Finite element modeling of frictionally restrained composite interfaces

The use of special interface finite elements to model frictional restraint in composite interfaces is described. These elements simulate Coulomb friction at the interface, and are incorporated into a standard finite element analysis of a two-dimensional isolated fiber pullout test. Various interfacial characteristics, such as the distribution of stresses at the interface, the extent of slip and delamination, load diffusion from fiber to matrix, and the amount of fiber extraction or depression are studied for different friction coefficients. The results are compared to those obtained analytically using a singular integral equation approach, and those obtained by assuming a constant interface shear strength. The usefulness of these elements in micromechanical modeling of fiber-reinforced composite materials is highlighted

The effects of crack surface friction and roughness on crack tip stress fields

A model is presented which can be used to incorporate the effects of friction and tortuosity along crack surfaces through a constitutive law applied to the interface between opposing crack surfaces. The problem of a crack with a saw-tooth surface in an infinite medium subjected to a far-field shear stress is solved and the ratios of Mode-I stress intensity to Mode-II stress intensity are calculated for various coefficients of friction and material properties. The results show that tortuosity and friction lead to an increase in fracture loads and alter the direction of crack propagation

Local-global analysis of crack growth in continuously reinforced ceramic matrix composites

The development is described of a mathematical model for predicting the strength and micromechanical failure characteristics of continuously reinforced ceramic matrix composites. The local-globe analysis models the vicinity of a propagating crack tip as a local heterogeneous region (LHR) consisting of spring like representation of the matrix, fibers and interfaces. This region is embedded in an anisotropic continuum (representing the bulk composite) which is modeled by conventional finite elements. Parametric studies are conducted to investigate the effects of LHR size, component properties, interface conditions, etc. on the strength and sequence of the failure processes in the unidirectional composite system. The results are compared with those predicted by the models developed by Marshall et al. (1985) and by Budiansky et al. (1986)

Força lumbar en jugadors d'hoquei sobre herba

Introducció: El dolor lumbar té una alta prevalença entre els esportistes, s’ha relacionat amb dèficits en la força extensora lumbar, i el fet de patir-ne representa un obstacle important per a la pràctica d’esports d’alta intensitat. Mètode: S'ha mesurat la força lumbar en dos grups de practicants d'hoquei sobre herba mitjançant màquina MedX® i un test de resistència isomètric lumbar. Resultats: Entre ambdós grups els resultats han estat molt homogenis. Els dos tests no presenten relació entre si, ni amb les característiques biomèdiques dels jugadors (edat, índex de massa corporal o VO2màx). Els nivells de força màxima i resistència isomètrica obtinguts han estat superiors als de referència entre sedentaris. D’una manera característica, entre els jugadors d'hoquei la relació entre la força extensora del tronc en flexió (M) respecte la força extensora del tronc en extensió (m) és més gran que en altres esportistes (ràtio M/m > 1,6, mentre que en la població normal és 1,4) a causa probablement de la posició en semiflexió pròpia de l'hoquei. Conclusió: Els resultats dels test de força extensora lumbar tenen unes característiques pròpies entre els jugadors d’hoquei

Fuerza lumbar en jugadores de hockey hierba

Introducción: El dolor lumbar tiene una alta prevalencia entre los deportistas, se ha relacionado con déficits en la fuerza extensora lumbar, y el hecho de padecerlo representa un obstáculo importante para la práctica de deportes de alta intensidad. Método: Se ha medido la fuerza lumbar en 2 grupos de practicantes de hockey hierba mediante máquina MedX® y un test de resistencia isométrico lumbar. Resultados: Entre ambos grupos los resultados han sido muy homogéneos. Los 2 tests no presentan relación entre sí ni con las características biomédicas de los jugadores (edad, índice de masa corporal o VO2máx). Los niveles de fuerza máxima y resistencia isométrica obtenidos han sido superiores a los de referencia entre sedentarios. Característicamente, entre los jugadores de hockey hierba la relación entre la fuerza extensora del tronco en flexión (M) respecto a la fuerza extensora del tronco en extensión (m) es mayor que en otros deportistas (ratio M/m > 1,6, mientras que en la población normal es 1,4) debido probablemente a la postura en semiflexión propia del hockey. Conclusión: Los resultados de los test de fuerza extensora lumbar tienen unas características propias entre los jugadores de hockey hierba

