Load interaction effect on fatigue crack growth in through-wall cracked pipes under large scale yielding: Experimental and numerical investigation

Highlights

• Load interaction effect on fatigue crack growth under large scale yielding conditions.

• Crack growth measurement in through-wall cracked pipe tests under variable large amplitude fatigue loading.

• No crack acceleration or retardation observed due to the load change in experiment.

• Numerical FE debond analysis to simulate through-wall cracked pipe test for further understanding.

• No crack closure observed from FE simulation under variable large amplitude fatigue loading.

Conclusion: Load interaction under large scale yielding did not affect the fatigue crack growth for through-wall cracked pipe owing to no crack closure

Abstract

In this paper, the load interaction effect on fatigue crack growth under large scale yielding conditions is investigated, via combined experimental and finite element (FE) analyzes of a through-wall cracked pipe. For the fatigue load amplitude, two amplitudes were applied to the through-wall pipe, 75% and 85% of the experimental maximum load under monotonic loading. For the loading type, periodic overload, high-low and low-high loading sequences were applied. It is experimentally found that the change of crack growth rate due to load interaction does not occur under large scale yielding, in contrast to the behavior under small scale yielding. Numerical analysis using FE debond analysis further shows that crack closure does not occur in the through-wall cracked pipe under large scale yielding fatigue loading, which can explain no load interaction effect on fatigue crack growth, observed in the present experiment.

Introduction

Structural integrity assessment under seismic loading is essential for structural stability evaluation of a piping system. The seismic loading can be characterized by its high strain rate and large-amplitude cyclic loading. It has been shown that large-amplitude cyclic loading causes more severe degradation effect on fracture toughness than the strain rate [[1], [2], [3], [4], [5]]. Furthermore, for very low cycle fatigue loading condition caused by large-amplitude cyclic loading, fracture behavior of a cracked pipe can be different from that of a small specimen due to the constraint effect and thus the full-scale pipe test would be required to understand fracture behavior. Therefore, many full-scale pipe experiments under large-amplitude cyclic loading simulating seismic loading have been conducted to investigate fracture behavior in many countries (US [[5], [6], [7], [8]], Japan [[9], [10], [11], [12]], India [[13], [14], [15], [16], [17]] and Korea [18]). These experiments have focused on the effects of the loading type (such as the load- or displacement-controlled cyclic loading) [[5], [6], [7], [8],[13], [14], [15],18], cyclic loading variables (such as the load/displacement amplitude and load ratio) [9,10,[13], [14], [15]], material (base or weld) [9,10,16,17] and so on. However, in most of cases, constant amplitude and load ratio conditions were applied, but not variable amplitude and load ratio conditions.

The load interaction effect on the fatigue crack growth has been well known and has been analyzed experimentally as well as analytically [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54]]. The crack retardation effect (decrease in the crack growth rate) was observed in the high-low loading sequence (decrease in the load amplitude) [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]] and in the overload case [[27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54]]. On the other hand, the crack acceleration effect (increase in the crack growth rate) was observed in the low-high loading sequence [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. Such crack retardation and acceleration effects due to the load interaction can be well described by the crack closure phenomenon [19,20,[55], [56], [57], [58], [59]]. Many researchers attempted to identify the crack closure phenomenon by using the strain gage [[60], [61], [62], [63]], Digital Image Correlation (DIC) [[48], [49], [50], [51]] and SEM [64]. For a cracked specimen, the crack tip deformation, crack opening load and residual stress were difficult to observe from the fracture surface. In this respect, the crack closure was determined via FE analysis by using inversion of stresses at crack tip [42,43,65,66], last contact [27,[65], [66], [67],69] and contact force [68] in the FE analysis.

The above-mentioned works were conducted under the small-scale yielding condition, but the crack growth behavior under large scale yielding conditions is expected to differ from that under small scale yielding. Works related to the load interaction effect on fatigue crack closure under large scale yielding are quite limited. The FE analysis for a plate with a hole showed the decrease of the crack opening stress, as the maximum stress approached the yield strength [70,71]. The crack opening stress was further reduced for completely reversed cyclic loading (R=-1) under elastic-plastic condition. For plane strain FE analysis of a crack problem, crack closure was hardly observed for completely reversed cyclic loading (R=-1) when the maximum cyclic stress was close to the yield strength [72]. Experiment was performed using a specimen with the corner crack under two different fatigue loading conditions; small scale and large scale yielding conditions [73]. The crack opening and closure behaviors were investigated using experimentally measured load-displacement curves. Under the completely reversed cyclic loading, the crack was open even at the minimum load under the large scale yielding but was usually close at the zero load under the small-scale yielding. In Ref. [74], two specimens with different sizes were used to investigate the crack closure effect. A small size specimen with an artificial small surface crack and large size specimen with an actual surface crack were tested under large scale yielding fatigue loading conditions. The crack closure behavior was observed by measuring the relative displacement of the crack front using the digital image correlation. It was found that the crack opening load was approximately equal to the minimum load in both specimens under cyclic loading, suggesting that crack closure did not occur. The effect of crack closure on fatigue crack growth in a cracked pipe was also observed in [75]. The fatigue crack growth for cracked pipe was predicted by using various pipe evaluation methods (GE/EPRI [76], LBB.NRC [77], LBB.ENG2 [78,79]). The predicted fatigue crack growth depending on the crack closure showed a significant difference. Review of existing works on the crack closure phenomenon under large scale yielding suggests that more systematic work is needed for clarification.

In this paper, combined experimental and FE analyzes are conducted to investigate the effect of load interaction on fatigue crack growth of pipe under large scale yielding. For experiment, fatigue crack growth test using a through-wall cracked pipe (TWC) made of SA508 Gr.1a low-alloy steel under the large scale yielding condition was performed. For cyclic loading, periodic overload and block loading containing the high-low and low-high loading sequence were applied. The applied load was 75% and 85% of the maximum load determined from the same pipe test under monotonic loading. Section 2 presents the material and pipe test data. The effect of load interaction on very low cycle fatigue crack growth is also analyzed in Section 2, based on the experimental data. Section 3 explains the FE debonding analysis for TWC pipe with experimental validation. Based on the FE debonding analysis, the crack closure behavior for the TWC pipe under large scale yielding fatigue is also explained in Section 3. Finally, the conclusion is given in Section 4.