Cracks, microcracks and fracture in polymer structures: Formation, detection, autonomic repair

Polymer-based materials are extensively used in various applications such as aircrafts, civilian structures, oil and gas platforms and electronics. They are, however, inherently damage prone and over time, the formation of cracks and microscopic damages influences the thermo-mechanical and electrical properties, which eventually results in the total failure of the materials. This paper provides an overview of the principal causes of cracking in polymer and composites and summarizes the recent progress in the development of non-destructive techniques in crack detection. Furthermore, recent progress in the development of bio-inspired self-healing methods in autonomic repair is discussed.

Crack Damage in Polymers and Composites: A Review

Assessment of different solution methods for receding contact problems in functionally graded layered mediums

The aim of this paper was to examine the continuous and discontinuous contact problems between the functionally graded (FG) layer pressed with a uniformly distributed load and homogeneous half plane using an analytical method and FEM. The FG layer is made of non-homogeneous material with an isotropic stress–strain law with exponentially varying properties. It is assumed that the contact at the FG layer-half plane interface is frictionless, and only the normal tractions can be transmitted along the contacted regions. The body force of the FG layer is considered in the study. The FG layer was positioned on the homogeneous half plane without any bonds. Thus, if the external load was smaller than a certain critical value, the contact between the FG layer and half plane would be continuous. However, when the external load exceeded the critical value, there was a separation between the FG layer and half plane on the finite region, as discontinuous contact. Therefore, there have been some steps taken in this study. Firstly, an analytical solution for continuous and discontinuous contact cases of the problem has been realized using the theory of elasticity and Fourier integral transform techniques. Then, the problem modeled and twodimensional analysis was carried out by using ANSYS package program based on FEM. Numerical results for initial separation distance and contact stress distributions between the FG layer and homogeneous half plane for continuous contact case; the start and end points of separation and contact stress distributions between the FG layer and homogeneous half plane for discontinuous contact case were provided for various dimensionless quantities including material inhomogeneity, distributed load width, the shear module ratio and load factor for both methods. The results obtained using FEM were compared with the results found using analytical formulation. It was found that the results obtained from analytical formulation were in perfect agreement with the FEM study.

Investigation of continuous and discontinuous contact cases in the contact mechanics of graded materials using analytical method and FEM

This article reviews fretting wear damage in industries and in the contact mechanics of coated systems. Micro-slip motion resulting in fretting damage is discussed along with major experimental factors. The experimental factors, including normal force, relative displacement, frequency and medium influence are directly compared. Industrial solutions to reduce fretting damages are then discussed. The contact mechanics of a coated system are reviewed to quantify stress states in a coating layer and the substrate. Finally, a literature review on simulation for fretting is carried out. This review study provides useful methods and practical solutions to minimize fretting wear damage.

Literature Review on Fretting Wear and Contact Mechanics of Tribological Coatings

A new computational method based on the J k -integral is put forward for the purpose of conducting fracture analysis of orthotropic materials subjected to hygrothermal stresses. By utilizing the constitutive relations of plane orthotropic hygrothermoelasticity, an alternative expression for the J k -integral is derived to replace the general limit definition. A numerical procedure is developed and integrated into a finite element analysis software to implement the proposed form of the J k -integral. Temperature and specific moisture concentration fields, which are required in fracture calculations, are also computed through finite element analysis. Numerical results are generated by considering an embedded crack in a polymer matrix fibrous composite laminate, that is subjected to steady-state hygrothermal loading. Comparisons of the mixed-mode stress intensity factors computed by the J k -integral based method to those evaluated via the displacement correlation technique demonstrate that, the proposed for...

Hygrothermal Fracture Analysis of Orthotropic Materials Using J k -Integral

A singular integral equation (SIE) approach and a finite element method are developed for the solution of the frictional sliding contact problem between a finite-thickness laterally graded solid and a rigid stamp of an arbitrary tip-shape considering the plane strain assumption. An exponential shear modulus variation is introduced through the lateral direction. The field variables are obtained applying the Fourier transformation techniques on the governing partial differential equations. A surface displacement gradient is then utilized to derive a SIE of the second kind. A numerical solution of the SIE is performed by using a collation method and the Gauss quadrature integration techniques for the flat, triangular and circular stamp profiles. Finite element analyses (FEA) of the same contact problems are also performed upon selection of the augmented Lagrange contact-solver in ANSYS. For the incomplete (triangular and circular) stamp problems, an iterative algorithm is developed in order to obtain practically computational solutions for any desired contact lengths. Successful convergence of the SIE results and excellent consistency between the SIE and FEA results are attained, that indicate the reliability of both methods. The change in the thickness is shown to alter the contact behavior of the laterally graded solid significantly.

Solution of the plane contact problem between a finite-thickness laterally graded solid and a rigid stamp of an arbitrary tip-profile

Fracture parameters of an inclined surface crack in a graded half-plane subjected to sli-
ding frictional contact are investigated in this study. The problem is modelled via the finite
element method (FEM) under the plane strain assumption employing a newly developed di-
splacement boundary condition. The shear modulus of the half-plane is graded exponentially
through the thickness direction by means of the homogeneous finite element approach. The
augmented Lagrange algorithm is selected as an iterative contact solver. The mixed mode
stress intensity factors (SIFs) which are induced by the contact stresses are evaluated utili-
zing the Displacement Correlation Technique (DCT). The accuracy of the present procedure
is ensured comparing the SIF results to those calculated by an analytical method for verti-
cal surface cracks. Additional SIF curves are generated as functions of the crack inclination
in order to reveal the effects of non-homogeneity, punch location, crack length and friction
coefficient. The prominent conclusion of this study indicates that the crack orientation has
profound effects on the behavior of SIF curves, which has not been presented in any study
published so far. Hence, consideration of crack inclination is proved essential to successfully
predict fracture behavior of a graded medium under frictional contact.

/pdf/inclined-surface-cracks-in-a-graded-half-plane-subjected-to-3t824clfvc.pdf

Inclined surface cracks in a graded half-plane subjected to frictional sliding contact

A new finite element procedure is developed for the analysis of sliding contact problems involving spatially varying coefficient of friction. The problem is implemented using APDL (ANSYS Parametric Design Language) considering the Augmented Lagrange method as the contact solver. Upon discretization of the contact interface into multiple contact pairs, a sequence of steps is followed to evaluate the resultant friction force required for the sliding contact. As a case study, heterogeneous-friction contact problem between an orthotropic laterally graded half-plane and a rigid flat stamp is investigated under plane strain assumption. The proposed iterative procedure is proved reliable by comparing the results to those generated by a SIE (Singular Integral Equation) approach for isotropic laterally graded half-planes. Extra results are presented to reveal the effects of problem parameters on the contact stresses and the friction force. The paper outlines a convenient numerical solution for an advance sliding contact problem, and the results can be used in validation purposes of experimental and analytical studies.

/pdf/a-finite-element-procedure-for-sliding-contact-problems-2cspixk04n.pdf

A Finite Element Procedure for Sliding Contact Problems Involving Heterogeneous Coefficient of Friction

Onur Arslan

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Hygrothermal Fracture Analysis of Orthotropic Materials Using J k -Integral

Solution of the plane contact problem between a finite-thickness laterally graded solid and a rigid stamp of an arbitrary tip-profile

Inclined surface cracks in a graded half-plane subjected to frictional sliding contact

A Finite Element Procedure for Sliding Contact Problems Involving Heterogeneous Coefficient of Friction

Onur Arslan