Schwingfestigkeit von Schweißverbindungen aus der Aluminiumlegierung AlMg4,5Mn (AA5083) nach dem Konzept der Mikrostützwirkung

Fatigue strength of welded joints in AlMg4.5Mn (AA5083) aluminium alloy with the micro-support approach For the calculation of the fatigue strength of welded joints in aluminium the micro-support approach of Neuber was applied. Fatigue tests with unnotched and notched specimens of the base and weld metal (deposite) of AlMg4.5Mn (AA5083) were carried out in order to derive the unknown microsupport length in dependency on the number of cycles. For the application of the concept, results of fatigue tests with geometrical similar welded joints with full and partial penetration were available, for which stress concentration factors had been calculated. The comparison of the experimental and calculated fatigue strength shows that the endurance strength is well assessed. An expansion of the concept to lower numbers of cycles is possible.     

Effect of mean stress on fatigue properties of 1800 MPa-class spring steels

Fatigue tests were conducted for 1800 MPa-class spring steels at various stress ratios. For comparison, similar fatigue tests were conducted for conventional steels whose tensile strength was lower than 1200 MPa. The spring steels exhibited fish-eye fractures, and the origins of these fractures were oxide, TiN and the matrix itself. In contrast, the conventional steels never exhibited fish-eye fractures. The fatigue strength of these steels decreased monotonously as the stress ratio increased, when the fatigue strength was evaluated in terms of stress amplitude. However, the fatigue strength degradation was less than that expected from a modified-Goodman line, and the best fit line was obtained by connecting the fatigue limit at zero mean stress to true fracture strength instead of tensile strength. This research also reviewed application of a power low to the stress ratio effect evaluation. In these results, the difference between the spring and conventional steels was negligible.     

高强度合金抗疲劳应用技术研究与发展

评述了超高强度钢、高强度Al合金和Ti合金表面完整性抗疲劳应用技术的研究和发展。高强度合金疲劳性能对应力集中敏感，不适当的加工工艺和切削热等造成的表面损伤和高拉应力使其疲劳和应力腐蚀性能损失殆尽。先进的表面完整性加工尤其是表面改性可显著提高疲劳性能，如激光冲击使7475-T761拉-拉疲劳寿命提高约89％，7075-T6裂纹扩展速率降低到原来的1／1500；超声喷丸使超高强度钢低周疲劳强度提高约50％，Ti7Al4Mo合金高周疲劳强度提高约15％；表面超硬化可使Vasco X-2M齿轮钢接触疲劳寿命提高30～35倍等。     

Ermüdungsverhalten ausgewählter Werkstoffe und Bauteile bei sehr hohen Schwingspielzahlen

Very High‐Cycle Fatigue of Selected Materials and Components Results of several fatigue tests using a servohydraulic testing machine (VHF 50 D) and a spring testing machine are presented. Investigations were carried out at room temperature with. – smooth and notched specimens made of aluminium wrought alloy EN AW 6082,. – screws M 8 made of EN AW 6056,. – smooth specimens made of die cast magnesium AZ91 hp (here also tests at 125 °C) and. – shot‐peened helical compression springs of four different high strength steels. Tests were done with a frequency of 20 Hz (spring testing machine) and between 250 Hz and 400 Hz (VHF50D) up to a maximum number of cycles of N = 1,5 x 10⁹. Crack initiation sites were investigated and could be found at high number of cycles below the surface for smooth magnesium and aluminium specimens and also for most of the shot‐peened helical compression springs, but not for screws and other notched specimens.     

Einfluß einer Oberflächenkaltverfestigung auf die Schwingfestigkeit und das Wechselverformungsverhalten der normalisierten Kohlenstoffstähle Ck 15 und Ck 45 unter Zugdruck-und Umlaufbiegebeanspruchung

Effect of Surface Cold Working on Fatigue Strength and Cyclic Stress-Strain Behaviour of Normalized Plain Carbon Steels Ck 15 and Ck 45 under Compression-Tension and Rotating Bending Load Investigations on deep rolled specimens of normalized plain carbon steels showed that surface could working can improve the fatigue strength by about 15% in rotating bending tests and by about 5% in cyclic compression-tension tests. The better endurance properties are mainly caused by a reduced amplitude of plastic strain. From microscopical examinations of Ck 15 surfaces was deduced that crack initiation in deep rolled specimens resembles by far with that in specimens not surface treated. During fatigue tests the dislocation structure produced by deep rolling approached that developing in samples without cold worked surface.     

