Timothy J. Foecke

Transmission electron microscopy observations of deformation and fracture in nanolaminated Cu-Ni thin films

Abstract Deformation and fracture of nanolaminates have been investigated by in-situ transmission electron microscopy (TEM) straining of cross-sectioned epitaxial Cu/Ni nanolaminates grown on Cu(001) single-crystal substrates. Initial deformation is accommodated by confined layer slip. With continued straining, unstable fracture occurs, creating a mixed-mode crack that propagates across the nanolaminate interfaces. Further straining results in stable crack growth with intense plastic deformation ahead of the crack tip extending over many bilayers in the direction of crack growth. However, the plastic zone is confined within a small distance normal to the crack. Post-mortem TEM, in general, did not reveal the presence of dislocations in the crack wake, except when the crack was deflected. By comparison, the plastic zone size in the substrate was greater by several of orders of magnitude.

5754 Aluminum Sheet Deformed Along Bi‐Linear Strain Paths

Sheet specimens of aluminum alloy 5754 were deformed along a series of bi‐linear, equal‐biaxial and uniaxial, strain paths while simultaneously measuring stress‐strain behavior. Using the measured crystallographic texture before and after deformation, the VPSC model that incorporates texture evolution was used to simulate the flow stress and hardening behavior. Including latent hardening of multiple slip planes allowed the model to explain the decrease in flow stress when changing from equal‐biaxial to uniaxial deformation. However, the model did not capture the details of the drop in flow stress nor the magnitude of the plastic hardening after the change in deformation mode. This is likely due to room temperature recovery between the two steps of testing.

In situ TEM observations of fracture in nanolaminated metallic thin films

Fracture of single crystal nanolaminated thin films has been investigated through in situ straining of cross-sectional samples of Cu/Ni nanolaminates grown on Cu (001) single crystal substrates. The earlest stages of deformation exhibits a confined layer slip mechanism. With continued straining, unstable fracture occurs creating a mixed-mode crack that propagates across the nanolaminate, roughly perpendicular to the interfaces. Eventually, stable crack growth with intense plastic deformation ahead of the crack tip occurs over many bilayers in the direction of crack growth. Simultaneously, plasticity was seen to spread only 1 or 2 bilayer distances normal to the crack, creating an extremely localized plastic zone. Transmission electron microscopic (TEM) examination after the test did not reveal the presence of dislocations in the crack wake, except where severe crack deflection was observed. By comparison, the plastic zone size in the substrate was greater by several of orders of magnitude.

Advanced Biaxial Cruciform Testing at the NIST Center for Automotive Lightweighting

Modeling of sheet metal forming operations requires mechanical properties data at very large tensile strains and various biaxial strain paths. Typically these data are developed along strain ratio paths that are linear and monotonic, but actual forming strain paths are nonlinear and not necessarily monotonically increasing. A unique planar-biaxial testing facility at the National Institute of Standards and Technology (NIST) has been designed to address non-linear strain paths and other long standing measurement needs. The system uses a combination of four independently controlled hydraulic actuators, with either displacement, force, or strain feedback control, to deform the material, while measurements of the material response is accomplished through a unique combination of digital image correlation and X-ray diffraction. Results of commissioning tests are presented for displacement and force control along different axes. The system was able to deform the sample in the elastic and plastic regimes. The results show the difference between the displacement and strain paths followed, as well as some unexpected behavior (e.g. buckling). Other expanded system capabilities for future use are briefly described.

Assessment of Structural Steel From the World Trade Center Towers. Part I: Recovery and Identification of Critical Structural Elements

The National Institute of Standards and Technology conducted a 3-year,

Assessment of Structural Steel From the World Trade Center Towers, Part II: Analysis of Images for Forensic Information

16 million investigation of the World Trade Center (WTC) disaster in response to the events of September 11, 2001. A primary goal of the WTC investigation was to explore the building materials and the construction and technical conditions that contributed to the outcome of the tragedy. This is one of a series of papers that describe various facets of the investigation involving the recovery and identification of the structural components and the evaluation of the steel with regard to damage and degradation as a result of the events of the day. This paper covers unique aspects of the recovery of the structural elements and the subsequent identification of their original as-built locations within the towers. A total of 236 pieces of WTC steel were cataloged, including several exterior and core columns from the impact and fire floors of WTC 1 and WTC 2.

