In today’s automotive design environment, balancing vehicle structural safety against the need for lightweighting has become a ubiquitous challenge. Systems such as seat belt restraints, air bags, antilock braking (ABS), adaptive headlights, and emergency stop assistance technologies have made vehicles safer based on the reduced number of accident deaths per mile driven. But with all of those systems in place, the U.S. still has between 30,000 to 40,000 motor vehicle accident deaths each year, and that trend has remained relatively constant for the past three decades. In order to address this challenge, Tesseract Structural Innovations in Fayetteville, AR, has developed a unique lightweighting technology, known as UDU. This aluminum foam technology is designed to meet the need for improved passive structural safety in vehicles, while at the same time creating a new playbook for lightweighting in many structural areas of the vehicle, such as electric vehicle (EV) battery packs.
Designing for Impact Absorption
The patented UDU technology is a lightweight, modular composite structure comprised of a structural skin or “scaffold” integrated with a metallic foam or similar material. This modular composite configuration creates enormous design flexibility for applying the technology to automotive applications. The structural portion is generally made from high strength aluminum alloy materials, such as extrusions, impact extrusions, or castings. However, other materials, such as advanced high-strength steels (AHSS) and carbon fiber reinforced plastic (CFRP), can also be used for the skin material.
A key component of the technology is stabilized aluminum foam from Cymat Technologies, an innovative materials technology company located in Mississauga, Canada. The company employs a proprietary process to manufacture their unique Stabilized Aluminum Foam (SAF), an advanced lightweight recyclable material that provides a wide array of features, including customizable density and dimensions, energy absorption, and thermal and acoustic insulation. Cymat produces SAF for architecture, blast mitigation, automotive, transportation, and industrial markets. The company’s SmartMetal division assists in the integration of this technology into kinetic energy management systems, such as the UDU.
Tesseract discovered that when the structural skin and aluminum foam are “tuned” to take advantage of the inherent constant-force crush regime of the aluminum foam, a near ideal energy absorber can be constructed. This means that for a given design space, UDU can absorb as much energy as is physically possible without injuring vehicle occupants or collapsing vehicle structural members, such as the A-pillar or the side sill. This allows forces passing through the structure to reach a designed limit. Above that force limit, it crushes at almost a constant force, with the kinetic energy being converted into strain energy inside the structure.
Figure 1 shows data from an actual crush test, in which the UDU skin was designed to limit the crush force to a maximum of 75,000 lbs. The area under the red line represents the maximum amount of energy that can be absorbed over a one-inch crush distance in that particular design space. The green test curve shows that the force has been limited to just less than the maximum allowable. In other words, the skin structure has done its job. By adjusting the stiffness and density of the internal foam material, the middle portion of the green crush curve can be lifted up to approach the ideal energy absorber.
Since the UDU technology is modular, this constant force crush behavior can be maintained over relatively long distances. The segments can be stacked and clustered to create virtually an infinite number of shapes and sizes. In the case of a vehicle front crash, it can be designed to crush as much as a meter of length, if required, to limit forces. In the case of a side crash in an EV, the structure can be designed to limit intrusion into the battery compartment while absorbing the crash energy and directing forces to existing load path members.
Tesseract has demonstrated the efficacy of the UDU technology in crash situations through finite element analysis (FEA) simulation and prototype testing. To illustrate how the structure can improve crash behavior, some of these case studies are presented here.
Small Overlap Crash Protection for a Full-size Pickup Truck
The most lethal of all vehicle crashes is the small overlap (SOL) front crash, and it is also one of the most common accident types. Also known as the narrow overlap crash, the SOL crash condition occurs when only the outer 25% of the front of the vehicle impacts another vehicle or another object, such as a tree or a utility pole. This outer 25% of the front profile lines up roughly with the wheel well on both sides of the vehicle. Unfortunately, the wheel well of most vehicles is designed with little structural support to manage crash forces and absorb crash energy. On average about 10,000 people per year are killed in SOL front crashes in the U.S.
The UDU technology was first conceived to improve the outcome of these kinds of crashes. Using a National Highway Traffic Safety Administration (NHTSA) public domain research model for the 2014 Chevy Silverado pickup, Tesseract designed an UDU structure to replace the shotgun inner fender member of the truck. A simple bracket was designed to integrate the structure within the existing vehicle (Figure 2).
