62 in the absence of someone overseeing who goes in and out of the vehicle. The body structure itself is made from five AHSS material groups. Each of them plays a particular role in structural support and crash safety depending on its zone, the direction in which crashes will come from, and how vehicle dynamics will play out in each crash. Specifically, the A, C and D-pillars, along with the doorsill and roof rail, are press-hardened steels, available in tensile strengths of up to 2000 MPa. These are hot-formed by heating them, inserting them into a die and press-forming them, and in the SEM1 they are chosen to give minimal intrusion into the passenger compartment during a crash. “We use press-hardened steels where we need to form shapes that can’t be achieved through the less expensive process of cold-forming,” Coates explains. “And when press-hardened steels are rapidly cooled after press-forming, they form martensitic steels – very highstrength materials with tensile strengths of about 1500-1900 MPa,” he says. “We use them in some anti-intrusion beams and panels, in front of and behind the passengers, to minimise intrusion during an impact and reduce the risk of serious injuries to the occupants.” The floors and longitudinal beams are made from dual-phase, complex-phase, and third-generation steels that have been optimised to be as formable as possible while still retaining mechanical strength. The roof rails and console are made from high-strength, low-alloy, bake-hardenable steels. Overall, around 200 different steel structural parts are assembled to comprise the SEM1’s body structure and enable it to meet the various regulatory crash safety tests. “One of the biggest technical challenges for instance were the frontal crash tests,” McGregor recounts. “There are four front-load cases or tests that the car had to meet, one being a frontal full-width barrier [FFB] crash to see if the car could decelerate and crumple gently during an impact with a rigid barrier, and if the airbags and seatbelts would do their jobs. “Even tougher was a SORB [small overlap rigid barrier] test, where the SEM1 hits a barrier at 64 kph but only 25% of the vehicle’s width makes contact with it, so high strength and stiffness are needed for it to ‘bounce’ off the barrier – that’s a conflicting requirement on the physics of the structure with the FFB test.” The front of the SEM1 therefore features longitudinal ‘crush rails’ that are made from dual-phase steels that crumple in a manner acceptable for FFB test requirements, while above those sit a vertical dash brace made from 1900 MPa press-hardened steel that minimises intrusions of metals or other items into the passenger compartment or battery during crush loads. Behind the dash brace is another 1900 MPa presshardened steel component: this is a front strut brace tested to support SORB-type load barrier reactions. “That section alone took about 300 CAE crash modelling sessions to optimise its design,” McGregor says, “We believe it is a world first to design a vehicle this small that can pass a SORB test with good safety ratings and minimal intrusions, but if autonomous robotaxis are to pass muster with safety authorities then it has to be done. Such was the scale of the development and simulation that we published a technical journal on all of it at the SAE Japan conference.” In addition to the mechanical strength in this and the other sections across the SEM1, the project partners spent considerable effort identifying ways to remove weight to ensure the vehicle could run for extended periods on its allelectric powertrain without being energyinefficient or slow. The result is that the vehicle’s top speed is actually 130 kph, which is capped by software for safety on city roads. “Each grade of AHSS has different minimum thicknesses, and parts such as body panels, fenders and doors can go down to 0.6 mm, which really helps save weight,” McGregor says. In addition, the battery is designed with a unique carrier frame that, unlike conventional enclosures, dispenses with a cover in favour of bolting it underneath the SEM1 frame’s floor. This, with other and smaller measures, saved 37% of the weight compared to conventional sealed battery packs without affecting safety. This also makes it easy to inspect the modules and cells visually. So, if an intrusion into the pack should still occur, instead of discarding the whole pack (as is common practice), only the damaged module needs to be replaced. August/September 2023 | Uncrewed Systems Technology The taxi’s structure has around 200 different steel parts, with the material and geometry for each having been carefully chosen to meet critical safety standards
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