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The purpose of present work is to deepen the understanding of crack propagation, non-propagation, and fatigue limit. Quantitative prediction of fatigue crack non-propagation in constant amplitude loading is attempted through extensive 3D elastic–plastic finite element analysis of near-threshold propagation of short cracks nucleating from surface defects. Level of plasticity-induced crack closure is used to estimate the effective stress intensity factor range, which is evaluated in terms of non-propagation potential. It is shown that plasticity-induced crack closure alone is adequate in producing a local minimum in the development of effective stress intensity factor range, which gives a prerequisite to crack arrest.

The microstructure and nodule count of large-size nodular cast iron components vary spatially. These variables are qualitatively known to affect the fatigue limit, yet no model exists to quantify the effects. Some of the physical aspects, such as the clustering of graphite nodules and the role of ferrite microhardness in ferritic–pearlitic nodular cast iron fatigue, have been unclear in the literature. This paper aims to clarify and quantify these aspects. In the absence of casting defects, the largest ferrite with a crack initiating graphite is shown to be the physical, and statistical, explanation for the mixed grade fatigue limit.

The influence of load ratio on the high and very high cycle fatigue (VHCF) strength of Ck45M steel processed by thermomechanical rolling integrated direct quenching was investigated. Ultrasonic fatigue tests were performed under uniaxial and torsional loading at load ratios of R = −1, 0.05, 0.3, and 0.5 with smooth specimens and specimens containing artificially introduced defects. Up to 2 × 10⁵ cycles, failure originated from surface aluminate inclusions and pits under both loading conditions. The prevailing fracture mechanisms in the VHCF regime were interior crack initiation under uniaxial loading and surface shear crack initiation under torsional loading. The mean-stress sensitivity and the fatigue strength were evaluated using fracture mechanics approaches. Equal fatigue limits for uniaxial and torsional loading were determined considering the size of crack initiating defects and the appropriate threshold condition for Mode-I crack growth. Furthermore, the mean-stress sensitivity is independent of loading condition and can be expressed by \( {\sigma}_{\mathrm{w}}(R)={\left.{\sigma}_{\mathrm{w}}\right|}_{R=-1}\cdotp {\left(\frac{1-R}{2}\right)}^{0.63} \) and \( {\tau}_{\mathrm{w}}(R)={\left.{\tau}_{\mathrm{w}}\right|}_{R=-1}\cdotp {\left(\frac{1-R}{2}\right)}^{0.63} \).

In this article, the continuum-based high-cycle fatigue analysis method, introduced by Ottosen, Stenström and Ristinmaa in 2008, is extended to cases where the stress history is a stochastic process. The basic three-parameter Ornstein–Uhlenbeck process is chosen for stress description. As a practical example, the theory is applied in both finite and infinite life cases. A definition for the safety factor is introduced, which is reduced to a minimization problem of the value for the endurance surface. In the stochastic case, the values of the endurance surface form a stochastic process and the cumulative distribution function can be constructed for its maximum values.

A reduced micromorphic model is formulated in the scope of crystal plasticity and crystalline cleavage damage. The finite strain formulation utilizes a single additional microvariable that is used to regularize localized inelastic deformation mechanisms. Damage is formulated as a strain-like variable to fit the generalized micromorphic microslip and/or microdamage based formulation. Strategies of treating slip and damage simultaneously and separately as non-local variables are investigated. The model accounts for size-effects that simultaneously affect the hardening behavior and allow to predict finite width damage localization bands. The results show that the micromorphic extension introduces extra-hardening in the vicinity of grain boundaries and slip localization zones in polycrystals. At the single crystal level slip band width is regularized. Two ways of dealing with damage localization were identified: An indirect method based on controlling width of slip bands that act as initiation sites for damage and a direct method in which damage flow is regularized together with or separately from plastic slip. Application to a real martensitic steel microstructure is investigated.

Fretting is small amplitude reciprocating sliding between surfaces, and it may quickly causes surface cracks, which can continue growing under cyclic loads, until the structure breaks entirely as a result of the fretting fatigue. Fretting can also produce hardened wear particles as a result of adhesive wear, which then accelerates abrasive wear. In this case, the community uses the term fretting wear. The design of heavily loaded contacts, susceptible to fretting, is a difficult task because there is no generally accepted design guide. More extensive fretting research is needed to create them. This paper introduces detailed design phases for a equipment (rig) for a variable normal force fretting test. Supporting high radial and normal forces such that there is minimal run-out between the specimens was the most significant design challenge. The combination of a hydrostatic radial bearing and elastic torque shaft was selected for the detail design phase based on FE-analyses, calculations, and overall evaluation. The frame of the test rig consists of the main frame, which supports mainly the normal force and two torque frames, which support torque cylinders. Many solutions, which were found to be working in the current "ring-ring" apparatus of Tampere University, could be utilized in the new test rig like the tapered connections of the specimens, the elastic rod of the torque lever, axial displacement plate, and contact pressure adjustment system. The designed test rig enables fretting tests with 0 Hz to 20 Hz cycle frequency so that normal and tangential force or displacement can be controlled independently of each other. The normal force cannot change from compression to tension dynamically, but the adhesive force of the contact can be measured by slowly increasing the tension force. The designed fretting test rig fulfills all essential requirements, which were set.

