Anssi Laukkanen

The Deformation, Strain Hardening, and Wear Behavior of Chromium-Alloyed Hadfield Steel in Abrasive and Impact Conditions

Abstract The alloying of Hadfield steels aims at enhanced mechanical properties and improvements in the wear resistance. In this work, the impact and abrasive properties of a chromium-alloyed high-manganese Hadfield steel were experimentally studied using a wide variety of testing techniques and characterization methods. In addition, an in-service sample was characterized to identify the wear and hardening mechanisms in a real application (jaw crusher). The dynamic mechanical behavior of the steel was determined using the Hopkinson split bar technique. The abrasion properties were studied with three-body abrasion tests using several different natural abrasives. The effects of existing plastic strain and normal loading on the surface hardening and wear rate were further investigated with scratch testing. High-velocity impact testing was performed to evaluate the effect of pre-strain on the impact wear behavior of the material. It was shown that the dynamic loading affects both the yield behavior and the strain hardening rate of the studied steel. The connection between pre-strain, hardness, and wear rate in abrasion was established. In impact conditions, plastic straining of the surface layer first has a positive effect on the wear resistance, but when strain hardening reached the observed ductility limit, it showed an adverse effect on the material’s performance. The addition of chromium and an increase in the manganese content from the nominal ASTM Hadfield composition provided some improvements in the strength, ductility, and surface hardening of the studied steel.

Improved crack growth corrections for J–R-curve testing

Abstract The ASTM J–R-curve testing standard contains an incremental crack growth correction to J, whereas the ESIS P2 test procedure contains a correction for the total crack growth. The ASTM expression is only applicable to single specimen J–R-curve testing in contrast to the ESIS expression which is also applicable for multi-specimen tests. The ASTM expression is, however, considered to be more accurate than the ESIS expression. It is of interest to unify the test procedures so that irrespective of standard, the same result would be obtained. Having this goal in mind, the different crack growth corrections have been examined and compared both with each other as well as with other analytical and numerical estimates, applicable both in incremental as well as integral form. As a result a new correction is proposed, which is capable of unifying the different testing standards.

Modeling and Verification of Creep Strain and Exhaustion in a Welded Steam Mixer

Structures operating in the creep regime will consume their creep life at a greater rate in locations where the stress state is aggravated by triaxiality constraints. Many structures, such as the welded steam mixer studied here, also have multiple material zones differing in microstructure and material properties. The 3-dimensional structure as such in addition to interacting material zones is a great challenge for finite element analysis (FEA), even to accurately pinpoint the critical locations where damage will be found. The studied steam mixer, made of 10CrMo 9-10 steel (P22), has after 100 000 hours of service developed severe creep damage in the several saddle point positions adjacent to nozzle welds. FE-simulation of long term behaviour of this structure has been performed taking developing triaxiality constraints, material zones and primary to tertiary creep regimes into account. The creep strain rate formulation is based on the logistic creep strain prediction (LCSP) model implemented to ABAQUS, including primary, secondary and tertiary creep. The results are presented using a filtering technique utilising the formulation of rigid plastic deformation for describing and quantifying the developing “creep exhaustion”. INTRODUCTION The structural integrity of high temperature welded structures has been widely studied and published, often however as rather simple girth weld cases [1]-[10]. In these the development of stress (von Mises or maximum principal stress) and strains (axial or hoop) are followed and conclusions regarding critical locations are usually drawn from the locations of maximum stresses or strains. Furthermore simulations are often based on steady state creep strain rates and the impact of triaxiality constraints and multiaxial creep ductility are seldom taken into account. As a consequence these studies often fail to pinpoint the locations where service exposed components actually would develop creep damage. In the design stage this could become a problem emerging later in the life of the component, as it did with the mixer studied here at about half the desired (design) life. The influence of creep ductility exhaustion under multiaxial conditions has

Effective interface model for design and tailoring of WC–Co microstructures

Interface structures are a key feature in developing modern composite material solutions with ever improved performance. We present a nano-microstructural modelling approach for the tungsten carbide (WC)–Co system which can include the interface structures of WC–Co and various other phases present in the microstructure, utilising a methodology which combines imaging-based and synthetically generated nano-microstructures into an effective interface model for predicting the behaviour and properties of the resulting composite material. The effective model comprises of a local model of the WC/Co interface interacting with a larger-scale model of the WC–Co microstructure. The results provide a linkage between the interface character of cemented carbide microstructures and their properties, for example with respect to compressive strength, fracture toughness and wear resistance. The methodology presents a multiscale formalism for carrying out performance and application-driven evaluation and tailoring of composite interfaces and mesostructures, carried out on the basis of the emerging engineering material properties.

Modelling and testing of elastomer impact deformation under high strain rates

Abstract Elastomers are frequently used in applications involving repeated impacts of hard abrasive particles on surfaces at high or moderate strain rates. Operational conditions of components experiencing erosive and impact type wear are sometimes difficult to mimic in laboratory conditions, and as such, it is common to approach behaviour and lifetime predictions by meticulously studying different types of single impacts. This is particularly true for characterising and modelling of high strain rate impact events between hard particles and a wearing elastomer surface. This work presents and applies such a methodology for two specific elastomer materials: a natural rubber (NR) and styrene–butadiene rubber (SBR) compounds. The elastomer materials are subjected to quasistatic and dynamic testing conditions for determination of hyper- and viscoelastic material properties. The results are used in an iterative calibration procedure for establishing related constitutive models by applying the Ogden and Prony series models.

