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45 Table 16. NCHRP Report 350 criteria for MGA test. TTI Test MGA MGA 405421-1 C08C3-027.1 C08C3-027.2 Impact Conditions Speed (mph) 63.1 30.0 62.1 Angle (deg) 25.5 26.0 25.5 Exit Conditions Speed (mph) 34.2 12.9 43.2 Angle (deg) 16.0 11.4 18.7 Occupant Kinematics Longitudinal OIV (m/s) 7.1 3.8 6.1 Lateral OIV (m/s) 4.4 3.3 3.7 Longitudinal Ridedown (G) -7.9 -3.3 -6.1 Lateral Ridedown (G) 8.4 -1.9 -5.6 Vehicle Kinematics 50 ms X Acceleration (G) -5.3 -2.9 -5.5 50 ms Y Acceleration (G) 4.3 -2.6 -3.1 50 ms Z Acceleration (G) -4.8 1.8 -4.1 Guardrail Deflections Dynamic deflection (ft) 3.3 1.4 7.2 Static Deflection (ft) 2.3 1.2 3.3 Vehicle Rotation Maximum Roll (deg) -10 Not reported 29.7 Maximum Pitch (deg) -4 Not reported 12.1 Maximum Yaw (deg) 42 Not reported -10.2 outcome, i.e., redirection vs. vaulting, as well as the increased 11 inches of deflection. A small number of simulations with deflection of the supporting posts. rail deflection only were also conducted for 3 and 6 inches of deflection. Higher extents of rail only deflection were not con- sidered since it was unlikely that the posts would be unaffected 10.2.4 Conclusions of the Crash Tests as well. Rail and post deflection is typically produced by a low- The outcome of the crash tests demonstrated that there severity impact. Therefore, the best way to reproduce this are limits to the amount of damage that can be sustained by damage would be to simulate such an impact. Low-speed guardrails while still maintaining its functional capacity. The impacts in the range of 3060 kph (18.637.3 mph) with an test series showed that a guardrail with 14.5 inches (368.3 mm) impact angle of 25 degrees were used to cause 3, 6, 9, and of rail and post deflection in a guardrail represents an unaccept- 11 inches of deflection in the rails, often with concurrent post able condition that warrants high-priority repair. The perfor- deflection as well. In some models, artificial constraints were mance of guardrail with lower amounts of deflection was introduced to prevent post motion so that the effects of the evaluated with finite element models, as described in the rail deflection could be studied in isolation. An example of following sections, to determine the limit of deflection that a completed full-scale model with 6 inches of rail and post can be allowed. deflection is shown in Figure 40. 10.3 Evaluation Through Finite Element Modeling The crash tests show that 14 inches of lateral post and rail deflection is a damage level which requires repair. The level of damage below 14 inches of post/rail deflection which may be acceptable was investigated by finite element simulation. A series of simulations was conducted to determine how much deflection could be permitted in a strong-post w-beam guardrail without compromising the safety of the system. All simulations were conducted with the LS-DYNA software as described earlier in this report. Simulations with com- Figure 40. Simulated guardrail with rail and bined rail and post deflection were conducted for 3, 6, 9, and post deflection.