The metallurgical analysis of wrought iron from the RMS Titanic

A meaningful modeling analysis of the collapse of the World Trade Center towers required documentation of the damage of the buildings due to the aircraft impacts. Images accumulated during the National Institute of Standards and Technology National Construction Safety Team investigation were analyzed for information regarding structural damage and failure modes. A number of recovered and identified components survived the building collapse and subsequent handling during recovery in essentially the same condition as they were post-impact. The modes of failure for bolted and welded connections of the outer wall were documented, as was the condition of the spray-applied fire-resistant material (fireproofing) that had been sprayed onto the structural steel during erection of the building. Finally, images taken of portions of the towers at the locations of identified recovered components were studied to determine whether they were exposed to post-collision fires and to establish the extent of the exposure.

Finite Element Modeling of Deformation Behavior of Steel Specimens under Various Loading Scenarios

The discovery of the RMS Titanic has led to a number of scientific studies, one of which addresses the role that structural materials played in the sinking of the ship. Early studies focused on the quality of the hull steel as a contributing factor in the ships rapid sinking, but experimental results showed that the material was state-of-the-art for 1911. Instead, it was suggested that the quality of the wrought iron rivets may have been an important factor in the opening of the steel plates during flooding. Here the quality of RMS Titanic wrought iron is examined and compared with contemporary wrought iron obtained from additional late 19th-/early 20th-century buildings, bridges, and ships. Traditional metallurgical analysis as well as compositional analysis, mechanical testing, and computer modeling are used to understand the variation in the mechanical properties of wrought iron as a function of its microstructure.

Constitutive Modeling based on Evolutionary Multi-junctions of Dislocations

Lightweighting materials (e.g., advanced high strength steels, aluminum alloys etc.) are increasingly being used by automotive companies as sheet metal components. However, accurate material models are needed for wider adoption. These constitutive material data are often developed by applying biaxial strain paths with cross-shaped (cruciform) specimens. Optimizing the design of specimens is a major goal in which finite element (FE) analysis can play a major role. However, verification of FE models is necessary. Calibrating models against uniaxial tensile tests is a logical first step. In the present study, reliable stress-strain data up to failure are developed by using digital image correlation (DIC) technique for strain measurement and X-ray techniques and/or force data for stress measurement. Such data are used to model the deformation behavior in uniaxial and biaxial tensile specimens. Model predictions of strains and displacements are compared with experimental data. The role of imperfections on necking behavior in FE modeling results of uniaxial tests is discussed. Computed results of deformation, strain profile, and von Mises plastic strain agree with measured values along critical paths in the cruciform specimens. Such a calibrated FE model can be used to obtain an optimum cruciform specimen design.

Interpretation of Diffraction Data from In Situ Stress Measurements during Biaxial Sheet Metal Forming

A latent hardening model based on binary junction-induced hardening can effectively describe the anisotropy measured in multiaxial tests. However, this approach still has some descriptive and predictive limitations. Recent findings show that binary junctions generated by interactions of pairs of dislocations can only induce short-term hardening effect due to the unzipping process of binary junctions. By contrast, multi-junctions, which are formed via multiple interactions of dislocations, can exert a strong and enduring influence on the hardening of polycrystals. In this study, we extend the modeling of dislocation junctions from the binary to multi-junctions, and implement this evolution into a self-consistent visco-plastic model. An application of this model for predicting the yield surface and texture evolution of AA5754 during uniaxial and plane strain loadings is given as a demonstration of the capabilities of the evolutionary binary-multi junction approach.