Fatigue and Creep

Abstract

In Chap. 12 we described the monotonic behavior of a composite under ambient temperature conditions of loading. There are many applications of composites where cyclic fatigue and high-temperature, i.e., creep conditions prevail. Accordingly, in this chapter we go further in complexity and describe the fatigue and creep behavior of composites. Fatigue is the phenomenon of mechanical property degradation leading to failure of a material or a component under cyclic loading. The operative word in this definition is cyclic. This definition thus excludes the so-called phenomenon of static fatigue, which is sometimes used to describe stress corrosion cracking in glasses and ceramics in the presence of moisture. Creep refers to time-dependent deformation in a material, which becomes important at relatively high temperatures (T > 0.4T _m, where T _m is the melting point in kelvin). We first describe fatigue and then creep of composites.

Problems

• 13.1. List some of the possible fatigue crack initiating sites in particle, short fiber, and continuous fiber reinforced composites.

• 13.2. What factors do you think will be important in the environmental effects on the fatigue behavior of fiber reinforced composites?

• 13.3. Acoustic emission can be used to monitor damage in carbon fiber/epoxy during fatigue. Under steady loading conditions the damage is controlled by fiber failure and one can describe the acoustic emission by

  $$ \frac{{{\text{d}}N}}{{{\hbox{d}}t}} = \frac{A}{{{{\left( {t + T} \right)}^n}}} $$

  where N is the total number of emissions, t is the time, T is a time constant, and A is constant under steady loading conditions. Taking n = 1, show that log t is a linear function of the accumulated counts (Hint: see Fuwa M, Harris B, Bunsell AR (1975) J Appl Phys 8: 1460).

• 13.4. Discuss the effects of frequency of cycling in regard to hysteretic heating in PMCs and CMCs.

• 13.5. Discuss the fatigue behavior an aramid fiber reinforced PMC is subjected to fatigue at negative and positive stress ratio (R).

• 13.6. Which one will have a better creep resistance in air: an oxide/oxide composite or a nonoxide/nonoxide system? Explain your answer.

• 13.7. Diffusional creep involving mass transport becomes important at low stresses and high temperatures. Discuss the importance of reinforcement/matrix interface in creep of a composite under these conditions.

• 13.8. Assume that the creep of fiber and matrix can be described by a power-law and that a well bonded interface exists. Assume also that the strain rate of the composite is given be the volume weighted average of the strain rates of the fiber and matrix. Derive an expression for the strength of such a composite.

• 13.9. In some composites, residual thermal stress distribution obtained at room temperature on cooling from the high processing temperature results in compressive radial gripping at the interface. Discuss the effect of high temperatures or creep in such a composite.