Tips for Testing

Extensometers

• When mounting a strain gage extensometer, make sure the flexible arm is on the bottom.
• Be sure the extensometer is mounted straight and on-center of the specimen.
• Attach your extensometer properly to get best test results and avoid slippage errors.
• Use proper clip sizes for attaching to specimens.
• Support extensometer cables with cable cleats.

General

• ASTM E4 recommends on-site verification of your system at least once a year. Similar recommendations are provided in ISO 75001. Instron Professional Services provides verification services: Force, Speed, Strain, Temperature, Crosshead Speed, Crosshead Displacement, Hardness, Torsion, and Alignment. For further information, please visit our Calibration and Verification Home Page.

Load Cells

• A basic thumb rule is to warm up a load cell a minimum of 15 minutes. This is a good thing to remember especially if your load cells are 5kN or less. For larger load capacity cells, warm them up about 25 minutes. Following these guidelines will ensure stable load readings. When power is first applied to the load cell, its strain gages heat up and expands the material the gages are bonded to. This thermal expansion causes the load signal to “drift” until the normal operating temperature is reached. Eventually, the temperature of the gage and the material to which it is bonded will equalize. At this point, the warming up is complete, thermal expansion stops, and the load signal becomes stable.
• Do not put your load cell in environmental chambers or ovens. Most Instron load cell specifications restrict operation between 20ºF (-6ºC) and 120ºF (49ºC). The load cells have built-in temperature compensation, but it only works properly within normal operating temperatures. Additionally, if the load cell exceeds its maximum temperature, the humidity protection coating on the strain gages can melt and load signals will then vary with humidity.
• If testing less than 2% of capacity, good testing practice dictates you should change to a lower capacity load cell. This is because, if your system is rated at 0.5% accuracy and full scale is 100 lbs (445 N), the best your machine can measure 2 lbs (8.9 N) is some value between 1.5 lbs (6.7 N) and 2.5 lbs (11.1 N). Whereas, if the full scale were 5 lbs, it can measure 2 lbs somewhere between 1.95 lbs (8.67 N) and 2.05 lbs (9.12 N).

Specimens

• Always clamp specimens in upper grip first when that grip is attached using a flexible adapter. Hold the grip body and align the specimen to the jaw faces while clamping the upper grip. When you release your hand, the adapter self aligns the specimen into the fixed lower grip. You can now clamp the lower grip without moving the specimen. If, however, you clamp the specimen in the fixed grip first, the specimen is stressed as the adapter moves while clamping the upper grip.
• Once a specimen is clamped in place, do not balance the load. Balance the load before closing the second grip. Any load that appears after the second grip closes is a real load applied to the specimen and should not be balanced out. However, you can use the jog key to reduce the load on the specimen. But, remember…jogging changes the specimen’s gage length.
• Use Load Protect function to protect specimen from damage.