Über den Einfluß von Probendicke und -geometrie auf die Ermüdungsrißausbreitung und die Bruchzähigkeit von Polycarbonat

The Influence of Specimen-thickness and Geometry on Fatigue Crack Propagation and Facture Toughness of Polcarbonate This study deals with the influence of specimen-thickness and geometry of the fracture-behaviour of polycarbonate at room-temperature. With decreasing thickness, an increasing resistance against fatigue crack propagation and an increase of the fracture toughness under static loading conditions was observed. Additional J-integral-evalutions showed a good agreement with the values of the fracture toughness. Scanning electron microscopy studies of the facture surfaces showed the sequence of damage and te individual micromechanisms of fracture. Based on th latter, detailed informations were given for the interpretation of the mechanical properties as a function of specimen-geometry.     

Definition,Messung und Anwendung bruchmechanischer Kennwerte von grauen Gußeisen

Definition, Measurements, and Application of Fracture Mechanical Properties of Grey Cast Irons Fracture mechanical properties were determined of cast irons with lamellar (GGL) and globular (GGG) graphite in a perlitic as well as a bainitic-austenitic matrix. Besides conventional and fracture mechanical tensile tests special attention was paid to fatigue crack growth. Ultrasonic loading appeared to be useful if at small amplitudes a large number of cycles is required for the observation of crack growth. A microscopic investigation of microstructure before and after cracking aided the interpretation of the origin of the measured properties. While the properties of the GGG-materials can be understood by the concepts of elastic-plastic fracture mechanics, lamellar graphite does cause a particular behavior. Notable is the non-linear elastic behavior, crack branching, and formation of satellite cracks, which provide difficulties in defining critical stress intensities K_c as well as the threshold for the start of fatigue crack growth δK_th. Stress induced transformation of residual austenite at the crack tip can retard crack growth by local increase in volume. Proposals are made for a reasonable definition of fracture mechanical properties as well as their application in design.     

Korrosion von grauem Gußeisen in konzentrierter Schwefelsäure

Corrosion of Grey Cast Iron in Concentrated Sulphuric Acid The Pauling process of regenerating spent sulphuric acid is an example of industrial application of the corrosion resistance of grey cast iron in boiling strong sulphuric acid. The corrosion behaviour of cast iron in boiling sulphuric acid depends on the corrosion potential. At a high corrosion potential, the material shows a stable passive state. In a study on the influence of the alloy components Si, Cu and Sn on the corrosion resistance of grey cast iron, the following results were derived: In the active state, cast iron containing more than 1,4-1,5% Si exhibits a corrosion maximum in 97% boiling sulphuric acid, whereas alloyed grey cast iron with low Si content displays in the active state better corrosion behaviour over the whole concentration range from 93–98% H₂SO₄. When the alloyed cast iron is in the passive state, corrosion resistance increases as the silicon content is reduced and the sulphuric acid concentration is raised.     

Durchstrahlungsbilder-Vergleichsreihe von Gußfehlern in Chemieapparaten aus Gußeisen mit Lamellengraphit

Reference series of radiographs of casting defects in chemical apparatus made of grey cast iron Chemical apparatus of grey cast iron as reaction vessels or storage tanks continue to be of major importance. When subjected to corrosive attack, they must not show excessive casting defects, otherwise the wall thickness will be reduced too much locally. Radiographic testing is generally used for the quality inspection of such apparatus. In order to evaluate casting defects in radiographs, a reference series of radiographs was produced and its system discussed. This series enables not only an evaluation of radiographs, but also makes it possible to fix quality requirements.     

Verfestigung und Verschleißverhalten von weißen Gußeisen bei abrasiver Beanspruchung

Work Hardening and Abrasion Resistance of White Cast Irons White cast irons are frequently used against abrasive wear under extreme loading conditions. The microsructure and the mechanical properties of these alloys play an important role to control their wear rates. Regarding the mechanical properties, it has been reported that the bulk hardness alone is not enough to describe the wear behaviour. Generally, it has been shown that the work hardening induced by the wear process has an essential effect on the abrasion resistance. In this investigation, using a pin abrasion test, the wear behaviour of Ni-Hard 4 and high chromium white cast irons has been studied. The results of this study contribute to discuss the correlation mechanisms between work hardening and abrasive wear behaviour.     

