Experimental and stochastic approaches to assessing the strain demand of pipelines and flexibility requirements for coatings

A criterion for selecting a coating for an energy pipeline is that the coating should have a suitable flexibility to meet the high strain demand during hydrostatic testing and during field bending. This requires knowledge of the level of strain demand for the pipeline, and also the maximum strain that could be
tolerated by the coating system. Whereas average strains imposed during manufacturing and construction are reasonably well predicted, there is insufficient understanding on the factors leading to localised deformation of the pipe. Significant work has been carried out in the past to develop tests for assessing
the coatings’ ability to handle a certain amount of strain based on bend testing, tensile testing and burst testing. However, there is a concern as to whether these tests properly represent localised micro-strains associated with construction activities including field bending and pressure testing, particularly pressure testing of pipelines designed for operation at 80% of specified minimum yield strength (SMYS). Consequently coatings considered "suitable" for modern pipelines may fail. The first issue discussed in this paper is main factors affecting strain localisation. The non-deterministic distributions of heterogeneities over the pipe provide a ground to consider the mechanisms of localisation as a stochastic process. An approach is proposed to quantify the maximum localised strain demand through cold field bending and hydrostatic experiments. Another issue discussed in this paper is the experimental assessment of coating flexibility under the effects of localised strains. Preliminary mandrel tests have been carried out to assess the uniformity of the imposed strain. Although mandrel testing has been shown to be a useful method for relative comparison of coating flexibility, it has several weaknesses that could significantly affect the reliability and reproducibility of the results.

Effects of combined silicon and molybdenum alloying on the size and evolution of microalloy precipitates in HSLA steels containing niobium and titanium

The effects of combined silicon and molybdenum alloying additions on microalloy precipitate formation in austenite after single- and double-step deformations below the austenite no-recrystallization temperature were examined in high-strength low-alloy (HSLA) steels microalloyed with titanium and niobium. The precipitation sequence in austenite was evaluated following an interrupted thermomechanical processing simulation using transmission electron microscopy. Large (~ 105 nm), cuboidal titanium-rich nitride precipitates showed no evolution in size during reheating and simulated thermomechanical processing. The average size and size distribution of these precipitates were also not affected by the combined silicon and molybdenum additions or by deformation. Relatively fine (< 20 nm), irregular-shaped niobium-rich carbonitride precipitates formed in austenite during isothermal holding at 1173 K. Based upon analysis that incorporated precipitate growth and coarsening models, the combined silicon and molybdenum additions were considered to increase the diffusivity of niobium in austenite by over 30% and result in coarser precipitates at 1173 K compared to the lower alloyed steel. Deformation decreased the size of the niobium-rich carbonitride precipitates that formed in austenite.

Compositional effects of large graphene oxide sheets on the spinnability and properties of polyurethane composite fibers

Recent advances in wearable electronics, technical textiles, and wearable strain sensing devices have resulted in extensive research on stretchable electrically conductive fibers. Addressing these areas require the development of efficient fiber processing methodologies that do not compromise the mechanical properties of the polymer (typically an elastomer) when nanomaterials are added as conductive fillers. It is highly desirable that the addition of conductive fillers provides not only electrical conductivity, but that these fillers also enhance the stiffness, strength, stretchability, and toughness of the polymer. Here, the compatibility of polyurethane (PU) and graphene oxide (GO) is utilized for the study of the properties of elastomeric conductive fibers prepared by wet-spinning. The GO-reinforced PU fibers demonstrate outstanding mechanical properties with a 200-fold and a threefold enhancement in Young's modulus and toughness, respectively. Postspinning thermal annealing of the fibers results in electrically conductive fibers with a low percolation threshold (≈0.37 wt% GO). An investigation into optimized fiber's electromechanical behavior reveals linear strain sensing abilities up to 70%. Results presented here provide practical insights on how to simultaneously maintain or improve electrical, mechanical, and electromechanical properties in conductive elastomer fibers.

An enhanced method for determining the maximum strain a pipeline coating could tolerate

An enhanced mandrel bend testing method has been proposed for the evaluation of the maximum strain level that could be tolerated by an organic coating, and for the understanding of localised coating deformation and cracking behaviours under nonuniform mechanical strains. The aim is to develop a practical method that is suitable for selecting pipeline coatings in order to ensure that the selected coatings have sufficient flexibility to meet the high strain demand during the construction, hydrostatic testing and operation of high pressure pipelines. Two new mandrel bend testing setups have been designed by employing either centre or end clamps in order to improve the uniformity of strain distributions over coated steel coupons, and by using strain gauges to perform in situ measurements of local strains. A series of tests have been carried out to evaluate the new method for testing the flexibility of selected epoxy based pipeline industry coatings. Preliminary computational simulation has also been carried out for assisting the interpretation of mandrel bending test results.

