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Methods for analyzing the causes of mechanical failures: 1. Macroscopic inspection of faulty components. Macroscopic inspection primarily addresses two issues: first, the nature of the failure; second, its qualitative causes. It is the basis for conducting various fault analyses and one of the keys to the success of overall fault diagnosis. When macroscopically inspecting a fractured component, the following sequence of operations is generally followed: (a) First, observe the entire device or component with the naked eye to understand the function of the faulty part, so that the analyst can gain a comprehensive understanding of the relationship between the faulty system and the faulty component (i.e., an overall perspective); important details must be photographed. (b) Determine the failure characteristics of the faulty component based on macroscopic observations. If there is significant deformation near the fracture surface and shear lips are present on the fracture, this is a characteristic of ductile fracture. Conversely, if there is no obvious deformation in the fracture surface and no shear lips, it is a brittle fracture; when the faulty component is subjected to alternating stresses and shell or wave-pattern formations can be observed on the fracture surface (which are usually difficult to detect in ultra-high strength steels), it is a fatigue fracture. There are various other physical characteristics that can also be used to determine the attributes of the corresponding fault. When observing with the naked eye, a magnifying glass with a magnification of 50x or less is usually used. (c) Determine the location of the fracture source by tracing the tear texture of the fracture surface. For example, based on the radial convergence points and the small spikes of the chevron (ridge) pattern (i.e., steel plates with no slots on both sides), the location of the fracture origin can be determined. (d) Analyze the relationship between fracture characteristics and component shape, in order to determine the mode of loading (tension, compression, bending, torsion, alternating, etc.), as well as the stress magnitude, the degree of stress concentration, and the relationship with the direction of principal stresses. (e) Whether there are any abnormal non-fibrous fracture areas on the fracture surface. This type of fracture morphology, especially when it appears in or near the crack initiation site, may be caused by metallurgical defects such as casting, forging, or welding defects; it may play a direct role in the occurrence of failures. 2. Microscopic examination of the fracture surface of failed components: Microscopic examination, also known as micro-fractographic analysis or microscopic metallography of fracture surfaces, can be conducted using optical microscopes, transmission electron microscopes, scanning electron microscopes, and scanning Auger electron spectrometers. Based on the characteristics of the fracture pattern, the failure mechanism of the material or component can be determined. 3. Metallographic inspection: Metallographic inspection, along with electron metallographic observation, is an essential method in the fault analysis process; sometimes it is even more important than electron microscope techniques, and should be regarded as a standard analytical approach. The main factors that cause failures—processing techniques such as heat treatment, surface treatment, casting, welding, etc.; material defects resulting from improper processing methods and steps; and damage caused by changes in operating conditions and environment during use—can all be identified through metallographic testing. The microstructure has a significant impact on the properties of materials, and premature fracture may be related to abnormal structures. Sometimes, this damage is related to unwanted components or to the conditions of use. For example, the aging of low-carbon steel can cause the precipitation of iron nitride, or the condensation of gases in the steel. Metallographic examination can provide reliable information regarding the characteristics of cracks, particularly their mode of propagation, as well as the relevant factors influencing their initiation and growth. 4. Non-destructive testing: After visual inspection, non-destructive testing techniques can be used as needed to study and analyze faults. Common non-destructive testing techniques include dust testing, liquid penetrant testing, and eddy current testing. These techniques are used to detect surface cracks and discontinuities; they are simple to apply and highly effective. Internal defects can be detected using radiography or ultrasound. 5. Experimental stress analysis: Experimental stress analysis techniques can be used to determine the stress conditions on components and the stress levels that cause damage. The following methods have relatively good practical value. (a) The brittle coating method is effective for addressing the following issue: identifying small areas of high strain ； Determine the principal strain direction ； Approximate values of tensile strain and compressive strain are determined. It has become a standard strain measurement tool for use in general laboratories and in the field. (b) The electrocathode strain measurement method can detect uniaxial strain as well as static and dynamic strains in plane-strain fields, and is applicable to both metallic and non-metallic structures. It is a mature method that is widely applied and has extensive experience in use. (c) Photoelastic coatings are also used for stress measurement in laboratories. In this technique, a birefringent coating that controls thickness is applied to the component under test using a reflective coating adhesive, and then measured using a photoelastic analysis device. This method requires special equipment, and can be recorded using single-frame color film, color negative film, or color motion picture film. (d) X-ray surface residual stress measurement is a direct, non-destructive method for measuring stress, and it is only applicable to crystalline materials. Stress changes are determined by the angular changes in X-ray diffraction lines caused by stressed crystalline materials. There are now specialized X-ray stress measuring instruments that can be used to non-destructively measure the surface stress of components. 6. Chemical composition analysis. Chemical analysis includes four types: routine, local, surface, and microarea. Due to local variations in chemical composition caused by the metallurgical process, or the presence of relatively high levels of harmful elements as residues, the scrap rate during the manufacturing process increases. Gas analysis is used to determine whether the concentrations of oxygen, nitrogen, hydrogen, and gaseous elements (arsenic, antimony, bismuth, lead, tin) exceed permissible levels. Gas analysis is used to determine the contents of oxygen, nitrogen, and hydrogen elements in metals. Oxygen and nitrogen can cause strain aging and quenching aging, while welding, cathodic cleaning, electroplating, and pickling can all lead to the penetration of hydrogen into the metal, thereby causing hydrogen embrittlement. 7. Mechanical property testing: Once the load applied to the faulty component is known (whether through estimation or based on design data), the mechanical properties of the material must be measured again in order to conduct further strength verification. For the failure analysis of components, it is almost always necessary to determine the hardness and mechanical properties of the materials. Since hardness measurement is simple and easy to perform, it is often one of the most useful methods for analyzing the causes of failures. The obtained data can be used to: (a) determine whether the heat treatment meets the requirements ； (b) Estimate the tensile strength of metal materials, especially steel ； (c) Examine the changes and hardening caused by overheating, carbon deposition, carburizing, nitriding, and work hardening, etc. In addition to comparing the tested tensile and impact properties with the values specified in the manual, it is sometimes necessary to conduct measurements of mechanical properties at slightly higher or lower temperatures, in order to determine whether the components are exposed to overheating during use. It is also necessary to determine other properties related to the mechanism of failure, such as fracture toughness, fatigue strength, creep strength, and susceptibility to stress corrosion cracking. Generally speaking, cases of damage caused by insufficient tensile strength are not common. Therefore, mechanical property testing mainly serves to conduct re-inspections and rule out concerns regarding damage caused by mechanical properties. The purpose of applying fracture mechanics analysis in fault analysis is to determine, through the testing and analysis of fracture toughness, the crack size that can be tolerated for the safe use of a component, as well as to determine the remaining service life of a component with cracks. The former is used to determine whether the fracture toughness of the finished material of the component is appropriate; if it is not, measures must be taken to increase the fracture toughness of that component in order to prevent the same type of failure from occurring again ； The latter involves determining how much longer a component with existing cracks can be used, so as to avoid misjudging its premature retirement. In short, when performing fault analysis on certain large components, these issues should be taken into consideration.