Multi-directional Crack Research Articles

Magnetic pulse welding: Interface of Al/Cu joint and investigation of intermetallic formation effect on the weld features

This paper attempts to highlight emerging issues when joining dissimilar metal pairs by magnetic pulse welding. For this purpose, joint of Al/Cu and Al/Al welded with the same parameters are compared. Al/Cu combination involves a formation of an interfacial intermetallic phase whereas Al/Al joint is bonded by metal continuity at the interface. The intermetallic is found to be amorphous, nanoporous, and damaged by a multi-directional cracks. The formation of such features is respectively explained by a melting confined at the interface followed by high rate cooling, cavitation phenomenon and solidification shrinkage. Mechanical characterizations of the Al/Cu joint show brittle fracture at the interface with fragmentation of the [0] intermetallics (glass type rupture). The intermetallic defects such as pores and cracks strongly reduce the breaking load level when compared to the Al/Al case. On the other hand, the Al/Al joint shows ductile fracture with plastic deformation of the interface.

On microstructure and flexural strength of metal–ceramic composite cladding developed through microwave heating

A domestic multimode microwave applicator was used to develop carbide reinforced (tungsten-based) metal–matrix composite cladding on austenitic stainless steel substrate. Cladding was developed through microwave irradiation of the preplaced clad materials at 2.45GHz for 420s. Clads show metallurgical bonding with substrate by partial dilution of materials. Back scattered images of clad section confirm uniformly distributed reinforced particles in the metallic matrix. Presence of WC, W2C, NiSi, NiW and Co3W3C phases was detected in the clad. Flexural characteristics show two distinct load transitions attributable to deformations of the matrix and the reinforced particles. Clads fail at the upper transition load; further load is taken by the SS-316 substrate. Clads exhibit good stiffness and good adhesion with the substrate. Multi directional cracks were observed at the clad surface; on further loading, cracks get propagated into the clad thickness without getting peeled-off. Mechanism of clad development has been introduced.

A Numerical Investigation of Thermal-related Matrix Shrinkage Crack and Delamination in Composite T-Piece Specimens Using a Modified Interface Cohesive Model

This article numerically investigated the curing temperature effect on matrix shrinkage crack and the effect of matrix shrinkage cracking on delamination in composite T-piece specimens using a modified interface cohesive model with thermal effects accounted. Thermal relative coefficient was introduced to produce the relative thermal displacement in the formulation of interface cohesive elements. The thermal shrinkage crack in the deltoid region of T-piece was simulated. Effect of this thermal initial cracking on the prediction of dominated delamination [Chen, J., Ravey, E., Hallett, S., Wisnom, M. and Grassi, M. (2009). Prediction of Delamination in Braided Composite T-Piece Specimen, Composites Science and Technology, 69(14): 2363–2367; Chen, J. (2011) Simulation of Multi-directional Crack in Braided Composite T-Piece Specimens Using Cohesive Models, Fatigue & Fracture of Engineering Materials & Structures, 34(2): 123–130.] of T-piece under T-pull loading case was also studied. The investigation indicated that some improper restraints to T-piece specimens during the curing process will induce so called thermal shrinkage cracks in the deltoid region of T-piece. This sort of thermal related matrix shrinkage crack has limited effect on the capacity of T-piece to resist T-pull loading. Radius laminates are the main load carrier in the T-pull loading case. This modeling investigation supplied considerable information for the design and manufacture of T-piece related composite components under pulling condition. Further investigation considering loading cases such as bending and combination with bending and T-pull is suggested in the future work to explore general effects of thermal related matrix shrinkage cracking on delaminating.

Simulation of multi-directional crack growth in braided composite T-piece specimens using cohesive models

This paper presents an approach to the prediction of multi-directional crack growth in braided composite T-piece specimens using cohesive zone models. Failure predictions given by the cohesive model together with experimental results are presented. The effects of single and multi-directional crack model on failure prediction are investigated. Results show that both models agree with experimental data, single-crack model agrees with the maximum value of experimental data, and multi-directional crack model agrees with the mean value. The simulation of multi-directional crack growth of T-piece specimens is important in the study of their failure mechanism. Investigation reported in this paper indicates that prediction of failure loads by the multi-directional crack model is necessary in the design of some particular composite components which have similar features to the T-piece specimens.

Space-Averaged Constitutive Model for HPFRCCs with Multi-Directional Cracking

This paper focuses on numerical modeling of high-performance fiber-reinforced cementitious composites (HPFRCCs), specifically polyvinyl alcohol engineered cement composites (PVA-ECCs) in the context of a space-averaged, fixed-crack approach. Compression, tension, and shear models are proposed. The compression and tension models include internal unloading and reloading paths. The shear model considers the shear stress transfer contributed by surface friction and fiber bridging in a phenomenological manner. The applicability of the models is verified against recent experiments on precracked PVA-ECC plates subjected to principal stress rotation, demonstrating that the proposed models replicate various responses of the plates. The degradation of initial stiffness and overall strength of plates with precracks at different angles is represented well. Lastly, this paper demonstrates the ability of the models to replicate the average strains spanning bidirectional multiple cracks occurring at the bottom surface of the precracked plates.

