Recent Developments In Computational Fracture Mechanics At Cardiff

The following most recent developments in computational fracture mechanics at Cardiff University are reviewed: hybrid crack element (HCE) which can give directly the stress intensity factor (SIF) as well as the coefficients of higher order terms in the plane linear elastic crack tip asymptotic field; extended finite element method (XFEM) which avoids using a mesh conforming with the crack as is the case with the traditional FEM and gives highly accurate crack tip fields; penalty function technique for handling point loads; and compressed sparse row (CSR) storage scheme for efficient implementation of the above techniques. Possible future improvements are also discussed

Simulation of the flow of self-compacting concrete in the V-funnel by SPH

The flow of self-compacting concrete (SCC) mixes through a V-funnel is simulated from the moment the gate is opened until the time when the light was first seen in the bottom opening through observation from the top (discharge time, tv-funnel). For this simulation, the three-dimensional mesh-less smooth particle hydrodynamics (SPH) computational approach is chosen in which the SCC mix is treated as a non-Newtonian Bingham-type fluid. The numerical simulation results for the discharge time agree very favourably with those recorded in the laboratory on a range of normal strength SCC mixes with cube compressive strengths from 30 to 80 MPa with a maximum size crushed coarse aggregate of 20 mm. In contrast, the two-dimensional simulation of the V-funnel test is found to underestimate significantly the discharge time. The statistics of the distribution of the coarse aggregate particles equal to or larger than 8 mm in the mix (this limit is set by the number of particles used in the SPH simulations) shows that the mix remains homogeneous with no settlement of the heavier particles during the flow and after the flow has stopped

A Criterion for Brittle Failure of Rocks Using the Theory of Critical Distances

This paper presents a new analytical criterion for brittle failure of rocks and heavily overconsolidated soils. Griffith’s model of a randomly oriented defect under a biaxial stress state is used to keep the criterion simple. The Griffith’s criterion is improved because the maximum tensile strength is not evaluated at the boundary of the defect but at a certain distance from the boundary, known as the critical distance. This fracture criterion is known as the Point Method, and is part of the Theory of Critical Distances, which is utilized in fracture mechanics. The proposed failure criterion has two parameters: the inherent tensile strength, ó0, and the ratio of the half-length of the initial crack/flaw to the critical distance, a/L. These parameters are difficult to measure but they may be correlated with the uniaxial compressive and tensile strengths, óc and ót. The proposed criterion is able to reproduce the common range of strength ratios for rocks and heavily overconsolidated soils (óc/ót=3-50) and the influence of several microstructural rock properties, such as texture and porosity. Good agreement with laboratory tests reported in the literature is found for tensile and low confining stresses.The work presented was initiated during a research project on “Structural integrity assessments of notch-type defects", for the Spanish Ministry of Science and Innovation (Ref.: MAT2010-15721)

A new approach to the design of RC structures based on concrete mix characteristic length

A new approach to the design of reinforced concrete (RC) structures is proposed. It does not rely on the traditional characteristic compressive strength of the concrete mix which is the basis of all current codes for the design of RC structures. Instead, the approach is based on the characteristic length of the concrete mix that has its origins in the concepts of fracture mechanics. Based on the research done in Cardiff University over the past 6 years on long and short beams and slender columns, it is shown that this new approach leads to a substantial reduction in the amount of reinforcing steel needed in RC structures made from high strength concrete mixes without jeopardising their ductility. This provides conclusive evidence that the current design code provisions for reinforcement based on the mix characteristic compressive strength grossly overestimate the requirements for high strength mixes leading to wastage of steel, reinforcement congestion and high cost of construction. The adoption of this new design approach, which is based on sound physical principles, should help promote the use of high performance, durable and sustainable concrete in the construction industry without increasing the cost of construction or compromising the safety of structures