Variation and complexity of materials, designs, and structures is accelerating at an incredible rate, beyond what individual sensors can grasp. Full-field optical strain provides a true understanding of the complete structural response of these complex designs. This full-field data brings a more holistic reality to computer models, allowing designers to precisely advance their designs. This capability is crucial for the development of our industries.
Twenty years ago, advanced designs used a few standard materials and traditional configurations that had worked for generations. Today, every component can be of a different material, metals to composites, each well-suited for their applications, then bonded, welded, riveted, or screwed together, with sound dampening materials, elastomers, foams, and thermoplastics, creating increasingly complex structures. Current computer models use many assumptions when these assembly complexities arise, leading to weaknesses in the final design.
Optical strain is material independent and measures the response of the integrated system, so designers can get a measure of the true response and strength of their designs. They can truly understand the capabilities of their designs and also understand unanticipated design weaknesses before manufacturing and final structural testing.
Boeing uses optical strain to fully understand their complete range of material properties that their manufacturing specifications allow. When they allow +/- 3 degrees of ply orientation, they can model the true material strength variation in their designs based on precision optical strain testing of each of those parameters. Better material properties, allows for better designs. NASA Glenn reported that they get more consistent material properties with optical strain over any other method.
Subassembly designs are made up of real parts with real manufacturing variations. As parts are assembled, they are made to fit to the final design, with inherent stresses built into these components, which should be compensated for in the design, if even known. Optical strain allows for more complete understanding of the assembly process.
Structural testing is the ultimate test of vehicle performance, with holistic measurements critical for the full understanding of the true structural response of your vehicle. The method directly aligns to CAD, so all data is precisely in vehicle coordinates. Results show the complete strain and 3D displacement data.
Optical strain is highly fast and efficient compared to traditional methods, allowing one person to do the work of teams in a fraction of the time. Boeing engineering estimates that it is 50x less labor than traditional gauges, and 10x less expensive. It also allows you to see the strain vectors so that you can fully understand the load vectors effecting your structures.
Optical Strain Method
NIAR calls optical strain the FEA of testing because it provides the continuous 3D deformation and strain field across the complete structure. It actually measures the 3D coordinates of each point across the surface of structure, which is the 3D shape, and the changes are the 3D displacement of the structure, with local displacements being strain. In 3D, these local displacements become the full strain tensor of material response, as if you had 10,000 rosette strain gauges in each field of view; each field of view is worth $1M of strain gauges.
The 3D shape is measured dynamically in 6-DOF (Degrees of Freedom), so complete 6-DOF deformations and the total strain tensor is known in 3D space and time. Optical strain measures a surface and the true strain tensor across the material surface with the desired accuracy from far finer than, or equivalent to, a traditional gauges. In many cases, you really do not know what you are measuring with a strain gauge. The method sees the development of strains going by strain gauges, and local material responses, to crystalline responses in metals, that are just seen as variance or errors in single point sensors.
Optical strain measures 3D coordinates in time, so it is used for any combination of complex displacements, 6-DOF tracking, strain, vibration, modal analysis, etc., with a non-contact, simple to use method, providing immensely more data than costly individual mechanical sensors. You measure across the structure the entire strain response, as well as the displacements that are causing the strains, with the data aligned to directly to the CAD model, so every point of the measurement is precisely in your CAD coordinates.
Validation and Certification
Optical strain was introduced more than 30 years ago and has developed into a precision technology governed by iDICs (International Digital Image Correlation Society) directed by NIST and Sandia National Labs. In the U.S., NASA was an early adopter of the technology and was the core technology used for the model validation measurements for the Return to Flight of the Space Shuttle in 2004, with validation paper, with a broad application review. [1,2]
A strong validation was performed by Lawrence Livermore National lab.  The USAF adopted it initially for thermal strain studies on the B-2 Stealth Aircraft,  and then for broad applications. Boeing was an early adopter for aerospace manufacturing, starting 2006, and finally presenting at 2012 ASNT and their ARAMIS Optical Strain certification at 2016 ASTM. 
The NIAR was the first user of ARAMIS Optical Strain in the U.S., and lead to the FAA Technical Center Fuselage Test Facility implementing the technology in 2004. [ 6, 7]
It is also covered in the ASNT Handbook.  Northrop-Grumman also performed an independent precision verification of ARAMIS Optical Strain for the structural testing of the James Webb Space Telescope,  for which they saved NASA $2M, and the telescope is working well now.