Fatigue strength of high strength dual-phase steel sheet

Fatigue tests have been carried out on lean-alloyed dual-phase steels with tensile strengths ranging from 300–800 MPa. Smooth specimens and specimens with punched holes were tested. The fatigue strength of dual-phase steel was found to be similar to that of other types of steel (eg solution hardened or microalloyed steels) of equal tensile strength. The fatigue strength increases with increasing yield strength. For notched specimens it is also related to the yield ratio. Work and bake hardening increase the fatigue strength of smooth specimens in proportion to the increase in yield strength. For notched specimens this effect is less and is dependent on the yield ratio. Bake hardening of material which was not work hardened also increased the fatigue strength. The notch sensitivity of low yield ratio dual-phase steel is found to be low. The notch sensitivity seems to increase with increasing yield ratio.     

Static and Fatigue Properties of the Cold and Warm Compacted Sintered Aluminium Alloy Alumix 431 (Al‐5.5 Zn‐2.5 Mg‐1.5 Cu)

In this work, the densification behaviour, sintering characteristics, hardness, strength, tensile and fatigue properties of the Alumix 431 alloy (Al, Zn, Mg and Cu alloy) produced using the conventional press and sinter process in different pressures and temperatures are investigated. For this purpose, specimens were produced under pressures between 300 and 500 MPa at 50 MPa intervals at room temperature (RT), and 80 °C in the first step. In the second step, specimens were produced at the compaction pressure of 230 MPa under RT and 180 MPa with 80 °C for tensile and fatigue testing which provided same densities. Specimens were sintered under five different sintering conditions. The dimensional change in warm compacted samples is lower than that of cold compacted samples. Tensile and fatigue properties of warm (180 MPa pressure) and cold (230 MPa pressure) compacted specimens are almost equal due to the same densities.     

Enhancing high-cycle fatigue properties of cold-drawn Fe–Mn–C TWIP steels

High-cycle fatigue properties of cold-drawn twinning-induced plasticity (TWIP) steel, a favored candidate for replacing fully pearlitic (FP) steels in wire applications, were investigated. The high-cycle fatigue tests were conducted on cold-drawn TWIP and FP steels that had comparable ultimate tensile strength for comparison. Fatigue strength of both TWIP and FP steels increased with the tensile strength, but the TWIP steel cold-drawn to a tensile strength of 1.5 GPa exhibited a very low fatigue ratio (a ratio of fatigue strength to tensile strength) which deviated far from the predicted linear relationship. Fracture surface analysis showed that crack initiation mainly occurred at the ferrite matrix in FP steels, while either at grain or twin boundaries in TWIP steels where a large density of dislocations piled up during cold drawing. In the case of TWIP steels, the presence of inclusions at grain boundaries led to high local stress concentration and caused early intergranular fatigue cracking as notch sensitivity increased with tensile strength. Subsequent annealing after cold-drawing effectively increased fatigue strength of TWIP steels. It was suggested that TWIP steel revealing both high tensile strength and excellent high cycle fatigue strength could be a promising alternative for replacing conventional FP steels.     

Influence of activation energy and sensitivity to hydrogen embrittlement on fatigue strength degradation by irreversible hydrogen in high‐strength steels

Fatigue tests were conducted for cold‐drawn eutectoid steels having different activation energies for irreversible hydrogen trap sites and sensitivities to hydrogen embrittlement; the purpose was to investigate the mechanism of fatigue strength degradation by irreversible hydrogen. The fatigue strength of a sample with low activation energy was decreased by irreversible hydrogen in the material, whereas the fatigue strength of a sample with high activation energy was not. When the activation energies for irreversible hydrogen were almost equal, a higher sensitivity to hydrogen embrittlement induced fatigue strength degradation by irreversible hydrogen. Therefore, fatigue strength degradation by irreversible hydrogen depends on the activation energy for irreversible hydrogen trap sites and sensitivity to hydrogen embrittlement.     

Entwicklung mit Hilfe des Warmpreßverfahrens hergestellter Sinterstähle mit optimaler Warmstreckgrenze,Dauerfestigkeit und Kerbschlagzähigkeit – Teil 2

Development of Hot-Forged Sintered Steels with Optimum Yield Point at Higher Temperature, Fatigue and Impact Strength . The material characteristic values of hot-forged sintered steels are reported. Besides the usual properties, (ultimate tensile strength, yield strength and elongation at room temperature). The behaviour of materials at higher temperature is of great importance. Further, fatigue strength and impact strength must be considered. The influence of heat treatment on these properties were investigated and brought to a correlation with the formation of microstructures. The hot forging P/M preform requires a special attention on alloying elements and alloying techniques. Optimum properties results when using completely alloyed powders or mixed powders by employing special master alloys. This and further problems involving alloying techniques and particularly the behaviour of carbon as an alloying element is reported in details. The correlation between mechanical properties and fracture behaviour is discussed.     

