A study has been made of the influence of a superimposed mean shear stress on the capability of some multiaxial high cycle fatigue criteria to predicting fatigue behavior of 42CrMo4 and 34Cr4 alloy steels. Five selected critical plane-based criteria, namely Matake (M), Susmel & Lazzarin (S&L), Findley (F), Carpinteri & Spagnoli (C&S) and Liu & Mahadevan (L&M), were applied to a number of published experimental fatigue resistance limit tests, involving synchronous sinusoidal in-phase and out-of-phase bending and torsion. Applying to the same loading conditions a mesoscopic scale-based criterion proposed by Papadopoulos (P), one could verify that predictive capability of such an approach is almost invariably superior to those associated with the M, S&L, F, C&S and L&M models. As the Papadopoulos criterion is independent of mean shear stress, it seems appropriate to conclude that the inclusion of such a stress as loading parameter in the critical plane-based models does, in fact, exert a negative influence on their predictive capability. Finally, it is worth mentioning that, except for the Matake, S&L and L&M criteria, the other critical plane-based criteria exhibit a dependence of the fatigue resistance in pure torsion with respect to a superimposed mean shear stress, in disagreement with well-established experimental observations.

The purpose of this work is to evaluate mechanical behavior of sisal fiber reinforced cement mortar. The composite material was produced from a mixture of sand, cement and water. Sisal fibers were added to the mixture in two different lengths. Mechanical characterization of the composite and the plain mortar was carried out using three point bend, compression and impact tests. Specimens containing parallel sided notches of different root radii were loaded in three point bending in order to determine the effect of the fibers on the material fracture toughness in the presence of discontinuities. According to the results, while fiber reinforcement leads to a decrease in compressive strength, J-integral calculations at maximum load for the different notch root radii have indicated, particularly for the case of long fibers, a significant superiority of the reinforced material in comparison with the plain cement mortar, in consistence with the impact test data.

A study has been made of the effect of short randomly dispersed polypropylene fibers on the fracture behavior of lightweight structural concrete. Using unnotched and precracked beams subjected to bend loading, it was possible to determine the toughness factor TF and to estimate specific J integral parameters, namely J IC and J MAX corresponding to the onset of fracture initiation and to the attainment of ultimate load, respectively. The results have indicated a considerable improvement in the fracture behavior associated with the presence of fibers. More specifically this improvement was manifested by a 400% increase in both TF and J IC.

A 2D finite element analysis was applied to calculate shear and von Mises equivalent stresses developed, under masticatory loading, in an upper central incisor restored with cast gold post and carbon fiber reinforced epoxy resin post. Based on the predicted shear stress levels, it is concluded that the gold post model is more prone to shear failure along the post-dentin interface. Whereas shear stress concentration also occurs in both models at the core-crown interface, the stress level predicted there is higher for the carbon fiber post model which would be more susceptible to crown displacement. Finally, the prediction of von Mises equivalent stress indicates a non-uniform distribution, with the stress preferentially concentrated in the gold post along its interface with the tooth dentin. For the carbon fiber post restored model, on the other hand, the von Mises stresses are more uniformly distributed achieving its maximum level in the tooth dentin.

The use of intra-radicular posts for rebuilding of damaged teeth is a normal practice in contemporary dentistry. However, dental roots restored with posts are subjected to the risk of failure under occlusal loads, particularly in cases of small dentin thickness. This study adopted the finite element analysis to compare the elastic stress distribution in simulated endodontically treated maxillary central incisor restored with two different esthetic posts, a ceramic post and a prefabricated fiber glass post. Under masticatory load, the shear stress and von Mises equivalent stress were determined for the different regions of the two models. The results demonstrated that stress concentrations occurred mainly in the cervical dentin in the prefabricated fiber glass post model. The ceramic post model presented stress concentration in a region limited to the proper post adjacent to its apical end, thus preserving the root dentin.

The aim of this work is to evaluate the applicability of the Wheeler and Willenborg models to predicting fatigue crack growth retardation in a flash welded structural steel subjected to a single overload during constant amplitude (CA) fatigue crack propagation test. Compact tension specimens, in different microstructural conditions, were subjected to a single overload at a given crack length during CA fatigue loading and crack growth rate da/dN vs. the stress intensity factor range deltaK was monitored, evidencing the retardation in crack propagation over an interval of crack length. The size of the delay zone as well as the number of the delay cycles were predicted by both the Wheeler and Willenborg models and then compared with the experimental data. Finally, the results are presented and discussed focusing on the comparison between the predictions made by the two models in the light of the experimental data.