Modeling and analysis of aging behavior of concrete structures in nuclear power plants

The design rules for reactor containments and similar safety structures in nuclear power plants have been established decades ago based on their initial material and structural conditions, and design-based assumptions. However, as these plants begin to approach the end of their original design life and transition to the life-extension phase, analytical methods are needed to evaluate potential aging-related effects. In the case, of pre-stressed concrete containments, for example, the concrete is subject to long-term creep in the presence of time-varying loads due to tendon relaxation and possible re-tensioning, periodic leak-rate testing and service-and weather-dependent thermal cycling. Also, some PWR plants are facing the prospect of replacing steam generators, which require the cutting of large opening in the pre-stressed containment. Performing structural modification for such purposes should be guided by conducting detailed numerical simulations to avoid introducing new, or obscuring pre-existing, damage. This requires the use of qualified, well-validated analytical methodology with robust material-behavior representations of damage states, including the ability to model pre-existing cracks and other service-induced damage conditions. Such a methodology has been developed considering a wide range of structural behavior and service conditions. This paper describes the behavioral modeling attributes of this methodology and its application to the diagnostic evaluation of the effects of long-term service and environmental conditions on concrete infrastructures in nuclear power plants, including structural retrofitting and rehabilitation of reactor containments, and proof of performance for life-extension service. Reinforced concrete, despite its apparent simplicity, presents one of the most difficult problems in constitutive modeling and numerical simulation. The difficulties in constitutive modelling of concrete structures stem from the fact that cracks can form at relatively low stress levels. This leads to highly complex interaction between post-cracking behavioral regimes, which include: multidirectional cracking, crushing, shearing along rough crack surfaces and the consequential formation of rebarreacted compression normal to crack surfaces. One observable manifestation of this interaction is the potential for split cracking parallel to the compressive stress field imposed by the tendon in a pre-stressed containment, especially if the tendons are located close to the external surface of the containment wall where concrete confinement is small. Such split cracking could grow with time and may lead to wall delamination, with potentially serious consequences for the containment's design-basis function. The ageing-dependent constitutive properties of the concrete material considered are: tensile strength, post-cracking shear behavior, and compressive strength, with both, hardening effects due to the maturity of concrete compressive strength and softening effects due to environmental degradation, temperature, internal micro-cracking and past-loading-related macro cracks. A robust analytical treatment of these properties in a general three-dimensional stress-strain constitutive formulation that recognizes the interaction with the reinforcement is fundamental to the degree of fidelity of the predicted response to true physical behavior.

Controlled mechanical fracture for fabricating microchannels with various size gradients

We present a simple method to generate cracks with controllable size (depth and width) and space gradients using deep surface oxidation and anisotropic mechanical stretching. To generate a thick oxidation layer (<∼7 µm), a polydimethylsiloxane (PDMS) slab of uniform or varying thickness was exposed to UV/ozone for less than 30 min in the UV-C wavelength including wavelengths of 185 and 254 nm. Subsequently, the PDMS slab was wrapped on a cylindrical support (radius: 11 mm) to apply a uniform bending strain (<21%), resulting in equally separated, anisotropic cracks over a large area. By modulating initial oxidation depth and applied bending stress, cracks of varying sizes and spaces were formed on a single PDMS slab. Furthermore, multiple, sequential cracks were generated by increasing the strain in a step-wise fashion and multi-directional cracks by applying the strain with an orientation angle. Finally, size and space-varying cracks were formed between two adjacent large channels in an interconnected format by selective masking and irreversible bonding.

Multi-directional modeling of crack pattern in 2D R/C members

A macro-scale approach to R/C modeling is proposed in this paper by formulating a comprehensive model, that describes R/C behavior in the uncracked stage (solid concrete) and in the cracked stage, the latter with either unidirectional cracking (primary cracks) or with bidirectional cracking (primary and secondary cracks) or even multi-directional cracking. The secant stiffness matrix is formulated by means of a direct procedure, based on the assumption that the solid concrete and the reinforcement work in parallel, while the solid concrete between the cracks and the cracks themselves work in series. The resistant mechanisms active at the crack interface are introduced by means of their highly nonlinear laws, that are taken from the literature and are based on well-documented tests. The reliability and accuracy of the proposed model are checked against a few well-documented tests on 2D R/C members failing past the formation of secondary cracks.

Three-Dimensional Cyclic Behavior Simulation of RC Columns under Combined Flexural Moment and Torsion Coupled with Axial and Shear Forces

Full three-dimensional nonlinear finite element analysis based on the path-dependent non-orthogonal multi-directional crack model is applied to RC columns subjected to combined cyclic flexure/shear and torsion. This loading causes non-orthogonal crack-to-crack mechanistic interactions accompanying opening/closure and shear slip along crack planes. The analytical results show that the proposed methodology is able to simulate the highly nonlinear behavior of RC columns under cyclic loading, including not only load-carrying capacity but also complex restoring force characteristics and post-peak softening after peak strength. Sensitivity analysis is also carried out to examine the effectiveness of the transverse reinforcement, which simultaneously resists both transverse shear and torque moment. The current design procedure for determining the necessary and sufficient amount of web reinforcement is reviewed and the necessity of performance assessment is raised.

