فهرست مطالب یونس علیزاده

In this paper, the effect of notch depth on the J-integral in the plates made of Bainitic functionally graded and weakened by U-notch under mode I loading was investigated. Functionally graded steels (FGSs) consist of ferrite (α), austenite (M), bainite (β) and martensite (M) co-existing in different configurations and produced via electro slag remelting (ESR). In the present work, a Bainitic functionally graded steel in the form of divider configuration was made and the J-integral was investigated. Moreover, the effect of notch depth on the critical load and critical J-integral was investigated. The results show a good agreement between the Finite element results taken from commercial ABAQUS software with the experimental ones. The results show that the critical fracture load decreases by increasing the notch depth. Moreover, the critical J-integral increases by increasing the notch depth.

Molecular dynamics simulations are used to study the mechanical properties of single-walled carbon nanotube reinforced polyvinyl pyrrolidone matrix. The effects of nanotube diameter and chirality on the elastic moduli of carbon nanotube reinforced nanocomposites are studied. It is shown that zigzag nanotube reinforced polymers have higher longitudinal elastic modulus than their armchair counterparts. For example, embedding (5,5) and (9,0) SWCNTs whose diameters are close to eachother in polymer matrix lead to the elastic modulus of 78.43 and 81.55 GPa, respectively. Besides, increasing diameter results in decreasing longitudinal Young’s modulus. Because of disability of the existing finite element approaches to study the behavior of polymers containing atom types other than carbon, based on molecular dynamics simulations, a finite element method is proposed. The results of the proposed method are in good agreement with the results of molecular dynamics simulations. The correlation coefficient of diameter and Young's modulus obtained from molecular dynamics simulations is equal to -0.8375. Moreover, the correlation coefficient of diameter and Young's modulus computed by finite element method is obtained as -0.8781.

In this paper, fracture behavior of functionally graded material weakened by U-notches under mode I loading has been investigated. Electro slag remelting process has been used to produce functionally graded specimens in a notch arrester configuration. Hardness test has been utilized to define the position of each layer. Mechanical properties, including elastic modulus and poisson's ratio, vary along the width of U-notched specimens. The critical fracture load (Fcr) was achieved by performing three point bending examination and using force-displacement curve.
Then, the process simulation was done by finite element software. Firstly, Jcr of each specimen was calculated by using critical value of strain energy averaged over a well-defined control volume. Then the critical fracture load was evaluated by means of the J Integral criterion. In this research, the effect of the notch root radius, for a fixed notch depth, on the Jcr value as well as the critical fracture load has been studied. To compare fracture behavior of the FG specimens with the corresponding homogeneous ones, have the same properties with the notch root layer in FG specimen, Fcr and Jcr value of each have been investigated. The average difference between the predicted Fcr by J integral criteria and experimental results is 17.84%. Finally, the effect of number of graded area layers on Jcr and Fcr has been investigated. The results shows, the value of Jcr and Fcr do not be affected while the number the number of layers more than 20.

Hot pressing of metal powders is used in production of parts for similar properties to wrought materials. Since residual porosity, inhomogeneous properties, dimensional and geometrical instability and therefore reduction in mechanical strength are the main problems in powder metallurgy components which are due to friction between powder particle interfaces and powder compact and die walls. Because of this, access to parts with high density and homogeneous structure is the great object in powder metallurgy. One of the remedies can be employment of ultrasonic vibrations which is thought to result in increased rates of densification and therefore higher efficiency of the process in increase of part density and strength. To evaluate this solution, this paper deals with the effects of high power longitudinal ultrasonic vibrations on the densification of AA1100 aluminum and Ti-6Al-4V titanium alloy powder under constant applied stress and different temperatures. The effects of powder type and process temperature on the densification behavior and ultrasonic efficiency are discussed. For experimental tests, setup of ultrasonic assisted hot pressing of powders were designed and fabricated. The results show that applying ultrasonic vibrations leads to obtaining higher relative density. In addition, it is found that the effect of ultrasonic vibrations is greater for Ti-6Al-4V powders. However, the temperature has different effects on the ultrasonic vibrations efficiency in two types of powders.

