Redistribution Of Internal Stresses Research Articles

Which Design Concept for Prestressed Steel?

Prestressed steel structures are those in which, during manufacture, assembly, or exploitation, deliberate stresses are produced of precise magnitude, direction, and period of duration. The most significant aims of prestressing are: enlargement of the elastic range in which the structure works; redistribution of internal stresses or forces; improvement of stability; increase of fatigue resistance; decrease in deformations; wider use of high strengthsteels. Some examples of the many types of prestressed steel structures and methods of prestressing are: rigid basic structures (girders, trusses, frames, masts, towers, etc.) prestressed by high strength tendons; systems or networks of prestressed flexible strings (hanging roofs and walls, etc.); multi-layer and hybrid beams or vessels (simultaneous use of different materials as concrete and steel, carbon steel and quenched-tempered steel, etc.); statically indeterminate structures prestressed by enforced displacement of redundant restraints (usually by enforced shifting of some redundant supports or by compelled assembly of some elements fabricated with planned dimension "inaccuracies"); removal or exploitation of residual, secondary, or other "parasite" stresses (from welding, temperature treatments, mechanical operations with steel in cold state, unwanted constructional rigidity of some details, etc.). Prestressed structures utilizing tendons are the most widely used and most economical.

A model for uniaxial creep based on internal-stress redistribution

The changes of creep-strain rate associated with primary and tertiary creep are attributed to a process of internal-stress redistribution between the strong and weak portions of material while for each portion the creep rate depends only on its stress. An expression for the primary-creep curve is derived which is shown to be in substantial agreement with the usual empirical (time)1/3 power law. The model describes the changes in creep rate when the stress is constant and implies as a natural extension a method of calculating creep rate when the stress changes because memory effects are attributed solely to changes of internal stress. The calculated rates obtained according to the method show the creep-recovery phenomenon and in this respect are superior to the ‘hardening hypotheses’ which fail to simulate creep recovery but are normally used for stress-redistribution calculations. According to the model the strain during primary creep is dependent on the stress derivative of the secondary creep rate. Analysis of results for the aluminium alloy DTD 5070A has helped confirm the model by showing a correlation between these quantities.

Basis of the local strains theory (review. 1)

A study is made of the relationship between the stress and strain tensors, temperature, time and the material characteristics in the most general case for a complex loading path. The failure and loss of stability of actual structures corresponds to a complex loading path, when the change in the stress tensor components is not proportional to a single parameter. The same applies to the microphenomena of failure, when the destruction of the material is preceded by a complex redistribution of internal stresses and strains. The phenomenological basis of the theory is developed. This makes it possible to solve a number of practical problems more correctly than has so far proved possible. These problems include problems of the buckling of beam sections and shells. The local strains theory is a variant of the theory of plasticity that allows the solution of the complex loading problem and makes it possible to unify the theory of plasticity and the theory of deformation of theonomic materials. The review is based on more than twenty publications dealing with questions relating to the local strains theory.

Influence of decarburization on the properties of case hardened steel

1. The existing method of deoxidizing salt baths with ferrosilicon does not prevent decarburization of heated articles. 2. The occurrence of the decarburized layer on the surface of case hardened articles leads to a redistribution of internal stresses (decrease of compression stresses and the occurrence of tensile stresses in the surface area of the case hardened layer), and this decreases the fatigue resistance of the case hardened steel. 3. To decrease decarburization during case hardening and during heat treatment of case hardened articles, one should avoid heating the articles to high temperatures for long periods of time. Therefore, normalization of case hardened articles at high temperatures is undesirable.