This Small Business Innovation Research (SBIR) Phase I project will determine the feasibility of a novel ultrasonic non-destructive method for detecting and characterizing defects such as fatigue cracks in metals. Existing ultrasonic methods have difficulty detecting cracks in the presence of the typical surface pitting found on pipes, pressure vessels, storage tanks and other steel plate and beam structures, particularly if access to the inner or outer surface is limited or costly. The proposed method, Shear Polarization Contrast (SPC), uses a proprietary transducer to generate and analyze shear waves with arbitrary polarization, generated on one surface and reflected from the far surface back to the transducer, to distinguish highly polarization-sensitive features such as cracks (highly anisotropic) from benign pitting and corrosion that are less polarization-sensitive (relatively isotropic). In Phase I, polarization sensitivity will be analyzed for various combinations of defect orientation and size in the presence of varying degrees of pitting to develop and validate a predictive model for crack detection threshold and sizing accuracy. After Phase II prototype development, the anticipated Phase III laboratory and field instruments will allow thorough non-destructive evaluation of components and structures, including the interior and both surfaces, while only requiring access to one surface.
The broader impact/commercial potential of this project will be improved safety and protection of capital investment in critical civil, energy, petrochemical and industrial infrastructure. This project will incorporate novel techniques to electronically steer the polarization of acoustic shear waves to analyze and characterize their interaction with the physical, internal micro-structure of engineering materials, providing new analytical capabilities for Non-Destructive Testing, materials analysis and forensic engineering. For example, inaccessible inner surfaces of plate and beam components in aging critical infrastructure such as bridges could be inspected for cracks, the remaining useful life of these structures estimated, and a plan for reinforcement, replacement or demolition could be implemented. Until now, this capability did not exist. Resultant commercial equipment will be a field-deployable system integrating a proprietary Electromagnetic Acoustic Transducer (EMAT), specialized electronics and software. EMATs require minimal test surface preparation, allowing production to continue for even high temperature processes, reducing or possibly eliminating loss of production and the inconvenience of emptying vessels. The system?s competitive advantage is the unique ability to test and characterize cracks in the interior volume and on both surfaces, as well as classify cracks located under and within pitted and rough areas, only requiring access to one surface.
Research Outcome: This SBIR Phase I research has proven the feasibility of Shear Polarization Contrast (SPC) as a new ultrasonic Non-Destructive Evaluation (NDE) method for locating and sizing cracks in a wide range of engineering structures and components. Through finite element model simulation and analysis of experimental observations, a theoretical explanation of SPC was developed that established a method of detecting cracks as well as estimating crack depth. Additional simulated cracks were then analyzed and the SPC measurements were found to be within 15% of actual depth, which is within typical NDE requirements. Phase I experiments also confirmed that SPC offers significant technical advantages over existing NDE methods and has excellent potential to improve test speed and convenience, increase sensitivity to smaller early-stage defects, and detect cracks that are usually obscured within pitting and corrosion. These technical advantages, plus the reduced need for test preparation described below, could significantly reduce test costs and/or encourage more frequent testing. By replacing other NDE methods in some circumstances and expanding testing where it was previously too expensive or difficult in others, SPC is expected to improve safety and potentially save millions of dollars per annum by sustaining United States industrial, energy, commercial and transportation infrastructure. Technology: SPC is based on a proprietary Electro-Magnetic Acoustic Transducer (EMAT) that has the unique ability to produce and measure ultrasonic shear waves that have variable polarization angle (i.e., the direction of particle motion). The EMAT (and associated instrumentation and software) allows a material under test to be subjected to two shear waves having a common propagation path but orthogonal shear wave polarizations. Cracks modify the material’s elasticity in a highly directional way, and the interaction between the crack and each of the two shear waves propagating along the crack surface depends on whether the polarization is perpendicular (most affected) or parallel (least affected) to the crack surface. After the two shear waves have passed through a region containing a crack, there are increased differences, or contrasts, between the two ultrasonic signals. Crack depth is found by analyzing the received ultrasonic signal contrasts (measured as relative differences in phase and amplitude) over a range of frequencies and analyzing interference patterns due to slight differences in propagation velocity caused by the crack. Anticipated Commercial Benefits: Detecting surface cracks on inaccessible surfaces is presently difficult and sometimes impossible without resorting to expensive and cumbersome NDE methods. For example, detecting small inner-surface cracking on large storage tanks often requires direct access to the surface, so the vessel must be taken out of service, emptied, and the inner surface must be cleaned and prepared. SPC will take advantage of the fact that many engineering components and structures have two generally-parallel surfaces (examples include pipe and tank walls, plates and gussets, the web and flanges of I-beams, C-channel, box sections, etc.). This allows for an ultrasonic test beam to be generated by the SPC EMAT on whichever surface is more convenient or accessible, travel to the far surface, and reflect back to the EMAT to be analyzed. Any crack on the surface nearest or opposite the EMAT and in the intervening column of material will interact with the shear waves and be detected. In this way, the inner surface of a storage tank could be tested from the outside surface without emptying the tank. Similarly, by removing the need to access both the inner and outer surfaces, SPC could significantly reduce inspection time and costs for pipes and pressure vessels, ship and aircraft hulls, and the plate and beam components in aging infrastructure such as bridges and steel-frame buildings. Another major problem in NDE is distinguishing between dangerous surface cracks and benign surface corrosion and pitting. Defects masked by pitting have led to deadly, expensive and environmentally damaging failures. In Phase I bench testing, the highly directional effect of a crack on a material’s acoustoelastic properties was found to produce a high contrast as the shear polarization angle varied, compared to much smaller contrasts produced by pitting and corrosion that are generally non-directional. The excellent differentiation between small cracks and pits is unique to SPC, and provides opportunities for market leadership. EMATs provide several additional benefits compared to the typical transducers used for ultrasonic testing: fluid/gel couplants are not necessary since EMATs create and detect ultrasonic waves electromagnetically in the material’s surface; EMATs typically require minimal surface preparation; paint and other coatings have little or no effect; and EMATs can be designed for much higher surface temperatures. These characteristics can reduce test preparation and down-time in applications such as manufacturing and industrial processing. In fact, for many applications, SPC will allow testing to be performed while production continues, saving test preparation costs and eliminating lost revenues from production down-time.
