In our last blog on the topic of corrosion, we discussed how corrosion control can be a “headache”, and how controlling corrosion implies controlling the man-made contributors such as corrosion fatigue, which is a very dominant corrosion factor. Nowadays, integrated computational material engineering (ICME) and recent advances in computational capabilities can alleviate that “headache”. VPS-MICRO^® is a probabilistic ICME tool that deals with fatigue problems. It builds a Virtual Twin^® of the microstructures of any existent component, product or a fleet of products.

VPS-MICRO is a multiscale tool that can receive data from continuum simulations, such as nodal or element solutions from Finite Element Analysis (FEA), and extends the simulation to the microstructural level. One can now accurately consider the microstructural complexities that make geometrically-identical test specimens have significantly different fatigue lives, even in well-controlled laboratory tests. The fatigue life variation caused by the microstructure is compounded by other variations in actual products in the field. VPS-MICRO overlays complex microstructures (with probabilistically-generated material properties) onto the continuum solutions, and runs simulations by transitioning cracks from nucleation to crack growth to final failure. Monte Carlo formulae are used to generate the complex microstructure: grain sizes, defects, orientations, frictional strengths, etc. The theory of VPS-MICRO is based on cyclic crystal plasticity, microstructural small flaw fracture mechanics, and long crack linear elastic fracture mechanics (LEFM).  The main output result from VPS-MICRO is the probability of failure of the tested component. It also provides more detailed information about the failed components, such as the expected statistical distributions of lives (in cycles), micro-cracks and the features on the fracture surface (i.e. grain sizes and orientations, frictional strengths and micro-stresses). All of these outputs can be visualized by using VPS-MICRO’s post-processor.

In VPS-MICRO, corrosion is considered one of the microstructural complexities. Corrosion results in geometrical microfeatures on the corroded surface. These microfeatures induce high stress concentrations (k_t) that initiate cracks more easily than a smooth surface. As an example, corrosion features in aluminum occur as big pits (macro-pits) that encompass smaller pits (micro-pits). These pits result in high stress concentration gradients that die out over a very thin layer into the depth. Stress gradients caused by micro-pits are called micro-gradients, while stress gradients caused by macro-pits are called macro-gradients. Micro-gradients can have a higher stress concentration than macro-gradients, due to the extremely rough topology on the microstructural scale, but they also die out faster.

Although the high stress occurs over a very small surface layer, corroded surfaces can reduce fatigue life by several orders of magnitude. This k_t occurs over a small scale, where the variation in the material’s microstructure greatly influences nonhomogeneous properties. In VPS-MICRO, these stress concentrations are applied concurrently to the crack growth models (nucleation, short crack, and long crack) as the crack grows. Microstructural small flaw fracture mechanics (SFFM) must be used to properly account for the thin layer of roughened surface. Continuum analysis methods do not work at this scale. To learn more about the theory behind stress and crack propagation adopted in VPS-MICRO, check out this example.

From previous projects, VPS-MICRO has provided reliable predictions of the lives of corroded components. The predicted results were within acceptable bounds of the experimental results, which highlights the benefits of using VPS-MICRO to build Virtual Twins of potentially corroded components. Employing this technology reduces the number of empirical physical tests required to qualify a component. VPS-MICRO provides time- and cost-effictive assessments of materials and designs. This decreases lead time, increases return on investment (RoI), and most definitely cures the corrosion control headache!

We previously discussed how corrosion can be “the good guy” for some applications when degradation is beneficial, such as in self-deteriorating medical implants. Also, how it can be “the bad guy” for some other applications where degradation impairs the structural integrity, like in airplane structures and oil and gas pipelines and vessels. As the bad guy, its impairing effect needs to be mitigated; and as the good guy, corrosion needs to be intentionally designed. Thus, both need to be managed and controlled. Corrosion control eats up time and money in both cases. Controlling corrosion, as the bad guy, has prompted manufacturers to develop a corrosion management system (CMS) to mitigate the effect of corrosion, since its global cost exceeded US$ 2.5 trillion in 2013 (3.4% of the global GDP). However, this CMS is not only applicable to unintended corrosion. It also can be adopted when corrosion is part of the design (the good guy). However, CMS requires advanced labs, massive amounts of data gathering and experiments, and years of experience, which in return consumes too much time and money.

