Deflection, in materials science, is the amount a material deflects under compressive load. It is a displacement measurement with units expressed in millimeters or inches. This post goes over frequently asked questions around deflection measurements and equipment used to measure them.

Which mechanical properties are calculated from deflection data?

Deflection data can be obtained from compressive loading, thus from test types such as compression testing, shear testing, bend testing, and cyclic testing. A force-deflection curve is generated using data from the force transducer (load cell or a pressure transducer) and a deflectometer. Properties computed from a force-deflection curve are listed below:

  • Deflection
  • Compressive strength
  • Flexural strength
  • Shear strength
  • Yield strength
  • Yield point
  • Chord modulus
  • Secant modulus
  • Core shear modulus
  • Total work

What methods are used to measure deflection?

Deflection can be measured by using:

  1. Crosshead position
  2. A deflectometer
  3. Other displacement transducer/sensor

Refer to the ADMET Material Testing Glossary for more terms.

What is a deflectometer?

Displacement transducers are used for more accurately determining the deflection of the specimen. A deflectometer is a displacement transducer that is in direct contact with the specimen, usually at the center, and automatically record the displacement or the change in displacement as a function of the load on the specimen or of the elapsed time from the start of the test.

Is there a difference between deflection measurements using crosshead position and a deflectometer or a displacement transducer?

Yes, deflection data obtained using crosshead position differs from data obtained using a deflectometer or a displacement sensor. Thus, it is important to note the method of deflection measurement in test reports.

Requirements for quality control in production environments are usually met by measuring deflection using crosshead position. However, more accurate measurement may be obtained by using a displacement transducer such as a deflectometer.

What are the specifications of deflectometers?

Unlike extensometers, deflectometers do not come with a specified gauge length. They still have the travel range option to select from. ADMET deflectometers are offered with a wide range of travel ranges, in metric and SI:

  • Metric: 1mm, 4mm, 6mm, 12mm, 25mm, 50mm
  • SI: 0.05”, 0.15”, 0.25”, 0.50”, 1.00”, 2.00”

What are the specifications of displacement transducers?

Unlike deflectometers, displacement transducers are set up in a way that the transducer itself is not in direct contact with the specimen. Instead, they are mounted on transducer holders that are usually apart of a fixture, a compression cage, or a loading plate.

ADMET offers displacement transducers with travel ranges from 5mm to 100mm.

How is a deflectometer used?

Mount the specimen in the test fixtures, align it as needed, then install the deflectometer. ADMET deflectometers feature a single arm attached to a spherical contact tip that is positioned on the specimen to measure the deformation on the specimen during testing. Our deflectometers include a magnetic base that can be placed on the desired surface in the testing area. 

What are the calibration requirements of a deflectometer?

While the test is running at the specified speed, it is important to ensure the deflectometer is free of inertia. Standard method to verify the deflectometer and the data acquisition or the controller system it is connected to is ASTM E83. Note that ASTM E83 is intended for extensometer calibration, thus Table 1 of Practice E83 references gauge length data which is applicable to extensometers, but not to deflectometers. If otherwise indicated, ADMET Calibration team utilizes an effective gauge length of 50mm to satisfy the requirements in Table 1, as recommended by the standard. 

For modulus testing, it is recommended to use at the minimum a Class B2 instrument. For other strain measurements, most standardized test methods recommend the requirements of at least a Class C instrument.

Featured System: eXpert 2600 Electromechanical UTM

ADMET eXpert 2600 Electromechanical Testing Systems are used to perform a variety of tests including compression, bend, shear and more. ADMET controllers provide the option to collect deflection data from the crosshead position of these machines as well as from instruments such as deflectometers and displacement sensors. Gallery below showcases the use of eXpert 2600 systems for ASTM/ISO/EN test methods. Click on a picture to find out more about the equipment conforming to each standardized test method.