Rasko Ojdrovic

WEIGHT FUNCTIONS FROM MULTIPLE REFERENCE STATES AND CRACK PROFILE DERIVATIVES

Abstract Two modifications for implementation of the weight function method are presented. It is proposed to assume the derivative of crack opening displacements instead of the crack profile in the form of a series, as is commonly done, and to compute the unknown coefficients in the series from one or more known stress intensity factors. The new approach eliminates differentiations and reduces the number of integrations as compared with the commonly used method. The new method is tested by exploring the influence of assumed reference load cases and combinations of reference cases on the accuracy of computed stress intensity factors for a variety of applied loads on an edge crack. Use of two known stress intensity factors is shown to give excellent accuracy of computed weight functions and stress intensity factors, well within 1% for most load cases considered.

Influence of aggregate size on fracture toughness of concrete

Abstract Employing an extension of the splitting tensile by using a notched cylinder specimen, we have studied effects of initial notch length and maximum aggregate size on fracture toughness of concrete. Experimental results show that maximum aggregate size does influence ductility, with increasingly ductile behavior associated with increasing aggregate size. The results also support previous work in that initial notch length and maximum load do not yield a constant value for fracture toughness, whereas maximum linear load and initial notch length minimize the effects of slow crack growth and do produce a more constant value.

Process zone analysis for the single-edge notched specimen: Part I. Process zone size and crack profile

A systematic analysis of the process zone is presented. Relations among the process zone size, load and crack opening displacement (COD) are derived using the weight function method and a power function closing pressure distribution in the process zone. These relations are specialized and used to study the behaviour of the single-edge notched (SEN) specimen loaded in tension and in bending. This study shows that the method presented here may be successfully used to analyze the fracture of finite-size specimens.

Effect of age on splitting tensile strength and fracture resistance of concrete

Abstract Concrete cylinders aged 1,2,3,5,7,14 and 28 days were loaded to failure in indirect tension. Similar cylinders, but with a central notch cast in the diametral plane of loading, were also loaded to fracture. The splitting tensile strength of the notched cylinders was determined, and its behavior with age was compared with that of unnotched cylinders. Apparent fracture toughness values were calculated as a function of age. It appears that strength and fracture resistance follow similar aging patterns for the concrete tested.

Process zone analysis for the single-edge notched specimen: Part II. Process zone growth and crack propagation

Process zone growth and crack propagation in the single-edge notched (SEN) specimen are studied using the relations among applied load, notional crack and process zone lengths, and crack opening displacement derived in the first part of this work [1]. Process zone growth is simulated by increasing the notional crack length while keeping the traction-free crack length constant. A model for crack propagation based on either critical crack tip opening displacement (CTOD) or critical process zone length, as criteria for traction-free crack extension is proposed. The influence of closing pressure distribution, initial traction-free crack length, and crack extension criterion on the behavior of load vs. CMOD curves is discussed. The present model can be used to model load-deformation behavior from initial loading through softening to failure of nonlinear materials, as is verified by comparing the theoretical and experimentally determined load vs. crack mouth opening (CMOD) curves for concrete beams.

Composite versus Stand-Alone Design Methodologies for Carbon Fiber Lining Systems

Fiber Reinforced Polymer (FRP) composite lining systems are used by major municipalities throughout the United States to structurally rehabilitate and upgrade large diameter pipelines. For internal Carbon Fiber Reinforced Polymer (CFRP) lining systems addressing prestressed concrete cylinder pipes (PCCP), there are two design approaches utilized relative to interaction with the host pipe structure. These approaches are referred to as stand-alone and composite. For a stand-alone design, the carbon fiber takes 100% of the loads acting on the pipeline system with no reliance on the host pipe for structural integrity. Composite designs rely on the carbon fiber lining system and inner concrete core of the PCCP to interactively provide a structural system to resist the loads. A composite design approach relies on the inner core to resist bending and buckling due to external loads such as soil cover, water table, vehicular loads and vacuum pressure. When applicable, this type of design can be more cost-effective because the amount of carbon fiber materials utilized can be less than stand-alone design. This paper presents design limit states and includes information from recent research, development, and testing. It discusses factors to be considered, potential challenges and best practices for determining stand-alone versus composite designs for carbon fiber lining systems.

Fifteen Years of Lessons Learned: Design and Construction of CFRP Liners for Large Diameter Pipelines

Strengthening of pressure pipelines using carbon fiber reinforced polymer (CFRP) materials has been an accepted repair method since the late 1990’s. CFRP composites are high strength, non-corrosive and durable materials and can add considerable structural capacity which makes them very suitable for pressure pipeline strengthening. However, there are several keys to success and best practices with regard to material selection, design, construction and quality control. There are lessons learned through installations which have taken place over the past fifteen years. These include adoption of criteria for material selection, new design philosophies, termination end detailing and critical points during the construction process. This paper provides state of the art information regarding materials, design considerations, installer experiences and current best practices. Field case studies provide a comprehensive review of the use of CFRP composites. 1. BACKGROUND Over the past fifteen years inspection methods for large diameter pipelines have evolved to include more accurate methods, failure risk analysis and repair prioritization. These advancements have provided utility owners with critical information regarding the exact location of distressed pipes and caused an increase in demand for targeted rehabilitation technologies. Based on this repair prioritization information, utilities have taken proactive steps in advance of failure to replace or repair the distressed pipes with an unacceptably high risk of failure. There are several repair options for concrete pressure pipes which include encasing the degraded pipe