Fathi Aref Ibrahim Darwish

On the prediction of fatigue crack retardation using Wheeler and Willenborg models

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 DK 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.

On the Strengthening of Cement Mortar by Natural Fibers

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.

Virtual analysis of stresses in human teeth restored with esthetic posts

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.

On the Relationship Between J -Integral and Crack Tip Opening Displacement in Elastic-Plastic Fracture Mechanics

The relationship between J-integral (J) and crack tip opening displacement (δ), considered fundamental for elastic-plastic fracture mechanics, can be established based on prior knowledge of the constraint factor m, which depends on the work hardening exponent and the material’s yield strain. Both J and δ were simultaneously determined at fracture initiation and at different points along the resistance curves for a number of structural steels. The corresponding m values were calculated and then compared with the predictions made by different models. The results indicate that the experimentally determined m values are in fair agreement with the predictions made by ASTM over the whole range of flow parameters considered in this study. The Hutchinson-Rice-Rosengren singularity-based predictions result in overestimating m for steels considered to be of low strength and high strain hardening exponent. Predictions made by other models are predominantly higher in comparison with their experimental counterparts.

Stress prediction in a central incisor with intra-radicular restorations

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.

Fracture behavior of polymeric fiber reinforced lightweight structural concrete

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 JIC and JMAX 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 JIC.

Fatigue crack propagation in structural steel after flash welding

The study was carried out to evaluate the effect of the tempered martensite fraction in the fatigue behaviour of type R4 structural steel as used, quenched and tempered, in the manufacture of chains for offshore mooring systems. As the manufacturing process for these uses flash welding, the study also includes welded joints of the same steel. Different fractions of martensite were achieved by means of variations in the austenitization temperature of the material and the fatigue resistance of the tempered test pieces was evaluated by means of the kinetics of crack growth of crack under cyclic loading at a constant amplitude. The results indicated that an increase in the amount of ferrite was associated with an increase in the number of cycles to material failure, and for the same microstructural condition, the fatigue life of the welded joint was always lower than that of the base material. Finally, the fatigue behaviour of the test pieces is presented and discussed on the basis of the mechanical properties and toughness of the material.

Predicting Fatigue Crack Retardation Following Overload Cycles

Dating back to the beginning of the seventies, a number of models have been proposed to predict fatigue crack growth rate under variable amplitude loading. This effort was motivated by earlier observations that the application of an overload is followed by crack growth retardation over a crack length increment. The model of Willenborg, which belong to the group of yield zone models, incorporate interaction effects and is characterized by introducing crack tip plasticity. Although interaction models are generally considered to be applicable for high strength alloys with limited ductility, empirical verification of the predictions made by these models is rather limited. Accordingly, the present study was initiated in an effort to test the validity of the Willenborg model for predicting fatigue crack growth retardation in an R3 grade structural steel following the application of overload cycles. As this grade steel is used for fabricating offshore mooring chains, the study was extended to also include flash welded joints taken from the chain links.

Mechanical characterization of sisal reinforced cement mortar

This work aims at evaluating the 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 different lengths. Mechanical characterization of both the composite and the plain mortar was carried out using three point bend, compression, and impact tests. Specimens containing notches of different root radii were loaded in three point bending in an effort to determine the effect of the fibers on the fracture toughness of the material. The results obtained indicate that, 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.