Creep Life Prediction of Ceramic Components Subjected to Transient Tensile and Compressive Stress States

The desirable properties of ceramics at high temperatures have generated interest in their use for structural applications such as in advanced turbine systems. Some of these ceramic components, such as vanes and rotors, are subjected to concurrent tensile and compressive stress fields. Design lives for such systems can exceed 10,000 hours. Such long life requirements necessitate subjecting the components to relatively low stresses. The combination of high temperatures and low stresses typically places failure for monolithic ceramics in the creep regime. The objective of this paper is to present a design methodology for predicting the lifetimes of structural components subjected to concurrent transient tensile and compressive creep stress states. In this methodology, failure generally starts at or near the most highly stressed point and subsequently propagates across the section. The creep rupture life is divided into two stages. The first is called the stage of latent failure. During this stage the damage accumulates until it becomes critical at some point within the component, and failure begins. Damage due to compressive stresses is assumed to be negligible. Subsequently, the second stage, named the propagation of failure, takes place. Component failure occurs at the end of this stage when the total carrying capacity of the structure is expended. This methodology utilizes commercially available finite element packages and takes into account the time varying creep stress distributions (stress relaxation). The creep life of a component is divided into short time steps, during which, the stress distribution is assumed constant. The damage is calculated for each time step based on a modified Monkmon-Grant creep rupture criterion. Failure is assumed to commence when the normalized accumulated damage at a point in the body is equal or greater than unity. For tensile/compressive stress states, rupture is assumed to take place when the damage zone is large enough so that the component is no longer able to sustain load. The corresponding time will be the creep rupture life for that component. Flexural and C-ring data of siliconized silicon carbide KX01 material are used to test the viability of this methodology. The NASA integrated design code CARES/Creep (Ceramics Analysis and Reliability Evaluation of Structures/Creep) which utilizes this damage accumulation model was used for this purpose. It was found that the methodology described in this paper yielded reasonable creep rupture life predictions given the amount of scatter in the data.