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Chen J.,MINES ParisTech Center of materials | Madi Y.,MINES ParisTech Center of materials | Madi Y.,EPF College of Engineering | Morgeneyer T.F.,MINES ParisTech Center of materials | Besson J.,MINES ParisTech Center of materials
Computational Materials Science | Year: 2011

Plasticity and fracture mechanisms of a 2198 Al-Cu-Li thin sheet alloy having a thickness equal to 6 mm are investigated. Two heat treatments are studied: T351 and T851. Mechanical tests are carried out on flat specimens including smooth tensile samples and U-notched specimens. Test data are used to identify the parameters of constitutive equations describing plastic anisotropy. The microscopic fracture surfaces of the different specimens are observed using scanning electron microscopy. Smooth and notched samples exhibit a slant fracture surface. Two microscopic fracture mechanisms are identified: fibrous fracture involving grain boundary decohesion and dimple fracture. Observed fracture modes depend on specimen geometry (notches increase stress triaxiality and favor dimple fracture) but also on loading direction. Loading along the rolling direction leads to predominant fibrous fracture. Reducing sheet thickness to 2 mm also favors fibrous fracture. Finally a localization indicator based on Rice's analysis of bifurcation is used to analyse finite element simulations and predict observed fracture plane orientations. © 2010 Elsevier B.V. All rights reserved.


Briere V.,EPF College of Engineering | Briere V.,University of Pittsburgh | Harries K.A.,University of Pittsburgh | Kasan J.,HDR | Hager C.,University of Pittsburgh
Construction and Building Materials | Year: 2013

In prestressed concrete elements, bond between pretensioned strands and the surrounding concrete is attributed to adhesion, mechanical interlock and the friction and 'wedge-action' attributable to radial expansion of the strand following release. The latter effect is referred to as the Hoyer effect and is the subject of this paper. The Hoyer effect contributes primarily to the transfer of the initial prestressing force to the concrete thereby affecting the transfer length. The transfer of large prestressing forces, particularly at an early concrete age, can lead to local cracking associated with bursting stresses or splitting associated with transfer of the strand force through bond. An analysis of embedded strand behavior based on fundamental principles of mechanics clearly demonstrates the importance of strand dilation characteristics in the development of concrete stresses and therefore the transfer of prestressing forces. An experimental program demonstrates that the dilation of seven wire strand is affected by not only the Poisson's ratio for steel but also the 'tightening' of the helical wires and effects of bearing between these wires. The resulting dilation of a seven wire strand is larger than Poisson's ratio for steel and is found to be a function of strand diameter. A discussion of design code provisions for prestressing strand transfer length is included. North American practice, while conservative, does not explicitly address the mechanics of stress transfer. Improvement in this regard could result in more efficient prestressed concrete structures. European practice, on the other hand, appears to reasonably capture the Hoyer effect. © 2012 Elsevier Ltd. All rights reserved.


Cheng C.,Hefei University of Technology | Cheng X.,Anhui Economic and Management Institute | Niu Z.,Hefei University of Technology | Recho N.,EPF College of Engineering | Recho N.,CNRS Pascal Institute
European Journal of Mechanics, A/Solids | Year: 2016

Due to the material or geometrical discontinuities, the stress, electric displacement and magnetic induction may become theoretically infinite and singular at the vertexes of magneto-electro-elastic (MEE) V-notches, where the mechanical failure or dielectric breakdown may initiate from. Based on the assumption of the asymptotic expansions of the physical fields near the vertex, the characteristic differential equations with respect to the singularity order are derived from the equilibrium equations and Maxwell equations. After a set of variable replacement, these non-linear differential equations are transformed into the linear ones. The traditional iterative method for solving the transcendental equation is avoided. The mechanical, electric and magnetic boundary conditions together with interfacial continuity conditions are also expressed by the combination of the singularity order and characteristic angular functions. The singularity characteristic analyses for MEE V-notches are transformed into a problem of solving characteristic ordinary differential equations with variable coefficients. The singularity orders and characteristic angular functions can be derived by introducing the interpolating matrix method to solve the established characteristic equations. Herein, the singularities for the in-plane and anti-plane MEE V-notches are respectively investigated. The influence of the poling direction on the singularities of MEE V-notches is discussed. The role of the volume fraction of the BaTiO3 inclusions on the singularities of MEE V-notches is studied. The obtained results can be used to design MEE products for reducing the singularity induced by the V-notch. © 2015 Elsevier Masson SAS. All rights reserved.


Cheng C.,Hefei University of Technology | Cheng C.,EPF College of Engineering | Niu Z.,Hefei University of Technology | Recho N.,EPF College of Engineering | And 2 more authors.
Fatigue and Fracture of Engineering Materials and Structures | Year: 2013

A coupled model resulting from the boundary element method and eigen-analysis is proposed in this paper to analyse the stress field at crack tip. This new combine method can yield several terms of the non-singular stress in the Williams asymptotic expansion. Then the maximum circumferential stress (MCS) criterion taken the non-singular stress into account is introduced to predict the brittle fracture of cracked structures. Two earlier experiments are re-examined by the present numerical method and the role of the non-singular stress in the brittle fracture is investigated. Results show that if more terms of non-singular stress are taken into account, the predicted crack propagation direction and the critical loading by MCS criterion are much closer to the existing experimental results, especially for dominating mode II loading conditions. Moreover, numerical results manifest that Williams series expansion can describe the stress field further from the crack tip if more non-singular stress terms are adopted. © 2012 Wiley Publishing Ltd.


Cheng C.Z.,Hefei University of Technology | Cheng C.Z.,EPF College of Engineering | Niu Z.R.,Hefei University of Technology | Recho N.,EPF College of Engineering | And 3 more authors.
Computers and Structures | Year: 2011

The stress computational accuracy of internal points by conventional boundary element method becomes more and more deteriorate as the points approach to the boundary due to the nearly singular integrals including nearly strong singularity and hyper-singularity. For calculating the boundary stress, a natural boundary integral equation in which the boundary variables are the displacements, tractions and natural boundary variables was established in the authors' previous work. Herein, a natural stress boundary integral equation (NSBIE) is further proposed by introducing the natural variables to analyze the stress field of interior points. There are only nearly strong singular integrals in the NSBIE, i.e., the singularity is reduced by one order. The regularization algorithm proposed by the authors is taken over to deal with these singular integrals. Consequently, the NSBIE can analyze the stress field closer to the boundary. Numerical examples demonstrated that two orders of magnitude improvement in reducing the approaching degree can be achieved by NSBIE compared to the conventional one when the near boundary stress field is evaluated. Furthermore, this new way is extended to the multi-domain elasticity problem to calculate the stress field near the boundary and interface. © 2011 Elsevier Ltd. All rights reserved.

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