【ASME Analysis and Design】Requirements and steps for ASME analysis and design

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This post was last edited by B0SS on 2018-4-11 at 20:44. The ASME analysis and design standards are developed based on the failure modes of a component. To determine the safety of a component, it is necessary to consider the following 4 failure modes, with all of them having to be satisfied: 1) Prevention of plastic collapse, 2) Prevention of local failure, 3) Prevention of instability, 4) Prevention of failure under cyclic loading. 1. Prevention of plastic collapse? There are three methods to prevent plastic collapse of structures: 1) elastic stress analysis ； 2) Ultimate load analysis ； 3) Elasto-plastic stress analysis. 1) Elastic stress analysis to prevent plastic collapse of components requires evaluating three types of stress: overall membrane stress (Pm), local membrane stress (PL), and membrane plus bending stress (PL+Pb). For Pm, it is generally ensured through conventional calculations; therefore, only the latter two types of stress need to be evaluated. In many cases, it is difficult to distinguish between PL and Pb, and they are often classified as secondary stress (Q), which can easily lead to recklessness and affect the safety of the structure. It should be noted that when evaluating secondary stresses, the stresses derived from the operational condition analysis must be used ； The ASME standard adopts the fourth strength theory – the energy theory of shape change, whereas JB/T4732 uses the third strength theory – the maximum shear stress theory. 2) Ultimate load analysis: The ASME VIII-2 code has introduced Load and Resistance Factor Design (LRFD), applying it to ultimate load and elastic-plastic analysis, and no longer uses twice the elastic slope as a criterion for evaluation. Based on the load combinations and load coefficients from the ultimate load analysis in Table 5.4, they are classified into Load Condition Combination 1: overall criterion, Load Condition Combination 2: local criterion, and Load Condition Combination 3: hydraulic testing conditions. During analysis, it is necessary to consider the load combinations and coefficients for each combination of operating conditions; only when the solution for each condition converges can the limit load analysis of the structure be considered satisfactory. 3) Elastoplastic stress analysis: Elastoplastic stress analysis also incorporates the use of load and resistance coefficient designs. According to Table 5.5 in ASME VIII-2, the load condition combinations and load coefficients used for elastoplastic stress analysis include overall criteria, local criteria, and hydraulic testing conditions. The structure is safe as long as the sum of the forming strains of all units and the total equivalent plastic strain is less than or equal to the triaxial strain limit. The comparison between elastic analysis and elastoplastic analysis is as follows: (1) Elastic stress analysis is a linear analysis in which the material remains within the linear elastic range; the theory of small deformations is applied, the stiffness matrix does not change as the structure deforms, and nominal stresses and strains are obtained ； Elasto-plastic stress analysis is a type of material and geometrically nonlinear analysis that requires the input of actual stress-strain curves to account for strain hardening in the material. During the iterative solution process, the stiffness matrix of the structure is continuously updated as the structure deforms, thereby establishing new equilibrium iteration equations; the resulting stress and strain values are more accurate in reflecting actual engineering conditions. (2) During the solution process, the elastic stress analysis design is relatively simple; only it is necessary to apply the appropriate loads ； The elastic-plastic stress analysis setup is relatively more complex, as it requires the specification of various nonlinear solving parameters to ensure the convergence of the structure; these include factors such as time steps, large deformation switch, linear search, Newton-Raphson options, automatic time step size, time step prediction, and loading methods. (3) During the post-processing stage, the elastic stress analysis requires customizing a path; after linearizing the stresses along this path, stress intensity assessment is performed. Stress linearization is highly dependent on grid division and the operator, exhibiting considerable randomness. In cases where the stress conditions are complex, it may be impossible to classify the stress accurately according to its type, which can lead to hasty decisions ； Elastoplastic stress analysis eliminates the need for stress linearization and stress classification; it requires calculations and comparisons of multi-axis strain limits, total equivalent plastic strain, and structural forming strain in accordance with ASME standards, making the processing process somewhat more complex than that of elastic stress analysis. (4) Key focus areas: Elastic stress analysis focuses on the stress intensity of the structure as well as the various stress components after linearization ； Elastic-plastic stress analysis focuses on whether the calculations converge, whether the curves representing stress, strain, plastic strain, etc., along the loading time course are smooth, as well as on parameters such as the multi-axis strain limit, total equivalent plastic strain, and structural deformation strain. This is mainly because the two methods use different