Category Archive: Strain Measurement

Optical Strain Measurement: What to Know

Optical strain measurement is a non-contact measurement technique that employs digital image processing principles. Compared to traditional strain measurement methods, this technique provides a more streamlined solution for testing operations. The increase in testing efficiency is due largely to the elimination of the need for the modification of parts for lead-wire routing and sensor attachment. 

Optical strain measurement offers engineers insight into strain behavior by providing a detailed map of the strain field. This is in direct contrast to more traditional measurement methods, which rely on single-point measurements with only presumed correlations to analytical model measurements.

Essentials of Optical Strain Measurement

Optical-strain-measurementDespite having existed for a number of years, it is only recently that the utilization of optical strain measurement techniques on a regular basis became practical. This change is largely attributed to the technological advances in the realms of computation power and digital imaging. Significant enhancements to these technologies allowed them to meet the requirements for the image processing-based measurement techniques on which optical strain measurement relies.

A wide variety of strain mapping methods have grown in popularity over the past several years. These techniques determine a part’s surface strain by observing how patterns printed on the part’s surfaces move during deformation. 

At TDI, we developed a Rotor Optical Strain System (ROSS) to facilitate optical strain measurement procedures. Using the ROSS, our team targets an area on the test rotor’s surface and marks it with a distinct engineering pattern. The use of this pattern, combined with our proprietary de-rotating system for image capturing, makes it easy to calculate strain via a digital image correlation (DIC) technique—i.e., an experimental technique that measures stress-stain. 

Benefits of Optical Strain Measurement

As optical strain measurement utilizes digital image correlation (DIC), it presents several significant advantages over other methods, including:

  • More comprehensive insight into strain behavior. Rather than relying on single-point measurements, this method provides engineers with a more detailed strain field map that offers a fuller picture of the strain field and structural performance of the tested product. 
  • More reliable measurements. Optical strain measurement eliminates the need to assume correlations between actual measurements and measurements from the standard analytical model that is typically required for single-point measurement techniques.
  • Less need for parts modification. As a non-contact technique, optical strain measurement eliminates the need to accommodate lead wires, strain gages, and slip rings. This elimination saves on parts cost and allows engineers to modify, inspect, or reconfigure tests (e.g., swapping turbine blades) with ease.
  • Less risk of unexpected failure. As this technique eliminates the need for strain gages and lead wires, it reduces the risk of measurement failure due to component failure and increases the likelihood of collected data maintaining its integrity.
  • More potential for future applications. On the whole, optical strain measurement offers a much greater degree of applicability than alternate measurement techniques. The necessary equipment can easily be set up in stand-off positions; this, in turn, allows for strain measurement even under extreme environmental or operating conditions (e.g., high temperatures).

While optical strain measurement has been used for well over a decade on static components, applying the process to high-speed rotation components is relatively new. That’s why we seek to improve measurement technologies to position optical strain measurement as a cost-effective tool for a bevy of high-speed testing and production applications.

Test Devices’ Optical Strain Measurement Services

At Test Devices, we’ve developed an application for the optical strain technique for high-speed rotating parts. Our work on this innovative and forward-thinking process presents a clear opportunity for our clients. 

Interested in partnering with Test Devices, Inc. to learn more about your parts’ capability and performance? Looking to weed out weak components and strengthen the parts you rely on in your industry and application? Request a quote today to find out more about our optical strain measurement services.

Optical Strain Measurement Vs. Traditional Strain Gauging

In mechanical engineering, “strain” refers to the degree and the way a structure deforms under a load. An understanding of the strain behavior, combined with the knowledge of the failure mechanisms of structural materials, allows intricate yet robust designs of modern, high-performance aerospace machines, including rocket and jet engines.

High-speed rotors, such as jet engine parts, are subject to very high stress induced by centrifugal force. The need to lighten the parts to enhance the performance of the engine must be balanced with its durability and structural integrity. Engineers must, accurately and confidently, know the stress/strain state of the rotor to make the critical design decisions.

What Is Traditional Strain Measurement?

Strain gauges are the traditional instruments employed for measuring strain. With this approach, a gauge is attached to the material being tested using an appropriate adhesive. For the purposes of spin testing, the test parts must be modified, and special tooling has to be designed to allow lead wire passages and slip rings or telemetry systems pass data from the rotating parts to the data acquisition system. Strain gauges are laborious to implement and prone to premature failure during the test, resulting in higher overall test costs, schedule overrun and less reliable test data.

Further limitation of the strain gauges is that it is a point measurement and is blind to the behavior of the surrounding strain field behaviors. The readings from a gauge placed on a certain position of a test rotor must be interpreted accurately to perform a meaningful comparison against an FEA model.

Optical Strain Measurement Applications in the Aerospace Industry

Test Devices Inc. has been interested in a non-destructive test and manufacturing process – the Rotating Optical Strain System (ROSS) – for some time. The ROSS is a non-contact strain measurement system which eliminates limitations, is less costly and provides more complete data, possibly allowing the measurement at higher temperatures needed for fully understanding the engine parts.

The ROSS will unlock a wealth of new information for material and component designers. The data is valuable for validating (or refining) the numerical models used in designing jet engine parts and gas turbine rotating parts as well as understanding the details of the failure mechanisms limiting the performance of existing parts.

Combined with advanced spin testing capabilities, Test Devices provides a unique testing resource for civil and military jet engine/propulsion system developers. The ROSS can accelerate the development of advanced materials and manufacturing capabilities across the gas turbine industry. It can provide the most relevant data to reduce risks in currently active turbine engine development programs.

Learn More

Optical strain instruments have been on the market for years, commonly used to measure strain in static objects. But in recent years, they’ve been steadily rising in popularity as more and more industry professionals begin to use them to measure strain in high-speed spin tests.

To learn more about how these optical strain gauges can benefit your testing and production processes, reach out to the team at Test Devices today.