Reliability-based failure analysis of brittle materials

The reliability of brittle materials under a generalized state of stress is analyzed using the Batdorf model. The model is modified to include the reduction in shear due to the effect of the compressive stress on the microscopic crack faces. The combined effect of both surface and volume flaws is included. Due to the nature of fracture of brittle materials under compressive loading, the component is modeled as a series system in order to establish bounds on the probability of failure. A computer program was written to determine the probability of failure employing data from a finite element analysis. The analysis showed that for tensile loading a single crack will be the cause of total failure but under compressive loading a series of microscopic cracks must join together to form a dominant crack

Reliability based analysis of contact problems

The Batdorf model is modified to include the reduction in shear due to the effect of compressive stresses on the crack face. This new formulation was used to obtain the probability of failure of ceramic components under contact stress conditions. The combined effect of the surface and volume flaws are included in the analysis. Due to the nature of the fracture of brittle materials under compressive loading, the component is modeled as a series system in order to establish bounds on the probability of failure

Numerical calibration of the stable poisson loaded specimen

An analytical calibration of the Stable Poisson Loaded (SPL) specimen is presented. The specimen configuration is similar to the ASTM E-561 compact-tension specimen with displacement controlled wedge loading used for R-Curve determination. The crack mouth opening displacements (CMOD's) are produced by the diametral expansion of an axially compressed cylindrical pin located in the wake of a machined notch. Due to the unusual loading configuration, a three-dimensional finite element analysis was performed with gap elements simulating the contact between the pin and specimen. In this report, stress intensity factors, CMOD's, and crack displacement profiles are reported for different crack lengths and different contacting conditions. It was concluded that the computed stress intensity factor decreases sharply with increasing crack length, thus making the SPL specimen configuration attractive for fracture testing of brittle, high modulus materials

Refugees and their preferences for relocation

Despite popular portrayals, most refugees are highly immobile once they have left their countries of origin. They are subject to the decisions of various governance actors, which affects their individual agency, sometimes trapping them in a state of involuntary immobility in host countries. This has a bearing on their preferences for relocation to a third country. We argue refugees’ preferences for relocation are a function of their perception of the effectiveness of governance actors in managing refugee situations. UN agencies often take on traditional public responsibilities for refugee populations, such as providing healthcare and education. Host governments’ responsibilities, in contrast, are commonly limited to respecting non-refoulement and providing security. Based on these competencies, we can distinguish between refugee perceptions of institutions as bestowing purely or impurely ‘agency-enhancing’ benefits. We expect these different perceptions affect refugees’ well-being in the host state, their perceived ability and aspiration to move onward, and their preferences regarding options outside legal frameworks. We test our expectations with an original survey among Syrian refugees hosted in Lebanon. We find purely agency-enhancing benefits are associated with a lower aspiration, but a higher perceived ability for onward movement. By contrast, impurely agency-enhancing benefits are associated with higher aspiration and a lower perceived ability for onward movement. Using a conjoint experiment, we also find that more positive perceptions of institutions are associated with a lower willingness to consider onward movement outside legal frameworks

Porous Media Approach for Modeling Closed Cell Foam

In order to minimize boil off of the liquid oxygen and liquid hydrogen and to prevent the formation of ice on its exterior surface, the Space Shuttle External Tank (ET) is insulated using various low-density, closed-cell polymeric foams. Improved analysis methods for these foam materials are needed to predict the foam structural response and to help identify the foam fracture behavior in order to help minimize foam shedding occurrences. This presentation describes a continuum based approach to modeling the foam thermo-mechanical behavior that accounts for the cellular nature of the material and explicitly addresses the effect of the internal cell gas pressure. A porous media approach is implemented in a finite element frame work to model the mechanical behavior of the closed cell foam. The ABAQUS general purpose finite element program is used to simulate the continuum behavior of the foam. The soil mechanics element is implemented to account for the cell internal pressure and its effect on the stress and strain fields. The pressure variation inside the closed cells is calculated using the ideal gas laws. The soil mechanics element is compatible with an orthotropic materials model to capture the different behavior between the rise and in-plane directions of the foam. The porous media approach is applied to model the foam thermal strain and calculate the foam effective coefficient of thermal expansion. The calculated foam coefficients of thermal expansion were able to simulate the measured thermal strain during heat up from cryogenic temperature to room temperature in vacuum. The porous media approach was applied to an insulated substrate with one inch foam and compared to a simple elastic solution without pore pressure. The porous media approach is also applied to model the foam mechanical behavior during subscale laboratory experiments. In this test, a foam layer sprayed on a metal substrate is subjected to a temperature variation while the metal substrate is stretched to simulate the structural response of the tank during operation. The thermal expansion mismatch between the foam and the metal substrate and the thermal gradient in the foam layer causes high tensile stresses near the metal/foam interface that can lead to delamination

