### Filtern

#### Dokumenttyp

#### Schlagworte

- Cyclic J-integral (2)
- Fatigue crack propagation (2)
- IBESS (2)
- Bruchmechanik (1)
- Crack driving force (1)
- Crack initation (1)
- Cyclic R-curve analysis (1)
- Elastic follow-up (1)
- Elastic-plastic fracture mechanics (1)
- Fatigue S-N curve (1)

#### Organisationseinheit der BAM

In diesem Beitrag soll das Validierungskonzept der IBESS-Prozedur, sowie erste Ergebnisse und Erkenntnisse der noch gegenwärtig laufenden Validierung vorgestellt werden. Die Vorstellung erfolgt dabei am Beispiel der im Projekt eingesetzten geschweißten Stumpfstoß- sowie Kreuzstoßverbindung. Neben der Validierung des analytischen bruchmechanischen Modells zur Ermittlung der Schwingfestigkeit, steht auch die Validierung von Kriterien zur Interaktion und Koaleszenz von kurzen Mehrfachrissen im Fokus.

Approximation of the crack driving force for cracks at notches under static and cyclic loading
(2017)

The work deals with the efficient calculation of the elastic-plastic crack driving force (J-integral for monotonic loading andΔJ-integral under cyclic loading) for short cracks at notches as essential parameter for the reliable static and fatigue assessment of notched structures. The J- or ΔJ-integral is calculated based on analytical solutions for stress intensity factors, estimated by means of well-known weight function solutions in the case of cracks under power-law stress distributions. A plasticity-correction function is applied to the stress intensity factors to obtain the final expression of the crack driving force. The comparison between analytical solutions and finite element calculations in case of cracks at the weld toe in welded joints shows good agreement.

The acronym IBESS stands for "Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen" which, translated from German, means "integral fracture mechanics determination of the fatigue strength of welds". the method introduced in this study is the outcome of a German Research cluster in which eight partners were involved. A list of them is found at the end this study. The IBESS method is characterized by a number of partially novel aspects and elements of fracture mechanics applied to the evaluation of fatigue stength of welds. The most important ones are: (a) Determination of fatigue crack propagation for mechanically/physically short and long cracks. (b) Determination of an elastic-plastic crack driving force for the treatment of mechanically short cracks. To that purpose an analytical expression for the cyclic J-integral was developed and validated against finite element results. (c) The gradual build-up of the crack closure phenomenon is determined by using cyclic R-curves which describe the crack size dependency of the fatigue crack propagation threshold in the physically short crack growth regime. (d) A physically meaningful initial crack size is defined for total life consideration. It is based on a two-criteria approach. Based on a cyclic R-curve analysis, the crack size at crack arrest is determined as a lower bound. If, however, a pre-existing crack-like defect is larger than this, its dimensions define the initial crack size. (e) Multiple crack propagation at the weld toe is considered. (f) In conjunction with this, the variation of the weld toe geometry is considered in a stochastic model. (g) As a result, both the fatigue limit (defined for 107 loading cycles) and the finite life (high cycle) fatigue S-N curve are obtained statistically. (h) At various analysis steps, parametric equations have been developed which allow for analytical calculations instead of complete stochastic analyses based on finite elements which are unrealistic even at present. (i) The method has been validated with a large number of S-N curves including two materials, three weldment types with two geometries, each referring to differnt manufacturing technologies and the as-welded and stressrelieved state. (j) Althrough not finally solved, an extended discussion is provided on the issue of welding residual stresses including their redistribution under cyclic loading. (k) A number of simplifications is proposed at lower analyses levels which, however, partly lack complete validation by now.

Welding residual stresses have an impact on the performance of welded structures, on their fracture resistance, their resistance against fatigue crack propagation and, most important, their fatigue strength and fatigue lifetime. The present paper provides an overview on the issue mainly from the point of view of the application of fracture mechanics to the determination of the fatigue strength as the topic of this Special issue. Besides own experimental and theoretical data a comprehensive discussion is provided in that context which includes the definition and interaction of short- and long-range (or reaction) residual stresses, the effect of cyclic mechanical loading and its treatment in fracture and fatigue analyses.

The cyclic J-integral (∆J-integral) is a crack tip parameter of elastic-plastic fracture mechanics which can be used as governing parameter for the description of fatigue crack growth (FCG) in metallic structures. In this contribution, it is applied for modelling FCG in weldments. The ∆J-integral is determined by means of analytical approximation formulas as well as numerical methods. An analytical solution, which takes into account effects of the local ligament plasticity, was derived. This solution is based on well established methods such as R6, BS7910 and SINTAP which were modified for cyclic loading. It incorporates methods for the description of short crack closure behaviour as well as the well known analytical (long) crack closure function of Newman. A specific code was written to evaluate the ∆J-integral numerically in the course of finite element based crack growth simulations. The code was first validated for an infinite plate with centre crack by applying elastic and elastic-plastic material behaviour. Next, the ∆J-integral was calculated for cracks in various butt and cruciform welded joints. The results were compared with the results of the derived analytical approximation formula. A good accordance was achieved between the results.

This book provides a comprehensive and thorough guide to those readers who are lost in the often-confusing context of weld fatigue. It presents straightforward information on the fracture mechanics and material background of weld fatigue, starting with fatigue crack initiation and short cracks, before moving on to long cracks, crack closure, crack growth and threshold, residual stress, stress concentration, the stress intensity factor, J-integral, multiple cracks, weld geometries and defects, microstructural parameters including HAZ, and cyclic stress-strain behavior. The book treats all of these essential and mutually interacting parameters using a unique form of analysis.