Fracture control and structural integrity (FraCSI) education and research

Abstract

Over the past six decades, fracture research has evolved into a coherent interdisciplinary field of research and education motivated by the need for structural integrity assurance and safe operation of components that are part of air, ground and water transportation systems and power generation systems. This paper explores opportunities and challenges that lie ahead and a strategy for meeting them. The design, material selection and choice of inspection techniques, inspection criteria and intervals are seldom considered concurrently leading to designs that often take more time from start to finish than afforded by product design cycle times particularly when new and innovative materials are involved in a competitive market place. This need must be addressed with a systematic approach to educating fracture control and structural Integrity (FraCSI) engineers; this paper explores this need and the case for shaping the field into an even more cohesive discipline.

1. Introduction

The interdisciplinary field of Fracture Control and Structural Integrity (FraCSI) aims to achieve safe operation of engineering components and structures by engaging several disciplines in science and engineering. FraCSI analyses focus on design evaluation and assessment of margins between safe and normal operation and the possibility of fracture. This is sometimes considered in a probabilistic frame-work and expressed as probability of failure.

FraCSI has evolved as a field at the interface of several disciplines including Mechanical, Materials, Aerospace, and Civil Engineering, Mechanics, Physics, and Mathematical and Computational Sciences and is now also touching Biomedical Engineering and Geology. The field has continued to prosper over the past six decades while continuously expanding its capabilities to address new classes of deformation, fracture and crack initiation and crack growth problems. Past thirteen ICF conferences since 1965 have contributed immensely to keep the field in the fore-front of science and engineering research. However, rapidly changing business environment characterized by a need to reduce development cycle of new products, e.g. aircraft systems, power generation systems and others has put on added pressure of expediting commercialization of new ideas on all scientific and engineering disciplines including FraCSI. Insertion of new materials is particularly seen by designers as a long lead-time item and often the critical element in new product design cycle with the need being unmet thus far. This must be addressed by FraCSI professionals and it can only happen if FraCSI analysis backed by appropriate design codes is part of front-end considerations during design.

The past six decades have seen significant progress in experimental capabilities for materials characterization, molecular and atomistic scale computational modelling, nondestructive inspection techniques, scientific understanding of the fracture phenomenon within the crack tip process zone in the form of damage mechanics, finite element analysis as well as new developments and better understanding of the limitations of single parameter linear and nonlinear fracture mechanics. Similarly, there has been progress in understanding of crack initiation phenomena related to fatigue, creep and environmental effects and their synergistic effects. However, a question arises that deserves in-depth probing. Have we maximized the potential of these developments in the design of next generation structural materials and/or highly efficient structural components? For example, we can now estimate and measure as well as compute the concentration of hydrogen in the crack tip region in ferritic and austenitic stainless steels because of progress in molecular dynamics and atomic scale ab initio calculations; but, can we then use this information to quantitatively estimate what that means to damage tolerance of a pressure vessel as mandated by ASME design codes for design of vessels used to store hydrogen at high pressures? The answer is no and there is a disconnect there that must be addressed.

We are also at the cross-roads where few may even believe that the field FraCSI has now matured and does not need the same level of attention in research resource allocations. Conversely, it can be argued that there is greater than ever need for various disciplines within the umbrella of FraCSI to coalesce more effectively than in the past to deliver implementable methods and analyses tools that can be inserted in the direct-path of design of future engineering systems and thereby accelerating the design process by minimizing the number of iterations needed and the time between iterations. Similarly, there is an issue on how long it takes to develop and qualify new materials for use in primary structural components. Can this new material development cycle be compressed in time to match with the typical product development cycle? Other relevant questions are about how we educate our young scientists and engineers to be well prepared for the future, and how FraCSI professionals should be organized in industry and in research laboratories to address the challenges described above? This paper explores some of these very important questions.

2. Examples of future projects whose success depends of FraCSI expertise

In a new paradigm, design, materials selection, and specification of inspection techniques, inspection criteria and intervals as a part of good maintenance practices, must be concurrently addressed such that suboptimal designs that are either too bulky and expensive or in the other extreme carry a higher than tolerable risk of fracture, can be avoided. There are numerous examples of where these needs are essential but we will focus on three that are within the realm of the author’s expertise.

