Journal of Applied Mechanics

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J_ID: JAM DOI: 10.1115/1.4030589 Date: 15-May-15

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Journal of Applied Mechanics

Editorial Biographical Sketch of Alan Needleman (2015)

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Alan Needleman’s scientific career started out from a void— literally. A void branched out into various directions and continued to seed many new developments in computational mechanics of materials. In fact, at the age of 70, but still very much alive and inspiring as ever, Alan Needleman is the proud founder of several “schools” or “families.” This special issue is an account of the symposium on “New Frontiers in the Mechanics of Materials,” which was held August 5–8, 2014 at the Mines ParisTech premises in Paris. Alan Needleman obtained his Ph.D. from Harvard University in 1970, under supervision of Professor John Hutchinson. His first article, taken from his thesis, is on “Void growth in an Elastic–Plastic Medium” and was published here in Ref. [1], the Journal of Applied Mechanics, in 1972. The topics in this paper— periodic array of voids, large deformation plasticity, and nonlinear finite element computation—laid the foundation of much of Alan’s subsequent research. While appointed as Assistant Professor in the Mathematics Department at MIT, Alan focussed his attention on numerical methods for nonlinear problems. During that time, he met Viggo Tvergaard at, what now is, the Technical University of Denmark. There, they embarked on a lifelong collaboration; initially on buckling in structures, but soon shifting to bifurcation and localization problems in tensile bars and sheets. While the latter are characteristic phenomena in the failure of ductile engineering materials, final fracture requires a fracture mechanism. Around the time that Alan joined Brown University in 1975, he and Viggo picked-up on earlier suggestions that void nucleation and growth are key to ductile fracture. They adopted and extended the yield function for porous plastic material developed by Gurson (as a student of J. Rice at Brown) to account for the entire chain of the nucleation, growth, and coalescence of voids. This model—now often referred to as the Gurson-Needleman-Tvergaard model—is at the heart of the so-called local approach to fracture. As it proved to be able to simulate real, complex fracture phenomena, such as cup-cone fracture of a tensile bar [2], it has become adopted by many engineers and materials scientists as a day-today tool of analysis. At Brown University, interactions with colleagues like Rice, Shih, Asaro, and Suresh led Alan to expand his interests in a range of materials problems. With Rice, he was the first to develop a variational principle and associated numerical method to study the interaction between plastic void growth and grain boundary diffusion at high temperatures [3]. This has been instrumental in subsequent developments in the community of creep rupture. The collaboration with Asaro enabled an old idea by Taylor to Journal of Applied Mechanics

become practically feasible: the description of plasticity in terms of slip on discrete slip systems. Their illuminating idea in the early 1980s was to use a power law viscoplasticity formulation in order to avoid the nonuniqueness associated with the use of classical critical resolved shear stress rules [4]. This, together with the advent of modern computing power, made crystal plasticity computations possible and, thus, opened the road to studying hitherto inaccessible physical phenomena such as texture development. Nowadays, crystal plasticity is being used in academic research all over the world, as well as in some large industrial labs. Alan also played a key role in the next wave of innovation in plasticity, when experiments on metal thin wires and sheets in the mid 1990s revealed that the resistance to plastic deformation tends to increase with decreasing size. These phenomena could not be described by existing plasticity models, not even crystal plasticity. He recognized the potential of a description in terms of the motion of the carriers of plasticity, dislocations. Together with Van der Giessen, he devised a formulation of discrete dislocation plasticity [5] that combined the best of two worlds: analytical closed-form solutions for individual dislocations in infinite space and a finite element solution of a supplementary field that accounts for boundary conditions. The versatility of the method for solving boundary value problems was demonstrated by the prediction of plasticity size effects at length scales of tens of micrometers or smaller (including the well-known but poorly understood Hall-Petch grain size effect). The initial group of a few postdocs at Brown working on this, including authors Deshpande and Benzerga, expanded rapidly, especially when their students also adopted discrete dislocation plasticity; another school was born. Even though the physics of materials became the leading inspiration, Alan’s prominent innovations deal with computational methods. Void nucleation led him to develop the idea of cohesive zones (or “cohesive surfaces,” as he prefers) to the stage where it have become a standard tool [6]. The mixed-mode version he developed together with student X.-P. Xu is a landmark [7]. Alan has also been leading in three-dimensional nonlinear finite element simulations. The 3D ductile fracture studies carried out together with Viggo in the early 1990s were dynamic, in order to avoid the excessively large size of 3D finite element stiffness matrices. Within this context, Alan and Viggo were among the first in computational mechanics to use massively parallel computing [8]. Using essentially similar techniques, Alan has reached his current hallmark on the long journey from individual voids to real ductile failure: still together with Viggo, he has been the first to show that proper simulations taking into account void nucleation, growth, and coalescence predict fracture surfaces whose self-affine properties agree with experiments [9]. This work closes a longstanding gap between the materials science community, which emphasizes physical mechanisms, and experimental/theoretical physics, on

