Al Ghorbanpoor

BOUNDARY ELEMENT ANALYSIS OF CRACK GROWTH FOR MIXED-MODE CENTER SLANT CRACK PROBLEMS

Abstract A boundary element analysis of crack growth for a linear mixed-mode center slant crack problem is presented in this article. The analysis was performed using multidomain boundary element scheme and quadratic quarter point boundary elements on each side of the crack. The crack growth direction is predicted by the maximum principal stress theory, however the Crack Surface Relative Displacement (CSRD) method is used to eliminate the calculation of the stress intensity factors which are normally required. The BEM results showed good agreement with the analytical and a very fine mesh FEM solutions.

Rapid-Construction Technique for Bridge Abutments Using Controlled Low-Strength Materials

The time required for building bridge abutments is one of the main obstacles facing rapid bridge construction. For typical span bridges, this can be remedied by using controlled low-strength materials (CLSM) as backfill materials placed behind full-height, precast concrete panels that are integrated with the CLSM backfill via steel anchors. The CLSM bridge abutments can be constructed in a short time as they require neither heavy machinery for excavation and compaction nor piling equipment. In addition to the speedy construction, the ability to use by-product material, such as fly ash and foundry sand, in CLSM backfill translates into greater economy and the potential for a sustainable design. This paper describes the behavior of an instrumented laboratory, large-scale CLSM bridge abutment with full-height, precast concrete panels that was subjected to a monotonically increasing sill pressure. The experiment showed that the CLSM bridge abutment is capable of carrying typical bridge loads with a large safety margin and with negligible deformations.

Magnetic-based NDE of steel in prestressed and post-tensioned concrete bridges

This paper addresses a study, funded by the Federal Highway Administration (FHWA), the U.S. Department of Transportation (DOT), that is currently underway at the University of Wisconsin-Milwaukee. The objective of the study is to develop an automated non-destructive testing system based on the magnetic flux leakage principle that would allow assessment of the condition of reinforcing and prestressing steels in concrete bridge components. Corrosion or cracking of steel within concrete members will be detected and evaluated. The system will be used as a self clamping and moving sensing device that can be installed on a concrete girder. Data from the sensing device is transmitted via a wireless communication system to data recording/analysis equipment on the ground. The sensing device may also be operated manually to allow inspection of local areas such as the end bearing or cable anchorage locations in cable bridges. Through performing a correlation analysis of recorded data, an assessment of the condition of the member under test is made. Reference data base for the correlation analysis is established through laboratory and field testing with known conditions.

Field Test of Magnetic Methods for Corrosion Detection in Prestressing Strands in Adjacent Box-Beam Bridges

AbstractMagnetic methods are progressing in the detection of corrosion in prestressing strands in adjacent precast, prestressed concrete box-beam bridges. This study is the first field trial of magnetic strand defect detection systems on an adjacent box-beam bridge. A bridge in Fayette County, Ohio, that was scheduled for demolition was inspected. The prestressed box beams had significant strand corrosion. The corroded strands showed discontinuities and a reduced cross-sectional area. These changes are reflected in the magnetic signatures of the prestressing steel. Corrosion in the prestressing steel was detected using two magnetic methods; namely, the magnetic flux leakage method and the induced magnetic field method. The purpose of these tests was to demonstrate the ability of the magnetic methods to detect hidden corrosion in box beams in the field and tackle the logistic problem of inspecting box beams from the bottom. The inspections were validated by dissecting the bottom of the box beams after the ...

Practical assessment of magnetic methods for corrosion detection in an adjacent precast, prestressed concrete box-beam bridge

Magnetic methods are progressing in the detection of corrosion in prestressing strands in adjacent precast, prestressed concrete box-beam bridges. This study is the first field trial of magnetic strand defect detection systems on an adjacent box-beam bridge. A bridge in Fayette County, Ohio, which was scheduled for demolition, was inspected. Damage to prestressed box-beams is often due to corrosion of the prestressing strands. The corroded strands show discontinuities and a reduced cross-sectional area. These changes, due to corrosion, are reflected in the magnetic signatures of the prestressing steel. Corrosion in the prestressing steel was detected using two magnetic methods, namely the ‘magnetic flux leakage’ (MFL) and the ‘induced magnetic field’. The purpose of these tests was to demonstrate the ability of the magnetic methods to detect hidden corrosion in box-beams in the field and tackle the logistic problem of inspecting box-beams from the bottom. The inspections were validated by dissecting the bottom of the box-beams after the inspections. The results showed that the MFL method can detect hidden corrosion and strand breaks. Both magnetic field methods were also able to estimate corrosion by detecting the effective cross-sectional area of the strand in sections of the beams. Thus, it was shown that the magnetic methods can be used to predict hidden corrosion in prestressing strands of box-beams.

