Stephen W. Banovic

Constitutive Relations for AA 5754 Based on Crystal Plasticity

Constitutive equations for the multiaxial stress-strain behavior of aluminum alloy 5754 sheets were developed, based on crystal plasticity. A Taylor-based polycrystal plasticity model, a tangent formulation of the self-consistent viscoplastic model (VPSC), and an N-site viscoplastic model based on the fast Fourier transform (VPFFT) were used to fit a single slip system hardening law to the available data for tension, plane strain, and biaxial stretching. The fitting procedure yields good agreement with the monotonic stress-strain data, with similar parameter values for each model. When simulating multiaxial tests using the developed hardening law, models that allow both stress and strain variations in grains give better predictions of the stress-strain curves. Furthermore, generally, the simulated texture evolution is too rapid when compared to the experiments. By incorporating a more detailed neighbor interaction effect, the VPFFT model predicts texture evolution in better agreement with experiments.

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.

Through-Thickness Texture Gradient in an Annealed Al-Mg Alloy Sheet

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.

Assessment of structural steel from the World Trade Center towers, part III: Physical damage caused by impact of aircraft

Through-thickness crystallographic texture of a rolled and subsequently annealed Al-Mg continuous cast (CC) alloy was examined. Results showed that the annealed sheet developed a through-thickness texture gradient with the form of a stronger cube texture at the surface when compared to the center layers and varied R orientations through the thickness. While the through-thickness texture gradient was not attributed to the shear related texture, it was still related to the extent of deformation within the sheet.

Assessment of structural steel from the World Trade Center towers, part IV: Experimental techniques to assess possible exposure to high-temperature excursions

This paper describes damage characteristics and failure modes of the recovered structural steel components from the World Trade Center (WTC) towers WTC 1 and WTC 2. Knowledge of damage to the steel elements played an important role in the investigation by ascertaining the response of the structural steel of the towers to the impact of the planes and by providing information on exterior column damage for correlations with impact damage modeled in the baseline structural performance and aircraft impact damage analysis. Results focus on the five exterior panel sections in the impact region of WTC 1 and the four core columns from near the impact regions of WTC 1 and WTC 2.

Assessment of structural steel from the World Trade Center towers, part V: forensic photographic evidence of tower collapse initiation and progression

Recovered structural steel from the World Trade Center was examined as part of the National Institute of Standards and Technology investigation to provide data on potential temperature excursions seen by the steel for input and validation of the fire and thermal finite element models. While numerous experimental techniques were appraised for use during this study, two proved to be practical: assessment of the primer paint on the structural elements and examination of the steel microstructure. Results from these two techniques are presented. Evaluation of primer paint from 21 exterior panel sections, which represent approximately 3% of the panels from fire-involved floors, was conducted and indicated that only three locations may have experienced temperatures over 250°C. Steel microstructures taken from these and other areas on exterior panels exposed to pre-collapse fires showed no evidence of exposure to temperatures exceeding 625°C for times longer than the detectable lower limit of 15 min. The lack of high-temperature excursions observed during this analysis may be related to the protection afforded by intact spray-applied fire-resistant material on the components at the time of exposure.

Dynamic Flow Stress Measurements for Machining Applications

Photographic forensic evidence for the global collapse of the World Trade Center (WTC) towers was developed as part of the National Institute of Standards and Technology (NIST) WTC collapse investigation. The libraries accumulated during the Federal Emergency Management Agency/American Society of Civil Engineers study and NIST investigations were searched for images that showed gross building distortions preceding or during the collapse events. Frames or images showing draw-in of the cast face of WTC 2 and the south face of WTC 1 were analyzed to map the distortion field, which was tracked over time. The inward distortions increased in magnitude as the building burned, reaching a maximum at the time of collapse. Evidence of dropped floors within the towers before global collapse was developed, as was evidence of the kinking and rotation of the top portion of WTC 2 just before and during collapse.

Evolution of Deformation-Induced Surface Morphologies Generated in Fe-Based Sheet

Metals undergo a combination of rapid loading and rapid heating during normal and high speed machining processes. Constitutive models for these materials, however, generally lack any information regarding kinetics of thermally-induced transformations, such as austenite formation in carbon steels, that can have profound effects on their mechanical viscoplastic behavior. The NIST electrically-pulse-heated Kolsky bar was developed specifically to probe material response under conditions approaching those present during machining operations. We have achieved heating rates in excess of 1,000 °C/s combined with strain rates above 1,000 s−1 with this system. This paper presents recent experimental results for AISI 1045 and AISI 1075 steel using the pulse-heated Kolsky bar, and examines some aspects of the uncertainty of the method.

Experimental observations of evolving yield loci in biaxially strained AA5754-O

The complex strain states that exist within a real metal stamping are likely to generate different surface morphologies when compared to the same level of plastic strain produced via single pass deformation. This study quantifies the surface morphology that develops when an as-received traditional plain carbon steel sheet is deformed under two different single-pass, in-plane stretching operations. Roughness measurements performed in the as-received condition with a high resolution scanning laser confocal microscope revealed that an initial surface roughness did not appear to influence the deformation generated with biaxial strain. However, the initial surface roughness could have affected the deformation generated with uniaxial strain. The roughness data were fitted to a probability density function (PDF) and resulted in a near-ideal Gaussian distribution of the surface profile heights. This analysis revealed that the shape of the PDF curve changes monotonically with surface roughness and plastic strain and that the accuracy of the rms roughness (Rq) parameter strongly depends on the quality of the Gaussian fit.