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**Special Overview of the Control and Prevention of Brittle Failure in Crane Steel Structures: Dalian Heavy Industry Lifting Group Co., Ltd.**
Zou Sheng and Xu Hongze discuss how to avoid brittle failure during the design and manufacturing process of crane steel structures, proposing effective control and preventive measures to ensure structural integrity.
**1. Introduction**
Crane steel structures are subjected to dynamic loads, making them particularly vulnerable to brittle failure. This type of failure occurs without visible signs of overload, even under safe load conditions, and can be sudden and catastrophic. To prevent such failures, it is essential to focus on improving the design, selecting appropriate materials, and avoiding stress concentrations or cracks that may form during manufacturing.
Stress concentrations, arc scratches, welding cracks, and damaged edges from punching or shearing can all contribute to crack initiation and propagation. Therefore, these factors must be carefully controlled during the production phase.
When the rigidity and strength of a structure are insufficient, rapid damage can occur. The selection of high-strength steel should be done with caution, as improper choices can lead to accidents and increased costs. Factors like the type of stress, structural weight, loading speed, plate thickness, and ambient temperature all influence the choice of steel with adequate impact resistance.
**2. Notes on the Selection of Steel Structure Materials**
**2.1 Effect of Stress Type and Size on Material Selection**
Different parts of a steel structure experience varying stress states. Static tensile or compressive stresses differ significantly from multi-axial stress states. Fatigue load cycles, such as pulsating or symmetric cycles, have a greater impact on material selection than simple static loads. Residual stresses from welding, structural weight, and stress concentration also play a key role.
In cases of complex stress and significant stress concentration, materials resistant to fatigue and less sensitive to stress concentration should be selected.
For a given material, there exists a critical stress value below which brittle failure will not propagate. However, this value varies with temperature and impact toughness. If the stress level is too low, it may lead to oversized and uneconomical designs. Therefore, ensuring sufficient stiffness and strength (including dynamic and fatigue strength) while considering initial defects, the primary factors for material selection are impact toughness and ambient temperature.
**2.2 Influence of Plate Thickness on Material Selection**
Thicker plates have lower impact energy due to metallurgical effects and increased stress concentration. Standards often provide mechanical properties for specific thicknesses, but these may not always apply directly. For example, plates thicker than 26 mm typically have lower impact energy, requiring careful material selection.
**2.3 Influence of Environmental Temperature on Material Selection**
Environmental temperature significantly affects the performance of steel. At lower temperatures, the impact toughness of steel decreases, increasing the risk of brittle fracture. The transition temperature—where the material shifts from ductile to brittle behavior—is critical in material selection. If the ambient temperature is below the transition temperature, brittle failure becomes more likely.
**3. Matters Needing Attention in Steel Structure Design**
Designs typically assume elastic behavior and consider stress concentration factors. Under cyclic loading, fatigue life is evaluated based on stress concentration. Stiffness and vibration characteristics are also important.
Modern finite element analysis helps identify stress concentrations, allowing designers to optimize local structures. Avoiding abrupt changes in geometry and using proper welding techniques are essential to reduce crack initiation and propagation.
Welding quality is crucial, as fatigue cracks in the heat-affected zone can lead to brittle failure. Proper weld selection, process parameters, and preheating are vital for preventing cracks.
**4. Matters Needing Attention in the Manufacturing Process**
During manufacturing, any imperfections such as arc scratches, welding cracks, or sheared edges can initiate cracks. Preheating before welding and controlling residual stresses through aging or heat treatment help mitigate these risks.
Flame heating for deformation correction must be handled carefully to avoid thermal shock. Residual stress relief is especially important for thick plates and high-carbon steels.
**5. Conclusion**
Material selection must align with structural stress conditions, plate thickness, and environmental temperature. The FEM code provides detailed guidance for crane design. While brittle failures are rare today, their potential consequences remain severe, necessitating continued attention in design and management.
Author: Zou Sheng
Address: 169 Bayi Road, Xigang District, Dalian
Reference: "Material Handling Equipment," 2004 (8)