The effect of overloads on fatigue crack propagation measured by DIC, BEMI and synchrotron (#55)
M. Marx1, M. Thielen1, C. Motz1
1 Saarland University, Materials Science and Engineering, Saarbruecken, Saarland, Germany
A key factor to understand fatigue crack propagation is knowing the local effective applied stresses at the crack tip, the driving forces of cracks. Because those local stresses are hardly accessible in experiments, the stress intensity factor according to Paris and Erdogan  is used to relate the crack propagation rate, crack parameters and the external load in a simple way. Nevertheless, in various cases this simple law fails e.g. for variable amplitude loading, beginning after occurrence of an overload (OL). The mechanisms of the OL effect, residual stresses (RS) in front of the crack tip and plasticity induced crack closure (PICC) due to the plastic wake, have been studied for a long time without a final description of the effect.
In this work, a crack in S960Q steel has been followed before OL, after OL, at maximum retardation and recovery. Additionally, the effect of just one relaxing cycle of complete unloading was investigated. We present the mapping of stress fields and strain fields induced by OLs on a fatigue crack and their influence on transient crack growth retardation. This investigation was performed by calibrated magnetic Barkhausen noise (MBN) microscope  and digital image correlation (DIC) based on in-situ SEM imaging . Both results were compared to synchrotron x-ray diffraction .
We observed a strong correlation of the local fatigue crack growth rate (Fig. 1a) with the micro RS distribution obtained from 2D mappings by MBN with a spatial resolution of 10 μm (Fig. 1b): The RS field after the overload explains in the first instance the successive retardation while the DIC results reveal the influence of these RS on crack tip’s opening reactions and strain fields under external loads. While strain fields show a strong decrease due to the OL, differences in crack opening stresses remain rather low at first, but prevail in the further part of the OL region. The influence of PICC and RS could be distinguished by careful examination of crack tip opening displacement (CTOD) and crack tip strain fields measured by DIC. At the beginning, RS determine the crack propagation. With increasing growth through the OL region, closure becomes more important since a larger part of the crack is affected by the OL PICC while the RS becomes less pronounced as the crack tip strain fields exceed the OL region. However, at an R-ration of 0.5 just after one cycle of relaxation by unloading to zero load, the crack retardation effect decreases enormously (Fig. 2). The decrease in the crack propagation effect is just to half the prior speed instead of one tenth of the crack growth rate before the overload. This is very important and has to be taken into account for a statistical loading amplitudes and order effects.
Calculations based on the changes of the effects described the observed retardation in fatigue crack growth accurately. These are not claimed to build a properly predictive model, but the combined measurements open the possibility of physical interpreting underlying effects and provide thereby useful information for more sophisticated modelling and simulation approaches. Concluding we interpret the results as follows: The RS effect on the strain fields can be associated to be more significant than PICC at maximum retardation with a change of mechanisms on reacceleration.
 Paris P., Erdogan F., J. Basic Eng 85, 4 (1963) 528-533
 Sheikh Amiri M., Thielen M., Rabung M., Marx M., Szielasko K., Boller C., J. Magn. Magn. Mater., 372 (2014) 16-22
 Thielen M., Marx M., Sheikh-Amiri M., Boller C., Motz C., Materials Performance and Characterization 5, 3 (2016)
 Thielen M., Schaefer F., Gruenewald P., Laub M., Marx M., Meixner M., Klaus M., Motz C., Int. J. Fatigue 121 (2019) 155–162
Keywords: crack closure, overload, fatigue crack propagation
Determining the influence of overloads on the cyclic material behaviour of nodular cast iron (#17)
C. Bleicher1, J. Hesseler1, R. Wagener1, H. Kaufmann1, T. Melz1
1 Fraunhofer LBF, Darmstadt, Germany
To reduce weight and increase the power as well as the utilization of nodular cast iron components e.g. for wind turbines and heavy industry parts, locally higher loads need to be withstand by the material. This becomes crucial, when additional overloads coming from a storm or a misuse influence the structure of thick-walled components causing high local elastic-plastic deformations. In this case, the cyclic, elastic-plastic material behavior and its development under cyclic loading are important to consider during component design.
