Stress Concentrations and Notch Sensitivity in Woven Ceramic Matrix Composites Containing a Circular Hole—An Experimental, Analytical, and Finite Element Study

Abstract

In this paper, the stress concentrations associated with a circular hole and subjected to uniform tensile loading in woven ceramic matrix composites (CMCs) have been determined experimentally through strain gauge measurements. The stress concentrations were calculated from the measured strain distributions surrounding the holes during the initial loading period. Then, the load‐carrying capacity of notched CMCs with various hole radius to width ratios (a/w) has been determined. The objective is to study the stress concentrations due to circular notch, and then evaluate the notch sensitivity in woven CMCs. Both quasi‐isotropic [0/90/+45/−45]s and cross‐ply [0/90/0/90]s laminates of woven SiC/SiNC composites were considered. The experimental data of stress concentrations were compared with the results obtained from a complex variable method and finite element analysis. The notched strength data were also analyzed on the basis of some existing theories. The results show significant stress concentrations in the range of 3.75–4.75 near the notch edges during the early stage of loading. But in most cases, the notched strength data show notch insensitivity in CMCs, particularly with small hole diameters. This occurs mostly due to stress redistribution effects during inelastic deformation in ceramic matrix composites. The degree of notch insensitivity in CMCs is observed to decrease with comparatively larger diameter holes. For notch insensitivity, the limiting value of a dimensionless parameter n, comprising of fracture toughness, un‐notched strength, and width of the specimen, has been determined on the basis of existing notch sensitivity theories. The stress concentration factors are seen not to be influenced by the lay‐up sequence of the laminate. Reasonable agreement was observed between the theory and experimentally determined stress concentration factors, particularly for small hole diameters.

Influence of heat treatment and fiber orientation on the damage threshold and the fracture behavior of Nextel fiber-reinforced Mullite-SiOC matrix composites analysed by acoustic emission monitoring

“…As a consequence, the toughness of the composites decreases and the notch sensitivity increases. For crack extension driven by stress concentrations, the failure of CMCs can be related to the well-known Griffith criterion [42,43], which could explain the linear correlation:…”

Effects of Tow‐Scale Holes on the Mechanical Performance of a 3D Woven C/SiC Composite

“…The UTS generally decreases with increasing a (for fixed a/b and a/w ) and increasing a/w (for fixed a and a/b ). Such effects have been found in most composite systems for specimens with circular holes of size a ≥ 1 mm . Where attempts have been made to compute the stress and strain distributions ahead of holes at the onset of fracture (using an appropriate elastic/plastic constitutive law), the local conditions appear to be consistent with a point stress failure criterion.…”

“…In some applications, it is envisioned that the components will contain geometric (stress‐concentrating) features such as holes, for cooling, fuel passage, or attachment. For this reason, a great deal of study has been undertaken previously to characterize the notch sensitivity of strength of a wide range of CMCs, including C/C, C/CAS, oxide/oxide, and SiC/SiC systems …”

“…Despite popular notions of “notch‐insensitivity,” even the toughest CMCs exhibit some dependence of ultimate tensile strength (UTS) and other performance metrics on notches or holes. The degree of sensitivity depends on: (i) the intrinsic properties of the composite, notably its (generally anisotropic) elastic, plastic, and fracture characteristics; and (ii) the notch size and shape, characterized by the principal radii a and b (for elliptical holes) and the ratios a/b and a/w ( w being the half specimen width) . The UTS generally decreases with increasing a (for fixed a/b and a/w ) and increasing a/w (for fixed a and a/b ).…”

Carbon–Carbon Composites

“…The notch effects, such as stress concentration, local damages, stiffness, and strength loss, are critical mechanical properties for structure designation. The notch effects of advanced materials are initially investigated in the ceramic matrix composites (CMCs) [1][2][3][4][5]. Three classes of stress redistribution mechanisms can lead to the excellent notched strength performance of CMCs: class I, matrix cracking and fiber failure; class II, matrix cracking; class III, shear damage by matrix cracking [2].…”