Compression (physics) | EPFL Graph Search

In mechanics, compression is the application of balanced inward ("pushing") forces to different points on a material or structure, that is, forces with no net sum or torque directed so as to reduce its size in one or more directions. It is contrasted with tension or traction, the application of balanced outward ("pulling") forces; and with shearing forces, directed so as to displace layers of the material parallel to each other. The compressive strength of materials and structures is an important engineering consideration. In uniaxial compression, the forces are directed along one direction only, so that they act towards decreasing the object's length along that direction. The compressive forces may also be applied in multiple directions; for example inwards along the edges of a plate or all over the side surface of a cylinder, so as to reduce its area (biaxial compression), or inwards over the entire surface of a body, so as to reduce its volume. Technically, a material is unde

Postbuckling behavior and imperfection sensitivity of elastic-plastic periodic plate-lattice materials

Fani Derveni

Advances in additive manufacturing have enabled a new generation of materials with advantageous properties inherent to their architecture. Recently, architected materials with periodic arrangements of plates, called plate-lattice materials, have been developed to reach theoretical least upper bounds for stiffness and strength of isotropic materials. This work investigates the buckling behavior of several plate-lattice architectures with relative densities between 0.5% to 25% when subjected to uniaxial compression. Finite element unit cell models with shell elements and periodic boundary conditions are used to simulate the buckling and post-buckling behavior of plate-lattices made from an elastic-perfectly plastic parent material. Five plate-lattice architectures with cubic symmetry are investigated - three anisotropic architectures (simple cubic, body-centered cubic, face-centered cubic) and two isotropic architectures (simple cubic and body-centered cubic combination, simple cubic and face-centered cubic combination). Geometric imperfections of varying amplitude are applied to determine the imperfection sensitivity of these plate-lattice materials, and to calculate their buckling knockdown factors. Consistent with the behavior of the elastic-plastic plate building blocks that comprise the lattice, this investigation reveals that plate-lattice materials are generally imperfection insensitive when their constituent plates become thinner and first bifurcation occurs in the elastic range away from the yield stress. When this occurs, the initial post-buckling behavior is stable and the ultimate strength can be many times higher than the initial buckling stress. (C) 2021 Published by Elsevier Ltd.

ELSEVIER2022

Damage evolution in coated cemented carbides under compressive contact loads

Maria-Simona Moldovan

This work deals principally with two important issues and their interrelation: the evolution of damage in coated cemented carbide tools used for cold forming, and the assessment of mechanical behavior of cemented carbides under compressive contact loads. Damage evolution in ironing tools is qualitatively investigated by planar and cross-sectional microscopical observations. Several mechanisms of damage are identified by taking into account the circumferential distribution along the surface of the ironing tools: coating wear and/ fracture, plastic deformation and fracture of the substrate. In the context of damage location in the substrate, the emphasis is put on the study of damage following all processing steps in substrate and coating preparation. This analysis aims at determining whether the substrate is weakened or not by the mechanical and chemical pre-treatment prior to coating. Spherical indentation testing is used as an experimental procedure to reproduce the compression stress state during cold forming. Large indenter tips (300 microns) are employed for testing cemented carbides in an effort to minimize the effects of material structural inhomogeneities. The behavior of cemented carbides under compressive loads is governed by the reversibility of deformation induced by indentation. A unique feature revealed by analysis of experimental load-displacement indentation curves is the positive energy balance, in that a gain of energy is evidenced after a loading-unloading cycle. Two hypotheses are developed for interpreting such a behavior: the strain induced martensitic transformation of the cobalt ductile phase and the thermal residual stresses in the composite. Substrate weakening subsequent to mechanical and chemical pretreatment prior to coating is expected to have a particular role on the indentation behavior of cemented carbides. The impact of substrate damage on the indentation parameters is identified by a parametric study using finite element simulations in which the damage is considered uniform. In reality, the damage distribution over the surface is rather irregular. In this context, a statistical approach is used for assessment of the material indentation parameters. The failure of coating/substrate systems under compressive contact loads is investigated by finite element simulations. The first failure events are identified by considering that system failure occurs either by substrate plastic deformation or coating fracture. The response of coated systems to spherical indentation is synthesized in damage diagrams which can predict whether the coating or the substrate fails first. The possibility for using such diagrams for assessing the evolution of damage in coated cemented carbide tools is highlighted.

EPFL2008

Déformabilité et capacité portante des colonnes en béton armé

Serge Dal Busco

For buildings which are stabilized by core or shear walls, the vertical members are generally subjected to compressive normal forces N and certain imposed angles of rotation θ. Similar in this respect are non slender bridge piers. These rotations originate from, on one hand, the length variations of the horizontal elements (beams, slabs) due to shrinkage, temperature and prestressing, while on the other hand the bending of these elements due to the vertical loading. Thus, in this case, it is a problem of imposed deformations. However, the usual methods of calculation do not take this fact into account and bring the computation back to a question of forces. In this case, the design is mostly based on the resistance of individual sections, with the use of moment-axial load interaction diagrams MR-NR. In this thesis, a different method is proposed which takes account of the problem of imposed deformations, which concerns equally well the real behaviour of the structure. This proposal is limited to the cases where the slenderness ratio λcr is less than or equal to 50 (λcr ≤ 50). The analysis of a column is carried out considering the vertical load N and the imposed end rotations θ, due to the interaction of horizontal load carrying members. This method allows one to create structures with a greater performance, thanks to the larger distance between expansion joints and to a reduction in the overall dimensions of the columns. This equally has the consequence that the columns are almost always able to be fixed ended by the horizontal members, thus permitting a reduction in their slenderness λcr and which eliminates the costs of the hinge devices. However, it is mostly necessary to resort to high percentages of reinforcement to transmit the elevated vertical loads. This concept of the association of N and 0 is applied to both the examination of the ultimate limit state (loading capacity and deformability) and to serviceability (aptitude of service). The first verification consists of assuring that the column possesses the required deformability to permit the imposed rotations, while transferring the normal force, with the required security. It is possible to obtain this by using adequate transverse reinforcement at the fixed ends of the column, in such a fashion that plastic hinges would be able to form there. The first part of this work concentrates on the development of a method which allows the determination of the characteristics of such a transverse reinforcement (shape and configuration, spacing and diameter of stirrups), as well as an investigation of the numerous parameters influencing the behaviour of columns at the ultimate limit state. In the second part, an explanation is given on how to control the behaviour of a column, when it is subjected to service loading and imposed rotations, in order to verify its serviceability. This explanation takes into account the nature of the rotations, caused by the variations in temperature or by long term phenomena, such as shrinkage and creep. Its application is easy as it is given in the form of charts. Finally, a method for the practical cases is proposed to the designer. It contains the notes and charts to design and verify reinforced concrete columns with different levels of reinforcement.

EPFL1988