To assess the material’s local elastic-plastic material behavior under the influence of overloads, strain-controlled fatigue tests under constant and variable amplitude loading were performed at alternating loading, R = -1, with unnotched specimens removed from cast blocks, a main frame and two planet carriers of wind turbines made of EN-GJS-400-18U-LT, EN-GJS-700-2, ADI-800 and ADI-900.
By using two different load time histories with a bulky and a more linear form developed from local hot spots at a wind turbine’s hub and a main frame in a first step the lightweight potential could be determined by comparing the S-N curves for constant and variable amplitude loading. In order to analyze the influence of an additional overload during usage, in additional test series a maximum overload of 0.5 and 1.0 % total strain under tensile loading were included in the load sequence and the results compared to those without overload.
Keywords: nodular cast iron, overloads, ADI, wind energy, thick-walled
Lifetime extension due to initial quasi-static overloads (#43)
R. Szlosarek1, A. Rudolph1, J. Köckritz1, M. Kröger1
1 TU Freiberg, Institute for Machine Elements, Engineering Design and Manufacturing, Freiberg, Germany
The lifetime of fluctuating loaded components is often determined by the stress concentration around notches. A lifetime extension can be achieved through design modifications by mitigating the notch. This can be reached by using larger transient radii or by introducing relief notches. These options are not always feasible due to limited available space. Another possibility is the introduction of residual stresses by shot blasting, deep rolling or surface hammering. All these methods require an additional work step in the production process and are also only limited suitable to introduce residual stresses around notches. The presented work will show a possibility of increasing the lifetime by using initial quasi-static overloads. The advantage of this method is that the residual stresses are applied exclusively in the region of the notches. Hence, the surface qualities and dimensions of the functional surfaces of the component will not be modified. Flat and round samples made from ductile steels are used to demonstrate the increase in lifetime under tensile and compressive loads. Different stress ratios, notch geometries and overloads are presented. The obtained results show a significant increase in lifetime. The degree of extension depends on the overload amplitude, the stress concentration and the stress ratio. The dependencies of these parameters to the lifetime were overserved in several test series.
Numerical simulations using the finite element method were performed to understand the phenomena. Therefore, elastic-plastic material models were used to simulate the initial overload and the following fluctuating loads. Using the simulation models, it is possible to explain the lifetime extension by a changing mean stress during the fluctuating load due to the residual stress. Using the numerical results, it was possible to optimize the initial overloads to maximize the lifetime.
Keywords: residual stress, overload, simulation, notch effect
The influence of rigid obstacles on fatigue life-time of critical part of axle trailers (#112)
J. Steinhübl1, M. Šulko2
1 KNOTT spol. s r.o., Development & Design, Modra, Slovakia
The operation of the vehicles, which also includes trailers, causes significant stress amplitudes, especially in the material of the bogie support parts. The current trend of optimizing the chassis components in trailer technology follows the trends of the automotive industry in order to reduce the weight of the trailer as much as possible due to its weight gain while maintaining the original vehicle category. Statistics on the use of trailers of category O1 and O2 show that their use in full load at maximum permissible weight is a maximum of 20% of their operating conditions. On the other hand, there are very often cases of overloading of trailers up to 20% above their maximum permissible weight. At the design stage, it is not always possible to estimate and anticipate all non-standard phenomena and impacts in the actual design operation and to analyze their impact on the resulting dynamic strength and fatigue life of its supporting parts.
The paper is focused on the analysis of the impact of overcoming of rigid obstacles on fatigue strength and durability of the critical cross-section of the trailer axle. On the basis of direct measurements in operation, the density of obstacles, the speed of the passage and the tire pressure are analyzed. Measures to increase the fatigue strength of the axle are presented.
Kliman,V, 1984: Fatigue Life Prediction for a Material Under Programable Loading Using Cyclic Stress-Strain Properties. Materials Science & Engineering, 68, 1984 s.1
Amzallag,C.-Gerey,.P.-Robert,J.L.-Bahuaudt,J 1994: Standardization of the Rainflow Counting Method for Fatigue Analysis. International Journal of Fatigue, 16, 1994, s.287
Morrow,J.D.,1968: In: Fatigue Design Handbook. Red.:Graham,J.A. Warendale, PA, USA, Society of Automotive Engineerings. 1968, s.21
Keywords: fatigue, real operation, axle, measurement