Memorial Tributes: National Academy of Engineering, Volume 21
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  • ALAN COTTRELL 1919-2012


    ALAN HOWARD COTTRELL died February 15, 2012, at the age of 92. Over a period of some 70 years the impacts of his work on the basic understanding of materials and its application to engineering structures, his academic leadership, his role as scientific advisor to the British government, and his contributions to safe nuclear energy were all immense.

    Sir Alan was born in Birmingham (UK) on July 17, 1919, the elder son of Albert and Elizabeth Cottrell. He attended Moseley Grammar School and then read metallurgy at Birmingham University, graduating in 1939. He was put on war work and introduced to the serious problem of the cracking of tanks’ armor plating at electric arc welds, which he solved. This early experience no doubt influenced his lifelong interest in fracture and structural integrity.

    He was made lecturer at Birmingham in 1943 and in 1944 married Jean Elizabeth Harber, a marriage that lasted happily for 55 years. They had one son, Geoffrey, in 1951, and much later adopted a daughter, Ioana. It is said that one of Alan’s classic books, Dislocations and Plastic Flow in Metals, published in 1953 (Clarendon Press), was written during sleepless nights with baby Geoffrey. 

    Toward the end of the war Alan prepared a new lecture course, “Theoretical Structural Metallurgy” (which formed the basis of another classic book he wrote at this time), in which he discussed the structure and properties of metals in terms of the behavior of constituent atoms and electrons. The course was very influential and ahead of its time. It contributed greatly to transforming a hitherto rather qualitative subject into a quantitative discipline and was an important step in achieving his ambition to transform metallurgy into materials science. He was a brilliant lecturer, conveying complex phenomena in simple terms.

    After the war Alan started research on the plastic properties of metals, with a view to establishing the role of crystal line defects, called dislocations, in determining mechanical properties. The yield point of structural steel was of major interest, and he explained it in terms of the interaction of interstitial carbon and nitrogen atoms with the dislocations (Cottrell locking). There followed explanations of the yield drop, strain aging, the role of grain boundaries, blue brittleness of iron, the temperature dependence of the yield stress in steels, and pinning effects in face-centered cubic crystals.

    There were also seminal contributions in other areas. In a series of elegant temperature cycling experiments, with Robert Stokes on aluminum and M.A. Adams on copper, he showed that the relatively small temperature-dependent part of the flow stress is proportional to the main temperature-independent part (the Cottrell-Stokes Law), which was explained in terms of dislocations cutting through other dislocations. This led to the “forest” model of flow stress. This is without doubt one of the most important contributions to understanding of work hardening and stimulated much further research. 

    In addition, he explained Robert Cahn’s experiments on the recovery of bent crystals of zinc by the process of “polygonization” and introduced a new mechanism, the Lomer-Cottrell lock, whereby a dislocation formed by the interaction of two dislocations at the intersection of two slip planes in face-centered cubic crystals would transform into a strong barrier to further slip. He was also interested in the interaction of point defects with dislocations, and, with Robert Maddin, carried out seminal experiments on quench hardening of aluminum.

    These and other investigations were all pioneering studies carried out over a period of just ten years. They are an impressive achievement and remarkable for their physical insight and lasting impact, and for showing the way critical experiments should be carried out. Alan’s contributions in this field are second to none.

    His work contributed much to making the Birmingham Department famous as a leading center for the science of metals. He was given a personal professorship in 1949 at the age of 30, and in 1955 was elected to the Royal Society at the early age of 35.

    That year he also accepted the post of deputy head of the Metallurgy Division at the UK Atomic Energy Establishment at Harwell, because he expected to find problems there of national importance that were in his field. His aim was to advance the understanding of radiation damage relevant to the development of nuclear power reactors.

    Radiation damage in uranium rods, in the graphite core in Magnox civil nuclear reactors in the United Kingdom, was of particular concern. Swelling and growth of uranium were studied and in a brilliantly designed experiment Cottrell, with A.C. Roberts, showed that creep under neutron irradiation would produce a large buckle in the fuel rod within a few weeks. This led to a redesign of the fuel rods in the reactors.

