Zhuo Zhuang

Fracture in mechanism-based strain gradient plasticity

Abstract In a remarkable series of experiments, Elssner, Korn and Ruehle (Scripta Metall. Mater. 31 (1994) 1037) observed cleavage fracture in ductile materials, a phenomenon that cannot be explained by classical plasticity theories. In this paper we present a study of fracture by the theory of mechanism-based strain gradient (MSG) plasticity (Gao et al., J. Mech. Phys. Solids 47 (1999b) 1239); Huang et al., J. Mech. Phys. Solids 48 (2000a) 99). It is established that, at a distance much larger than the dislocation spacing such that continuum plasticity is applicable, the stress level in MSG plasticity is significantly higher than that in classical plasticity near the crack tip. The numerical results also show that the crack tip stress singularity in MSG plasticity is higher than that in the HRR field, and it exceeds or equals to the square-root singularity. This study provides a means to explain the observed cleavage fracture in ductile material.

Taylor-based nonlocal theory of plasticity: Numerical studies of the micro-indentation experiments and crack tip fields

Abstract Recent advances in strain gradient plasticity have provided a means to quantitatively characterize the experimentally observed size effect at the micron and submicron scales. The introduction of strain gradients in the constitutive model has increased the order of governing equations and therefore require additional boundary conditions in some theories of strain gradient plasticity. Is it possible to develop a micro-scale plasticity theory that preserves the structure of classical plasticity? The Taylor-based nonlocal theory (TNT) of plasticity (Int. J. Solids Struct. 38 (2001), 2615) was developed from the Taylor dislocation model for this purpose. We have proposed a finite element method for TNT plasticity, and have applied it to study micro-indentation experiments. The micro-indentation hardness predicted by TNT plasticity agrees very well with the indentation hardness data. We have also studied the crack tip field in TNT plasticity, and have found that the stress level in TNT plasticity is significantly higher than that in classical plasticity. This provides an alternative mechanism for cleavage fracture in ductile materials observed in Elssner et al.s experiments (Scripta Metall. Mater. 31 (1994) 1037).

The recent development of analysis methodology for rapid crack propagation and arrest in gas pipelines

Several dynamic analysis issues relating to rapid crack propagation (RCP) and arrest in gas piplines were developed recently, and are presented in this paper. This is based on a fluid/structure/fracture interaction package, PFRAC. Some developments have been implemented into this finite element code to simulate the behavior of the fractured pipes. The criteria for crack initiation, propagation and arrest have been discussed. As the crack propagates along the pipeline, the gas pressure decompression ahead of the crack tip and an efficiency of a linear decay behind the crack have been used in the computation. For the calculation of crack driving force G, the numerical approaches using the nodal force release and energy balance methods are described. This paper also presents a novel analysis methodology that has been developed to investigate the suitability of crack arrestors. Several numerical results for the cracked steel pipes with arrestors are presented along with comparisons with pipes that do not have arrestors.

Determination of material fracture toughness by a computational/experimental approach for rapid crack propagation in PE pipe

Based on an investigation of the Small Scale Steady State (S4) test, an integrated computational/ experimental approach has been developed in order to assess the fracture behaviour of polyethylene (PE) gas distribution pipe material during rapid crack propagation (RCP). This paper describes the use of the results obtained from the S4 test and program modified from PFRAC (Pipeline Fracture Analysis Code) to evaluate the fracture toughness of the material, Gd, which could not be directly obtained from the test, and to predict critical pressure, pc, for RCP in a full scale PE pipe. The contact algorithms are developed to consider the opening pipe wall impact against a series of containment rings and the capabilities of PFRAC are also extended. Since Gd is evaluated, the investigations are made on it to the effect of temperature and wall thickness. In addition, procedures to evaluate the critical pressure for the S4 test pipe are also discussed.

Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals

Significance Producing strong and defect-free materials is an important objective in developing many new materials. Thermal treatments aimed at defect elimination often lead to undesirable levels of strength and other properties. Although monotonic loading can reduce or even eliminate dislocations in submicroscale single crystals, such “mechanical healing” causes severe plastic deformation and significant shape changes. Inspired by observing an easier pullout of a partly buried object after shaking it first, we demonstrate that “cyclic healing” of the small-volume single crystals can indeed be achieved through repeated low-amplitude straining. The cyclic healing method points to versatile avenues for tailoring the defect structure and strengthening of nanoscale metal crystals without the need for thermal annealing or severe plastic deformation. When microscopic and macroscopic specimens of metals are subjected to cyclic loading, the creation, interaction, and accumulation of defects lead to damage, cracking, and failure. Here we demonstrate that when aluminum single crystals of submicrometer dimensions are subjected to low-amplitude cyclic deformation at room temperature, the density of preexisting dislocation lines and loops can be dramatically reduced with virtually no change of the overall sample geometry and essentially no permanent plastic strain. This “cyclic healing” of the metal crystal leads to significant strengthening through dramatic reductions in dislocation density, in distinct contrast to conventional cyclic strain hardening mechanisms arising from increases in dislocation density and interactions among defects in microcrystalline and macrocrystalline metals and alloys. Our real-time, in situ transmission electron microscopy observations of tensile tests reveal that pinned dislocation lines undergo shakedown during cyclic straining, with the extent of dislocation unpinning dependent on the amplitude, sequence, and number of strain cycles. Those unpinned mobile dislocations moving close enough to the free surface of the thin specimens as a result of such repeated straining are then further attracted to the surface by image forces that facilitate their egress from the crystal. These results point to a versatile pathway for controlled mechanical annealing and defect engineering in submicrometer-sized metal crystals, thereby obviating the need for thermal annealing or significant plastic deformation that could cause change in shape and/or dimensions of the specimen.

Simulation-Based Unitary Fracking Condition and Multiscale Self-Consistent Fracture Network Formation in Shale

Hydraulic fracturing (fracking) technology in gas or oil shale engineering is highly developed last decades, but the knowledge of the actual fracking process is mostly empirical and makes mechanicians and petroleum engineers wonder: why fracking works? (Ba zant et al., 2014, “Why Fracking Works,” ASME J. Appl. Mech., 81(10), p. 101010) Two crucial issues should be considered in order to answer this question, which are fracture propagation condition and multiscale fracture network formation in shale. Multiple clusters of fractures initiate from the horizontal wellbore and several major fractures propagate simultaneously during one fracking stage. The simulation-based unitary fracking condition is proposed in this paper by extended finite element method (XFEM) to drive fracture clusters growing or arresting dominated by inlet fluid flux and stress intensity factors. However, there are millions of smeared fractures in the formation, which compose a multiscale fracture network beyond the computation capacity by XFEM. So, another simulation-based multiscale self-consistent fracture network model is proposed bridging the multiscale smeared fractures. The purpose of this work is to predict pressure on mouth of well or fluid flux in the wellbore based on the required minimum fracture spacing scale, reservoir pressure, and proppant size, as well as other given conditions. Examples are provided to verify the theoretic and numerical models. [DOI: 10.1115/1.4036192]

Stability analysis of the propagation of periodic parallel hydraulic fractures

When multiple fractures are propagating simultaneously, the fracture spacing may coarsen gradually resulting from loss of stability of the fracture system, which can also occur during the propagation of multiple hydraulic fractures (HFs). In this paper, the stability of the propagation of periodic parallel HFs in brittle solids is investigated based on a representative unit cell consisting two HFs. The fractures are driven to propagate by the inside fluid flow and fluid–solid coupling effect is involved. Both the stress interaction acting in the solid medium and the flow of fluid medium can influence the stability. Stability criterions are given based on the variations of the rates of change of the stress intensity factor with respect to facture velocities. Stability results are obtained by solving the system with fully coupled numerical method which considers the deformation of solid medium, fracture propagation, fluid flow in fractures, fluid partitioning into each fracture. Based on the numerical results, the influences of propagation regimes, fracture lengths and the fracture distance on the stability of HFs are investigated.