1. Effects of fixture configurations and weld strength mismatch on J-integral calculation procedure for SE(B) specimensPrimož Štefane, Stijn Hertelé, Sameera Naib, Wim de Waele, Nenad Gubeljak, 2022, izvirni znanstveni članek Opis: This work presents the development of a J-integral estimation procedure for deep and
shallow cracked bend specimens based upon plastic ηpl factors for a butt weld made in an S690 QL
high strength low alloyed steel. Experimental procedures include the characterization of average
material properties by tensile testing and evaluation of base and weld metal resistance to stable tearing
by fracture testing of square SE(B) specimens containing a weld centerline notch. J-integral has been
estimated from plastic work using a single specimen approach and the normalization data reduction
technique. A comprehensive parametric finite element study has been conducted to calibrate plastic
factor ηpl and geometry factor λ for various fixture and weld configurations, while a corresponding
plastic factor γpl was computed on the basis of the former two. The modified ηpl and γpl factors were
then incorporated in the J computation procedure given by the ASTM E1820 standard, for evaluation
of the plastic component of J and its corresponding correction due to crack growth, respectively. Two
kinds of J-R curves were computed on the basis of modified and standard ηpl and γpl factors, where
the latter are given by ASTM E1820. A comparison of produced J-R curves for the base material
revealed that variations in specimen fixtures can lead to ≈10% overestimation of computed fracture
toughness JIc. Furthermore, a comparison of J-R curves for overmatched single-material idealized
welds revealed that the application of standard ηpl and γpl factors can lead to the overestimation of
computed fracture toughness JIc by more than 10%. Similar observations are made for undermatched
single material idealized welds, where fracture toughness JIc is overestimated by ≈5%.
Ključne besede: metal weld, strength mismatch, fracture, plastic correction factors, fixture rollers, J-R resistance curve
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2. Application of normalization method to fracture toughness testing of welds with pronounced strength heterogeneity : doctoral disertationPrimož Štefane, 2022, doktorska disertacija Opis: This doctoral dissertation presents the results of an extensive fracture testing programme of
welds with pronounced strength heterogeneity. Purpose of this programme was to determine
fracture toughness of heterogeneous welds that contain a midplane crack. Application of
standardized fracture testing methods in heterogeneous welds might lead to overestimation or
underestimation of fracture toughness and consequentially to inaccurate assessment of
structural integrity. Reasons for that are variations in mechanical properties of different
material regions in the weld which have a significant impact on development of deformation at
the crack tip, and consequently on the crack driving force. Experimental procedures in scope
of this research include fabrication of weld sample plates, that were welded with MAG process.
The welds were fabricated using two different electrodes, one with higher and one with lower
mechanical properties, with respect to base material S690QL in order to replicate extreme
variations of mechanical properties in the weldment. Fabricated welds were then characterized
in detail using metallography, three-point bend impact testing, indentation hardness
measurements and tensile testing of flat miniature and round bar standard tensile specimens.
Resistance of welds to stable tearing was investigated by fracture testing of square surface
cracked SE(B) specimens containing a weld midplane notch. J-integral has been estimated from
plastic work, using the normalization data reduction method that is included in standard ASTM
E1820. The advantage of the normalization data reduction method is that no special equipment
or complex testing method is needed to measure ductile crack growth during fracture testing.
The ductile crack growth is determined directly from the load-displacement record, by applying
appropriate calibration function and physical lengths of initial and final cracks that were
measured post-mortem with the nine-point method. Several correction factors had to be
calibrated in order to successfully implement the normalization data reduction method to
fracture testing of welds with pronounced strength heterogeneity. For that reason, parametric
finite element analyses were conducted for several weld configurations. Finite element models
incorporated plane strain conditions in order to provide calibrated factors that comply with
plane strain equations included in ASTM E1820. Additionally, crack tip constraint has been
extensively analysed and correlated with the plastic deformation fields. This clarified altered
deformation behaviour of modelled welds in comparison with the base material and
corresponding effect on fracture toughness. Finally, calibrated factors were applied to
computation of J-integral from data that were measured during fracture testing. J-R resistance
curves were constructed for the tested heterogeneous welds and compared to the ones of the
base material. This directly showed the effect of variations of mechanical properties on the
weld fracture behaviour.
Ključne besede: weld, strength mismatch, fracture, normalization data reduction technique, plastic
correction factors, test fixture, SE(B) specimen, J-R resistance curve
Objavljeno v DKUM: 10.01.2023; Ogledov: 778; Prenosov: 206
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3. Determination of lower bound fracture toughness of a high strength low alloy steel welded jointNenad Gubeljak, 2005, izvirni znanstveni članek Opis: The use of high-strength low-alloy steels for high performance structures (e.g. pressure vessels and pipelines) requires high strength consumables to produce an overmatched welded joint. This globally overmatched multipass welded joint contains two significantly different microstructures, as welded and reheated. In this paper, the influence of weld metal microstructure on fracture behaviour is estimated in comparison with the fracture behaviour of composite microstructures (as welded and reheated). The lower bound of fracture toughness for different microstructures was evaluated by using the modified Weibull distortion. The results, obtained using specimens with crack front through the thickness, indicated low fracture toughness, caused by strength mismatching interaction along thevcrck front. In the case of through thickness specimens, at least one local brittle microstructure is incorporatedin the process zone at the vicinity of the crck tip. Hence, unstable fracture occurred with small or without stable crack propagation. Despite the fact that the differences between the impact toughness of a weld metal and that of base metal are insignificant, the future toughness of a weld metal can be significantly lower.
Ključne besede: welded joints, fracture mechanics, Crack tip opening displacement, high strength seeel weld, mismatch effects, fracture toughness, resistance curves
Objavljeno v DKUM: 01.06.2012; Ogledov: 2489; Prenosov: 111
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