Low Fracture Energy Research Articles

Effect of Hydrogen Corrosion on the Brittle Fracture Resistance of an Oil Line Pipe 530 mm in Diameter Made of 12GSB Ferritic–Pearlitic Steel

The structure, the static tensile properties, the impact toughness, and the fracture mechanisms of the 12GSB pipe steel specimens cut from the defect regions in a pipeline 530 mm in diameter are studied. Hydrogen corrosion is found to affect the anomalous character of the temperature dependences of the impact toughness and the fraction of ductile component in a fracture surface. Sulfur is shown to substantially influence the ductile crack propagation energy in the transverse direction and to weakly influence this energy along the rolling direction. The development of hydrogen corrosion in the pipeline is found to be accompanied by a significant increase in the reduced fraction of intergranular fracture (15.8–35%) in brittle fracture surfaces, and this fraction in the separation plane reaches 50.5%. The relation between the impact toughness of the transverse specimens and the fraction of ductile component in a fracture surface has an anomalous character: this relation has two linear segments, the fracture energies in which differ by an order of magnitude. The segment with a low fracture energy (4.4–16.1 J/cm2) characterizes the action of the sear component of separation in the rolling plane, and the segment with a high fracture energy (38–246 J/cm2), the action of a ductile mode I fracture mechanism. The impact toughness anisotropy coefficient is found to be high, Ka = 2.31.

Preparation of V8C7-Fe/iron dual-scale composite via two-step in situ reaction

To overcome the disadvantages of low toughness and low fracture energy of conventional particle reinforced steel/iron-based composites, a novel V8C7-Fe/iron dual-scale composite was designed and fabricated via a two-step in situ reaction at 1150 °C for 5 min and 1100 °C for 12 h (denoted as 1150 °C-5 min + 1100 °C-12 h). The composite consists of a bundle-shaped V8C7-Fe dual-scale structure and cast iron matrix. Predominant features of the bundle-shaped V8C7-Fe dual-scale structure, which has a diameter of approximately 3 mm, include a toughness of the α-Fe phase and a high volume fraction of V8C7 particles that have different sizes and morphologies. The interface of the V8C7-Fe dual-scale structure/matrix has distinct gradient distribution characteristics. From the center of the V8C7-Fe dual-scale structure to the matrix, the morphology of the V8C7 particles evolves from nearly spherical to cubic. This is mainly because of changes in the growth environment that lead to the competitive growth between different crystal faces and orientation attachment growth during nucleation and growth. In addition, from the center of the V8C7-Fe dual-scale structure to the matrix, the hardness gradually decreases from 1306 HV0.2 to 410 HV0.2. For the V8C7-Fe/iron dual-scale composite, the yield strength, maximum compressive strength, and maximum compressive strain are approximately 1150 MPa, 1390 MPa, and 15.2%, respectively. Compared to cast iron (420 MPa, 830 MPa and 21.8%, respectively), the V8C7-Fe/iron dual-scale composite achieves both high strength and reasonable toughness.

Impact of different levels of oxidative aging on engineering properties of asphalt mixes at low temperatures

Asphalt pavements at cold climates or regions with high-temperature fluctuation are prone to cracking due to the induced thermal stress within the mix. The properties of asphalt mixtures at low temperatures, including stiffness, tensile strength, and fracture energy are of prominent importance in the initiation and propagation of the low temperature or thermal cracks in the pavement. The influence of aging on changing mechanical properties of mixes has been well recognized. This study aimed to gain more insight into how aging affects the low-temperature behavior of asphalt mixtures. Overall, 15 types of asphalt mixes with three binder contents, two aggregate types, and three different binder types (conventional and using two modifiers) were assessed at both short-term and long-term aging conditions using the dynamic modulus test (at −15, 0, and 20 °C), the semi-circular bending test (at −5°C), and the indirect tensile strength test (at −10 °C). In general, aging increased stiffness and reduced relaxation potential of mixes and affected the fracture and strength properties of mixes, adversely. The main finding of this study was that the influence of aging on the thermal cracking critical properties of asphalt mixes was not the same for mixes with different aggregate sources, mixes at different binder content, and mixes with modifiers. Appropriate selection of mix ingredients and volumetric properties were found to be crucial to ensure the resistance of the mix to thermal cracking over the long-term service life of the pavement.

