James Watson, Maclyn McCarty, and Torsten Wiesel

Torsten Wiesel (right) with Professor Emeritus Maclyn McCarty (center), co-author of the paper with Oswald Avery and Colin MacLeod, and James D. Watson, director of Cold Spring Harbor Laboratory, 1994 Photo by Leif Carlsson To commemorate the fiftieth anniversary of the discovery at The Rockefeller University that genes are made of DNA - considered by many to be the single most important biological discovery of the twentieth century - the university has kicked off a year-long series of events that were running through May 1994. The celebration was formally inaugurated in November 1993 with a lec­ture by Nobel laureate James D. Watson, best known for discovering the double-helical structure of DNA. See also Search Winter 1994, vol. 4, no. 1https://digitalcommons.rockefeller.edu/group-portraits/1013/thumbnail.jp

Fatigue characterization of Poly Vinyl Chloride (PVC) foam core sandwich composite using the G-control method

This paper presents experimental results from cyclic crack propagation tests performed on sandwich specimens with glass/epoxy face sheets and Poly Vinyl Chloride (PVC) foam cores using the G-controlled cyclic energy release rate (ΔG) test procedure. The face material was tested in tension, compression and shear to determine in-plane and out-of-plane mechanical properties, such as Young’s modulus, Poisson’s ratio and shear modulus. These properties were then used in an analytical model of the mixed-mode bending sandwich specimen to calculate compliance and energy release rate. Finite element analysis was used to determine the mode-mixity of the crack loading. Experimental crack growth cyclic tests were carried out on pre-cracked mixed-mode bending sandwich specimens with H45, H100 and H160 PVC foam cores under two mode-mixities (mode I and mode II dominant). Post-mortem analysis was performed on tested specimens, highlighting the influence of mode mixity and foam density on the crack path. Crack propagation diagrams showing da/dN versus ΔG curves were obtained to establish the Paris-Erdogan relation for each material combination tested at the two mode-mixities. Results showed constant crack growth rates for all the materials tested and revealed the influence on mode-mixity on crack propagation speed and foam density (higher foam density, slower crack propagation). </jats:p

Implementation of a Plastically Dissipated Energy Criterion for Three Dimensional Modeling of Fatigue Crack Growth

Fatigue crack growth is simulated using three dimensional elastic-plastic finite element analysis. The crack extension per load cycle, da/dN, as well as crack front profile changes (crack tunneling) under cyclic loading is not specified as an input but evaluated based on a condition that relates plastically dissipated energy to a critical value. Simulation of cyclic crack propagation in a middle-crack tension M(T) specimen using this implementation captures the well established, experimentally obtained crack growth rate reduction accompanying a single overload event. The analysis predicts that the single overload also affects the crack front profile, where a tunneling crack propagates with a flatter crack front in the overload affected zone

Characterization of Fracture Toughness G (sub c) of PVC and PES Foams

The fracture behavior of polyvinyl chloride (PVC) and polyethersulfone (PES) foams has been examined using the single-edge notch bend and the double cantilever beam (DCB) tests. PVC foam densities ranging from 45 to 100 kg/m3 and PES foam densities ranging from 60 to 130 kg/m3 were examined. The PVC foams failed in a linear elastic brittle manner, whereas the PES foams displayed much more ductility and substantially larger toughness at a comparable foam density. The cell wall thickness of the PES foams was almost twice the thickness of the PVC foams which may have contributed to the high fracture toughness here defined as critical energy release rate (G c). The PES foam, further displayed low initiation toughness, due to the sharp artificial crack tip and large toughness corresponding to propagation from a natural crack. The results show that the ductile PES foams have toughness close to its solid counterpart whereas the toughness of the PVC foams falls substantially below its solid counterpart

Implementation of a Plastically Dissipated Energy Criterion for Three Dimensional Modeling of Fatigue Crack Growth

Fatigue crack growth is simulated using three dimensional elastic-plastic finite element analysis. The crack extension per load cycle, da/dN, as well as crack front profile changes (crack tunneling) under cyclic loading is not specified as an input but evaluated based on a condition that relates plastically dissipated energy to a critical value. Simulation of cyclic crack propagation in a middle-crack tension M(T) specimen using this implementation captures the well established, experimentally obtained crack growth rate reduction accompanying a single overload event. The analysis predicts that the single overload also affects the crack front profile, where a tunneling crack propagates with a flatter crack front in the overload affected zone

In Situ Analysis of Crack Propagation in Polymer Foams

This article presents an experimental study on the microscopic mechanisms associated with crack propagation in closed cell polymer foams. A brittle, slightly cross-linked polyvinyl chloride (PVC) foam of density 60 kg/m3 and a ductile thermoplastic polyether sulfone (PES) foam of density 90 kg/m3 were examined. The PVC and PES foams have similar cell size (≈0.7 mm) but the cell edges of the PES foam were much thicker than those in the PVC foam. Overall, it was observed that the elements of both foams fractured in an extensional mode. Crack propagation in the PVC foam was inter-cellular, where agglomerates of very small cells formed a region of weakness. Damaged cell walls were observed on both sides of the crack plane. For the PES foam, craze-like deformation bands were observed in the highly stretched region ahead of the blunted crack tip. Further ahead of the crack tip, highly stretched cells were observed. Fracture occurred predominantly through the center of the cells in the PES foam