Modification of Coal Samples with Bursting Liability Subjected to Microwave Irradiation

This paper introduces the innovative technique to release the bursting liability of coal seam via microwave irradiation. To verify the feasibility of this environment-friendly technique, a series of laboratory tests incorporating acoustic emission (AE) investigation were carried out. Test results indicated that both the uniaxial compressive strength (UCS) and bursting energy index of raw and water-soaked coal samples were significantly reduced. In particular, the bursting liability was reduced by one level when the values of UCS were compared, the evidence of which is the variation of wave velocities of tested coal samples. It can also be found from the events and hits in the complete stress-strain curve and the cumulative curve of acoustic emission that the elastic modulus of the raw and water-soaked coal samples subjected to microwave irradiation decreased by 58.42% and 29.63%, respectively. This facilitates the entry into the stage of stable crack propagation more quickly, the growth rate and size of the cracks were slower and more uniform, and there were no smaller coal fragments ejecting during the failure process of the coal samples. Meanwhile, the proportion of high-energy events released in coal samples experienced a decline after the treatment of the microwave. Moreover, microwave heating principally promoted the initiation and expansion of microcracks in coal samples under the influence of microwave power of 1 kW and a heating time of 120s, which may cause the overall damage of large fractures to break into multiple small and medium cracks. Based on the experimental results, the conceptual process of using microwave in weakening the bursting liability of coal seam was then proposed, which will be the meaningful reference for microwave-assisted oil recovery and coal bed methane production.

Theoretical limits in detachment strength for axisymmetric bi-material adhesives

Reversible dry adhesives rely on short-ranged intermolecular bonds, hence requiring a low elastic modulus to conform to the surface roughness of the adhered material. Under external loads, however, soft adhesives accumulate strain energy, which release drives the propagation of interfacial flaws prompting detachment. The tradeoff between the required compliance, for surface conformity, and the desire for a reduced energy release rate, for better strength, can be achieved with a bi-material adhesive having a soft tip and a rigid backing. This design strategy is widely observed in nature across multiple species. However, the detachment mechanisms of these adhesives are not completely understood and quantitative analysis of their adhesive strength is still missing. Based on linear elastic fracture mechanics, we analyze the strength of axisymmetric bi-material adhesives. We observed two main detachment mechanisms, namely (i) center crack propagation and (ii) edge crack propagation. If the soft tip is sufficiently thin, mechanism (i) dominates and provides stable crack propagation, thereby toughening the interface. We ultimately provide the maximum theoretical strength of these adhesives obtaining closed form estimation for an incompressible tip. In some cases, the maximum adhesive strength is independent of the crack size, rendering the interface flaw tolerant. We finally compare our prediction with experiments in the literature and observe good agreement.

Evaluation of Monkman–Grant strain as a key parameter in ductility exhaustion damage model to predict creep rupture of grade 92 steel

Conventional strain-based numerical prediction assumes that failure occurs when ductility is exhausted or accumulation of creep strain reaches the critical failure strain. Due to instability at the onset of rupture, the failure strain value appears to be scattered and leads to the erroneousness in prediction. In this paper, a new local constraint-based damage model incorporating the Monkman–Grant ductility, as a measure of strain during uniform creep deformation stage, was implemented into a Finite Element (FE) model to predict the creep damage and rupture of Grade 92 steel under uniaxial and multiaxial stress states. The prediction was applied on plain and notched bar specimens with various notch acuities. The uniaxial stress-dependent Monkman–Grant (MG) failure strain was adopted in the FE to simulate the influence of the constraints which were induced by the creep damage. The implication of reduced failure strain in long-term creep time on the rupture prediction is discussed. The multiaxial MG failure strain of the notched bar, which has a lower value than uniaxial failure strain due to the geometrical constraint, was estimated based on the linear inverse relationship between normalised MG failure strain and normalised triaxiality factor. It was found that the results obtained from the proposed technique were in good agreement with the experimental data within the scatter band of ± factor of 2. It was shown that MG failure strain can be used as an alternative to strain at fracture. MG strain outweighed strain at fracture because the determination of its value only required short-term testing to be performed. In most cases considered in the present investigation, the rupture-type fracture was predicted, however, there was evidence that under high constraint and low stress, stable crack propagation occurred before fracture. The location of the maximum creep damage was found to be dependent on the creep time, geometry or acuity level of the specimen. For sharp notch specimen, the failure was initiated near the notch root, however, as the notch radius increased, the initiation location moved further away towards the specimen centre.

