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51 Table 21. Maximum rail tensions in rail and post deflection simulations. Maximum Rail Tension (kN) % Increase Over Undamaged Separation No separation Separation No separation allowed allowed allowed allowed Undamaged 237.4 0.0% 3 in. Rail and 247.1 258.1 4.1% 8.7% Post 6 in. Rail and 282.9 229.2 19.2% -3.5% Post 9 in. Rail and 292.6 237.6 23.3% 0.1% Post 11 in. Rail and 261.1 235.5 10.0% -0.8% Post under quasistatic loading. However, rupture may also occur negatively affect the crash performance. The research team's at lower rail tensions. Localized tearing is possible in impacts conclusions are the following: of this type, but the research team's model was not configured to accurately compute element tearing resulting from local- Crash tests demonstrated that 14.5 inches (368 mm) of ized stress concentrations and did not include failure criteria post and rail deflection with a damage length of 36 feet for the steel components. The model was meshed using large (11 meters) was a damage level requiring high-priority re- element sizes 0.40.6 in (1040 mm), which were appropri- pair. Two full-scale crash tests were conducted to evaluate ate for determining vehicle dynamics but were too coarse to the limits of acceptable rail and post deflection in crash- realistically model the initiation and propagation of tears. damaged strong-post w-beam guardrail. The damaged bar- As an alternative, the tension carried by the rails was used to rier failed to contain a Chevrolet 2500 pickup truck that determine the relative risk of rail rupture. impacted the damaged section at 62 mph (100 km/hr) and The tensions for the rail and post simulations are tabu- 26.4 degrees. The vehicle vaulted over the guardrail and lated in Table 21 under the column for separation allowed came to rest upright behind the barrier. A critical factor in simulations. The tension for the rail deflection only did not the outcome of the test was the failure of a post near the vary significantly from the undamaged simulation. However, area of impact to separate from the rails during impact. all of the post and rail deflection simulations showed an Finite element simulations were employed to investigate the increase in rail tension compared to the undamaged simu- acceptability of damage levels below 14.5 inches (368 mm) lation, with the tension steadily increasing to a maximum of of rail and post deflection. Simulations were conducted for 292.6 kN at 9 inches of deflection. Although this tension was post and rail deflection varying from 3 to 11 inches (76 to below the 410 kN limit of w-beam rail, rupture can occur at 279 mm). A series of simulations were run in which a sin- a lower tension (Ray et al., 2001). The higher tension carried gle post was prevented from separating from the rail. In this by the damaged rails implied that there was a modest increase simulation series, the vehicle experienced a significant roll in the chance that a rail rupture would occur. beginning at 6 inches (152 mm) of deflection and eventually The tension was also tabulated for the simulations in which rolled over when the deflection reached 11 inches (279 mm). post separation was not permitted. These tensions are listed The crash performance of rail with 3 inches of deflection was under the "No separation allowed" column. The recorded not markedly different than undamaged rail. maximum tensions were not much different than that of the Finite element simulations were conducted of impacts into undamaged simulation. Because the connection between the guardrail with rail deflection between two adjacent posts. post and rail was maintained, more of the crash energy was No post deflection was permitted in the first impact. The transmitted to the posts. posts were free to move however in the second impacts of these simulations. Rail deflection of 3 and 6 inches between the posts was investigated. The vehicle and guardrail per- 10.5 Conclusions formance in these simulations were almost unchanged from This study has examined the crash performance of strong- the undamaged simulation. These results support the con- post w-beam guardrail with rail and post deflection from a clusion that the contributions of the post during an impact previous impact. The MGA crash tests and finite element were important. simulations of second impacts into damaged guardrails have Rail tension was examined in all simulations as an indi- shown that the combination of rail and post deflection can cator of rail rupture potential. The tension carried by the