What are the main types of failure for pressure vessels?

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As the title suggests, these are notes I took during my own exam preparation. I’d like to share them with everyone for discussion: What are the main types of failures in pressure vessels? Answer: Failures of pressure vessels can be classified by their causes, and there are three main types of failures: short-term failure modes, long-term failure modes, and cyclic failure modes. Among them: short-term failure modes include: (1) brittle fracture ； (2) Ductile fracture ； (3) Joint leakage caused by excessive deformation ； (4) Crack formation or ductile tearing caused by excessive local strain ； (5) Elastic, plastic, or elastoplastic instability (collapse) ； Long-term failure modes include: (6) creep fracture ； (7) Creep – Excessive deformation at mechanical joints or resulting in unacceptable load transfer ； (8) Creep instability ； (9) Erosion, corrosion ； (10) Environment-promoted cracking (such as stress corrosion cracking, hydrogen-induced cracking) ； (11) Ductile plastic deformation ； (12) Alternating plasticity ； (13) Elastic strain fatigue (medium and high-cycle fatigue) or elastoplastic strain fatigue (low-cycle fatigue) ； (14) The environment contributes to fatigue. For pressure vessel standards, the following failure modes must be considered at least when determining design criteria and design methods: brittle fracture, ductile fracture, joint leakage, elastic or plastic instability, and creep fracture. What are the general characteristics of brittle fracture in pressure vessels? What is the cause of it? What are its preventive measures? Answer: Fracture is a sudden type of failure that is highly destructive. Since it occurs under relatively low stress levels, it is also referred to as low-stress failure. The characteristics of brittle fracture are as follows: (1) Vessels that suffer from brittle failure generally do not exhibit significant elongation deformation, and the wall thickness of such vessels also does not decrease ； (2) The crack is parallel and perpendicular to the direction of the principal stress. The reduction of area near the fracture is very small, generally not exceeding 3%. On the fracture surfaces of containers with relatively thick walls, chevron-shaped or radial patterns are often observed; these are one of the main macroscopic characteristics of brittle fracture ； (3) Fractures always begin to form at the stress concentration points in the structure – areas where there are defects or sudden changes in geometry. Once the crack reaches a critical size, it expands rapidly, causing the structure to break ； (4) Failure usually occurs in an instant, with a very high rate of fracture; the pressure inside the container cannot be released through a small crack. As a result, brittle failure often results in the object breaking into pieces, and these pieces tend to fly out ； (5) Most brittle fracture accidents occur at lower temperatures. The main causes of brittle fracture are: (1) poor toughness of the material ； (2) Inherent stress concentration in containers and weld joints ； (3) Residual stress exists ； (4) There was inappropriate cold working deformation during the manufacturing process. The main measures to prevent brittle fracture are: (1) reducing stress concentration in the structure and welds, and eliminating various defects in the container as much as possible. Therefore, in processes such as welding design, material selection, welding execution, and inspection, care must be taken to minimize stress concentration as much as possible. The joints should have a smooth transition to prevent defects such as cracks and undercutting ； (2) The container material should have good toughness. The material should be selected based on the operating conditions of the container (primarily temperature), so that it meets the toughness criteria specified in the standards during service. Avoid rapid pressure changes in the container during starting and stopping, as this can cause a significant decrease in the toughness of the material ； (3) Eliminate residual stress. Therefore, during design and manufacturing, the welds must be arranged reasonably; appropriate measures should be taken during welding; and heat treatment should be performed after welding as needed ； (4) Strengthen non-destructive testing of manufactured and in-service assets. Conducting regular inspections and non-destructive testing in accordance with **relevant regulations is a highly important measure to ensure that containers do not suffer from brittle failures. What are the typical characteristics of ductile failure in pressure vessels? What is the cause of it? What are its preventive measures? Answer: The characteristics of ductile failure include: (1) after the container fails, there is significant plastic deformation of its walls; the residual volume deformation rate and the maximum circumferential elongation can reach 10%, with some cases even reaching 20% ； (2) The fracture surface is dark gray and fibrous, without any shiny metallic luster ； The port is not level, and it forms a 45° angle with the direction of the principal stress ； Torn-like ； (3) After failure, cylindrical containers generally take on a bulged