Mechanical Models with Interval Parameters

In this paper we consider modelling of composite material with inclusions where the elastic material properties of both matrix and inclusions are uncertain and vary within prescribed bounds. Such mechanical systems, involving interval uncertainties and modelled by finite element method, can be described by parameter dependent systems of linear interval equations and process variables depending on the system solution. A newly developed hybrid interval approach for solving parametric interval linear systems is applied to the considered model and the results are compared to other interval methods. The hybrid approach provides very sharp bounds for the process variables - element strains and stresses. The sources for overestimation when dealing with interval computations are demonstrated. Based on the element strains and stresses, we introduce a definition for the values of nodal strains and stresses by using a set-theoretic approach

Topology Optimization and Analysis of Thermal and Mechanical Metamaterials

To take advantage of multi-material additive manufacturing technology using mixtures of metal alloys, a topology optimization framework is developed to synthesize high-strength spatially periodic metamaterials possessing unique thermoelastic properties. A thermal and mechanical stress analysis formulation based on homogenization theory is developed and is used in a regional scaled aggregation stress constraint method, and a method of worst-case stress minimization is also included to efficiently address load uncertainty. It is shown that the two stress-based techniques lead to thermal expansion properties that are highly sensitive to small changes in material distribution and composition. To resolve this issue, a uniform manufacturing uncertainty method is utilized which considers variations in both geometry and material mixture. Test cases of high stiffness, zero thermal expansion, and negative thermal expansion microstructures are generated, and the stress-based and manufacturing uncertainty methods are applied to demonstrate how the techniques alter the optimal designs. Large reductions in stress are achieved while maintaining robust strength and thermal expansion properties. An extensive analysis is also performed on structures made from two-dimensional lattice materials. Numerical homogenization, finite element analysis, analytical methods, and experiments are used to investigate properties such as stiffness, yield strength, and buckling strength, leading to insights on the number of cells that must be included for optimal mechanical properties and for homogenization theory to be valid, how failure modes are influenced by relative density, and how the lattice unit cell can be used to build macrostructures with performance superior to structures generated by conventional topology optimization

Fresh state stability of vertical layers of concrete

The production of cement is currently associated with around 5% of global carbon emissions. The manufacture of functionally graded structural elements, where the material composition is varied over the volume, allows the use of cement to be optimized and minimized. Horizontal gradation of material properties can be achieved by casting vertical layers having homogeneous composition. However, a key problem in this application is the control of the fresh state deformations of the layers. This study investigates for the first time the fundamental problem of the fresh state stability of concrete prisms that consist of two vertical layers of different mixes. Original experiments are designed to invoke stable and unstable behaviour. Two novel limit-state models are formulated to assess the stability of the system as a function of material properties and geometry. The results show that a relationship between system stability, geometry and material parameters exists, and that it is captured by the presented models

Space environmental effects on graphite-epoxy compressive properties and epoxy tensile properties

This study characterizes the effects of electron radiation and temperature on a graphite-epoxy composite material. Compressive properties of the T300/934 material system were obtained at -250 F (-157 C), room temperature, and 250 F (121 C). Tensile specimens of the Fiberite 934 epoxy resin were fabricated and tested at room temperature and 250 F (121 C). Testing was conducted in the baseline (nonirradiated) and irradiated conditions. The radiation exposure was designed to simulate 30 year, worst-case exposure in geosynchronous Earth orbit. Mechanical properties tended to degrade at elevated temperature and improve at cryogenic temperature. Irradiation generally degraded properties at all temperatures

Variational approach to probabilistic finite elements

Probabilistic finite element method (PFEM), synthesizing the power of finite element methods with second-moment techniques, are formulated for various classes of problems in structural and solid mechanics. Time-invariant random materials, geometric properties, and loads are incorporated in terms of their fundamental statistics viz. second-moments. Analogous to the discretization of the displacement field in finite element methods, the random fields are also discretized. Preserving the conceptual simplicity, the response moments are calculated with minimal computations. By incorporating certain computational techniques, these methods are shown to be capable of handling large systems with many sources of uncertainties. By construction, these methods are applicable when the scale of randomness is not very large and when the probabilistic density functions have decaying tails. The accuracy and efficiency of these methods, along with their limitations, are demonstrated by various applications. Results obtained are compared with those of Monte Carlo simulation and it is shown that good accuracy can be obtained for both linear and nonlinear problems. The methods are amenable to implementation in deterministic FEM based computer codes

Long term integrity for space station power systems

A study was made of the High Temperature Design Codes ASME N47, British R5, and the French RCC-MR Rules. It is concluded that all these codes provide a good basis of design for space application. The new British R5 is the most complete since it deals with the problem of defects. The ASME N47 was subjected longer to practical application and scrutiny. A draft code is introduced, and a proposed draft for high temperature design in which attempts were made to identify gaps and improvements is suggested. The design is limited by creep characteristics. In these circumstances, life is strongly affected by the selected value of the factor of safety. The factor of safety of primary loads adopted in the codes is 1.5. Maybe a lower value of 1.25 is permissible for use in space. Long term creep rupture data for HAYNES 188 is deficient and it is suggested that extrapolation methods be investigated

Essential Elements for an Early Warning System to Detect Flowslides in Pyroclastic Deposits

Air-fall pyroclastic deposits on steep slopes in Campania (Southern Italy) are periodically sub-jected to rainfall-induced landslides that may evolve into catastrophic flowslides. To protect built-up areas, Early Warning Systems (EWSs) were implemented. Existing EWSs are essentially based on pluviometric thresholds or models which are unable to accurately monitor the physical phenomena which are responsible for flow-slide generation in pyroclastic deposits. Over the last 20 years, landslides with no evolution in flow-slide occurred in this area and the alarms generated by existing EWSs in the cases of rainfall were false and very expensive, thus, lowering population trust in EWSs. To improve the existing EWSs, two complex mod-els for pyroclastic soils from Cervinara and Sarno slopes are proposed in the paper, capable of simulating physical phenomena (such as, the saturation increase due to rainwater infiltration, mechanical degradation and undrained instability), control instability phenomena (landslide) and evaluate the post-failure evolution