.