2.1. Energy storage for hydrogen/fuel cell propelled ground transportation systems

The key benefits of hydrogen fueled cars, especially in urban areas, consist of the following¹:

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Reducing greenhouse gas emissions.
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Advancing renewable power using hydrogen for energy storage and transmission.
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Highly efficient energy conversion.
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Fuel flexibility—use of diverse, including clean and renewable fuels.
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Reducing air pollution.
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High reliability and grid support capabilities.
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Suitability for diverse applications.
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Quiet operation.
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Low maintenance needs.

Key challenges, on the other hand, consist of the following [1]:

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The desired range of 300-400 miles/tank determines the capacity of onboard fuel tanks to compete with vehicles on the market, see Fig. 1.
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Volumetric energy density considerations require high hydrogen pressures in the on-board as well as ground storage fuel tanks, see Fig. 2.
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The cost of producing and delivering hydrogen from zero- or near-zero-carbon sources must be reduced.
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To reduce filling time for vehicles, ground storage tanks must operate at pressures to fill on board tanks in few minutes.

All these challenges are addressable and the FraCSI approach is critical in several ways in meeting them as discussed next.

To meet the design targets of volumetric density of stored energy in the form of H₂ gas, the operating pressures must be raised to 1000 bar as can be seen from Fig. 2. Also, to keep the costs of storage tanks down, we must utilize ferritic steels with medium alloy content that are hardenable, inspectable, and easily processed in the form of large diameter (∼400 mm ID) seamless pipes with wall thicknesses in the range of 30 to 40 mm, and lengths in the range of 10 m. This application requires what is known as Type II cylinders in which a seamless steel tube acts as liner and the liner is wrapped by a jacket of either a carbon fiber/epoxy composite or an ultra-high strength steel wire. The liner material must be resistant to hydrogen embrittlement at high operating pressures and the design must meet all applicable American Society of Mechanical Engineers (ASME) design codes for pressure vessels, specifically ASME Section VIII-Division 3 with special requirements for service in hydrogen, article KD-10 [1].

Fig. 1.
Sales of vehicles as related to their range in miles between successive refueling.

Fig. 2.
Energy density of stored hydrogen in terms of gravimetric (MJ/Kg) and volumetric measures (MJ/L).

The biggest FraCSI challenge in this case is understanding and quantifying the effects of hydrogen embrittlement in ferritic steels employed here. The issues include the propagation of any manufacturing flaws that escape detection at the time of manufacturing and how variables such strength, hydrogen pressure, hydrogen purity levels, load ratio and frequency of the loading cycle affect its propagation rate. Figure 3 shows the damage processes that operate at the crack tip in hydrogen gas environment [2]. Models are available to model gas transport, diffusion of hydrogen in the plastically deformed and high geometrically necessary dislocation (GND) density region near the crack tip leading hydrogen enhanced localized plasticity (HELP), and the hydrogen enhanced decohesion (HEDE) region. An example of such data as a function of load ratio is shown in Fig. 4. In spite of this very resource intensive (3,4) and hard to obtain data, it is incomplete in many respects and insufficient for meeting applicable design codes [1] in the following ways.

Fig. 3.
Crack tip degradation mechanisms operating in gaseous hydrogen environment [2].

American Society for Testing and Materials (ASTM) provides specifications [5] for A372 Grade J Class 70 steels that are acceptable for use in pressure vessels by the American Society for Mechanical Engineers (ASME) design codes but are known to have degraded fatigue crack growth resistance in the presence of high pressure hydrogen [3,4]. The need is for the understanding of the damage kinetics model to be combined with experimental data shown in Fig. 4 to provide practical engineering models with capabilities to both interpolate and extrapolate the effects of the variables mentioned above that influence the FCGR behavior in H₂ environment. After five decades of research on hydrogen embrittlement, such models are elusive. Compensating for gaps in knowledge and understanding by conducting experiments is both time consuming and expensive.

Fracture research is believed to be highly interdisciplinary but, it has lacked a systems approach that addresses real and actual problems. The field today has sufficient breadth so the people must choose their specialty within FraCSI creating a void in systems thinking that brings the best of scientific understanding to solving real engineering problems. Materials selection for a long time had the same issues until Mike Ashby and his group provided engineering tools to address the gaps in technology².

