Volume 60, Issue 10

In order to consider cyclic material behavior in the course of the design process of cyclically loaded components and safety relevant parts, the importance of local strain-based fatigue design approaches has been growing continually. For the damage impact of load-time histories on components such as chassis parts, standard service loads with amplitudes settled in the high cycle fatigue and very high cycle fatigue regimes as well as overloads and misuse with load amplitudes from the low cycle fatigue regime, have to be considered, in order to perform a reliable fatigue life estimation. Therefore, a continuous fatigue life curve from the LCF to the VHCF regime, which covers all relevant damage mechanisms, is required. Hence, a fatigue life curve from low cycle fatigue (LCF) across to high cycle fatigue (HCF), and culminating at very high cycle fatigue (VHCF), derived from a combined method using strain- and load-controlled fatigue tests, will be discussed. This continuous fatigue life curve for aluminum wrought alloys, based on the evolution of the behavior of elastic-plastic material as well as the results of high frequency testing up to VHCF, will be presented.

The fatigue notch factor is the quotient of the nominal stress endurance limits of unnotched and notched specimens. It represents the notch sensitivity of a material or a component under cyclic loading. For example, the fatigue notch factor is used in analytical strength assessments with nominal stress and is also suitable as a quality criterion for materials and entire components. On the one hand, an analytical approach to rate the reliability of the fatigue notch factor is derived assuming a log-normal distribution for the population of the endurance limits of the unnotched and notched specimens. The analytical approach provides the best achievable result and can be used to rate real experiments. On the other hand, the staircase method, which is often used to determine endurance limits, is simulated using Monte Carlo simulations. From the simulations, information about the reliability of the real test can be gathered, which can be compared to the analytical approach. In conclusion, in this paper the reliability of the fracture of two originally log-normally distributed random variables each used in an own staircase test is examined. The result is compared to an analytical approach. In former articles the scope has only been on single test series, the examination of error propagation, e.g, by using the staircase method and building the fracture of the two results afterwards, is missing in former examinations. The staircase method is a procedure to estimate fatigue notch factors with reliabilities comparable to those of the analytical approach. To increase the number of evaluable tests, runouts should be reused.

Austempered ductile iron (ADI) is a heat-treated ductile iron with improved tensile strength and toughness. Therefore, ADI is an interesting material for the construction of technical parts. However, for practical use, a strength assessment is necessary. An often-used option for an analytical strength assessment is the FKM-guideline. This guideline includes the strength assessment for steel, cast iron and aluminum materials, but it has not yet been especially investigated for ADI. In this work the use of the guideline for ADI is investigated. On the basis of experimental data, the accuracy of the guideline is examined and the limitations are discussed. Furthermore, the parameters that are used for the calculation in the guideline can be adjusted for the improvement of the accuracy of the strength assessment. In this way in particular, the forecast for shear stresses and the high strength materials can be improved.

A large variety of methods for estimating cyclic material properties from quasi-static material properties can be found throughout the literature. Therefore, the user usually faces the problem of determining which methods would best suit his purpose. To help answer this question, a massive database containing test results from quasi-static and cyclic tests is gathered and used for a quality rating of the different methods. This rating results in the recommendation to use the uniform material law for steel materials, the modified park-song's method for wrought aluminum and the method of variable slopes 2006 for thin steel sheets. In addition, the accuracy of these estimation methods is compared to experimental results using deviation ranges in the total strain life curve and in the cyclic stress-strain curve.

In the first part of this paper, a large variety of methods for estimating cyclic material properties from quasi-static material properties were rated concerning their accuracy. For this, a massive database containing test results from quasi-static and cyclic tests has been gathered and used to compare estimated and experimental properties. In this second part of the paper, the experience of how different estimation methods work and the database itself are used to develop a new estimation method, the so-called FKM method. This new method is able to estimate cyclic material properties for steel, cast steel and wrought aluminum and thus exceeds the known estimation methods if the quality criteria from part 1 are applied.

Tests with constant load amplitudes are used to characterize the fatigue strength behavior of material specimens and components. The S-N curve derived from these test results describes the relationship between the load amplitude and the corresponding cycles to failure. Different concepts for carrying out and evaluating fatigue tests make it difficult to compare the results from different research institutes. The aim of the new version of the German standard DIN 50100:2016 is to define a procedure for determining an S-N curve for metallic alloys that does not allow any scope for interpretation. It is assumed that the test results are subject to logarithmic normal distributions in both load and cycle direction. It is further assumed that the S-N curve in the high-cycle fatigue regime and the long-life fatigue regime can be approximated by a bilinear function. For the determination of the straight line of finite life, the pearl string method and the load level method are available for determining the position parameter and the slope of the power function according to Basquin. Long-life fatigue strength is determined using the staircase method and forms the knee point of the S-N curve on average with the straight line of finite life. For the long-life fatigue regime, a horizontal course or a decrease with low inclination, depending on the material group examined, is assumed. In addition, DIN 50100:2016 contains information on the accuracy of the estimation of the mean values and the scatter of the characteristic values according to the sample size. The goal of achieving comparability of S-N curves is supported by extensive examples. An English translation of DIN 50100:2016 is available also.

