Microcrack modeling given temperature history *

Thermal Fatigue Project

The shelf-life of solid rocket motors (and other objects containing friable materials) is limited by the slow development of microcracks during storage. These microcracks are initiated and encouraged by thermal stresses and the resulting thermal fatigue due to temperature cycling. In the case of solid rocket motors, these cracks may propagate throughout the propellant so that, upon ignition, pieces of unconsumed propellant may dislodge and cause catastrophic failure of the rocket.

Instrumented Sensor Technology (IST) manufactures an integrated temperature sensor and recorder that is small enough to be embedded within a rocket's propellant. These units can be interrogated to determine the propellant's temperature history over many years. IST desires to add an algorithm to the unit's software so that it can, in addition to storing data, use the stored data to determine if the propellant has exceeded its useful shelf-life. In this way, the interrogator does not need any detailed knowledge of the rocket to determine the propellant's condition.

Hence, IST has an interest in developing a model which will predict the strain due to thermal gradients, the fatigue due to cycles of strain, the onset of microcrack formation, and the propagation of cracks within the bulk of a friable material. The number and size of cracks would then be used to estimate the condition of the stored material. The model should have the ability to include the effects of a storage vessel that is constructed from a different material than the bulk of the object. For example, a friable material may be contained in a steel drum that has different thermal properties resulting in a unique strain condition at the boundary. Also the vessel may have several basic shapes; cylindrical with flat ends, cylindrical with hemispherical ends, spherical, and rectangular.

This project has two goals. First, to identify a finite list of material parameters and construct a crack model so that, given a recorded temperature history, one can determine whether the material has exceeded its operational shelf-life. The model should predict the onset of crack formation and follow the propagation of the cracks.

Second, to design a experimental protocol and specify a test object that will enable IST to verify the model. The object should be chosen based on availability, its well known properties, and ability to show obvious signs of crack formation. (Translucent plastic might make a good test object since cracks may be directly visible under polarized light.) The protocol should subject the object to a temperature range that is easily produced and monitored (-40 C to 100 C would be desirable) and specify a sufficient number of cycles so that significant thermal damage due to crack formation has occurred.

The ideally completed project deliverable would be a crack model and experimental protocol.

*This summary prepared by R. J. Lambert of Instrumented Sensor Technology, Okemos, MI, and R. E. Svetic

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