DECLARATION 2 PUBLICATIONS
2.2 Experimental methods
2.2.4 Residual stress and fatigue strength of welded structures
2.2.4.5 Factors affecting fatigue strength
25 | P a g e as Mode II stress problems. Load-carrying cruciform fillet joints are considered to be Mode I (Meneghetti, 2012)2.
Meneghetti (2013) provided a simplified method to measure Mode III stresses at the weld toe, by correlating the FEA technique with the notch SIF values. Tube-to-flange fillet welded joints subjected to torsional loading were evaluated using LEFM. Similarly, Lu (2002) examined the extent of the influence of WRS on the fatigue life of welded components. Low-cycle fatigue tests in a cantilever setup were conducted. Fatigue cracks occurred at the weld toe in all the experiments, and ratcheting strain was highest at the weld toe. The fatigue life of materials is therefore reduced in the presence of ratcheting3 (Lu, 2002).
26 | P a g e Discontinuities: In a case where continuity of structure cannot be satisfied by membrane forces alone, a discontinuity is said to exist. Discontinuities are potential nests of localised stress concentration. The three main categories of discontinuities in welded pressure vessel structures are as follows (Chaudhari and Belkar, 2014):
a. Geometric discontinuities, which comprise abrupt changes in the curvature radius, connections (e.g. nozzles) and material thickness.
b. Load discontinuity, which refers to an abrupt change in load type or intensity.
c. Material discontinuity, which constitutes an abrupt change in the mechanical properties of a material.
d. Metallurgical discontinuities, which involve a change in microstructure from the PM to HAZ to the fusion zone.
The stress magnification at the discontinuity is called the stress concentration. The factor by which it is magnified, relative to the nominal stress, is referred to as the stress concentration factor (SCF) or SIF. The calculation can be represented as follows:
SIF x nominal stress = max stress = hotspot stress
In welded structures, the weld notches at the toe and root, which are often characterised by imperfections due to the sub-optimal joining process, are locations of high stress concentration that can act as crack initiators. The question of whether the weld toe or root crack type would dominate the fatigue criterion of the welded structure is a function of several variables. These include the ratio of throat-to-plate thickness, type of loading, weld geometry and residual stress (Fricke, 2012).
Nozzle–shell joints: Pressure vessels are generally exposed to fatigue loading resulting from pressure fluctuations, mechanical loading or thermal loads (Pasha et al., 2008). Furthermore, in pressure vessel structures, circular holes made in the shell for the purpose of plugging nozzles increase the maximum stress experienced by the vessel. This is a result of stress concentration and high magnitude of localised stresses around such a geometrical discontinuity (Paraschiva et al., 2016). The sudden change in geometry and direction at the nozzle–shell joint of the pressure vessel causes high stress concentrations, thereby increasing the chance of failure during operations. A study of the effect of nozzle openings on stress concentrations showed that the two variables were proportionally related. Vessels with larger nozzles experience higher maximum stresses than vessels with smaller nozzles. Stress concentration also increases with an increase in the nozzle angle (Paraschiva et al., 2016).
27 | P a g e Lewinski (2016) considered the effect of the configuration and geometry of the manhole to the stress state of the pressure vessel. As manhole thickness increases, the stress decreases, up to a certain level of thickness – in that study, 25.8 mm. Thereafter, the stress starts to increase with increasing thickness. Where the shape of the manhole is elliptical, the equivalent stresses are larger than those for a manhole having consistent radius.
Pasha et al. (2008) evaluated the fatigue life of an operational pressure vessel using a combination of numerical modelling and experiments. Reasonable agreement was noted between the numerical and experimental results. The pressure vessel tested by Pasha et al. (2008) was designed for a fatigue life of 4500 cycles; the fatigue life analysed through numerical modelling was 5234 cycles, and the actual fatigue life was 5051 cycles. Furthermore, the stresses on the pressure vessel shell, flat head and nozzle as analysed by the numerical methods were acceptably close to the experimental results. The numerical results showed that the nozzle’s inner corner was the weakest spot for fatigue. The actual fatigue failure during experiments occurred at the same nozzle corner (Pasha et al., 2008).
Peng (2011) applied a combination of experimental and finite element methods to determine methods for controlling welding distortion in nozzle-shell weld joints of stainless steel pressure vessels. Two welding processes of GTAW and SMAW were selected for the study given their popularity in stainless steel pressure vessel welding applications. The results showed that GTAW showed higher levels of distortion than SMAW. The alternating (i.e. welding internal diameter and external diameter in an alternating fashion) welding sequence was found to generate the least welding distortion. Petrovic et al (2011) proposed a procedure to determine the stress state of the pressure vessel shell that is provoked by a torque loading at the free end of the nozzle. The developed method showed that maximum stresses appear on the envelope near the nozzle.
Baloc et al. (2015) examined the influence of two nozzles welded onto the shell, regarding the stress profile of the pressure vessel. The authors combined experimental and numerical methods.
The comparison of experimental and simulation results showed a difference of up to 17.4%, which is considered acceptable. Xuedong et al. (2002) studied shaped compact specimens simulating high-strain characteristics of pressure vessel nozzles to understand their fracture fatigue behaviour. The effect of welding-induced residual stress and performance deterioration of the material adjacent to the weld joint rendered the area of the nozzle susceptible to fracture and fatigue damage in the pressure vessel (Xuedong et al., 2002).
Residual stress in welded structures occurs because of heterogeneous plastic deformations, thermal contractions and phase transformation. The contribution of residual stress in fatigue failure properties of the component occurs through the so-called mean stress effect. Mean stress
28 | P a g e results in material experiencing repeated excursions into the plastic range, even for small loading amplitudes. Residual stress in welded joints acts as mean stress, and facilitates the stress- controlled repeated excursions into the plastic range; in turn, this process causes degradation and failure resulting from accumulated deformation or ratcheting (Lu, 2002). The material behaves in a linear elastic manner for low-amplitude fatigue cycles in the absence of mean stress.
Al-Mukthar (2010) considered the effect of residual stress on crack propagation in welded components. The author observed that fatigue cracks can develop and propagate around the weld joint of a structure during service life, even if the dynamic stresses are well below the yield limit.