Radu Negru

• PhD
• Assoc. Professor at Polytechnic University of Timişoara

In this work, the variation of the stiffness and strength with the weave pattern orientation of a twill carbon fibre weave pre-impregnated system was investigated. Singe-ply sheets were manufactured using vacuum-assisted curing at a temperature of 120 °C for 8 hours. Specimens were cut at 0°, 15°, 30°, 45°, 60°, 75° and 90° respectively, and subjec...

In this article, the yielding and plastic flow of a rapid-prototyped ABS compound was investigated for various plane stress states. The experimental procedures consisted of multiaxial tests performed on an Arcan device on specimens manufactured through photopolymerization. Numerical analyses were employed in order to determine the yield points for...

In this paper, the mechanical behavior of hyperelastic foams used in athletic footwear soles is investigated for compression and tensile loadings. Specimens were cut using profiled tools from aged footwear manufactured by five different companies. Based on the experimental results, the modified Ogden hyperelastic strain-energy density function was...

This work investigates the mechanical behaviour of woven carbon composites using a multi-scale approach. With the commercial software Digimat, the woven structure was developed based on microscopic observations (yarn pattern and density). Starting from the geometry, a periodic structure was generated and the material constants were determined using...

Polycarbonate thermoplastic materials have found their usage in many automotive interior type products (e.g. cluster instruments, Head-Up displays etc.), due to their visual aspects offered to the products and to the good mechanical properties possessed, especially in forms of short fiber reinforced composites. This paper presents the tensile behav...

In this work, the mechanical behaviour of composite materials consisting of fly ash-based geopolymer reinforced with wood fibres is investigated for compressive and flexural loadings. The gathered test data were used to calibrate constitutive models for the geopolymer, consisting of the exponential Drucker–Prager yield criterion coupled with the co...

This paper investigates the failure strain as a dependence of the stress triaxiality and the Lode angle parameter for polyurethane rigid foams (PUR) of two densities (100 and 300kg/m3). Tests were carried out in tension for various configurations, resulting in different states of stress triaxiality at various Lode angles in the critical areas. The...

Composite resin materials for posterior restorations are becoming more and more popular. According to the market, Ormocer materials are more expensive than composite resin this paper is comparing the resistance to fracture between nanohybrid composites and Ormocers under vertical forces to evaluate their cost efficiency in the dental cabinets. 20 t...

In this article, the response of lightweight cellular structures with open cells is investigated through finite element analysis and mechanical testing in compression. The structures consist of tessellations of Kelvin cells (tetrakaidecahedron), which were designed having three parameters that determine their relative density: strut thickness, stru...

This work aims to characterize mechanical properties (strength and fracture toughness) of an alumina-based ceramic foam exhibiting a pore volume fraction around 88% with a multi-scale porosity ranging from 300 to < 2 µm with a slight interconnectivity of cells. Young’s modulus determined by three different methods (resonance, ultrasonic and compres...

The paper presented a series of experiments to improve the performance of rapid prototyping processes. The paper presents aspects regarding the realization of polymeric parts by PolyJet rapid prototyping technology. Achieving oversized parts on printers whose print size is small is subject to additional demand for the designer and the technology en...

In this work, a numerical simulation of the behavior of the magnesium alloy AM50 in tension and torsion is performed. Preliminary experimental procedures were carried out in order to determine the material characteristics and the stress–strain response. Based on the experimental data, three material models were developed: linear elastic with isotro...

Flash-Free Orthodontic Adhesive System represents the latest technology in the field of orthodontics. In the present study the bonding technique is applied on CAD/CAM produced resin crowns. Ten resin blocks were shaped after an upper premolar crown design from the 3Shape Library, and they were divided in two groups: on Group 1, control, Clarity Adv...

The latest technology in the field of orthodontic materials is represented by the Flash-Free Orthodontic Adhesive System. In the present study this novel technique is compared with the conventional direct bonding method. Ten extracted human premolars were separated into two groups: in the Group 1, APC Flash-Free Adhesive Coated Appliance System (3M...

This study investigates the elastic response of a tessellated structure based on cylindrical struts generated along the wireframe of a truncated octahedron (the Kelvin cell). The structure has two variable dimensions (strut thickness and strut length), the relative density being plotted as a function of the given parameters and ranged between 6.7 1...