Finite element analysis of coating adhesion failure in pre-existing crack field

Abstract The influence of a pre-existing crack field on coating adhesion failure in a steel surface coated with a 2 μm thick titanium nitride (TiN) coating was investigated by finite element method modelling and simulation. The stress and strain fields were determined in contact conditions with a spherical diamond tip sliding over the coated surface at a loading of 8 N. One crack in or at the coating increased the maximum tensile stresses with six times from 82 to 540 MPa when the crack was vertical through the coating or L shaped and with nine times when the crack was horizontal at the coating/substrate interface. A simulated multicrack pattern relaxed the tensile stresses compared to single cracks. The results indicate that a cracked coated surface needs to have about five to nine times higher adhesive and cohesive bonds to resist the same loading without crack growth compared to a crack free surface. For optimal coated surface design, the strength of the adhesive bonds between the coating and the substrate in the vertical direction needs to be 50% higher than the cohesive bonds within the coating and the substrate in the horizontal direction. The first crack is prone to start at the top of the coating and grows vertically down to coating/substrate interface, and there it stops due to the bigger cohesion within the steel material. After this, there are two effects influencing that the crack will grow in the lateral direction. One is that steel cohesion is normally bigger than the coating/interface adhesion, and the second is that there are higher tensile stresses in the horizontal than in the vertical cracks. Several vertical cracks can stop the horizontal crack growth due to stress relaxation.

Fracture Toughness Transferability Study Between the Master Curve Method and a Pressure Vessel Nozzle Using Local Approach

Local approach methods are to greater extent used in structural integrity evaluation, in particular with respect to initiation of an unstable cleavage crack. However, local approach methods have had a tendency to be considered as methodologies with ‘qualitative’ potential, rather than quantitative usage in realistic analyses where lengthy and in some cases ambiguous calibration of local approach parameters is not feasible. As such, studies need to be conducted to illustrate the usability of local approach methods in structural integrity analyses and improve upon the transferability of their intrinsic, material like, constitutive parameters. Improvements of this kind can be attained by constructing improved models utilizing state of the art numerical simulation methods and presenting consistent calibration methodologies for the constitutive parameters. The current study investigates the performance of a modified Beremin model by comparing integrity evaluation results of the local approach model to those attained by using the constraint corrected Master Curve methodology. Current investigation applies the Master Curve method in conjunction with the T-stress correction of the reference temperature and a modified Beremin model to an assessment of a three-dimensional pressure vessel nozzle in a spherical vessel end. The material information for the study is extracted from the ‘Euro-Curve’ ductile to brittle transition region fracture toughness round robin test program. The experimental results are used to determine the Master Curve reference temperature and calibrate local approach parameters. The values are then used to determine the cumulative failure probability of cleavage crack initiation in the model structure. The results illustrate that the Master Curve results with the constraint correction are to some extent more conservative than the results attained using local approach. The used methodologies support each other and indicate that with the applied local approach and Master Curve procedures reliable estimates of structural integrity can be attained for complex material behavior and structural geometries.© 2003 ASME

Crystal Growth in Polyethylene by Molecular Dynamics: The Crystal Edge and Lamellar Thickness

We carried out large-scale atomistic molecular dynamics simulations to study the growth of twin lamellar crystals of polyethylene initiated by small crystal seeds. By examining the size distribution of the stems—straight crystalline polymer segments—we show that the crystal edge has a parabolic profile. At the growth front, there is a layer of stems too short to be stable, and new stable stems are formed within this layer, leading to crystal growth. Away from the edge, the lengthening of the stems is limited by a lack of available slack length in the chains. This frustration can be relieved by mobile crystal defects that allow topological relaxation by traversing through the crystal. The results shed light on the process of polymer crystal growth and help explain initial thickness selection and lamellar thickening.

A crystal plasticity approach for shear banding in hot rolled high-strength steels

A crystal plasticity approach with a phenomenological shear banding mechanism incorporated in a conventional dislocation crystal plasticity model is presented. In the developed framework, the hardening and softening relations are considered both within and between the deformation mechanisms. The study aims to increase the understanding of the importance of hot rolling texture to the shear banding propensity in martensitic steels. In the single crystal simulations performed for selected common rolling textures, it was found that shear band activation and the magnitude of softening are dependent on the initial orientation of the crystal. In general, softening-related shear banding in single crystals was shown to be well reproduced by the model at high plastic strains and high strain rates.

Predicting stiffness and strength of birch pulp – Polylactic acid composites

This paper studies failure of birch pulp–polylactic acid composites. Stiffness and strength are calculated using the theory of short fibre composites and the results are compared to experimental data. The results differed from the experimental values by 0–6%. With less aligned fibres the short fibre theory is not feasible. The performance of the 40 wt% birch pulp – polylactic acid composite is predicted with X-ray microtomography based finite element modelling, and the results are compared with experiments. Stiffness results differed from experiments by 1–17% . By adding into the models a third material phase representing the interface between the fibres and the matrix, the stress–strain curve of the composite was obtained with good accuracy. The work presents finite element modelling methodology of wood plastic composites and the critical further steps needed in order to assess the stress–strain behaviour, strength and stiffness. Tools for comparing different wood plastic composite microstructures are also presented.