Volume 60, Issue 5

Two structural steels were cold deformed and tested via tensile tests to investigate the dependence of cold deformation on liquid metal embrittlement during hot-dipping. It was found that the reduction in area of tensile specimens with a low degree of deformation showed, when tested in liquid zinc, the highest degradation compared with a tensile test in air that had been heated to 450 °C. An increasing degree of deformation leads to a decreasing tendency to form brittle cracks. The material conditions of steel S355J2 with the highest degree of deformation manifested nearly no influence on liquid metal embrittlement. For steel S460N, the amenability to a reduction in area of the highly deformed material was degraded when compared to tests in hot air. The reason for these differences in the extent of liquid metal embrittlement during hot-dipping might have to do with a difference in microstructure, in particular with respect to the grain size and the deformation of the microstructure.

Metal foam is a high-porosity engineering material which has many outstanding properties such as lightness, high specific strength and stiffness, large energy absorption during impact and good thermal transportation. Impregnation of metal foams with polymers produces new types of composites such as interpenetrating phase composites (IPCs) and co-continuous composites, due to an interconnection on the macroscopic level of individual phases as a co-continuous 3-D network. The coexistence of the metal and polymer phases allows each to contribute its prominent properties to the composite. This novel composite material is a potential candidate for applications in the automotive and aerospace industries. The present study aims to develop two IPCs from open-cell Al foams of 20 ppi impregnated with silicone or epoxy resin. The compressive behavior and energy absorption characteristics of IPCs are also examined and compared. The results showed that although both IPCs have a similar foam structure with similar density, the disparities in the properties of impregnated polymers lead to distinct mechanical properties. The combination of the Al foam and polymers, both silicone and epoxy resin, yield stiffer IPCs than either of the two individual materials alone. Higher stiffness was found in IPCs with epoxy resin, owing to the brittle nature of the resin. Energy absorption capacity was also increased when compared with the original Al foam.

Active thermography via electromagnetic excitation is a non-destructive evaluation method with a wide range of applications. It is a quick and reliable means for detecting inhomogeneities like cracks at or close to the surface of conductive components utilizing infrared imaging. Electric current can be used in two ways for thermography: In induction thermography, a current is coupled to the component without contact by passing an alternating current through a coil which is in close proximity to the component inspected; in conduction thermography, the current is coupled directly with the component. In this paper, we present a review of the basics of this non-destructive evaluation method along with several component examples and examples of inspection systems. In particular, automated testing is regarded. Additionally, sufficient detectability of cracks under low and high residual stress is discussed and compared with other surface testing methods.

Fatigue is one of the most significant causes of spring damage. Since fatigue tests play a crucial role in predicting the lifespan of springs, many special test device and test techniques have been describ ed in the literature. In this paper, the significant design and manufacturing steps of a newly developed eccentric spring fatigue testing device are introduced. This device is particularly suitable to perform fatigue life tests on high rigidity springs, such as mold springs. This low-cost device can be used on springs at many different load ratings. In this study, the fatigue process of metal springs and a case study regarding the development and manufacturing process of a new fatigue testing device are described. Furthermore, the fatigue test procedure on metal springs and some test results generated by the spring fatigue test device are also presented.

This study investigates the effects of process parameters on the quality of products fabricated by fused deposition modeling (FDM), such as surface roughness and tensile strength. Polylactic acid (PLA) samples were built on a FDM machine at various layer thicknesses, nozzle temperature and deposition head velocity. The effect of cooling the samples during the process was also considered. The experimental study was performed according to a mixed type Taguchi L16 orthogonal array. The effectiveness of each parameter was also discussed by an analysis of variance (ANOVA). The tensile strength results were compatible with the optical images of the fracture surfaces while the surface roughness results were compatible with the surface topography of the parts along the thickness. The two dominant quality characteristics were found to be layer thickness and deposition head velocity. Lower layer thickness values yielded higher tensile strength and lower surface roughness. Use of a cooling fan and nozzle temperature were found to be the least effective parameters. Finally, the results indicated that tensile strength and surface quality of the FDM samples improved about 25 %, and 12 %, respectively at optimal process conditions.

AA6XXX aluminum alloys are widely used in the automotive and aeronautic industries and for food packaging thanks to their lightness, specific strength, and corrosion resistance. The mechanical properties of AA6XXX alloy increase through an age hardening process thanks to the formation of Mg 2 Si particulates. These particulates enhance mechanical strength and likewise change corrosion resistance. In this study, the corrosion behavior of AA6063-T4 alloy in acidic and alkaline media was investigated and the results were compared with AA6063-T651 alloy. Acidic media was prepared in a 0.1M HCl solution; alkaline media was prepared in a 0.1M NaOH solution. Corrosion tests were performed for 360 hours in both solutions. Corrosion properties were determined by measuring weight loss and surface roughness values of the samples and pH values of the solutions. In both environments, artificially aged samples were found to have higher corrosion rates. There was a direct relationship between corrosion loss and surface roughness values.

