Prediction method for low-cycle fatigue crack growth velocity and direction of offshore engineering structure

Abstract

The invention relates to a method for simultaneously calculating expansion speed and direction in three-dimensional space for the low-cycle fatigue crack growth of an offshore engineer structure / component. The method includes such step as 1. obtaining stress / strain field at crack tip by finite element method; 2. adopting a strain-life curve to determine the fatigue life cycle number of finite element elements under different strain / stress states, and the plastic damage evolution model is established based on irreversible thermodynamic method; 3, calculating the damage degree of each element, correcting the constitutive material of each unit, and deleting the failed unit after the damage reaches the damage threshold. by inputting the load state into a plastic damage evolution model; 4, repeating that above process according to the loading sequence continuously, and the damage element continues to initiate from the crack tip to failure; and 5, comparing the element deletion process withthe load history, the crack propagation speed and direction in the low cycle fatigue process of the component / structure can be obtained at the same time.

Description

technical field [0001] The invention relates to the field of structural mechanics fatigue crack analysis, in particular to a method for predicting the propagation speed and direction of low-cycle fatigue cracks in marine engineering structures. Background technique [0002] At present, the marine structure under the complex marine environment has been subjected to variable multi-axial dynamic loads for a long time, and cracks will be generated at the stress concentration points during service, which will become the key factor of structural failure, and its low cycle (strong load) fatigue crack growth rate is much faster than High-cycle fatigue largely determines the load-bearing performance and life of marine engineering structures / components. Accurately predicting the growth rate and direction of low-cycle fatigue cracks has become one of the core tasks of related engineering design and research and development. [0003] In the design of marine engineering structures, the t...

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Problems solved by technology

[0003] In the design of marine engineering structures, the traditional fatigue life calculation method first analyzes the hot spots and stresses of the structure, obtains the load history by rainflow counting, etc., and linearly superimposes the fatigue damage under different loads according to the stress-life curve, and the hot spot stress calculation does not consider the crack area Stress singularity, the rainflow counting method is difficult to deal with the load history effect, the stress-life curve is obtained by the smooth test uniaxial test, on the one hand, the influence of cracks on the stress-life is not considered, and on the other hand, the actual existence of the structure is not considered multiaxial stress state, and the structural fatigue life under multiaxial stress is often much lower than that of uniaxial stress

In the calculation of fatigue crack growth, the Paris curve is mostly used for high cycle fatigue, and is directly related to the direction and mode of crack growth, because this type of characteristic directly affects the state of the crack tip stress / strain field; under multiaxial loading conditions, the critical surface method and energy method is close to the fatigue test results of smooth specimens, but the crack tip has singularity, and the above method is homogenized in a certain area, so it is difficult to be directly used for crack growth; the configuration force method needs to cooperate with the fatigue life curve (such as the Coffin-Mansion method) The crack growth direction and velocity are calculated separately, and the change of material properties during the fatigue process is not considered. The existing experimental results show that the plastic zone of the crack tip is constantly evolving under the low-cycle fatigue load of elastic-plastic metal materials. The plastic evolution of this type of material means that With the appearance of damage, the load-bearing characteristics (elastic modulus, yield surface, etc.) will change accordingly, directly affecting the stress-strain characteristics of the corresponding area

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