Fall 2019 > Cover Story

Cracking the Code of Materials


Asbestos, a word that seems alien to the younger generation, is a naturally occurring fibrous mineral that was commonly used for insulation and fireproofing in buildings because of its excellent tensile strength and versatility. Asbestos was widely used in construction projects in Hong Kong, particularly during the 1970s, when residential and public housing were in great demand.

Asbestos is an inexpensive building material, but society has paid a high price for its use. Since early 1980s, many places around the world, including Hong Kong, have banned the use of asbestos because inhalation of asbestos fibers can lead to mesothelioma (a type of cancer) or asbestosis, which can result in death.

This is when Prof. MAI Yiu-Wing came into the picture. Mai, a highly regarded materials scientist, is now a University Chair of Mechanical Engineering at the University of Sydney and an IAS Senior Visiting Fellow.



\\ No material is perfect. One way to construct new materials is to ‘deconstruct’ them, to analyze how they break.”\\

Back in the 1970s to 1980s, Mai was commissioned by a world-leading Australian industrial building materials firm, James Hardie & Coy Pty. Ltd., to develop a new asbestos-free material. His pioneering work on cellulose fiber cement, which is a mixture of fibers from radiata pine, sand and cement, turned out to be Mai’s most impactful and notable research achievement, and it has had a lasting effect on the science of fiber composites.

Cellulose fiber cement is a high-performance, durable and harmless advanced material widely used for façade cladding and roofing. Its discovery also led to the development of the scientific research field of “quasi-brittle fracture mechanics,” which paved the way for Mai’s research on the fracture mechanics of a range of engineering materials such as cementitious materials and their fiber composites, stitched/z-pinned composite laminates and coarse-grained ceramics.

Fracture Mechanics

Throughout the ages of human civilization—from the Stone, Bronze and Iron Ages to the present—a myriad of materials has been discovered, developed and deployed. Newly innovated man-made materials, such as polymers, carbon fibers and graphene, etc., have transformed our lives tremendously, and they are shaping the future.

“Yet, no material is perfect,” said Mai. As a materials scientist, Mai’s focus is not simply on the discovery of new materials. His research involves the study of the properties and characteristics of known materials to determine or improve their utility.


“In fact, one way to construct new materials is to ‘deconstruct’ them, to analyze how they break,” he said. “My research on fracture mechanics is largely about understanding the relationships between cracks or defects, stresses and toughness of a wide range of advanced materials under different operational conditions of stress, temperature and environment.”

Mai added that fracture mechanics, which is the study of the propagation of cracks in materials, is not only a tool used to improve the performance of mechanical components, but also vital in understanding fatigue under cyclic stresses, also known as fatigue failure, in vehicles such as aircrafts, ships and cars and in other infrastructures such as bridges and natural gas transmission pipelines.


“The accident of Aloha Airlines Flight 243 that happened 31 years ago is perhaps one of the most famous cases of metal fatigue contributing to cracking and accident,” Mai recalled. He used the tragedy of the “unsinkable” Titanic as another example to demonstrate the importance of fracture mechanics.


“Material failure was one of the key reasons for Titanic’s rapid sinking after the collision,” said Mai. Indeed, expeditions surveying the shipwreck later revealed that the below-freezing seawater temperature, high-speed collision with the iceberg and sulfur content of the hull steel plates caused the brittle fracture and eventually ended the maiden and final voyage of Titanic.1



Sailing Around the “Material” World

Mai, who has specialized in the field of materials science for over 40 years, has traveled far and wide over the course of his research life and has encountered many pioneers in the field.

During his graduate studies, Mai was deeply inspired by Prof. Charles GURNEY, his PhD supervisor at the University of Hong Kong. Gurney was best known for his research on crack propagation, in which he focused on energetics and the laws of thermodynamics to understand crack resistance, crack stability and crack path. He had worked with the renowned English aeronautical engineer Prof. Alan Arnold GRIFFITH (who shared the honor of “father of fracture mechanics” with Prof. George R. IRWIN) at the Royal Aircraft Establishment in Farnborough, UK.

