This paper proposes a methodology for analytically determining the initiation of ductile fracture due to the nucleation, growth, and coalescence of voids. Since structural damages in the shipping packages of radioactive materials are judged to be mainly caused by ductile fracture rather than shear fracture due to shear band localization, the proposed methodology has potential use in establishing strain-based failure criteria. The proposed methodology is based on the concept that, to ensure its structural integrity, a package should be designed within the maximum load-carrying capability. The load-carrying capabilities for various states of stress can be determined from the load-displacement relationships obtained from the numerical simulations of various specimen tests. As a result, the maximum equivalent plastic strain corresponding to the maximum load-carrying capability can be expressed in terms of stress triaxiality. This paper demonstrates that it is possible to analytically determine the effective plastic strains at the damage initiation in the state of multiple stresses where the load-carrying capacity is at maximum. By considering both material and geometrical nonlinearity in the mathematical representations of structures, the maximum load-carrying capabilities can be calculated as long as the stress-strain data is given.

This paper proposes an alternative methodology to determine the failure criteria for use in dynamic simulations of radioactive material shipping packages in the events of hypothetical accident conditions. The current stress failure criteria defined in the Nuclear Regulatory Guide 7.6 [1] and the ASME Code, Section III, Appendix F [2] for Level D Service Loads are based on the ultimate strength of uniaxial tensile test specimen rather than on the material fracture in the state of multi-axial stresses. On the other hand, the proposed strain-based failure criteria are directly related to the material failure mechanisms in multi-axial stresses. In addition, unlike the stress-based criteria, the strain-based failure criteria are applicable to the evaluation of cumulative damages caused by the sequential loads in the hypothetical accident events as required by the Nuclear Regulatory Guide 7.8 [4].

This paper discusses the evaluation of the structural integrity of the SAFKEG radioactive material package to ensure hazardous material confinement during the postulated accident event of forklift truck collision. When a SAFKEG Package carried by a forklift truck traveling at 7 miles per hour collides with another forklift truck moving at the same speed but in the opposite direction, the structural response of the package components represents a complex problem. Because of its complex geometric configuration and complicated contact conditions between the neighboring component interfaces, the problem can not be solved using the implicit numerical scheme that would involve solving a large number of simultaneous equations through numerical iteration. In addition, material degradation and failure caused by collision can create severe convergence difficulties in the implicit analysis. Consequently, the explicit solution method is used in the present analysis, and thus the problem has to be treated as a dynamic one even though the inertia effect is insignificant due to the low speed of the forklift trucks. Applying the explicit dynamic technique to quasi-static problems involving very large deformation and material degradation require special considerations in overcoming solution convergence and extremely long computing time. The techniques used to overcome these difficulties are discussed in this paper. This paper also discusses the development of the constitutive models of the thermal insulating and shock absorbing materials used in the SAFKEG. The resin-bounded cork material is represented by the “crushable foam” plasticity with volumetric hardening. On the other hand, the material model of the brittle foam is developed by using the combination of the Mohr-Coulomb and Drucker-Prager plasticity theories. The analysis utilizes the finite-element method and the ABAQUS/Explicit Computer Code, version 6.3. A combination of the “General Contact” method and the “Contact Pair” method is employed to simulate the complicated interface variations among the neighboring components of the model.

In general, the design process involves envisioning and developing concepts for a component or system, combining these fractions into an integrated whole and evaluating the final design against functional requirements. A major challenge is developing components or systems to a level of maturity that permits feasibility evaluation of the integrated whole while optimizing opposing performance functions (e.g., thick for strength, but thin for heat transfer). Economic pressure often drives design concepts to conservative bases early in the process. The approach presented in this paper is a highly costeffective means of developing alternative design solutions for given set of design requirements -in this case a radioactive materials (RAM) transportation package.

The design of radioactive material (RAM) shipping packages are designed to withstand the mechanical and thermal loads specified in Title 10 Code of Federal Regulations Part 71 (10CFR71). The Hypothetical Accident Conditions (HAC) specified in 10CFR71 include a 30-foot free drop, a crush by an 1100-pound plate, a 40-inch free drop onto a round bar and a 30-minite fire. Furthermore, in accordance with the Nuclear Regulatory Commission Guide 7.8, these loads should be applied sequentially. The current common practice is to base the thermal modeling on the un-deformed (or un-damaged) packaging configuration and thermal stresses are neglected in structural analyses. This paper presents a methodology to simulate the coupled thermal and structural responses and to evaluate the accumulated damages caused by both sequentially applied thermal and mechanical loads of the Hypothetical Accident Conditions. Part 1 of the paper discusses the thermal analysis for the Normal Conditions of Transport (NCT) to establish the initial temperatures of the shipping package. It also discusses the subsequent analyses of closure-bolt tightening preload analysis and 30-foot drop analysis.