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Foundation ::
Failure Analysis ::
CARES_LIFE
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CARES_LIFE
Ceramics Analysis and Reliability Evaluation of Structures Life Prediction Program
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SOURCE CODE AVAILABLE
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Successful application of advanced ceramics depends on proper characterization of material properties and the use of probabilistic brittle material design methodology. CARES/LIFE is a computer program for predicting the probability of a monolithic ceramic component's failure as a function of its time in service.
Probabilistic component design involves predicting the probability of failure for a thermomechanically loaded component from specimen rupture data. Typically these experiments are performed using many simple geometry flexural or tensile test specimens. A static, dynamic, or cyclic load is applied to each specimen until fracture. Statistical strength fatigue parameters are then determined from these data.
Using these parameters and the results obtained from a finite element analysis, the time-dependent reliability for a complex component geometry and loading is then predicted. Appropriate design changes are made until an acceptable probability of failure has been reached. This design methodology combines the statistical nature of strength-controlling flaws with the mechanics of crack growth to allow for multiaxial stress states, concurrent (simultaneously occurring) flaw populations, and subcritical crack growth (SCG).
Failure as a function of service time
CARES/LIFE predicts the probability of failure of a monolithic ceramic component as a function of service time. It assesses the risk that the component will fracture prematurely as a result of subcritical crack growth. The effect of proof testing components prior to service is also considered. CARES/LIFE is coupled to commercially available finite-element programs such as ANSYS, ABAQUS, MSC/NASTRAN, and COSMOS/M. It also retains all of the capabilities of the previous CARES code, which include fast-fracture component reliability evaluation and Weibull parameter estimation from inert strength (without SCG contributing to failure) specimen data. CARES/LIFE can estimate parameters that characterize SCG from specimen data as well.
Finite-element heat transfer and linear-elastic stress analyses are used to determine the component's temperature and stress distributions. The reliability at each element is calculated assuming that randomly distributed volume flaws and/or surface flaws control the failure response. The overall component reliability is the product of all the element survival probabilities. CARES/LIFE generates a data file containing element risk-of-rupture intensities (a local measure of reliability) for graphical rendering of the structure's critical regions.
Weibull equation
CARES/LIFE describes the probabilistic nature of material strength, using the Weibull cumulative distribution function. The Weibull equation is based on the weakest-link theory (WLT). WLT assumes that the structure is analogous to a chain with many links. Each link may have a different limiting strength. When a load is applied to the structure such that the weakest link fails, then the structure fails.
The effect of multiaxial stresses on reliability is predicted by using the principle of independent action (PIA), the Weibull normal stress averaging method (NSA), or the Batdorf theory. For the PIA model the reliability of a component under multiaxial stresses is the product of the reliability of the individual principal stresses acting independently. The NSA method involves the integration and averaging of tensile normal stress components evaluated about all possible orientations and locations. This approach is a special case of the more general Batdorf theory and assumes the material to be shear insensitive.
The Batdorf theory combines the weakest link theory and linear elastic fracture mechanics (LEFM). A user-selected flaw geometry and a mixedmode fracture criterion are required to model volume- or surface-strengthlimiting defects. Mixed-mode fracture refers to the ability of a crack to grow under the combined actions of a normal load (opening mode) and shear load (sliding and tearing modes) on the crack face.
In CARES/LIFE, the relations describing subcritical crack growth are directly incorporated into the PIA, NSA, and Batdorf theories. Subcritical crack growth is modeled with the power law, the Paris law, and the Walker law for static and constant-amplitude cyclic loading. These laws use experimentally determined parameters which are material- and environmentsensitive. Predicted lifetime reliability of structural ceramic components depends on Weibull and fatigue parameters estimated from widely used tests involving flexural or tensile specimens. CARES/LIFE estimates fatigue parameters from naturally flawed specimens ruptured under static, cyclic, or dynamic (constant stress rate) loading. Fatigue and Weibull parameters are calculated from rupture data of three-point and four-point flexure bars, as well as tensile specimens. For other specimen geometries, a finite element model of the specimen is also required when estimating these parameters.
CARES/LIFE carries the NASA case number LEW-16018. It was originally released as part of the NASA COSMIC collection.
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