Editorial

4 pages, 179 KiB

Open AccessEditorial

Technological Aspects in Fatigue Design of Metallic Structures

by Martin Leitner

Metals 2023, 13(3), 610; https://doi.org/10.3390/met13030610 - 18 Mar 2023

Viewed by 778

Traditional manufacturing processes, such as welding and casting, and modern techniques, such as additive manufacturing, can significantly affect the local material properties of metallic materials [...] Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

Research

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17 pages, 13860 KiB

Open AccessArticle

Fatigue Assessment of Wire and Arc Additively Manufactured Ti-6Al-4V

by Sebastian Springer, Martin Leitner, Thomas Gruber, Bernd Oberwinkler, Michael Lasnik and Florian Grün

Metals 2022, 12(5), 795; https://doi.org/10.3390/met12050795 - 04 May 2022

Cited by 6 | Viewed by 1870

Abstract

Wire and arc additively manufactured (WAAM) parts and structures often present internal defects, such as gas pores, and cause irregularities in the manufacturing process. In order to describe and assess the effect of internal defects in fatigue design, this research study investigates the [...] Read more.

Wire and arc additively manufactured (WAAM) parts and structures often present internal defects, such as gas pores, and cause irregularities in the manufacturing process. In order to describe and assess the effect of internal defects in fatigue design, this research study investigates the fatigue strength of wire arc additive manufactured structures covering the influence of imperfections, particularly gas pores. Single pass WAAM structures are manufactured using titanium alloy Ti-6Al-4V and round fatigue, tensile specimen are extracted. Tensile tests and uniaxial fatigue tests with a load stress ratio of R = 0.1 were carried out, whereby fatigue test results are used for further assessments. An extensive fractographic and metallographic fracture surface analysis is utilized to characterize and measure crack-initiating defects. As surface pores as well as bulk pores are detected, a stress intensity equivalent

∆ K_{e q v}

transformation approach is presented in this study. Thereby, the defect size of the surface pore is transformed to an increased defect size, which is equivalent to a bulk pore. Subsequently, the fatigue strength assessment method by Tiryakioğlu, commonly used for casting processes, is applied. For this method, a cumulative Gumbel extreme value distribution is utilized to statistically describe the defect size. The fitted distribution with modified data reveals a better agreement with the experimental data than unmodified. Additionally, the validation of the model shows that the usage of the

∆ K

modified data demonstrates better results, with a slight underestimation of up to about −7%, compared to unmodified data, with an overestimation of up to about 14%, comparing the number of load cycles until failure. Hence, the presented approach applying a stress intensity equivalent transformation of surface to bulk pores facilitates a sound fatigue strength assessment of WAAM Ti-6Al-4V structures. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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14 pages, 11121 KiB

Open AccessFeature PaperArticle

Microstructural Impact on Fatigue Crack Growth Behavior of Alloy 718

by Christian Gruber, Peter Raninger, Jürgen Maierhofer, Hans-Peter Gänser, Aleksandar Stanojevic, Anton Hohenwarter and Reinhard Pippan

Metals 2022, 12(5), 710; https://doi.org/10.3390/met12050710 - 21 Apr 2022

Cited by 3 | Viewed by 1496

Abstract

Alloy 718 for forged parts can form a wide range of microstructures through a variety of thermo-mechanical processes, depending on the number of remelting processes, temperature and holding time of homogenization annealing, cogging and the number of forging steps depending on the forming [...] Read more.

Alloy 718 for forged parts can form a wide range of microstructures through a variety of thermo-mechanical processes, depending on the number of remelting processes, temperature and holding time of homogenization annealing, cogging and the number of forging steps depending on the forming characteristics. In industrial practice, these processing steps are tailored to achieve specific mechanical and microstructural properties in the final product. In the present work, we investigate the dependence of the threshold of stress intensity factor range ΔK_th on associated microstructural elements, namely grain size and distribution. For this purpose, a series of tests with different starting microstructures were performed at the falling stress intensity factor range, ΔK, and a load ratio of R = 0.1 to evaluate the different threshold values. Fracture initiation and crack propagation were analyzed afterward using scanning electron microscopy of the resulting fracture surfaces. In order to obtain comparable initial conditions, all specimens were brought to the same strength level by means of a two-stage aging heat treatment. In the future, this knowledge shall be used in the context of simulation-aided product development for estimating local fatigue crack propagation properties of simulated microstructures obtained from forging and heat treatment modeling. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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24 pages, 9373 KiB

Open AccessArticle

Fatigue Crack Initiation and Propagation Relation of Notched Specimens with Welded Joint Characteristics

by Moritz Braun, Claas Fischer, Jörg Baumgartner, Matthias Hecht and Igor Varfolomeev

Metals 2022, 12(4), 615; https://doi.org/10.3390/met12040615 - 02 Apr 2022

Cited by 9 | Viewed by 3188

Abstract

This study focuses on predicting the fatigue life of notched specimens with geometries and microstructure representative of welded joints. It employs 26 series of fatigue tests on welded and non-welded specimens containing notches located in different material zones, including the parent material, weld [...] Read more.

