Optical Strain Measurement

The Issue

Traditional strain measurement techniques collect data only from a selected number of locations. Decisions on where to locate strain gages are typically based on analytical models or “educated guesses.” Often, the manner in which a gage is mounted, and the interpretation of the resulting data, is a topic of debate.

Why It Matters

The bonding of strain gages and lead wires in traditional testing makes the process of traditional strain measurement both costly and time-consuming. Bonded strain gages and lead wires are subject to large “g” forces on rotating hardware, and often to elevated temperature conditions. Unfortunately, gages may occasionally detach during testing, compromising the data obtained. It is very difficult to strain gage “tight” locations reliably. Furthermore, bonding a gage to a small feature could alter its deflection upon loading.

What You Can Do

Test Devices’ customers can now capture strain data over a full component during spin testing. Optical strain measurement can measure and record surface strain at thousands of locations simultaneously. Collected data can act as a standalone resource, or to supplement data generated by traditional in-situ methods such as strain gaging, growth probes and post-test dimensional measurement.

The strain map generated with an optical system provides a richer and more complete picture of any component deformation. Optical strain measurement is ideal for studying small or complex component features such as turbine blades and impellers.

Figure 1 – Optical Strain Measurement

Advantages of Optical Strain Measurement

Reduced cost and ease of test set-up: Eliminating gages and lead wires allows engineers to easily modify, inspect or reconfigure a test (e.g. swapping turbine blades).

Reduced risk: As a non-contact technique, optical strain measurement eliminates the costly, time-consuming process associated with bonding of strain gages and lead wires in traditional testing. Data accrued during optical strain measurement is less likely to be compromised, as gage detachment is not a factor.

Greater applicability: Optical strain measurement equipment can be set up in stand-off positions; outside of a test chamber entirely, if sightline ports are available. This enables strain measurement during testing under extreme conditions (e.g. high temperatures or aggressive environments) that would damage contact and proximity gages.

Optical strain measurement has been used for over a decade on static components. Applying it to very high-speed rotational components is relatively new, so Test Devices is addressing challenges and developing improvements to make optical strain measurement an economical “go-to” tool for a wide range of high-speed testing and production applications.