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1

Comparison of two models of numerical simulation of low cycle fatigue of a plastic material with small rigid inclusions

Temis J M, Azmetov K Kh, Fleming W J

Journal of Theoretical and Applied Mechanics

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2013

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Vol. 51 nr 2

463--474

EN

Theory of Cells and a numerical FE method based on strain cyclic plasticity, damage model and technology of died elements were used for a prediction of the fatigue life of a metal matrix composite material. Results of calculations were compared with experimental fatigue data. It was shown that the predicted fatigue life of MMC using the method of cells was in close agreement with the experimental results for life outside of low cycle fatigue regime of 1000 cycles or less. The results obtained from the mathematical simulation procedure show that the failure occurs in several steps – the process of damage accumulation in the material and the process of crack growth. The results of prediction of time of the composite material full fracture are in good agreement with experimental data. The comparison show that both the numerical method and the theory of cells can be used to predict fatigue life of MMC to within an acceptable degree of accuracy.

PL

W pracy zastosowano teorię komórkową i numeryczną metodę elementów skończonych opartą na odkształceniowo cyklicznej plastyczności, wprowadzono model zniszczenia oraz uwzględniono technologię wytłaczania do określenia trwałości zmęczeniowej kompozytów metalowo-ceramicznych. Wyniki obliczeń porównano z badaniami doświadczalnymi. Zaobserwowano, że wyniki teoretyczne uzyskane z zastosowaniem teorii komórkowej były w zgodzie z eksperymentem dla niskocyklowego obciążenia zmęczeniowego, tj. dla 1000 cykli i poniżej. Rezultaty otrzymane w drodze symulacji numerycznych modelu matematycznego wskazały, że zniszczenie zmęczeniowe przebiega w kilku etapach – w wyniku akumulacji uszkodzeń i wskutek wzrostu szczeliny. Obliczenia czasu do pełnego pęknięcia kompozytu pokryły się z wynikami doświadczalnymi w dobrym stopniu. Po porównaniu efektywności metody numerycznej oraz teorii komórkowej stwierdzono w podsumowaniu, że obydwie metody mogą być stosowane do wyznaczania trwałości zmęczeniowej metalowo-ceramicznych materiałów kompozytowych z uzyskaniem zadawalającej dokładności.

2

Simulation of diffusion bonding defects influence on low- cycle fatigue of nonhomogeneous structures

Temis J.M., Azmetov H.H., Timofeev D.S.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

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2011

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z. 99

62-64

EN

Diffusion bonding (DB) is a technological process providing the bond between different as well as similar materials without melting. The provided bond possesses characteristics up to 95...99% of the respective factors of the parent material. However, even the thoroughly carried out DB contains defects. Because of incorrect choice or coating of the stop-off material and fragile oxide phase unbonded areas, cleavages and microcracks can occur in the bond. Modeling of their formation and influence on DB characteristics meets various difficulties due to the dependence on DB conditions. There- fore the most widespread defect - microvoids, which formation depends on asperities of the bonding surfaces, is considered. Voids are determined as cavities (fig. 1a, b) in the bond plane, which contain neither oxides, nor stop- off. Because of the surface diffusion during DB, microvoids are rounded to nearly spherical form, so they could be approximated as spherical cavities.

3

Plasticity models for materials and structures LCF simulation

Temis J.M.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

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2011

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z. 99

65-67

EN

Mathematical simulation of processes prior to low-cycle fatigue (LCF) crack initiation and describing its propagation is actual for many high-loaded structures in conditions of nonstationary cyclic loading. Success of LCF lifetime prediction depends on solution results of a number of related problems: creation of plasticity models, adequately describing processes taking place during alternating elasto-plastic deformation; formulating criterions of low-cycle fatigue damage; development of effective numerical methods of simulation. Phenomenon of LCF is directly coupled with the process of plastic deformation of structure, because development of failure has two stages. The first stage of hidden damage accumulation in the construction material is prior to the second – crack nucleation and propagation. During cycle loading in process of crack nucleation, propagation and growth alternating nontationary plastic deformation occurs near crack end, leading to damage accumulation with following material fracture and crack propagation. In this paper current state of the problem is examined considering modern conception of methods and models of non-isothermal deformation during cyclic loading [1,2]. Low-cycle fatigue phenomenon formulates tight requirements to models accuracy and numerical methods efficiency. LCF criterions could be formu- lated being dependent only on cycle plastic deformations (high level of stress or strain), or both on plastic strains and alternating elastic strains of the cycle (transition region between LCF and classical fatigue). Process of alternating or complex loading in plastic zone leads, depending on "deformation history", to changing of original elastic and plastic material parameters: modulus of elasticity, Bauschinger's effect parameters, nonlinear parameters of hard- ening. According to parameters taken into account, scalar or vector models of damage is considered.