This study focuses on predicting the fatigue life of notched specimens with geometries and microstructure representative of welded joints. It employs 26 series of fatigue tests on welded and non-welded specimens containing notches located in different material zones, including the parent material, weld metal, and heat-affected zone. Overall, 351 test samples made of six structural steels are included in the present evaluation. For each individual specimen, the stress concentration factor, as well as the stress distribution in the notched section, was determined for subsequent fracture mechanics calculation. The latter is employed to estimate the fraction of fatigue life associated with crack propagation, starting from a small surface crack until fracture. It was shown that the total fatigue life can be realistically predicted by means of fracture mechanics calculations, whereas estimates of the fatigue life until macroscopic crack initiation are subject to numerous uncertainties. Furthermore, methods of statistical data analyses are applied to explore correlations between the S–N curves and the notch acuity characterized by the notch radius, opening angle, and the stress concentration factor. In particular, a strong correlation is observed between the notch acuity and the slope of the S–N curves. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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20 pages, 7282 KiB

Open AccessArticle

Damage-Based Assessment of the Fatigue Crack Initiation Site in High-Strength Steel Welded Joints Treated by HFMI

by Yuki Ono, Halid Can Yıldırım, Koji Kinoshita and Alain Nussbaumer

Metals 2022, 12(1), 145; https://doi.org/10.3390/met12010145 - 12 Jan 2022

Cited by 10 | Viewed by 2234

Abstract

This study aimed to identify the fatigue crack initiation site of high-frequency mechanical impact (HFMI)-treated high-strength steel welded joints subjected to high peak stresses; the impact of HFMI treatment residual stress relaxation being of particular interest. First, the compressive residual stresses induced by [...] Read more.

This study aimed to identify the fatigue crack initiation site of high-frequency mechanical impact (HFMI)-treated high-strength steel welded joints subjected to high peak stresses; the impact of HFMI treatment residual stress relaxation being of particular interest. First, the compressive residual stresses induced by HFMI treatment and their changes due to applied high peak stresses were quantified using advanced measurement techniques. Then, several features of crack initiation sites according to levels of applied peak stresses were identified through fracture surface observation of failed specimens. The relaxation behavior was simulated with finite element (FE) analyses incorporating the experimentally characterized residual stress field, load cycles including high peak load, improved weld geometry and non-linear material behavior. With local strain and local mean stress after relaxation, fatigue damage assessments along the surface of the HFMI groove were performed using the Smith–Watson–Topper (SWT) parameter to identify the critical location and compared with actual crack initiation sites. The obtained results demonstrate the shift of the crack initiation most prone position along the surface of the HFMI groove, resulting from a combination of stress concentration and residual stress relaxation effect. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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23 pages, 2992 KiB

Open AccessFeature PaperArticle

Design Implications and Opportunities of Considering Fatigue Strength, Manufacturing Variations and Predictive LCC in Welds

by Mathilda Karlsson Hagnell, Mansoor Khurshid, Malin Åkermo and Zuheir Barsoum

Metals 2021, 11(10), 1527; https://doi.org/10.3390/met11101527 - 26 Sep 2021

Cited by 2 | Viewed by 2395

Abstract

Fatigue strength dictates life and cost of welded structures and is often a direct result of initial manufacturing variations and defects. This paper addresses this coupling through proposing and applying the methodology of predictive life-cycle costing (PLCC) to evaluate a welded structure exhibiting [...] Read more.

Fatigue strength dictates life and cost of welded structures and is often a direct result of initial manufacturing variations and defects. This paper addresses this coupling through proposing and applying the methodology of predictive life-cycle costing (PLCC) to evaluate a welded structure exhibiting manufacturing-induced variations in penetration depth. It is found that if a full-width crack is a fact, a 50% thicker design can result in life-cycle cost reductions of 60% due to reduced repair costs. The paper demonstrates the importance of incorporating manufacturing variations in an early design stage to ensure an overall minimized life-cycle cost. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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18 pages, 5187 KiB

Open AccessArticle

Digital Scanning of Welds and Influence of Sampling Resolution on the Predicted Fatigue Performance: Modelling, Experiment and Simulation

by Gustav Hultgren, Leo Myrén, Zuheir Barsoum and Rami Mansour

Metals 2021, 11(5), 822; https://doi.org/10.3390/met11050822 - 18 May 2021

Cited by 14 | Viewed by 1979

Abstract

Digital weld quality assurance systems are increasingly used to capture local geometrical variations that can be detrimental for the fatigue strength of welded components. In this study, a method is proposed to determine the required scanning sampling resolution for proper fatigue assessment. Based [...] Read more.