Shear Modulus for Nonisotropic, Open-Celled Foams Using a General Elongated Kelvin Foam Model

An equation for the shear modulus for nonisotropic, open-celled foams in the plane transverse to the elongation (rise) direction is derived using an elongated Kelvin foam model with the most general geometric description. The shear modulus was found to be a function of the unit cell dimensions, the solid material properties, and the cell edge cross-section properties. The shear modulus equation reduces to the relation derived by others for isotropic foams when the unit cell is equiaxed

The Role of Crack Formation in Chevron-Notched Four-Point Bend Specimens

The failure sequence following crack formation in a chevron-notched four-point bend 1 specimen is examined in a parametric study using the Bluhm slice synthesis model. Premature failure resulting from crack formation forces which exceed those required to propagate a crack beyond alpha (min) is examined together with the critical crack length and critical crack front length. An energy based approach is used to establish factors which forecast the tendency of such premature failure due to crack formation for any selected chevron-notched geometry. A comparative study reveals that, for constant values of alpha (1) and alpha (0), the dimensionless beam compliance and stress intensity factor are essentially independent of specimen width and thickness. The chevron tip position, alpha (0) has its primary effect on the force required to initiate a sharp crack. Small values for alpha (0) maximize the stable region length, however, the premature failure tendency is also high for smaller alpha (0) values. Improvements in premature failure resistance can be realized for larger values of alpha (0) with only a minor reduction in the stable region length. The stable region length is also maximized for larger chevron based positions, alpha (1) but the chance for premature failure is also raised. Smaller base positions improve the premature failure resistance with only minor decreases in the stable region length. Chevron geometries having a good balance of premature failure resistance, stable region length, and crack front length are 0.20 less than or equal to alpha (0) is less than or equal to 0.30 and 0.70 is less than or equal to alpha (1) is less than or equal to 0.80

The Journey Home: Flight Related Factors on Refugee Decisions to Return

Normative practice for forced displacement is to voluntarily repatriate refugees once conditions are stable in the country of origin, which typically translates to the end of violence. However, Syrian refugees have been returning over the past few years even though there is yet to be a definitive end to the Syrian civil war. Therefore, this paper asks how refugees form decisions on when and whether they should return despite ongoing violence and instability in their country of origin? For now, we focus upon one part of the picture: how prior exposure to violence in the country of origin affects their subsequent decision to return home from their host country. To explore this relationship, we designed an original survey, implemented among Syrian refugees hosted in Lebanon (N=2,000), to causally identify the effects of prior conflict exposure on refugees’ decisions to return. We find that Syrian refugees are more willing to leave Lebanon and return home when they have prior experience of violence in Syria. We explain this initially counterintuitive finding as reflecting that they better understand their tolerance to violence, because they are “experts” and are more capable of assessing risk. In contrast, refugees who were not directly exposed to violence before fleeing their homes are more unsure of the threats associated to returning and are unwilling, therefore, to accept the risk of doing so

The Journey Home: Flight Related Factors on Refugee Decisions to Return

The international refugee regime promotes voluntary repatriation as the preferred solution to refugee crises. It is commonly held that it is safe for refugees to return once conditions are stable in the country of origin, which typically translates to when the violence between combatants ceases. However, the empirical record suggests that refugee returns are far from uniform in relationship to the presence or absence or level of violence in a conflict setting. In other words, we know remarkably little about the conditions under which refugee returns actually occur. In response to this shortfall in knowledge, we ask: how do refugees form decisions on when and whether they should return despite ongoing violence and instability in their country of origin? We focus upon one crucial part of the picture, in particular: how does prior exposure to violence in the country of origin affect refugees’ subsequent decisions to return? To explore this relationship, we designed an original survey, implemented among 2,000 Syrian refugees hosted in Lebanon to causally identify the effects of prior conflict exposure on refugees’ decisions to return. We find that Syrian refugees are more willing to leave Lebanon and return home when they have prior experience of violence in Syria. We explain this counter-intuitive finding as a reflection of these particular refugees as “experts” who are better able to understand and assess their risk tolerance of violence. In contrast, refugees more removed from violence before fleeing their homes harbor more uncertainty of the threats associated with returning and are unwilling to accept the risk of doing so