Fig. 4.
FCGR behavior of A372 Grade J Class 70 steel used in hydrogen storage vessels [3,4].
2.2. Materials for use in high temperature components

Structural integrity of hot gas-path sections of aircraft turbines (Fig. 5)³ is a primary design concern because of the harsh environment that these components are subjected to during service. These include transient and steady-state thermal stresses, hold times and static and cyclic stresses due to external loading in fracture critical locations. Design concerns include creep deformation (primary, steady-state, and tertiary creep) and rupture, creep-fatigue and environmental effects, varying material properties due to thermal gradients, complex crack geometries and variable amplitude loading. Thus, this is perfect example for use of FraCSI for selecting the right material and a set of safe operating conditions during service. Blades for land-based gas turbines are made of directionally solidified (DS) materials as shown in Fig. 6 in which the elastic and creep properties are orthotropic adding another degree of concern. The design cycle of jet engines is of the order of 7 years while new material qualification can take much longer, limiting designers of jet engines to use of existing materials!

Analytical frame-work exists for predicting creep-fatigue crack growth in creep-ductile materials but a more rigorous frame-work is needed to extend the concepts to creep-brittle materials in which there is a competition between environmental effects and creep damage in advancing the cracks [6]. For the DS materials, we need to account for grain boundaries and differences in crystallographic orientations on the creep deformation at the crack tip as the crack transitions from one grain to the next.

Thinking about components that operate at elevated temperatures in general, some progress has been made in the assessment of crack growth in welds under creep-fatigue loading but future studies must consider microstructural gradients and transition layers between the weld metal and the base metal and cracks that meander from one region of the weld to another. Fundamental studies are needed to better understand creep-fatigue-environment interactions.

Field validation of creep-fatigue model predictions should be given a high priority [6].

Fig. 5.

Inside of aircraft gas turbine showing the hot section components [3].

2.3. Prognostics of rupture in soft tissue-abdominal aortic aneurysms (AAA)

FraCSI approach can potentially play an important role in developing accurate prognostic tools for the occurrences of rupture in blood vessels such as in the abdominal aorta. Ruptures result from degradation of the vessel walls due to age and disease. Schematic representation of abdominal aneurysm is shown in Fig. 7⁴. Rupture in AAA has a mortality rate of 90% and there are 20,000 new cases of aneurysms diagnosed every year in the United States alone. The current criterion used for intervention is quite simplistic and states that the risk for rupture is high when the aneurysmal diameter reaches 5 cm or more [7] without regard to individual differences person to person. Consequently, ruptures occur that may be preventable.

Fig. 6.

Directionally solidified land based gas turbine blade showing elongated grains along the blade axis.

Fig. 7.

Schematic of a normal and aneurysmal aorta [4].

Figure 8 shows the stress-strain behavior of tissue material excised from aortas of 5-month old pigs that are healthy. Tests were subsequently also performed on degraded aortic material that had been exposed to a collagenase solution that degrades the collagen fibers. The microstructure of the healthy and degraded tissue is also shown in Fig. 8 [8,9]. It is obvious from Fig. 8 that the degradation results in much lower load carrying capability of the tissue material. Aneurysms form because of degradation that occurs in localized regions of the blood vessels and those regions deform preferentially compared to the relatively healthier tissue.

Fig. 8.

(Top) uniaxial stress-strain behavior of material excised from a healthy porcine aorta compared to the same material exposed to collagenase solution that accelerates its degradation for various periods ranging from 4 hours to 16 hours and (bottom) microstructure of the aorta wall showing the collagen fibers in a matrix of elastin in the (A) healthy and degraded states (B) 4 (C) 8, (D) 12 (E) 16 hours exposure to collagenase [Alfaori, [8]].