Experimental Investigation on the Interfacial Debonding between FRP Sheet and Concrete under Medium Strain Rate

Fiber-reinforced polymer (FRP) composites have been widely used to strengthen the existing reinforced concrete (RC) structures to against static and dynamic loads. During the past decades, the interfacial bond behavior between FRP and the concrete substrate under static load has been systematically investigated by experimental and numerical approaches. In contrast, the interfacial bond performance under dynamic loads, e.g., impact and explosive loading, is still far away from well known, especially taking the strain rate effect into account. In this contribution, the single-lap shear test is conducted to sixty specimens at the medium strain rate between 1.0E−4/s and 5.0E−3/s. The effects of various system parameters, including the strain rate, concrete strength, type of FRP and adhesive, on the interfacial fracture energy, peak shear stress, FRP strain distribution, interfacial shear stress, and effective bond length, are thoroughly investigated. It has been revealed that the strain rate and concrete strength can significantly affect the interfacial fracture energy and peak shear stress. The specimen with CFRP sheet possesses higher interfacial shear stress but lower fracture energy than that with BFRP sheet. The adhesive with lower elastic modulus is helpful to improve interfacial energy dissipation under dynamic load. The effective bond length decreases with concrete strength and strain rate, mainly between 75 mm and 90 mm, which is significantly shorter than that under static load. Inspired from the Kulkarni and Shah model, a new model is proposed to evaluate the interfacial fracture energy and peak shear stress with respect to the strain rate, and the estimated values agree well with the experiments.

Evaluation of performance properties of 50% recycled asphalt mixtures using three types of rejuvenators

In this study Cyclogen, Rapiol and waste cooking oil was used as rejuvenators to improve properties of aged bitumen. The effects of these rejuvenators on asphalt mixtures containing 50% of reclaimed asphalt pavement specimens was evaluated using dynamic creep, resilient modulus, low temperature fracture energy and moisture susceptibility tests and the results was compared to recycled mixtures without rejuvenator and control HMA mixtures. The results showed that the rejuvenators are effective in alleviating the aging effect of RAP bitumen. Waste cooking oil performed better than other studied rejuvenators followed by Cyclogen and Rapiol. However, moisture resistance was impaired when rejuvenators were used.

Experimental Investigation on Fatigue and Low Temperature Properties of Asphalt Mixtures Designed with Reclaimed Asphalt Pavement and Taconite Aggregate

Using reclaimed asphalt material for rehabilitation and construction of new asphalt pavements is currently a common practice not only in view of the economic benefits associated with this process but also because of the reduced exploitation of natural resources. For this reason, road authorities have implemented recommendations and guidelines to regulate the use of reclaimed asphalt pavement (RAP) and other recycled materials such as industrial by-products. Nevertheless, the combined use of different recycled materials is not commonly addressed. In this paper, the effect of adding RAP and taconite (a mining by-product) on fatigue and low temperature properties of asphalt mixture was investigated with two different testing geometries: indirect tensile (IDT) and semi-circular bending (SCB). Fatigue behavior, creep stiffness, relaxation modulus, low temperature fracture energy, and fracture toughness were also evaluated, computed, and then compared. A more brittle behavior was observed for mixture prepared with RAP material, however, the mechanical performance was not significantly different for mixtures containing 20% RAP alone and in combination with 50% taconite compared with conventional asphalt mixtures designed with virgin material. This was not the case when RAP content was increased up to 50%, showing a substantially poorer response both in terms of fatigue and low temperature characteristics and suggesting the RAP had a dominant effect. The present exploratory research seems to support the idea of combining RAP and different industry by-products, such as taconite, as long as the RAP content is kept below a specific threshold.