Laser treatment of cemented carbide cutting tool

Cemented carbide tools are widely used in machining industry due to their superior properties. Laser processing of the tool surface provides thermal integration of binding agent with carbide compounds in the surface region. This in turn improves the microstructure in the region irradiated by the laser beam. In the present study, laser heating of cemented carbide tool surface is carried out. Temperature field in the irradiated region is obtained and temperature gradients as well as cooling rates in the surface region are predicted. Microstructural analysis prior and after the laser heated surface carried out. Fracture toughness of the laser treated surface is measured using indentation tests. It is found that Co and TaC melt in the early heating period due to their relatively lower thermal conductivity than WC. High cooling rates and temperature gradients below the surface are responsible for high thermal stresses developed in this region. Multi directional cracks are observed at the surface and formation of tungsten nitride (WN) occurs in the surface region as illustrated by XRD measurements.

Finite element analysis and behaviour of pre-cracked reinforced concrete members in shear

Reinforced concrete members may be subjected to cracking damage due to environmental attack and past loading. Recent experiments conducted by the authors show that the shear behaviour of pre-cracked beams differs greatly from the uncracked ones. In an uncracked beam, the shear failure is governed by a single diagonal crack. On the other hand, the shear failure of pre-cracked beams is governed by a mutual contribution of pre-crack and diagonal crack or by the pre-crack only. The behaviour of all the pre-cracked beams can be consistently explained by the concept of shear anisotropy along the pre-crack plane. The pre-cracked beam problem poses challenges to the numerical analysis of reinforced concrete members. The finite element method (FEM) is discussed in this paper. It is made clear that a crack model that can handle the pre-cracked beam problem must be able to record and transfer pre-crack and other state variables over load steps, treat multi-directional cracks with mutual interaction and treat shear anisotropy along the pre-crack plane. The four-way fixed crack approach is proposed for numerical analysis. The analysis target includes non-orthogonal two-way crack interaction, three-way crack interaction and crack interaction in which two cracks form an arbitrary angle.

Finite element analysis and behaviour of pre-cracked reinforced concrete members in shear

Reinforced concrete members may be subjected to cracking damage due to environmental attack and past loading. Recent experiments conducted by the authors show that the shear behaviour of pre-cracked beams differs greatly from the uncracked ones. In an uncracked beam, the shear failure is governed by a single diagonal crack. On the other hand, the shear failure of pre-cracked beams is governed by a mutual contribution of pre-crack and diagonal crack or by the pre-crack only. The behaviour of all the pre-cracked beams can be consistently explained by the concept of shear anisotropy along the pre-crack plane. The pre-cracked beam problem poses challenges to the numerical analysis of reinforced concrete members. The finite element method (FEM) is discussed in this paper. It is made clear that a crack model that can handle the pre-cracked beam problem must be able to record and transfer pre-crack and other state variables over load steps, treat multi-directional cracks with mutual interaction and treat shear anisotropy along the pre-crack plane. The four-way fixed crack approach is proposed for numerical analysis. The analysis target includes non-orthogonal two-way crack interaction, three-way crack interaction and crack interaction in which two cracks form an arbitrary angle.

MULTI-DIRECTIONAL FIXED CRACK APPROACH FOR HIGHLY ANISOTROPIC SHEAR BEHAVIOR IN PRECRACKED RC MEMBERS

Recent experiments on pre-cracked beam conducted by the authors pose challenge to the numerical analysis in the field of reinforced concrete mechanics. The numerical requirements of pre-cracked beam problem are identified as (1) multi-step loading paths and path-dependency transfer, (2) multidirectional cracks with crack interaction and (3) highly anisotropic shear along pre-crack plane. Four-way fixed crack approach is judged to fulfil the above numerical requirements. The finite element analysis of pre-cracked beam is conducted. It is verified that the four-way fixed crack approach can reliably reproduce the experimental results of pre-cracked beam

Shear Strength of Reinforced Concrete Plates and Shells Determined by Finite Element Analysis Using Layered Elements

Transverse shear failures of reinforced concrete plates and shells are analyzed using a layered shell element that has been modified to more accurately model shear behavior. The transverse shear stress distribution through the shell thickness is obtained by equilibrium relations, Three‐dimensional failure criteria are used to predict transverse shear failure. The element is able to predict shear failures for plates and shells with and without shear reinforcement. A new cracking model based on crack friction, shear dilatancy, and crack surface deformability is used for some of the results. The analytical results are compared with experimental results for beams, plates, and shells subjected to concentrated forces. Good agreement between analysis and experiment is obtained for plates with and without shear reinforcement and for shells without reinforcement. An improved multidirectional cracking model is needed for shells with shear reinforcement.