Deformation field variations in equal channel angular extrusion due to back pressure

Flow lines were analysed in aluminium alloy 6061 during equal channel angular extrusion (ECAE) in a 90° die with and without the application of back pressure during pressing. The lines appeared much more rounded when a back pressure was applied compared to the case of conventional ECAE testing. With the help of an analytic flow function, the deformation field was obtained. It is shown that back pressure slightly lowers the total strain, strongly increases the size of the plastic zone and significantly reduces the plastic strain rate.

Strain rate effect of high purity aluminum single crystals: experiments and simulations

In order to study the strain rate effect on single crystal of aluminum (99.999% purity), aluminum single crystals are fabricated and subjected to uniaxial compression loading at quasi-static and dynamic strain rates, i.e., from 10-4 s-1 to 1000 s-1. The orientation dependence is also investigated with single slip or multi slip. The stress-strain curves of pure Al single crystals along two orientations and at different strain rates are obtained after measuring initial orientation using the Laue Back-Reflection technique. Crystal Plasticity Finite Element Method (CPFEM) with three different single crystal plasticity constitutive models is used to simulate the deformations along two orientations under various strain-rates. The classical and two newly developed single crystal plasticity models are used in the investigation. The simulation results of these models are compared to experimental results in order to study their abilities to predict finite plastic deformation of single crystalline metal over a wide strain rate range.

Bimetallic copper-aluminium tube by severe plastic deformation

A new process of joining tubes from different materials to a bimetallic tube based on a combination of large shear and high hydrostatic pressure is proposed. It results in improved mechanical locking of surface asperities, along with enhanced diffusivity owing to the ultrafine-grained microstructure produced. This is augmented by a temperature increase due to heat release associated with mechanical work. Electron microscopy characterization of the interface and the adjacent regions supports the hypothesis of enhanced interdiffusion.

Dynamic strain-induced ferrite transformation (DSIT) in steels

The goal in the heat treatment or thermomechanical processing of steel is to improve the mechanical properties. For structural steel applications the general aim is to refine the ferrite grain size as this is the only method that improves both the strength and toughness simultaneously. For conventional hot rolling and accelerated cooling processes, it is difficult to refine the grain size below 5. μm without extensive alloying. However, it has been found that inducing transformation during deformation (i.e. dynamic transformation) can lead to grain sizes of the order of 1. μm, even in very simple steel compositions. The exact mechanism(s) for this transformation process are still being debated, and this has also been complicated by recent studies where such grain sizes can be obtained by static transformation from austenite that has been heavily deformed at low temperatures prior to the transformation. This chapter reviews the various major studies related in particular to dynamic transformation and considers the contributions from the deformed austenite structure developed prior to the transformation and the potential for dynamic recrystallisation of the ferrite. A key factor is proposed to be the early three-dimensional impingement of the ferrite which also provides an insight into cases where ultrafine grains are achieved statically.

Strain localisation patterns under equal-channel angular pressing

Instabilities of plastic flow in the form of localised shear bands were experimentally observed to result from equal-channel angular pressing (ECAP) of magnesium alloy AZ31. The appearance of shear bands and their spacing were dependent on velocity of the pressing and applied back-pressure. A generic gradient plasticity theory involving second-order strain gradient terms in a constitutive model was applied to the case of AZ31 deformed by ECAP. Linear stability analysis was applied to the set of equations describing the deformation behaviour in the process zone idealised as a planar shear zone. A full analytical solution providing a dispersion relation between the rate of growth of a perturbation and the wave number was obtained. It was shown that the pattern of incipient localised shear bands exhibits a spectrum of characteristic lengths corresponding to admissible wave numbers. The interval of the spectrum of wave numbers of viable, i.e. growing, perturbations predicted by linear stability analysis was shown to be in good agreement with the experimentally observed spectrum. The effect of back-pressure applied during ECAP was also considered. The predicted displacement of the shear band spectrum towards lower wave numbers, shown to be a result of the decreased shear strain rate in the shear zone, was consistent with the experimentally observed increase of the band spacing with increased back-pressure. A good predictive capability of the general modelling frame used in conjunction with linear stability analysis was thus demonstrated in the instance of the particular alloy system and the specific processing conditions considered.