Entwicklung mit Hilfe des Warmpreßverfahrens hergestellter Sinterstähle mit optimaler Warmstreckgrenze,Dauerfestigkeit und Kerbschlagzähigkeit – Teil 1

Development of Hot-Forged Sintered Steels with Optimum Yield Point at Higher Temperature, Fatigue and Impact Strength . The material characteristic values of hot-forged sintered steels are reported. Besides the usual properties, (ultimate tensile strength, yield strength and elongation at room temperature). The behaviour of materials at higher temperature is of great importance. Further, fatigue strength and impact strength must be considered. The influence of heat treatment on these properties were investigated and brought to a correlation with the formation of microstructures. The hot forging P/M preform requires a special attention on alloying elements and alloying techniques. Optimum properties results when using completely alloyed powders or mixed powders by employing special master alloys. This and further problems involving alloying techniques and particularly the behaviour of carbon as an alloying element is reported in details. The correlation between mechanical properties and fracture behaviour is discussed.     

Ultrahigh strength-ductile medium-Mn steel auto-parts combining warm stamping and quenching & partitioning

A novel application of quenching and partitioning (Q&P) treatment to warm stamping of a warm-rolled medium Mn steel was investigated. The results show that Q&P could improve yield strength of auto-parts from the formation of carbides and twinned martensite, and reduce the yield point elongation for carbon partitioning during Q&P process and a higher dislocation density. Regardless of stamping temperature either the single austenite or dual-phase region, an extraordinary product of strength and ductility (≥24 GPa·%) was achieved, which is two or three times higher than those of hot-stamped boron steels. The combined warm rolling and warm stamping process with Q&P treatment may have potential to implement the application of medium Mn steels for ultrahigh strength-ductile auto-parts.     

Fatigue assessment of cut edges in high strength steel – Influence of surface quality

The influence of surface roughness on the fatigue strength in high strength steels and different cutting processes are studied. Fatigue testing is conducted on S700 and S960 material for different plate thicknesses cut in dog bone specimens using oxygen, plasma, laser and waterjet cutting. The surface roughness is measured for all specimens and residual stress measurements are carried out. Estimations of the fatigue strength are made based on the measured surface roughness and the ISO 9013:2002 standard for thermal cutting quality tolerances. The testing shows a 15–70 % increase in the fatigue strength compared to the estimation, proving a weak connection between the surface quality levels in ISO 9013:2002 and the fatigue test results. Different codes and design recommendations (IIW, EC3 and EN 13001) for fatigue strength of cut surfaces are compared with the fatigue test results which clearly shows an increased fatigue strength with enhanced quality and steel grades. However, the codes and design recommendations do not allow for any fatigue strength improvement with improved quality and increased yield strength.     

Umformtechnologie: Prozesse – Werkstoff‐ charakterisierung – Simulation – Verifikation

Sheet forming technology – processes, materials, simulation and verification Hydroforming and Deep Drawing represent leading technologies for forming sheet metal components. The Materials Branch of the University of Duisburg‐Essen works since more than 10 years in the field of Hydroforming and showed that Hydroforming increases the strength and that the weldings of hydroformed tubes normally exhibit a same (fatigue) strength as the base material. For an improvement of the economics of hydroforming spliced tubes were considered and standard hollow nodes for tubes nodes structures were developed and a proposal was made to produce these tubes in variable tools with segments or lamellas. A significant increase in economics of Hydroforming and of Deep Drawing of components is achieved by an introduction of the principles of an integral (cooperative) development of products. At the University of Duisburg‐Essen the following stages are run through: CAD (the Material Branch uses PRO/ENGINEER®), forming simulation – for that PAM‐STAMP® (ESI) and PATRAN MARC MENTAT® (MSC) are used – FEM strength calculation and EVICD for a consideration of variable service loading. Forming simulation needs as basic material data the yield curve, the parameters of anisotropy and the Forming Limit Curve (FLC). For a determination of the FLC in Deep Drawing Tests suitable Nakazima specimens were developed by applying the principles of cooperative product development. The specimens could then successfully be validated in Deep Drawing Tests, which were instrumented by the advanced 3D‐forming‐analysis‐system AUTOGRID inProcess (VIALUX). Various other forming simulations were also performed and some general rules for the performance of forming simulations were formulated. Finally, reverse engineering is briefly discussed.     

Dauerfestigkeit von Sinterwerkstoffen

Fatigue Strength of Sintered Materials Based on recently published fatigue test results it is now for the first time possible to estimate fatigue diagrams for sintered steels. Fatigue strength of sintered steels is mainly influenced by density and alloying content, production parameters being of lesser importance.     