FEA simulations run using the LS-Dyna software showed that the 2014 Chevy Silverado truck, with the wheel well mounted UDU structure, absorbed almost 25% more kinetic energy than the baseline production model (Figure 3). This increased energy absorption would translate into a significant improvement in SOL crash performance for the truck (Figure 4). Furthermore, all of this improvement was achieved with a minimal net mass addition and minimal changes to the existing body structure.
Semi-Truck Underrun and Overrun Protection
Every year in the U.S. almost 4,000 people are killed in accidents involving big-rig trucks. Many of those accidents involve either the large truck overrunning a smaller vehicle or a smaller vehicle underrunning the big-rig trailer. There are a number of structures that have been designed to prevent the underrun/overrun conditions on semi-trucks. However, these structures tend to be very rigid and stiff, and they don’t generally address the need for energy absorption to protect the occupants of the smaller vehicles that collide with the semi-trucks.
For one such application, Tesseract designed an UDU crash box structure to replace an existing steel energy absorber for a semi-truck front overrun device (Figure 5). This crash box saved more than 7.0 lbs per truck (3.5 lbs per part) over the existing steel version, while increasing energy absorption by over 100% (Figure 6). The all-aluminum component fit into the same design space as the existing steel part, and it used the same attachment hardware. It was a true plug-and-play design that required no modifications to the existing truck structure.
Electric Vehicle Battery Protection
As vehicles are converted from “internal combustion engines to all-electric drivetrains, lithium batteries have become the power source of choice. Most plug-in EVs have a large battery pack that is packaged under the floor pan of the occupant compartment. With EVs becoming more widely used for everyday driving purposes, the need to increase the driving range between battery charges has become more necessary. Making the battery pack as large as possible is a priority with OEMs. In addition, the lithium-ion battery pack must be protected during a crash. This is particularly true in the event of a side crash, where body structural members of the vehicle could intrude into the battery pack space and cause the battery pack to be punctured resulting in a fire.
To effectively protect the EV battery pack during an impact, such as a side crash into a utility pole, the vehicle structure must limit the amount of intrusion into the battery space while simultaneously absorbing energy to keep the side forces on the driver and passengers at a survivable level. Tesseract’s sill beam UDU structure has accomplished that dual purpose goal.
Figure 7 shows the results of an FEA simulation of a pole drop test onto an UDU sill beam structure designed for EV battery protection. This structure consisted of four layers of aluminum skin, three layers of which are filled with Cymat aluminum foam. The first layer of the side sill allowed the pole to embed into the structure and distribute the load over a larger area than would have otherwise happened. The second and third layers had a stiffness gradient that promoted sequential crushing of the layers. The fourth layer acted as a reaction beam to limit total displacement toward the battery pack and to allow complete crushing of the previous three layers.
The total energy absorbed by the side sill beam structure was increased by the pole embedment (Figure 8), which effectively increases the crush distance for the UDU. Pole embedment into the structure at the outset of the crash, and subsequently during the crash, generates bonus energy absorption. The structure was able to limit the intrusion into the battery pack space, resulting from the side pole impact simulation, to an amount less than the OEM specification for the vehicle. The side sill also absorbed over 40% more energy during the side pole impact than the existing EV body structure.
Though the side sill structure modeled was a lightweight aluminum skin filled with Cymat aluminum foam, the skin could have also been fabricated with AHSS, CFRP, or a variety of other materials. This design flexibility provides the option to design EV battery protection into the vehicle body structure and/or directly into the battery pack itself.
The UDU’s ability to absorb a significant amount of energy without injuring vehicle occupants or collapsing other connecting structural members gives automotive structural designers a powerful tool for creating passive crash safety protection in the vehicle body and frame, while still pursuing lightweight strategies. Light vehicle applications for the UDU technology include bumper beam crash boxes, battery enclosure protection, and front, side, and rear crash improvement. Applications are also possible for heavy trucks, including underrun protection and overlap crash protection. Tesseract is also exploring opportunities for use of the UDU technology in aerospace and defense applications.
Editor’s Note: This article first appeared in the December 2021 issue of Light Metal Age. To receive the current issue, please subscribe.