Figure 9 shows the considerations for a FraCSI approach to the development of a prognostics tool that can be used to reliably predict AAA rupture. This begins with the development of constitutive equations that include visco-elastic and visco-plastic material response, and considerations relating to large deformations, inhomogeneity, and anisotropy in the material behavior. With finite element codes that are available today, such capabilities can be developed. A suitable fracture/rupture criterion will also have to be found. The similarities and differences between the behavior of human and porcine tissue will have to understood. The latter task is non-trivial because access to healthy and diseased aortas are difficult. Therefore, predictions should be validated by comparing them to results from successive CT images from the same patient. The latter ability will be a confidence builder in the prognostic tool and the collected data can be used to improve the accuracy of the constitutive relations derived from model materials such as from pigs.

Fig. 9.

Considerations in building a FraCSI based prognostic tool for predicting the growth and rupture of AAA.

3. Thoughts on FraCSI education

Amidst all the information/knowledge that we want to pass on to our students, we must never forget that the larger goal of education is to develop the student mind to solve societal problems. We cannot teach the students everything they should know to be professionals for 50 years but we can teach them how to learn. There are two quotes that for me have served as guiding principles as of late. They are:

‘Nothing is of more importance to the public weal than to form and train up youth in wisdom and virtue. Wise and good men (or women) are, in my opinion, the strength of a state: much more so than riches or arms, which under management of ignorance and wickedness, often draw destruction, instead of providing for the safety of a people’ – Benjamin Franklin, 1750

‘Wisdom is a combination of knowledge and the ability to process knowledge to make good decisions. One without the other is not wisdom’ – Ashok Saxena, 2009

Keeping in mind that we should use FraCSI education to simultaneously build specialized skills and wisdom, we continue to explore appropriate formal and continuing education in the field of FraCSI.

3.1. Formal FraCSI education leading to advanced degrees

The evolution of a discipline often follows the sequence of first being a multi-disciplinary program in which experts in different disciplines come together to solve a complex engineering problem that cannot be addressed within the scope any single discipline. Fracture is a prime example of this where the disciplines of mechanics, materials science and engineering, mechanical, civil, and aerospace engineering have collaborated to move the field forward. Often with time, such initiatives evolve into a more cohesive interdisciplinary program in the form of a research center that offers cross-listed courses and shared and centralized research facilities and opportunities for collaboration. This then can evolve into a single discipline that includes several embedded sub-disciplines. FraCSI has certainly taken the first two steps but not the third, so, it is interesting to explore where we stand on that and whether it is a goal even worth pursuing.

Organization of future education and research must enable the development of FraCSI professionals who at the front-end are adept at working at the interface of various types of expertise needed to address complex problems before handing the problems over to the scientific experts. These professionals must also be good at translational research for integrating solutions developed by domain experts into the needs of design engineers. All other established disciplines have such technology integrators as part of design teams and now that FraCSI has evolved into a discipline, it needs its own systems engineers.

An important consideration in exploring the viability of FraCSI as a single discipline is the answer to the question whether there is a clear advantage in realizing future opportunities if all the activities were within a single administrative entity. Further, even if FraCSI becomes a technical discipline by itself, it may for practical reasons such as lack of resources, not be possible everywhere. A high performing and reliable but expensive automobile may be considered a technical marvel but is expected to be present only in few garages! There may be reasons to have FraCSI as a discipline in certain companies, universities and laboratories that can afford it and benefit from it sufficiently to justify the cost. But for other smaller organizations, it may not be suitable and similarly not for all institutions. Here, we proceed to envision FraCSI as a discipline and what it would entail and the leave the decision about its affordability to individual organizations whether industry/research laboratories or educational institutions. The related FraCSI sub-disciplines then could include:

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Constitutive relations, physics of deformation and damage, and stress analysis will address all types of linear, nonlinear, and time-dependent stress-strain behavior accounting for all types of material deformation mechanisms including multiaxial, isotropic and anisotropic properties, inhomogeneities in microstructures at meso, micro, and atomic scales, as necessary. Engineering materials today are complex enough to warrant such capability.
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Materials testing and characterization will address material characterization from microstructural and physical and mechanical properties perspectives, and will also include forensics for determining the root causes of fracture and the underlying mechanisms. Development of test methods to determine materials properties for use in design and establishing conditions for safe operation during service.
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Nondestructive evaluation (NDE) and sensors are crucial in determining the design life of new components and remaining life in the case of components already in service. New approaches and understanding in this fast-moving field are expected and the potential for exploiting the use of this technology is perhaps the most underutilized in FraCSI. Risk of fracture can be effectively mitigated by an aggressive maintenance, online monitoring, and in-service inspection program. However, such a program can benefit tremendously from analysis and prognostics.
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Data management systems are needed to capture material data along with the pedigree information about the material tested that are produced at considerable expense, collecting service operating data, and data from integrated sensors that might provide information about condition assessment. Almost all disciplines such as aerospace, mechanical, electrical, and chemical include expertise in data management and analysis as applied to their specific systems that should be part of FraCSI education also.

