scholarly journals Capturing and Micromechanical Analysis of the Crack-Branching Behavior in Welded Joints

Metals ◽  
2020 ◽  
Vol 10 (10) ◽  
pp. 1308
Author(s):  
Wenjie Wang ◽  
Jie Yang ◽  
Haofeng Chen ◽  
Qianyu Yang
Keyword(s):  
Crack Propagation ◽  
Welded Joints ◽  
High Stress ◽  
High Strength ◽  
Crack Branching ◽  
Stress Concentrations ◽  
Ductile Materials ◽  
Pulling Forces ◽  
Crack Propagation Process ◽  
Combined Action

During the crack propagation process, the crack-branching behavior makes fracture more unpredictable. However, compared with the crack-branching behavior that occurs in brittle materials or ductile materials under dynamic loading, the branching behavior has been rarely reported in welded joints under quasi-static loading. Understanding the branching criterion or the mechanism governing the bifurcation of a crack in welded joints is still a challenge. In this work, three kinds of crack-branching models that reflect simplified welded joints were designed, and the aim of the present paper is to find and capture the crack-branching behavior in welded joints and to shed light on its branching mechanism. The results show that as long as there is another large enough propagation trend that is different from the original crack propagation direction, then crack-branching behavior occurs. A high strength mismatch that is induced by both the mechanical properties and dimensions of different regions is the key of crack branching in welded joints. Each crack branching is accompanied by three local high stress concentrations at the crack tip. Three pulling forces that are created by the three local high stress concentrations pull the crack, which propagates along with the directions of stress concentrations. Under the combined action of the three pulling forces, crack branching occurs, and two new cracks initiate from the middle of the pulling forces.

Fatigue Crack Propagation Limit Curves for S690QL and S960M High Strength Steels and their Welded Joints

2018 ◽  
Vol 1146 ◽  
pp. 44-56 ◽  
Author(s):  
János Lukács ◽  
Ádám Dobosy ◽  
Marcell Gáspár
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Propagation ◽  
Crack Growth ◽  
Fatigue Crack Propagation ◽  
Welded Joints ◽  
High Strength Steels ◽  
High Strength ◽  
Base Materials ◽  
Propagation Limit

The objective of the paper is to present the newest results of our complex research work. In order to determination and comparison of the fatigue resistance, fatigue crack growth tests were performed on different grades of S690QL quenched and tempered, and S960TM thermomechanically rolled high strength steels.15 mmand30 mmthick base materials were used for our investigations. Welded joints were made from these base materials, using gas metal arc welding with matching, overmatching, and undermatching filler metals. In the paper, the performance of the welding experiments will be presented, especially with the difficulties of the filler material selection; along with the results of the fatigue crack growth examinations executed on the base materials and its welded joints. Statistical aspects were applied both for the presenting of the possible locations of the cracks in the base materials and the welded joints and for the processing of the measured data. Furthermore, the results will be compared with each other, and the possibility of derivation of fatigue crack propagation limit curves will be referred.

Fatigue Crack Propagation Limit Curves for High Strength Steels and their Application for Engineering Critical Assessment Calculations

2014 ◽  
Vol 891-892 ◽  
pp. 563-568 ◽  
Author(s):  
János Lukács ◽  
Marcell Gaspar
Keyword(s):  
Fatigue Crack ◽  
Crack Propagation ◽  
Crack Growth ◽  
Fatigue Crack Propagation ◽  
Welded Joints ◽  
Research Work ◽  
High Strength Steels ◽  
High Strength ◽  
Critical Assessment ◽  
Propagation Limit

There are different prescriptions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth. The research work aimed (i) to determine fatigue crack propagation limit curves for high strength steels and their welded joints, based on the Paris-Erdogan law; (ii) to use the determined limit curves for engineering critical assessment (ECA) calculations. Experiments were performed on different high strength steels and their welded joints; and the propagating cracks in the specimens represent the different possible locations of the real cracks in the structural elements. Fatigue crack growth tests were executed byΔK-decreasing and constant load amplitude methods. The evaluation process consists of six steps, and by means of the selected values a statistical method can be proposed for determination of the limit curves. Engineering critical assessment calculations were performed on a welded structural element having crack like defects.

Strength of Elastomers. A Perspective

1978 ◽  
Vol 51 (2) ◽  
pp. 225-252 ◽  
Author(s):  
Thor L. Smith
Keyword(s):  
High Speed ◽  
Elevated Temperatures ◽  
High Stress ◽  
High Strength ◽  
Extension Rate ◽  
Stress Concentrations ◽  
Chain Mobility ◽  
Growing Crack ◽  
High Elongation ◽  
Progressive Growth