This paper investigates the compressive and flexural behavior of a closed-cell fly ash-based geopolymer foam with a density of 240 kg/m³. The influence of loading speed (0.1, 10, 100, 250 and 500 mm/min) and anisotropy (in-plane and out-of-plane directions) on the compressive mechanical properties at room temperature were investigated. Under compre...

In this work, a two-dimensional approach is considered in the evaluation of the elastic properties of an alumina foam exhibiting a porosity of around 87%. Preliminary experimental procedures were performed for the determination of the Young's modulus. The stiffness was evaluated with the help of microstructural models consisting of regular structur...

Foam materials have gained a lot of attention in recent years due to their excellent energy absorption capabilities. This paper investigates the compressive and flexural behaviour of a closed-cell fly ash-based ceramic foam with a density of 240 kg/m 3. The influence of loading velocity (0.1, 10, 100, 250 and 500 mm/min) and anisotropy (in-plane an...

Notch effect in porous polyurethane rigid (PUR) materials is analyzed using a ring shaped specimen containing a single radially U-notch of different tip radii. If the specimen is subjected to a diametrically compressive load, various mixed mode I/II conditions are acquired depending on the notch orientation angle. A comparison between our experimen...

The use of magnesium alloys in the automotive industry increased in the last decade because of their low weight and relative good mechanical properties. However, the variable loading conditions require a good fatigue behavior. This paper summaries the fatigue properties of magnesium alloys and presents new fatigue curve results for die cast AM50 ma...

The aim of this in vitro study was to assess a new design that, to our knowledge, we have introduced (patent pending) for indirect dental veneers. Their effect on the retention and adhesive properties at their interfaces have been studied. Fourteen high performance polymeric (PEEK) dental veneers have been elaborated using a computer-aided-design (...

The Averaged Strain Energy Density (ASED) criteria is applied herein to reinterpret the fracture data of PUR foams. Four type of specimens were used in fracture tests. The ASED parameters were determined based on micromechanical models. The volume control for cracked components is represented by a circle with the centre at the crack tip for all typ...

In this paper, the mechanical behavior in three-point bending was investigated for four different sandwich beams composed of aluminum alloy 1050 H24 faces and Necuron rigid polyurethane foams. The tests were conducted according to the recommendations of ASTM C393-00. Three different failure modes were observed (yielding of faces, face and core inde...

Implants are one of the most innovative and less invasive prosthetic procedures available. Almost all types of edentations can, nowadays, be treated with an implant-retained prosthetic piece, either fixed or removable. But regardless of the development in the industry over the last decades and the high success rate-estimated at over 95%-there are s...

Local Strain Energy Density represents an engineering approach for assessing the brittle fracture of cracked and notched components. Experimental determination of fracture parameters (critical value of deformation energy Wc in a local finite volume around the notch tip and the radius of the control volume Rc) represents a key issue. The paper prese...

Cellular plastics are light weight structures with many applications in civil, aeronautical, automotive and mechanical engineering. Properties of cellular materials depend on the properties of the solid material, on the shape and dimensions of the cellular structure and on the relative density of the cellular material. Most of cellular plastic mate...

Two types of adhesively bonded joints were experimental and numerical investigated. Firstly, the adhesives were characterized through a set of tests and the main elastic and mechanical properties were obtained. After that, the stress distributions at interface and middle of adhesive layer were determined using a linear elastic FEA. The numerical da...

This paper assesses three methods (Theory of Critical Distances, Volumetric method and Strain Energy Density) for the lifetime prediction of notched components in medium-cycle fatigue. The predictions are compared with experimental fatigue data, obtained by authors by testing aluminium 2024-T3 specimens with semi-circular and two types of V-notches...

Local strain energy density and theory of critical distances represent two recent approaches in assessing the brittle fracture of cracked and notched components. Experimental validation of these methods was mainly conducted on materials with brittle behaviour (ceramics and PMMA). In this paper the applicability of these methods to another class of...

Sealant materials are typically employed in dentistry in order to prevent the development of cavities on the teeth. They prevent bacterial adhesion to enamel, thus arresting the development of demineralization and of caries. In this study, the critical zone of the interface between different sealant materials and the results of the dental work for...

This paper investigates the fracture properties and notch effect of PUR materials with four different densities. The asymmetric semi-circular bend specimen was adapted to perform mixed mode fracture toughness tests. This semi-circular specimen with radius R, which contains an edge crack of length a oriented normal to the specimen edge, loaded with...