Theoretical study on the (1-012) deformation twinning and cracking in coarse-grained magnesium alloys

This paper theoretically and systematically investigates: (1) the effect of local transformed strains within deformation twinning on twin intersection; (2) the fracture mode based on type I co-zone tensile twin intersection in coarse-grained magnesium alloys, as well as the impacts of twin intersection and grain diameter on interfacial crack nucleation along twin boundaries; and (3) the influence of the local stresses arising from the encountered twin bands on crack growth. A novel dislocation-based strain nucleus model and a Green's function method, which are applicable to any material with local transformations in which elastic properties are reasonably approximated as isotropic, are specifically employed to model the concentrated transformed strain and calculate the local stress field resulting from deformation twinning and the stress intensity factors at crack tips in the magnesium alloys, respectively. In addition, an electron backscatter diffraction (EBSD) measurement is provided for crack nucleation originating from Type I co-zone tensile twin intersection. The theoretical modeling indicates: (i) the local strains within barrier twins strongly dictate the growth of incident twins and enhance the twin propagation stress; (ii) larger grains favor brittle fracture. More specifically, the dislocation reactions and pile-ups at the junctions between tensile twins can result in interfacial crack nucleation and growth along the twin boundaries, which is a brittle fracture mode based on lower twinning stress and stress concentration in the coarse-grained magnesium alloys; and (iii) the direction of crack propagation is easily changed by high-density twin bands and twin intersections owing to the local strains.

Microstructure characteristics of the (111) oriented grains in a Fe-30Ni-Nb model austenitic steel deformed in hot uniaxial compression

The work presents a detailed investigation of the microstructure characteristics of the (111) oriented grains in a Fe-30Ni-Nb austenitic model steel subjected to hot uniaxial compression at 925 °C at a strain rate of 1 s- 1. The above grains exhibited a tendency to split into deformation bands having alternating orientations and largely separated by transition regions comprising arrays of closely spaced, extended sub-boundaries collectively accommodating large misorientations across very small distances. On a fine scale, the (111) oriented grains typically contained a mix of "microbands" (MBs) closely aligned with {111} slip planes and those significantly deviated from these planes. The above deformation substructure thus markedly differed from the microstructure type, comprising strictly non-{111} aligned MBs, expected within such grains on the basis of the uniaxial compression experiments performed using aluminium. Both the crystallographic MBs and their non-crystallographic counterparts typically displayed similar misorientations and formed self-screening arrays characterized by systematically alternating misorientations. The crystallographic MBs were exclusively aligned with {111} slip planes containing slip systems whose sum of Schmid factors was the largest among the four available slip planes. The corresponding boundaries appeared to mainly display either a large twist or a large tilt component.

Evolution of elastic modulus in roll forming

Roll forming is a continuous process in which a flat strip is incrementally bent to a desired profile. This process is increasingly used in automotive industry to form High Strength Steel (HSS) and Advanced High Strength Steel (AHSS) for structural components. Because of the large variety of applications of roll forming in the industry, Finite Element Analysis (FEA) is increasingly employed for roll forming process design. Formability and springback are two major concerns in the roll forming AHSS materials. Previous studies have shown that the elastic modulus (Young’s modulus) of AHSS materials can change when the material undergoes plastic deformation and the main goal of this study is to investigate the effect of a change in elastic modulus during forming on springback in roll forming. FEA has been applied for the roll forming simulation of a V-section using material data determined by experimental loading-unloading tests performed on mild, XF400, and DP780 steel. The results show that the reduction of the elastic modulus with pre-strain significantly influences springback in the roll forming of high strength steel while its effect is less when a softer steel is formed.

The effect of ageing on the compressive deformation of Mg-Sn-Zn-Na alloy

The influence of ageing on deformation twinning in an extruded Mg-6Sn-3Zn-0.04Na alloy is investigated. In-situ compression tests have been carried out using high resolution synchrotron X-Ray Diffraction (XRD) to measure the influence of precipitates on twining activity. Synchrotron experiments revealed the increase in the critical resolved shear stress of twinning with ageing. The compressive yield strength (along the extrusion direction) of the aged sample increased by ∼ 150% over the non-aged specimen. To obtain statistical insight into the twinning activity, the microstructure of the non-aged and aged samples (200°C, 24 hours) deformed up to ∼1% plastic strain was studied using optical microscopy. A higher number of thinner twins were observed in the microstructure of the aged sample compared to the non-aged sample.

On the work-hardening behaviour of a high manganese TWIP steel at different deformation temperatures

In the current study, the work-hardening behaviour of a high manganese TWIP steel was investigated at different deformation temperatures. At room temperature, the steel exhibited an excellent combination of mechanical properties due to a unique work-hardening behaviour. There were four distinct stages observed in the work-hardening behaviour as a result of complex dynamic strain induced microstructural reactions consisted of dynamic recovery, dislocation dissociation, stacking fault formation, mechanical twining and dynamic strain aging. An increase in the deformation temperature significantly influenced the microstructure evolution, resulting in a remarkable alteration in the work-hardening behaviour. Consequently, the mechanical properties of the TWIP steel were gradually deteriorated with the deformation temperature. The mechanical twins appeared to have a restricted influence on the work-hardening behaviour of the TWIP steel at room temperature and remarkably diminished with the temperature. The enhanced work-hardening behaviour was mostly attributed to the interaction of glide dislocations with stacking faults.