Comprehensive Design of Steel Structure Fatigue Calculation and Structural Requirements: From Stress Amplitude Method to Brittle Fracture Prevention Design

Comprehensive Design of Steel Structure Fatigue Calculation and Structural Requirements: From Stress Amplitude Method to Brittle Fracture Prevention Design

Steel structures subjected to repeated dynamic loads, alternating stresses and cyclic working conditions are prone to fatigue crack initiation and progressive expansion, which further induce structural local damage or overall failure. Fatigue failure of steel structures is typical brittle failure without obvious macroscopic deformation, featuring strong concealment and suddenness, which seriously endangers structural service safety and durability. Combined with domestic and international mainstream design specifications including GB 50017, Eurocode 3 Part 1-9 and ISO fatigue design series standards, this document systematically elaborates the core theory of steel structure fatigue calculation based on the stress amplitude method, cumulative fatigue damage verification mechanism, whole-process brittle fracture prevention design strategy and standardized structural detailing requirements, forming a complete and standardized technical system for steel structure anti-fatigue safety design.

1. Basic Overview of Steel Structure Fatigue Failure

Structural fatigue refers to the irreversible damage accumulation phenomenon of steel materials and connecting details under long-term repeated cyclic loads. Different from static strength failure, fatigue failure occurs under cyclic stress far lower than the static yield strength of steel, and is mainly affected by cyclic stress amplitude, load cycle times, structural detail form, material toughness and welding residual stress.

Fatigue failure of steel structures generally undergoes three stages: micro-crack initiation at stress concentration points, stable expansion of internal cracks, and instantaneous brittle fracture of sections. Weld joints, hole openings, section sudden changes and structural connection discontinuities are typical fatigue vulnerable details, which are the key control objects of anti-fatigue design and structural optimization.

Steel structure fatigue design is divided into constant amplitude fatigue verification and variable amplitude cumulative fatigue damage verification. The stress amplitude method is adopted as the core calculation basis, abandoning the traditional maximum stress control principle, which scientifically conforms to the essential law of cyclic fatigue damage accumulation.

2. Core Principle and Calculation System of Stress Amplitude Method

2.1 Basic Definition of Fatigue Stress Amplitude

The stress amplitude method takes cyclic stress variation amplitude as the core control index of fatigue design, ignoring the influence of average stress in elastic working state. For steel structural components and welded connections under elastic cyclic loading, fatigue damage is mainly determined by the alternating stress range rather than the peak stress value.

The stress amplitude is defined as half of the difference between the maximum and minimum cyclic stress. For structural fatigue verification under dynamic loads such as cranes, bridge structures and wind vibration, effective stress amplitude statistical analysis shall be carried out according to actual load spectrum characteristics to eliminate invalid low-amplitude stress cycles.

2.2 Constant Amplitude Fatigue Calculation Criteria

Constant amplitude fatigue is applicable to structural working conditions with stable cyclic load and fixed stress variation range. According to standard specification systems, the fatigue strength verification formula is based on allowable stress amplitude control. The calculated actual stress amplitude of key details shall not exceed the standard allowable fatigue stress amplitude corresponding to the detail category and design cycle times.

Welded and non-welded components adopt differentiated calculation criteria. Welded details have obvious residual stress and stress concentration effects, so fatigue strength is mainly controlled by stress amplitude without average stress correction; non-welded smooth components can appropriately consider the beneficial influence of average stress on fatigue performance.

Combined with plate thickness correction coefficients and structural detail classification coefficients, the allowable stress amplitude is dynamically modified to adapt to the fatigue strength attenuation effect of thick plates and complex connection forms.

2.3 Variable Amplitude Cumulative Fatigue Damage Calculation

Actual engineering steel structures are mostly subjected to variable amplitude random cyclic loads. Based on the Miner linear cumulative damage theory, the fatigue damage degree under multi-stage variable amplitude loads is quantitatively calculated. The cumulative damage ratio is the superposition of the damage contribution of each stress amplitude level.

When the total cumulative damage value is less than 1.0, the structural fatigue performance meets the design requirements; when the value exceeds 1.0, it is judged that fatigue damage accumulation is excessive, and fatigue failure risk exists within the design service life. This method realizes quantitative evaluation of random fatigue damage of complex steel structures.

2.4 S-N Curve Fatigue Strength Classification System

The S-N curve (stress amplitude-cycle number curve) is the basic database for steel fatigue design. According to structural detail forms, welding types and connection modes, steel structural details are divided into multiple fatigue strength categories. Each category corresponds to an independent S-N curve, defining the corresponding relationship between allowable stress amplitude and fatigue cycle times.

High-quality smooth welding details and mechanical connection details have higher fatigue classification grades and larger allowable stress amplitudes; welded joints with defects, abrupt section changes and stress concentration details belong to low fatigue grades, requiring stricter stress amplitude control and cycle limitation.

3. Key Influence Factors of Structural Fatigue Performance

Structural Stress Concentration
Local geometric discontinuities such as welding toe, hole edge, section mutation and stiffener end will cause sharp stress concentration, which is the primary cause of fatigue crack initiation. Anti-fatigue design must optimize structural lines to reduce local stress peak values.