Abstract The strength and extensibility of an elastomer depend on its overall viscoelastic properties, as reflected in the time and temperature dependence of stress-strain curves, and also on those discrete processes, including crack formation and growth, that culminate in high-speed crack propagation. The discrete processes determine the lifetime of a specimen; the viscoelastic characteristics affect the dependence of stress on deformation. The interplay between these effects causes strength and extensibility to depend strongly on test conditions. An elastomeric network composed solely of highly mobile chains is very weak indeed and fractures at a low elongation. This characteristic differs diametrically from that expected of an idealized network of mobile chains. If such a network were stretched, stress concentrations and unbalanced forces at the molecular level, which can result from short chains, entanglements, and network imperfections, would be vitiated rapidly by stress-biased segmental diffusion, especially at the elevated temperature. Therefore the network should be able to withstand a high elongation and thus a high stress. Hence, the low strength always exhibited by a single-phase non-crystallizable elastomer at elevated temperatures is incompatible with the characteristics ascribed to a network in the molecular theory of rubber elasticity. A network of mobile chains is weak for two reasons. First, microcracks develop readily in a stretched specimen. Their formation is usually attributed to stress concentrations near heterogeneties either within or on the surface of a specimen. Second, and most importantly, a microcrack—once it forms—encounters little resistance to growth because the chains are highly mobile. High strength results not because microcracks do not develop but because their growth is impeded. Unless processes that impede growth come into play, a microcrack enlarges rapidly and catastrophic propagation soon follows. When chain mobility is relatively low, the dissipation of energy through viscoelastic processes near the tip of a slowly growing crack retards its progressive growth. But this source of strength is rather ineffective except within narrow ranges of temperature and extension rate, or time scale more generally. Thus, high strength and toughness result from other mechanisms that impede crack growth. Effective mechanisms usually come into play and impart toughness if colloidal particulate fillers or plastic domains are present, except at low concentration.

A study of the deformation and fracture of single-crystal gold films of high strength inside an electron microscope

1960 ◽  
Vol 255 (1281) ◽  
pp. 218-231 ◽  
Keyword(s):  
Plastic Deformation ◽  
Tensile Strength ◽  
Electron Microscope ◽  
Single Crystal ◽  
High Stress ◽  
High Strength ◽  
Gold Wire ◽  
Detailed Examination ◽  
Stress Concentrations ◽  
High Stress Level

Single-crystal films of gold in (111) orientation, and 500 to 2000 Å in thickness, have been prepared by an evaporation technique. A device has been constructed to allow these films to be strained in a controlled manner while under observation inside the electron microscope (Siemens Elmiskop I). It is shown, by the absence of observable plastic deformation, that the films deform elastically up to abnormally high strain values. This is confirmed, in the case of 500 Å films, by precision electron diffraction measurements, which indicate elastic strains as high as 1 to 1·5%. This represents a tensile strength several times that of hard-drawn gold wire. The high tensile strength occurs despite the presence of a high density of dislocations. Failure occurs once the elastic limit is exceeded. Detailed examination of the fractured specimens reveals that highly localized plastic deformation occurs immediately before fracture. The nature of the fracture process has been deduced from the micrographs, and it is shown that the catastrophic failure occurs as a result of the high stress level which exists when plastic deformation occurs, coupled with the stress concentrations which occur as localized thinning takes place.

Micromechanisms of Deformation and Fracture in a Ti3Al-Nb Alloy

1990 ◽  
Vol 213 ◽  
Author(s):  
A.S. Akkurt ◽  
G. Liu ◽  
G.M. Bond
Keyword(s):  
Crack Propagation ◽  
Material Characterization ◽  
High Stress ◽  
Crack Branching ◽  
In Situ Tem ◽  
Phase Boundaries ◽  
Deformation And Fracture ◽  
Characterization Studies ◽  
Fracture Processes

ABSTRACTThe object of this study has been to gain a greater understanding of deformation and fracture processes in a Ti-24Al-11Nb (at %) alloy. The in-situ TEM deformation technique has been used to observe these processes as they occur. Material characterization studies revealed the existence of three different crystal structures in the material (α2,B2 and orthorhombic (0)). Slip is first initiated in the B2 and 0 phases. Although dislocations are observed in the α2 phase in the deformed material, they are seen only in high-stress regions and only in some laths. While numerous dislocations may be injected into the corresponding grain and phase boundaries, slip does not easily propagate into the α2 phase. Cracks are seen to originate most frequently in the regions transformed fully to α2 laths, and sometimes in the regions that contain α2 laths at prior β grain boundaries. Failure of grain or phase boundaries in the B2 or 0 phases where no α2 is present is not observed. Crack branching in the α2 phase is common, particularly in the vicinity of phase boundaries. Crack propagation in the B2 and 0 phases occurs by plastic thinning, and cracks formed in the α2 lath regions are either stopped in phase boundaries or blunted in the neighboring 0 or B2 grains.

scholarly journals Review of Fatigue Assessment Methods for Welded Steel Structures

2018 ◽  
Vol 2018 ◽  
pp. 1-16 ◽  
Author(s):  
Boris Fuštar ◽  
Ivan Lukačević ◽  
Darko Dujmović
Keyword(s):  
Welded Joints ◽  
Fatigue Crack Initiation ◽  
High Stress ◽  
Steel Structures ◽  
Assessment Methods ◽  
Stress Concentrations ◽  
Additional Material ◽  
Welded Steel ◽  
Fatigue Assessment ◽  
Steel Joints

Due to high stress concentrations, welded joints represent the most common locations of fatigue crack initiation in steel structures that are prone to fatigue. Welding affects material properties by the process of heating, cooling, and combining basic and additional material. Since welding is the primary process of joining elements in steel structures, it is obvious that fatigue assessment during the design and maintenance process becomes inevitable. There are many fatigue assessment methods of welded joints, but their precision remains questionable. This paper represents a review of the most common fatigue assessment methods used for welded steel joints. As a result of this review, areas that require additional research are highlighted.

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