Voids nucleation processes

Voids can be formed by either entrapment of air mechanically or by one of two nucleation processes. Mechanical entrapment could include (1) entrained gas bubbles from the resinmixing operation, (2) bridging voids from large particles or particle clusters (quenched DDS curing agent, airborne particles, or paper release agent), (3) voids from wandering tows, fuzz... 

There was one problem with this program. Various experiments suggested that the voids in the neutron (reactor) irradiated materials contained helium. Helium is formed rapidly during neutron irradiation as a decay product. Harkness et had shown that the swelling is a nucleation-controlled process. As the accelerator ion bombardment experiments, intended to mimic the reactor neutron bombardments experiments, do not produce helium within the sample, it is important to ask what effect helium has on the void nucleation problems. 

A nucleation process is generally described assuming the initial formation of the voids to be in accordance with the classical nucleation theory. It can be assumed that nucleation occurs between resin and fiber or resin and added particle (heterogeneous nucleation) or within the resin itself (homogeneous nucleation). 

When the dissolved gas reaches a saturation limit in the polymer, it becomes supersaturated and finally diffuses out of the polymer system to form voids or bubbles [17-19]. The formation of bubbles represents a nucleation process in which voids are formed either without nucleating agent (self-nucleation process) or with solid nucleating agents at the liquid-solid interface (heterogeneous process). 

An example for which the division into cohesive and process zones is not obvious is the problem of crack advance by void nucleation and coalescence in a ductile material [51]. Here, the cohesive zone might be taken to encompass all the cavities that contribute to crack growth. However, whether any cavitation off the fracture plane is associated with a process zone or with an extended cohesive zone is a matter of choice. Alternatively, the cohesive zone might be limited... 

Concentrations of hydrostatic stresses ahead of and equivalent plastic strains at the local carbide crack tips arise in both mode I and mode II loadings as shown in [5] and [11] - [13]. They give rise to the assumption of localized void nucleation and growth in both failure modes (Fig. 12). These processes are known from experiment to control the failure process unaer mode I loading [1]. The results presented suggest a crack propagation by void nucleation, growth and coalescence as shown in Fig. 13. 

Layadi et al. have shown, using in. situ spectroscopic ellipsometry, that both surface and subsurface processes are involved in the formation of /xc-Si [502, 503]. In addition, it was shown that the crystallites nucleate in the highly porous layer below the film surface [502, 504], as a result of energy released by chemical reactions [505, 506] (chemical annealing). In this process four phases can be distinguished incubation, nucleation, growth, and steady state [507]. In the incubation phase, the void fraction increases gradually while the amorphous fraction decreases. Crystallites start to appear when the void fraction reaches a maximum... 

Fig. 11 Craze in commercial polystyrene showing the characteristic steps nucleation through void formation in a pre-craze zone, growth of the fibrillar structure of the widening craze by drawing-in of new matrix material in the process zone, and final breakdown of the fibrillar matter transforming a craze into a crack (the crack front is more advanced in the center of the specimen, shielded by a curtain of unbroken fibrils marked by the arrow). The fibril thickness depends—of course—on the molecular variables, the strain rate-stress-temperature regime of the crazing sample and on its treatment (preparation, annealing) and geometry (solid, thin film) for PS typical values of between 2.5 and 30 nm are found [1,60,61]...

In order to start the multiscale modeling, internal state variables were adopted to reflect void/crack nucleation, void growth, and void coalescence from the casting microstructural features (porosity and particles) under different temperatures, strain rates, and deformation paths [115, 116, 221, 283]. Furthermore, internal state variables were used to reflect the dislocation density evolution that affects the work hardening rate and, thus, stress state under different temperatures and strain rates [25, 283-285]. In order to determine the pertinent effects of the microstructural features to be admitted into the internal state variable theory, several different length scale analyses were performed. Once the pertinent microstructural features were determined and included in the macroscale internal state variable model, notch tests [216, 286] and control arm tests were performed to validate the model s precision. After the validation process, optimization studies were performed to reduce the weight of the control arm [287-289]. 

It seems reasonable to assume that crazing is a process which can occur quite naturally in any orientation hardening material, which exhibits plastic instability at moderate strains and in which the yield stress is much higher than the stress required for the nucleation of voids (cavitations). 

---