2014 Vol.4(4)

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Highlight Review
Mesoscopic characterization and modeling of microcracking in cementitious materials by the extended nite element method
Junjie Huang, Mingxiang Chen, Jian Sun
Theoretical and Applied Mechanics Letters  2014, 4(4): 041001. doi: 10.1063/2.1404101
[Abstract](502) [PDF 5397KB](7)
This study develops a mesoscopic framework and methodology for the modeling of microcracks in concrete. A new algorithm is first proposed for the generation of random concrete meso-structure including microcracks and then coupled with the extended finite element method to simulate the heterogeneities and discontinuities present in the meso-structure of concrete. The proposed procedure is verified and exemplified by a series of numerical simulations. The simulation results show that microcracks can exert considerable impact on the fracture performance of concrete. More broadly, this work provides valuable insight into the initiation and propagation mechanism of microcracks in concrete and helps to foster a better understanding of the micro-mechanical behavior of cementitious materials.
Solid Mechanics
Editorial: Recent advances in fracture research
Xiqiao Feng, Shouwen Yu
Theoretical and Applied Mechanics Letters  2014, 4(4): 041002. doi: 10.1063/2.1404102
[Abstract](435) [PDF 22KB](3)
Asymptotic solutions for buckling delamination induced crack propagation in the thin film-compliant substrate system
Tongqing Lu, Tiejun Wang
Theoretical and Applied Mechanics Letters  2014, 4(4): 041003. doi: 10.1063/2.1404103
[Abstract](482) [PDF 717KB](8)
In a thin film-substrate system in-plane compressive stress is commonly generated in the film due to thermal mismatch in operation or fabrication process. If the stress exceeds a critical value, part of the film may buckle out of plane along the defective interface. After buckling delamination, the interface crack at the ends may propagate. In the whole process, the compliance of the substrate compared with the film plays an important role. In this work, we study a circular film subject to compressive stress on an infinitely thick substrate. We study the effects of compliance of the substrate by modeling the system as a plate on an elastic foundation. The critical buckling condition is formulated. The asymptotic solutions of post-buckling deformation and the corresponding energy release rate of the interface crack are obtained with perturbation methods. The results show that the more compliant the substrate is, the easier for the film to buckle and easier for the interface crack to propagate after buckling.
Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature
Mattias Calmunger, Guocai Chai, Sten Jo-hansson, Johan Moverare
Theoretical and Applied Mechanics Letters  2014, 4(4): 041004. doi: 10.1063/2.1404104
[Abstract](491) [PDF 3248KB](2)
In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel (AISI 316L) and one nickel-base alloy (Alloy 617) have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromechanisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650℃.
Dissimilar ultrasonic spot welding of Mg-Al and Mg-high strength low alloy steel
V. K. Patel, D. L. Chen, S. D. Bhole
Theoretical and Applied Mechanics Letters  2014, 4(4): 041005. doi: 10.1063/2.1404105
[Abstract](519) [PDF 5069KB](4)
Sound dissimilar lap joints were achieved via ultrasonic spot welding (USW), which is a solid-state joining technique. The addition of Sn interlayer during USW effectively blocked the formation of brittle Al12Mg17 intermetallic compound in the Mg-Al dissimilar joints without interlayer, and led to the presence of a distinctive composite-like Sn and Mg2Sn eutectic structure in both Mg-Al and Mg-high strength low alloy (HSLA) steel joints. The lap shear strength of both types of dissimilar joints with a Sn interlayer was significantly higher than that of the corresponding dissimilar joints without interlayer. Failure during the tensile lap shear tests occurred mainly in the mode of cohesive failure in the Mg-Al dissimilar joints and in the mode of partial cohesive failure and partial nugget pull-out in the Mg-HSLA steel dissimilar joints.
Fatigue behaviour of coke drum materials under thermal-mechanical cyclic loading
Jie Chen, Zihui Xia
Theoretical and Applied Mechanics Letters  2014, 4(4): 041006. doi: 10.1063/2.1404106
[Abstract](490) [PDF 8064KB](2)
Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes damage in coke drums in the form of bulging and cracking. There were some studies on the fatigue life estimation for the coke drums, but most of them were based on strain-fatigue life curves at constant temperatures, which do not consider simultaneous cyclic temperature and mechanical loading conditions. In this study, a fatigue testing system is successfully developed to allow performing thermal-mechanical fatigue (TMF) test similar to the coke drum loading condition. Two commonly used base and one clad materials of coke drums are then experimentally investigated. In addition, a comparative study between isothermal and TMF lives of these materials is conducted. The experimental findings lead to better understanding of the damage mechanisms occurring in coke drums and more accurate prediction of fatigue life of coke drum materials.
High resolution ultrastructure imaging of fractures in human dental tissues
Tan Sui, Siqi Ying, Gabriel Landini, Alexander M. Korsunsky
Theoretical and Applied Mechanics Letters  2014, 4(4): 041007. doi: 10.1063/2.1404107
[Abstract](442) [PDF 11137KB](3)
Human dental hard tissues are dentine, cementum, and enamel. These are hydrated mineralised composite tissues with a hierarchical structure and versatile thermo-mechanical properties. The hierarchical structure of dentine and enamel was imaged by transmission electron microscopy (TEM) of samples prepared by focused ion beam (FIB) milling. High resolution TEM was carried out in the vicinity of a crack tip in dentine. An intricate "random weave" pattern of hydroxyapatile crystallites was observed and this provided a possible explanation for toughening of the mineralized dentine tissue at the nano-scale. The results reported here provide the basis for improved understanding of the relationship between the multi-scale nature and the mechanical properties of hierarchically structured biomaterials, and will also be useful for the development of better prosthetic and dental restorative materials.
