Nenad Gubeljak

The use of ethylene/propylene copolymers as compatibilizers for recycled polyolefin blends

Compatibilizing effects of ethylene/propylene (EPR) diblock copolymers on the morphology and mechanical properties of immiscible blends produced from recycled low-density polyethylene (PE-LD) and high-density polyethylene (PE-HD) with 20 wt.-% of recycled poly(propylene) (PP) were investigated. Two different EPR block copolymers which differ in ethylene monomer unit content were applied to act as interfacial agents. The morphology of the studied blends was observed by scanning- (SEM) and transmission electron microscopy (TEM). It was found that both EPR copolymers were efficient in reducing the size of the dispersed phase and improving adhesion between PE and PP phases. Addition of 10 wt.-% of EPR caused the formation of the interfacial layer surrounding dispersed PP particles with the occurrence of PE-LD lamellae interpenetration into the layer. Tensile properties (elongation at yield, yield stress, elongation at break, Youngs modulus) and notched impact strength were measured as a function of blend composition and chemical structure of EPR. It was found that the EPR with a higher content of ethylene monomer units was a more efficient compatibilizer, especially for the modification of PE-LD/PP 80/20 blend. Notched impact strength and ductility were greatly improved due to the morphological changes and increased interfacial adhesion as a result of the EPR localization between the phases. No significant improvements of mechanical properties for recycled PE-HD/PP 80/20 blend were observed by the addition of selected block copolymers.

Comparison between Fracture Behaviour of Pipe-Line Ring Specimens and Standard Specimens

Thethin walled structures as pipe-line are often unsuitable for standard testingof fracture toughness. One possibility is applying non-standard modifiedspecimens with simple testing procedure, but measured fracture behaviour isconsequence of loading conditions and geometry of specimen. In this paper thedifferences in fracture behaviour of single edge notch bending (SENB) and ringpipe-line bended specimens are discussed. Especially uneven fatigue crack frontas consequence of complex fatigue loading caused different fracture behaviour,than standard single edge notch bending (SENB) specimens. The stress-strainconditions at the crack tip are analysed by finite element modelling. Thecritical crack tip opening displacement has been determined as a crack tipsurface strain-relaxation by using stereo-optical grading method. Comparisonbetween CTOD-R curves of both types of specimens shows difference in crackdriving force.

Why do some titanium-alloy total hip arthroplasty modular necks fail?

BACKGROUND Increased modularity in total hip arthroplasty (THA) through extra junction between the neck and the femoral stem is gaining popularity among orthopaedic community. However, the advantage of the additional junction is shadowed by an increased risk of mechanical failure. The aim of this study was to describe the exact mechanism of fracture of the modular femoral neck in an uncemented stem. METHODS Clinical, metallurgical, and mechanical analysis including finite-element modelling and elemental-sensitive tissue analysis with the micro-PIXE method was performed on two patients treated with fully modular primary THA made from Titanium alloy of the same oval taper-cone design. In patient A revision was performed 7.8 years after the unilateral primary procedure because of modular femoral neck fracture, while patient B was left-side revised 15 years after the bilateral primary procedure because of aseptic loosening of the femoral stem. RESULTS Body weight was 30% higher and the arm of implanted modular femoral neck was 51% longer in patient A compared to patient B. Therefore, the stress ratio on the modular femoral neck of patient A was calculated to be 2.45 times higher than in patient B, preventing cold welding and producing taper damage and degradation at the neck-stem junction. Large clusters of metallic debris containing Titanium and Vanadium from the alloy were present in the periprosthetic soft tissues of patient A. CONCLUSIONS Patients with higher body mass index treated with fully modular Ti-alloy THA may be at increased risk to experience catastrophic failure of the device. Orthopaedic surgeons should avoid using long necks whenever possible, as these are especially prone to develop a vicious circle starting with the fretting process and crevice corrosion at the taper-cone connection, leading to crack initiation and crack propagation, accelerated by the increased vulnerability of the Ti-alloy in biologic media, ultimately ending as fracture at the typical site. Serum Ti concentration may represent a rough estimation of taper degradation and patients with elevated levels should be warned and followed accordingly.

Effect of fracture path on the toughness of weld metal

The crack propagation direction may affect weld metal fracture behavior. This fracture behavior has been investigated using two sets of single edge notched bend (SENB) specimens; one with a crack propagating in the welding direction (B×2B) and the other with a crack propagating from the top in the root direction (B×B) of a welded joint. Two different weld metals were used, one with low and one with high toughness values. For Weld Metal A, two specimen types have been used (B×B and B×2B) both with deep cracks. The weld metal A (with high toughness values) has reasonably uniform properties between weld root and cap. The resulting J-R curves show little effect of the specimen type, are ductile to the extent that the toughness exceeds the maximum Jmax, value allowed by validity limits and testing is in the large –scale yielding regime. In the case of weld metal B (with low toughness values) with two specimen types (B×B and B×2B) the B×B specimen has shallow cracks while the B×2B specimen has deep cracks. Both resulting J-R curves show unstable behavior despite the fact that the types of specimen and their constraints are different. The analysis has shown that crack propagation direction is most influential for a weldment with low toughness in the small scale yielding regime, whereas its influence diminishes due to ductile tearing during stable crack growth and large scale yielding. The results have shown that these effects are different in both the crack initiation phase and during stable crack growth, indicating a dependence on weld metal toughness and the microstructure of the weld metal. It can be concluded that, if resistance curves during stable crack growth do not show differences in both notch orientations, the fracture toughness values of the whole weld metal can be treated as uniform.