Tiivistelmä Artikkelissa tarkastellaan äärettömään elinikään liittyvää varmuuskertoimen laskentaa kontinuumimekaniikkaan perustuvassa jännitysväsymismallissa. Jännityshistoria voi olla joko deterministinen tai stokastinen. Varmuuskertoimen laskenta redusoituu kestävyysfunktion maksimiarvon etsimiseen. Stokastisen jännityshistorian tapauksessa myös kestävyysfunktion arvot muodostavat stokastisen prosessin, jonka maksimiarvon todennäköisyysjakauma voidaan muodostaa. Menetelmää havainnollistetaan sekä yksinkertaisella yksidimensioisella että moniakselisella teollisella esimerkkilaskennalla.

Fretting may deteriorate contacts in terms of wear and cracking. Fretting-induced material degradation and frictional behaviour was studied experimentally in a large flat-on-flat fretting contact with a moderate nominal normal pressure and in different sliding conditions. The materials studied were cast iron, structural steel, and quenched and tempered steel. The results for all materials revealed a severely deformed microstructure, a tribologically transformed structure with high hardness, an oxidized third body layer and cracking. The initial peak in the coefficient of friction followed by stabilization occurred in all material pairs in gross sliding conditions. With low utilization of friction, only a limited amount of damage occurred. The results for each of these materials were compared.

A novel approach for inferring the underlying non-metallic inclusion distribution from fatigue test fractography is presented. It is shown that the non-metallic inclusion size distribution obtained from fatigue testing differs from the extreme value distributions, which do not take fatigue into account. Fatigue, as a process, acts as a filter for the observed inclusions, and by taking advantage of this allows us to extract more refined information from the fractography using statistical inference. The emphasis in this paper is on analysis of axial fatigue testing of smooth specimens. The concepts presented here apply to all fatigue testing where the data from fracture surfaces is collected.

This article presents a robust Finite-Element-Method-based wear simulation method, particularly suitable for fretting contacts. This method utilizes the contact subroutine in a commercial finite element solver Abaqus. It is based on a user-defined contact formulation for both normal and tangential directions. For the normal contact direction, a nodal gap field is calculated by using a simple Archard’s wear equation to describe the depth of material removal due to wear. The wear field is included in the contact pressure calculation to allow simulation of wear and contact stress evolution during the loading cycles. The main advantage of this approach is that all contact variables are accessible inside the routine, which allows full coupling between normal and tangential contact variables. Also, there is no need for mesh modifications during the solution. This makes the implementation flexible, robust and particularly suitable for fretting cases where friction and tangential contact stiffness play an essential role. The method is applied to the bolted joint type fretting test case. The methodology is also fully applicable to complex real component simulations.

Fretting fatigue and wear may exist if two parts have small amplitude relative rubbing between the contacting surfaces. A peak in the coefficient of friction typically occurs during the first thousands of loading cycles in dry fretting contact with quenched and tempered steel. This peak is related to adhesive friction and wear causing non-Coulomb friction and high local contact stresses possibly leading to cracking. The focus of the study is the effect of different experimental methods on the frictional behavior of the fretting contact between the steel surfaces. The use of pre-corroded specimens and contact lubrication delayed and reduced the initial peak. However, a pre-added third body layer removed the peak completely.

It has been recently discovered that there is a periodical axial impact phenomenon in a running engine crankshaft. Bending of the shaft causes significant extension of the crankshaft and impact to the engine block through the axial thrust bearing. The aim of this work is to study impact-induced energy fluctuations in a complex-shaped Wärtsilä sixteen vee 32 engine crankshaft by using an explicit finite element method (FEM) during the first 25 ms after impact. Using the FEM allows us to study real components used in industry, and analyze their dynamics in the transient phase. In conclusion, we found interesting results that can be used as guidelines for a full-scale crankshaft measurement instrumentation plan. The full-scale measurements will be performed later in the Wärtsilä Oy facility at Vaasa, Finland. The main finding is that a substantive amount of energy is trapped in the head region and the first two crank pins of the crankshaft, which can affect crankshaft durability regarding high-cycle fatigue.