Analyzing various models of Circadian Clock and Cell Cycle coupling

The daily rhythm can influence the proliferation rate of many cell types. In the mammalian system the transcription of the cell cycle regulatory protein Wee1 is controlled by the circadian clock. Zamborszky et al. (2007) present a computational model coupling the cell cycle and circadian rhythm, showing that this coupling can lead to multimodal cell cycle time distributions. Biological data points to additional couplings, including a link back from the cell cycle to the circadian clock. Proper modelling of this coupling requires a more detailed description of both parts of the model. Hence, we aim at further extending and analysing earlier models using a combination of modelling techniques and computer software, including CoSBI lab, BIOCHAM, and GINsim

Mechanistic Modeling of Unbonded Concrete Overlay Pavements

An unbonded concrete overlay (UBCO) system is a Portland cement concrete (PCC) overlay that is separated from an existing PCC pavement by an asphalt concrete (AC) interlayer. Current UBCO design procedures are based on empirical equations or highly simplified mechanistic models. To overcome the limitations, fracture mechanics concepts, specifically the finite element method-based cohesive zone model (CZM), are introduced in this research as a new paradigm for analyzing UBCOs with the ultimate goal of establishing a more rational design procedure. To illustrate the advantages of a fracture mechanics-based approach to design, specific attention is paid to but one type of failure associated with pavement structures: reflection cracking. The design against reflection cracking approach relies on a load-carrying capacity equivalency between the designed UBCO and a reference newly designed single layer PCC pavement. An illustrative fracture mechanics-based design procedure for UBCOs is developed and proposed by a large number of crack propagation simulations of both the UBCO composite and the reference single layer pavement. Preliminary comparisons of the results with field observations suggest that the fracture mechanics paradigm offers promise for improved design of UBCOs against reflection cracking and other potential loading conditions that could be analyzed using nonlinear fracture mechanics models. It is recommended that an experimental program be established to assess the accuracy of the model predictions, and additional experiments and three-dimensional fracture mechanics simulations be considered to provide additional insights as to whether UBCOs can be “thinned-up”.Department of Civil Engineering, University of MinnesotaBallarini, Roberto; Liao, Minmao. (2012). Mechanistic Modeling of Unbonded Concrete Overlay Pavements. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/148549

THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 91-GT-94 Stability Analysis of Bridged Cracks in Brittle Matrix Composites

ABSTRACT The bridging of matrix cracks by fibers is an important toughening mechanism in fiber reinforced brittle matrix composites. This paper presents the results of a non-linear finite element analysis of the Mode-I propagation of a bridged matrix crack in a finite size specimen. The composite is modeled as an orthotropic continuum and the bridging due to the fibers is modeled as a distribution of tractions which resist crack opening. A critical stress intensity factor criterion is employed for matrix crack propagation while a critical crack opening condition is used for fiber failure. The structural response of the specimen (load-deflection curves) as well as the stress intensity factor of the propagating crack are calculated for various constituent properties and specimen configurations for both tensile and bending loading. By controlling the length of the bridged crack results are obtained which highlight the transition from stable to unstable behavior of the propagating crack

Closed-path J-integral analysis of bridged and phase-field cracks

We extend the classical J-integral approach to calculate the energy release rate of cracks by prolonging the contour path of integration across a traction-transmitting interphase that accounts for various phenomena occurring within the gap region defined by the nominal crack surfaces. Illustrative examples show how the closed contours, together with a proper definition of the energy momentum tensor, account for the energy dissipation associated with material separation. For cracks surfaces subjected to cohesive forces, the procedure directly establishes an energetic balance à la Griffith. For cracks modeled as phase-fields, for which no neat material separation occurs, integration of a generalized energy momentum (GEM) tensor along the closed contour path that traverses the damaged material permits the calculation of the energy release rate and the residual elasticity of the completely damaged material