scholarly journals Validation Study on the Statistical Size Effect in Cast Aluminium

Metals ◽  
2020 ◽  
Vol 10 (6) ◽  
pp. 710
Author(s):  
Matthias Oberreiter ◽  
Sebastian Pomberger ◽  
Martin Leitner ◽  
Michael Stoschka
Keyword(s):  
Fatigue Strength ◽  
Size Effect ◽  
Bulk Material ◽  
Assessment Model ◽  
Cast Material ◽  
Stressed Volume ◽  
Strength Assessment ◽  
Cast Aluminium ◽  
Distribution Parameters ◽  
And Shifts

Imperfections due to the manufacturing process can significantly affect the local fatigue strength of the bulk material in cast aluminium alloys. Most components possess several sections of varying microstructure, whereat each of them may inherit a different highly-stressed volume (HSV). Even in cases of homogeneous local casting conditions, the statistical distribution parameters of failure causing defect sizes change significantly, since for a larger highly-stressed volume the probability for enlarged critical defects gets elevated. This impact of differing highly-stressed volume is commonly referred as statistical size effect. In this paper, the study of the statistical size effect on cast material considering partial highly-stressed volumes is based on the comparison of a reference volume V 0 and an arbitrary enlarged, but disconnected volume V α utilizing another specimen geometry. Thus, the behaviour of disconnected highly-stressed volumes within one component in terms of fatigue strength and resulting defect distributions can be assessed. The experimental results show that doubling of the highly-stressed volume leads to a decrease in fatigue strength of 5% and shifts the defect distribution towards larger defect sizes. The highly-stressed volume is numerically determined whereat the applicable element size is gained by a parametric study. Finally, the validation with a prior developed fatigue strength assessment model by R. Aigner et al. leads to a conservative fatigue design with a deviation of only about 0.3% for cast aluminium alloy.

scholarly journals Fatigue strength characterization of Al-Si cast material incorporating statistical size effect

2018 ◽  
Vol 165 ◽  
pp. 14002 ◽  
Author(s):  
Roman Aigner ◽  
Martin Leitner ◽  
Michael Stoschka
Keyword(s):  
Size Effect ◽  
Extreme Value Distribution ◽  
Ct Scanning ◽  
Element Analysis ◽  
Cast Material ◽  
Stressed Volume ◽  
Alloy Casting ◽  
Weakest Link ◽  
Testing Region ◽  
Strength Characterization

Cast aluminium components may exhibit material imperfections such as shrinkage and gas pores, or oxide inclusions. Therefore, the fatigue resistance is significantly influenced by the size and location of these inhomogenities. In this work, two different specimen geometries are manufactured from varying positions of an Al-Si-Cu alloy casting. The specimen geometries are designed by means of shape optimization based on a finite element analysis and exhibit different highly-stressed volumes. The numerically optimized specimen curvature enforces a notch factor of only two percent. To enable the evaluation of a statistical size effect, the length of the constant testing region and hence, the size of the highly-stressed volume varies by a ratio of one to ten between the two specimen geometries. Furthermore, the location of the crack initiation is dominated by the comparably greatest defects in this highly-stressed volume, which is also known as Weibull’s weakest link model. The crack initiating defect sizes are evaluated by means of light microscopy and modern scanning electron microscope methods. Finally, the statistical size effect is analysed based on the extreme value distribution of the occurring defects, whereby the size and location of the pores is non-destructively obtained by computed tomography (CT) scanning. This elaborated procedure facilitates a size-effect based methodology to study the defect distribution and the associated local fatigue life of CPS casted Al-Si lightweight components.

scholarly journals The effect of microstructural heterogeneities on the High Cycle Fatigue scatter of cast aluminium alloys: from an elementary volume to the structure

2018 ◽  
Vol 165 ◽  
pp. 14006 ◽  
Author(s):  
Driss El Khoukhi ◽  
Franck Morel ◽  
Nicolas Saintier ◽  
Daniel Bellett ◽  
Pierre Osmond
Keyword(s):  
Size Effect ◽  
Aluminium Alloys ◽  
Fatigue Testing ◽  
Numerical Approach ◽  
Stressed Volume ◽  
Cast Aluminium ◽  
Critical Defect ◽  
Loaded Material ◽  
Critical Defect Size ◽  
Lost Foam

Cast Al-Si alloys have been widely used in automobile applications thanks to their low density and excellent thermal conductivity. A lot of components made of these alloys are subjected to cyclic loads which can lead to fatigue failure. Furthermore, the well know size effect in fatigue, whereby the fatigue strength is reduced in proportion to an increase in size, can be important. This is caused by a higher probability of initiating a crack in larger specimens (i.e. statistical size effect). This paper analyses the role of casting defects on the statistical size effect. For that, a uniaxial fatigue testing campaign (R=0.1) has been conducted using two cast aluminium alloys, fabricated by different casting processes (gravity die casting and lost foam casting), associated with the T7 heat treatment, and with different degrees of porosity. Different specimens (smooth and notched) with different stressed volumes have been investigated. The first part of this article is dedicated to the experimental characterization of the statistical size effect in both alloys via the concept of the Highly Stressed Volume. The second part investigates the effect of the Highly Stressed Volume on the critical defect size via diagram of Kitagawa-Takahashi. The results show that the presence of statistical size effect is strongly linked to the characteristics of the pore population present in the alloy. A numerical approach, linking the observed pore distribution to the volume of loaded material, is proposed and discussed.

scholarly journals Probabilistic Surface Layer Fatigue Strength Assessment of EN AC-46200 Sand Castings