Laser cladding of Stellite 31 powder on AISI 316L plate size of 50 × 50 × 25 mm using diode laser powers of 3.5, 4.0, 4.5 and 5.0 kW was produced. The wear tests of clad layers were investigated according to ASTM G99. Equiaxed and elongated grains were found in clad layer using laser powers of 3.5, 4.0 kW and 4.0, 5.0 kW, respectively. Compressive residual stress was detected at the surface of the clad layers. They decreased as laser power increased. After polishing, compressive residual stress and the wear resistance of the clad layers subjected to higher laser power were higher than those subjected to lower laser power. The friction coefficient of the clad layer subjected to higher laser power was lower than that subjected to lower laser power.

This study proposes to design a road support structure subjected to high-strain-rate loading occurring during vehicle collisions. The strain-rate affects both the hardening process and the material failure process. For modeling the strain-rate influence on material, various mathematical material models are used including the Johnoson-Cook model. The main goal of the study is to presenta method for determining the parameters for the Johnson-Cook damage model, a hybrid method which requires Charpy impact tests on a standard specimen with an annular notch and a series of calculations using finite element method simulating the test. The advantages of the presented method compared to existing methods are the high availability of the equipment, simple and quick processing of results and significantly lower costs.

This paper aims to determine the role of in-nugget/residual Alclad in the friction stir spot welds of 1.6 mm thick Alclad AA2219 aluminum alloy via an assessment of microstructure, mechanical properties and fracture modes of welds. Alclad redistribution/dispersion within the stir zone of the alloy is varied by using different tool profiles (pinless and conical pin tools) and welding parameter combinations. The results reveal that a pinless tool facilitates the retention of Alclad within the weld's effective joint line/width while no substantial residual Alclad is observed in the effective bonded width of a conical pin weld. The increase in tool depth improves the weld strength of a pinless weld. The fracture morphology of a pinless weld is influenced by the in-nugget Alclad content. Interfacial fracture with midpoint defect, interfacial fracture and nugget pull-out are the fracture modes of pinless welds while a conical pin weld fails by circumferential nugget shear failure. A direct correlation exists between nugget rotation and lap shear failure load. Maximum lap shear failure loads of 4.0 kN and 2.1 kN are obtained in pinless and conical pin welds at the optimum welding parameter combinations of 1500 rpm – 0.8 mm – 8 s and 1400 rpm – 0.43 mm – 4 s, respectively.

In this study, the effect of cooling rate on microstructure, mechanical properties and residual stress of 7075 aluminum alloy was investigated. The influence of cooling rate on microstructure, hardness, electrical conductivity of 7075 aluminum alloy was investigated using a Jominy end quench test. Water at three different temperatures (20 °C, 50 °C, 75 °C) and polymer solutions of varied concentrations (5vol.-% and 25vol.-%) were used as a quenching medium. The changes of hardness, electrical conductivity and microstructure properties of the specimens with an increase in distance from the quenched surface were investigated comparatively for different quenching mediums. Tensile tests were applied to determine the effect of the quenching rate on mechanical properties of the specimens. Residual stress was measured using the ESPI hole drilling technique to understand the influence of cooling rate. The results show that the cooling rate decreases as the distance from the quenched surface, water temperature and polymer concentration increases. The changes in material properties such as hardness, electrical conductivity along the material profile decrease as water temperature and polymer concentration increase. Although the hardness and mechanical properties of the material decrease as the cooling rate decreases, the values obtained are convenient for conditions of industrial usage. Residual stress was significantly eliminated by quenching in hot water or polymer solution at a high concentration.

Failure and stress analyses of adhesively joined composite pipe systems of varied orientation angles under internal pressure have been carried out using the 3-D finite element method (FEM) and experimentally. Composite pipes adhesively joined without sleeves were examined. The composite pipes were produced from E-glass fiber and adhesively joined using the adhesive types DP410 and DP490. The numerical model was created using ANSYS software and the efficiency of the model was verified by the experimental results. The finite element analyses (FEA) and experimental tests were carried out to predict the failure internal pressure. The internal failure pressures were obtained from experimental tests and compared with the numerical results. Radial, tangential, axial and shear stress values were obtained via numerical analyses in composite pipes and in the adhesive layers. In addition, the von Mises stress distributions in the adhesive were obtained. The effects of the orientation angles were investigated in the interface region between the pipes and the adhesive. The most effect parameters were determined for the composite pipe system joined at various pipe radii. It was found that the composite pipes could be used under high internal pressure, a fact that is important for industrial applications.

