Fatigue failures create enormous risks for all engineered structures, as well as for human lives, motivating large safety factors in design and, thus, inefficient use of resources. Inspired by the excellent fracture toughness of bone, we explored the fatigue resistance in metastability-assisted multiphase steels. We show here that when steel microstructures are hierarchical and laminated, similar to the substructure of bone, superior crack resistance can be realized. Our results reveal that tuning the interface structure, distribution, and phase stability to simultaneously activate multiple micromechanisms that resist crack propagation is key for the observed leap in mechanical response. The exceptional properties enabled by this strategy provide guidance for all fatigue-resistant alloy design efforts.

http://science.sciencemag.org/content/sci/355/6329/1055.full.pdf

Bone-like crack resistance in hierarchical metastable nanolaminate steels

Metastability-based high entropy alloy design opens a new strategic path for designing high-strength materials. However, high strength is always coupled with poor damage tolerance under cyclic load...

https://www.tandfonline.com/doi/pdf/10.1080/21663831.2018.1523240

Metastability-assisted fatigue behavior in a friction stir processed dual-phase high entropy alloy

This study presents a comparative analysis of the high-cycle fatigue behaviors of butt weld joints in advanced high-strength steels with different strength levels and weld bead geometry. Welded joints were made using a gas metal arc welding (GMAW) process on dual-phase steels (DP440 and DP590) and martensitic steel (MS) with tensile strengths of 440, 590, and 1500 MPa, respectively. The microstructures with the lowest hardness were found at the base metal, the sub-critical heat-affected zone (HAZ), and the fusion zone for DP440, DP590, and MS weldments, respectively. Fatigue failure of specimens without weld beads occurred at the points of lowest hardness, and fatigue life exhibited the order MS>DP590>DP440, similar to the order of lowest hardness values in each weldment. However, the introduction of high weld beads resulted in very short, similar fatigue lives for all welded joints and fracture occurred at weld toe due to the overwhelming stress concentration effect. A transition from geometry-governed fracture toward microstructure-governed fracture was investigated by varying weld bead heights.

Microstructure and fatigue performance of butt-welded joints in advanced high-strength steels

A study was done to investigate the effect of tungsten (W) addition on the microstructure, tensile properties and low cycle fatigue (LCF) properties of 9Cr-lMo steels at 298K and 873K. Four steels containing different amounts of W (0W, 1.2W, 1.8W and 2.7W) were normalized at 1323K for 1 hour and tempered at 1023K for 1 hour. Microstructural analysis revealed that no significant differences wete observed in their tempered martensitic microstructure of 0W, 1.2W and 1.8W alloys, but d-ferrite was observed to form at the prior austenite grain boundaries of the 2.7W alloy. With the increase in W content, yield and tensile strength increased at both temperatures. Low cycle fatigue life also increased with the W content up to 1.8%, but decreased in the 2.7W alloy, which was primarily due to the presence of soft d-ferrite acting as the crack initiation site. The fatigue life at 873K was reduced compared to that at 298K, due not only to the decrease in strength at high temperature but also to the formation of oxide layers along the slip bands, which increases slip irre-versibility during cyclic deformation.

Effect of W addition on the low cycle fatigue behavior of high Cr ferritic steels

A study has been made to investigate the fatigue properties of a high-strength spring steel in relation to the microstructural variation via different heat treatments. Rotating–bending fatigue and fatigue crack growth (FCG) tests were conducted to evaluate the fatigue properties, and a transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) unit was used to characterize the tempered microstructure. The results indicate that the fatigue endurance σf increases with increasing tempering temperature, reaching a maximum at 450°C, then decreases. The increase of σf is mainly attributed to the refined distribution of precipitation, together with the structural uniformity of tempered martensite. The softening of tempered martensite due to excessive precipitation accounts for the decrease of σf. By contrast, the FCG results show an insensitivity of the stage-II growth behavior to the microstructural changes for the whole range of tempering temperature tested. The insensitivity is interpreted in terms of the counterbalancing microstructure-dependent contributions to the FCG behavior.

Microstructural influence on fatigue properties of a high-strength spring steel

The work reported here results from a study on the effect of martensite content on the high cycle fatigue performance in which possible correlations between the quasi-static and fatigue properties have been sought. Commercial grade SAE 5115 carburizing steel was used in this study

The effect of martensite content on the fatigue behaviour of a ferritic-martensitic steel

This study presents a novel method for accurately predicting the dynamic behavior of multistory frame buildings under earthquake ground motion. The proposed method allows approximately estimating the inter-story drift ratio, a crucial parameter strongly associated with building damage, its distribution along the building height, and its maximum value location. An equivalent continuous beam model with a rotation at the base, consisting of a combination of a shear beam and a flexural beam, is proposed to achieve this. This model derives closed-form solutions for the building’s dynamic characteristics. The lateral deformations along the height of frame buildings subjected to a given earthquake load, particularly the inter-story drift ratio profiles, and the maximum inter-story drift ratio parameter, are investigated. The proposed continuous model requires two dimensionless parameters: the lateral stiffness ratio (α) and the rotation at the base (θ), representing the drift ratio of the first story. For the expression of the lateral stiffness ratio (α) coefficient, a simple equation is also proposed using the beam-to-column stiffness ratio (ρ, or Blume coefficient) associated with the framed (discrete) system. Various building models are employed to validate the proposed method, demonstrating its applicability to both high-rise and low-rise building configurations. With the results obtained, it is shown that the proposed continuous model can be used not only for high-rise or multistory building models but also for low-rise building models.

/pdf/accurate-estimation-of-inter-story-drift-ratio-in-multistory-d1crv9zx.pdf

Accurate Estimation of Inter-Story Drift Ratio in Multistory Framed Buildings Using a Novel Continuous Beam Model

Klasik kiriş teorilerinde kirişler bir boyutlu kabul edilir. Bu teoriye göre mesnetler tarafsız eksendedir. Ancak pratik uygulamalarda kirişler tarafsız eksenlerinden farklı noktalardan mesnetlenmektedir. Literatürde üniform yük etkisinde eksantrik mesnetlenme durumuna sahip kirişler için düzenlenmiş kiriş teorisi geliştirilmiştir. Bu çalışmada tekil yük altındaki eksantrik mesnetli kirişler MacLaurin serileri kullanılarak analitik olarak çözülmüştür. Eksantrik mesnetlerin kirişin eğilme rijitliği üzerindeki etkileri araştırılmıştır. Elde edilen analitik denklemler, mesnetlerin kiriş derinliğinde farklı konumlarında (eksantrisite) kirişlerin eğilme analizi üzerindeki etkisini araştırmak için kullanılmıştır. Bulgular, kirişlerin eğilme rijitliğinin eksantrik mesnet durumundan önemli ölçüde etkilendiğini göstermektedir. Elde edilen sonuçlar Sonlu Eleman çözümleri ile karşılaştırılmıştır.

H. Türker

Papers

The effect of martensite content on the fatigue behaviour of a ferritic-martensitic steel

Accurate Estimation of Inter-Story Drift Ratio in Multistory Framed Buildings Using a Novel Continuous Beam Model

Investigation of Eccentrically-Supported Simple Beam under Point Load