Kurzfassung Die Resonante Frequenzsweep-Thermografie (RFST) basiert auf dem Prinzip der lokalen Defektresonanz (LDR) in Verbindung mit der bekannten Ultraschall-angeregten Thermografie. Lokale Defektresonanzen treten immer dort auf, wo ein Bauteil lokal aufgrund eines Defektes geschwächt wird. Dieser Bereich besitzt dann eigene, sehr viel höhere Resonanzfrequenzen und kann mit diesen sehr effizient angeregt werden. Dies führt zu einer energieeffizienten Ultraschallthermografie mit einfachen Anregungsquellen. Da jedoch Defektresonanzfrequenzen üblicherweise unbekannt sind, muss breitbandig angeregt werden. Während eines langsamen Frequenzsweeps im unteren und mittleren Kiloherzbereich werden Defekte, sobald ihre Resonanzfrequenz erreicht wird, angeregt und die thermische Antwort aufgrund von Rissuferreibung oder viskoelastischer Erwärmung an der Oberfläche von einer Thermografiekamera detektiert. Trotz der sehr niedrigen integralen Wärme können mithilfe einer Fourier-Filterung Defekte mit hohem Signal-Rausch-Verhältnis detektiert werden. Im Rahmen dieser Arbeit wird die RFST auf eine Vielzahl von Flachbodenbohrungen in PMMA und Impactschäden in CFK angewendet. Dabei variiert bei ersteren sowohl der Radius als auch die Ligamentdicke, während letztere sich durch Impact-Energie bzw. Schadensfläche unterscheiden. Aufgrund der großen Anzahl an Prüfkörpern konnte eine Probability of Detection (PoD)-Analyse durchgeführt und das Verfahren RFST somit validiert werden.

Before an implantable medical device passes market clearance, a number of mechanical tests need to be conducted in order to verify the mechanical safety of the product. With regard to total hip replacement stems, these are subject to fatigue tests both in the proximal region of the neck as well as in the distal region of the shaft. Despite these tests, we experienced two clinical fractures of the same product in our clinic. It was shown that these fractures were initiated by a laser engraving of the manufacturer with subsequent crack growth. The basic question raised in the present work was to find out why fracture may not have occurred during experimental preclinical testing. Hence, a supplementary finite element analysis was conducted considering the same hip stem under the prescribed conditions of the test standard in different variations. It was found that the choice of orientation and side (left or right leg) strongly influences stress in the hip stem which has not yet been prescribed in test standards. Depending on the side, only compressive stress may be acting in the experimental situation on the locations under risk, thereby inhibiting clinically relevant crack growth and concealing an actual fracture risk.

Uniaxial stress-controlled ratcheting experiments of PTFE (polytetrafluoroethylene) under cyclic compressive loads with multiple load steps were performed. The influence of stress rate, temperature, stress amplitude, loading sequence and peak holding on the compressive ratcheting behavior have been discussed systematically. Results indicate that ratcheting deformation increases significantly when enhancing temperature and stress amplitude and reducing stress rate. Only a slight, accumulated deformation occurs during the first 20 or so cycles at room temperature (RT), but it always turns to shakedown. Moreover, ratcheting strain is clearly influenced by the loading sequence. When greater compressive deformation was obtained during the prior load step due to a greater stress level (such as higher temperature or a lesser stress rate), increased compressive strain hardening and deformation resistance could be produced, which in turn restricts or even reduces ratcheting deformation in the pursuing load step. It is of great interest that no initial cyclic stress strain curve was observed under cyclic compression at peak holding time. This indicates that creep recovery due to anelastic relaxation decreases the ratcheting rate of PTFE material under compressive creep-fatigue conditions. This research provides an important testing approach and data for PTFE material subjected to repeated compressive loads influenced by temperature modulation.

In this study, a multi-objective design approach with accelerated methodology was developed for a B-pillar (side door pillar) in which the intrusion velocity was decreased and the crash energy absorbed. The B-pillar material characteristics were determined using a drop tower test to accelerate the design process instead of a vehicle crash test. A finite element simulation of the drop tower test was conducted, and the results obtained from the simulation were confirmed with the test results. The side impact finite element model was simulated according to the Euro NCAP test protocol, and the B-pillar was divided into two sections using the results obtained from the analysis. Tailor rolled blank and Tailor welded blank B-pillar crash simulations were performed, and the results were compared to examine the intrusion velocity. Alternative design solutions for single and multi-material B-pillars were conducted to design an optimum B-pillar structure. The design functions were created using the radial basis function method, and the failure criteria were determined for the analyses. Optimization problems for weight minimization and maximum energy absorption were established and solved using meta-heuristic methods. The approach suggested in this study can be used in accelerated B-pillar designs.

