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Publication# Strain-based analysis on the influence of local undercut geometry on fatigue crack initiation life

Abstract

The local undercut defects at the weld toe provide a potential initiation site for fatigue cracks and significantly impact the structure's fatigue strength. The influence of continuous undercut depth on fatigue performance is widely studied, but the research on the influence of local undercut geometry is limited. In this research, the influence of the 3D undercut geometry on the fatigue strength of welded joints is investigated using the elasto- plastic finite element analysis, with explicit consideration of geometry and plasticity effect. A simplified parametric model is created to represent the realistic undercut geometry based on high-resolution measurement data. The combination of the geometric parameters for the realistic undercut models was filtered from a full- factorial design of six undercut geometry parameters. The influence of the individual geometric parameters of the local undercut defect such as depth, radius, width, and length on the fatigue performance of weld is systematically investigated. Based on the results, a three-dimensional undercut geometry index is proposed in the current study and found suitable for estimating fatigue crack initiation life. Furthermore, the research provides insights into fatigue strength-based quality criteria for welds with local undercut defects.

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Fatigue (material)

In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.

Geometry

Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician who works in the field of geometry is called a geometer. Until the 19th century, geometry was almost exclusively devoted to Euclidean geometry, which includes the notions of point, line, plane, distance, angle, surface, and curve, as fundamental concepts.

Geometric mean

In mathematics, the geometric mean is a mean or average which indicates a central tendency of a finite set of real numbers by using the product of their values (as opposed to the arithmetic mean which uses their sum). The geometric mean is defined as the nth root of the product of n numbers, i.e., for a set of numbers a1, a2, ..., an, the geometric mean is defined as or, equivalently, as the arithmetic mean in logscale: Most commonly the numbers are restricted to being non-negative, to avoid complications related to negative numbers not having real roots, and frequently they are restricted to being positive, to enable the use of logarithms.

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