Untersuchungen Zum Temperaturwechselverhalten von Gußeisen - Teil II: Werkstoffwissenschaftliche und metallphysikalische Interpretation

Test examing the temperature shift behaviour of cast iron - Part II: Interpretation according to materials science and metallphysics The first part of this article discusses the most important influences on temperature shock behaviour of cast iron materials and experimental results of the examinations. The presented second article offers an interpretation of theoretical as well as practical examination results according to materials scientific views. A method will be presented determining the lifetime of thermal shock stressed components using activation-energy-stress diagrams and statisticall significant regression-relationships.     

Rechnerischer Festigkeitsnachweis der Ermüdungstragfähigkeit von vergüteten und einsatzgehärteten Stirnrädern

The work presents a calculative proof of strength for compensated and case hardened spur gears taking into account the different stress conditions in the tooth root and on the tooth flank. The basis of the proof of stregth is the local comparision of the occured stresses and the allowed stresses in the spur gear. The complex stress condition on the tooth flank is illustrated. On the flank occours a three dimensional stress condition with turnig principial stress systems. Furthermore the influences on this stress condition of residual stresses and of the contact of technical surfaces are discussed. Especially the described complex stress condition on the tooth flank asks for a calculation with an adequate criterion of failure. For this purpose a variant of the Schubspannungsintensitätshypothese (SIH) is proposed. The applicability of the proposed model is verified with numerous results of experiments. Some of the calculations are presented here. Altogether there is a good conformity between the test results and the calculated results especially for the ammount of the fatigue limit, the kind of damage (e.g. tooth root breackage, pitting, tooth breackage in the region of the flank) and the position of the beginning of the damage.     

Ermüdungsrißwachstum in Kerben

Fatigue Crack Growth in Notches Nowadays it is wellknown that an important part of the fatigue life time, usually differenciated in crack initiation and crack growth, is often controlled by fatigue crack growth of cracks in notches. An elastic-plastic on the J-integral based crack growth model considering the crack opening and closure phenomenon will be described to determine crack growth of cracks in notches between crack initiation and failure. Experimental results and finite element analysis were used to verify the developed model.     

Chevronproben zur näherungsweisen Bestimmung der Bruchzähigkeit

Chevron Specimen for the Estimation of Fracture Toughness Fracture toughness is a material property which is presently used in many industrial areas, either as material selection criteria or as material quality requirement. In some areas, nuclear power plants and aerospace, it is also a design parameter for design against catastrophic failures. Determination of the fracture toughness in accordance with ASTM E 399 is relatively elaborate. Depending on the material concerned, a certain minimum material cross section is required to obtain the necessary size of the specimen. Many semi-finished product forms of the different materials can not be tested for fracture toughness due to the specimen size requirements. For these reasons, alternative test methods were sought of which testing of chevron-notched specimens is one method. In the work to be presented, the test method to determine fracture toughness via chevron-notched specimens is briefly described. The most frequently used chevron-notched specimens are shown together with loading grips to be used in conjunctions with universal testing machines. Certain effects associated with some of the chevronnotched specimens are pointed out which result in a large difference between the fracture toughness determined in accordance with ASTM E 399 and that obtained via chevron-notched specimens. The aim of our research effort is to develop a chevron-notched specimen geometry which furnishes fracture toughness values compatible with K_Ic values without complicating the test method. Such a chevronnotched specimen is presented and the fracture toughness values obtained from these specimens of 7475-T 7351 and different Ti-alloys are compared to the K_Ic values obtained in accordance with ASTM E 399 for the same materials.     