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the other hand, who have claimed fractality of fracture surfaces since many years. The above summary of Alan’s scientific legacy, obviously, cannot provide a complete overview of the >325 articles in some of the best journals in the field. Suffice it to note that he has been recognized by ISI (Science Citation Index) as a highly cited author in both the fields of Materials Science and Engineering. This is just one of the many honors he received. In 2006, Alan received two awards which are named after giants in the field of plasticity: the Prager Medal from the Society for Engineering Science and the Drucker Medal from ASME. A few years later, the latter society bestowed on him the highest recognition in the field of applied mechanics—the Timoshenko Medal, named after one of the founders of the modern finite element method. Other recent honors include his membership of the American Academy of Arts and Sciences (2007), and the receipt of honorary doctor degrees from the Technical University of Denmark and from the Ecole Normale Superieur de Cachan, located near Paris. The latter give credit to his successful collaborations: primarily with Viggo (for more than 40 years!), but also with many others who have had the privilege of helping build one of Alan’s schools (i.e., discrete dislocation plasticity, in case of the authors). Alan not only recognizes the power of synergy in research collaborations but also enjoys the social interaction. For the same reason, coffee breaks during scientific meetings serve a double purpose: Alan enjoys talking to his colleagues and while doing so he picks up ideas for unexplored areas. Even though his priority in professional life was in research, Alan has performed numerous professional activities, including for instance serving in numerous review committees, editorial advisory boards and in advisory committees of organizations (such as ASME). His main administrative duty, however, has been to serve as Dean of Engineering at Brown University for three years from 1988 (however, he managed to not let this affect his scientific productivity). After retirement from Brown, he has actively helped in expanding the research portfolio of the department of Materials Science and Engineering at the University of North Texas in Denton, and more recently at Texas A&M University. Having grown up in Philadelphia and having spent the rest of his life around Boston, his move to Texas was a bit of a surprise. But there was a very simple reason: to be near his grandchildren who

live in Denton. Work was and is an important element of Alan’s life, but his family—his wife Wanda, their children Daniel and Deborah, plus their grandchildren—is at the center and fill whatever void is left.

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References [1] Needleman, A., 1972, “Void Growth in an Elastic–Plastic Medium,” ASME J. Appl. Mech., 39(4), pp. 964–970. [2] Tvergaard, V., and Needleman, A., 1984, “Analysis of the Cup-Cone Fracture in a Round Tensile Bar,” Acta Metall., 32(1), pp. 157–169. [3] Needleman, A., and Rice, J. R., 1984, “Plastic Creep Flow Effects in the Diffusive Cavitation of Grain Boundaries,” Acta Metall., 28(10), pp. 1315–1332. [4] Peirce, D., Asaro, R. J., and Needleman, A., 1983, “Material Rate Dependence and Localized Deformation in Crystalline Solids,” Acta Metall., 31(12), pp. 1951–1976. [5] Van der Giessen, E., and Needleman, A., 1995, “Discrete Dislocation Plasticity: A Simple Planar Model,” Modell. Simul. Mater. Sci. Eng., 3, pp. 689–735. [6] Needleman, A., 1987, “A Continuum Model for Void Nucleation by Inclusion Debonding,” ASME J. Appl. Mech., 54(3), pp. 525–531. [7] Xu, X.-P., and Needleman, A., 1994, “Numerical Simulations of Fast Crack Growth in Brittle Solids,” J. Mech. Phys. Solids, 42(9), pp. 1397–1434. [8] Mathur, K. K., Needleman, A., and Tvergaard, V., 1994, “Ductile Failure Analyses on Massively Parallel Computers,” Comput. Methods Appl. Mech. Eng., 119(3–4), pp. 283–309. [9] Needleman, A., Tvergaard, V., and Bouchaud, E., 2012, “Prediction of Ductile Fracture Surface Roughness Scaling,” ASME J. Appl. Mech., 79(3), p. 031015.

A. Amine Benzerga 䊏, Texas A&M University, College Station, TX 䊏, e-mail: [email protected] Vikram S. Deshpande 䊏, University of Cambridge, Cambridge 䊏, UK e-mail: [email protected]

Zernike Institute Erikfor Van der Giessen 䊏, Advanced Materials University of Groningen, Groningen 䊏, The Netherlands e-mail: [email protected]

9747 AG

Venue of the symposium, Mines ParisTech (arrow), at the heart of the Latin Quarter near the Luxembourg gardens

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Attendees of the Needleman Symposium. Seated (from left to right): S. Forest, J.-B. Leblond, A. Huespe, A. Staroselsky, A. Benallal, L. Anand, J. W. Hutchinson, J. R. Rice, A. Needleman, L. Banks-Sills, V. Tvergaard, L. B. Freund, A. Pineau, chet, D. Needleman, K. Ravi-Chandar. First row standing: A. A. Benzerga, T. Y. Bre Siegmund, E. Bittencourt, J. Remmers, M. Ristinmaa, R. J. Clifton, Y. Huang, R. Banerjee, E. Bouchaud, L. Nicola, J. Llorca, A. Molinari, N. A. Fleck, H. Gao, M. Falk, J. R. Willis. Second row: Cao, L. Ponson, W. A. Curtin, C. Volokh, J. W. Kysar, D. Coker, J. R. Greer, V. S. Deshpande, R. McMeeking, Z. Suo. Third row: A. Nakatani, O. Allix, D. Balint, C. Niordson, E. van der Giessen, S. Osovski. Attended but absent from picture: M. Ortiz, S. Suresh, C. F. Shih.

The Needleman family (partial) during the banquet on the Seine River

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