Rakenteiden Mekaniikka

Alkusanat

Jarkko Niiranen — Mon, 27 Oct 2025 00:00:00 +0200

Transient thermal stress FE-analysis method development

Mon, 27 Oct 2025 00:00:00 +0200

This research paper addresses the challenges of thermomechanically loaded components in four-stroke medium-speed engines, focusing on exhaust pipe failures due to low-cycle thermal fatigue. Wärtsilä's shift towards 100 % renewable energy has altered engine operating conditions, leading to new challenges in exhaust components subjected to fluctuating thermal conditions. The study focuses on the transient method's ability to detect phenomena during heating and cooling in stress and temperature histories, optimizing the transient analysis definitions and providing some principles for design modifications in thermal stress problems. A case study of nodular cast iron exhaust manifold is used as an example. Traditional methods using cyclic steady-state temperatures have been found insufficient, prompting the development of a more accurate transient method that uses measured temperatures during the engine's thermal cycle. The paper compares conventional steady state heat transfer analysis and two transient heat transfer analyses for defining thermal boundary conditions. The temperatures in the first transient analysis are defined accurately from the measurements, leading to more realistic results and long calculation time. The second transient analysis is improved to offer a balanced method between accurate thermal boundary definitions and shorter calculation time. The transient method reveals higher stress amplitudes in previously low-stress zones, identifying the actual critical points on the exhaust pipe.

Phase field method for brittle fracture implemented with polygonal finite elements

Timo Saksala, Mahmood Jabareen , Reijo Kouhia — Mon, 27 Oct 2025 00:00:00 +0200

The aim of this article is to model fracture propagation in brittle materials, such as rocks and concrete, with the phase field approach. The hybrid formulation of the phase field theory is adopted because it enables using an ad-hoc, or a problem specific, crack driving force, here of Mohr–Coulomb type, to correctly model brittle materials under compression or shear. Hybrid formulations are variationally inconsistent because the crack driving force is not the same as the one used in the underlying energy functional. They are, however, thermodynamically consistent, and computationally cheap since they allow to use a linear balance of momentum equation within the robust staggered scheme to solve the coupled system for the phase field and the displacement field. The phase field method is implemented with 2D polygonal finite elements based on the Wachspress interpolation functions. As numerical examples, typical test cases of notched samples under mode I and II loadings are simulated. Finally, a slope stability problem is solved as an engineering application.

Analysis of flow behavior of bioinks outside the 3D-printing nozzles

Ashish Pawar, Ashvin Chaudhari, Eero Immonen — Mon, 27 Oct 2025 00:00:00 +0200

The major challenge in extrusion-based bioprinting for medical application is printability, which largely depends on the flow behavior of bioinks just outside the nozzle. This flow behavior is influenced by several factors, including nozzle dimensions, bioink density, bioink viscosity, surface tension of the bioink-air interface, and the desired printing speed and structure. Accurately predicting the flow behavior of bioinks outside the nozzle in advance can reduce the costs associated with experimental testing. In this work, Volume of Fluid (VOF) method under Finite Volume method (FVM) framework is used to study the flow behavior outside a single nozzle. Computational Fluid Dynamics (CFD) simulations are conducted to analyze the behavior of bioinks outside the printing nozzles and flow behaviors are compared with literature. Initial simulations are performed using water due to its well characterized rheological and physical properties, and its widespread use as a reference medium in bioink formulations. The effect of all process parameters on the flow outside the nozzle was analyzed using water as the working fluid. By applying two non-dimensional numbers, Reynolds number and Weber number, flow demarcation regimes are established for water. Furthermore, simulations are performed for boinks to predict their printability. The model predictions for the qualitative flow behavior of bioinks at different temperatures matches well with experimental data from the literature.

On the derivation of constant-coefficient partial differential equations for elastic shells

Mika Malinen — Mon, 27 Oct 2025 00:00:00 +0200

Here the problem of formulating a representative model problem of shell theory is considered. We study two ways to obtain a constant-coefficient expression for the strain energy density function of a linearly elastic shell. The first formulation has already been given in the context of the analysis of boundary layers in thin shells, while the other is introduced here. It appears that the essential difference between the formulations is that the constant-coefficient expressions for the strains given here depend on four geometric parameters instead of the two parameters of curvature needed by the earlier derivation. The source of this discrepancy is investigated and shown to be related to the properties of the metric tensors that are attainable by means of different parametrizations of a given surface.