Joona Vaara

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• Senior Structural Analyst at Wärtsilä

A model for the probabilistic cyclic R-curve has been derived. The model is based on the commonly used hypothesis of consecutive microstructural barrier fronts defining the erratic behavior of microstructurally short cracks and the transition to physically short cracks with declining importance of the microstructural features. The model can describ...

This work explores the effects of underloads on physically short fatigue cracks propagating under near-threshold zero–tension loading in various constraint conditions. A finite element model is employed to model the transient behavior of plasticity-induced crack closure and residual stress, from which propagation behavior can be inferred. The expec...

A crack propagation finite-element model of interior defect-initiated short cracks under near-threshold loading conditions is used to investigate the effect of plasticity-induced crack closure in the presence of a single overload and underload. The effect of an overload predicted by the present model is qualitatively very similar to what is commonl...

The big end bearing of a connecting rod is one of the most highly loaded bearings in marine engines. This paper presents a feasibility study and optimization of the geometrical parameters of a bush-cut bronze bearing for connecting rods in large-bore marine engines. An accurate simulation workflow was developed and carried out in Abaqus to analyze...

The effect of a rough, textured surface and contact orientations on the fretting behavior of self-mated martensitic 34CrNiMo6 +QT steel was examined via a large annular flat-on-flat contact. A friction study accompanied by microscopy analyses was performed to provide a clear insight into fretting characteristics. In gross sliding, the rough surface...

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 de...

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...

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 an...

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 an...

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 kestävyysfun...

The mechanical and thermal properties of ferritic–pearlitic ductile irons vary widely according to their silicon and pearlite contents. Thus, different combinations of silicon and pearlite affect components’ lifetime under mechanical and thermal stress. An excellent example of the usage of such irons is combustion engine cylinder heads. They experi...

Welding always induces discontinuities and imperfections in the structure that allows for potential fatigue cracks. Welding effects thermal strains, which yield to residual stresses of the structure that have a noticeable effect on the fatigue behaviour of the structure. Welding inexorably leads to microstructure and geometry changes in the welding r...

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...

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...

The use of ductile irons in thermo-mechanically loaded components is increasing, necessitating more knowledge of material properties at intermediate temperatures. A study of the mechanical properties of ductile irons at intermediate temperatures was conducted, investigating the effect of different pearlite fractions along with silicon content tests...

An initial friction peak typically occurs in a dry self-mated quenched and tempered steel fretting contact in gross sliding conditions. The peak is related to adhesive friction and wear, which causes non-Coulomb friction. An early surface degradation including cracks may occur. To avoid such a peak, different media were studied using a flat-on-flat...

A hierarchical Bayesian approach to analysing fatigue test data including reinserted specimens is proposed. It is found that the inference model is capable of fitting the fatigue damage model to the observed data well. After the addition of rerun specimen data, the results show a significant change in the predictive SN curves. For the analysed 40Cr...

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 f...

Residual stresses remain in manufactured mechanical components after the forces related to the manufacturing process have been removed. Beneficial compressive residual stresses can be induced using shot peening, cold expansion of holes, and low plasticity burnishing, for example. The purpose of this review is to determine the relevant phenomena and...

Artikkelissa tarkastellaan evoluutioyhtälöpohjaisen jännitysväsymismallin stokastista laajennusta. Esitetty malli on muodostettu yleisten kontinuumimekaniikan periaatteiden mukaisesti ja on siten luonnostaan moniakselinen ja käsittelee kaikki jännityskomponentit ekvivalentilla tavalla. Malli soveltuu myos mielivaltaiselle kuormitushistorialle. Esim...

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 term...

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 temp...

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...

A simulation method to predict the reliability of clamped metal contacts under cyclic loading is presented. The main idea is to predict the development of contact condition of a joint by simulating a spatially variable coefficient of friction (COF) and wear. Frictional energy dissipation drives the COF evolution rule, and classic Archard's equation...

It has been shown that quenched and tempered steel in gross-sliding fretting conditions, with tens of microns of slip amplitude, leads to fretting induced cracking and high and non-Coulomb friction. At low tangential load levels, there was only insignificant cracking. However, the running condition tends to change from stick to gross-sliding with a...

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 env...

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 t...

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...

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...

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 co...

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...

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...

Multiscale microstructural and micromechanical modeling has arisen as a candidate to improve upon the classical methodologies for evaluation of fatigue crack initiation and propagation, both concerning improving our understanding of the fundamental material deformation and damage processes as well as in establishing more accurate design rules for e...

This article describes the fatigue testing of QT steel in ultrasonic frequencies. Different possibilities, challenges and prejudices are reviewed.

Professor Y. Murakami has been a dominant gure in the eld of fatigue, pinpointing the most important things for research. However, in order to further improve the prediction methods, knowing the history, discussions and decisions in the fatigue community are necessary. This paper aims to give a brief and historical review on various high-cycle fati...

ONERA fatigue model identification has been carried out for the nodular cast iron material. Selected fatigue model considers mean stress effect, temperature dependency, multiaxiality and non-linear damage cumulation due to variable amplitude loading. Fitting of model parameters was carried out using the Z-set software package.

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.

The finite element simulation of a cylinder head has been carried out with Abaqus Standard using Z-mat material model, with thermal boundary conditions coming from combined conjugate heat transfer and gas-exchange simulations. The fatigue post-processing of results has been done with Z-post software using ONERA fatigue model. The resulting lifetime...