Ausgehend von dem bekannten Konzept der lokalen Dauerfestigkeit werden Probleme bei dessen Anwendung auf martensitische Randschichten, insbesondere Laserhartungsschichten, erortert.



Es wird eine Methode fur die Berechung der Dauerfestigkeit randschichtgeharterer Proben bzw. Bauteile aus martensitisch hartbaren Stahlen vorgestellt, welche den Einflus der Oberflachenrauheit auf die Dauerfestigkeit berucksichtigt



Fur die Bewertung der Mittel- und Eigenspannungsempfindlichkeit wird ein neuer Berechnungsansatz vorgeschlagen.



Die Grenzen des Konzept werden aufgezeigt und seine Nutzlichkeit an Beispielen demonstriert.



The local fatigue strength concept and its application to martensitic surface layers, especially laser hardened layers Starting from the known local fatigue strength concept problems of its application to martensitic surface layers, especially laser hardened layers have been discussed.



A method for calculation of the fatigue limit of surface hardened specimens or components from quench-hardenable steels has been proposed, including the influence of surface roughness on fatigue strength.



There is given a new formula for estimation of the mean and residual stress sensitivity.



The limits of the concept have been shown and its practical use has been demonstrated exemplarily.

Das Konzept der lokalen Dauerfestigkeit und seine Anwendung auf martensitische Randschichten, insbesondere Laserhärtungsschichten

The assessment of fatigue life under variable amplitude loading is always carried out by use of cumulative damage hypotheses, and here mostly Miner’s rule is applied. However, comparison of actual with predicted fatigue lives often show large variations of accuracy due to problems associated with load interaction effects [1]. The present investigation is dedicated to this problem in the field of crack initiation life.

Crack Initiation Life Predictions Based on Elastic-Plastic Fracture Mechanics of Short Cracks

This research work involved the prediction of the fatigue life of an automotive rear
suspension twistbeam assembly fitted to a vehicle travelling over a customer correlated
durability route. This was achieved by making use of the integrated concepts of scaling and
superposition of linear static finite element analysis being driven by experimental load data
- the so called 'quasi-static time domain' approach. A study of the free body diagram of the
twistbeam resulted in an indeterminate load set of some 24 components, with experimental
data indicating that a state of static unbalance existed. Subsequent to developing a
matrix-based generalised method ofload cell calibration to confum the foregoing, a modal
technique was developed to partition the experimental data into a static load set, causing
elastic deformations, and a rigid load set, imparting rigid body accelerations. The
semi-independent characteristics of the twistbeam necessitated the coupling of large
structural displacements with inertia relief. This required extensive modifications to the
current techniques and led to the development and use of a three dimensional functional
response matrix in place of the conventional two dimensional one. Recommendations
concerning appropriate finite element boundary conditions were also formulated to handle
these effects. Finally, the limitations of the uniaxial fatigue model were revealed under the
application of a set of tools for analysing the biaxiality and mobility of the maximum absolute
principal stress.

Using experimental loads with finite element analysis for durability predictions

In Low Cycle Fatigue two variables are usually applied to describe the state of a material : 


The damage variable D, 0≤D≤DF, defining the damage of the material, often without any detailed physical explanation. DF is the damage variable at fracture, DF≤1.0.


The number of cycles N. NF is the number of cycles to fracture for a given stress range ∆σ or strain range ∆e. Manson and Coffin gave for a strain controlled test the relation 

$${{\text{N}}_{\text{F}}}\text{ }\Delta {{\varepsilon }^{\beta }}=\text{const}\text{., 1}\text{.5}\le \beta \le \text{2}\text{.0}\text{.}$$

(1)





see e.g. the review by Manson and Halford, [1].

Some Remarks to the Application of Damage Mechanics to Low Cycle Fatigue

In order to minimize the expenses of the development of engineering components and to reduce the risk of their operation, reliable fatigue life prediction methods have to be available in the design stage. Presently two groups of methods can be distinguished which explicitly account for the local stresses/strains in the fatigue critical areas: the so-called notch analysis concepts or "local approaches," and new developments which are based on finite element methods (FEM). In the first part of the paper typical areas in application of the notch analysis methods and some aspects of their further development are described. The significance of the short crack stage is briefly characterized from a physical and an engineering point of view. In the second part the present state in the development of a new three-dimensional elastic-plastic FEM-based fatigue analysis concept, FEMFAT, is described. FEMFAT uses a material model according to MrOz's proposal, and the fatigue damage is calculated on the basis of the plastic work as performed at the fatigue critical locations of components. A calculation scheme for the damage increments under arbitrary out-of-phase loading based on uniaxial data is proposed. Two examples of the application of FEMFAT are given.

Some Contributions to the Further Development of Low Cycle Fatigue Life Analysis Concepts for Notched Components under Variable Amplitude Loading

In der vorliegenden Arbeit werden neuere im Schrifttum mitgeteilte Methoden (Verfahren nach T. H. Topper u. Mitarbeiter und nach K. J. Miller und Mitarbeiter) zur Beschreibung des Ausbreitungsverhaltens kleiner Risse an Kerben betrachtet und in der Weise aufbereitet, das sie unmittelbar in der Praxis anwendbar sind. Es erfolgt eine ausfuhrliche Darstellung der Grundlagen und des Anwendungsbereichs der Methoden, ferner ein Vergleich mit konventionellen Losungsansatzen auf der Grundlage der linear-elastischen Bruchmechanik (LEFM).



Anschliesend werden die derzeit noch bestehenden Beschrankungen in den physikalischen Grundlagen der betrachteten Methoden angegeben. Es werden Weiterentwicklungsschwerpunkte im Hinblick auf die allgemeine Anwendung der Methoden auf Betriebsfestigkeitsprobleme aufgezeigt.



Evaluation of Recent Models for Short Fatigue Cracks



In the present study some new procedures for the evaluation of the propagation behavior of small cracks at notches which were originally proposed by T. H. Topper et al. and by K. J. Miller et al. are considered. They are presented in a manner that they can immediately be used for practical applications. The basis of the procedures and their ranges in application are considered and they are compared to conventional fracture mechanics analyses.



In a second part of the study still existing limitations in the physical basis of the procedures are outlined and some aspects regarding the extension of the range in application of the procedures to variable amplitude loading histories are given.

J. Foth

Papers

Some Contributions to the Further Development of Low Cycle Fatigue Life Analysis Concepts for Notched Components under Variable Amplitude Loading

Darstellung und Bewertung von Modellen für die Ausbreitung kurzer Risse bei Schwingbeanspruchungen