Extremely Low Cycle Fatigue Research Articles

Experimental study on ductile crack initiation in compact section steel columns

In order to develop a verification method for extremely low cycle fatigue (ELCF) of steel structures, the initiation mechanism of ductile cracks is investigated in the present study, which is the first step of brittle fracture, occurred in steel bridge piers with thick-walled sections. For this purpose, a total of six steel columns with small width-thickness ratios were tested under cyclic loading. It is found that ductile cracks occurred at the column base in all the specimens regardless of cyclic loading histories subjected. Moreover, strain history near the crack initiation location is illustrated and an index of energy dissipation amount is proposed to evaluate deformation capacity of structures.

Thermal fatigue behaviour of Al2O3-DBC substrates under high temperature cyclic loading

Purpose The purpose of this paper is to study the phenomenology of Al2O3-DBC substrate thermal-cracking under different high temperature cyclic loadings. The extremely low cycle fatigue (ELCF) life prediction model for ductile materials was used to describe the thermal fatigue life of Al2O3-DBC substrates. Design/methodology/approach Four groups of thermal cycling tests using Al2O3-DBC substrates with 0.65 mm thick copper were conducted using different peak temperatures. The failure samples were observed by optical microscope. The thermal plastic strain distribution in the Al2O3-DBC substrates was analyzed using a finite element method with the Chaboche model for describing plastic deformation of copper. The ELCF life prediction model was used to predict the life of Al2O3-DBC substrates under high temperature cyclic loadings. Findings Interface cracking was observed to initiate at the short edge of the bonded copper and deviated into the ceramic layer when the crack grew beyond the critical length of 0.1-0.8 mm. The interface crack deviated into the ceramic layer at different thickness and grew parallel to the interface layer between the ceramic layer and copper layer. The crack propagation stopped after certain cycles. The copper layer with 10-20 μm thick alumina inside was not split away totally from the ceramic layer. The ELCF life prediction model could predict the life of Al2O3-DBC substrates well under high temperature cyclic loading. The material constants in the extremely low fatigue life prediction model were obtained using thermal fatigue tests results. Research limitations/implications The influence of copper layer thickness and ceramic layer thickness on thermal cracking characteristics of DBC substrate should be studied in the future. Failure models should also be further investigated. Originality/value The failure model of Al2O3-DBC substrates under high temperature cyclic loading was studied. A method for predicting the life of the substrate samples under high temperature cyclic loading was proposed.

Fatigue properties of 316 stainless steel and its failure due to internal cracks in low-cycle and extremely low-cycle fatigue regimes

Abstract It is important to know residual fatigue strength of materials in the extremely low-cycle fatigue (ELCF) regime in order to assess structural integrity after large cyclic strain caused by an earthquake. In the ELCF regime, it was pointed out that not only surface cracking but also additional bulk damage reduces the fatigue life. In this study, the fatigue strength of stainless steel in the low-cycle and ELCF regimes and the effects of bulk damage on fatigue strength were investigated. Type 316 stainless steel was subjected to axial cyclic strain up to strain amplitude of 6%. Also, the change in fatigue life was investigated when the surface layer of the specimens was removed during the fatigue tests. It was shown that not only the surface cracking but also the bulk damage affected the fatigue life. It was also shown that the bulk damage consisted of two factors: internal cracking and local damage. Internal cracks were observed on the fractured surface when the strain amplitude was more than 1%, and in some cases the specimen was fractured by these internal cracks. Local damage reduced the incubation period before crack initiation and fatigue life decreased as the strain amplitude increased. In all cases, the experiment conducted under the strain amplitude e a = 0.5%, neither the internal cracks nor the local damage affected the fatigue life.