The impacts of type and proportion of five different asphalt modifiers on the low temperature fracture toughness and fracture energy of modified HMA

Low temperature fracture toughness and fracture energy are two important measures that could be used to investigate the impacts of using asphalt modifiers on the performance of asphalt pavements in cold regions. The aim of this research was to identify the impacts of using various proportions of five different asphalt modifiers on the fracture toughness and the fracture energy of Hot Mix Asphalts (HMA) under mode I loading and at low environmental temperatures. The asphalt modifiers used for this purpose were: Elastoplastomer Polymer Strings (EPS), Parafiber, Sulfur Polymer, Polyolefin-Aramid Compound Structural Fibers (PACSF) and Sasobit. These modifiers were individually used at three different proportions to produce Semi-circular Bend Specimens containing vertical edge crack. Each specimen was then tested under symmetric monotonic three-point bend loading at -15°C. The results indicated that, except for the EPS, both measures were increased with an increase in the modifier proportion. The most increase in the both measures was observed in the specimens modified with the PACSF, closely followed by specimens modified with the Parafibers. The least increase in these two measures were observed in the specimens modified with the Sulfur Polymer. The results indicated the applicability of examined modifiers to improve the resistance of HMA to crack initiation and crack growth at low temperatures.

Tough Nano‐Architectured Conductive Textile Made by Capillary Splicing of Carbon Nanotubes

Flexible electronics require electrically conductive and mechanically reliable nanoscale thin films. However, thin metal films have low fracture energy, which limits the performance of flexible devices. We demonstrate the design and synthesis of highly conductive, strong and tough nano‐architectured textile by capillary splicing of aligned carbon nanotubes (CNT). Owing to the strong van der Waals forces among CNTs, the pristine CNT network has average strength of 170 MPa. The average fracture energy of the textile is 16 kJ/m2, 50 folds higher than metal nanofilms. The high toughness results from crack bifurcations and friction hysteresis in a dissipation zone propagating several millimeters ahead of the crack tip. This material is suitable for applications ranging from smart skin and flexible sensors.

Dissimilar Al/steel friction stir spot welding: To penetrate into the lower steel sheet or not?

There has always been a question about which condition results in superior mechanical properties in friction stir spot welds: (i) when the tool tip penetrates into the lower sheet or (ii) when the tool tip does not touch it. Given the question, the effects of the tool penetration depth were investigated on the microstructure and mechanical behaviour of dissimilar Al-5083/St-12 joints produced by friction stir spot welding (FSSW). Two different processes named as non-penetrating and penetrating FSSW were used. Optical and scanning electron microscopes as well as microhardness and tensile–shear tests were utilised to examine the obtained joints. The results showed that the penetrating process led to the stronger joints with lower fracture energy. The differences were studied in the light of the joining mechanisms and the formation of Al/Fe intermetallic compounds.

Effect of stoichiometric ratios for synthesized epoxy phenolic novolac (EPN) resins on their physicochemical, thermomechanical and morphological properties

Purpose The purpose of this paper is to study the effect of various stoichiometric ratios for synthesised epoxy phenolic novolac (EPN) resins on their physicochemical, thermomechanical and morphological properties. Design/methodology/approach In the present study, EPN (EPN-1, EPN-2, EPN-3, EPN-4 and EPN-5) resins were synthesised by varying five types of different stoichiometric ratios for phenol/formaldehyde along with the corresponding molar ratios for novolac/epichlorohydrin. Their different physicochemical properties of interest, thermomechanical properties as well as morphological properties were studied by means of cured samples with the variation of its stoichiometric ratios. Findings The average functionality and reactivity of EPN resin can be controlled by controlling epoxy equivalence as well as cross-linking density upon its curing as all of these factors are internally correlated with each other. Research limitations/implications Epoxy resins are characterised by a three-membered ring known as the epoxy or oxirane group. The capability of the epoxy ring to react with a variety of substrates imparts versatility to the resin. However, these resins have a major drawback of low toughness, and they are also very brittle, which limits their application in products that require high impact and fracture strength. Practical implications Epoxy resins have been widely used as high-performance adhesives and matrix resins for composites because of their outstanding mechanical and thermal properties. Because of their highly cross-linked structure, the epoxy resin disables segmental movement, making them hard, and it is also notch sensitive, having very low fracture energy. Social implications Epoxy resin is widely used in industry as protective coatings and for structural applications, such as laminates and composites, tooling, moulding, casting, bonding and adhesives. Originality/value Systematic study has been done for the first time, as no exact quantitative stoichiometric data for the synthesis of EPN resin were available on the changes of its different properties. Thus, an optimised stoichiometric composition for the synthesis of the EPN resin was found.