Bruchzähigkeitsverhalten von Magnesium‐Druckgusslegierungen

Fracture Toughness of Pressure Die Cast Magnesium Alloys The increasing use of pressure die cast alloys for the manufacturing of automotive components requires materials values, which describe the mechanical behaviour comprehensively. There is especially a lack of data for the design of safety relevant components. In this paper fracture mechanics investigations are presented to assess the crack resistance of the alloys AZ91, AM50 and AE42 on the basis of the multispecimen test method. The investigations occurred on SENB and CT‐specimens taken from pressure die cast plates of the same dimensions. The results reveal a clear correlation between the microstructure, especially the concentration and the distribution of intermetallic phases, and the crack resistance. Because of the cast‐induced defects like microshrinkage and gas inclusions the good intrinsic properties of the alloys can not be completely exploited.     

Ermüdung und Mikrostruktur von koronaren Stents

Fatigue and Microstructure of Coronary Artery Stents During implantation the structure of coronary artery stents is subjected to distinct plastic deformation. Subsequently the implant has to sustain up to 700 million cycles induced by the cyclic diameter change of coronary arteries. Regions of high deformation show characteristic deformation structures like slip traces or extrusions and intrusions of grains on the surface without failure of the material. In order to gain information about the microstructural and microtextural evolution during deformation microstructure characterisation by means of scanning electron microscopy as well as transmission electron microscopy and single grain orientation determination were carried out. The analysis of cyclic deformation with different frequencies is of special interest. Several correlations between microstructure and deformation state are observed. Few deformation characteristics occur in the solution annealed stents and increase after dilation. Inhomogeneous deformation behaviour depending on the deformation of individual grains is observed. After cyclic deformation further changes in the microstructure can be observed. These changes caused by cyclic deformation are sensitive to load frequency and generate the formation of a completely different microstructure. Knowledge of microstructure evolution caused by distinct plastic deformation and fatigue in so called oligocrystalline structures is basis for ongoing development in stent optimization.     

Untersuchungen zum Temperaturwechselverhalten von Gußeisen – Teil I: Einflußfaktoren auf das Temperaturwechselverhalten,experimentelle Untersuchungen und deren Ergebnisse

Test examining the temperature shift behaviour of cast iron – Part I: Influence on temperature shift behaviour experimental studies, and their results The first part of this article describes and discusses the most important influence on the temperature shift behaviour of cast iron materials. For some unalloyed and low – alloyed cast iron types the resistance to thermal shocks is measured using a test unit observing the temperature shift behaviour. Metallographic studies on tested materials round up the statements judging materials behaviour under thermal testing.     

Die Messung der Bruchzähigkeit eines Vergütungsstahles mit Hilfe der Teilentlastungsmethode

Application of the Unloading Compliance Method in Fracture Toughness Measurements of a Quenched and Tempered Steel The application of the J-integral leads to a considerable reduction of material needed for establishing toughness data. The number of specimens is further reduced by measuring the crack resistance curve in a single specimen. This requires monitoring the crack extension during the experiment. A comparison of several methods showed that the unloading compliance technique is particularly suitable for that purpose. This method was applied for measuring the toughness as a function of location in a large forging of a steel 25 NiCrMo 14 5. Good toughness values (about 200 MPam^1/2) with little scatter were found over the whole cross section of the forging.     

Qualitätssicherung bei der Herstellung von Gußeisen mit Kugelgraphit nach dem Meehanite-Verfahren – am Beispiel von Armaturen für die chemische Industrie

Quality Assurance in the Production of Nodular Cast Iron According to the Meehanite Process – with Particular Regard to Valves for the Chemical Industry The application of nodular cast iron for valves, specially for the chemical industry, has attracted particular interest. This is due to the fact that manufacturing methods have reached a high degree of perfection at reasonable costs. Interest is focused on ductile grades GGG-35.3 and GGG-40.3 per DIN 1693 and, with restrictions, on GGG-40 and GGG-50. Maintenance of the desired level and close tolerances of material properties in the casting is the main condition for industrial application. The most important factors in this respect are the manufacturing methods and quality assurance systems employed. Regarding these factors the Meehanite process offers optimum conditions. For control purposes it is important that the properties of the separate test bar and of the casting correspond. Tests carried out on a casting taken from routine production of a Meehanite foundry prove that this is possible. For approval of a foundry as a supplier of castings of this kind the application of a suitable manufacturing method and the presence of a reliable quality control system should be the main criteria.