Fourth International Conference on Material and Component Performance under Variable Amplitude Loading
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Welding and Riveting

Session chair: Nagode, Marko, Professor (Germany)
 
Shortcut: B
Date: Monday, 30. March 2020, 11:00
Room: Hall B
Session type: Oral

Contents

11:00 B-01

Linear damage accumulation of self-pierce riveted joints (#107)

L. Masendorf1, M. Wächter1, S. Horstmann2, M. Otroshi2, A. Esderts1, G. Meschut2

1 Clausthal University of Technology, Institut für Maschinelle Anlagentechnik und Betriebsfestigket, Clausthal-Zellerfeld, Lower Saxony, Germany
2 Paderborn University, Laboratorium für Werkstoff- und Fügetechnik, Paderborn, North Rhine-Westphalia, Germany

Joining technology is regarded as a key technology for reducing energy consumption and CO2 imitation as well as the use of innovative materials and development of new, resource-saving products. Punch riveting is a widely used and established joining process in many sectors. The white and brown goods, electrical engineering, construction and, in particular, the automotive industry are some of the sectors mentioned here.

Since the design and assessment of punch rivet components with regard to structural durability can only be carried out experimentally using prototypes due to a lack of experience and calculation concepts, the improvement of this uneconomical and time-consuming procedure is the goal of this contribution.

Therefore, a numerical simulation and design method for cyclically loads punched riveted joints shall be introduced. This concept shall be based on the notch strain concept.

The following steps are necessary to achieve the goal shown above:

  1. Tensile tests on all materials involved in the joint for determination of tensile strength and quasi-static stress-strain curves
  2. Estimation of the cyclic material properties from the tensile strength in order to obtain the strain-life curve and the cyclic stress-strain curve
  3. Estimation of mean stress sensitivity from the tensile strength to conduct an amplitude transformation for variable amplitude loadings.
  4. Execution of a 2D forming simulation of the joining process to determine the geometry and the stresses and degrees of deformation present in the connection
  5. Transferring the results of the forming simulation into a static-mechanical load simulation for determining the relation between the external load and the elastic-plastic strain at the critical point
  6. Estimation of the service life by means of the damage parameter Wöhler curves calculated from the strain-life curve

In order to verify the simulation and calculation method, service life investigations have been carried out on punched riveted components under constant and variable amplitude load.

The test results, as well as the workflow through the fatigue assessment and its accuracy in estimation the fatigue life will be shown in this contribution.

Keywords: punch rivet, notch strain conept, structural durability
11:20 B-02

Application of an Integral Treatment of Butt Joints for the Fatigue Life Assessment under Variable Amplitude Loading (#68)

B. Möller1

1 Fraunhofer Institute for Structural Durability and System Reliability LBF, Materials and Components, Darmstadt, Hesse, Germany

The fatigue assessment of welded joints according to recommendations and standards is based on fatigue (FAT) classes. These FAT classes were derived from experimental results of specimens and components, but do not include the low cycle fatigue regime and, hence, an elastic-plastic description of the material behaviour. The typical fatigue life of structures within the design of truck cranes is related to the low cycle fatigue (LCF) regime and often focuses on critical details of welded joints. Butt joints are essential load-carrying welds. An example of highly stressed structures is the telescopic boom of truck cranes.

The analysis of existing service measurements already showed that service loading of highly loaded crane structures can be approximated by Gaussian-like load spectra with loads related to the low cycle fatigue regime.

For a fatigue life assessment in the transition from the low cycle to the high cycle fatigue regime, an integral treatment of butt joints has been introduced. This integral treatment is based on the cyclic material behaviour, but characterises butt welds by cyclic stress-strain curves and strain-life curves using representative volume elements to describe a material integral considering the range from base material across the weld to the base material with the help of strain-controlled fatigue testing. A strain-based approach using damage parameters to assess modelled stress-strain paths is used for the fatigue life estimation. This approach, using the integral treatment of the butt joint, is not just applied for the verification under constant amplitude loading, but also for the fatigue life estimation under variable amplitude loading of the Gaussian-like load spectra.

Keywords: integral treatment, strain-based fatigue life assessment, high-strength fine-grained steels, welded butt joints, gaussian load spectrum
11:40 B-03

Analysis of the notch effect components of weld joint (#85)

V. Chmelko1, M. Harakaľ2, P. Zlabek3

1 Slovak University of Technology, Applied Mechanics and Mechatronics, Bratislava, Slovakia
2 Knorr-Bremse AG, Munchen, Germany
3 University of West Bohemia, RTI, Pilsen, Czech Republic

In general, the welded joints are multifactorial notches. This study presents the methodology of experimental separation of total notch factor into two basic components: geometrical influence (when the weld can be seen on the surface) and technological factors like (internal defects of the weld joint, structural changes of the material and residual stress). Proposed methodology of this separation comes out from the cyclic test of three specimens – group with an existing weld joint, group without any weld joint (base material) and another group where the outer geometry of the weld joint was removed (see figure). 

In the article, there will be shown the effect of weld geometry on the weld joint fatigue life-time which will be obtained by FEM methods as well as there will be presented the methodology and discussion of practical usage for obtained results.

specimens
specimens for analysis of the notch effect components
S-N curves
S-N curves of 3 groups of specimens - methodology of separation of weld joint notch factor components
Keywords: weld joint, notch factor, life time
12:00 B-04

Extremely low cycle fatigue assessment model for structural steel and its application to welded joints (#122)

T. Hanji1, K. Tateishi1

1 Nagoya University, Department of Civil and Environmental Engineering, Nagoya, Japan

Low cycle fatigue damage was observed in steel bridge members, for instance, in the 1995 Southern Hyogo Prefecture Earthquake and the 2011 off the Pacific coast of Tohoku Earthquake. In a strong seismic event, steel members may undergo repetitive inelastic deformation. Simultaneously, extremely large cyclic strains are locally introduced to welded joints, which can cause low cycle fatigue cracks. In order to establish a design method against low cycle fatigue, fatigue strength of steel members, particularly that of welded joints, should be assessed as precisely as possible.
In conventional low cycle fatigue tests, hourglass shaped specimens are commonly used. The hourglass shaped specimen has an advantage of being free from buckling of the specimen. On the other hand, since the specimen has only one tested section at the minimum sectional area, it is difficult to use it in testing fatigue strength of welded joints. Then, the authors have newly developed a low cycle fatigue testing system, in which a steel plate specimen is subjected to bending deformation and image analysis technique is applied to measure strains in the specimen [1]. The testing system has enabled low cycle fatigue tests with introducing extremely large strains into the specimen, even though it includes welded parts.
There have been several studies on low cycle fatigue of plain steel material which indicate that fatigue life under variable strain amplitude can be predicted with Miner’s rule and Manson - Coffin’s relationship. In an extremely large strain field corresponding to the fatigue life of less than ten cycles, however, their applicability has not been sufficiently confirmed. Besides, the fatigue strength of welded joints under variable strain amplitude has not been well investigated.
In this study, low cycle fatigue tests under variable strain amplitude on plain steel material were conducted with the newly developed testing system. Based on the test results, a simple low cycle fatigue assessment model in the extremely large strain field was proposed by introducing a damage mechanics concept. And then, its applicability to welds was indicated through low cycle fatigue tests on welded joints under constant and variable strain amplitudes.

References

[1] K. Tateishi and T. Hanji (2004) Low cycle fatigue strength of butt-welded steel joint by means of new testing system with image technique, International Journal of Fatigue, Vol.26(12), pp.1349-1356.

Keywords: Extremely low cycle fatigue strength, Welded joint, Damage mechanics, Miner’s rule