“Prof. Gurney was a great mind and taught me how to think originally and broadly in doing research. I was encouraged to read widely, from thermodynamics to philosophy of science, and from poetry to literature. He won the A. A. Griffith Medal and Prize by the UK Institute of Materials, Minerals and Mining in 1993,” Mai recalled. As the Chinese saying goes, “a famous teacher trains a fine student” (名師出高徒). Mai was awarded the same A. A. Griffith Medal and Prize in 2016 for his research results which have influenced engineering practice with global impact.

Mai’s energy-focused work on fracture mechanics was extended at the University of Michigan, Ann Arbor, where he started his postdoctoral work with Prof. Tony ATKINS on boron fiber composites and the thermal shock of cutting-tool ceramics. Atkins, who developed the world’s toughest boron-epoxy continuous-filament-reinforced composite by cleverly engineering their interfaces, had been Professor Emeritus of Mechanical Engineering at the University of Reading.

“Atkins had a dynamic personality and was always full of energy and ideas,” Mai said. “His research focused on the deformation flow and fracture of all materials. Using the Griffith-Gurney approach for elastic-plastic fracture, he extended it to the cutting of hard and soft solids. He taught me to be broad and enthusiastic in research, and to think both laterally and vertically. We co-authored Elastic and Plastic Fracture: Metals, Polymers, Ceramics, Composites, Biological Materials, a classic book published in the mid-1980s.”


\\ “As a researcher, I was taught that it is important to have a focus on social and economic benefits beyond academic concerns.”\\

Mai’s journey in the pursuit of knowledge was far from its end, and he continued his postdoctoral research on the fracture and fatigue mechanics of polymers at Imperial College London, UK. There, he worked with Prof. Gordon WILLIAMS, currently Emeritus Professor and Senior Research Investigator in the Department of Mechanical Engineering at Imperial. Williams published the book Fracture Mechanics of Polymers in 1982, and his research gradually moved towards cutting theory, a method for measuring the fracture energy of materials, which describes the toughness of soft materials like foodstuffs, plastics and polymer nanocomposites.

“Along with Prof. Williams and other researchers, we worked on the cutting behavior of polyolefins with different molecular weights, and epoxies with different degrees of crosslinks and with polystyrene microspheres,” said Mai. Their research team studied the effects of rake angle, cut depth and tool sharpness as well as chip bending on the fracture energy obtained from cutting theory, and their findings provided a basic understanding of the machining behavior of these soft materials and other polymers and their nanocomposites.


What Mai has learned from Williams is more than research. “As a researcher, I was taught that it is important to have a focus on social and economic benefits beyond academic concerns. The research of Prof. Williams on fracture in materials has led to many International Organization for Standardization (ISO) and American Society for Testing and Materials (ASTM) standards. This is living proof of the translational influence of research on industry and society that leads to new practices, test standards and protocols from the results of basic research.”

Since then, a belief in social responsibility has been instilled in the heart of Mai. Together with Prof. Brian COTTERELL, an expert in material crack kinking and crack path, Mai led the research program on fracture mechanics and composite materials at the Centre for Advanced Materials Technology based in the School of Aerospace, Mechanical and Mechatronic Engineering at the University of Sydney. They extended the essential work of fracture (EWF) method from sheet metals to polymers and blends, showing it to be suitable as a standard method for fracture toughness evaluation. EWF is now used for the toughness measurement of thin metal sheets, newsprint and paper in many countries.


Opening Up a New Direction

After conducting research on the mechanics of various materials for four decades, at the turn of the century, Mai breathed new life into his work by extending his focus from fiber composites to ceramic- and polymer-nanocomposites, which have countless applications.

When he took up a senior position at City University of Hong Kong from 2000 to 2002, Mai and colleagues worked on superhard and bioactive surface coatings with the support of the Hong Kong Industrial Technology Centre Corporation (HKITC). The layered structure on steel and tool inserts by physical vapor deposition (PVD) improves the adhesion and reduces the residual stress. Superhard coating technology has benefited the watch and clock industry in Hong Kong and the automotive industry in China. “Multi-layered Superhard Nanocomposite Coatings” is one of two patents that Mai holds.