Metals ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 616
Author(s):  
Sebastian Pomberger ◽  
Matthias Oberreiter ◽  
Martin Leitner ◽  
Michael Stoschka ◽  
Jörg Thuswaldner
Keyword(s):  
Heat Treatment ◽  
Surface Layer ◽  
Fatigue Strength ◽  
Treatment Condition ◽  
Heat Treatment Condition ◽  
Assessment Model ◽  
Sand Cast ◽  
Strength Assessment ◽  
Long Life ◽  
Cast Surface

The local fatigue strength within the aluminium cast surface layer is affected strongly by surface layer porosity and cast surface texture based notches. This article perpetuates the scientific methodology of a previously published fatigue assessment model of sand cast aluminium surface layers in T6 heat treatment condition. A new sampling position with significantly different surface roughness is investigated and the model exponents a 1 and a 2 are re-parametrised to be suited for a significantly increased range of surface roughness values. Furthermore, the fatigue assessment model of specimens in hot isostatic pressing (HIP) heat treatment condition is studied for all sampling positions. The obtained long life fatigue strength results are approximately 6% to 9% conservative, thus proven valid within an range of 30 µm ≤ S v ≤ 260 µm notch valley depth. To enhance engineering feasibility even further, the local concept is extended by a probabilistic approach invoking extreme value statistics. A bivariate distribution enables an advanced probabilistic long life fatigue strength of cast surface textures, based on statistically derived parameters such as extremal valley depth S v i and equivalent notch root radius ρ ¯ i . Summing up, a statistically driven fatigue strength assessment tool of sand cast aluminium surfaces has been developed and features an engineering friendly design method.

Areal fatigue strength assessment of cast aluminium surface layers

2020 ◽  
Vol 133 ◽  
pp. 105423 ◽  
Author(s):  
S. Pomberger ◽  
M. Stoschka ◽  
R. Aigner ◽  
M. Leitner ◽  
R. Ehart
Keyword(s):  
Fatigue Strength ◽  
Surface Layers ◽  
Strength Assessment ◽  
Cast Aluminium ◽  
Fatigue Strength Assessment

scholarly journals Modification of a Defect-Based Fatigue Assessment Model for Al-Si-Cu Cast Alloys

Materials ◽  
2018 ◽  
Vol 11 (12) ◽  
pp. 2546 ◽  
Author(s):  
Roman Aigner ◽  
Martin Leitner ◽  
Michael Stoschka ◽  
Christian Hannesschläger ◽  
Thomas Wabro ◽  
...  
Keyword(s):  
Fatigue Strength ◽  
Fatigue Testing ◽  
Threshold Value ◽  
Fatigue Behaviour ◽  
Mechanical Tests ◽  
Assessment Model ◽  
Fatigue Assessment ◽  
Strength Assessment ◽  
Assessment Approach ◽  
Long Life

Cast parts usually inherit internal defects such as micro shrinkage pores due to the manufacturing process. In order to assess the fatigue behaviour in both finite-life and long-life fatigue regions, this paper scientifically contributes towards a defect-based fatigue design model. Extensive fatigue and fracture mechanical tests were conducted whereby the crack initiating defect size population was fractographically evaluated. Complementary in situ X-ray computed tomography scans before and during fatigue testing enabled an experimental estimation of the lifetime until crack initiation, acting as a significant input for the fatigue model. A commonly applied fatigue assessment approach introduced by Tiryakioglu was modified by incorporating the long crack threshold value, which additionally enabled the assessment of the fatigue strength in the long-life fatigue regime. The presented design concept was validated utilising the fatigue test results, which revealed a sound agreement between the experiments and the model. Only a minor deviation of up to about five percent in case of long-life fatigue strength and up to about 9% in case of finite-lifetime were determined. Thus, the provided extension of Tiryakioglu’s approach supports a unified fatigue strength assessment of cast aluminium alloys in both the finite- and long-life regimes.

Microporosity and statistical size effect on the fatigue strength of cast aluminium alloys EN AC-45500 and 46200

2017 ◽  
Vol 707 ◽  
pp. 567-575 ◽  
Author(s):  
Martin Leitner ◽  
Christian Garb ◽  
Heikki Remes ◽  
Michael Stoschka
Keyword(s):  
Fatigue Strength ◽  
Size Effect ◽  
Aluminium Alloys ◽  
Cast Aluminium

scholarly journals On the Statistical Size Effect of Cast Aluminium

Materials ◽  
2019 ◽  
Vol 12 (10) ◽  
pp. 1578 ◽  
Author(s):  
Roman Aigner ◽  
Sebastian Pomberger ◽  
Martin Leitner ◽  
Michael Stoschka
Keyword(s):  
Size Effect ◽  
Statistical Distribution ◽  
Assessment Model ◽  
Validation Data ◽  
Stressed Volume ◽  
Fatigue Assessment ◽  
Fatigue Design ◽  
Critical Defect ◽  
Cast Alloys ◽  
Set Up

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.

A probabilistic Kitagawa-Takahashi diagram for fatigue strength assessment of cast aluminium alloys

2019 ◽  
Vol 745 ◽  
pp. 326-334 ◽  
Author(s):  
R. Aigner ◽  
S. Pusterhofer ◽  
S. Pomberger ◽  
M. Leitner ◽  
M. Stoschka
Keyword(s):  
Fatigue Strength ◽  
Aluminium Alloys ◽  
Strength Assessment ◽  
Cast Aluminium ◽  
Fatigue Strength Assessment
Sign in / Sign up

Export Citation Format

Share Document