Figure 10 shows a pyramidal frame-work of engineering education from undergraduate to MS and PhD levels. FraCSI education must conform to these guidelines. The basic educational unit is one credit hour that consists of 50 minutes of lecture classes or 2.5 hours of laboratory instructions per week for 16 weeks, the duration of a normal semester. At the undergraduate level, there is probably room only for 3 or 4 credit hour elective courses at the dual undergraduate/graduate levels taught through case studies involving structural integrity assessments to support design, failure analysis, material selection, nondestructive inspection technique selection and criterion for accept/reject decisions. These courses could also serve as gateway to advanced courses in the same area that will serve as coursework to support graduate degrees. Following this path will produce FraCSI professionals who are well prepared to contribute to complex projects to assure structural integrity of systems.

Fig. 10.
Educational pyramid and requirements for graduate/undergraduate degrees.
3.2. Continuing education

Continuing education that is delivered online on topics related to FraCSI is an important aspect of maintaining currency and a way of spreading and sharing specialized expertise. This is needed more than ever before because people are living longer and careers are expected to span over at least 50 years. Because of the breadth of the field, multiple experts are needed to adequately cover the required range of expertise to deliver courses at the graduate level. This is an opportunity for ICF to facilitate the development and offering of such courses to its international audience. A portal for sharing case studies used for educational purposes would also be a valuable resource.

4. Summary and conclusions

This paper addresses some ideas on the changes that are needed, both in educational institutions and in research laboratories that will likely produce professionals that are well positioned to become integrators of FraCSI expertise and can add value in the front end of the design process for future systems. Examples of the design of advanced power plant equipment that operate at higher temperatures, systems for storage of hydrogen at pressures of 1000 bar safely in urban areas, and rupture in aneurysms in abdominal aortas are used as examples of where a consolidated systems effort can result in significant dividends for the society at large. All examples have important fracture control and structural integrity issues to be resolved. The paper also addresses important issues related to education at the graduate and undergraduate level as well as opportunities for collaboration and continuing education in the field. The main conclusions are:

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Use of H₂ fueled ground transportation systems have great potential for significantly reducing greenhouse gas emissions and air pollution. Fracture control and structural integrity (FraCSI) issues relating to hydrogen embrittlement and translating our understanding of it into engineering design holds the key to its future development.

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Insertion of new and innovative materials such as in highly efficient jet engines must be accelerated and it can only happen if FraCSI professional are ready to assume the responsibility by bringing all aspects of risk abatement into a consolidated framework.

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Fracture/rupture control and risk assessments can play a major role in biomedical applications such as in developing prognostic tools for predicting rupture in abdominal aorta aneurysms (AAA).

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FraCSI education needs to be modernized especially with respect to simultaneous considerations of different material types and deformation phenomena. Sharing of more case studies perhaps facilitated by ICF, and providing more opportunities for continuing education by creating online courses would be good contributions.

Footnotes

Acknowledgements

I am deeply indebted for the contributions of several of my current colleagues and the ones that I have had an opportunity to work with for the past 45 years who have shaped my thinking and approach to fracture research and education. I have been blessed throughout my career to have been associated with great mentors and colleagues for whom the balance between good science and engineering judgement was of utmost importance. Finally, I would like to dedicate this paper to the memories of Edward T. Wessel and Paul Paris who for me were great sources of inspiration for four decades. They both epitomized the balance between a sharp scientific mind with a deep appreciation for the needs of engineers and have left a definite imprint in several areas of the field of fracture mechanics.

Conflict of interest

None to report.

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