Failure mechanisms of single-crystal silicon electrodes in lithium-ion batteries.

Long-term durability is a major obstacle limiting the widespread use of lithium-ion batteries in heavy-duty applications and others demanding extended lifetime. As one of the root causes of the degradation of battery performance, the electrode failure mechanisms are still unknown. In this paper, we reveal the fundamental fracture mechanisms of single-crystal silicon electrodes over extended lithiation/delithiation cycles, using electrochemical testing, microstructure characterization, fracture mechanics and finite element analysis. Anisotropic lithium invasion causes crack initiation perpendicular to the electrode surface, followed by growth through the electrode thickness. The low fracture energy of the lithiated/unlithiated silicon interface provides a weak microstructural path for crack deflection, accounting for the crack patterns and delamination observed after repeated cycling. On the basis of this physical understanding, we demonstrate how electrolyte additives can heal electrode cracks and provide strategies to enhance the fracture resistance in future lithium-ion batteries from surface chemical, electrochemical and material science perspectives.

High tensile strength fly ash based geopolymer composite using copper coated micro steel fiber

As a ceramic-like material, geopolymers show a high quasi-brittle behavior and relatively low fracture energy. To overcome this, the addition of fibers to a brittle matrix is a well-known method to improve the flexural strength. Moreover, the success of the reinforcements is dependent on the fiber-matrix interaction. In this present study, effects of micro steel fibers (MSF) incorporation on mechanical properties of fly ash based geopolymer was investigated at different volume ratio of matrix. Various properties of the composite were compared in terms of fresh state by flow measurement and hardened state by variation of shrinkage over time to assess performance of the composites subjected to flexural and compressive load. The fiber-matrix interface, fiber surface and toughening mechanisms were assessed using field emission scan electron microscopy (FESEM) and atomic force microscopy (AFM) through a period of 56days. Test results confirmed that MSF additions could significantly improve both ultimate flexural capacity and ductility of fly ash based geopolymer, especially at early ages without an adverse effect on ultimate compressive strength.

Use of crumb rubber in lieu of binder grade bumping for mixtures with high percentage of reclaimed asphalt pavement

Use of recycled/reclaimed asphalt pavement (RAP) leads to economic as well as environmental benefits. However, increasing the percentage of RAP in a new asphalt pavement may hinder the economic benefits due to the necessity of expensive softer binders used to compensate for RAP stiffening effect. Alternatively, this problem may potentially be overcome by use of another recycled material: crumb rubber (CR). In this study, linear viscoelastic, thermal and fatigue cracking properties of mixtures prepared with different rubberised asphalt binders were compared with the soft binders. The scope of the testing included asphalt mixtures produced from the following binders; (i) devulcanised rubber, (ii) CR terminally blend, (iii) CR wet process, (iv) soft binder (PG 58-34) and (v) base (control) binder (PG 58-28). The results of the fatigue tests indicated that CR-modified asphalt mixtures made with the base binder (PG58-28) outperformed the mixture made with grade bumped (PG58-34) binder. Similar results were observed in low-temperature cracking tests. The results indicated that the CR-modified binders can be used in lieu of grade bumping of binders in high RAP mixtures. In fatigue cracking tests, among the three CR technologies, wet process outperformed terminally blend and devulcanised rubber at high temperatures and high strain levels at low temperatures. At relatively low strain levels, terminally blend was the best performer in low temperature. In low-temperature cracking tests, devulcanised rubber exhibited highest tensile strength, but lowest fracture energy. Wet process exhibited highest fracture energy and lowest strength.