Welding always has a deteriorating effect on fatigue strength in structures under dynamic loading. Weld joints induce discontinuity in structure geometry and the microstructure. Welding induced discontinuity, and defects allow for potential fatigue cracks that lead to the failure of welded parts or structures. The laser welding process differs from conventional arc welding in process and joint type. The most significant advantage in laser welding comes from the deep penetrating mode of welding, which also brings challenges to the soundness of the weld. The benefits of laser welding are most evident in the manufacture of sheet metal products such as sandwich panels. In literature, laser welding is generally dealt with by using different parts of the overall process without taking the fatigue point of view into account. In this article, the process of laser welding is discussed, while keeping fatigue strength in perspective. The fatigue data of laser welded joints is studied in order to find defect distribution that explains fatigue strength distribution in tests. The suitability of traditional fatigue assessment for laser welding is also discussed.

We propose a decision triggered data transmission and collection (DTDTC) protocol for condition monitoring and anomaly detection in the industrial Internet of things (IIoT). In the IIoT, the collection, processing, encoding, and transmission of the sensor readings are usually not for the reconstruction of the original data but for decision making at the fusion center. By moving the decision making process to the local end devices, the amount of data transmission can be significantly reduced, especially when normal signals with positive decisions dominate in the whole life cycle and the fusion center is only interested in collecting the abnormal data. The proposed concept combines compressive sensing, machine learning, data transmission, and joint decision making. The sensor readings are encoded and transmitted to the fusion center only when abnormal signals with negative decisions are detected. All the abnormal signals from the end devices are gathered at the fusion center for a joint decision with feedback messages forwarded to the local actuators. The advantage of such an approach lies in that it can significantly reduce the volume of data to be transmitted through wireless links. Moreover, the introduction of compressive sensing can further reduce the dimension of data tremendously. An exemplary case, i.e., diesel engine condition monitoring, is provided to validate the effectiveness and efficiency of the proposed scheme compared to the conventional ones.

This thesis promotes simulation-driven design process. It means, in practice, a fact-based methodology, where the project core team makes the design process decisions based on the simulated facts instead of gut feelings. At the same time, this thesis will, hopefully, work as introductory material for new employees, and parts of it work as teaching material in machine design courses. This thesis builds on knowledge gained from the practical work experience during the past twelve years while working in the Wärtsilä R&D and Engineering organization. All the method development exists because there has been a need for it. The results presented in this thesis are valuable to Wärtsilä, who has been the early adopter of the simulation-driven design process. In Wärtsilä’s engine development projects the new presented methodology is in use. Wärtsilä-31 product was the first using the new methodology, and the results speak for itself, it is the most efficient 4-stroke engine in the world. Rest of the Finnish industry will benefit from the results as well. All of these new findings will eventually merge into the machine design teaching in the University of Oulu. Thus, the next generation designers will have the new set of tools when they enter the job market., Tiivistelmä Tässä väitöskirjassa esitellään simulointivetoista tuotekehitystä. Käytännössä se tarkoittaa, että projektin ydinryhmä tekee päätökset simulointituloksien perusteella vakiintuneiden käytäntöjen sijaan. Kuvaillut menetelmät perustuvat kahdentoista vuoden työkokemukseen Wärtsilän moottorien tuotekehitysosastolla. Väitöskirjan tulokset ovat arvokkaita Wärtsilälle, joka on jo varhain ymmärtänyt simulointivetoisen tuotekehityksen edut. Kaikki esitellyt menetelmät on kehitetty todelliseen tarpeeseen, ja ne ovat käytössä Wärtsilän uusien moottorien tuotekehitysprojekteissa. Wärtsilä 31 -moottori oli ensimmäinen tuote, jonka kehityksessä näitä uusia menetelmiä käytettiin, ja tuloksena syntyi maailman parhaalla hyötysuhteella varustettu moottori. Wärtsilä tulee käyttämään tätä väitöskirjaa uusien työntekijöiden perehdyttämismateriaalina, mutta myös muulla suomalaisella teollisuudella on mahdollisuus hyötyä sen tuloksista. Toivon mukaan väitöskirjassa esitellyt uuden menetelmät päätyvät ammattikorkeakoulujen ja yliopistojen koneesuunnittelun opetukseen ja antavat seuraavan sukupolven koneensuunnittelijoiden käyttöön nykyaikaiset työkalut, kun he astuvat työmaailmaan.

