Publications

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Various loading and measuring configurations have been developed in Hopkinson bar fracture toughness experimental techniques. It is well known that several fundamental issues, such as force equilibrium, pulse shaping, stress-wave propagation, etc., must be evaluated in order to obtain a reliable measurement. In our previous work of characterizing Mode II dynamic fracture toughness of a woven composite, highly sensitive polyvinylidene fluoride (PVDF) force transducers were employed to check the forces on the front wedge and back spans in a SHPB ENF experiment. The results show that proper pulse shaping is necessary so the specimen can achieve stress equilibrium before the crack starts to propagate. This study addresses the issue that stress wave propagates through the non-uniform section, which is between the incident and transmission bars including the specimen, loading wedge, and supporting fixture. The transmitted signals are compared with PVDF measurements, and also with numerical simulations of stress waves propagate through supporting fixture and down to the transmission bar. © The Society for Experimental Mechanics, Inc. 2013.

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Predicting failure of thin-walled structures from explosive loading is a very complex task. The problem can be divided into two parts; the detonation of the explosive to produce the loading on the structure, and secondly the structural response. First, the factors that affect the explosive loading include: size, shape, stand-off, confinement, and chemistry of the explosive. The goal of the first part of the analysis is predicting the pressure on the structure based on these factors. The hydrodynamic code CTH is used to conduct these calculations. Secondly, the response of a structure from the explosive loading is predicted using a detailed finite element model within the explicit analysis code Presto. Material response, to failure, must be established in the analysis to model the failure of this class of structures; validation of this behavior is also required to allow these analyses to be predictive for their intended use. The presentation will detail the validation tests used to support this program. Validation tests using explosively loaded aluminum thin flat plates were used to study all the aspects mentioned above. Experimental measurements of the pressures generated by the explosive and the resulting plate deformations provided data for comparison against analytical predictions. These included pressure-time histories and digital image correlation of the full field plate deflections. The issues studied in the structural analysis were mesh sensitivity, strain based failure metrics, and the coupling methodologies between the blast and structural models. These models have been successfully validated using these tests, thereby increasing confidence of the results obtained in the prediction of failure thresholds of complex structures, including aircraft.

The Engineering Sciences Center at Sandia National Laboratories provided an independent peer review of the structural analysis supporting the National Transportation Safety Board investigation of the August 1, 2007 collapse of the I-35W Bridge in Minneapolis. The purpose of the review was to provide an impartial critique of the analysis approach, assumptions, solution techniques, and conclusions. Subsequent to reviewing numerous supporting documents, a SNL team of staff and management visited NTSB to participate in analysis briefings, discussions with investigators, and examination of critical elements of the bridge wreckage. This report summarizes the opinion of the review team that the NTSB analysis effort was appropriate and provides compelling supporting evidence for the NTSB probable cause conclusion.

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This paper describes the analyses and the experimental mechanics program to support the National Aeronautics and Space Administration (NASA) investigation of the Shuttle Columbia accident. A synergism of the analysis and experimental effort is required to insure that the final analysis is valid - the experimental program provides both the material behavior and a basis for validation, while the analysis is required to insure the experimental effort provides behavior in the correct loading regime. Preliminary scoping calculations of foam impact onto the Shuttle Columbia's wing leading edge determined if enough energy was available to damage the leading edge panel. These analyses also determined the strain-rate regimes for various materials to provide the material test conditions. Experimental testing of the reinforced carbon-carbon wing panels then proceeded to provide the material behavior in a variety of configurations and strain-rates for flown or conditioned samples of the material. After determination of the important failure mechanisms of the material, validation experiments were designed to provide a basis of comparison for the analytical effort. Using this basis, the final analyses were used for test configuration, instrumentation location, and calibration definition in support of full-scale testing of the panels in June 2003. These tests subsequently confirmed the accident cause.

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A new direction for the national laboratories is to assist US business with research and development, primarily through cooperative research and development agreements (CRADAs). Technology transfer to the private sector has been very successful as over 200 CRADAs are in place at Sandia. Because of these cooperative efforts, technology has evolved into some new areas not commonly associated with the former mission of the national laboratories. An example of this is the analysis of fabric structures. Explicit analyses and expertise in constructing parachutes led to the development of a next generation automobile airbag; which led to the construction, testing, and analysis of the Jet Propulsion Laboratory Mars Environmental Survey Lander; and finally led to the development of CAD based custom garment designs using 3D scanned images of the human body. The structural analysis of these fabric structures is described as well as a more traditional example Sandia with the test/analysis correlation of the impact of a weapon container.

A Senate Committee requested assistance from Sandia in determining the adequacy of the investigation of the incident aboard the USS IOWA. This currently unexplained explosion occurred in Turret 2 of the battleship on April 19, 1989, killing 47 crewmen. The investigation included material characterization of debris found after the explosion, ignition experiments to characterize the propellant, and analytic modeling of the mechanics, interior ballistics and ignition. The analytic modeling is described in this paper. The modeling of the incident was concerned with the mechanics of the ramming equipment used to load the 16 inch guns, and the interior ballistic and ignition of the propellant. Many separate analyses were performed to explain the crushing of the propellant grains, the dynamics and location of ignition of the propellant train, and the presence of damage after the incident. The goal of this modeling was to assess the feasibility of the various events in the turret, and to identify the cause of the incident. An item of particular interest was damage to the rammer control handle quadrant. The US Navy conjectured that the blast propelled the rammermans seat into the quadrant in such a way as to suggest low speed ram during the incident. The speed of the ram was discovered to be very important in determining the probability of ignition during an overram, and an analysis of the rammermans seat motion was completed. In order to understand how the seat impacts the quadrant, a three-dimensional finite element analysis was completed using ABAQUS/Explicit. The loading of the seat was due to two-phase gas and propellant flow through the bag train and into the turret volume. The results showed that impact onto the quadrant probably first occurred at the rear, dislodging it from its mount. This analysis was pivotal in the examination of the incident, and was the final evidence that the cause of the explosion could not be conclusively determined.

The three-dimensional structural analysis of reentry vehicles presents a considerable challenge to the analyst. This is due to the mechanics of the problem as well as the incorporation of results from several disciplines into the environmental description of the problem. Separate results from aero-analyses, frequently computed in one-dimensional format must be combined into a three-dimensional format suitable for a structural finite element analysis. Features required for the analysis include the ablated thickness of the heatshield structure, as well as pressure on the vehicle and the temperature distribution through the heatshield. By combining these environments, a complete description of all factors which affect the structural performance of reentry vehicles are included into one analysis. This paper presents a method of analyzing the structural response of reentry vehicles using the complete three-dimensional environmental load description.