Recent progress in medium-Mn steels made with new designing strategies,a review

After summarizing the relevant researches on the medium Mn steels in references, two new targets on the tensile properties have been defined. One is that both transformation-induced(TRIP) and twinninginduced plasticity(TWIP) could be realized for the steel with a relatively low Mn content, which exhibits the similar tensile properties to the classical TWIP steels with higher Mn content. The other is to achieve ultrahigh ultimate tensile strength(1.5 GPa) without sacrificing formability. To achieve these goals,new designing strategies was put forward for compositions and the processing route. In particular, warm rolling was employed instead of the usual hot/cold rolling process because the former can produce a mixture of retained austenite grains with different morphologies and sizes via the partial recrystallization. Consequently, the retained austenite grains have a wide range of mechanic stability so that they can transform to martensite gradually during deformation, leading to enhanced TRIP effect and then improved mechanic properties. Finally, it is succeeded in manufacturing these targeted medium Mn steels in laboratory, some of them even exhibit better tensile properties than our expectation.     

Fatigue and fracture of a Ni–P amorphous alloy thin film on the micrometer scale

Fracture and fatigue tests have been performed on micro‐sized specimens for microelectromechanical systems (MEMS) or micro system technology (MST) applications. Cantilever beam type specimens with dimensions of 10 × 12 × 50 μm³, approximately 1/1000th the size of ordinary‐sized specimens, were prepared from a Ni–P amorphous thin film by focused ion beam machining. Fatigue crack growth and fracture toughness tests were carried out in air at room temperature, using a mechanical testing machine developed for micro‐sized specimens. In fracture toughness tests, fatigue pre‐cracks were introduced ahead of the notches. Fatigue crack growth resistance curves were obtained from the measurement of striation spacing on the fatigue surface, with closure effects on the fatigue crack growth also being observed for micro‐sized specimens. Once fatigue crack growth occurs, the specimens fail within one thousand cycles. This indicates that the fatigue life of micro‐sized specimens is mainly dominated by a crack initiation process, also suggesting that even a micro‐sized surface flaw may be an initiation site for fatigue cracks which will shorten the fatigue life of micro‐sized specimens. As a result of fracture toughness tests, the values of plane strain fracture toughness, K_IC, were not obtained because the criteria of plane strain were not satisfied by this specimen size. As the plane strain requirements are determined by the stress intensity, K, and by the yield stress of the material, it is difficult for micro‐sized specimens to satisfy these requirements. Plane‐stress‐ and plane‐strain‐dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This indicates that fracture mechanics is still valid for such micro‐sized specimens. The results obtained in this investigation should be considered when designing actual MEMS/MST devices.     

Mn-Si-Cr系无碳化物贝氏体/马氏体复相高强钢的研究进展

无碳化物贝氏体/马氏体复相高强钢具有比同等强度马氏体钢更优异的韧性和塑性,被广泛应用到轨道交通、机械、建筑等领域。文章概述了低成本Mn-Si-Cr系无碳化物贝氏体/马氏体复相钢近年来在合金化设计、工艺设计、微观组织、强韧化机理、强塑化机理、延迟断裂及疲劳性能等方面取得的研究成果。特别介绍了近年来笔者在BQP工艺处理CFB/M复相钢方面的工作进展,经过BQP处理之后,CFB/M复相钢显示了更优异的强度、塑性、韧性和疲劳性能的匹配。最后简单介绍了Mn-Si-Cr系无碳化物贝氏体/马氏体复相钢在不同领域的应用情况,特别是其在重载高速铁路领域的应用现状和前景。     

Fatigue properties of self‐piercing riveted multi‐rivet joints in steel and aluminum sheets

Self‐piercing riveting (SPR) is an important joining technology for connecting steel and aluminum sheets. In this paper, AA6111 aluminum alloy and DP780 high‐strength steel were adopted to study the influence of fatigue on remaining static strength and energy absorption properties on self‐piercing riveting multi‐rivet joints. The results showed that energy absorption capacity of the specimens decreased significantly after high cycle fatigue. Fatigue reduced the remaining static lap shear strength of riveted specimens. Scanning electron microscopy (SEM) was used to analyze the cross section of fatigue specimens fractured by static tension. The results showed that fretting wear was found at the contact area between rivet and aluminum sheets. Fatigue bands and fatigue cracks appeared in fatigue specimens after high cycle fatigue, while those with low cycle fatigue specimens did not appear. Small cracks weaken the strength of the aluminum sheet, resulting in the static tensile strength of the riveted specimen with high cycle fatigue is lower than that of other fatigue specimens.