High cycle fatigue (HCF) is a frequently limiting failure mechanism of machine elements and modern high strength steels. Present day design rules rely on semi-empirical methods, guidelines and utilization of macroscopic analysis means in origin, such as fracture mechanics. The resulting challenge is that short crack regime, critical for HCF in terms of lifetime of components and products, is somewhat poorly handled. This is an outcome of the fact that the present means and methodologies do not explicitly account for effects arising from material microstructure, an oversight micromechanics aims to rectify. Micromechanical modeling operating on fatigue at the scale of material microstructure necessitates the introduction of suitable means to describe the mechanisms of cyclic plastic deformation and microstructural morphologies, considered critical for HCF especially at the early stages of micro-crack nucleation and damage evolution towards and within the short crack regime. In current work, a crystal plasticity based approach with combined hardening is utilized to capture the respective deformation response utilizing full field modeling. The modeling is carried out for both simplified prior austenite grain like microstructures as well as complex imaging based martensitic quenched and tempered steel microstructural models. A fully coupled damage modeling scheme is introduced to track damage nucleation and evolution at the scale of the studied microstructures. Crack closure is included within the approach to track behavior of microstructure scale defects under, e.g., fully reversed loading, more realistically. Model calibration is addressed and application cases involving damage and crack growth both under monotonic and cyclic loading are presented. The results demonstrate how the coupling of damage to crystal plasticity modeling can be utilized to identify and track the evolution of microstructure scale damage mechanisms in complex martensitic microstructures.

The influence of ferritic and pearlitic microstructures to dynamic strain aging (DSA) of ductile iron was investigated by means of strain-controlled cyclic tests performed at temperatures ranging from room temperature to 450°C, strain rates of 10⁻⁵ to 10⁻² s⁻¹ and at three different strain amplitudes along with tensile tests. Results indicated that all tested microstructures exhibit strain aging effects at elevated temperatures in some form, similar to ferritic-pearlitic steels. Additionally, static tensile tests show hardening at intermediate temperatures, minima in elongation and serrations in stress–strain curves. Cyclic data indicates DSA related changes in stresses at 250°C and 350°C at smaller strain rates, while the effects become more suppressed at higher strain rates.

In this work, an extension of a previously developed continuum based high-cycle fatigue model is enhanced to also capture the low-cycle fatigue regime, where significant plastic deformation of the bulk material takes place. Coupling of the LCFand HCF-models is due to the damage evolution equation. The high-cycle part of the model is based on the concepts of a moving endurance surface in the stress space with an associated evolving isotropic damage variable. Damage evolution in the low-cycle part is determined via plastic deformations and endurance function. For the plastic behaviour a non-linear isotropic and kinematic hardening J2-plasticity model is adopted. Within this unified approach, there is no need for heuristic cycle-counting approaches since the model is formulated by means of evolution equations, i.e. incremental relations, and not changes per cycle. Moreover, the model is inherently multiaxial and treats the uniaxial and multiaxial stress histories in the same manner. Calibration of the model parameters is discussed and results from some test cases are shown.

This article describes implementations of beam elements to JuliaFEM. The theory is briefly introduced, and the usage of beam elements is introduced with a usage example that involves a natural frequency calculation of a formula race car frame. The calculation results were compared to results from a commercial program, and their consistency is excellent.

This article briefly describes a new programming language Julia and a new innovative Finite Element Method (FEM) solver JuliaFEM. We selected an easy to understand example of a linear elasticity problem as a method for this introduction. We go through the example step by step and provide a detailed explanation of the different phases of the solution steps. The main result presented here demonstrates the scripting possibilities of JuliaFEM, both pre- and post-processing.

An optimization routine was applied to high pressure fuel pipes to avoid resonance in a heavily vibrating environment. The optimization process and also the natural frequency calculations in every iteration were completely performed with the high-level programming language Julia; the optimization process was performed with the JuMP optimization environment, and the frequencies where calculated with JuliaFEM finite element method solver platform. The benefit of this kind of embedded implementation is a quick response which yields a pleasant development environment to focus on the essential—the choice of the optimization strategy.

Tiivistelmä Artikkeli on koostettu kolmesta kurssin 920002J, Tieteellinen tutkimus ja etiikka, kevään 19.03.–10.04.2018 esseestä, jotka käsittelevät tutkimuksen eettisiä kysymyksiä ja haasteita. Teknillisen mekaniikan alalla tieteen etiikasta keskustellaan aivan liian vähän, ja tämän artikkelin tarkoituksena on toimia keskustelun avaajana toivottavasti samalla tuoden myös uusia näkökulmia aiheeseen. Osa näistä eettisistä asioista on Suomessa syntyneelle, eläneelle ja suomalaisen arvomaailman omaavalle henkilölle itsestäänselvyyksiä.