Introduction: The ceramic materials defects are often located in the proximal or connectors areas of the ceramic fixed partial dental prosthesis. Objectives: The present study compares the performances of three imagistic evaluation methods to identify and to locate the ceramic defects. Methods: 37 metal ceramic fixed partial prosthesis were obt...

The paper is focused on failure assessment of the shaft of a pumped storage unit. The shaft failure occurred during normal operation of the hydropower unit after 35 years of service. Three different methods are applied in order to assess its lifetime. Firstly, a finite element analysis was performed in order to obtain the static stress distribution...

Using the asymmetric semi-circular bend specimen (ASCB) a set of mixed-mode fracture tests were carried out in the full range from pure mode I to pure mode II. The tests were conducted on two polyurethane materials characterized by different properties. The fracture parameters were obtained from experiments and are compared with the predictions bas...

The key ingredient for any turbine runners design procedure and failure analysis is the material and its mechanical and fatigue properties. Starting from this observation, a series of static and dynamic tests were performed on the T10CuNiCr180 stainless steel after more than three decades of service. The fatigue tests were carried out under constan...

This paper presents the numerical determination of stress intensity factor for a 3D cracked T – joint using FRANC 3D software. The analysis preformed were linked between the finite element analysis using Abaqus software to obtain the stress state used in Franc 3D for stress intensity factor determination in crack resence in structure. For many appl...

Using the asymmetric semi-circular bend specimen (ASCB) with vertical crack, a series of mixed mode I/II fracture tests were performed on two different rigid polyurethanes materials. The experimental results obtained for brittle fracture are compared with the theoretical predictions based on MTS and SED criteria. The results demonstrate that the SE...

Shafts represent an important component of hydraulic units, like pumps and turbines. Most of the hydraulic equipments in Romania are operating from more than 30 years. Consequently, failure occurs in some shafts due to fatigue initiated in the stress concentration area. This paper presents a pump shaft failure case and its fatigue life estimation u...

In this paper the hydraulic stresses induced in a Francis turbine runner blade by steady fluid flow were investigated. Based on the one-way coupled simulation, the approach consists of a fluid flow analysis which provides the distribution of the fluid pressure on the blade, followed by the structural finite element analysis. The three dimensional t...

Dental onlays are restorations used to repair rear teeth that have a mild to moderate amount of decay. They can also be used to restore teeth that are cracked or fractured if the damage is not severe enough to require a dental crown. The use of onlays requires less tooth reduction than does the use of metal fillings. This allows dentists to conserv...

Imagistic investigation of the metal-ceramic crowns and fixed partial prostheses represent a very important issue in nowadays dentistry. At this time, in dental office, it is difficult or even impossible to evaluate a metal ceramic crown or bridge before setting it in the oral cavity. The possibilities of ceramic fractures are due to small fracture...

The aim of this paper is the application of two methods for notch fatigue life assessment, methods which are based on finite element analysis: the theory of critical distances and the volumetric method. Firstly, un-notched and notched specimens (for three different geometries) were tested in tension under constant-amplitude loading. The use of theo...

The influence of impregnation on mechanical properties of polyurethane foams at room temperature has been experimentally evaluated in both static and impact loading conditions. The impregnation layer increases the flexural modulus and changes the foam behaviour at impact. For un-impregnated foam the energy absorbed to fracture is equal to the impac...

Imagistic investigation of the metal-ceramic crowns and fixed partial prostheses represent a very important issue in nowadays dentistry. At this time, in dental office, it is difficult or even impossible to evaluate a metal ceramic crown or bridge before setting it in the oral cavity. The possibilities of ceramic fractures are due to small fracture...

The aim of this study was to analyze the interface between the amalgam and composite resin restorations using a noninvasive method like the optical coherence tomography working in Time Domain. The conclusions point out the importance of investigation the integrity of every interface after every direct restoration into the oral cavity.

The performance of engineering components and structures is strongly influenced by the interaction between design, manufacture and materials. A methodology to assess the integrity of a circular collector is presented by investigating the failure of copper lamellas. The investigated circular collector, having 315 copper lamellas, is one of the main...

Fixed partial prostheses as integral ceramic, polymers, metal-ceramic or metal-polymers bridges are mainly used in the frontal part of the dental arch (especially the integral bridges). They have to satisfy high stress as well as esthetic requirements. The masticatory stress may induce fractures of the bridges. These may be triggered by initial mat...

