Low-cycle fatigue of Nicalon™-fiber-reinforced ceramic composites

The effects of matrix strength on the notched and unnotched tensile properties of a family of porous-matrix oxide composites are examined both experimentally and theoretically. Experiments are performed on three composites, distinguished from one another by the amount of binding alumina within the matrix. Increases in alumina concentration produce elevations in unnotched tensile and shear strengths, but the benefits are offset by an increase in notch sensitivity. The degree of notch sensitivity is rationalized on the basis of a model that accounts for interactions between notch tip tensile and shear bands. The model predictions are cast in terms of the ratio of the notch length to a characteristic bridging length scale. These results, in turn, form the basis for a simple analytical formula for notched strength, accounting for effects of elastic anisotropy and finite sample size. The utility of this formula in predicting notched strength is assessed. Issues associated with bridging law shapes and bridging length scales are addressed. The effect of alumina concentration on notch sensitivity is discussed in terms of its influence on the bridging length scale, dictated by the interplay between the unnotched tensile strength, the longitudinal Young's modulus, the degree of in-plane elastic anisotropy, and the fracture energy. The net result is a decreasing bridging length scale and hence increasing notch sensitivity as the matrix is strengthened with alumina.

Section: Analysis Of Notch Sensitivitymentioning

confidence: 99%

Strength and Notch Sensitivity of Porous‐Matrix Oxide Composites

Mattoni¹,

Zok²

2005

“…14. The calculations were performed for the Nicalon/MAS, in large part because of the availability of data on the bridging law following fiber bundle failure in this material. 37,38 The stress required for initiation of the localized band was calculated using the point stress failure criterion, with the characteristic distance, d, assumed to range from 0 to 0.75 mm. As a → ϱ, all of the curves for the center-hole specimen asymptotically converge to 1/k p where k p is the stress concentration evaluated at a net section stress equal to the unnotched UTS.…”

Section: (4) Transitions In the Strength-limiting Processmentioning

confidence: 99%

“…This traction law was selected because it most closely approximates the pullout behavior of Nicalon/MAS once all of the fibers have failed. 37 The form of the pullout curve is a manifestation of the exponential distribution of pullout lengths coupled with a constant sliding stress. The bridging parameters were obtained from a fiber bundle pullout test, performed on a deeply notched tensile bar of the Nicalon/MAS composite.…”

Section: May 1999mentioning

confidence: 99%

“…The bridging parameters were obtained from a fiber bundle pullout test, performed on a deeply notched tensile bar of the Nicalon/MAS composite. 37 The values used in the calculations are 0 ‫ס‬ 100 MPa, ⌫ ‫ס‬ 40 kJ/m 2 , and E ‫ס‬ 130 GPa. Similar pullout characteristics have been reported independently for a comparable laminated CMC.…”

Section: May 1999mentioning

confidence: 99%

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Notch‐Sensitivity of Fiber‐Reinforced Ceramic‐Matrix Composites: Effects of Inelastic Straining and Volume‐Dependent Strength

McNulty

Zok

Genin

et al. 1999

The effects of circular holes and sharp notches on the tensile strength of two Nicalon‐reinforced ceramic composites have been investigated. The influence of inelastic straining on the redistribution of stress has been elucidated through measurements of the local strains in the regions of high stress concentration, coupled with finite element simulations of the test geometries, using a nonlinear constitutive law appropriate to ceramic composites. The scale dependence of strength has been inferred from tests performed on specimens of varying size. The utility of two failure models that incorporate both the inelastic straining and the scale dependence has been assessed: one based on the point stress failure criterion and the other on weakest‐link fracture statistics. Both approaches provide a reasonably consistent description of the experimental measurements. Some of the implications and limitations associated with the failure models are discussed.

“…This form is suggested by the present data as well as pullout data for fiber-reinforced ceramic composites. 15,16 With this assumption made, the traction law becomes…”

Section: (2) a Micromechanical Model Of Fiber Pulloutmentioning

confidence: 99%

In‐Plane Fracture Resistance of a Crossply Fibrous Monolith

McNulty

Begley

Zok

2001

The in-plane fracture resistance of a crossply Si 3 N 4 /BN fibrous monolith in the 0°/90°and ؎45°orientations is examined through tests on notched flexure specimens. The measurements and observations demonstrate the importance of fiber pullout following fiber fracture. The mechanical response is modeled using a crack-bridging approach. Two complementary approaches to evaluating the bridging law are developed: one based on a micromechanical model of fiber pullout and the other based on the load versus crack mouth opening displacement response of the flexure specimens following fracture of all fibers. Both approaches indicate that the bridging law follows an exponential form, characterized by a bridging strength and an effective pullout length. An assessment of the bridging model is made through comparisons of simulations of the load-displacement response with those measured experimentally.