Explosives-free Laboratory Blast SimulationMTS R&D engineer Marty Gram discusses a safe, economical and information-rich alternative to field blast testing.
Q: What challenges does blast-resistance testing typically involve?
Gram: To effectively develop and validate blast-resistant structures, engineers require highly reliable and repeatable test results. The current means of blast testing, which requires the initiation of an actual blast in the field using explosives, does not adequately address these requirements.
Testing with explosives involves the observation of extreme forces during an event that lasts only milliseconds. This event produces a fireball that obscures much of the phenomena test engineers wish to observe, making it difficult or impossible to conduct useful real-time visual observation, either with the naked eye or high-speed video. The blast often damages any transducer attached to the specimen, making it difficult to determine how the structural damage progresses in the specimen.This means of testing is also costly, labor-intensive, non-repeatable and dangerous.
Q: How have these shortcomings been resolved?
Gram: In close collaboration with the University of California San Diego (UCSD), MTS has developed new blast simulation technology that allows explosions to be precisely replicated in a laboratory through the use of hydraulic actuation, without the use of high explosives.
Q: How does this new technology work?
Gram: The UCSD system employs four high-force blast generators (BGs) that propel steel plates equipped with contoured elastomeric springs into a test specimen at high velocity. Each BG is composed of a hydraulic actuator and several control valves, accumulators and transducers, capable of accelerating an impacting mass to 30 m/s (1200 in/s) within a stroke of about 1 m (40 in). This produces a 1 to 5 ms pulse with a typical peak pressure loading of 35 MPa and an impulse of 14 kPa-s over the surface of the specimen.
A close-coupled pressure line accumulator provides the high flow high response valve with the flow necessary to accelerate the impacting mass to a precise velocity, and a close-coupled return line accumulator accommodates flow from the retract valve. Both valves are servo-controlled using LVDTs on the valve position. Precise control of the actuators allows simultaneous impacts with 4 or more blast generators; this allows testing of very large specimens such as building or bridge columns.
Accelerometers on the impacting mass measure the impact force and various transducers on the specimen measure the impact and response.
This technique has proven itself amazingly accurate. When comparing post-test data between lab simulations and their corresponding live explosive loads, the correlation, as measured by specimen deformation, was nearly identical.
Q: How are control and data acquisition accomplished?
Gram: During blast simulation, a computer-based control system provides valve commands to generate the desired impact velocity and timing of the four BGs. Control and setup transducers include three pressure transducers on each BG; LVDT stroke transducers on the control valves; and a stroke transducer used to control actuator position and measure impact velocity. Accelerometers mounted on the impacting mass record impact data for calculating force and impulse values.
Output from all control system transducers is digitally recorded at 4 kHz for each test. Accelerometer output is recorded at 800 kHz. High-speed data acquisition, triggered by the control system, can be recorded at 52 channels of 14-bit data at 1 MHz.
The UCSD system also includes two high-speed video cameras, capable of capturing 5,000 frames per second of unobstructed, fireball-free visual specimen failure information.
Q: What are the benefits of this blast simulation technology?
Gram: By moving their testing into the laboratory and eliminating the use of explosives, test engineers have a much safer, repeatable and information-rich means of assessing a structure’s blast-resistance.
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