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46 10.3.1 Validation of the Finite Element Model MGA Crash Test Simulation of MGA C08C3-027.2 MGA Crash Test The model was validated using the MGA test series in which two Chevrolet 2500 pickup trucks impacted a guardrail (MGA, 2008a; 2008b). By using multiple crash tests, the acceptability of using the finite element approach to model a wide range of crash conditions could be assured. t = 0 ms t = 0 ms The MGA crash tests were conducted to evaluate the per- formance of a guardrail with rail and post deflection. In the first test, a 30 mph (47 kph) impact was used to create 14.5 inches (368 mm) of deflection. The second test was performed accord- ing to NCHRP Report 350 standards with the impact occur- t = 120 ms t = 125 ms ring at the same point as the previous test. Because of the damage incurred by the first impact, the second test resulted in a failure due to the vehicle vaulting over the guardrail. These MGA crash tests were invaluable as a source of validation data for the finite element models. A series of photos from the second crash test and associ- t = 242 ms t = 250 ms ated simulation is shown in Figure 41. For this test, there was visually good agreement between the real crash test and the finite element model. The models were also compared by the NCHRP Report 350 criteria, as shown in Table 17. The first MGA impact, a low-speed collision intended to t = 360 ms t = 350 ms cause a minor amount of deflection, was successfully repro- duced. A simulation speed of 32 mph (52 km/hr) was required to reproduce the 14.5 inches (368 mm) of deflection observed in the 30 mph (48.3 km/hr) crash test. For the second MGA crash test, initial attempts at reproducing the results were t = 490 ms t = 500 ms unsuccessful. After an investigation, a critical factor in the outcome of the crash test was found to be the failure of a sin- gle post, located roughly 12.8 feet (3.9 meters) downstream of the impact point, to separate from the rail during both the first and second impacts. The addition of a constraint on t = 690 ms t = 700 ms the same post in the finite element model resulted in a drastic change in the predicted outcome of the impact, changing a Figure 41. Comparison of finite successful crash into a failure with the vehicle vaulting over the element simulations against second guardrail. Occupant impact velocities and ridedown accelera- MGA crash test. tions were below the NCHRP Report 350 limits in all the crash tests and simulations. 12.5 feet (3.8 meters) downstream, which maximized the ef- fect on vehicle performance. 10.3.2 Results of Finite Element Simulations In the first set of simulations, a guardrail with combined The MGA tests demonstrated that the separation of posts rail and post deflection of 3, 6, 9, and 11 inches (76, 152, from the rails can radically change the crash performance of 229, and 279 mm) was tested. The NCHRP Report 350 test strong-post w-beam guardrail. Finite element modeling may values recorded for each simulation are shown in Table 18. not be able to accurately predict which behavior will occur in Despite the huge difference in performance between the a real crash when relevant factors such as soil strength or bolt MGA test simulation of 14.5 inches of deflection and the position are not known. The approach was to bracket the undamaged simulation, there was very little variation in crash performance by conducting two series of simulations. performance between the simulations of lesser damage. In the first series, the rails and posts were allowed to separate. Even the simulation with 11 inches of deflection yielded In the second series, a single post was prevented from sepa- virtually the same crash results and test values as the un- rating. The post to which this constraint was applied was damaged simulation.

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47 Table 17. Validation of finite element tion of the vehicles in each of these crash test simulations at simulations against second MGA crash test. 700 ms after impact. The vehicle began to move upward and roll with increasing amounts of prior deflection damage. The MGA Crash MGA Crash Test C08C3- Test vehicle eventually rolled onto its side when the deflection dam- 027.2 Simulation age reached 11 inches (279 mm). However, even at 6 inches Impact Conditions Speed (kph) 99.9 100.0 (152 mm) of deflection, the roll was very high and reached over Angle (deg) 26.4 26.4 35 degrees before the vehicle began to recover. Exit Conditions Figure 43 shows the local vehicle velocity at the center of Speed (kph) 69.5 57.0 Angle (deg) 18.7 5.7 gravity as a function of time for both the normal and fixed post Occupant simulations. There was almost no difference in the velocity Impact Velocity X (m/s) 6.1 9.3 between the undamaged simulation and the unmodified rail Impact Velocity Y (m/s) 3.7 5.4 and post deflection simulations. All of the exit speeds were Ridedown X (G) -6.1 -10.4 Ridedown Y (G) -5.6 -5.4 in the range of 3135 mph (5056 km/hr). The velocities for 50 ms Average X (G) -5.5 -10.0 the simulations with a fixed post were a little more varied. 