    Another area studied was the radiation embrittlement of structural steels, resulting in a rise of the brittle-ductile transition temperature. This work has a direct bearing on the integrity of pressure vessels in pressurized water reactors of current design as well as in the older Magnox reactors in the United Kingdom. In addition, Cottrell wrote a review article in 1956 on the effects of neutron irradiation on metals and alloys, which was very influential at the time.

    These studies led him to consider the problem of brittle cleavage of steels. Experimental evidence showed that cleavage cracks were nucleated by plastic deformation. In a famous paper in 1958 he described an ingenious mechanism for reducing elastic energy by the coalescence of dislocations on intersecting slip planes for the nucleation of cleavage cracks on cube planes in the body-centered cubic lattice. The difficult step in brittle fracture was therefore the propagation of the crack nuclei across the grains. This led to the identification of refinement of grain size as the important factor in increasing not only yield strength (as recognized by Norman J. Petch) but also toughness. This fact plays an important role in the development of modern steels.

    On October 10, 1957, a reactor at Windscale caught fire during a gentle heating to anneal damage due to displaced carbon atoms in the graphite core—the Wigner energy released in this process heated up the graphite so much that it caught fire. For this national emergency Cottrell set up a laboratory in a few weeks and, with his team, unravelled the problem and was able to give assurance that the UK Magnox reactors would be immune to this self-heating effect.

    In 1958 Alan accepted an invitation to become head of the Department of Metallurgy at the University of Cambridge. He modernized the department by bringing in new people and new equipment and by teaching the subject from the atomic point of view. Since 1965 the subject has included both metallurgy and materials science. The new structure has stood the test of time.

    Alan also started two new research projects, on field-ion microscopy and on superconducting alloys, which he predicted correctly to become an important growth area in materials science. His research focused on (1) brittle fracture of structural steel at freezing temperatures, responsible for many tragic accidents to ships and bridges, and (2), with Anthony Kelly, the physics of fibrous composites, which although made from brittle materials could be very strong and resistant to fracture. 

    These studies led to the development of new materials such as fiberglass and carbon fiber. Much later Alan advised then Prime Minister Edward Heath to use carbon fiber for the spars of his boat. All these teaching and research activities transformed the Cambridge department into a world-class institution.

    Alan’s work on fracture included the development, with Bruce A. Bilby and K.H. Swinden, of the theory of elasticplastic cracks and the elucidation of the basic processes of failure at the tip of a sharp notch. A toughness parameter (critical crack opening displacement) was identified for a metal containing a crack, when extensive plastic yielding occurred at the high stresses at the crack tip, which was characteristic of the material, and which, when measured in a test piece, could be used to predict behavior in a large structure. This represented an important advance in understanding and ensuring structural integrity and had an enormous impact in this field.

    Alan was also interested in studying fracture on the atomic scale. With W.R. Tyson and Tony Kelly he considered factors determining whether a material with a sharp crack would fail in a brittle or ductile manner, enabling materials to be classified as inherently brittle or ductile. Their classic 1967 study, “Ductile and Brittle Crystals” (Philosophical Magazine, 15(135):567–586), stimulated much further research, particularly on nucleation of dislocation loops at crack tips.

    In 1964 Alan moved to become Sir Solly Zuckerman’s deputy in the UK Ministry of Defense. Although most reluctant to leave the department and the university, he had become concerned with the need to invigorate British manufacturing industry with scientific technology, and felt that Whitehall was the place to do this. Working on the defense review by UK Secretary of State for Defense Denis Healey, Alan led studies by the Army, Navy, and Air Force on problems such as the excessive cost of a military presence in the near and far East. This led to the cancellation of the UK government’s East of Suez Policy. In 1966 he followed Zuckerman to the Cabinet Office as deputy chief scientific advisor. There he tackled various problems with scientific aspects, including environment and pollution, the Advanced Passenger Train, and the Torrey Canyon disaster.

    In 1971 Alan was knighted and became chief scientific advisor. His position became complicated with the arrival of Victor Rothschild and his Central Policy Review staff, proposing to make the work of the UK Research Councils (responsible for government-funded research in the universities) more related to national needs. Alan’s crucial suggestion that the Research Councils should remain independent was accepted and led to a compromise “customer-contractor” policy.

    But Alan became increasingly uncomfortable with the machinations of Whitehall politics. He played it straight and used his powerful intellect to make his case, however unpopular. He was clear about one thing: Knowledge is power in Whitehall.

