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Internal Stress Determination in Composites

#G-511


Direct Determination of Inherent Stresses in Composite Materials

Tech Area / Field

  • MAT-COM/Composites/Materials
  • PHY-SSP/Solid State Physics/Physics

Status
3 Approved without Funding

Registration date
17.04.2000

Leading Institute
Tbilisi Architectural and Civil Engineering Institute, Georgia, Tbilisi

Collaborators

  • Freie Universität Berlin, Germany, Berlin\nUniversity of Sheffield, UK, Shiffield\nStanford University, USA, CA, Stanford\nUniversity of Portsmouth, UK, Portsmouth

Project summary

Direct Determination of Inherent Stresses in Composite Materials

In composite materials which in the process of formation of their structures undergo transitions from a liquid phase into a solid one, particularly in concrete, the Inherent stresses are developed which are also results of the interaction with environment (shrinkage, temperature and humidity oscillations).

This is a cause of significant Inherent stresses in the body of concrete which affect in high degree its most important characteristics and are able to cause serious disturbances of integrity of the construction up to its complete destruction. The constructions under varying action of external aggressive factors (temperature, ice formation, humidity, radiation, etc.) are subject to permanent risks. This is particularly true in regard with the constructions, having considerable extensions, and consequently, strong gradients of stresses, such as, for example, nuclear reactor envelops.

Urgency, demonstrated on concrete as an example, may be extended to all other composite materials.

Actually, up to present, practically no registration of Inherent stresses is made during designing reinforced concrete constructions due to unclearness of the real picture of the distribution of Inherent stresses throughout the body of concrete, since the methods of their determination, as analytically, so experimentally, in the majority of cases are based on measurements of shrinkage deformations and, thus, are indirect ones, containing considerable, and often, principal errors. Analytical models of stress fields in concrete are not adequate to the real picture not only because of complexity of the structure, but also due to varying in time properties of the material that change the character of the connection between deformations and stresses.

The aims of this project are the following:

- to establish the nature and character of the development of Inherent stresses in concrete (it is meant as their distribution throughout the body of concrete, so their development in time) during the complete cycle of shrinkage and swelling.

- to find cause and result connection between the well-known curve of complete shrinkage deformations and expected nonmonotone curve developed at this stress. To represent the calculations of the regularities found in the form of a computer software.

- to reveal the most dangerous zones of stress concentration in the body of concrete.

- to establish the connection between the character of developed Inherent stresses and the concrete composition.

- to study the relaxation in time of stresses, caused by loads.

- to work out, on the basis of above-mentioned, the recommendations for taking into account the factors of Inherent stresses by designing norms and standards of constructions, particularly concerning important objects.

We have elaborated and tested during several years of work a method of direct measurement of stresses at particular points, that allows one to follow the development of strained states as in the process of solidification, so in the processes of moisturizing and drying of concrete. This makes possible to carry out the monitoring of stresses during several years at any particular point in a block, which causes anxiety (what has been proved in practice).

This method is unique because it completely satisfies the requirements to the measurements of unloaded stresses: to be susceptible to stretching stresses as well (along with compressing ones), high sensitivity, small dimensions, absence of the zero drift. It is based on the use of magnetoelastic effect. The priority of creating this method belongs to us.

Small dimensions (1cm3) and the direct cohesion of the sensitive elements of the transducer with the material enables the direct perception of Inherent stresses at any point of a concrete block without introduction of the distortions, caused by alien body or contact surface.

The measurements are remote, they can be carried out at a considerable distance from the construction, that may be located in an aggressive or radioactive surroundings.

The transducer is introduced into a concrete object during the latter's manufacturing; that enables one to follow the development of the stresses from the very first moment of their formation. It does not react to deformations, what is a shortcoming of the majority of stress gauges. It is characterized by the independence of its indications from the changes in concrete elasticity module, by the selectivity of stress directions. The stability of readings in time allows to watch the construction during several years.

Maybe the most significant and unique peculiarity of this transformer is its capability to measure, along with compressive stresses, stretching ones as well - the complete reversibility of the indicator's reading - the property that is so necessary in checking Inherent stresses that are periodically changing, even in their direction.

It will be investigated various kinds of concrete with additional chemical components of different function that will completely compatible with the transducer.

This will enable to carry out large scale task for constructing of a multifactor picture of the state of a concrete at a particular moment in time and at a particular point; to establish and represent in the form of a software interconnection and interinfluence of the most significant durability and deformation properties of the concrete and its composition.

Principal picture of the stress field distribution in a concrete matrix with indicating coefficients for accounting various factors will be constructed. It was revealed the part of the relaxation of stresses in time, that is usually very difficult to calculate.

The authors are confident of proving their theory of gradiental distribution of shrinkage stresses in a concrete body, one of the manifestation of which is the existence of stresses and deformations of opposite signs observed by us during the conducted research (i.e. at deformations of compression, in some layers of concrete construction, there were observed stretching stresses). So paradoxical at the first glance a phenomenon is completely explainable by above theory. It this we rely on the achievements of physicochemical mechanics.

At the second stage of the work, it is planned to observe relaxation in time of stresses, caused by external loading of constant value.

In actual constructions Inherent, as well as load caused stresses are operating simultaneously and they are not additive.

All analytical and indirect methods of assessment the values of relaxation stresses have general significant shortcomings: they cannot completely take into account the influence of environment, heterogeneity of material, its composition, the degree of hydratation of binding materials, variations in elasticity module in time, uninstantaneousness of loading in actual conditions. Besides they do not take into account stress gradient across cross-section of the construction caused by non-uniform distribution of moisture.

That’s why the relaxation will be studied in dependence from environment humidity.

By means of juxtaposition of the results of both experiments, it will be possible to reveal the most dangerous zones in important constructions.

Actually for the first time it will be possible to represent the resulting, complex, three-dimensional field of stresses (caused by various causes) in loaded concrete.

The obtained actual picture of the distribution of stresses in the concrete body and its development in time will serve to the lowering of the risks of damage, optimal choice of construction form and concrete composition during the process of designing.

This method enables also the possibility of investigation Inherent stresses, developed in the processes of freezing-melting of (concrete) material and also in the repeated cycles of its heating-saturation processes.


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