With regard to CMS, the National Association of Corrosion Engineers (NACE) proposed a hierarchical pyramid that can be implemented. The pyramid consists of many elements and procedures that span from upper management down to testing practices that are tailored to control corrosion. Corrosion alone is seldom the cause of degradation and failure. The structural failure is induced by manmade—controllable—factors such as dissimilar material galvanic corrosion, stress corrosion cracking and corrosion fatigue. Controlling corrosion means controlling those manmade factors. It is the combination of corrosion and loading that has been the dominant factor driving failure for many industries such as healthcare, oil and gas, weapon systems and infrastructure and aerospace. For example, the Department of Defense (DoD) estimated that its cost of unintended corrosion, that is mainly caused by corrosion fatigue, exceeded US$ 21 billion in 2010. Assessment of corrosion fatigue requires collecting a massive amount of information and data about the microscopic structure of the materials and their reliability. That requires years of experience and running a huge number of experimental tests. Consequently, huge amounts of time and money need to be spent in order to design, make decisions and implement action items. However, in the past few years, computer modelling in material science has advanced to the point that we can start to predict corrosion damage based on the underlying physics instead of relying on large test programs. Using integrated computational materials engineering (ICME), which combines industrial knowledge with physics, we can reduce the time and money needed for the decision-making process.

VEXTEC has developed the Virtual Twin^® that predicts the probability of degradation down to the microstructural level, where damage starts. The latest version of VEXTEC’s software VPS-MICRO^® provides a probabilistic physics-based assessment of materials exposed to corrosion fatigue. VPS-MICRO is a versatile software. The software combines our historical knowledge of corrosion and fatigue with physics-based computations. It is multifunctional and can interact with the different elements of a corrosion management system depending on the type of requested results. Engaging VPS-MICRO as a Virtual Twin can provide a time and cost-efficient evaluation of new or existing materials and designs. It not only reduces the corrosion control cost, but it increases the return on investment; less amount of experimental testing, faster evaluation, and less hassle during the decision-making process. The technology adopted in VPS-MICRO will be discussed in the concluding blog in this series.

The topic of corrosion makes recurring appearances in the media; it seems that when you hear about one corrosion-related problem, invariably there will be others reported-on at around the same time. There has recently been a spate of articles confirming that corrosion is currently a headache to the oil and gas sector (undersea bolt failures), as well as to the aviation sector (corrosion-induced fatigue of turbine engine blades in the new Dreamliner aircraft). Oftentimes these stories are first published by financial-leaning news outlets (Wall Street Journal, CNN Money, Bloomberg), a result of the high visibility and cost that these incidents bring in terms of replacement and downtime to their respective industries. Enough of these stories circulating over the span of a few news cycles will make any investor wary, and will prompt questions on what is being done from a regulatory standpoint to restore confidence in companies’ operations. This is particularly true when these reports of corrosion failures have impacts (real, or perceived) on public and environmental safety.

Of course, corrosion is not a new phenomenon. We have been observing the process of corrosion for centuries in our manmade structures, and have developed ways to physically mitigate its effects (painting, inspection methods, et cetera). However, it has only been in recent history that we a) have deeper understanding of the electrochemical processes that describe corrosion, and b) have the industrial engineering prowess to design and build ever greater machines and superstructures that help make modern life possible (economically-available energy sources and air travel, being prime examples). The confluence of these two factors drive the need for more development of mechanistic approaches to corrosion mitigation, through the use of computer-assisted modeling and simulation.

To that end, more and more resources are being appropriated for the research of these corrosion mechanisms in many of the materials that are used today. For example, members of the LIFT Consortium (Lightweight Innovations for Tomorrow) have begun work on the development of new models and a material properties database that will allow for more accurate simulations of corrosion in aluminum alloys used in aerospace and other transportation sectors (focusing on aluminum alloys containing copper, lithium, magnesium, manganese, and zinc). The materials database will be characterized to such a degree so that precise information is obtained about the interaction between microstructure and corrosion. The team will begin with the characterization of the industry’s workhorse alloys, and then extend work to evaluate newer alloys crated using various manufacturing techniques. The goal is to mitigate corrosion in a broad spectrum of aluminum alloys through improved simulator capabilities.

However, only half of the equation is being studied by LIFT: the corrosion impact on metals…with no discussion of how that corrosion introduces damage states, from which stress corrosion cracking and other types of corrosion-fatigue can arise. VEXTEC has pioneered development of a software for the U.S. Navy that predicts the statistical distribution of stress corrosion cracking in an alloyed aluminum microstructure that has been exposed to a corrosive environment. This software serves as a basis for all types of materials that are impacted by corrosion: the material modelers can provide the inputs of the corroded damage states into the VEXTEC software, which will in turn simulate the result of in-service loading on the durability of the critical structures of interest.

Until such time as corrosion has been completely removed as a mechanism in a critically-stressed component (and that time is not approaching anytime soon), it isn’t enough to just model the corrosion characteristics…we must also be able to effectively model the subsequent damage growth throughout the component’s service life.