In a recent IAS Distinguished Lecture titled “On Thermal Conductive Polymer Composites as Underfill Materials for Electronic Packaging,” Mai shared with his audience a major international joint research project of his research team supported by the National Natural Science Foundation of China. The project reviews the state-of-the-art advances regarding the key requirements of underfill materials for microelectronic devices. Underfills are “formulated as 100% solids compositions, i.e., liquid resins and hardeners are used without solvents or volatile additives.”2 They are materials applied to improve the structural integrity and reliability of flipchip packages.​​​​​

“The surface mounting technology for integrated circuits (IC) was very popular in the 1990s,” Mai said. “Today, flipchip packaging technology is commonly used, but this technology has a major challenge which is caused by thermomechanical stresses from large mismatches of the coefficients of thermal expansion between silicon chips, solder joints and substrate during thermal cycling of electronic components.”

The performance reliability and stability of smaller yet more powerful microelectronic devices are affected by basic issues such as thermal heat dissipation, stress redistribution, signal transmission and mechanical protection. In Mai’s lecture, he pointed out that new approaches have been proposed to overcome such problems, which can affect the performance reliability and stability of these new microelectronic devices. However, these approaches still cannot be effectively applied to the complex underfill process that is an integral part of electronic packaging.

“Fine control of the combined mechanical-thermal-electrical-processing-dielectric performance criteria of site-specific underfill materials for flipchip packaging has proven very challenging. The challenges we face require further work for solutions,” Mai said.


Taking Stock

Inarguably, Mai’s research on materials science has had significant impacts on engineering practice. The seminal contributions to fracture mechanics and advanced composite materials that he has made as an international leader was probably something that Mai had never dreamt of when living an ordinary life as the son of a shipyard carpenter.

Entering technical school to fulfill his father’s wishes, Mai acquired basic engineering concepts and technical skills like woodwork, metalwork, physics and geometry, which laid a solid foundation for his future studies in mechanical engineering at the University of Hong Kong.

Reflecting on his research life, Mai humbly said that it was a series of chance encounters that have shaped him. What made him the proudest is not his own achievements but those of his students who have become leaders in their fields. “I believe in Newton’s Law: what goes up must come down,” said Mai with a gentle smile. “At this stage of my career, I hope that my research can benefit our society, and that my teaching can pave the way for students to meet the changing needs of society. I always encourage my students to avail themselves of every opportunity to learn and discover things outside the realm of scientific fields.”

1.

“What Really Sank the Titanic?” Gannon, Robert, Popular Science, February 1995, 246 (2) 49-55
2.

Encyclopedia of Materials: Science and Technology (Second Edition), 2001, 8332-8335

Prof. MAI Yiu-Wing

Prof. Mai received his PhD, DSc and DSc (honoris causa) degrees from the University of Hong Kong in 1972, 1999 and 2013 respectively. In 1976, Mai joined the University of Sydney, where he obtained his DEng in 1999, and is now a University Chair and Professor of Mechanical Engineering.

Throughout the years, Mai has maintained close ties to academic institutions in Hong Kong and China. From 1993 to 1995, as the Acting Head, Mai helped build up the Mechanical Engineering (currently known as Mechanical and Aerospace Engineering) Department and the Advanced Engineering Materials Facilities at HKUST. Mai also held positions in many local and overseas universities including the University of Hong Kong, City University of Hong Kong, Hong Kong Polytechnic University, Peking University, South China University of Technology, and others.

As a highly cited researcher in materials science, Mai's main research interests include (1) materials science and engineering covering processing-structure-property relations, manufacturing and development of innovative materials; (2) smart materials, eco-materials and biomimetics; (3) nanomaterials and nanoengineering; (4) fracture and fatigue mechanics of materials and structures; (5) design, characterization and mechanics of interfaces/interphases; and (6) tribology and surface engineering and science.

Mai’s devotion to scientific research has gained international prestige. He is a recipient of the Takeo Yokobori Gold Medal of the International Congress on Fracture (2013), the A. A. Griffith Medal and Prize of the Institute of Materials, Minerals and Mining (2016), the Scala Award and World Fellowship of the International Committee on Composite Materials (2015), and the AGM Michell Medal of the Engineers Australia (2016).

In 2010, Mai was made a Member of the Order of Australia for his service to engineering. He was also elected to the Fellowships of the Australian Academy of Technological Sciences and Engineering in 1992, the Australian Academy of Science in 2001, the UK Royal Society in 2008 and the UK Royal Academy of Engineering in 2011. In 2017, he was elected Foreign Member of the Chinese Academy of Engineering.