MIXED MODE FATIGUE CRACK GROWTH BEHAVIOUR UNDER MICROSTRUCTURAL VARIATION IN FLASH-BUTT WELDS

The strength loss due to welding process poses a high risk of catastrophic rolling contact fatigue failure in the heat affected zones of flash-butt rail welds. Accurate characterisation of fatigue crack growth behaviour in such regions can provide a database for developing safer and more efficient maintenance strategies. This extended abstract details an experimental study on fatigue crack growth behaviour in flash-butt welds in a hypereutectoid rail steel with a hardness level of over 400 HV. Groups of mixed mode fatigue crack growth tests were carried out at parent rail region, partially spheroidised region, fully spheroidised region, re-austenitised region and bond line region. Fractographic analysis was performed to aid the application of the marker band method as well as to analyse the morphology of fracture surfaces. Once all experiments are finished, an equivalent stress intensity factor formula will be fitted to quantify the mixed mode crack driving force in different regions, and modifications of crack growth direction prediction criteria will be proposed for crack growth under the influence of microstructural variation. The current work will provide a reliable database for predicting rolling contact fatigue crack growth at different regions in flash-butt rail welds.
EXTENDED ABSTRACT

FRACTURE TOUGHNESS CHARACTERIZATION OF 316L STAINLESS STEEL WELDED PLATES AT LIQUID NITROGEN (77 K) AND LIQUID HELIUM (4 K) TEMPERATURES

In the framework of a collaborative project between ASME, NASA, and NIST, quasi-static fracture toughness tests were performed at liquid nitrogen temperature (77 K, or 196 °C) and liquid helium temperature (4 K, or -269 °C) on precracked SEN(B) specimens extracted from the centers of four separate lots of welded 316L stainless steel plates. Although the plates were produced in accordance with the same specifications from the same material (316L), large differences in fracture toughness have been observed, with the toughest weld exhibiting almost twice the critical toughness of the least tough at 77 K (219 kJ/m2 vs. 113 kJ/m2), and about seven times the critical toughness of the least tough at 4 K (146 kJ/m2 as compared to 21 kJ/m2). Charpy absorbed energies previously obtained at 77 K for three of the four welds were found to be strongly linearly correlated with fracture toughness at both test temperatures, with an exception represented by the fourth weld, which provided the highest impact toughness and the second lowest quasi-static fracture toughness. Dynamic toughness measurements at impact loading rates were also performed on precracked SEN(B) specimens, in order to deconvolute the roles of strain rate and notch sensitivity on the fracture properties.
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EFFECT OF BOLT PRELOAD ON FRETTING FATIGUE BEHAVIOUR OF DOUBLE LAP BOLTED JOINTS WITH CLASS B SURFACE FINISH IN HIGH-CYCLE FATIGUE: EXPERIMENTAL AND NUMERICAL INVESTIGATION.

Slip-critical bolted connections are prone to developing fretting damage between the contacting surfaces. In this study, the effect of two different bolt preload values, 90 and 145 kN, on the fretting fatigue behaviour of steel double-lap bolted joints with a Class B (shot-blasted) surface under high-cycle fatigue conditions is investigated experimentally and numerically. The experimental results show that increasing the bolt preload decreased the total fretting fatigue life significantly. Moreover, the proposed numerical method was able to predict crack initiation and crack propagation behaviours successfully.
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THE TIP FIELDS OF SHARP V-NOTCH UNDER CREEPING CONDITION CONSIDERING OUT-OF-PLANE EFFECT

Notches in creeping solids at high temperatures have drawn considerable attention due to their importance in structural integrity assessment. Understanding the three-dimensional (3D) effect on the notch tip field is important for the fracture mechanics analysis of engineering materials and structures. This paper presents an asymptotic solution for 3D sharp V-notched structures subjected to mode Ⅰ creep loading condition.
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CONSTANT CTOA DETERMINATION FOR STABLE DUCTILE CRACK GROWTH AND ITS APPLICATION TO RUNNING FRACTURE CONTROL FOR GAS TRANSMISSION PIPELINE

The crack tip opening angle (CTOA) has been used as a reliable fracture toughness to characterize stable crack growth for thin-wall structures in low-constraint conditions. Recently, it has been found that CTOA can be also utilized as a robust fracture parameter to describe arrest fracture toughness for gas transmission pipelines in modern ductile steels. This is a great improvement of the traditional fracture control technology for gas pipelines, where a Charpy-vee notch (CVN) impact energy based two curve model developed at Battelle (BTCM) was used to determine the arrest toughness. While the CVN-based BCTM is not applicable to modern pipeline steels with grades X70 and above, the CTOA-based BTCM works well for these high grades, but requires constant CTOA. This work develops four methods to determine constant CTOA using the single edge notched bend (SENB) specimens, including a load-displacement linear fit method, a logarithmic load-displacement linear fit method, a stable tearing energy method, and a J-differentiation method. The test results for A285 carbon steel show that these CTOA methods can determine nearly identical critical CTOA values over stable ductile crack growth using the SENB specimens.
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FAILURE ANALYSIS AND RESIDUAL LIFE ESTIMATION USING A MIXED METHOD OF X-RAY FRACTOGRAPHY AND SIMULATION

In order to guarantee passengers safety while improving rolling stock maintenance, the French railroad company, SNCF, studies the evolution of cracks that can propagate in fatigue loading situations, particularly for locomotive axles. This research aims to determine the crack propagation history, which is a determining factor in maintenance, studying fracture surfaces while combining X-ray fractography analysis and numerical methods for variable amplitude loads
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