Crack initiation propagation model

A statistical analysis based on General Linear Model (GLM) was developed to analyse the influence of loading and off-axis angle on damage of composite laminates. The void content in the composite specimens acted as stress raiser resulting in cracks initiation, propagation and failure as tensile loads progressively applied. Analysis of Variance (ANOVA) was performed for void contents to check the statistical differences caused by the experimental errors. 

A method of predicting failure based on the concepts of stress and fracture mechanics is the cohesive zone method. The cohesive zone model has been used increasingly in recent years to simulate crack initiation, propagation, and failure. The cohesive zone model allows multiple cracks to be modelled and the direction of crack propagation need not be known in advance however, cohesive zone elements need to be present at all possible crack paths. Cohesive zone models follow a traction-separation constitutive law to predict failure initiation, damage, and failure. Several shapes for the traction-separation law have been presented in the literature, with the bilinear, exponential, and trapezoidal shapes, as shown in Fig. 25.14, being the most commonly used for strength prediction. 

An improvement in the toughness of thermosets can be favored by rubber or thermoplastic particles, which operate both in crack initiation and propagation mechanisms. The different toughening mechanisms can act simultaneously and can be modeled quantitatively. 

Figure 27.19 shows the variation of U/A as a function of A, according to the proposed model of crack initiation and propagation energies. From the intercept of this plot, a fracture energy value of 4.7 kJ/m2 can be obtained for ABS. This value is much lower than that determined above, using the method developed for brittle fracture or the constant fracture energy consideration. 

These reactions are known in industrial processes and occur simply by decomposition and in gas phase. However, it is a nonelementary chain reaction and occurs in different elementary steps. For reactions such as thermal cracking in gas phase, there are several kinetic models, but the most interesting one is the model of Rice-Herzfeld. The main idea is that in the first step of the reaction, the abstraction of the H from the reactant molecule occurs and subsequently there is decomposition with formation of a new radical. The process takes place throngh the steps of initiation, propagation, and termination. 

Figure 15.18 shows crack speed measurements on a model joint pulled with a steady load. The crack initiated at the comer of the joint as expected. It then speeded up steadily, until it penetrated about one joint thickness along the interface. Then it travelled at constant speed in accord with Equation (15.5). From these results, the initiation force was found to be 1.39 times higher than the propagation load. This result fitted an approximate theory based on the idea that the strain energy in the shaded region of Fig. 15.18 ranained constant as the crack initiated. This gave the initiation force... 

By means of experimental data on welded pipes made from SAE 304 (1.4301) a model for intergranular stress corrosion cracking in high purity high temperature water was derived. Crack initiation- as well as crack propagation-phases were treated. 

SCC occurs when the mechanical stress is high enough. According to several SCC models (Sec. 5.5), the macroscopic stress level is not necessarily the mechanical parameter that controls the cracking process. However, from an engineering point of view, it is accepted that the stress corrosion cracks initiate when the stress exceeds a threshold Oj, and propagate when the stress intensity factor is in excess of a threshold i-scc-... 

In addition, this model is unable to predict the specificity of the SCe systems, since in many systems, periodic film breakdown by plastic strain or creep does not lead to any crack initiation or propagation. 

DFT, ab initio and model chemistry calculations have been performed on the chain initiation, propagation and termination reactions for the cracking of propane. From the methodological point of view, it was shown that the DFT BMK results of enthalpies of formation and energies of activation are very similar to the experimental values, with rms error of about 2 kcal/mol. For reactions involving... 

Crack propagation models are then presented for both short and long cracks. Anodic dissolution and hydrogen effects at the crack tip are analyzed. Finally, the possible relation between stress corrosion cracking and CF is shown for crack initiation and crack propagation processes. 

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