A Direct Method for the Evaluation of Lower and Upper Bound Ratchet Limits

European Journal of Mechanics - A/Solids

2018

A 90° back-to-back pipe bend structure subjected to cyclic in-plane bending moment and steady internal pressures is analysed by means of the Linear Matching Method (LMM) in order to create the limit, shakedown, and ratchet boundaries. The analyses performed in this work demonstrate that the cyclic moment has a more significant impact upon the structural integrity of the pipe bend than the constant pressure. Full cyclic incremental analyses are used to verify the structural responses either side of each boundary and confirm correct responses. In addition, the shakedown boundary produced by the LMM is compared to another shakedown boundary of an identical pipe bend computed by the simplified technique and it is shown that the LMM calculates results more accurately. Parametric studies involving a change of geometry of the pipe bends and loading type are carried out. From the studies of the geometry, two semi-empirical equations are derived from correlations of the reverse plasticity limit and the limit pressure with the bend characteristic. Finally, the results presented in this paper provide a comprehensive understanding of post-yield behaviours of the 90° back-to-back pipe structure under the combined loading as well as offering essential points to be concerned for the life assessment of the piping system.

Section: Numerical Proceduresmentioning

confidence: 99%

Shakedown, ratchet, and limit analyses of 90° back-to-back pipe bends under cyclic in-plane opening bending and steady internal pressure

Cho

European Journal of Mechanics - A/Solids

2018

“…Current lower bound ratchet methods also include the Hybrid method [14], the lower bound extension to the LMM [15], the Uniform Modified Yield (UMY) and Load Dependent Yield Modification (LDYM) methods [16] and the new methods proposed here. The lower bound extension to the LMM [15] uses the stress fields generated by the upper bound LMM and scales the stresses until the they satisfy the lower bound Melan theorem. Despite using a well-developed upper bound procedure, the lower bound can take a relatively large number of iterations to achieve strict convergence The Hybrid method generates an estimate for the cyclic stresses through the use of the direct cyclic analysis [17] currently available in ABAQUS [18].…”

Section: Upper and Lower Bound Ratchet Methodsmentioning

confidence: 99%

A Fully Implicit, Lower Bound, Multi-Axial Solution Strategy for Direct Ratchet Boundary Evaluation: Implementation and Comparison

Jappy¹,

Mackenzie²,

Journal of Pressure Vessel Technology

2013

Ensuring sufficient safety against ratcheting is a fundamental requirement in pressure vessel design. However, determining the ratchet boundary using a full elastic-plastic finite element analysis can be problematic and a number of direct methods have been proposed to overcome difficulties associated with ratchet boundary evaluation. This paper proposes a new lower bound ratchet analysis approach, similar to the previously proposed hybrid method but based on fully implicit elastic-plastic solution strategies. The method utilizes superimposed elastic stresses and modified radial return integration to converge on the residual state throughout, resulting in one finite element model suitable for solving the cyclic stresses (stage 1) and performing the augmented limit analysis to determine the ratchet boundary (stage 2). The modified radial return methods for both stages of the analysis are presented, with the corresponding stress update algorithm and resulting consistent tangent moduli. Comparisons with other direct methods for selected benchmark problems are presented. It is shown that the proposed method evaluates a consistent lower bound estimate of the ratchet boundary, which has not previously been clearly demonstrated for other lower bound approaches. Limitations in the description of plastic strains and compatibility during the ratchet analysis are identified as being a cause for the differences between the proposed methods and current upper bound methods.

“…Step 1: Estimation of saturated hysteresis loop using eDSCA within LMM framework There are various direct methods which can be used in conjunction with FEA for modelling and analysis of structures under high-temperature and mechanical load. The LMM has been developed for limit analysis [10], shakedown analysis [11], ratchet analysis [12,13], and recently to include steady-state cyclic behaviour with full creep-cyclic plasticity interaction [7,14]. The LMM ensures that both equilibrium and compatibility are satisfied at all stages.…”

Section: The Creep-fatigue Damage Assessment Proceduresmentioning

confidence: 99%

Creep–Cyclic Plasticity and Damage Assessment of an SS304 Weldolet

Puliyaneth

Journal of Pressure Vessel Technology

2021

Creep-fatigue and creep-ratcheting life assessment of an SS304 weldolet considering full creep-cyclic plasticity interaction is investigated using the extended Direct Steady Cycle Analysis (eDSCA) within the Linear Matching Method Framework (LMMF). The creep behaviour is modelled using the Norton relationship, which is modified by the Arrhenius rule to account for the temperature variation within the weldolet. The introduction of a creep dwell increases the reverse plasticity resulting from the creep relaxation. This leads to both creep-fatigue and creep-ratcheting damage mechanisms at different regions within the weldment. For thermal load dominated loading combinations, creep ratcheting due to both cyclically enhanced creep and creep enhanced plasticity are observed based on the dwell period. The effect of dwell period, load and temperature on the creep-fatigue and creep-ratcheting interaction of a weldolet are presented. The simultaneous presence of various damage mechanisms at different locations within the weldment highlights the importance and requirement of the proposed creep-cyclic plasticity investigations at weld locations.