On the mean stress sensitivity of cast aluminium considering imperfections

Aigner, Roman; Leitner, Martin; Stoschka, Michael

doi:10.1016/j.msea.2019.04.119

Cited by 15 publications

(9 citation statements)

References 75 publications

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“…Future work will deal with the implementation of the statistically distributed fatigue strength of the defect free material as well as the long crack threshold, as originally presented in [37], along with a mean stress sensitivity model, as provided in [53], in order to set up a holistic fatigue assessment map.…”

Section: Discussionmentioning

confidence: 99%

“…Thus, the cyclic R-curve can be implemented in the KTD by inserting Equation (4) in the Kitagawa–Takahashi relationship—see Equation (3). A preliminary study in [53] reveals this approach to be suitable to assess the mean stress sensitivity of both imperfect and near defect free material. Hereby, a three-dimensional KTD is set up by implementing a mean stress sensitivity approach, provided in a common guideline [22] and a generalized R-curve considering varying load stress ratios, as proposed in [54].…”

Section: Introductionmentioning

confidence: 99%

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On the Statistical Size Effect of Cast Aluminium

et al. 2019

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Manufacturing process based imperfections can reduce the theoretical fatigue strength since they can be considered as pre-existent microcracks. The statistical distribution of fatigue fracture initiating defect sizes also varies with the highly-stressed volume, since the probability of a larger highly-stressed volume to inherit a potentially critical defect is elevated. This fact is widely known by the scientific community as the statistical size effect. The assessment of this effect within this paper is based on the statistical distribution of defect sizes in a reference volume V 0 compared to an arbitrary enlarged volume V α . By implementation of the crack resistance curve in the Kitagawa–Takahashi diagram, a fatigue assessment model, based on the volume-dependent probability of occurrence of inhomogeneities, is set up, leading to a multidimensional fatigue assessment map. It is shown that state-of-the-art methodologies for the evaluation of the statistical size effect can lead to noticeable over-sizing in fatigue design of approximately 10 % . On the other hand, the presented approach, which links the statistically based distribution of defect sizes in an arbitrary highly-stressed volume to a crack-resistant dependent Kitagawa–Takahashi diagram leads to a more accurate fatigue design with a maximal conservative deviation of 5 % to the experimental validation data. Therefore, the introduced fatigue assessment map improves fatigue design considering the statistical size effect of lightweight aluminium cast alloys.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

On the Statistical Size Effect of Cast Aluminium

et al. 2019

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“…The results are summarized in Table 4. Considering the comparably high residual stresses at the crack initiation spot as an existing mean stress, they change the present mean stress state and affect the crack initiation, propagation and consequently the fatigue strength in great measure [69,70].…”

Section: In-depth Residual Stress Distributionmentioning

confidence: 99%

Effect of Post Treatment on the Microstructure, Surface Roughness and Residual Stress Regarding the Fatigue Strength of Selectively Laser Melted AlSi10Mg Structures

Schneller

Leitner

Pomberger

et al. 2019

JMMP

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This paper focusses on the effect of hot isostatic pressing (HIP) and a solution annealing post treatment on the fatigue strength of selectively laser melted (SLM) AlSi10Mg structures. The aim of this work is to assess the effect of the unprocessed (as-built) surface and residual stresses, regarding the fatigue behaviour for each condition. The surface roughness of unprocessed specimens is evaluated based on digital light optical microscopy and subsequent three-dimensional image post processing. To holistically characterize contributing factors to the fatigue strength, the axial surface residual stress of all specimens with unprocessed surfaces is measured using X-ray diffraction. Furthermore, the in-depth residual stress distribution of selected samples is analyzed. The fatigue strength is evaluated by tension-compression high-cycle fatigue tests under a load stress ratio of R = −1. For the machined specimens, intrinsic defects like pores or intermetallic phases are identified as the failure origin. Regarding the unprocessed test series, surface features cause the failures that correspond to significantly reduced cyclic material properties of approximately −60% referring to machined ones. There are beneficial effects on the surface roughness and residual stresses evoked due to the post treatments. Considering the aforementioned influencing factors, this study provides a fatigue assessment of the mentioned conditions of the investigated Al-material.

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“…An overview of these and further state-of-the-art common mean stress models can be found in [51]. The FKM approach features a reduced sensitivity at high load stress ratios and is therefore characterized by a bilinear formulation.…”

Section: Figurementioning

confidence: 99%

Notch Stress Intensity Factor (NSIF)-Based Fatigue Design to Assess Cast Steel Porosity and Related Artificially Generated Imperfections

Schuscha

Horvath

Leitner

et al. 2019

Metals

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Shrinkage porosities and non-metallic inclusions are common manufacturing process based defects that are present within cast materials. Conventional fatigue design recommendations, such as the FKM guideline (“Forschungskuratorium Maschinenbau”), therefore propose general safety factors for the fatigue assessment of cast structures. In fact, these factors mostly lead to oversized components and do not facilitate a lightweight design process. In this work, the effect of shrinkage porosities on the fatigue strength of defect-afflicted large-scale specimens manufactured from the cast steel G21Mn5 is studied by means of a notch stress intensity factor-based (NSIF-based) generalized Kitagawa diagram. Additionally, the mean stress sensitivity of the material is taken into account and establishes a load stress ratio enhanced diagram. Thereby, the fatigue assessment approach is performed by utilizing the defects sizes taken either from the fracture surface of the tested specimens or from non-destructive X-ray investigations. Additionally, a numerical algorithm invoking cellular automata, which enables the generation of artificial defects, is presented. Conclusively, a comparison to the results of the experimental investigations reveals a sound agreement to the generated spatial pore geometries. To sum up, the generalized Kitagawa diagram, as well as a concept utilizing artificially generated defects, is capable of assessing the local fatigue limit of cast steel G21Mn5 components and features the mapping of imperfection grades to their corresponding fatigue strength limit.

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On the mean stress sensitivity of cast aluminium considering imperfections

Cited by 15 publications

References 75 publications

On the Statistical Size Effect of Cast Aluminium

On the Statistical Size Effect of Cast Aluminium

Effect of Post Treatment on the Microstructure, Surface Roughness and Residual Stress Regarding the Fatigue Strength of Selectively Laser Melted AlSi10Mg Structures

Notch Stress Intensity Factor (NSIF)-Based Fatigue Design to Assess Cast Steel Porosity and Related Artificially Generated Imperfections

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