Digital weld quality assurance systems are increasingly used to capture local geometrical variations that can be detrimental for the fatigue strength of welded components. In this study, a method is proposed to determine the required scanning sampling resolution for proper fatigue assessment. Based on FE analysis of laser-scanned welded joints, fatigue failure probabilities are computed using a Weakest-link fatigue model with experimentally determined parameters. By down-sampling of the scanning data in the FE simulations, it is shown that the uncertainty and error in the fatigue failure probability prediction increases with decreased sampling resolution. The required sampling resolution is thereafter determined by setting an allowable error in the predicted failure probability. A sampling resolution of 200 to 250 μm has been shown to be adequate for the fatigue-loaded welded joints investigated in the current study. The resolution requirements can be directly incorporated in production for continuous quality assurance of welded structures. The proposed probabilistic model used to derive the resolution requirement accurately captures the experimental fatigue strength distribution, with a correlation coefficient of 0.9 between model and experimental failure probabilities. This work therefore brings novelty by deriving sampling resolution requirements based on the influence of stochastic topographical variations on the fatigue strength distribution. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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19 pages, 15026 KiB

Open AccessArticle

Influences of Residual Stress, Surface Roughness and Peak-Load on Micro-Cracking: Sensitivity Analysis

by Jairan Nafar Dastgerdi, Fariborz Sheibanian, Heikki Remes and Hossein Hosseini Toudeshky

Metals 2021, 11(2), 320; https://doi.org/10.3390/met11020320 - 12 Feb 2021

Cited by 3 | Viewed by 2060

Abstract

This paper provides further understanding of the peak load effect on micro-crack formation and residual stress relaxation. Comprehensive numerical simulations using the finite element method are applied to simultaneously take into account the effect of the surface roughness and residual stresses on the [...] Read more.

This paper provides further understanding of the peak load effect on micro-crack formation and residual stress relaxation. Comprehensive numerical simulations using the finite element method are applied to simultaneously take into account the effect of the surface roughness and residual stresses on the crack formation in sandblasted S690 high-strength steel surface under peak load conditions. A ductile fracture criterion is introduced for the prediction of damage initiation and evolution. This study specifically investigates the influences of compressive peak load, effective parameters on fracture locus, surface roughness, and residual stress on damage mechanism and formed crack size. The results indicate that under peak load conditions, surface roughness has a far more important influence on micro-crack formation than residual stress. Moreover, it is shown that the effect of peak load range on damage formation and crack size is significantly higher than the influence of residual stress. It is found that the crack size develops exponentially with increasing peak load magnitudes. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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20 pages, 15433 KiB

Open AccessArticle

Fatigue Performance of High- and Low-Strength Repaired Welded Steel Joints

by Jan Schubnell, Phillip Ladendorf, Ardeshir Sarmast, Majid Farajian and Peter Knödel

Metals 2021, 11(2), 293; https://doi.org/10.3390/met11020293 - 08 Feb 2021

Cited by 10 | Viewed by 2925

Abstract

Large portions of infrastructure buildings, for example highway- and railway bridges, are steel constructions and reach the end of their service life, as a reason of an increase of traffic volume. As lifetime extension of a commonly used weld detail (transverse stiffener) of [...] Read more.

Large portions of infrastructure buildings, for example highway- and railway bridges, are steel constructions and reach the end of their service life, as a reason of an increase of traffic volume. As lifetime extension of a commonly used weld detail (transverse stiffener) of these structures, a validated approach for the weld repair was proposed in this study. For this, welded joints made of S355J2+N and S960QL steels were subjected to cyclic loading until a pre-determined crack depth was reached. The cracks were detected by non-destructive testing methods and repaired by removal of the material around the crack and re-welding with the gas metal arc welding (GMAW). Then, the specimens were subjected to cyclic loading again. The hardness, the weld geometry, and the residual stress state was investigated for both the original- and the repaired conditions. It was determined that nearly all repaired specimens reached at least the fatigue life of the original specimen. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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22 pages, 8486 KiB

Open AccessArticle

Validation Study on the Statistical Size Effect in Cast Aluminium

by Matthias Oberreiter, Sebastian Pomberger, Martin Leitner and Michael Stoschka

Metals 2020, 10(6), 710; https://doi.org/10.3390/met10060710 - 27 May 2020

Cited by 7 | Viewed by 2366

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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26 pages, 18151 KiB

Open AccessFeature PaperArticle

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

by Sebastian Pomberger, Matthias Oberreiter, Martin Leitner, Michael Stoschka and Jörg Thuswaldner

Metals 2020, 10(5), 616; https://doi.org/10.3390/met10050616 - 09 May 2020

Cited by 3 | Viewed by 2531

Abstract

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 [...] Read more.

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

{\bar{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. Full article

(This article belongs to the Special Issue Technological Aspects in Fatigue Design of Metallic Structures)

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