Validation of the optical strain system and the measurement is covered by VDI 2634-1 & VDI 2626, which are used to validate every ARAMIS Optical Strain system in manufacturing,  and is available to be used to truly validate every measurement FOV, using the same method.
For strain gauge measurements, only the amplifier is actually certified, not the installed strain gauge, so it really is not a validated measurement; you must rely on the precision of the installers.
Standards that follow the basis of VDI standards include ASTM E-83, ISO 10360-1, and ISO 9513. Optical strain is a form of optical photogrammetry for measuring 3D coordinates, which is the basis of VDI 2634-1 that validates optical measurements in 3D coordinates, positioning a certified standard in multiple positions within the measurement volume. ARAMIS Optical Strain can track these positions in 6-DOF in real-time (live).
Optical strain uses Digital Image Correlation (3D-DIC) for measuring strain on the surface of a material or structure, which is the basis of VDI 2626, which validates that strains throughout the volume are precisely measurable, moving a DIC rigid pattern in the volume and maintaining zero strain. The error during these tests validates the accuracy of the optical strain measurement. These accuracy/sensitivity measurements can be documented within an ARAMIS optical strain measurement, for validated measurements, with certified calibration and certified equipment.
Optical Strain Implementation
Optical strain is an accepted and used technology by thousands of companies and universities around the world and is critical for the understanding of modern materials, structures, and designs. Its implementation is far easier than the deep experience and knowledge required for the implementation of traditional gauges, and is actually fully certifiable. Training and certification is available from iDICs governed by NIST and Sandia National Labs.
Optical strain systems, certified in manufacturing, set-up following basic rules, calibrated with a certified calibration standard, make precision full-field measurements very straight forward. The measurement itself can even be certified during the measurement, all of which is documented in the data set.
Certified setup and calibration is at least 50-times faster than with traditional sensors, and the Boeing report included that optical strain produces 100-times more data. This holistic data is critical for modern materials and structures in aerospace, automotive, microelectronics, biomechanics, and civil.
- Schmidt, T., Tyson, J., Revilock, D.M., Lyle, K., “Performance Verification of 3D Image Correlation using Digital High-Speed Cameras”, Proceedings of 2005 SEM Conference, Portland, OR, 2005.
- Tyson, J., Schmidt, T., Coe, D., Galanulis, K., “3D Image Correlation for Dynamic and Extreme Environment Materials Measurements Holistic Structure Measurements from the Laboratory to the Field”, Proceedings of 2005 SEM Conference, Portland, OR, June 7-9, 2005.
- LeBlanc, M.M., Florando, J.N., Lassila, D.H., Schmidt, T., Tyson, J., “Image Correlation Applied to Single Crystal Plasticity Experiments and Comparison to Strain Gage Data”, 2006.
- Bailey, J.T., Coate, J.E., Tashiro, R., “Evaluating the B-2 Aft Deck Measured Response to the External Environment”, Proceedings of 2008 USAF Aircraft Structural Integrity Conference, 2008.
- Grossnickle, J., Gordon, T., McCrary, K., Wanthal, S., “3-Dimensional Non-Contact Optical Strain Measurements for Structures Evaluation and Optimization”, ASNT Aero, St. Louis, MO, 2012.
- Tomblin, J., Seneviratne, W., Pillai, G.R., “Effects of Disbonds, Lightning Strikes, and Low-Velocity Impact Damage on Adhesively Bonded Composite Joints”, DOT/FAA/AR-09/4, 2009.
- Avitable, P., Niezrecki, C., Helfrick, M., Warren, C., Pingle, P., “Noncontact Measurement Techniques for Model Correlation”, Sound & Vibration, January 2010.
- Tyson, J., Schwartz, E.I., “Optical Measurement of Strain and Displacement”, ASNT Handbook, Vol. 10, p.507-513.
- Pokk, A., Gurden, C., “JWST Structural Test Monitoring, Instrumentation and Data Acquisition”, 30th Aerospace Testing Seminar, El Segundo, CA, 2017.
- Witzel, O., “Acceptance and Verification of DIC Systems with Reference to VDI/ VDE Guidelines”, Intl. Digital Image Correlation Society (iDICs), Portland, OR, 2019.
This article was written by John Tyson II, PE, President, Joesph Horn, Engineering Manager, and Steve Openshaw, Senior Applications Engineer, all at Trilion Quality Systems. For more information, visit here .