A Comprehensive Study of the Polypropylene Fiber Reinforced Fly Ash Based Geopolymer

As a cementitious material, geopolymers show a high quasi-brittle behavior and a relatively low fracture energy. To overcome such a weakness, incorporation of fibers to a brittle matrix is a well-known technique to enhance the flexural properties. This study comprehensively evaluates the short and long term impacts of different volume percentages of polypropylene fiber (PPF) reinforcement on fly ash based geopolymer composites. Different characteristics of the composite were compared at fresh state by flow measurement and hardened state by variation of shrinkage over time to assess the response of composites under flexural and compressive load conditions. The fiber-matrix interface, fiber surface and toughening mechanisms were assessed using field emission scan electron microscopy (FESEM) and atomic force microscopy (AFM). The results show that incorporation of PPF up to 3 wt % into the geopolymer paste reduces the shrinkage and enhances the energy absorption of the composites. While, it might reduce the ultimate flexural and compressive strength of the material depending on fiber content.

Frictional properties of incoming pelagic sediments at the Japan Trench: implications for large slip at a shallow plate boundary during the 2011 Tohoku earthquake

The 2011 Tohoku earthquake (Mw 9.0) produced a very large slip on the shallow part of a megathrust fault that resulted in destructive tsunamis. Although multiple causes of such large slip at shallow depths are to be expected, the frictional property of sediments around the fault, particularly at coseismic slip velocities, may significantly contribute to large slip along such faults. We have thus investigated the frictional properties of incoming pelagic sediments that will subduct along the plate boundary fault at the Tohoku subduction zone, in order to understand the rupture processes that can cause large slip in the shallow parts of subduction zones. Our experimental results on clayey sediment at the base of the sedimentary section on the Pacific Plate yield a low friction coefficient of <0.2 over a wide range of slip velocities (0.25 mm/s to 1.3 m/s), and extremely low fracture energy during slip weakening, as compared with previous experiments of disaggregated sediments under coseismic slip conditions. Integrated Ocean Drilling Program (IODP) Expedition 343 confirmed that the clay-rich sediment investigated here is identical to those in the plate boundary fault zone, which ruptured and generated the Tohoku earthquake. The present results suggest that smectite-rich pelagic sediment not only accommodates cumulative plate motion over interseismic periods but also energetically facilitates the propagation of earthquake rupture towards the shallow part of the Tohoku subduction zone.

A finite element method with mesh‐separation‐based approximation technique and its application in modeling crack propagation with adaptive mesh refinement

SUMMARYThis paper presents a FEM with mesh‐separation‐based approximation technique that separates a standard element into three geometrically independent elements. A dual mapping scheme is introduced to couple them seamlessly and to derive the element approximation. The novel technique makes it very easy for mesh generation of problems with complex or solution‐dependent, varying geometry. It offers a flexible way to construct displacement approximations and provides a unified framework for the FEM to enjoy some of the key advantages of the Hansbo and Hansbo method, the meshfree methods, the semi‐analytical FEMs, and the smoothed FEM. For problems with evolving discontinuities, the method enables the devising of an efficient crack‐tip adaptive mesh refinement strategy to improve the accuracy of crack‐tip fields. Both the discontinuities due to intra‐element cracking and the incompatibility due to hanging nodes resulted from the element refinement can be treated at the elemental level. The effectiveness and robustness of the present method are benchmarked with several numerical examples. The numerical results also demonstrate that a high precision integral scheme is critical to pass the crack patch test, and it is essential to apply local adaptive mesh refinement for low fracture energy problems. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Assessment of a microwave-assisted recycling process for the recovery of high-quality aggregates from concrete waste

