The contents of this issue reflect the rapid advances that have taken place in materials science and engineering, prompted by the demand for high quality performance materials by industry.

The contributions represent some of the latest developments in the field of Materials Characterisation, describing procedures for the assessment of physical and chemical properties of materials by experimental and computational methods. Apart from microstructural and macro-mechanical investigations on metallic substances, extensive coverage is also given to composites, biomaterials, polymers, ceramics and cementitious materials as well as to the effectiveness of various surface treatments. The wide range of topics includes interaction between disciplines, which is sometimes essential to achieving a proper understanding of material behaviour.

All issues, including this one, have been published in paper as well as digital format and are being widely distributed throughout the world

The Editors

Tallinn, Estonia

2017

An unalloyed ductile iron, which incorporates C and Si as major and Mn as minor alloying elements, is processed by a novel austempering process, in order to obtain superior mechanical properties. The samples are initially austenitized at 890°C for 20 min, then quenched into patented water-based quenching liquid at 180°C for 0.5, 2 and 3.5 s respectively, and austempered at 220°C for 240 min in an electric furnace. Optical microscopy (OM) and scanning electron microscopy (SEM) are performed to correlate the mechanical properties with microstructural characteristics. It is found that partial martensite can be formed firstly upon quenching, which will accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies during austempering. The prior martensite content increases with increasing holding time during quenching. A tensile strength of 1330MPa, an elongation of 3.13% and a hardness of 45HRC can be achieved by controlling the prior martensite content to 12%. SEM of fracture surfaces reveals a mixed ductile and cleavage rupture morphology type in all samples. The results indicate that the tensile behavior of the investigated ADI is mainly influenced by the content of prior martensite.

High-speed impacts such as ballistic and hurricane debris can cause severe damages due to the high kinetic energies in the impacting objects. A good understanding of the mechanism of high-speed impacts can help develop impact-resistant or protective systems. Experimental studies of high-speed impact problem, though valid and useful, are often limited and challenged by the large, nonlinear deformations and contacts involved in such problems. To this end, physical experiments are best used as a validation tool rather than an exploration tool for new system designs. In this study, nonlinear finite element simulations are performed to evaluate the response of metallic materials (e.g. steels) and non-metallic materials (e.g. woven fabrics) under high-speed impacts. In addition, the effects of layered structures of different types of materials as well as layer configurations are investigated.

A dynamic indentation experiment is presented for assessment of the adhesive behavior of a range of coatings in erosive defouling of commercial aircraft engines using CO$_2$ dry-ice. A series of experiments is presented in which particles made from a reference material (polyoxymethylene—POM) and from CO$_2$ dry-ice are made to impact compressor airfoils under a range of impact angle and velocity conditions. The airfoils investigated are coated with an indicator material (PTFE), which is typically used to visualise the defouling effect in large scale compressor defouling experiments. In addition, fouled compressor airfoils taken from service and coated with a fouling typically found in low-pressure compressor stages are investigated. The energy required for the reference particles (POM) to create a defouling effect for the different coatings is determined by an experimental evaluation of their coefficient of restitution. This energy requirement is assumed to be fouling specific. Empirical defouling functions are presented. They correlate the defouling effect for both particle materials under various impact conditions. The empirical correlations are developed into a simulation procedure to predict particle impact erosion and energy dissipation of coated surfaces in numerical indentation simulations.

This research presents analytical and mathematical modelling of coating failures within industrial components, structures, mobile assets and systems due to corrosive degradation and mechanical fracture. These failures lead to several surface problems; therefore, contact mechanics and electrochemistry approaches incorporating induced residual stresses have been adopted to develop a comprehensive solution for the prediction and prognostic of such failures. Experimental study of film cracking and its propagation into substrates, interfacial transient behaviours and film-substrate system has been conducted. A parallel study of corrosive degradation to include cathodic delamination, cathodic blistering and tribo-corrosion of films has been conducted. Experimental and analytical studies of induced residual stresses within the coating and their effects on failure mechanisms and propagation have been completed. A detailed investigation of elastic mismatch at the interfacial contact and interfacial crack tip field has been performed and a complex stress intensity factor is presented. Mathematical derivation of oscillatory singularity, mode mix and interfacial fracture criterion to include adhesion are presented. This paper presents novel mathematical modelling incorporating interfacial crack propagating, diffusion of corrosive species and cathodic blistering for prediction and prognoses of coating failures.

Corrosion causes damage to reinforcing steel in concrete structures and governs the service life of the structures. Currently, researchers are paying attention to modelling the behaviour of the bond between the concrete and steel interface of corroded reinforcement. The main objective of this paper is to study the recent research relevant to the bond behaviour at the interface between corroded ribbed bars and concrete and to identify the future research focus. Initially, the paper presents the mechanisms of corrosion damage of reinforced concrete by discussing corrosive agents, causes and effects. Then mechanisms of corrosion prorogation, mechanical properties of corroded reinforcing steel and effects of corrosion on bond degradation of reinforced concrete are discussed in details. Thereafter, recent experimental researches on bond degradation between reinforcement and concrete are reviewed. Previous studies have proposed formulae, which depend on cover, reinforcing bar diameter, concrete strength and corrosion level, to predict the ultimate bond strength. Effect of other parameters (i.e. type of the bars, bar spacing, crack size, aggregate size, type of loading, stress state and etc.) on bond strength have not been properly studied in literature. Bond strength against biaxial bending or combined load action has not been investigated. Finally, the paper concludes with the significance of testing naturally corroded test specimens, compared to the artificially corroded specimens, as well as discussing loading situations.

