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49 100 100 90 90 80 80 70 70 Velocity (kph) Velocity (kph) 60 60 50 50 40 Undamaged 40 Undamaged 30 3 Inch 30 3 Inch 6 Inch 6 Inch 20 20 9 Inch 9 Inch 10 11 Inch 10 11 Inch 0 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Time (s) Time (s) Figure 43. Vehicle velocities for rail and post deflection simulations (left) and the same simulations with one post prevented from separating (right). rail deflection only supports the earlier theory that the con- 10.4.2 Simulations of Rail and Post Deflection tributions of the posts may be more important in predicting In the simulations of the 3, 6, 9, and 11 inches (76, 152, 229, the outcome of a crash. and 279 mm) of rail and post deflection with no separation con- straints, minor rail and post deflection had very little effect on 10.4 Discussion the simulation results. The OIV, ridedown, and 50 ms average accelerations were satisfactory and the increases in maximum 10.4.1 Importance of Rail and Post Separation deflection were less than the increase in prior deflection. When A critical contribution to the vaulting of the vehicle in the the simulations were altered to prevent a post from separating MGA crash test was believed to be the failure of some of the from the rail, different outcomes were observed. The vehicle roll posts to detach from the guardrail. In the second MGA crash increased with increasing preexisting deflection. The vehicle test, a post failed to separate from the rail during impact. In a overturned during impact with a guardrail having 11 inches preliminary simulation of this crash, the post did separate (279 mm) of pre-existing rail deflection. Even for as little as from the rail, and the vehicle was successfully redirected. 6 inches of rail deflection, substantial rolling was observed. When a constraint was added to prevent the rail from sepa- When the rail and posts fail to separate, two different haz- rating from the post, the vehicle vaulted over the guardrail. ardous conditions can be created. If the post remains mostly The deflection of this post during impact was believed to have upright the vehicle may be at greater risk of snagging. Another pulled the rail downward which permitted the vehicle to vault possible outcome was reflected in the results of the MGA over the guardrail. crash test. If an unseparated post was deflected backwards and 1000 1000 Undamaged Undamaged 900 900 3 Inch 3 Inch 800 6 Inch 800 6 Inch 700 9 Inch 700 9 Inch Deflection (mm) Deflection (mm) 11 Inch 11 Inch 600 600 500 500 400 400 300 300 200 200 100 100 0 0 0 5000 10000 15000 20000 25000 0 5000 10000 15000 20000 25000 Downstream Distance from Post 9 (mm) Downstream Distance from Post 9 (mm) Figure 44. Maximum dynamic deflection for rail and post simulations (left) and the same simulations with one post prevented from separating (right).
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50 35 25 30 20 Length of Deflection (ft) 25 Rail Height (in) 15 20 15 10 10 5 Bottom of Rail 5 Top of Rail 0 0 0 2 4 6 8 10 12 14 16 0 3 6 9 12 15 Prior Deflection (in) Prior Rail Deflection (in) Figure 45. The height of the rails (left) and the length of damage (right) vs. the extent of prior deflection. downwards, as in the simulations with greater than 6 inches second impact. All of the measurements were made from the (152 mm) of deflection, the rail is pulled downward as well simulations with a separation constraint added. This situa- and the risk of vaulting is increased. tion represented the worst case scenario for vaulting because The vehicle behavior for both 3 and 6 inches (76 and the deflection of the post would pull the rail downward as it 152 mm) of prior rail deflection without post deflection deflected. was no different from that of the undamaged rail simula- Figure 45 shows that one consequence of an impact is that tion. The static and dynamic guardrail deflections were also the rail flattens. The bottom of the rail moved downward unchanged. These results provide further support for the from 15.3 inches (388.6 mm) to 12.6 inches (320 mm) above theory that the behavior of the posts in strong-post guardrail the ground surface. The top of the rail moved upward from systems can strongly influence the outcome of a crash test. 27.9 inches (709 mm) to 31.8 inches (808 mm). The maxi- mum height of the guardrail increased with increasing deflec- tion, indicating that the guardrail was becoming increasingly 10.4.3 Effects of Prior Damage on Rail Height flattened. The length of deflection also increased with increas- Existing literature has suggested that rail height can be a ing magnitude of deflection. These results indicate that, in major contributor to vaulting (Marzougui et al., 2007). The addition to rail height, the flattening of the rail and the dam- rails in the finite element simulations were examined to de- age length may also exert a significant influence on the crash termine whether the minor rail deflection incurred in the first outcome in these simulations. However, further testing will impact resulted in changes in the rail height that could be cor- be needed to draw any conclusions about the relative impor- related to the outcome of the simulated second impact. The tance of each of these factors on vaulting or rollover risk. hypothesis was that the pre-existing damage would lower the rail height and lead to the vehicle vaulting. 10.4.4 Evaluation of Rail Rupture Potential Figure 45 and Table 20 present the minimum height of the rail bottom, maximum height of the rail top, and the length Ray et al. conducted a study on rail rupture in crash tests of pre-existing deflection after the first impact but before the which showed that rails can carry up to 92.2 kip (410 kN) Table 20. The height of guardrails and length of damage in simulations with pre-existing damage. Minimum Height Maximum Difference in Length of of Height of Rail Max and Min Damage (m) Rail Bottom Top (mm) (mm) (mm) Undamaged 388 709 321 NA 3 in. 320 742 422 2.3 6 in. 317 755 438 3.5 9 in. 317 769 452 4.7 11 in. 314 781 467 5.8 14.5 in. 334 807 473 6.6