Creep-fatigue behaviour of aluminum alloy-based metal matrix composite

Metal Matrix Composite (MMC) represents a valuable option as structural material for different type of structures and components. Despite this they struggle to become widely adopted due to expensive manufacturing process and complex microstructural behaviour. When subjected to cyclic load conditions the structural response of MMC is not trivial, and becomes even more difficult when high temperature load is involved. Different failure mechanisms would happen and they are originated by the different material properties between the fibre and surrounding matrix. Among all, the mismatch of thermal expansion coefficient is recognized to be the dominant one. The significantly differing coefficients of thermal expansion between ceramic and metal give rise to micro thermal stresses, which enhance the initiation of matrix micro cracks. Their performance under varying load and high temperature is complex, and hence it is difficult to have a clear understanding of the structural responses, especially when fatigue and creep damages become the main failures of MMCs. To improve current understanding of the relationship between creep fatigue interaction of MMCs, the history of thermal and mechanical loading, and the creep dwell period, a highly accurate but robust direct simulation technique on the basis of the Linear Matching Method (LMM) framework has been proposed in this paper, and been applied to model the fatigue and creep behaviour of MMCs. A homogenised FE model is considered in all analyses, which consist of continuous silicon carbide fibres embedded in a square 2024T3 aluminium alloy matrix array. Various factors that affect creep and fatigue behaviours of composites are analysed and discussed, including effects of the applied load level, dwell period and temperature on the MMC’s performance. The effects of reversed plasticity on stress relaxation and creep deformation of MMC are investigated, and the behaviours of cyclically enhanced creep and elastic followup are presented. A detailed study of the creep ratchetting mechanism is also performed with the concentration on the impact of temperature and different loading conditions. The accuracy of the proposed method has been verified by detailed incremental finite element analyses using the commercial finite element solver Abaqus. Such verifications further improve the understanding of the failure mechanisms identified and discussed in this work

Influences of T-stress on constraint effect in mismatched modified boundary layer model for creep crack

Constraint effect plays an important role in assessing the stress field and the growth rate of creep crack in components under high temperature. The mismatched modified boundary layer (MMBL) model is extended to creep crack in this paper. For the MMBL model, the Q-parameters for different mismatch factors are studied under different T-stresses. The variation of the dimensionless T-stress in creep zone is given. The variations of open stresses with creep time for different mismatch factors are presented under different T-stresses. The comparisons of Q-parameter between homogeneous material and mismatched materials are made. The influences of mismatch factor on the constraint parameter are discussed. The influence of creep exponent on the open stress is also discussed

On creep fatigue interaction of components at elevated temperature

The accurate assessment of creep-fatigue interaction is an important issue for industrial components operating with large cyclic thermal and mechanical loads. An extensive review of different aspects of creep fatigue interaction is proposed in this paper. The introduction of a high temperature creep dwell within the loading cycle has relevant impact on the structural behaviour. Different mechanisms can occur, including the cyclically enhanced creep, the creep enhanced plasticity and creep ratchetting due to the creep fatigue interaction. A series of crucial parameters for crack initiation assessment can be identified, such as the start of dwell stress, the creep strain and the total strain range. A comparison between the ASME NH and R5 is proposed, and the principal differences in calculating the aforementioned parameters are outlined. The Linear Matching Method framework is also presented and reviewed, as a direct method capable of calculating these parameters and assessing also the steady state cycle response due to creep and cyclic plasticity interaction. Two numerical examples are presented, the first one is a cruciform weldment subjected to cyclic bending moment and uniform high temperature with different dwell times. The second numerical example considers creep fatigue response on a long fibre reinforced Metal Matrix Composite (MMC), which is subjected to a cycling uniform thermal field and a constant transverse mechanical load. All the results demonstrate that the Linear Matching Method is capable of providing accurate solutions, and also relaxing the conservatisms of the design codes. Furthermore, as a direct method it is more efficient than standard inelastic incremental finite element analysis

A study on the matching of constraint between steam turbine blade and laboratory specimens

The matching of constraint between laboratory specimens and actual cracked structures is a key problem of the accurate structure integrity assessment. Different laboratory specimens and the steam turbine blade with different constraints were selected, the matching of constraint between steam turbine blade and laboratory specimens was investigated. The results shown that the steam turbine blade with 2c = 50 mm, a/2c = 0.20 has a matching constraint with single edge-notched bend specimen with a/W = 0.6 and single edge-notched tensile specimen with a/W = 0.3. The steam turbine blade with 2c = 50 mm, a/2c = 0.25 has a matching constraint with single edge-notched bend specimen with a/W = 0.7. The steam turbine blade with 2c = 50 mm, a/2c = 0.30 has a matching constraint with single edge-notched bend specimen with a/W = 0.5 and single edge-notched tensile specimen with a/W = 0.1. The steam turbine blade with 2c = 50 mm, a/2c = 0.35 has a matching constraint with single edge-notched bend specimen with a/W = 0.4, compact tension specimen with a/W = 0.3 and central-cracked tension specimen with a/W = 0.7. The steam turbine blade with a = 15 mm, a/2c = 0.30 has a matching constraint with compact tension specimen with a/W = 0.7 and single edge-notched tensile specimen with a/W = 0.5. The steam turbine blade with a = 15 mm, a/2c = 0.40 has a matching constraint with compact tension specimen with a/W = 0.4. The steam turbine blade with a = 15 mm, a/2c = 0.50 has a matching constraint with single edge-notched bend specimen with a/W = 0.5

