Arc welds and their heat affected zones are heterogeneous due to the nature of the involved welding process. Nevertheless, the complexity of weld integrity assessments is often reduced by assuming homogeneous properties for weld metal and for heat affected zone metal. To evaluate this simplification, the authors aim to examine the effect of weld metal heterogeneity on the structural integrity of welded connections. As a part of this examination, the presented paper describes the characterization of heterogeneity in a selection of multi-pass steel arc welds. An extensive set of micro tensile tests has been performed. Significant differences in strength and yield-to-tensile ratio are observed between as-deposited and grain refined weld metal. Stress-strain curves of local microstructures are defined using a stressstrain model that describes two-stage strain hardening. This approach allows to quantify variations in stress-strain curve shape. The magnitude of the observed heterogeneity justifies the incentive for further research in the field of weld metal heterogeneity.
COBISS.SI-ID: 17749270
The effects of microstructural morphology on the fracture behavior of Ti-6Al-4V ELI (extra-low impurity) alloy in two different heat treatment conditions were examined. Alloy was solution treated above (ß ST) and below (alpha + ß ST) ß transus temperature followed by furnace cooling (FC) in order to obtain the fully lamellar and equiaxed microstructures. Tensile and fracture toughness tests were conducted. The crack tip opening displacement (CTOD) and strain distribution near the crack tip were measured on the compact tension (CT) specimen surface by digital stereometric method. The crack propagation resistance (CTOD-R) curves were developed by applying the modified normalization method and critical CTOD values were determined. To identify themicrostructural length scale controlling the fracture resistance of this alloy, the crack propagation path and fracture surface morphology were evaluated. It was found that the reduction in the characteristic microstructural dimension of an order of magnitude and significant change in the [alpha] phase aspect ratio contribute to drastic increase in the tensile properties and decrease in the crack initiation and propagation resistance. The fully lamellar microstructure displays slightly better biocompatibility because of the lower elastic modulus and superior fracture resistance. The enhanced crack propagation resistance of this microstructure is associated with the larger propensity for crack tip tortuousity, due to the coarser microstructural dimensions (lamellar colony size vs. primary [alpha] grain size). The difference in the crack propagation modes affects the shape and size of the actual crack tip strain distribution. These results were discussed correlating the complex multiple fracture mechanisms with the stress state in two microstructures.
COBISS.SI-ID: 17174550
A material with spatial variation in the elastic modulus E can have a much higher apparent fracture resistance and fracture stress than a comparable homogeneous material. The effect occurs due to the strong decrease of the crack driving force, which leads to crack arrest when the crack tip is located in the region with low elastic modulus. From the results of exemplary numerical studies and simple fracture mechanical considerations, models are derived in order to predict the fracture stress and fracture toughness of the inhomogeneous materials. It is shown that high values of fracture stress and fracture toughness can be reached if the amplitude of the E variation is high enough to provide crack arrest and the wavelength of the E variation is small. The beneficial effect of material property variations also occurs if the width of the compliant region is very thin and the loss in stiffness of the structure is almost negligible. The concept is applicable for various types of composite materials; examples are presented.
COBISS.SI-ID: 17637910
Belleville washer steel springs are characterized by long fatigue life, better space utilization, low creep tendency and high load capacity with a small spring deflection. In the case of a thicker spring, a higher loading and higher stiffness are obtained, but the deflection of the spring is reduced. In this case fatigue life is reduced and there is a very high probability that a Belleville washer spring can fail in a brittle manner, causing additional damage to the machinery. In order to prevent the fracture of a Belleville washer the elastomer filling was used on both free surfaces of the spring. Experimental testing and numerical analyses show that progressive loading characteristics were obtained when the elastomer filling was increasingly involved in the loading process. When the elastomer filling is compressed, the stresses in Belleville washer steel are reduced, because the majority of the deflection stress is taken by the elastomer instead of the steel.
COBISS.SI-ID: 17283094