A Contribution to Time-Dependent Damage Modeling of Composite Structures
The paper presents a new damage model for predicting stiffness loss due to creep loading and cyclic fatigue. The model, developed within a continuum damage mechanics framework, is based on the idea of a time-dependent damage spectrum, some elements of which occur rapidly and others slowly. The use of this spectrum allows a single damage kinematic to model creep and fatigue damage and to take into account the effect of stress amplitude, R ratio, and frequency. The evolution equations are based on similar equation than the one describing the viscoelasticity model and are relatively easy to implement. The new model is compared to the experimental results on carbon fiber/epoxy tubes. Quasi-static, creep and fatigue tests are performed on filament-wound tubular specimens to characterize the elastic, viscoelastic and plastic behavior of the composite material. Varying amounts of damage are observed and discussed depending on stress level and R ratio. The experimental work aims to develop and validate the damage model for predicting stiffness loss due to creep loading and cyclic fatigue.
Durability of tubular composite structures : fatigue and time-dependent damage modeling
L'objectif de cette etude est de developper un modele d'endommagement en fatigue en vue d'optimiser le dimensionnement d’un riser tout composite. Afin de mieux comprendre les phenomenes pertinents pour une telle structure, des etudes experimentale et analytique ont ete menees sur des tubes en fibre de carbone epoxyde. Des essais quasi-statiques et de fluage ont ete realises sur des eprouvettes afin de caracteriser le comportement longitudinal, en cisaillement et transverse ; des eprouvettes ont ete testees pour valider les modeles. Des essais de fatigue ont ete egalement realises, aux niveaux de contraintes et rapports de R differentes, pour determiner l’evolution de l’endommagement en fatigue. Un modele developpe par Perreux et al a ete choisi pour modeliser les phenomenes elastiques, viscoelastiques, plastiques, viscoplastiques et d’endommagement qui ont ete constates pendant les essais de caracterisation. Les resultats des eprouvettes ont permis d'obtenir les proprietes les parametres du modele, et le modele a ete valide par les resultats des autres empilements. Un nouveau modele d'endommagement a ete developpe pour prendre en compte l’endommagement du aux chargements cycliques ou de fluage. L’approche est basee sur un spectre temporel d'endommagement elementaire. Cette approche permet de prendre en compte les effets de la frequence de chargement et le rapport de R, et les simulations du modele concordent avec les resultats des essais. Finalement, les resultats de cette etude ont ete appliques au cas d'un riser de production tout composite.
Adolescent sexual offenders: incidence of childhood maltreatment, serious emotional disturbance, and prior offenses.
Adolescents incarcerated for sexual offenses were compared to those incarcerated for other crimes on measures of prior child abuse and neglect, serious emotional disturbance, and involvement in child welfare and the juvenile justice system. Sexual or physical abuse was more common than neglect among sexual offenders with fewer than three prior maltreatment reports. Sexual offenders were twice as likely to be receiving special education services for severe emotional disturbance, and were usually incarcerated later than other offenders. Implications for research and practice are discussed.