Ultimate Limit State Equations and Plasticity of Tubular Conical Transitions

Conical transitions have wide applications in wind turbine foundation as well as oil and gas jacket type of structures. The junctions where tubular and cone meet experience a sharp stress rise from shell edge effects. Like all structures experiencing sharp stress rises, fatigue considerations are critical. In addition to fatigue, the existing offshore structural design standards also require ultimate limit state checks. It is known from the lower bound theorem of plasticity limit analysis that the junction local edge effects do not impact the global capacity. Designing for the local junction ultimate limit state contains wide variations among existing design standards. In this paper, the design practices from API RP-2A, NORSOK N-004, and ISO 19902:2020 draft are assessed. They are compared to the shell plastic yield criteria of Hodge and Ilyushin. In addition, this paper provides a semi-analytical plasticity solution to determine junction plastic deformations. The formulation is based on cylindrical shell equations coupled with deformation plasticity theory. It is found that the growth of the junction plasticity zone is limited, which is consistent with the anticipation from the lower bound limit analysis theorem. The observations made in this paper show that the local junction plasticity is a secondary issue compared to other design considerations. Its ultimate limit state design equation can afford to be more lenient if chooses for future standards’ development.

Probabilistic LCF Risk Evaluation of a Turbine Vane by Combined Size Effect and Notch Support Modeling

A probabilistic risk assessment for low cycle fatigue (LCF) based on the so-called size effect has been applied on gas-turbine design in recent years. In contrast, notch support modeling for LCF which intends to consider the change in stress below the surface of critical LCF regions is known and applied for decades. Turbomachinery components often show sharp stress gradients and very localized critical regions for LCF crack initiations so that a life prediction should also consider notch and size effects. The basic concept of a combined probabilistic model that includes both, size effect and notch support, is presented. In many cases it can improve LCF life predictions significantly, in particular compared to E-N curve predictions of standard specimens where no notch support and size effect is considered. Here, an application of such a combined model is shown for a turbine vane.

Study on Shock Resistance of MEMS Devices with Different Stoppers

The shock resistance of the MEMS device can be improved by simplifying its structure, but it will reduce accuracy. A commonly implemented solution that strengthens the shock resistance is the use of stopper. However, the collision between MEMS structure and stopper in shock environment may lead to the failure of the device. Hence, stopper should have a fine protection performance. In this study, the design method and principle of the MEMS device in the shock environment were analyzed. It was pointed out that the reliability design methodology of the MEMS device based on statics theory was insufficient. Next, the response of MEMS device to shock was studied and the shock dynamics model was established. Based on the model, the shock response of the traditional design and designs with different stoppers were analyzed. At last, experiments were carried out and the protection performance of different stoppers was evaluated. Results show that the use of stopper can obviously improve the shock resistance of the device. Elastic stopper can strengthen the shock resistance of the device greatly because of the excellent protection ability, while hard stopper may cause the emergence of the sharp stress wave.

Effect of Blood Pressure on Closing Dynamics of Bileaflet Mechanical Heart Valves

Implantation of a bileaflet mechanical heart valve (BMHV) continues to be associated with a risk of thromboembolic complications despite anti-coagulation therapy1. This has been attributed to the structurally rigid design of the leaflets and valve mechanics combined with an intricate hinge mechanism for the rigid leaflets. The lack of a built in compliance within the valve mechanics presumably leads to sharp stress gradients within the flow as well as a violent closure of the valve often associated with the audible impact of the leaflets to the housing, and a potential for momentary cavitation of blood in the wake of leaflet impact.

Study on Designs of Stoppers for MEMS Devices in Shock Environment

Stoppers are commonly used to improve the shock resistance of MEMS devices. However, the collision between MEMS structure and stoppers in shock environment may lead to emergence of the stress wave, resulting in the failure of devices. Therefore, MEMS devices designed based on current statics theory is unreliable. After analyzing the method and principle for MEMS reliability design, the shock dynamics model was established. Based on the model, the response of the traditional design and designs with different stoppers to shock was researched. At last, protection performances of different stoppers were evaluated. Results showed that the use of stoppers could improve the shock resistance of the device obviously, but hard stoppers would cause to the emergence of the sharp stress wave. Elastic stoppers had excellent protection ability which could strengthen the shock resistance of the device greatly.

Stress Distribution in a Piezoelectric Material with an Elliptical Hole Subjected to Remote Uniform Shear Mechanical and Electric Loads

A two-dimensional electro-elastic analysis is performed on a transversely isotropic piezoelectric material with an elliptical hole, which is subjected to remote uniform shear forces, and remote electric field. Based on the impermeable electric boundary conditions, close form solutions are obtained by using the complex potentials method. Taking PZT-4 ceramic into consideration, the stress distributions around the neighborhood of the elliptical hole are given. It is shown that the hole geometry and the electric field are responsible for the shielding effect, there are sharp stress concentration near the hole.