Engineers and other decision-making agents utilize data about the materials of fabrication of load bearing structures to determine their durability, reliability and the overall safety. The data can be from a number of sources including the original manufacturing specifications, from manufacturing quality control, or from measurements done after the fact as part of condition assessment. Non-destructive testing (NDT) methods are appealing because they allow for estimating the characteristics and properties of assemblies and structures without damaging or jeopardizing the function of the structure during testing.
Non-destructive testing during condition assessment on existing structures in the field is very important to safety and the protection of the environment. We have a large inventory of existing infrastructures that may have changed from the time they were originally manufactured as well as existing infrastructures that would not meet the current standards of design and fabrication. One goal with condition assessment is to minimize the risk of a catastrophic event such as the break of a large oil or gas pipeline, the collapse of a bridge or the failure of a large pressure vessel. These events still occur too frequently in our society.
Non-destructive testing can be used to evaluate, among others, the existence and size of cracks, changes in material thickness for corrosion, and the properties of the materials. Properties of the materials that can be of interest include the chemistry, mechanical properties and the cracking resistance under the service environment and/or the cyclic loads.
Current industrial non-destructive techniques for mechanical properties can be limited in scope to measuring the hardness of a material by indentation, which provides an index of a material's resistance to penetration by a hard indentor or stylus. Although indentation testing is widely used, the traditional equipment provides a hardness value which is not a reliable measure of mechanical properties such as yield strength or ultimate strength, and provides no measure of ductility. A recent variation of the indentation hardness test uses a series of spherical indentations of progressively increasing depth at the same material location to provide an estimate of the stress-strain curve of the material. This technique requires generating multiple indents in each region of the structure where an estimate of the material properties is desired. Therefore, these series of indents have limitations with respect to the study of microstructural gradients, such as changes in properties through welds and surface modifications. This apparatus and method are detailed in U.S. Pat. No. 6,945,097 B2 dated 20 Sep. 2005. Another variation is instrumented indentation, whereby the reaction force on the stylus and its relative displacement is monitored during a loading and unloading cycle. The load-displacement information is then used to predict material hardness and elastic stiffness as described in Oliver and Pharr's 1992 paper, “An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation techniques.” More recently, Dao et al. utilized the load-displacement information along with numerical models to develop predictive algorithms for determining the complete stress-strain curve.
It is known in the prior art to use a hard indentor or stylus to deform materials by applying a vertical force and displacement and inducing a lateral movement of the indentor or stylus. These tests are often called scratch, or contact mechanics experiments. They introduce material and geometrical changes to the substrate surface. Contact mechanics tests have been used for material characterization throughout history, including in 1812 with the publishing and later broad adoption of the Mohs scale of mineral hardness. Over the past decades, advances in instrumentation to perform contact mechanic experiments have helped to elevate the amount of information that can be obtained through contact mechanics experiments. A number of test apparatus and methods have been developed and disclosed. However, the apparatus and techniques known to the inventors assume that the substrate can be brought at a desired angle with respect to the stylus.
Currently, contact mechanics tests are used to measure the strength of thin-films and coatings. This test is done by using a hard stylus to engage with the material while moving the stylus along the material's surface and controlling the load being applied to the stylus until failure occurs. This testing method is described in U.S. patent application Ser. No. 10/362,605, and is limited to select applications where materials utilize thin-films or coatings. This restriction makes the technology unsuited for assessing mechanical properties of common engineering materials. In addition to coating strength, recent academic research by A. T. Akono et al. has used contact mechanics tests in an attempt to correlate with the fracture toughness of materials. The implementation assumes that the crack forms at the apex of the stylus in-front of the direction of sliding. Contact mechanics tests have also been utilized to predict the yield strength and ductility of metals through the use of numerical modeling and dimensional analysis. All of these contact mechanics methods utilize existing laboratory testing devices and systems, but the underlying test apparatus is either too complex or not sufficiently accurate for broad commercial use. As a result, existing testing systems provide only partial solutions for evaluating mechanical properties.
Based on the above, contact mechanics experiments are not performed in the field or in industrial facilities as much as they could be if the capabilities were improved. Field testing solutions have been developed using indentation techniques. Examples include the King Portable Brinell Tester, Telebrinell Tester, Shear Pin Brinell Tester, Leeb (or rebound) Tester, and Automated Ball Indentation (ABI) Tester. These field devices use various methods of aligning the system with the structure being tested. Each method, however, requires the use of contact points that remain stationary. As a result, the devices must be connected and disconnected for each individual test location, or alignment of the devices is not maintained. Furthermore, these indentation testers provide limited information about the ductility of the material, especially within the heat affected zone of welded joints. Indentation testing also typically provides limited information with respect to the cracking resistance and toughness of the material under service conditions. The ductility of a material is an indication of how it will stretch or deform permanently before it breaks. The alternative solution for evaluating existing structures in the field is material removal for laboratory testing, which requires repair and limits the number of locations that can be tested without jeopardizing the integrity of the structure.
In some instances, the surface properties of the material that is measured through contact mechanics may not be representative of the bulk behavior. This is because gradients in properties may exist due to prior fabrication and manufacturing processes. These processes include heat treatments, cold forming, hot rolling, shot-peening, and others. There are currently no existing methods to systematically account for these gradients in mechanical properties, and therefore contact mechanics tests are only applicable for the small volume of material that is directly probed.