1331 低炭素鋼の極低サイクル疲労域における内部割れ増加機構(S21-4 疲労破壊機構,S21 金属材料の疲労特性と破壊機構)

1331 低炭素鋼の極低サイクル疲労域における内部割れ増加機構(S21-4 疲労破壊機構,S21 金属材料の疲労特性と破壊機構)

404 極低サイクル疲労特性に及ぼす内部パーライト割れの支配因子について

404 極低サイクル疲労特性に及ぼす内部パーライト割れの支配因子について

Extremely low cycle fatigue life prediction based on a new cumulative fatigue damage model

In order to predict an extremely low cycle fatigue (ELCF) life for annealed low carbon steel of S20C, a new cumulative fatigue damage model has been proposed. The model was based on the concept that the damage due to ductility exhaustion should be considered besides the damage generated by crack propagation in the ELCF regime where excessive strain was applied. It was found that the predicted fatigue life coincided well with the experimental results obtained by conventional fatigue tests and fatigue tests under mean strain conditions.

927 極低サイクル疲労特性に及ぼす内部パーライト割れの影響

927 極低サイクル疲労特性に及ぼす内部パーライト割れの影響

Fracture Mechanism of Ferritic Ductile Cast Iron in Extremely Low Cycle Fatigue.

Low cycle fatigue tests were carried out under push-pull loading conditions using Ferritic Ductile Cast Iron (FDI). In order to clarify the fatigue fracture mechanism of FDI in an Extremely Low Cycle Fatigue (ELCF) regime, an observation of the longitudinal sections of the partially fatigued specimens were performed. The fatigue process in the ELCF regime can be classified into three stages : (a) Generation of micro voids, caused by the debonding of the graphite-matrix interface. (b) Growth of the micro voids, perpendicular to the loading axis. (c) Formation of an internal macro crack, due to the coalescence of micro voids inside the material. Two types of microvoid coalescence processes have been observed. The first, due to the growth of internal micro-cracks and the second, due to internal necking of the matrix. The fatigue process of the FDI in the ELCF regime was accompained by the static fracture mechanism.

Transition of Fracture Mechanism of Ductile Cast Iron in Extremely Low Cycle Fatigue.

Low cycle fatigue tests were carried out over a wide range of plastic strain levels including an extremely low cycle fatigue (ELCF) regime on ferritic ductile cast iron under push-pull loading conditions. To evaluate the internally accumulated fatigue damage, the measumement of the density of the materials was also taken after a given number of strain cycles. Results show that ( 1 ) in the ELCF regime, the generation and the coalescence of microvoids, which originated from debonding of the graphite-matrix interface inside the material, lead to the final fracture of the specimen ; ( 2 ) in this case, the local density of the specimen, at the region where fatigue damage is concentrated, decreases gradually with strain cycling and its most drastic change occurs at the final stage of fatigue. This drastic decrease in density at that stage is caused by the rapid coalescence of microvoids inside the material.

Neutron irradiation effects on fatigue crack propagation in alloy 718 at 650°C: Smith, H. H. and Michel, D. J. J. Nucl. Mater. June 1987 149, (1), 109–112

This paper summarizes an extensive work of experimental and numerical studies of extremely low cycle fatigue (ELCF) on IN718 under room temperature. The ELCF is focused on low numbers of cyclic loading (fewer than 100 cycles) of fatigue failure. The IN718 cyclic plasticity behavior and the Bauschinger effect are studied and simulated using the well-known nonlinear kinematic hardening law by J.L. Chaboche and his co-workers under different strain amplitudes and different stress states. Moreover, the Vocé isotropic hardening law was used in combination with the Bai-Wierzbicki plasticity model. The Bai-Wierzbicki plasticity model was used to capture the effect of different stress states on ELCF based on the stress triaxiality and Lode angle parameters. On the other hand, the modified Mohr–Coulomb (MMC) ductile fracture model for monotonic loading was extended by a new damage evolution rule to cover the ELCF regime. A new parameter was introduced to represent the effect of the cyclic loading at ELCF. The new parameter is responsible to capture the change of non-proportional loading direction between the current stress and the backstress tensors. A comparison between the experimental data and the finite element simulation results (by Abaqus/Explicit) shows excellent correlations. Lastly, a fractographic examinations and fracture modes simulations are presented.