Effect of Strength of Mismatch Interface on Crack Driving Force

Abstract. A welded joint is a critical part of a welded structure with respect to defects, pores, geometry, misalignments, and mechanical anisotropy. Differences in the mechanical properties appear between weld metals, heat affected zone, and base metal. Crack growth resistance curves, e.g., determined in terms of crack tip opening displacement (CTOD), exhibit variations in the crack growth resistance of the welded joint as a consequence of the local variations in the material properties. Theoretical investigations have shown that also the local near-tip crack driving force (CDF) may deviate strongly from the far-field CDF, if the material properties vary locally. The reason is that the material inhomogeneity induces an additional CDF-term, called the material inhomogeneity term, Cinh, which leads to a shielding or anti-shielding of the crack tip. A model developed by Simha et al. is applied to evaluate the magnitude of Cinh in a welded joint with a strength mis-match interface. Both a stationary and a propagating crack are considered.

Prediction of the maximum load of pre-cracked welded bars using Charpy data according to the default level of the European SINTAP procedure

A number of tests have been carried out on (component-like) welded bend bars in order to establish the probability distribution of critical loads. This information serves as reference solutions for evaluating the Default level of the European flaw assessment procedure, structural integrity assessment procedure, which is based on the yield strength and Charpy data as the only input information often available to the user. Despite the poor input information, the procedure has shown to yield conservative but suitable predictions of the critical load distribution in the ductile-to-brittle transition range.

Residual stress state in pipe cut ring specimens for fracture toughness testing

Abstract Thin-walled pipes are not suitable for measuring fracture toughness parameters of vital importance because longitudinal crack failure is the most common failure mode in pipes. This is due to the impossibility to manufacture standard specimens for measuring fracture toughness, such as SENB or CT specimens, from the thin wall of the pipe. Previous works noticed this problem, but until now, a good and convenient solution has not been found or developed. To overcome this problem, very good alternative solution was proposed, the so-called pipe ring notched bend specimen (PRNB) [1–5]. Until now, only the idealized geometry PRNB specimen is analyzed, i. e., a specimen which is not cut out from an actual pipe but produced from steel plate. Based on that, residual stresses are neglected along with the imperfections in geometry (elliptical and eccentricity). The aim of this research is to estimate the residual stress state(s) in real pipes used in the boiler industry produced by hot rolling technique. These types of pipes are delivered only in normalized condition, but not stress relieved. Therefore, there are residual stresses present due to the manufacturing technique, but also due to uneven cooling after the production process. Within this paper, residual stresses are estimated by three methods: the incremental hole drilling method (IHMD), X-ray diffraction (XRD) and the splitting method (SM). Knowing the residual stress state in the ring specimen, it is possible to assess their impact on fracture toughness measured on the corresponding PRNB specimen(s).

Effect of Residual Stresses and Inclusion Size on Fatigue Resistance of Parabolic Steel Springs

Abstract The effect of inclusion size and residual stresses of spring steel grade 51CrV4 on fatigue lifetime in high cycle was investigated by experimental results obtained by fatigue testing. Since spring with obvious surface defects or inclusions at the surface are failed during pre-stress, the residual stress profile under surface becomes relevant for fatigue crack initiation and failure. Investigation shows that fatigue threshold for high cycle fatigue depends, besides residual stresses, on inclusion size and material hardness. In order to determine allowed size of inclusions in spring steel and the influence of residual stresses, the Murakamis and Chapettis models and concepts have been used. The stress loading limit regarding inclusion size and applied stress has been determined for loading ratio R = −1 and 0.1 on specimens with and without residual stresses.

Determination of Stresses in the Axle Pin Based on the Wind Model in the Wind Turbine

Even when all the details are provided, the design of wind turbines is a big challenge. The detailed knowledge of the deformation of vital wind turbine components caused by a wind with extreme operating gusts and other type of winds can be obtained only by performing measurements on the wind turbine. The vital component of the wind turbine is axle pin. This article presents the results of the measured changes in wind speed at the height at which strains were measured. The measured strains are used for calculating the stress. It is also shown that on the basis of the calculations on the wind model obtained with the GH Bladed software, the input data are obtained for the SolidWorks software, by which stress on the axle pin can be calculated. The final confirmation of the proper approach and the calculation is achieved by comparison between the calculated and the measured strains on the axle pin during exploitation of a wind turbine of nominal power output of 1 MW.

The Influence of the Weld Width on Fracture Behaviour of the Heterogeneous Welded Joint

Since welded constructions are widely used in engineering, a certain flaws in welded joints may occur either in process of welding or in exploitation period. Easiest way to prolong working life of such welded construction is to repair welded joint to eliminate possibility of construction failure. Process of repair welding usually gives heterogeneous welded joints because during process of repair additional material is introduced into welded joint, resulting in heterogeneity from the presence of materials in welded joint point of view. Such difference in materials usually results in yield strength difference between materials, represented with mismatch ratio, and it is commonly present in welds where high strength low-alloyed (HSLA) steels were welded. Since I butt welded joints are very common in welding, a systematic investigation of such welds is performed and presented in this paper. Therefore in this investigation the influence of present material in heterogeneous weld and geometry of weld is investigated in context of fracture resistance of welded joint represented as yield load solutions in the first place. A flaw in form of crack was implemented in such heterogeneous weld and using finite element method yield load solutions for different combinations of weld geometry and material strength are obtained and presented in form of diagrams.