Abstract

This paper presents a finite element that incorporates weak, strong and both weak plus strong discontinuities with linear interpolations of the unknown jumps for the modeling of internal interfaces. The new enriched space is built by subdividing each triangular or tetrahedral element in several standard linear sub-elements. The new degrees of freedom coming from the assembly of the sub-elements can be eliminated by static condensation at the element level, resulting in two main advantages: first, an elemental enrichment instead of a nodal one, which presents an important reduction of the compunting time when the internal interface is moving all around the domain and second, an efficient implementation involving minor modifications allowing to reuse existing finite element codes. The equations for the internal interface are constructed by imposing the local equilibrium between the stresses in the bulk of the element and the tractions driving the cohesive law, with the proper equilibrium operators to account for the linear kinematics of the discontinuity. To improve the continuity of the unknowns on both sides of the elements on which a static condensation is done, a contour integral has been added. These contour integrals named inter-elemental forces can be interpreted as a “do nothing” boundary condition (Coppola-Owen and Codina, 2011) published in another context, or as the usage of weighting functions that ensure convergence of the approach as proposed by J.C. Simo (Simo and Rifai, 1990). A series of numerical tests for scalar unknowns as a simple representation of more general numerical simulations are presented to illustrate the performance of the enriched elemental space.

Abstract

It is widely acknowledged that ultrasonic technology provides a considerable detection approach while describing the internal fractures in concrete, which is a serious issue when evaluating the durability performance of cement-based materials. This paper aims to introduce the work which is concerned with the use of ultrasonic computerized tomography (UCT) technique to evaluate the micro-cracks inside mortar. The result shows that UCT is a relatively promising method which is capable of imaging the positions of tiny cracks and local damage, moreover, the application of X-ray computerized tomography (XCT) also verifies the accuracy and reliability of UCT. Effective evaluating the internal cracks depends on the detection of attenuation of ultrasonic signals that perpendicular to internal cracks, furthermore, ultrasonic technology is more likely to detect the crack with larger surface of damage, but not sensitive to local fracture.

Abstract

Abstract

Abstract

When designing systems for the transportation and distribution of gas, safety is
one of the main issues to be considered. In order to minimize potential hazards, this subject
is treated in international regulations. Manufacturing characteristics of gas pipes and their
typical defects are described in the first part of this work. Safety concerns related to the
presence of real and potential crack type defects in gas pipes are presented. An actual gas
pipeline, which has been feeding natural gas to the city of Córdoba (Argentina) for more than
forty years, is considered as a study case. In order to determine the properties of this gas
pipeline, mechanical tests from material samples were carried out. The stress state associated
with “plane” type defects is determined using a model that is based on the material toughness of
the pipe, which is appropriate for studying the behavior of cracks. This model allows computing,
depending on the dimensions of the pipe, the transition pressure which separates possible
defects into two categories: defects which cause failure by breakage of the pipe and defects
which only cause gas leaks. The pipe failure pressure for passing and non-passing defects can
also be determined using this model. The system safety conditions are defined considering the
size of the defects and the work pressure. The range of lengths of possible defects, the size of
the defects that are critical, and the size of the defects which are tolerable for a fixed safety
coefficient, are determined. Finally, the problem of determining in a quick way, the reduced
pressure acceptable for the safe operation until repairs, of a system under the presence of a
crack of “non-tolerable size”, is addressed.
 

Abstract