Strain (chemistry)

Summary

In chemistry, a molecule experiences strain when its chemical structure undergoes some stress which raises its internal energy in comparison to a strain-free reference compound. The internal energy of a molecule consists of all the energy stored within it. A strained molecule has an additional amount of internal energy which an unstrained molecule does not. This extra internal energy, or strain energy, can be likened to a compressed spring. Much like a compressed spring must be held in place to prevent release of its potential energy, a molecule can be held in an energetically unfavorable conformation by the bonds within that molecule. Without the bonds holding the conformation in place, the strain energy would be released. Summary Thermodynamics The equilibrium of two molecular conformations is determined by the difference in Gibbs free energy of the two conformations. From this energy difference, the equilibrium constant for the two conformations can be de

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In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds (refer to figure on single bond rotation)

In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropane

In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are the monocyclic saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and c

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Delamination in cross-ply laminates: Identification of traction-separation relations and cohesive zone modeling

David Enrique Alanis Rojas, John Botsis, Joël Cugnoni, Ebrahim Farmand Ashtiani

Identification of traction separation relations is at the forefront of damage and fracture mechanics of composite laminates. Such relations are of particular interest in fracture of laminates consisting of layers of different fiber orientations. In this study, an iterative method based on internal strain measurements and parametric numerical modeling is employed to identify a traction-separation relation in quasi-static mode I dominated delamination of a cross-ply carbon fiber epoxy laminate. The results demonstrate that crack propagation is accompanied by a large bridging zone which is smaller than the zone in a uniaxial composite specimen of the same materials and linear dimensions. While the initiation value of the energy release rate (ERR) is the same, the ERR at steady state propagation and the maximum stress in the bridging zone are respectively 1.7 and 3.1 times larger than the corresponding values of the unidirectional specimen. The obtained traction-separation relation is employed in a cohesive zone model to predict the loading response and crack growth. The adopted approach is a step towards a better understanding of delamination in cross-ply composites. (C) 2015 Elsevier Ltd. All rights reserved.

Elsevier2015

Deformational behavior and damage mechanism of R-UHPFRC beam subjected to fatigue loading

Antoine Bassil, Eugen Brühwiler, Bartlomiej Wojciech Sawicki

The fatigue behavior of a reinforced UHPFRC (Ultra High Performance Fiber Reinforced Cementitious composite) T-shaped beam under four-point bending is investigated. The beam was subjected to a fatigue loading range equal to 49% of the static resistance and failed after 0.88 million cycles. It was instrumented with extensometers, strain gauges and distributed fiber optic sensors for strain monitoring. The fatigue process consists of three stages: with rapid, stable and again rapid growth of strains during 10%, 80% and 10% of total number of fatigue cycles, respectively. Except of the first 10%, this process takes place locally; therefore, it cannot be followed with the deflection measurement. During the stable stage, growth of strain occurs at minimum loading level in the fatigue cycle, indicating a fatigue damage process under tensile-compressive response of UHPFRC. Advanced fatigue crack propagation in the reinforcement bar determines the location of rupture of the beam. When the remaining cross-section of the rebar does not suffice to carry the tensile load, stress is transmitted to the encompassing UHPFRC causing its fast deterioration. Complete rupture of the rebar occurs only at the end of the test, when the beam collapses.

2021

A numerical approach to characterize the viscoelastic behaviour of fibre beds and to evaluate the influence of strain deviations on viscoelastic parameter extraction

Véronique Michaud, Christian Rytka, Vincent Werlen

The development of a robust material model able to accurately describe fibre bed compaction at different strain and strain rates is highly desirable because it is essential for the simulation of many composite manufacturing processes. In this study, we investigate the validity of an analytical viscoelastic model approach for different fabrics and at a wide range of strains, both in dry and wet conditions. We propose a numerical approach to determine the parameters of the material model that can overcome simplifications usually met with analytical approaches. We show that a three-branches Maxwell model with strain dependent stiffness and strain-rate dependent dampers can accurately describe the viscoelastic compaction behaviour of fibre beds at different strains and strain speeds, both in dry and wet conditions. We demonstrate that strain deviations have a considerable impact on the viscoelastic parameter extraction and should be taken into account.

2021