Metal forming is a conventional manufacturing process that a material with simple form is subjected to plastic deformation and emerged to industrial end products. Reduction of forming forces and improving of products quality have been a promising object for investigators and artisans. To accede this purpose, primary methods such as increasing material temperature and modern methods such as use of high power low amplitude ultrasonic vibrations were introduced. In ultrasonic assisted forming, high power ultrasonic transducer produces low amplitude high frequency mechanical vibrations which transmitted to material subjected to deformation and contacting surfaces of tool/workpiece. Results show reduction of forming forces and tool wear as well as improving surface integrity and dimensional stability that lead to increasing production rate and process efficiency. By regarding to importance and capability of ultrasonic assisted metal forming, this paper is concerned with application of ultrasonic vibration on metal forming processes. To this purpose, fundamental and mechanisms of application of high power ultrasonic were introduces and discussed. Also, industrial future of this technology as well as its advantages, range of application and its restriction were mentioned.

Nowadays, railway has been identified as the safest and most proper transportation system. Failure of rails, as one of the most importance elements of railway systems, can cause accidents and affect its safety. In this study, 55 steel samples extracted from UIC60 rail have been tested by Charpy impact test equipment in different temperatures ranged between -40 and 60 C. Then, fracture toughness of rail in different temperatures has been calculated by using known experimental relation between fracture toughness and measured fracture energy. Finally, the rail critical crack length has been determined by replacing calculated fracture toughness in formula of stress intensity factor. In order to restrict rail fracture and increase service life of the rail, critical crack length, in addition to none destructive tests for maintenance process of rails

In this paper, the mean value of strain energy density criterion has been used to predict the critical fracture load in specimens with U-shaped notch under bending loads. An equation for calculation of averaged strain energy density in this zone has been presented. Bending loads under mode I have been considered. Two cases have been studied: 1) the control volume only includes the semicircular arc of the notch root, and 2) the control volume includes rectilinear edge of the notch. The effect of notch root radius and notch depth on the mean value of strain energy density has been studied in two cases above and the critical fracture load rate has been predicted. Finite Element Analysis has been carried out for verification. The studies show that the critical fracture load increases and then decreases by increasing the notch root radius. Also, this parameter decreases exponentially by increasing the notch depth.

Many factors influence the tensile strength of the tendon in a tendon-bone tunnel complex. The purpose of this study is to determine the influence of tendon length & time of implantation of the tendon in the bone tunnel on the tensile strength of the tendon in a rabbit model. In this prospective study fourteen adult(age 6 months) White New Zealand rabbits(weight: 2.5-3 kg) underwent bilateral hindlimb(n=24) surgery in which the Extensor digitorum longus tendon was implanted into the tibial metaphyseal extra-articular bone tunnel. Based on the length of bone tunnel(1 or 2 cm) and duration of implantation(6 or 12 wks), the rabbits were divided into 4 groups. Seven rabbits were sacrificed after 6 weeks(groups 1 and 2) and 7 others were sacrificed after 12 weeks(groups 3 and 4) for tendon tensile loading test to determine ulitimate pull-out strenght. In group one(1 cm, 6wks) one tendon was ruptured and four were pulled out, in group two(2 cm, 6 wks) one tendon ruptured and six pulled-out, in group three(1cm, 12wks) one tendon was pulled out and five ruptured and in group four(2 cm, 12 wks) five tendons ruptured and one was pulled-out. The mean failure force after 6 wks. was 4.98N for group one and 9.37N for group two. After 12 wks. it was 6.36N for group three and 10.81N for group four. Failure force difference was significant between 1 and 2 cm tendon lengths after 12 wks. Tendon rupture site was significantly different after 6 & 12 wks. This study demonstrated that the tendon length within a bone tunnel influences tendon tensile strength after 12 weeks. In other words, increase in tendon length within the bone tunnel, increases it's resistance against tensile strenght with the passage of time. Time of implantation has an additive effect to tendon length on the tendon tensile strength of the tendon-bone tunnel complex.