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Roof-Crush Analysis of the Volvo XC40 using the Implicit Solver in LS-DYNA

During the development process of a new platform or car model, each design iterate is subjected to a large number of load cases, both dynamic as well as static. At Volvo Car Corporation, this process is almost entirely carried out using virtual testing by finite element analysis. The amount of physical prototypes is reduced to a minimum, and in many cases physical testing is limited to the component or sub-assembly level. Still, the final design must pass a number of physical tests and legal requirements, where roof crush is an important test of the structural integrity of the cab. The purpose of the FMVSS roof crush test is to “reduce deaths and injuries due to the crushing of the roof into the passenger compartment in rollover accidents” [6]. At Volvo Car Corporation, occupant safety is a fundamental element in all development projects since the start of the company, and the Volvo XC40 received a 5-star rating when tested by Euro NCAP [7]. The roof crush resistance is important with relation to safety in case of a roll-over accident, since the structural integrity of the car body makes the final line of defense, but many safety systems will interact in this case, from driver assist systems to electronic stability systems and restraint systems. The roof crush test will induce high stresses in many structural parts of the car body, for example the A-, B- and C-pillars, window frame and roof. This means that the analysis must be carried out meticulously, since the roof strength requirement may set design limits for many structural parts. Also new design concepts, such as composite roof panels or panorama glass roofs, imply new challenges for the roof crush analysis. The testing procedure according to FMVSS 216 [6] is specified as quasi-static (the time to complete the test is minimum 10, maximum 120 seconds), but has traditionally been run in explicit LS-DYNA in only a fraction of this time. From this viewpoint the roof-crush load case would be a typical application of implicit analysis, allowing the simulation of the test to get closer to the real test procedure. As a part of the ongoing method development work, it was decided to evaluate also the implicit technique, using the Volvo XC40 as a benchmark case. A previous study [5] indicated that it is possible to re-use FE-models originally created for crash load cases also for quasi-static load cases using the implicit solver in LS-DYNA with a reasonable modification effort. A previous study on implicit roof-crush analyses in LS-DYNA [1] indicated good correlation to explicit analyses, as well as reasonable performance with respect to solution time. Also, the publicly available examples [2][3] of implicit roof-crush analyses served as great inspiration in the present work.