Michael S. Jepsen

Fatigue of thin walled tubes in copper alloy CuNi10

The current work concerns the investigation of the fatigue resistance of CuNi10 tubes, which are frequently used in heat exchangers of large ship engines. The lifetime performances of the exchanger tubes are greatly affected by the environmental conditions, where especially the temperature fluctuations and the harsh chloride environment cause fatigue and corrosion problems, respectively. A failure of the tubes will trigger an instantaneous shutdown of the engine. Thus, the paper will focus on a model for fatigue life estimation of the CuNi10 material. In the current case of a ship engine, the temperature fluctuations occur when the engine starts and stops. The difference in temperature at cool down or warm up between the exchanger and the exchange media will cause straining of the tubes. To investigate this phenomenon, a series of strain-based fatigue tests are carried out. Results are evaluated by means of the ASTM E739 guideline and one-sided tolerance limits factor method. The tests show good fatigue resistance and the risk for a failure is low in aspect to the case of a ship heat exchanger.

A direct and fully general implementation of influence lines/surfaces in finite element software

Abstract In the paper a general and direct method for implementation of influence lines in finite element software is provided. Generally influence lines are applied to identify the most critical location and combination of live loads in civil engineering structures. The proposed method is based on the Muller-Breslau principle and the basic idea is to equate discontinuous displacement fields with consistent nodal forces, thus obtaining influence functions only applying a single load case without changing the geometry or boundary conditions of the finite element model. Initially the method is developed by means of some illustrative beam problems, where the consistent nodal forces for angular, lateral and axial displacement discontinuities for a Bernoulli-Euler beam element are derived. Finally it is shown that the method is fully general and efficient in identifying the influence functions of generalized stresses in e.g. plates and shells.

Nonlinear Motion Analysis of the Wavestar Wave Energy Converter With a Focus on the Structural Responses

The scope of this paper is to connect a nonlinear WEC numerical model with a structural response model. The numerical WEC model takes into account the nonlinear hydrostatic restoring moment of the Wavestar float. A parameterized structural model of the Wavestar arm is developed in ANSYS APDL. Based on the assumption that the structural displacements remain small, linear first order theory is used to calculate the structural response. The section moments and forces are thus superimposed according to the superposition law. The effect of the nonlinear hydrostatic restoring moment on the structural response is investigated. Moreover, an analysis is carried out which shows that reactive control, applied as a closed loop control, increases the section moments and shear forces.Copyright