    In 1973, in a minute to the UK Nuclear Power Advisory Board, and in 1974, in evidence to the Parliamentary Select Committee on Science and Technology, Alan expressed his concern about the integrity of the steel reactor pressure vessel, which is critical to the safety of the pressurized water reactor (PWR) promoted by Walter Marshall at that time for the UK Civil Nuclear Program. This caused quite a stir. In response Marshall set up a High-Level Pressure Vessel Committee in 1973 that examined the issue in great detail.

    In the early 1980s, after publication of the second Marshall Report, Alan felt satisfied that a robust safety case could be established provided the report’s recommendations were implemented. The report and Alan’s endorsement had a major impact on the enquiry about building a PWR at Sizewell and on getting UK Nuclear Installation Inspectorate approval, and led more generally to major advances in the requirements for ensuring the integrity of pressure vessels and other large safety-critical structures.

    Alan believed that nuclear energy is an important source of power and that the public should be able to form a rational view. To this end he set out the facts in simple terms in his 1981 book How Safe Is Nuclear Energy? (published by Heinemann Educational).

    In 1974 Alan accepted an invitation to become master of Jesus College, Cambridge. He was glad to return full time to his family and to academic life. He had to supervise a major revision of the college statutes and prepare for the admission of women. This proved a great success. 

    In 1977 he became vice chancellor for two years, during which he introduced the new chancellor, Prince Philip, to the intricacies of the operation of the university. On returning full time to college, his main activity was preparing for the arrival of Prince Edward, the Queen’s youngest son, who became an undergraduate in the college.

    In 1986 Alan returned to the Metallurgy Department, where he researched a new topic: the application of modern electron theory of metals to metallurgical problems, such as embrittlement of metals by certain impurities. He mastered the quite difficult theory and published in 1988 an excellent book, Introduction to the Modern Theory of Metals (published by the Institute of Metals), followed by an impressive set of papers on applications to important metallurgical problems and a book on Chemical Bonding in Transition Metal Carbides (Maney Publishing, 1995). During the last few years he published again on the plasticity of metals, particularly on creep.

    His accomplishments were recognized with numerous honors and awards—the Royal Society Hughes (1961) and Rumford (1974) Medals, the Platinum Medal of the UK Institute of Metals (1965), the Acta Metallurgica Gold Medal (1977), the Harvey Prize of the Technion (Israel) (1974), the Gold Medal of the American Society for Metals (1980), the Kelvin Gold Medal of the UK Institution of Civil Engineers (1986), and the Von Hippel Award of the Materials Research Society (1996). In 1996 he also received the Royal Society’s highest award, the Copley Medal; he was the first physical metallurgist to receive the medal since it was instituted in 1731. He also received 16 honorary degrees, including two from Cambridge University (ScD in 1976, LLD in 1996).

    He was a foreign honorary member of the American Academy of Arts and Sciences (1960), foreign fellow of the Royal Swedish Academy of Sciences (1970), and foreign member of the US National Academy of Sciences (1970) and US National Academy of Engineering (1976). He became a fellow of the Royal Society in 1955 (vice president in 1964, 1976, and 1977) and in 1979 was elected to the Fellowship of Engineering. 

    Alan Cottrell was the most outstanding and influential physical metallurgist of the 20th century. His concepts, techniques, and analysis form the basis of modern fracture mechanics applications to many materials systems. Through his pioneering research and as an educator, he influenced countless students, scientists, and engineers over the years and will continue to do so. His papers and books are remarkable for their clarity. In his studies he always knew what important questions to ask and how to answer them. He had a brilliant intellect which he retained to the end.

    He was also a kind, gentle, and sensitive person with a sense of humor, and he was very supportive. He was very eminent, but did not realize it and was very modest. He loved his family and was proud of Geoffrey working on nuclear fusion, which Alan considered to be an important future energy source. From 1996 he cared full time for his wife Jean, who suffered from Parkinson’s disease. Sadly, she died in 1999. Her loss affected him greatly.

    His lifetime achievement and impact have been immense, of which his family can be justly proud, and for which the rest of us are grateful. He is greatly missed by his loving family and by all of us who knew him and whose lives he touched. He will be remembered with great affection and admiration.

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