A car crash takes half a blink of an eye. Detailed knowledge of the crucial split-second when the crash impact energy flashes through a vehicle’s body and then onto the occupants’ bodies is critical to building safer cars.
For decades at automakers’ test tracks such as GM’s Milford Proving Ground in Michigan, crash test dummies have endured repeated near ballistic collisions to provide that key data to safety and vehicle engineers. Through the years, the familiar bull’s-eyed manikins of steel, rubber, vinyl, and foam have grown increasingly sophisticated, said Jack Jensen, Engineering Group Manager at the lab and GM Technical Fellow.
Fitted with the latest instrumentation, crash dummies have essentially become flight-data recorders, black boxes in human form, as the sensors and transducers are now self-contained and wireless, whereas previous dummies needed cables that could restrain free movement, Jensen said. “The new dummies can have 140 channels of data delivering at a rate around 10 samples a millisecond, which quickly fills huge files.”
Humanetics Innovative Solution of Plymouth, MI, is the leading supplier of anthropomorphic test devices, or ATDs.
ATDs are fitted with accelerometers, load cells, angular rate sensors, and displacement gauges, which “gives you so many g’s, so many Newtons of load, millimeters of deflection,” he explained. The deflection numbers, for instance, “let us measure the compression of the chest cavity or distance between the sternum and spine.” These metrics help define injury criteria for predicting the statistical risks of injuries.
“We have about 190 to 200 crash-test dummies in a wide range of sizes, shapes, and ages,” Jensen continued. Dummies are not cheap; they can go for $125,000, even a half-million dollars a copy.
More and more at the automakers nowadays, digital crash test dummies made of zeroes and ones are taking the big hits in virtual vehicles, he said. The simulated crash tests using computer-aided design (CAD) and engineering (CAE) systems to do finite element modeling and analysis allow GM engineers to learn earlier in the design process, before the hardware is available and built, and also optimize the performance across a broad range of test conditions.
“We still do a lot of physical crash tests here at Milford,” Jensen noted. The collisions are used to validate the models, confirm performance predictions, or when system capabilities are still evolving. Real dummies will be needed because simulations fail to capture everything that happens in a crash test or that an ATD can measure, so GM uses both physical and virtual dummies. They also use the software to model the behavior of physical dummies themselves because governments define safety regulations according to real-world crash tests.
“We might run a physical crash test to get a baseline regarding how the vehicle or safety system performs,” he said. “Then maybe we’d implement a series of incremental improvements using say, 20 computer simulations, and finally, build the new optimized vehicle or system for testing. But even though CAE allows us to reduce the number of physical collisions for analyzing a given engineering condition, the constant search for new hazardous conditions to consider means we’re doing more vehicle crashes than ever before.”
Virtual drivers and passengers
In 2003, Toyota introduced for public use the Total Human Model for Safety (THUMS) virtual human model software. THUMS, which was co-developed with Toyota Motor Central R&D Labs, is the most popular vehicle passenger safety simulation, said Jason Hallman, a vehicle safety engineer and biomedical engineer. The software today has 150 users and licensees including automakers, seat suppliers, research transport safety centers, and academic institutions.
“Right now we’re on THUMS v. 4, which is a finite-element physical framework in which the body’s hard and soft tissues—the bones, organs, muscles, ligaments, tendons—have been simulated in detail,” Hallman said. “The result is a model with 2 million elements.”
One of the academic institutions that uses THUMS 4 is a team of modelers at Wake Forest University: “We’ve reconstructed about eleven motor vehicle crashes—both frontal and side impacts—that caused acute injuries,” said Ashley Weaver, Assistant Professor of Biomedical Engineering. “By simulating real-world crashes, we can study the effect of vehicle design parameters, safety features, and occupant factors to improve safety.”
The THUMS system models three body categories: a large male, a medium man, and small woman, whereas two other classifications—young child and elderly female—are simulated by Collaborative Human Advanced Research Models v. 10 (CHARM 10), said Steve Ham, Toyota Senior Principal Engineer, who focuses on biomechanics.
“The technical background behind CHARM 10 and THUMS are substantially the same,” Ham said. “But you need specific models to represent children and the aged because their body geometries and the materials properties of their tissues are not the same as those average adults.” For a 10-year-old, the head-to-body ratios are different and the pelvic bone still has a weakness, a gap in it, he said. A 70-year-old woman typically has osteoporosis.
More specialized body types are needed, but because it can take several years to build one, Hallman and a team at the University of Michigan are developing a way to produce them quicker. Using body data from population studies, they mathematically morph existing finite-element models of skeletons and whole bodies across multiple body sizes and shapes. (See http://papers.sae.org/2016-01-1491/)
The greater scalability of the details would let engineers better fine-tune airbags, seatbelts, and passive safety systems to a wider range of bodies, Ham noted. Likewise, THUMS 5, which is being readied for release, will help evaluation of safety equipment by enabling more detailed analyses of post-crash injuries because it better simulates the body attitude and muscular state of a vehicle occupant when relaxed or braced for impact.