ABSTRACT

Accelerated tests are widely used by industry to derive information about long term material responses to a particular service environment. In the attempt to gather information about a material’s long term survivability, various methods have been used. The oilfield industry has been introduced to life estimation or life prediction of seal materials by companies or organizations using special test fixtures to obtain the performance of seal materials in a specified test environment. This paper presents information on conducting life estimation tests using existing test equipment that many companies and laboratories presently use in material evaluation.

INTRODUCTION

The design life of equipment may vary from five years to twenty-five years or more. It is often impractical to conduct tests for such extended times. Or there is a need for equipment in a specific environment in a relatively short time. It may be desirable for these and other reasons to conduct accelerated testing where results can be obtained in a relatively short time by testing at more severe conditions. Accelerated testing consists of utilizing test methods for shortening the life of products or materials, or hastening the degradation of their performance1. Typical accelerating stresses are temperature, mechanical load, thermal cycling or a combination of stresses. Some of the goals2 of accelerated testing are:

  • 1. Obtain a general indication of the type of material property changes which might occur in a specific environment.

  • 2. To rank-order a group of materials according to their relative tolerances to specific environment for the purpose of selecting the best material.

  • 3. To determine whether a particular material will survive a given environment for specified time period.

  • 4. To quantitatively predict long term material life or behavior in terms of time-dependent changes in a given property.

In determining whether a material will survive a specific environment, the material is subjected to an accelerated test simulating as much as possible the detrimental effects of the service environment for a desired time period in the service application. The assessment of deterioration can be made by measuring the physical properties of the material. The changes in hardness, modulus (stress at a stated elongation) and tensile strength measured at room temperature, are widely used to assess the degree of deterioration produced by heating the material for a given time at an elevated temperature in a service environment. Criteria, although arbitrary, can be adopted, such as the time to produce a certain percent loss of property at a given temperature or the temperature to produce a certain percent loss of property in a given time. It is desirable to carry out tests at several temperatures and for several times to get a true picture of deterioration.

An autoclave test can be used to compare the thermal aging characteristics of materials as measured by the change in some property of interest when exposed to various environments. A series of elevated temperatures are used to achieve the defined property change.

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