The aim of this study was to analyze the integrity of the ceramic veneers using a noninvasive method like the optical coherence tomography working in Time Domain. The obtained results were validated by Micro Computer Tomography. The conclusions point out the importance of investigation the integrity of every veneer before bonding in into the oral c...

p>Imagistic investigation of the metal-ceramic crowns and fixed partial prostheses represent a very important issue in nowadays dentistry. At this time, in dental office, it is difficult or even impossible to evaluate a metal ceramic crown or bridge before setting it in the oral cavity. The possibilities of ceramic fractures are due to small fractu...

Introduction. For the evaluation of marginal adaptation in class V composite restorations, several methods have been developed (bacterial penetration, fluid transport, clarification, penetration of radioisotopes, electrochemical methods and gas chromatography). Dye penetration tests (microleakage tests) however, are to be the most widely used due t...

The ceramic veneers are often use in order to increase the esthetic aspect of the frontal teeth when discolorations or old fillings are involved. But some of those ceramic veneers are fractured in a short period of time after they were bonded into the oral cavity. The aim of this study was to analyze the integrity of the ceramic veneers using a non...

A comparison between analytical and experimental results regarding the fracture strength of components with circular holes is presented. Two fracture criteria are assessed: teory of critical distance (TCD)) and a three parameters criterion. However TCD still remains a useful tool for engineering applications.

Holography offer new nondestructive possibilities for bridging the gap between in vitro and in vivo measurements in dentistry, and thus increase the possibility of achieving more accurate and sometimes more objective diagnosis and therapy. The use of stone and plaster study models is an integral part of any dental practice and is required for resea...

Optical Coherence Tomography (OCT) allows a better characterization of dental prostheses. The detection of substance defects within the ceramic layers for metal-ceramic prostheses was demonstrated. The detected defects have a large volume and therefore there is a high likelihood for fracture lines to be generated in the proximal areas of the cerami...

The performance of engineering components and structures is strongly influenced by the interaction between design, manufacture and materials. This paper presents the fracture of copper lamellas from a circular collector. The investigated circular collector, having 315 copper lamellas, is one of the main parts of an electric motor. The fracture of t...

The amount of available alveolar ridge is an important factor in planning to replace a tooth with an implant. Inadequate bone height and width may compromise the implant placement. Therefore, augmenting ridge defects is essential in the proper installation of the implant. In case an implant is placed in a defective ridge, additional augmentation wi...

The quality of the evaluation of implant insertion could be investigated by implant bone interface analysis. In this study the numerical simulation, tensional stamps and optical coherence tomography as a noninvasive method were used in order to evaluate these interfaces. The system contains two interferometers and one scanner. For each incident ana...

Interface damage characterization and interlaminar failure of sandwich specimens with two initial interlaminar defects (inserts) is done by using the digital image correlation method. Mode I tests reveal interesting particularities on damage localization and unstable crack propagation. After analyzing the experimentally obtained results, we propose...

Polyurethane foam materials are widely used as cores in sandwich composites, for packing and cushioning. The main characteristics of foams are light weight, high porosity, high crushability and good energy absorption capacity. The paper presents the experimental results obtained for the mechanical properties of polyurethane foams in different loadi...

Polyurethane foam materials are widely used as cores in sandwich composites, for packing and cushioning. This paper presents the experimental results obtained for the mechanical properties of polyurethane foams in different 3 loading conditions and the influence of impregnation on the mechanical properties. A 200 kg/m(3) density polyurethane foam w...

The complete dentures are currently made using different technologies. In order to avoid deficiencies of the prostheses made using the classical technique, several alternative procedures have been devised. In order to enhance the mechanical strength, complete denture bases are reinforced with fibres. Their material and structure vary wildly, which...

Scanning, also called digitizing, is the process of gathering the requisite data from an object. Many different technologies are used to collect three dimensional data. They range from mechanical and very slow, to radiation-based and highly-automated. Each technology has its advantages and disadvantages, and their applications and specifications ov...

A corner joint was investigated using finite element met hod. Since the geometry along the width of the corner joint is uniform and load does not change in that direction, the problem was assumed to be plane strain. In addition, it was assumed that the adherents (a luminum) and adhesive (epoxy resin) had linear properties. The joint was analysed un...

Digital image correlation and three-dimensional finite element investigations are done for analyzing strain concentration for a semicircular notch and two V-notches with different radii. Along the notch bisector normalized true opening strains are represented as a function of the normalized distance from the notch root. In the linear elastic domain...