Importance of toe irregularity for fatigue resistance of automatic welds

The early propagation stages of fatigue cracks along the toe of automatic bead-on-plate welds in a structural steel are studied. Three methods to introduce a controlled degree of waviness on the weld toe, to optimize the degree of crack interaction during fatigue growth, were tested: arc rotation, variable arc voltage and variable weld speed. The development of fatigue cracks in seven specimens was monitored using a strain-gauge method and beach marking. Crack mismatch and depth to coalescence were found to be much smaller in the case of straight welds, which also showed shorter propagation lives. The period of toe waves, as well as local toe geometry, strongly influence fatigue crack initiation and propagation lives. Best fatigue life improvements were obtained with arc rotation techniques.     

FATIGUE LIMIT OF BLUNT-NOTCHED COMPONENTS

The position and effective resistance of microstructural barriers and their relation to the fatigue strength of blunt-notched specimens are analysed and modelled for three low-carbon steel microstructures. A relationship for the notch size effect on the basis of the experimental evidence that the fatigue limit (both plain and notched) represents the threshold stress for the propagation of the nucleated microstructurally short cracks, was derived. The derived relationship characterizes the fatigue notch sensitivity by means of the parameter k_td defined as the stress concentration introduced by the notch at a distance d from the notch root surface equal to the distance between microstructural barriers, and was experimentally verified for two notch geometries in three microstructures: ferrite, ferrite–bainite and bainite–martensite.     

A technique to produce automatic welds with enhanced fatigue crack propagation lives under transverse loading

In order to contribute to the development of an automatic weld process that introduces a controlled degree of irregularity to the weld toe, as a means to enhance the development and interaction of cracks and improve fatigue resistance, the double arc rotating technique (DART) was developed. Beadon-plate welds were fabricated using this method. Weld toes were fatigue tested by three point bending in the transverse and longitudinal directions. The fatigue propagation behaviour of cracks growing from the weld toes, and its relationship with defined geometric parameters, were modeled using a fracture mechanics model. Compared with standard straight welds, fatigue life improved up to 111% when loaded in the transversal direction, while decreased about 10% in the longitudinal direction. A fatigue life prediction method for welds with controlled toe irregularity, considering defined toe geometry parameters, is presented, and results are discussed.     

CTOD-R curve construction from surface displacement measurements

The construction of a fracture resistance δ–R (or J–R) curve requires the appropriate measurement of crack-tip opening displacement (CTOD) as a function of crack extension. This can be made by different procedures following ASTM E1820, BS7448 or other standards and procedures (e.g., GTP-02, ESIS-P2, etc.) for the measurement of fracture toughness. However, all of these procedures require standard specimens, displacement gauges, and calibration curves to get intrinsic material properties. This paper deals with some analysis and aspects related to the measurement of fracture toughness by observing the surface of the specimen. Tests were performed using three-dimensional surface displacement measurements to determine the fracture parameters and the crack extension values. These tests can be conducted without using a crack mouth opening displacement-CMOD or load-line displacement gauge, because CMOD can be calculated by using the displacement of the surface points. The presented method offers a significant advantage for fracture toughness testing in cases where a clip gauge is not easy to use, for example, on structural components. Simple analysis of stereo-metrical surface displacements gives a load vs. crack opening displacement curve. Results show that the initiation of stable crack propagation can be easy estimated as the point of the curve’s deviation. It is possible to determine the deviation point if the crack opening displacement measurements are close to crack tip in the plastic zone area. The resistance curve, CTOD-R, is developed by the local measurement of crack opening displacement (COD) in rigid body area of specimen. COD values are used for the recalculation with the CMOD parameter as a remote crack opening displacement, according to the ASTM standard.     

Fatigue propagation threshold of short cracks under constant amplitude loading

In this study a threshold for fatigue crack propagation as a function of crack length is defined from a depth given by the position d of the strongest microstructural barrier to crack propagation, which defines the plain fatigue limit. The material threshold is estimated from the plain fatigue limit Δσ_eR, the position d of the strongest microstructural barrier and the threshold for long cracks, ΔK_thR. The threshold for eight different materials for which experimental results can be obtained from the literature was estimated. Good agreement was observed in all cases. Some quantitative analyses of the fatigue propagation behavior of short cracks are carried out and discussed.