Crash sleds test automakers' interior designs
Michael DeLeeuw, Instron Structural Testing -- Test & Measurement World, 3/1/2005
To those who understand its dynamics, the twisted mass of metal that results from an auto-crash test tells a revealing story. It is a story of complex forces exerting their influence on the car's structure, a tale of discovery in which nothing is random and all damage happens for a highly scientific and explicable reason. These forces combine with the design of the vehicle to determine the accelerations and motions of the passenger compartment and, ultimately, the potential for injury to its occupants.
Injuries are related to the design of the vehicle structure, which determines the shape and magnitude of the deceleration during a crash event. Another critical factor in determining injury levels is pitching motion—the rotation of the back of the vehicle upward during the crash. Yet, pitching is not the only consequence of a crash. A frontal crash often involves a combination of pitching and vertical motion. In fact, some cars don't pitch at all; instead of lifting, they squat.
Testing methodsOne problem for the auto industry has been the lack of practical, nondestructive methods to test the influence of pitching on occupant injuries. OEMs and suppliers have two choices to test the performance of safety devices in vehicles: crashing the vehicle into a wall or "barrier," or using a crash "simulator" (otherwise known by the more colorful term, "crash sled"). Crash sleds allow testing of interior passive safety components such as restraints, airbags, and seats, as well as selected subsystems using nondestructive techniques (Figure 1). Sled tests are far less expensive since they are nondestructive, but historically, they have been much less accurate than actual barrier tests. Studies have shown as much as a 40% difference in head injury criteria (HIC) values between a sled and a barrier due to the lack of pitching motion.
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| Fig. 1 Nondestructive crash sleds test interior passive safety components such as restraints, airbags, and seats. |
While federal safety standards are based on injury results of barrier tests, the use of a sled in development testing can save both time and money. This is particularly true for suppliers, where access to prototype parts and vehicles is restricted.To provide a sense of the economics involved, a problem found in the final development stages could require 10 additional prototypes to be crashed, at an additional cost of approximately $4 million. If the problem is not found in those stages, the average safety recall for an airbag component carries roughly $130 million in cost for the OEM. By contrast, a sled with pitching simulation can be installed and run at an average cost of $5000 per test.
Crash sleds—which OEMs and suppliers have been using for over 40 years—are far more cost-effective because the simulator does not actually crush the front end or destroy the structure. This is critical, since a $500,000 prototype must be completely destroyed to prove the performance of various safety systems.
In addition, with crash sleds, a large number of tests can be performed in a relatively short time frame, and because the test is easily repeatable, it enhances the validity of A/B comparison results. Multiple tests can be carried out with a range of specifications to determine the best design. This should lead to a greater probability for success when full-scale crash tests are eventually conducted. Therefore, there are several strong justifications for the use of a crash sled with high performance and test accuracy in safety development.
Improving sled accuracyDespite the industry's limited success with finding crash sleds that meet accuracy requirements, one leading automaker undertook the task of developing an effective pitching test. To achieve this goal, the automaker enlisted the help of Instron Structural Testing Systems (IST). With our expertise in the area of servohydraulic crash sleds and physical testing of structures and components for ground transportation, we designed two sophisticated crash simulators for installation at two R&D facilities for the automaker.
Design teams incorporating members of both companies were formed at each location. Both companies realized that the latest technologies available in mechanical design, modeling, software simulation, and digital closed-loop controlled electronics would be required to produce a machine capable of including vertical and pitching displacement in the simulator's capabilities.
The result of this collaboration was an enhanced servohydraulic crash sled using the core functions of IST's previous model. The sled features 100% computer control and uses best-in-class nonlinear software-modeling techniques. Since it features a point-and-click interface, users do not need extensive knowledge of complex control algorithms, making for a short learning curve. These features reduce the number of full-scale barrier tests required and provide shorter development times for new vehicles.
Like most crash-simulation sleds, the design works on the principle of reverse acceleration, in which a reinforced body, including interior safety components and test dummies, is accelerated in reverse from a standstill. The reverse acceleration creates the same loading history on the occupants that occurs during the actual crash event. One unique aspect is the combined use of high-pressure oil to power the system, and a force rating of 2.5 MN, making possible the implementation of a modular pitching unit as an additional option.
The sled can simulate longitudinal deceleration as well as vertical and pitching displacement during the crash event. Vertical and pitching motions are fully programmable within the capability of the system.
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| Fig. 2 The lower sled rides on a standard rail system. Injuries are related to the design of the vehicle structure, which determines the shape and magnitude of the deceleration during a crash event. |
The pitching system consists of a special pitching sled, engineered to mesh with a longitudinal accelerator. Two sleds, an upper and a lower, are connected by arms to allow for both vertical and pitching motion. The test buck is mounted to the upper sled; the lower sled rides on the standard rail system (Figure 2). Each corner of the upper sled features large sliding bearings that engage a secondary set of rails to control the pitching and vertical motion.
Each pitching rail is attached to a specially designed vertical actuator to precisely control its motion. To simulate pitching crash dynamics in real time, each vertical actuator delivers more than 400 kN of force and 3 m/s of velocity.
As soon as a test is initiated, the upper sled bearings are engaged in the pitching rails, and the motion of the upper pitching table is controlled by the pitching actuators. The connecting arms transfer the forces from the upper to the lower sled and to the main actuator piston, which is used to generate longitudinal acceleration. A typical test lasts less than 120 ms.
The new crash sled provides more accurate simulation of occupant kinematics and the injuries that occur in real-life crash events. These additional test capabilities play an integral role in the development of better airbags and restraint systems, substantially increasing the value of laboratory-based crash sled testing.
| Author Information |
| Michael DeLeeuw is the VP of Passive Safety Systems at Instron Structural Testing (IST). DeLeeuw graduated with a BSME from Worcester Polytechnic Institute and has worked in the automotive industry since 1988. Located in Detroit, MI, he has worked for Instron in various capacities for the past 17 years. |