An assessment of phase field fracture: crack initiation and growth

The phase field paradigm, in combination with a suitable variational structure, has opened a path for using Griffith’s energy balance to predict the fracture of solids. These so-called phase field fracture methods have gained significant popularity over the past decade, and are now part of commercial finite element packages and engineering fitness- for-service assessments. Crack paths can be predicted, in arbitrary geometries and dimensions, based on a global energy minimization—without the need for ad hoc criteria. In this work, we review the fundamentals of phase field fracture methods and examine their capabilities in delivering predictions in agreement with the classical fracture mechanics theory pioneered by Griffith. The two most widely used phase field fracture models are implemented in the context of the finite element method, and several paradigmatic boundary value problems are addressed to gain insight into their predictive abilities across all cracking stages; both the initiation of growth and stable crack propagation are investigated. In addition, we examine the effectiveness of phase field models with an internal material length scale in capturing size effects and the transition flaw size concept. Our results show that phase field fracture methods satisfactorily approximate classical fracture mechanics predictions and can also reconcile stress and toughness criteria for fracture. The accuracy of the approximation is however dependent on modelling and constitutive choices; we provide a rationale for these differences and identify suitable approaches for delivering phase field fracture predictions that are in good agreement with well-established fracture mechanics paradigms. This article is part of a discussion meeting issue ‘A cracking approach to inventing new tough materials: fracture stranger than friction’.

Characterizations of Crack Growth Behavior of Porous Polymer Membranes Using Digital Image Correlation and Finite Element Method

This study investigated the crack growth behavior of porous polymer membranes, experimentally and numerically, in order to clarify the criterion of crack growth. A notch was introduced into the membrane as an initial crack (pre-crack) and a uniaxial loading was applied for the stable crack propagation. During the test, crack propagation behavior was observed using a CCD camera and Digital Image Correlation (DIC) method. The strain around the pre-crack tip at the onset of crack propagation was measured experimentally using DIC method. It was clarified that large-scale yielding developed before the onset of crack growth. The stable crack propagation was observed for all tensile tests. In parallel, a homogenized model that mimicked porous polymer membrane was created using finite element method (FEM) in order to investigate stress/strain distribution around the crack tip. This study employed the yield criterion proposed by Deshpande and Fleck. The computed strain distribution was compared with that of experiment, showing a good agreement each other. By using strain distribution from DIC method and FEM computation, J-integral value was calculated to investigate the criterion of crack growth. Regardless of the initial crack length, it is found that the J-integral value at the initiation of crack growth becomes constant for all tests. It is concluded that we successfully determined the criterion of crack propagation of porous polymer membrane. Therefore, our study using DIC experiment and FEM computation is useful to clarify the crack growth behavior of porous polymer membrane and determines the criterion of crack propagation.

Embedding two-dimensional graphene array in ceramic matrix

Dispersing two-dimensional (2D) graphene sheets in 3D material matrix becomes a promising route to access the exceptional mechanical and electrical properties of individual graphene sheets in bulk quantities for macroscopic applications. However, this is highly restricted by the uncontrolled distribution and orientation of the graphene sheets in 3D structures as well as the weak graphene-matrix bonding and poor load transfer. Here, we propose a previously unreported avenue to embed ordered 2D graphene array into ceramics matrix, where the catastrophic fracture failure mode of brittle ceramics was transformed into stable crack propagation behavior with 250 to 500% improvement in the mechanical toughness. An unprecedentedly low dry sliding friction coefficient of 0.06 in bulk ceramics was obtained mainly due to the inhibition of the microcrack propagation by the ordered 2D graphene array. These unique and low-cost 2D graphene array/ceramic composites may find applications in severe environments with superior structural and functional properties.

Sideways and stable crack propagation in a silicone elastomer

We have discovered a peculiar form of fracture that occurs in a highly stretchable silicone elastomer (Smooth-On Ecoflex 00–30). Under certain conditions, cracks propagate in a direction perpendicular to the initial precut and in the direction of the applied load. In other words, the crack deviates from the standard trajectory and instead propagates perpendicular to that trajectory. The crack arrests stably, and thus the material ahead of the crack front continues to sustain load, thereby enabling enormous stretchabilities. We call this phenomenon “sideways” and stable cracking. To explain this behavior, we first perform finite-element simulations that demonstrate a propensity for sideways cracking, even in an isotropic material. The simulations also highlight the importance of crack-tip blunting on the formation of sideways cracks. Next, we provide a hypothesis on the origin of sideways cracking that relates to microstructural anisotropy (in a nominally isotropic elastomer). To substantiate this hypothesis, we transversely prestretch samples to various extents before fracture testing, as to determine the influence of microstructural arrangement (chain alignment and strain-induced crystallization) on fracture energy. We also perform microstructural characterization that indicates that significant chain alignment and strain-induced crystallization indeed occur in this material upon stretching. We conclude by characterizing how a number of loading conditions, such as sample geometry and strain rate, affect this phenomenon. Overall, this paper provides fundamental mechanical insight into basic phenomena associated with fracture of elastomers.

Cohesive Zone Modeling of Stable Crack Propagation in Highly Ductile Steel

A recent cohesive zone model is applied to the simulation of crack extension in austenitic stainless steel under large scale yielding conditions. The shape of the corresponding exponential traction-separation-relation can be modified in a wide range. In order to investigate the sensitivity regarding the cohesive zone parameters, a systematic parametric study is performed. The shape of the traction-separation envelope has a minor effect on the results compared to the cohesive strength and the work of separation. The aim is to fit experimental data by an appropriate choice of these parameters. Therefore, not only force-displacement curves should be used, but also crack growth resistance curves should be employed. A promising strategy for parameter identification is derived.