Turkey produces over 200,000 tons of walnut annually as the fourth largest producer in the world. Walnut shells amount to 40–60 % of the total weight, rendering over 100,000 tons of shells per year, mostly as waste. A limited quantity is utilized for fiberboard manufacturing, some as solid fuel or as scrubbing material in the chemical and metal industries. The purpose of this study is to develop low-cost polyester composites with enhanced physico-mechanical properties by using walnut cellulosic fibers as a reinforcement phase for polyester resin. The process not only makes production more economical but also reduces waste. The shells were ground and a detailed characterization of the waste fibers was carried out by scanning electron microscopy, X-ray diffraction, FTIR, laser diffraction and He gas picnometer techniques. The effects of the filler to matrix ratio as well as chemical treatment of walnut fibers on the physical and mechanical properties of the composite were determined. The composites were characterized by means of measurements for porosity, density, three point bending strength, impact resistance and hardness. The structure-property relationship of developed composites was discussed. Optimum physico-mechanical properties were obtained through a hydrophobic solution treatment of the powder since this provided better processability and enhanced interfacial adhesion.

Due to its excellent casting ability, spheroidal graphite cast iron is widely used in the production of complex automotive and machine products. In this study, the milling of spheroidal cast iron was carried out at different parameters. First, using the Taguchi statistical method, the parameters affecting surface roughness values were determined to b e cutting tool (65.54 %), then feed rate (19.65 %), and finally, with a minimum effect, cutting speed (7.31 %). Second, the amount of power consumed during chip removal was observed as being larger than the amount consumed when the machine was operating without a cutting load. Increases in the power index (PI) were measured by an amperemeter and it was determined that, as cutting speed was augmented, the amount of total power (P total ) consumed decreased, while, as feed rate was augmented, total power consumption increased. Total cutting energy (Pcutting) was increased by stepping up cutting speed. The PI was increased as a result of stepping up cutting speed and feed rate. On the basis of the cutting parameters, the Pcutting, P total , spindle torque (%) and specific energy consumption (SEC) value changes were calculated.

Kurzfassung In diesem Beitrag wird das Einsatzpotential der induktiv angeregten Shearografie mit dem der aktiven Thermografie zur zerstörungsfreien Prüfung von Klebverbindungen verglichen. Untersucht wird die Detektierbarkeit unterschiedlicher Fehlerbilder an Dickschicht- und Strukturverklebungen von Aluminium- und Tiefziehstahlsubstraten. Beide Verfahren zeigen für einen Einsatz bei strukturellen Klebungen gute Ergebnisse. Die Shearografie ermöglicht die Detektion von Fehlern, die zu lokalen Steifigkeitssprüngen in der Klebverbindung führen. Diese Steifigkeitssprünge können durch eine inhomogene Durchmischung oder durch Kissing Bonds verursacht werden. Der Nachweis dieser Fehler ist bei der angewendeten Messanordnung mittels Thermografie nur schwer bis gar nicht möglich. Mittels Thermografie ist hingegen eine eindeutige Detektion von ungleichmäßigen Klebschichtdicken möglich, die mittels Shearografie bisher nicht detektiert werden konnte. Bei den elastischen Klebungen zeigt die Thermografie bei vielen Fehlertypen deutlichere Messergebnisse als die Shearografie. Fehlertypen, die sich fast ausschließlich auf die Aushärtung des Klebstoffes auswirken, sind nur durch Shearografie nachweisbar. Da beide Verfahren unterschiedliche physikalische Grundlagen ausnutzen und sich dadurch für die Detektion unterschiedlicher Fehlertypen eignen, ist eine Kombination beider Verfahren sinnvoll. Die beiden Methoden ermöglichen so den Nachweis eines Großteils der für Klebverbindungen relevanten Fehlertypen.

Stainless steel is hard-to-cut due to its material specifications and many problems are encountered machining it. Nevertheless, stainless steel is frequently preferred in industry and its use increases. In this experimental study, the machining of AISI 430 ferritic stainless steel under dry, MQL (minimum quantity lubrication) and nanofluid MQL milling conditions was investigated. Nanofluids were prepared by adding 1 wt.-% and 2 wt.-% nano graphene to commercial vegetable cutting fluid. In these experiments, uncoated WC (tungsten carbide) and TiN (titanium nitride) coated WC cutting tools were used and MQL flow rates were selected as 20 ml × h −1 and 40 ml × h −1 . Surface roughness and cutting temperatures were measured and the effects of using nanofluid, MQL flow rate, and TiN coated cutting tools were specified. According to the experimental results, the MQL method was shown to be beneficial for reducing cutting temperature and surface roughness values and the nanofluid MQL yielded minimum values. Moreover, TiN coating and an increased MQL flow rate decreased cutting temperature and surface roughness.