shape with smaller ends and a larger middle part ； (4) It generally does not crack, that is, no fragments are produced or only a small number of them are formed, with only an opening appearing. The crack size depends on the expansion energy during container explosion ； (5) The actual bursting pressure of the container is close to the calculated bursting pressure, and this pressure is usually not less than the yield strength or tensile strength of the material. The causes of ductile failure include: (1) overfilling (for liquefied gas containers) ； (2) Overpressure during use ； (3) Reduction in container wall thickness. To prevent brittle failure accidents in pressure vessels, the most fundamental measure is to ensure that the total membrane stress on the vessel walls remains below the yield limit of the wall material under all circumstances. To this end, the key preventive measures include: (1) The pressure vessels used must be carefully designed to have sufficient thickness, so that the stresses on their walls stay within acceptable limits when the vessels operate at specified pressures ； (2) The container shall be equipped with a safety pressure relief device as specified, and it must be ensured that this device is sensitive and in good condition ； (3) Carefully follow the operating procedures for pressure vessels to prevent them from operating under excessive pressure ； (4) Strengthen the maintenance inspection of in-service containers, and take effective measures to prevent corrosion by corrosive media and the atmosphere. During inspection, if it is found that the wall of the container has been corroded to such an extent that its thickness has significantly decreased, or if significant plastic deformation of the container wall is observed during operation, its use should be stopped. What are the general characteristics of fatigue failure in pressure vessels? What is the cause of it? What are its preventive measures? Answer: The characteristics of fatigue failure generally include: (1) after the container fails, there is obvious plastic deformation of the wall, the diameter does not increase significantly, and most of the wall thickness also does not decrease markedly ； (2) The fracture surface of fatigue failure generally exhibits two distinct regions: one is the region where the fatigue crack originates and propagates, and the other is the final fracture region. Most pressure vessels have long stress variation cycles, resulting in slow crack propagation; therefore, the arc-shaped pattern of crack growth can sometimes still be observed. If the fatigue lines on the fracture surface are relatively distinct, it is also possible to determine the origin point of the fatigue crack based on them. It often occurs in areas where stress is concentrated ； (3) Fatigue failure of containers usually results in the formation of a crack, causing the container to leak and become ineffective ； (4) Rupture always occurs after the container has been subjected to repeated pressurization and depressurization. The main conditions for the occurrence of fatigue failure accidents are: (1) severe stress concentration exists in the containers and welded joints ； (2) The local stress at certain locations exceeds the yield strength of the material. The abnormal operating conditions that lead to fatigue failure accidents are mainly: (1) frequent start-up and shutdown of the vessel, causing it to be subjected to repeated stress ； (2) The pressure of the container varies or fluctuates within a wide range during operation ； (3) The operating temperature of the container undergoes periodic and significant changes, resulting in repeated thermal stress on the container walls ； (4) The container or its nozzles experience severe forced vibration, resulting in localized stresses ； (5) The container is subjected to periodic external loads. The fatigue failure of containers is caused by varying loads and excessive local stresses. To prevent such accidents, in addition to trying to avoid unnecessary frequent pressurization, depressurization, and large temperature fluctuations during operation, it is even more important to minimize local peak stresses during the design of the container, ensuring that they do not exceed the material’s endurance limit. If relatively high local stresses inevitably occur on the container, fatigue analysis and fatigue design should be carried out. What are the characteristics of creep failure in pressure vessels? What are the conditions under which creep failure occurs? Answer: The characteristics of creep failure generally include: (1) obvious deformation, the amount of which is related to the plasticity of the material at high temperatures ； (2) The fracture exhibits a brittle fracture morphology ； (3) A graphitization process occurs in the steel. The conditions for creep failure accidents include: (1) improper material selection, where materials with good plasticity at room temperature but brittleness at high temperatures are used, or ordinary carbon steel is employed in place of alloy steel with excellent creep resistance ； (2) An unreasonable structure, improper operation, or inadequate maintenance can all cause localized overheating. All criticism, corrections, discussions, additions, and improvements are welcome.