In this work, the nominal stress concept, the notch stress approach and two critical plane approaches are used to analyse the fatigue endurance of a pipe‐to‐plate welded joint subjected to complex loading histories. Both the pipe and the plate were made of S355JR steel. Starting from already known fatigue endurance curves obtained for the same specimens under pure bending and pure torsion, a first series of tests was conducted, in which specimens were loaded in bending for a given fraction of the estimated life and then in torsion until failure. A similar series of tests was then carried out by changing the loading order: specimens were firstly loaded in torsion for a given fraction of the estimated endurance and then in bending until failure. The whole test campaign was repeated for two different fractions of the estimated life, that is, 0.3 and 0.45, respectively. After that, additional three series of tests were carried out, in which the specimens were subjected to consecutive sequences of bending and torsion blocks of different lengths (short, medium and long, respectively); the relative length of the bending and torsion block in each series was determined in order to produce the same damage. The experimental results, in terms of total damage at failure, were analysed using the Palmgren–Miner hypothesis. For all the assessment methods, the characteristic endurance curves were firstly calibrated on the basis of finite element (FE) analyses and of the experimental results obtained under pure bending and pure torsion loadings. The observed damage at failure resulted always greater than 0.5 for all the employed methods and greater than 1 for most of the tests. The different methods gave similar results, with the critical plane methods giving a slightly more stable damage at failure and a correct determination of the failure location. For all the methods, the damage at failure slightly reduces as the block length shortens.

中文翻译：

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在不同长度的弯曲和扭转载荷块的作用下，焊接接头的疲劳寿命评估

在这项工作中，使用名义应力概念，缺口应力方法和两种临界平面方法来分析承受复杂载荷历史的管板焊接接头的疲劳强度。管和板均由S355JR钢制成。从相同的试样在纯弯曲和纯扭转下获得的已知疲劳耐久性曲线开始，进行了第一系列的测试，其中，试样在弯曲过程中承受了估计寿命的给定比例，然后进行扭转直至失效。然后，通过更改加载顺序进行了一系列类似的测试：首先以扭力加载样品，以获得估计耐力的给定比例，然后弯曲直至失效。整个测试活动都重复了估计寿命的两个不同部分，即0。3和0.45。此后，进行了另外三个系列的测试，分别对试样进行不同长度（分别为短，中和长）的弯曲和扭转块的连续试验。确定每个系列的弯曲和扭转块的相对长度，以产生相同的损伤。根据失效时的总损坏，使用Palmgren-Miner假设。对于所有评估方法，首先基于有限元（FE）分析以及在纯弯曲和纯扭转载荷下获得的实验结果来校准特征耐力曲线。对于所有采用的方法，观察到的失效破坏总会大于0.5，而对于大多数测试而言，总是大于1。不同的方法给出了相似的结果，而临界平面方法给出了在破坏时稍微更稳定的损坏以及对破坏位置的正确确定。对于所有方法，随着块长度的缩短，破坏时的破坏会略有减少。