criteria for evaluating the calculation results: the former employs a stress classification criterion, while the latter uses a local strain limit criterion. (5) In most cases, the linearization of dangerous sections in elastic stress analysis is forced to become a determination of whether plastic hinges will form at specific locations (unless it is an axially symmetric model or similar situation); it is not possible to determine whether the entire section will enter a plastic state. However, the presence of a finite number of disconnected plastic hinges does not prevent the structure from continuing to bear loads. Elastoplastic stress analysis takes into account the stress redistribution caused by deformation, allowing for an intuitive simulation of the phenomenon in which plastic hinges form a geometrically variable mechanism. Therefore, elastic stress analysis is more conservative than elastoplastic stress analysis, but the latter is more time-consuming and requires higher computer hardware. 2. Prevent local failure? Elastic analysis and elastoplastic analysis can be used to prevent local structural failure. 1) For elastic analysis, it is sufficient that the algebraic sum of the three stresses be less than 4S. 2) In elasto-plastic analysis, it is necessary to ensure that the sum of the forming strain of all elements and the total equivalent plastic strain under each load combination is less than or equal to the triaxial strain limit, so that the structure remains safe. 3. Prevent instability? Numerical analysis of the structure is performed to determine its buckling load; by dividing this value by the corresponding safety factor, the allowable external pressure on the structure can be obtained. The methods for this purpose include eigenvalue analysis and nonlinear analysis. Eigenvalue analysis belongs to linear buckling analysis and is actually impractical, whereas nonlinear analysis includes geometric nonlinearity, material nonlinearity, boundary condition nonlinearity, or contact analysis, which is more in line with engineering reality. 4. Prevent circular failure? 1) There are actually two types of instability in ratchet evaluation: during repeated loading, the same local area undergoes alternating plastic deformation in opposite directions each time. In such cases, a lack of stability leads to plastic fatigue, and failure occurs within a very short period of time; this is known as low-cycle fatigue failure. Another type is where the plastic deformation is not alternating back and forth but accumulates, which is called ratcheting. Failure is referred to as cumulative plastic deformation failure. Ratcheting does not occur when there is only one set of recurring loads. For a piece of equipment, ratchet evaluation is necessary whether it is a fatigue-prone device or not, as long as there are start-up and stop-up operations. The ratchet assessment is conducted under operating conditions, and it evaluates an amplitude value; it is somewhat similar to fatigue assessment, and its results are used as a coefficient in the fatigue assessment (fatigue damage coefficient). If the condition of being less than Sps is met, the fatigue loss coefficient can be set to 1 when performing fatigue analysis; otherwise, the fatigue loss coefficient needs to be modified. In the absence of thermal stress (or when no significant thermal stress is assumed), the combined primary and secondary stresses on the component must be less than or equal to Sps. Once it exceeds a certain value, it is considered that ratcheting will occur, meaning instability. Under thermal stress, there are two scenarios: in one case, the stress generated by all loads, including the thermal load, is less than Sps, in which case it is no different from the first scenario. When the value exceeds Sps due to the presence of thermal loads, a separate assessment can be carried out; in cases without thermal loads, the value must be below Sps, and the thermal stress generated by the thermal loads must be assessed separately. When using this method, it is necessary to limit the number of times the device is used, meaning that the device becomes a fatigue-prone device. The principle relied on for conducting a secondary stress assessment is generally the ratchet assessment. Specifically, in the absence of thermal loads, using the design pressure and temperature to assess secondary stresses is often conservative, as the design temperatures and pressures are generally more stringent than those under operating conditions ； When there are temperature differential stresses, the operating conditions are generally used alone to calculate the stresses resulting from temperature and other mechanical loads; in such cases, it is necessary to follow the ratchet assessment method strictly. 2) Fatigue assessment: Fatigue analysis is divided into elastic analysis and elastoplastic analysis. Generally, a stress analysis is conducted for cyclic loading conditions to determine the stress amplitude; corresponding corrections are then applied to this stress amplitude, and the fatigue life of the structure is obtained by referring to the appropriate fatigue curve. When there are two or more significant stress cycles, the effect of cumulative fatigue damage should be calculated. In the new version of the ASME 2017 standard, high-temperature equipment analysis per ASME NH has been introduced, providing analysis and design methods for creep and creep-fatigue, enabling the AMSE VIII-2 standard to conduct creep-fatigue analyses and thus making the technology more mature and advanced.