The causes of a composite roll fracture were investigated. The result of an FEM analysis showed that the occurrent stress could not have led to this fracture. Metallographic investigations in the area of the crack start site clearly showed non-metallic inclusions. These affect structural weak points and activate malfunctions. They significantly accelerated the cracking process due to the local notch effect which significantly reduced the performance of the material and thus the service life of the roll shaft.

Kurzfassung Heizöllagerbehälter aus Polyethylenformstoffen werden seit Beginn der 70-iger Jahre in Deutschland hergestellt. Die Hersteller empfehlen zur Gewährleistung der Sicherheit nach Ablauf von 30 Jahren den Ersatz dieser Behälter. Polyethylenformstoffe unterliegen im Laufe ihrer Nutzungsdauer der Alterung durch die Veränderung ihrer Eigenschaften. Das Ausmaß der Alterung und die Art des Abbauvorganges hängen im Wesentlichen vom chemischen Abbau des Polyethylens, der Wanddicke des Behälters und den Umgebungsbedingungen ab. Bisher lagen keine Untersuchungen zum Langzeitverhalten der Polyethylenformstoffe vor, insbesondere nach einer Nutzungsdauer der Tanks über 30 Jahre. Ziel der Untersuchungen war, den tatsächlichen Schädigungsgrad der Polyethylenformstoffe im Vergleich zu den unbelasteten Formstoffen zu ermitteln. Da die BAM bis Mitte der 90-iger Jahre für die Prüfungen und Gutachten zur Zulassung dieser Behälter zuständig war, sind die Werkstoffdaten noch vorhanden. Als Untersuchungsmethoden wurden die Bestimmung der Schmelze-Masse-Fließrate (MFR) und die Fourier-Transformation-IR-Spektroskopie (FTIR) von Tankausschnitten aus dem Bereich des Bodens, des Mantels und des Daches von 10 ausgesonderten Behältern aus den Polyethylenformstoffen A und B herangezogen. Die Messungen der MFR der Tankausschnitte aus diesen Bereichen zeigten Unterschiede in den Werten in Abhängigkeit vom Gewicht (5 kg oder 21,6 kg), welches eingesetzt wurde. Eine Abnahme der MFR-Werte wurde für die größere Anzahl der Tankwandungsausschnitte aus dem Formstoff B nach einer Lebensdauer der Tanks > 30 Jahre gemessen. Dieser Formstoff wurde hauptsächlich durch innere Alterung infolge Vernetzungen, Verzweigungen und Weichmacherverlust, und weniger durch oxidativen Abbau, geschädigt. Die FTIR-Messungen der Proben aus dem Bereich des Bodens und des Mantels der Tanks zeigten, dass die Intensität der asymmetrischen/symmetrischen CH 2 -Streckschwingungen im Bereich 2800–2900 cm −1 und der CH 2 -Biegeverformungsschwingung bei 1400 cm −1 aufgrund von Kettenspaltungen zugenommen haben. Die Intensität der Carbonylstreckschwingung C=O bei 1740 cm −1 ist niedrig. Sie ist ein Maß für die Oxidation des Polymerwerkstoffes.

A new extrusion technique, integrated dual-directional extrusion and spiral deformation(DDES), is proposed and successfully applied to AZ31 Mg alloys with no cracks present in the alloys. Combining finite-element analytic, optical microscopy, and X-ray diffraction techniques, it is found that the DDES process may cause cumulative strains of 1.88 in Mg alloy billet. The average grain size of the alloys can be refined from 300 μm to 3.5 μm at a temperature of 573 K, and the basal texture is also dramatically weakened from 53.6 to 6.52. These improvements are mainly attributed to dynamic recrystallization induced by heavily accumulated strain at the spiral region. Further compressive tests are carried out on processed AZ31 Mg alloys, and the superior compressive strength and fracture strain can attain 395 MPa and 30.6 %, respectively, rendering this new deformation technique into a promising approach for further governing properties of Mg-based alloys.

In this paper, the reasons for cracking in the longitudinal and spiral of Q125 casing in ultra-deep wells (UDWs) at the Tarim Oilfield were investigated for the first time ever. Up to know an evaluation of Q125 casing materials of UDWs were were limited to comparisons with WSP-2T and P110 via performance tests. In addition, an applicability evaluation of UDWs was carried out using MPC software and theoretical calculations. The results show that mechanical damage and stress corrosion cracking are the principal causes for longitudinal cracking. Spiral cracking can be the result of the manufacturing process. It should be added that the material out of which Q125 casing is produced manifests high strength, low toughness and great sensitivity to cracks, stress corrosion and low allowable limit crack size (ALCS) when compared with WSP-2T and P110. Owing to the complicated field environment of UDWs at the Tarim Oilfield, the causes of cracking failure have been analyzed, with the result that WSP-2T casing of a similar strength to Q125 casing ought to be selected in UDWs rather than Q125 casing with high strength and low toughness, which is significant for the development of UDWs at the Tarim Oilfield.