Strain rate effect of strength is a crucial factor for material characterization. Attempts have been made to evaluate strain rate effect by indentation tests. An indentation causes a non-uniform stress and strain field inside a specimen. This must induce a non-uniform strain rate field. However, little has been reported about strain rate distribution beneath the indenter. So far, various indenter control methods have been used. In previous studies, no direct comparisons were available as to how strain rate distribution was affected by different control methods. In this study, we report on the strain rate effect of indentation with two indenter control methods: constant loading rate (CLR) and constant indentation strain rate (CISR). The finite element method was designed to reproduce deformation caused by a conical indenter of a half apex angle of 70.3°. Pure aluminum (99.999 mass% purity), which showed high strain rate dependence of strength, was chosen as a specimen. Material properties were obtained from low strain rate (10–4, 10–2/s) to high strain rate (102/s) tests, and results were incorporated into a FEM analysis using the Cowper-Symonds equation. Four constant loading rates (from 0.7 to 350 mN/s) and constant indentation strain rates (from 0.006 to 6/s) were used, and both results were compared. Differences between both indenter control methods were displacement-dependent. Loading curvature, which has been defined as a material constant in the indentation, was calculated from load divided by square of displacement. Although loading curvatures were decreased with increasing displacement for CLR, they were constant for CISR. Results also showed that values of strain rate decreased as displacement increased for CLR, whereas they were the same for CISR. Similarities of both indenter control methods were found as follows. The highest strain rate regions were observed at the edge of the indenter. In addition, higher strain rate region was distributed hemispherically from the edge of the indenter.

An improved elasto-plastic characterisation technique relying on instrumented indentation data, accounting for frame compliance, spherical indenter imperfections and, most importantly, material pile-up at indentation edges was validated on experimental data from control specimens with known properties. The method was subsequently applied to the characterisation of weld regions for which traditional testing methods are not applicable. Variations of elasto-plastic properties were obtained from indentations on a butt-welded steel specimen spanning the three distinct weld regions. The consistency of these results and their sensitivity to variations of experimental data was examined and discussed.

A graded high-vanadium alloy composite coating was synthesized from premixed powders (V, Cr, Ti, Mo, Nb) on ductile iron (DI) substrate via atmospheric plasma arc surface alloying process. The resulted cross-section microstructure is divided into three distinct zones: upper alloyed zone (AZ) rich with spherical primary carbides, middle melted zone (MZ) with fine white iron structure and lower heat affected zone (HAZ). Spherical or bulk-like primary carbides with diameter < 1 μm in the AZ are formed via in-situ reactions between alloy powders and graphite in DI. Microstructural characterizations indicate that the carbides are primarily MC-type (M=V, Ti, Nb) carbides combined with mixed hardphases such as M2C, M7C3, M23C6, and martensite. Disperse distribution of spherical, submicron-sized metal carbides in an austenite/ledeburite matrix render the graded coating hard-yet-tough. The maximum microhardness of the upper alloyed zone is 950 HV0.2, which is five times that of the substrate. Significant plastic deformation with no cracking in the micro-indentations points to a high toughness. The graded high-vanadium alloy composite coating exhibits superior tribological performance in comparison to Mn13 steel and plasma transferred arc remelted DI.

Four-point bending is a standard test method that can be used to determine flexural properties of a material or for quality control. The ASTM and ISO test standards specify an allowable range of set-up parameters such as the coupon width, the support span and the load span that can be used to determine the flexural modulus. When angle-ply laminates are tested in four-point bending the apparent flexural modulus is over predicted due to the bending-twisting coupling and the interaction between the coupon and the test fixture. In the present study, the effect of the test configuration on the apparent flexural modulus of thin angle-ply laminates in four-point bending is evaluated for six different layups. It is shown that test set-ups that allow more twisting of the coupon will result in apparent flexural moduli that are closer to the theoretical value. The torsional moments induced by the test fixture are quantified, and it is shown that they are directly responsible for the increase in the apparent flexural modulus.

The use of the lignin–corundum hybrid fillers for phenolic resins was showed. The very important aspect of the use of lignin for phenolic resin composites is the reduction of phenol emission. The emission of phenol from the phenolic resin-hybrid filler composites were studied by headspace analysis. The physicochemical properties of the new hybrid fillers as well as the thermomechanical properties of the composites with them were examined. The surface properties of hybrid fillers were studied by inverse gas chromatography (IGC). The chemical structure of the new fillers was tested by Fourier transform infrared spectroscopy (FTIR). The dynamic mechanical thermal analysis (DMTA) was used to test the thermomechanical properties of the model composites for the use of abrasive tool production.

