Jalees Ahmad

A model for estimating nonlinear deformation and damage in ceramic matrix composites

The present paper addresses the estimation of global deformation of a continuous-fiber brittle matrix composite over a length much larger than the largest microstructural dimension of the composite, such as the fiber diameter and the fiber spacing. Initial microcracks and progressive microcracking of the matrix and interfaces under time-dependent load and temperature are considered. The model also includes consideration of inelastic deformation of one of the constituents. In this paper, the equations of the model are developed and applied to model the stress–strain behavior of several silicon carbide/silicon carbide ceramic matrix composites and the creep behavior of an oxide/oxide ceramic matrix composite. Comparison with test data shows that the model is able to capture the wide range of deformation behavior seen in these ceramic matrix composites.

Thermographic investigation of damage in ceramic matrix composites

As the engineering application of ceramic matrix composites progresses, a key part of the insertion effort is the non-destructive characterization. While most non-destructive evaluation is focused on the initial state based on the presence of defective conditions, the evolution of damage or change with exposure is more relevant. Thermography offers the benefit of fast inspection times with the option of finding defects or material changes based on the diffusivity of the material. A series of samples made out of ceramic matrix composite were inspected by thermography. Samples consisted of asfabricated and ones exposed to different conditions of temperature, stress and time. The results of this testing along with mechanical testing and analysis are presented and trends discussed.

An approach for nonlinear modeling of polymer matrix composites

In the present work, a mechanistic modeling approach is pursued for material characterization and for modeling inelastic deformation of polymer matrix composite components. The model attempts to capture the dominant micromechanical deformation mechanisms in laminated composites caused by matrix inelasticity at elevated temperatures. Given material characteristics of the constituent materials, the model can be used in predicting stress, time and temperature-dependent response of a composite under a broad range of thermal and mechanical load conditions. This article describes the modeling approach and examples of its use in a finite element analysis framework. Examples include analyses of simple test specimen coupons, stress concentration at holes and a structural element configuration of a polymer matrix composite. In each case, the model predictions are compared with the experimental measurements.

An Approach for Mechanistic Modeling of Ceramic Matrix Composites

A mechanistic modeling approach for material characterization and for life prediction of CMC components is described. The approach includes consideration of environment-induced degradation of CMC properties, progressive microcracking of the matrix material and interfacial damage. The mechanistic model has been embedded in a finite element analysis (FEA) framework to enable structural analyses of components and to analyze macro damage, such as cracks and delaminations. This paper describes the modeling approach and its application in the study of substructures. Examples include analyses of simple test specimen coupons, stress concentration at holes and a structural element configuration of a 2D woven SiC/SiC composite. The results include predicted and measured strain field near circular holes and global load-displacement behavior of structural elements. In each case, the model predictions are compared with the experimental measurements.Copyright

Ceramic Matrix Composite Interlaminar Fracture Toughness (Mode I-II) Characterization

The objective of this effort is to develop and demonstrate innovative interlaminar Mode I and Mode II fracture toughness analysis and test methods for ceramic matrix composites (CMC). Currently, there are number of American Society for Testing and Materials (ASTM) test standards for CMC’s at both ambient and elevated temperatures, including interlaminar tension and shear strength test methods. However, there are no standardized test methods for determination of interlaminar fracture toughness in CMC’s. Although research work exists on interlaminar Mode I and Mode II fracture toughness of various types of CMC’s, the test methods applied particularly in Mode II fracture toughness testing showed definite drawbacks and limitations. ASTM test standards for CMC’s may exhibit a zig-zag (wavy) crack path pattern, and fiber bridging. The experimental parameters that may contribute to the difficulty can be summarized as: specimen width and thickness, interface coating thickness, mixed mode failure evolution, and interlaminar defects. Modes I and II crack growth resistances, GI and GII, were analytically determined at ambient temperature using double cantilever beam (DCB) and End Notched Flexure (ENF) geometries. Three (3) CMC material systems were analyzed (Sylramic/IBN/MI, SiC/SiC CVI, and SiC/CAS). Several Finite Element (FE) based potential techniques were investigated: a) Multi-scale progressive failure analysis (MS-PFA); b) Virtual Crack Closure Technique (VCCT); and c) Contour Integral (CI). Advantages and disadvantages of each were identified. The final modeling algorithm recommended was an integrated damage and fracture evolution methodology using MS-PFA and VCCT. The analysis results (Fracture energy vs. crack length, Fracture energy vs. load, Fracture energy vs. crack opening displacement) matched the Mode I and Mode II coupon tests and revealed the following key findings. Mode I-DCB specimen: 1) Sylramic/IBN/MI failure mode is due to interlaminar tension (ILT) only in the interface section and a zig-zag pattern observed 2) VCCT crack growth resistance of Sylramic/IBN/MI is well matched to the test data and 3) SiC/SiC CVI failure mode is a mixed mode behavior (ILT to interlaminar shear (ILS). Mode II ENF specimen MS-PFA analysis suggests mixed mode behavior and the zig-zag pattern similar to Mode I coupon tests.Copyright

Thermal and destructive interrogation of ceramic matrix composites

Ceramic matrix composites are intended for elevated temperature use and their performance at temperature must be clearly understood as insertion efforts are to be realized. Most efforts to understand ceramic matrix composites at temperature are based on their lifetime at temperature under stress based on fatigue or creep testing or residual testing after some combination of temperature, stress and time. While these efforts can be insightful especially based on their mechanical performance, there is no insight into how other properties are changing with thermal exposure. To gain additional insight into oxidation behavior of CMC samples, a series of fatigue and creep samples tested at two different temperatures were non-destructively interrogated after achieving run-out conditions by multiple thermal methods and limited X-ray CT. After non-destructive analysis, residual tensile tests were undertaken at room temperature. The resulting residual properties will be compared against the non-destructive data. Ana...

Comparison of results from different NDE techniques from ceramic matrix composites with varying porosity levels

Ceramic matrix composites (CMCs) are attractive materials for use in advanced turbine engines. Due to the nature of available processing techniques, however, the amount and distribution of porosity in CMCs can vary greatly. This can be particularly true in parts with complex geometries. It is therefore important to characterize the porosity with non-destructive techniques and understand its effect on properties. A series of CMC samples were fabricated with varying levels of porosity and analyzed with different NDE techniques. The results were categorized and analyzed with respect to ease of interpretation and degree to which they could be quantified and used in models to determine the effects of defects. The results were also correlated with microstructural examination and mechanical properties.