50 ms Average Y (G) -3.1 -6.3 The vehicle in the 11-inch simulation retained the most speed 50 ms Average Z (G) -4.1 -6.5 Guardrail Deflections due to rolling on its side, which limited the amount of inter- Dynamic (m) 2.2 1.00 action with the guardrail. The 3-inch simulation vehicle showed Static (m) 1.0 0.80 the lowest amount of roll and lost more speed because of more Vehicle Rotations Max Roll (deg) 30 7.1 opportunities to interact with the posts. Max Pitch (deg) 12 11.5 The maximum deflection of the guardrail increased as the Max Yaw (deg) -10.2 -21.3 extent of rail and post deflection increased for both sets of simulations as shown in Figure 44. The increases were much larger for the simulations without separation due to the de- In the second set of simulations, the models were set up flecting posts pulling the rails out. However, for both sets, in an identical manner, except that a constraint was added each additional 3 inches (75 mm) in pre-existing deflection to a post located 12.5 feet (3.8 meters) downstream of the yielded only 0.81.6 inches (2040 mm) of extra dynamic de- impact point to prevent the post and rail from separating. flection. The limited effect of the pre-existing deflection was The NCHRP Report 350 test values recorded for each simu- attributed to the narrow range over which the damage was in- lation are shown in Table 19. Figure 42 shows the orienta- curred on the rail. Table 18. Simulation results for rail and post deflection with no rail and post separation constraints. Undamaged 3 in. Rail 6 in. Rail and 9 in. Rail and 11 in. Rail Model and Post Post Post and Post Deflection Deflection Deflection Deflection Impact Conditions Speed (kph) 100 100 100 100 100 Angle (deg) 25 25 25 25 25 Exit Conditions Speed (kph) 53 53 52 56 50 Angle (deg) 14.5 13.2 13.8 15.6 15.0 Occupant Impact Velocity X (m/s) 7.5 8.0 8.0 8.6 8.3 Impact Velocity Y (m/s) 5.5 5.6 5.5 5.5 5.9 Ridedown X (G) -11.8 -12.0 -12.2 -10.7 -12.8 Ridedown Y (G) -12.3 -13.0 -10.1 -12.0 -10.4 50 ms Average X (G) -6.7 -6.7 -6.8 -7.9 -7.1 50 ms Average Y (G) -6.8 -6.7 -6.5 -6.5 -6.8 50 ms Average Z (G) -3.8 -3.9 -3.0 -4.2 -4.6 Guardrail Deflection Max Dynamic (m) 0.69 0.72 0.74 0.76 0.78 Static Deflection (m) 0.55 0.62 0.55 0.60 0.64 Pre-existing deflection (m) 0.00 0.07 0.15 0.22 0.28 Vehicle Rotation Max Roll (deg) -14.4 -12.9 -13 -16.6 -13.2 Max Pitch (deg) -9.9 -10 -6.6 -5.6 10 Max Yaw (deg) 40.3 40 40 41 40.5

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48 Table 19. Simulation results for rail and post deflection with one rail and post separation constraint. Undamaged 3 in. Rail 6 in. Rail and 9 in. Rail and 11 in. Rail Model and Post Post Post and Post Deflection Deflection Deflection Deflection Impact Conditions Speed (kph) 100 100 100 100 100 Angle (deg) 25 25 25 25 25 Exit Conditions Speed (kph) 53 46 55 55 64 Angle (deg) 14.5 19.1 12.3 15.6 3.4 Occupant Impact Velocity X (m/s) 7.5 8.4 7.9 8.1 7.9 Impact Velocity Y (m/s) 5.5 5.5 5.1 5.2 5.4 Ridedown X (G) -11.8 -11.7 -13.2 -14.5 -8.8 Ridedown Y (G) -12.3 -10.4 -11.9 -11.7 -8.7 50 ms Average X (G) -6.7 -8.8 -8.2 -8.3 -7.0 50 ms Average Y (G) -6.8 -6.5 -6.2 -6.2 -6.0 50 ms Average Z (G) -3.8 -3.9 -3.7 4.6 5.2 Guardrail Deflection Max Dynamic (m) 0.69 0.82 0.86 0.86 0.90 Static Deflection (m) 0.55 0.64 0.66 0.67 0.77 Pre-existing deflection (m) 0.00 0.07 0.15 0.22 0.28 Vehicle Rotation Max Roll (deg) -14.4 32.1 35.5 39.7 Roll Max Pitch (deg) -9.9 -14.6 -19.8 22.7 28.2 Max Yaw (deg) 40.3 46.2 35.8 39.0 23.3 Rail Deflection Only Simulations. To determine the The NCHRP Report 350 test criteria were almost entirely relative contributions of the rails versus those of the posts, unchanged from the values recorded for the undamaged two simulations were conducted in which rail deflection was simulation. Between the undamaged and 6 inch rail only de- introduced between two adjacent posts. No post deflection flection simulation, the roll and pitch decreased by less than was permitted in the first impact. The posts were free to move 4 degrees and the maximum dynamic deflection increased by however in the second impacts of these simulations. These less than 3 percent. The longitudinal occupant impact veloc- rail deflection only simulations were limited to 3 and 6 inches ity showed the greatest increase, rising to 27 ft/s (8.2 m/s) (76 and 152 mm) of deflection since larger rail deflections from 24.6 ft/s (7.5 m/s), but was still within the recom- generally do not occur without also deflecting the posts. mended limit. The lack of change in crash test outcome for (a) Undamaged rail (b) Rail with 3" prior deflection (c) Rail with 6" prior deflection (d) Rail with 9" prior deflection (e) Rail with 11" prior deflection Figure 42. Damaged guardrail simulations after impact (t = 700 ms).