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1

Development of cumulative damage in structure materials

Čačko J.

Zeszyty Naukowe Politechniki Świętokrzyskiej. Mechanika

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2003

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Z. 78

63-68

EN

Comparision of a fatigue damage in two sets of specimens loaded under various stress levels is presented in the paper. Specimens were stressed under constant amplitude of total strain, whereby the first set was loaded in a low-cycle fatigue range and the second one in a high-cycle fatigue range. It follows, the specimens under low-cycle fatigue loading has endured lesser number of cycles to failure with greater number of initiated cracks than the other one. Materials of all specimens have performed cyclic softening with saturation character. The fatigue cracks have indicated a mixed both trans-crystallic and inter-crystallic nature.

PL

Porównano uszkodzenia zmęczeniowe w dwóch zbiorach próbek obciążonych do różnych poziomów naprężeń. Próbki były obciążone zmęczeniowo przy stałej amplitudzie całkowitych odkształceń w zakresie obciążeń nisko- i wysoko-cyklicznych. Okazało się, że próbki obciążone w zakresie nisko-cyklowym uległy zniszczeniu przy mniejszej liczbie cykli posiadając większą liczbę zainicjowanych szczelin niż te obciążone w zakresie wysoko-cyklowym. Wszystkie badane materiały wykazywały cykliczne osłabienie z tendencją do nasycania. Pęknięcia zmęczeniowe miały charakter zarówno śródkrystaliczny jak i międzykrystaliczny.

2

Evaluation of a fatigue cumulative damage in service life prediction

Čačko J.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

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2001

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z. 67

27-46

EN

Damage accumulation is evaluated according to a cumulative damage hypothesis, and the residual life is predicted with respect to a fatigue failure criterion. The estimation of cumulative damage under service loading is significantly influenced by a time loading history. The problem of the history effect can be converted to the problem of investigation of equivalent loading with non-zero stress/strain mean value. The conversion method is based in specification of an equivalent amplitude of the harmonic cycle with zero stress and strain mean values. At the same time, a relative damage that is accumulated in the course of equivalent cycle must be the same as the relative damage of actual cycle which has been identified by the rainflow method. The submitted approach enables to utilize the knowledge of cumulative damage process and fatigue behaviour prediction under harmonic loading and to use it for any complex operating loading spectrum. The base of the method and obtained results are presented in the paper.

3

Computer modelling of operating processes for both simulational and experimental investigation of fracture problems

Čačko J.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

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2001

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z. 67

7-26

EN

The problem of evaluation and computer modelling of operational processes with both continuous and discontinuous nature is presented in the paper. We suppose the knowledge of statistical characteristics of the continuous component in the frame of correlation theory - frequency probability density function of the distribution and power spectral density or autocorrelation function respectively. A simulation model is based on modelling of time-series with application for gaussian and nongaussian processes. Discontinuous events are assumed as a sequence of various impacts with Poissonian distribution, and they are superimposed to a continuous component. A cumulative damage is continuously evaluated regarding to a closed hysteresis loops counting. A fatigue damage mechanism is represented either by rainflow matrix or by a fatigue damage function. The function is specified using some hypothesis of a fatigue cumulative damage and fatigue failure criteria.