DECLARATION 2 PUBLICATIONS
2.2 Experimental methods
2.2.4 Residual stress and fatigue strength of welded structures
2.2.4.6 The Effect of WRS
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.
29 | P a g e SIF approach, the SIF range at which the crack occurs (ΔKeff) is calculated from Kapp, Kmax and K4op (Liljedahl et al., 2007).
According to ASTM E647, residual stress and/or crack closure may significantly affect FCG rate data, particularly at low SIFs and low stress ratios. However, such variables are not incorporated into the computation of ΔK. Residual stresses are thought to undergo a relaxation process under thermal or mechanical loading, due to cyclic plastic strain. Given the favourable effect of compressive residual stresses on fatigue properties, their relaxation through cyclic loading should be avoided (Ngiam, 2007). Lu (2002) argued that although certain design codes (e.g. ASME) allow the influence of residual stress in low-cycle fatigue range to be ignored – because it would likely relax to zero only after a few cycles, experimental analysis showed that this assumption did not hold for all low-cycle fatigue cases.
Rading (1993) observed that most of the fatigue cracks that developed in structural steel used in the fabrication of buses in Kenya originated from the HAZ region of the weld joints. Rading used a combination of experiments (i.e. ASTM E647 fatigue tests) and analytical methods (i.e. fracture mechanics) to evaluate the impact of welding on FCG behaviour of low-carbon structural steel.
The study established that the FCG rate in the weld metal, HAZ and parent metal differed depending on which stage (I, II, or II) the FCG occurred on – that is, the beginning, mid-range or threshold. Where the residual stresses had been released almost completely, the FCG rate (da/dN) was essentially the same throughout the parent metal and weld metal (Rading, 1993). The grain size of the microstructure affected the FCG rate. Given that the three main regions of the weld (i.e. weld metal, HAZ and parent metal) had different-sized grain microstructures, it was expected that the FCG rate would differ in each region accordingly (Rading, 1993). Rading’s study showed that the FCG rate was faster in the HAZ region than in the parent metal region. The HAZ grain size is finer than that of the parent metal, and hence the parent metal microstructure displays relatively high resistance to FCG.
Lu (2002) examined the extent of the influence of WRS on the fatigue life of welded components.
Tensile residual stresses that were greater than the yield stress were observed close to the weld toe area. Fatigue crack was first noticed after 800 cycles of a 5000-lb load, and through-thickness crack growth was realised at 980 cycles in a stainless-steel 304 material. The results showed that the dominant components of residual stress in a circumferential weld are hoop and axial stresses.
The socket welds were shown to have shorter fatigue life than butt welds (680 vs 980 cycles).
The four-pass weld bead was shown to have shorter fatigue life than the three-pass weld bead. Lu
4SIF at crack tip opening.
30 | P a g e (2002) concluded that residual stress at the weld toe acts as mean stress, causes ratcheting and reduces fatigue life.
Pasta and Reynolds (2007) applied the adjusted compliance ratio (ACR) method together with the online crack-compliance technique to determine the influence of WRS on FCG behaviour during fatigue testing of the titanium alloy structure. The ACR method allows crack closure effects due to WRS to be separated from effects due to other factors in the FCG data. FCG was measured using online crack compliance, and the influence of WRS on FCG behaviour was measured using the ACR method. This method uses residual stress SIF; and a constant stress- range test was performed in a servo-hydraulic machine. The results showed that longitudinal residual stress was highly tensile in the weld fusion zone, then it reduced in magnitude as the distance from the WCL increased, and eventually changed direction to becoming compressive.
Similarly, the FCG rate was higher in the weld fusion zone (tensile WRS) and lower outside the weld border line or weld toe (compressive WRS).
Rosenfeld and Kiefner (2006) investigated residual stress fields around the weld toe and weld root for multi-pass pipe-to-plate circumferential weld joints, their influence on fatigue resistance, and the effect of weld penetration. Two types of joints were examined, namely U-groove 3-pass and fillet 2-pass weld joints. The measured residual stress fields were further used in a fatigue life assessment for welded structures. The authors noted that previous studies showed that the weld root of the pipe-to-plate circumferential weld is under compressive stress, whereas the weld toe is under tensile stress. The fatigue life of the fusion zone is enhanced as a result of the presence of compressive stresses at the root. The authors concluded that the resistance to fatigue crack initiation is generally proportional to the ultimate strength properties of the material (Rosenfeld
& Kiefner, 2006).
Liljedahl et al. (2007) studied the residual stress re-distribution resulting from FCG in aluminium alloy 2024-T3 specimens. Three ΔK values were tested. For the medium-tension specimen, M(T), the FCG rate was significantly higher in the weld metal than the parent metal for ΔK = 6 and ΔK = 11. The gap between the two metals closed at ΔK = 15. For the compact tension specimen, CT, crack arrest at all three SIF ranges (ΔK) occurred at the weld metal. The micro-hardness profile suggested significant change in the microstructures of the weld metal, HAZ and parent metal. The residual stress fields were found to redistribute during FCG, and such distribution differed between M(T) and CT specimens. Results from the experiments were compared with results predicted using empirical methods; the CT results showed good agreement (Liljedahl et al., 2007).
31 | P a g e Ngiam (2007) presented the results of analytical and experimental work to investigate the favourable and the detrimental effects of surface residual stress on FCG in structural components.
Ngiam applied LEFM to assess the fatigue life in offshore structures, considering fillet and butt welds. The author concluded that incorporating residual stress fields into the processes to determine the fatigue properties of welded structures can significantly alter the FCG data.
Garcia et al. (2016) developed the FCG model using the LEFM and EPFM approaches to predict FCG behaviour in residual stress fields of aluminium sections. A combination of experiments and numerical modelling was used to evaluate crack propagation in residual stress fields of aluminium specimens. The fatigue tests were performed on a single-edge notched tension specimen using a servo-hydraulic machine. Four-point bending tests were performed on these specimens to introduce residual stress fields onto the material, and strain gauges were used for strain measurement. Strong correlation was noted between the residual stress results obtained through experiments and those obtained through finite element modelling. The EPFM method was found to have better accuracy than LEFM in matching the WRS field results from experiments. It was also established that compressive WRS fields have a deceleration effect on the FCG rate.
Bozic (2016) investigated the influence of residual stress on FCG behaviour in welded stiffened panel structures. The researcher determined Mode I total SIF (ΔKtot) based on the superposition rule of LEFM. To account for residual stress, the nominal stress ratio R was replaced by the effective SIF ratio Reff during empirical analysis. Residual stresses were found to be tensile along welded stiffeners and compressive between stiffeners.
Given that residual stress can be either beneficial or detrimental to fatigue properties of a welded structure, it is important to monitor the process of residual stress relaxation, especially in situations where benefits are derived from its presence. The two main forms of residual stress relaxation are mechanical relaxation and thermal relaxation. Mechanical relaxation depends on factors such as magnitude and gradient of initial residual stress and the extent of cold working.
Thermal relaxation is also influenced by the degree of cold working (Zabeen, 2012). Zabeen (2012) evaluated the stability of intentionally-induced compressive residual stresses on the surface of aircraft engine components under cyclic loading. The residual stress fields associated with mechanically-induced surface residual stress are likely to redistribute and relax as the fatigue crack propagates through the material. Relaxing surface-based compressive residual stresses would accelerate FCG rate. The author concluded that laser peening introduced compressive residual stress of up to 0.6 times the yield stress of the material in the longitudinal direction of the aerofoil specimen (Zabeen, 2012).
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