Thermal shock resistance behavior of a functionally graded ceramic: Effects of nite cooling rate
Zhihe Jin, Yuezhong Feng
Theoretical and Applied Mechanics Letters  2014, 4(4): 041008. doi: 10.1063/2.1404108
[Abstract](490) [PDF 59KB](2)
This work presents a semi-analytical model to explore the effects of cooling rate on the thermal shock resistance behavior of a functionally graded ceramic (FGC) plate with a periodic array of edge cracks. The FGC is assumed to be a thermally heterogeneous material with constant elastic modulus and Poisson's ratio. The cooling rate applied at the FGC surface is modeled using a linear ramp function. An integral equation method and a closed form asymptotic temperature solution are employed to compute the thermal stress intensity factor (TSIF). The thermal shock residual strength and critical thermal shock of the FGC plate are obtained using the SIF criterion. Thermal shock simulations for an Al2O3/Si3N4 FGC indicate that a finite cooling rate leads to a significantly higher critical thermal shock than that under the sudden cooling condition. The residual strength, however, is relatively insensitive to the cooling rate.
Wavelet method applied to specific adhesion of elastic solids via molecular bonds
Lei Zhang, Jizeng Wang, Youhe Zhou
Theoretical and Applied Mechanics Letters  2014, 4(4): 041009. doi: 10.1063/2.1404109
[Abstract](465) [PDF 132KB](4)
We propose a wavelet method to analyze the stochastic-elastic problem of specific adhesion between two elastic solids via ligand-receptor bond clusters, which is governed by a nonlinear integro-differential equation with a singular Cauchy kernel to describe the mean-field coupling between deformation of elastic materials and stochastic behavior of the molecular bonds. To solve this problem, Galerkin method based on a wavelet approximation scheme is adopted, and special treatment which transforms the singular Cauchy kernel into a smooth one has been proposed to avoid the cumbersome calculation of singular integrals. Numerical results demonstrate that the method is fully capable of solving the specific adhesion problems with complex nonlinear and singular equations. Based on the proposed method, investigations are performed to reveal the relation between steady-state pulling force and mean surface separation under different stress concentration indexes, which is crucial for assembling the overall constitutive relations for multicellular tumor spheroids and polymer-matrix microcomposites.
Energy barrier for configurational transformation of graphene nanoribbon on nanotube
Qifang Yin, Xinghua Shi
Theoretical and Applied Mechanics Letters  2014, 4(4): 041010. doi: 10.1063/2.1404110
[Abstract](483) [PDF 5912KB](1)
A graphene nanoribbon (GNR) has two basic configurations when winding on the outer surface of a carbon nanotube (CNT): helix and scroll. Here the transformation between the two configurations is studied utilizing molecular dynamics simulations. The energy barrier during the transformation as well as its relationship with the interfacial energy and the radius of CNT are investigated. Our work offers further insights into the formation of desirable helix/scroll of GNR winding on nanotubes or nanowires, and thus can enable novel design of potential graphene-based electronics.
Finding the buckling load of non-uniform columns using the iteration perturbation method
Aref Afsharfard, Anooshiravan Farshidianfar
Theoretical and Applied Mechanics Letters  2014, 4(4): 041011. doi: 10.1063/2.1404111
[Abstract](449) [PDF 208KB](4)
The aim of this study is to calculate the critical load of variable inertia columns. The example studied in this paper can be used as a paradigm for other non-uniform columns. The wavelength of equivalent vibratory system is used to calculate the critical load of the trigonometrically varied inertia column. In doing so, the equilibrium equation of the column is theoretically studied using the perturbation method. Accuracy of the calculated results is evaluated by comparing the solution with numerical results. Effect of improving the initial guess on the solution accuracy is investigated. Effects of varying parameters of the trigonometrically varied inertia and the uniformly tapered columns on their stability behavior are studied. Finally, using the so-called "perfectibility" parameter, two design goals, i.e., being lightweight and being strong, are studied for the discussed columns.
Fluid Mechanics
Numerical simulation of gap effect in supersonic flows
Song Mo, Haiming Huang, Guo Huang, Xiaoliang Xu, Zimao Zhang
Theoretical and Applied Mechanics Letters  2014, 4(4): 042001. doi: 10.1063/2.1404201
[Abstract](468) [PDF 5343KB](9)
The gap effect is a key factor in the design of the heat sealing in supersonic vehicles subjected to an aerodynamic heat load. Built on S-A turbulence model and Roe discrete format, the aerodynamic environment around a gap on the surface of a supersonic aircraft was simulated by the finite volume method. As the presented results indicate, the gap effect depends not only on the attack angle, but also on the Mach number.
Dynamics and Control
On the decomposition of forces
Dong Eui Chang
Theoretical and Applied Mechanics Letters  2014, 4(4): 043001. doi: 10.1063/2.1404301
[Abstract](445) [PDF 57KB](3)
We show that any continuously differentiable force is decomposed into the sum of a Rayleigh force and a gyroscopic force. We also extend this result to piecewise continuously differentiable forces. Our result improves the result on the decomposition of forces in a book by David Merkin and further extends it to piecewise continuously differentiable forces.