This study presents an innovative method for concrete waste up-cycling based on concrete weakening through microwave heating before impact crushing. Two series of tests were conducted in order to assess the influence of the aggregate properties (size distribution, mineralogical nature) and the influence of the operating conditions of the microwave heating pretreatment on concrete fragmentation; and thus to evaluate the feasibility and the robustness of this process. Experiments were carried out on lab-made, cylindrical concrete specimens and on no-slump concrete waste with a multimode cavity microwave equipment (2.45GHz, 6kW) and an impact crusher. Results showed that microwave heating always induced an embrittlement of concrete samples which resulted in lower fracture energy, higher fragmentation of samples and higher liberation of aggregates (i.e. free of cement paste). A microwave-assisted comminution process is therefore an effective recycling technique for the recovery of high-quality aggregates from concrete waste.

Effect of Rubber Modification on the Morphology and Properties of Novolac Nanostructures

Nanostructured organic materials have received considerable attention over the past decade. This nanostructure is responsible for their unusual acoustic, thermal, and mechanical properties. One of the most studying of this field is sol-gel polymerization of resorcinol-formaldehyde, followed by organic solvent exchange, supercritical drying. However, the conventional process involves long gelation time, expensive monomer of resorcinol, high cost of supercritical drying device and so on, which makes it some distance away from being promoted to commercial production. In this work, to reduce cost and time, the reaction of novolac resin and hexamine and ambient pressure drying method was used for preparation and drying of organic gel. To reduce time, the polymerization in saturated atmosphere of solvent vapor instead of conventional sol-gel polymerization presented. These nanostructure materials have a nanoporous structure that constructed by colloidal like particles gathered up in filament-shaped. Because of this nanoporosity and structure, these materials have very low fracture energy and could including them as brittle materials. The goal of this research produces these nanostructure materials using novel method and enhances fractural resistance. To gain this proposes, the feasibility of using acrylonitrilebutadiene rubbers (NBR) has been investigated. For studying of morphology and properties of this structure, we used SEM, FTIR and porosimetry analysis. The results shows that the addition of 0.5, 1 and 2 wt% NBR to Novolac nanostructure increased density and decrease the pore volume and pore size.

Synthesis and toughening behavior of bio-inspired nanocrystalline TiO2/polyelectrolyte nanolayered composites

Bio-inspired (nanocrystalline TiO2/polyelectrolyte (PE))4 nanolayered composites were synthesized by the layer-by-layer self-assembly and chemical bath deposition methods. The results show that the thicknesses of the TiO2 and PE layers in the as-synthesized are 41.3nm and 9.6nm, respectively, and the grain size of the TiO2 layer is about 5nm. For the type I cracking, the fracture toughness (KIC=1.18±0.67MPam1/2) of 41.3nm TiO2/9.6nm PE composite is larger than that of the bulk TiO2. For the type II cracking, the crack deflection occurred in the composite. This toughening behavior was attributed to the low interfacial fracture energy and the large ratio of the crack deflection length to the crack penetration length. Our finding demonstrates that the crack deflection is an effective way to toughen the bio-inspired nanocrystalline TiO2/PE nanolayered composites.

Impact fracture energy of structural steel welds constructed at low ambient temperatures

Welding at low atmospheric temperatures is frequently needed for the construction of steel structures in cold regions. In this work, the Charpy impact test in parallel with metallurgical observation was carried out to evaluate the low temperature impact fracture energy of structural steel welded joints constructed at low ambient temperatures. The impact fracture energy at low temperatures of structural steel joints welded at room temperature was also investigated for comparison. Standard V-notch Charpy impact specimens were prepared and tested under dynamic loading condition. The effects of low climatic temperatures during welding on the low temperature impact fracture energy of structural steel welded joints were explored based on the absorbed energies and the microstructures, from which the feasibility of welding in cold weather was assessed. Referring to the results, significant drop of the low temperature impact fracture energy at the heat affected zone occurs due to the rapid cooling effects. Nevertheless, it is confirmed that the impact fracture energy of structural steel welded joints constructed at low ambient temperatures can be guaranteed for use in cold regions provided that proper precautions such as the use of low-hydrogen electrodes and suitable thermal control are taken during the welding process.