Assessment of Corrosion of Steel in Concrete Structures by Magnetic Based NDE Techniques

Corrosion damage of steel in concrete structures is a major concern. NDE techniques based on variations in induced magnetic fields due to loss of steel have been shown to be an effective tool. This paper includes results from laboratory investigation, field tests, and numerical analysis based on this concept. Test specimens included reinforcing bars and prestressing cables with simulated flaws as well as flaws from real corrosion. Variations in the magnetic field were recorded as electrical signals that were characterized to aid in the detection of corrosion and evaluation of the condition of steel in concrete. It was found that the amplitude of the signals could be related to the extent of the corrosion. Loss of cross sectional area in bars and cables of approximately 3 percent could be detected by the technique. The results of a finite element analysis yielded a good agreement with those from the experiment. The technique offers significant capabilities for field assessment of the condition of steel in concrete structures.

Laboratory Assessment of Select Methods of Corrosion Control and Repair in Reinforced Concrete Bridges

Fourteen reinforced concrete laboratory test specimens were used to evaluate a number of corrosion control (CoC) procedures to prolong the life of patch repairs in corrosion-damaged reinforced concrete. These specimens included layered mixed-in chlorides to represent chloride contamination due to deicing salts. All specimens were exposed to accelerated corrosion testing for three months, subjected to patch repairs with various treatments, and further subjected to additional three months of exposure to accelerated corrosion. The use of thermal sprayed zinc, galvanic embedded anodes, epoxy/polyurethane coating, acrylic coating, and an epoxy patch repair material was evaluated individually or in combination. The specimens were assessed with respect to corrosion currents (estimated mass loss), chloride ingress, surface rust staining, and corrosion of the reinforcing steel observed after dissection. Results indicated that when used in patch repair applications, the embedded galvanic anode with top surface coating, galvanic thermal sprayed zinc, and galvanic thermal sprayed zinc with surface coating were more effective in controlling corrosion than the other treatments tested.

Crack tip displacement factor-D: an application to two dimensional linear elastic fracture mechanics

A fracture parameter, crack tip displacement factor D is introduced to characterize the two-dimensional linear elastic fracture mechanics problems. Similar to the conventional COD approach, the D factor can be determined directly from the crack surface displacement. The major advantage of using this fracture control parameter is that one can obtain it by evaluating the crack surface displacement from a numerical analysis or an experiment without the need for considering the stress field at the crack tip. Unlike the COD method, the introduced fracture parameter may be used for not only the simple opening mode but also for the sliding and tearing modes as well as for the mixed mode fracture problems. This article presents a detailed derivation of the D factor as well as some sample applications.

Nondestructive Testing of Steel Corrosion in Prestressed Concrete Structures using the Magnetic Flux Leakage System

The Magnetic Flux Leakage (MFL) method can be nondestructively used to disclose the location and extent of corrosion or fracture in prestressed strands in concrete structures. In this study, parameters with the greatest effect on the performance of the MFL system are investigated using numerical simulations. The MFL system under study is based on two permanent magnets to magnetize embedded strands and Hall-effect sensors to detect normal magnetic flux leakage. The system is assessed using magnetostatic and transient numerical analysis to effectively simulate the MFL system. Results have been verified by laboratory and field experiments. Both normal and axial Hall-effect sensors are modeled in simulations to better identify magnetic signals at the corrosion zone. The sensor lift-off and the magnetic field masking by lateral reinforcements on nearby pitting corrosion are addressed as two main drawbacks of using MFL systems to detect corrosion in prestressed concrete structures. To provide more details about the flux leakage interference between the pitting corrosion and lateral reinforcements in prestressed concrete structures, linear and oriented magnets/sensor arrays are proposed and analyzed numerically.

The role of oil seepage in fatigue crack growth of lubricated wearing systems

Abstract Lubricated rolling/sliding systems routinely undergo significant repair and replacement with substantial associated cost, due to cracking and pitting damages. While it is understood that these damages could be related to the effect of lubricating oil seepage inside cracks, to date no proper analytical solution for this special liquid–solid interaction problem has been presented. This paper discusses the role of oil seepage in fatigue crack growth of a lubricated sliding wear system. The problem of contact with asperities and pre-existing surface crack perpendicular to the contact surface in sliding wear, is analyzed by finite element method. In calculations, the incompressibility of the working fluid was assumed to determine the hydrostatic pressure between the crack faces during contact wearing. A parametric study is performed to investigate the effect of the length of each contacting asperity relative to the crack length, coefficient of friction and number of asperities between the contact surfaces, on the stress intensity factor KI and the oil pressure inside the crack. It is found that the lubricating oil seepage can significantly excite and accelerate the fatigue crack growth in wearing systems. From the results of this study, the effect of the lubricating oil seepage on pitting phenomenon can be clearly explained.