Isotropic materials have the same properties in all directions of space, with the same magnitude. Strict isotropy requires a spherical symmetry, hence a cubic unit-cell. All other crystal systems give rise to property anisotropy, i.e. direction dependence of properties and of their magnitude, although the anisotropy may often be weak enough to be quite insignificant. However, some materials show very strong anisotropy, owing to their layered structure, which is the result of unequal bond strength versus direction in space. Property anisotropy is usually the consequence of bonding anisotropy that gives anisotropic crystal growth, i.e. the crystals grow faster in some directions and slower in others, resulting in a crystallite shape that is often sheet-like (two-dimensional) or needle-like (one-dimensional). Many tin(II)-containing materials are found to have very strong low dimensionality: (1) SnF2/MCl (M = alkali metals and NH4) give needle shaped crystals even long hair-shaped. For example, in M3Sn5Cl3F10, the intersection of planes of lone pairs creates cleavage planes in two directions, giving needle shaped crystals. Extreme cases of two-dimensionality were observed in MSnF4, particularly in α-PbSnF4. Bonding anisotropy in tin(II)-containing materials is due to the tin stereoactive lone pair, when the lone pairs cluster in sheets, since no bonding to tin can take place in the lone pair direction. This gives rise to high preferred orientation of polycrystalline samples. The presentation will show how the anisotropy of the tin(II) quadrupole doublet, measured on polycrystalline samples subjected to an extremely enhanced preferred orientation, can be used to predict the direction of the lone pairs in the unit-cell and this, in turn, explain the direction of the cleavage planes. The presentation will focus on the use of X-ray diffraction and Mössbauer spectroscopy to characterize highly anisotropic phases and understand their structure-textural properties.

A new cement-based material is presented in this research contribution. The material consists in a fibre-reinforced self-compacting concrete with 100% of mixed recycled aggregate. Six different mixes were produced in two different conditions: (1) in a concrete plant in order to verify the adaptability of the existing equipment to produce and pour this material under real boundary conditions and (2) in laboratory controlled conditions. A physical (density, porosity, fibre distribution and orientation) and mechanical (compressive, tensile and post-cracking strengths, Young modulus) characterization involving 1,100 specimens was carried out. The results obtained permit to conclude that compressive concrete strength superior to 30 MPa can be achieved with certain ductility and tenacity. In based of these results, this material could be used in applications like foundations, ground-supported slabs, retaining systems and other elements with moderate structural responsibility.

In response to the nano-scale miniaturization trend of IC devices, advanced semiconductor processes require more stringent cleanliness. This study sets up a UHV measurement system for particles detection within a highly cleanliness testing chamber with a UHV slit valve to be test, and an experimental procedure is proposed and examined to investigate particle generation while this slit valve is in operations. Cycle numbers of 10,000, 20,000 and 40,000, respectively, are set for slit valve testing. A series of experiments are conducted to gather particles generation information and to clarify the possible causes and sources of dust particles and its concentrations and the particle sizes. The condensation particle counter (CPC) is used to measure the particle concentration and the differential mobility analyzer (DMA) is for particle sizes measurement. Besides, scanning electron microscopy (SEM) and atomic force microscope (AFM) are used to investigate the condensation behaviors on a witness wafer and the energy dispersive x-ray spectroscopy (EDS) is used to the surface characterizations of the slit valve O-ring. In atmosphere experiments, the particle sizes and size distributions are measured by CPC and SMPS instruments and the gathered results are compared to the measured particle sizes by SEM and AFM and are used to evaluate the assumptions of particle generating sources and mechanisms. Experimental results show that the particle sizes and particle concentrations increase as the cycle numbers increases. To examine the particle generations in vacuum, the particle sizes of the deposited particles on wafer are measured by SEM and AFM. The results are compared with the SMPS measurement.

Load capacities of pad eyes used in offshore lifting operations are generally determined based on the guidelines given in lifting standards. In 2012, NORSOK issued a new standard, R-002 ‘Lifting equipment’, to ensure that adequate safety requirements are complied with in connection with lifting operations on the Norwegian continental shelf. To ensure the accuracy of the followed design procedure, this paper presents a comparison of theoretical load capacities of 3.25-ton pad eyes with experimentally and numerically predicted load capacities. Several laboratory tests have been performed to conduct experimental analyses of the load capacities of pad eyes. These tests have included different pinhole sizes in the pad eyes, different strain directions of pad eye pinholes and different loads to which the pad eyes were subjected. Finite element (FE) simulation was performed for two different cases: with base plate and without base plates. The obtained numerical results show that the addition of the plate to the pad eyes increased the capacity of the pad eyes. It also shows that load capacity of the pad eyes gradually decreased with the increase in pinhole size. This shows the importance of following the standard’s requirements. The comparison of results shows that some of the load capacities provided by the FE analysis closer to the experimental and the theoretical results, while a few others were quite far from them. These differences have been comprehensively discussed in the latter part of the paper.