Creep composite

Improvement in the creep resistance of whisker-reinforced composites is modest compared to that obtained in particulate-reinforced composites. Creep... 

In this paper, the importance of particle and whisker reinforcement to creep and creep rupture behavior of ceramics is discussed. Particle and whisker additions generally increase both the fracture toughness and creep resistance of structural ceramics. These additions also act as nucleation sites for cavities. Cavities form preferentially in tensile specimens. This results in a creep asymmetry, in which composites creep faster in tension than in compression. As a consequence of cavitation, the stress exponent for creep in tension 6-10,... 

To gain a better understanding of the creep behavior of fiber-reinforced ceramics, a simple 1-D analytical approach will be used to examine the effects of constituent behavior on composite creep deformation and changes in internal stress. Since the derivation of the model provides valuable insight into the parameters that influence composite creep behavior, the derivation of the 1-D concentric cylinder model will be outlined first. 

Fig. 5.2 Comparison of creep behavior and time-dependent change in fiber and matrix stress predicted using a 1-D concentric cylinder model (ROM model) (solid lines) and a 2-D finite element analysis (dashed lines). In both approaches it was assumed that a unidirectional creep specimen was instantaneously loaded parallel to the fibers to a constant creep stress. The analyses, which assumed a creep temperature of 1200°C, were conducted assuming 40 vol.% SCS-6 SiC fibers in a hot-pressed SijN4 matrix. The constituents were assumed to undergo steady-state creep only, with perfect interfacial bonding. For the FEM analysis, Poisson s ratio was 0.17 for the fibers and 0.27 for the matrix, (a) Total composite strain (axial), (b) composite creep rate, and (c) transient redistribution in axial stress in the fibers and matrix (the initial loading transient has been ignored). Although the fibers and matrix were assumed to exhibit only steady-state creep behavior, the transient redistribution in stress gives rise to the transient creep response shown in parts (a) and (b). After Wu et al 1...

S.2.3.2 Parametric Studies Influence of Constituent Moduli and Creep-Stress Exponents on Composite Creep Behavior... 

The stress and temperature dependence of the composite creep rate is governed by the values of the activation energies and stress exponents of the constituents. The initial stress and temperature dependence of composite creep rate is governed by the values of n and Q for the constituent which has the higher creep rate the final stress and temperature dependence is governed by the values of n and Q for the constituent with the lowest creep rate. This is illustrated in Fig. 5.6d, which compares the stress and temperature dependence of the constituent creep rate with the initial and final creep behavior of the composite. 

Fig. 5.6 Relationship between the creep rate of a composite and the stress and temperature dependence of the creep parameters of the constituents.31 (a) Temperature dependence of constituent creep rate, (b) Stress dependence of constituent creep rate, (c) Intrinsic creep rate of constituents as a function of temperature and stress illustrating the temperature and stress dependence of the creep mismatch ratio. In general, load transfer occurs from the constituent with the higher creep rate to the more creep-resistant constituent, (d) Composite creep rate with reference to the intrinsic creep rate of the constituents. The planes labeled kf and em represent the intrinsic creep rates of the fibers and matrix, respectively.

Although analytical models are very effective tools for understanding how changes in the elastic and creep behavior of the constituents of a composite influence overall creep behavior, one must not blindly assume that accurate predictions of composite creep behavior can be obtained based upon creep experiments conducted on the individual constituents. For instance, even if a monolithic ceramic and a composite were processed under identical conditions, the fracture and creep behavior of the monolithic ceramic may be... 

Thus, when a stress a is applied to the composite, creep will occur until the strain has the value consistent with Eqn. (60). Numerical inversion of Eqn. (60) can be used to establish this strain. The stress-strain curve in Eqn. (60) has a stress maximum when... 

For the Mileiko18 model of composite creep leading to the steady-state creep rate for fixed fiber length given in Eqn. (63), a rudimentary fiber fragment length model gives... 

The procedure for obtaining the shift factor was proposed to Williams et al.3 by Ferry8 through constructing a series of creep plots at different temperatures and then looking to see what shift is required to reconstruct the composite creep plot (Figure 6.14). 

Composites are particularly susceptible to creep and relaxation effects in the throughthickness direction for the normal range of planar reinforcements. For loads normal to the plane of the composite, creep will occur, and in many bolted clamped joints the need to design the joint to allow for relaxation of the bearing stress is well recognised. 

The reinforcement effect of sisal fiber content on the flexural creep performance and flexural modulus of cellulose derivatives/ starch composites was studied by A1 Verez et al. [15]. Fiber content and temperature effects were also considered, taking into account various methods and equations. At short times, a creep power law was employed. A master curve with the Arrhenius model was used to determine the creep resistance at longer times and different temperatures. Good fitting of the experimental results with the four-parameter model was reported, leading to a relationship between the observed creep behavior and the composite morphology. The addition of sisal fibers to the polymeric matrix promoted a significant improvement of the composite creep resistance. 

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