Welding Residual Stress
Welding thermal processing produces high tensile residual stress at joints, which superimposes with external cyclic stress to accelerate crack expansion. Residual stress elimination treatment and welding process optimization are effective means to improve fatigue performance.

Material Toughness and Metallographic State
Steel with low toughness and poor plasticity has weak resistance to fatigue crack expansion. Tempered and fine-grained steel materials have better fatigue resistance and brittle fracture resistance, which are preferred for dynamic load structures.

Load Cycle Characteristics and Environmental Corrosion
Long-term alternating dynamic load, low-temperature environment and corrosive medium will significantly reduce the fatigue limit of steel structures and aggravate cumulative fatigue damage, requiring enhanced design safety margin and corrosion protection measures.

4. Systematic Brittle Fracture Prevention Design Technology

Fatigue failure of steel structures is essentially low-stress brittle fracture. Brittle fracture prevention design is the core guarantee to suppress sudden structural failure, forming a complementary safety system with fatigue calculation.

4.1 Material Toughness Matching Design

Select low-temperature resistant impact toughness steel according to the minimum service ambient temperature. Ensure that the Charpy V-notch impact energy meets the specification requirements at the lowest design temperature, avoid material cold brittleness transition, and eliminate the risk of low-temperature brittle fracture of components.

For important dynamic load structures and low-temperature service facilities, high-toughness fine-grained structural steel is mandatory to improve crack arrest performance and prevent rapid unstable expansion of fatigue cracks.

4.2 Crack Arrest and Anti-Explosion Structural Design

Set reasonable crack arrest plates and structural partition measures for long-span steel beams, box columns and continuous structural components. Block the continuous expansion path of local fatigue cracks, avoid overall chain brittle fracture of the structure, and improve structural redundancy and fault tolerance.

4.3 Welding Quality and Defect Control

Brittle fracture is extremely sensitive to welding defects. Strictly control welding residual height, undercut, porosity and incomplete fusion defects. Implement full coverage non-destructive testing for key fatigue details, eliminate inherent micro-defects at welding joints, and reduce crack initiation sources.

4.4 Low-Stress Robust Structural Layout

Optimize the overall structural force transmission path, avoid local excessive stress concentration and rigid constraint mutation. Adopt smooth transition structural details to reduce secondary stress and alternating stress amplitude, fundamentally suppress fatigue crack initiation and brittle fracture tendency.

5. Standardized Structural Detailing Anti-Fatigue Requirements

5.1 Welding Detail Structural Requirements

Optimize welding joint forms, avoid overlapping welds and multi-directional weld intersection. Adopt smooth transition welding toes and post-weld grinding treatment to reduce local stress concentration. For dynamic load components, avoid partial welding and intermittent welding, and ensure continuous and uniform weld forming. Strictly control weld size deviation and residual stress level to improve joint fatigue resistance.

5.2 Component Section and Transition Requirements

All section size changes, plate thickness transitions and component lap joints shall adopt smooth arc transition or gradient transition structures, without sharp corners and abrupt changes. The end of stiffeners and reinforcing plates shall be rounded and polished to eliminate stress concentration points, which is the key structural measure to improve fatigue life.

5.3 Bolt Connection Anti-Fatigue Requirements

High-strength bolt friction-type connections are preferred for dynamic load structural joints, avoiding bearing-type bolt connections with large gap deformation. Control bolt pre-tightening force uniformly to reduce joint slip and alternating impact stress. Optimize bolt arrangement spacing to avoid superposition of local stress concentration.

5.4 Opening and Local Structural Optimization

Avoid random opening and local cutting on fatigue-sensitive components. When openings are unavoidable, adopt circular arc transition and local reinforcement measures to make the stress field distribute uniformly and reduce cyclic stress amplitude at the opening edge.

6. Design Verification and Safety Control Principles

Complete fatigue calculation verification and structural detail optimization in the structural design stage. Classify fatigue risk levels according to structural importance, load cycle frequency and service environment. Implement stricter fatigue amplitude control standards and finer structural detailing requirements for key components and high-cycle dynamic load structures.

Establish a full-process safety control system of "fatigue stress amplitude calculation → cumulative damage verification → brittle fracture prevention design → structural detail optimization", realizing quantitative calculation, qualitative risk judgment and targeted structural reinforcement, and eliminate fatigue and brittle fracture hidden dangers at the design source.

7. Technical Summary

Steel structure anti-fatigue and anti-brittle fracture design is a systematic engineering integrating theoretical calculation, material selection and structural detailing optimization. The stress amplitude method and Miner cumulative damage theory form the core quantitative calculation system for fatigue design, which accurately evaluates the fatigue damage state of steel structures under constant and variable amplitude cyclic loads.

Scientific brittle fracture prevention design and standardized anti-fatigue structural details effectively solve the sudden failure risk of steel structures under long-term dynamic loads. Perfect combination of theoretical calculation and structural construction optimization can significantly improve the fatigue resistance, crack arrest performance and overall structural safety redundancy of steel structures, and provide reliable technical guarantee for the long-term stable service of dynamic load steel structures such as bridges, industrial workshops and large-scale public buildings.