Self-Aligned Concave Curve: Illumination Enhancement for Unsupervised Adaptation

Low light conditions not only degrade human visual experience, but also reduce the performance of downstream machine analytics. Although many works have been designed for low-light enhancement or domain adaptive machine analytics, the former considers less on high-level vision, while the latter neglects the potential of image-level signal adjustment. How to restore underexposed images/videos from the perspective of machine vision has long been overlooked. In this paper, we are the first to propose a learnable illumination enhancement model for high-level vision. Inspired by real camera response functions, we assume that the illumination enhancement function should be a concave curve, and propose to satisfy this concavity through discrete integral. With the intention of adapting illumination from the perspective of machine vision without task-specific annotated data, we design an asymmetric cross-domain self-supervised training strategy. Our model architecture and training designs mutually benefit each other, forming a powerful unsupervised normal-to-low light adaptation framework. Comprehensive experiments demonstrate that our method surpasses existing low-light enhancement and adaptation methods and shows superior generalization on various low-light vision tasks, including classification, detection, action recognition, and optical flow estimation. Project website: https://daooshee.github.io/SACC-Website/Comment: This paper has been accepted by ACM Multimedia 202

Shakedown analysis of a torispherical head with a piping nozzle under combined loads by the stress compensation method

Shakedown assessment is an important task in determining the load-bearing capacity of structures and evaluating their safety. The traditional shakedown analyses, which are based on the upper or lower bound shakedown theorem to establish the mathematical programming formulation and solve an optimisation problem, are difficult to apply in engineering practice owing to limitations of the computing scale and computational efficiency. In this study, a numerical shakedown analysis using the recently developed stress compensation method (SCM) is performed for a torispherical head with a piping nozzle, which is a typical structural component of pressure vessel equipment. The loads applied to the structural component include an internal pressure, axial force, twisting moment, out-of-plane and in-plane bending moments, and thermal loading, all of which vary independently of each other. Two- and three-dimensional strict shakedown boundaries for the torispherical head under different combinations of these loads are presented and analysed. In addition, the effect of a temperature-dependent yield strength on the shakedown domain is also investigated. These investigations demonstrate that the proposed SCM is capable of solving the practical shakedown problem for structures under complicated combined loads in industrial applications. The obtained results can provide guidance for the safe structural design of torispherical heads with piping nozzles

Learning Complicated Manipulation Skills via Deterministic Policy with Limited Demonstrations

Combined with demonstrations, deep reinforcement learning can efficiently develop policies for manipulators. However, it takes time to collect sufficient high-quality demonstrations in practice. And human demonstrations may be unsuitable for robots. The non-Markovian process and over-reliance on demonstrations are further challenges. For example, we found that RL agents are sensitive to demonstration quality in manipulation tasks and struggle to adapt to demonstrations directly from humans. Thus it is challenging to leverage low-quality and insufficient demonstrations to assist reinforcement learning in training better policies, and sometimes, limited demonstrations even lead to worse performance. We propose a new algorithm named TD3fG (TD3 learning from a generator) to solve these problems. It forms a smooth transition from learning from experts to learning from experience. This innovation can help agents extract prior knowledge while reducing the detrimental effects of the demonstrations. Our algorithm performs well in Adroit manipulator and MuJoCo tasks with limited demonstrations

Creep-fatigue and cyclically enhanced creep mechanisms in aluminium based metal matrix composites

An aluminium (Al 2024T3) matrix composite reinforced with continuous alumina (Al2O3) fibres is investigated under tensile off-axis constant macro stress and thermal cyclic loading. The micromechanical approach to modelling and three different fibre cross-section geometries have been employed. The effect of creep is included by considering three dwell times at the peak temperature of the thermal loading history. The presence of the hold time gives rise to different sources of failure such as cyclic enhanced creep and creep ratchetting. These failure mechanisms are carefully discussed and assessed. The linear matching method framework has been used for the direct evaluation of the crucial parameters for creep-fatigue crack initiation assessment at the steady cycle. A detailed representation of the steady-state hysteresis loops is provided by using the strain range partitioning and a method for dealing with multiaxiality is reported with regard to the algebraic sign of the Mises-Hencky equivalent stress and strain. All the results obtained have been benchmarked by fully inelastic step-by-step (SBS) analyses. The design of a long fibre metal matrix composite should consider not only the detrimental effect of their dissimilar coefficient of thermal expansion, but also the state of stress at the interface between the matrix and fibre