Making the strain

Structures or components subjected to quickly applied loads see proportionately high rates of strain. But the mechanism by which many materials deform is different at high strain rates than at lower rates. Consequently, quasistatic stress-strain test data may not accurately predict behavior at high strain rates. In fact, the use of such data in the building of dynamically loaded structures can lead to cautious, overweight designs or premature structural failures. Conversely, more optimal designs are possible when they are based on loading and strain rates that better reflect actual operation conditions.

High strain rates are generated in the lab using a variety of methods. Charpy impact pendulum machines employ an instrumented hammer that is swung into a notched square or rectangular section specimen bridged across a set of anvils positioned in the hammer path. The arrangement produces specimen strain rates of about 10 to 100 in./in.-sec, and corresponding impact energies to 600 J.

Drop-weight impact test rigs use a guided free-falling or spring-assisted weight and puncture probe. These machines develop similar strain rates but are capable of impact energies of about 60,000 J.

Hopkinson bars and gas guns are appropriate for testing at extremely high rates of strain. Both use pressurized gas to transmit force through a rod or projectile, respectively, and develop strain rates of 300 to 100,000 in./in.-sec. However, the Hopkinson bar rig can test only miniature cylindrical parts and is capable of about 10 J. Gas guns, on the other hand, can develop nearly 600,000 J, or about the energy of a 1-kg meteorite striking the earth at 4,000 km/hr.

Catapult rigs are another option. Here, a sledge fitted with components to be tested attaches to the end of a hydraulic ram. The ram is then accelerated into an obstacle. The arrangement produces strain rates of about 5,000 in./in.-sec and impact energies to 80,000 J. The use of such equipment is growing steadily, particularly within the automotive industry where test-method standardization has become a greater priority.

Servohydraulic high-rate strain-testing equipment, as from Instron Corp., Canton, Mass. (www.instron.com), is yet another option. Servohydraulic systems tend to be more versatile than traditional pendulum-impact and dropweight rigs. For example, Instron's VHS8800 produces strain rates from 1 to 1,000 in./in.-sec and impact energies to 10,000 J. It also does conventional static and quasi-static tests and may be configured for high-speed tension, bend, and impact tests at temperatures between –150 and 600?C. The use of large oil accumulators to drive the actuator piston lets the rig do constant velocity tests, something other high-rate testing equipment (except catapult rigs) can't do. It can also test a wide variety of specimen types, including seat-belt webbing materials and fastenings, automotive body and dashboard components, forging alloy compression specimens, and aircraft panel composite tensile specimens.

One company using the VHS8800 is Volvo Technology Corp. in Got?borg, Sweden (www.tech.volvo.se). Data from high-strain-rate tests on waistedtensile specimen will act as input for crash simulations. This is part of a collaborative effort with truck maker Volvo LV to investigate mechanical properties of several steel grades typically used in automotive manufacture. The equipment will also measure strain-rate sensitivity of joints made with welds, rivets, adhesives, and multiple combinations of these. Crush-testing segments of structures and models to verify crash simulation calculations is yet another facet of the project.

Such high-fidelity testing and simulation can significantly reduce the need for expensive crash tests. However, test velocities to 20 m/sec, as is the case with Volvo, pose both mechanical and controls engineering challenges to rig design, says Instron. For example, conventional servohydraulic test machines close the control loop with feedback from a position LVDT, load cell, or other transducer. But such schemes don't work well in high-strain rate systems. Actuators must be accelerated to top speed prior to load impact and tests may only last 20 to 50 msec. Operating under closed-loop control in such conditions is possible, but the actual velocity will lag the desired velocity by a considerable margin, necessitating long actuator strokes. Instron significantly reduced acceleration times by using open-loop control and high-flow proportional valves and custom software. This, in turn, shortens actuator stroke and boosts actuator and oil-column stiffness.

System resonance is another key metric. Impacts from tests cause all system components to oscillate at their natural frequency. Dynamic components are made as lightweight as possible to boost resonant frequencies to levels where the vibrations can be filtered out, typically 6 k to 8 kHz. Conventional strain-gage load cells are particularly susceptible to resonance. The use of extremely stiff piezoelectric load cells helps reduce resonant vibrations, says Instron. Further vibration rejection is possible by applying strain gages to the specimen itself. A static calibration within the elastic limits of the material effectively converts the test specimen into a miniature high-stiffness load cell that is unaffected by the resonance of other system components. The technique is most effective for cylindrical parts with a large crosssectional area ratio between the specimen shoulder and gage length.

Inertia of grippers and the hydraulic actuators driving them is another concern. This is particularly true for highstrain-rate tensile tests. Here, the actuator (with lower grip attached) must accelerate to the required velocity prior to touching the specimen. So-called lostmotion grips are the traditional way to do this. The top grip consists of a lowmass threaded adapter attached to the machine crosshead with a small-diameter pullrod. A low-mass flanged cylindrical grip body (upper slackrod) forms the lower grip. It attaches to the lower part of the specimen, and a tubular component (lower slackrod) surrounds it. Initiating actuator motion telescopes the lower grip assembly until reaching the desired actuator velocity. A flange on the upper slackrod then smacks a reduced diameter within the lower slackrod, forcing the specimen into tension at high speed.

While lost-motion grips are suitable for some materials, the grips tend to bounce as the upper slackrod initially accelerates beyond the desired actuator velocity. Appropriate damping materials can help quell the bouncing, though the arrangement significantly reduces specimen acceleration, an undesirable outcome. To avoid the problem, Instron has developed a patented Fastjaw grip. The specimen top is held by a low-mass, high-stiffness grip, similar to those in lost-motion designs. The bottom of the specimen, which is extended in length, sits within a U-shaped lower grip. The jaw faces preload with pretensioning bolts but are held apart until the required actuator velocity is reached. The single-piece jaws then snap shut on the specimen without bounce.