Fretting fatigue may lead to severe damage in machines. Adhesive material transfer spots in millimeter scale have previously been observed on fretted surfaces, which have been related to cracking. In this study, fretting-induced cracks formed in a large annular flat-on-flat contact are characterized. Optical and scanning electron microscopy of the fretting scar cross-section samples of self-mated quenched and tempered steel specimens revealed severe cracking and deformed microstructure. Two major cracks typically formed around an adhesion spot, which propagated at an oblique angle, regardless of the test parameters used. Millimeter-scale cracks were observed already within a few thousand loading cycles.

Designing contacts susceptible to fretting is a challenging task due to uncertainties related to friction. For example, coefficient of friction has shown to vary as a function of load cycles and so-called non-Coulomb friction can exist during individual load cycles. Concepts of stable and unstable friction are presented in this manuscript. Based on experiments, no fretting is to be expected if the utilization of friction is kept below unstable friction threshold. If contact is subjected to tangential load above this threshold, reciprocating slippage, fretting, is to be expected even if the contact was initially in stick. Experimental evidence for existence of such threshold is presented in form of friction data, slip data and fretting scars.

Summary This paper opens up the history of structural analysis and dynamics simulations of Wärtsilä engines. It cites already published articles and theses with some backgrounds information. It also discusses some of the backgrounds of the in-house tool development. Additionally, this paper presents the development of the computers and investment of the simulation capacity in order to understand how it has been the enabler of ever more complicated models. It lists the work done during fifty decades. The authors sincerely attempt to make this article as reader-friendly as possible, even though there are over 220 references, which of course demonstrates how dedicated Wärtsilä has been in supporting numerical simulations research in the past five decades.

Summary In this paper, two different commercial multibody dynamic (MBD) simulation software cases are studied. Due to the restrictions determined in the conditions of contract, the names of the software are not revealed, instead being called Software S and Software E. The central purpose of this research was to investigate the abilities of Software S in the simulation of a large engine, as a part of the strength analysis process. The abilities were studied by comparing the program with another, here called Software E, which is designed primarily for engine simulations. The capabilities of Software E have been proven after years of usage at Wärtsilä, resulting in its essential role in the strength analysis process today. The aim was to find the shortcomings and restrictions of Software S but also advantages it could bring to the strength analysis process for Wärtsilä. Similar simulation models were also built using both programs during this research. A 16-cylinder V-engine was selected as the subject because of its size in order to obtain further information about the behavior of the program when working with extensive model files. The components of the engine were flexible and were reduced FE models, also called super elements. The forces and contact situations that occur inside the engine were modeled using elements provided by the MBD programs. Different levels of detail of the modeling elements were used to obtain information about the flexibility of the program. The results obtained from time integrations were compared to ensure the similarity of both modeling elements used. Also, this paper reports the calculation times. In addition, a small-scale study was performed for Software S to clarify the effect of the modes used in time integrations towards results accuracy and calculation times. Simulation models were built successfully in both programs, and the results obtained correlated with each other on an adequate level. Significant differences appeared in the f

Fretting movement is dangerous for machines, because it can cause cracking and surface degradation. The aim of this work was to characterize fretting-induced material degradation in large flat-on-flat contacts without edge effects in a sliding direction using quenched and tempered steel 34CrNiMo6. The focus was on the adhesive contact spots, which were formed under a wide variety of operating conditions. Characterization methods were optical microscopy, Vickers hardness tests and scanning electron microscopy. Three different degradation areas were observed: a general deformation layer, a tribologically transformed structure and a third body layer. All the degradation phases have high hardness and low ductility compared to the base material. The formation and behavior of the degradation layers in different operating conditions were discussed.

JuliaFEM is an open source finite element method solver written in the Julia language. This paper presents an implementation of two common model reduction methods: the Guyan reduction and the Craig-Bampton method. The goal was to implement these algorithms to the JuliaFEM platform and demonstrate that the code works correctly. This paper first describes the JuliaFEM concept briefly after which it presents the theory of model reduction, and finally, it demonstrates the implemented functions in an example model. This paper includes instructions for using the implemented algorithms, and reference the code itself in GitHub. The reduced stiness and mass matrices give the same results in both static and dynamic analyses as the original matrices, which proves that the code works correctly. The code runs smoothly on relatively large model of 12.6 million degrees of freedom. In future, damping could be included in the dynamic condensation now that it has been shown to work.

A Bayesian sequential experimental design for fatigue testing was implemented on the basis on D-optimality and a non-linear continuous damage model. The design accounts for the whole range of testing levels.