Predicting damage and failure under low cycle fatigue in a 9Cr steel

Experimental data have been generated and finite element models developed to examine the low cycle fatigue (LCF) life of a 9Cr (FB2) steel. A novel approach, employing a local ductile damage initiation and failure model, using the hysteresis total stress–strain energy concept combined with element removal, has been employed to predict the failure in the experimental tests. The 9Cr steel was found to exhibit both cyclic softening and nonlinear kinematic hardening behaviour. The finite element analysis of the material's cyclic loading was based on a nonlinear kinematic hardening criterion using the Chaboche constitutive equations. The models’ parameters were calibrated using the experimental test data available. The cyclic softening model in conjunction with the progressive damage evolution model successfully predicted the deformation behaviour and failure times of the experimental tests for the 9Cr steels performed.     

Multiaxial fatigue damage parameter and life prediction for medium-carbon steel based on the critical plane approach

The tension–torsion fatigue characteristics were investigated under proportional and non-proportional loading in this paper. The fatigue cracks on the surface of multiaxial fatigue specimens were observed and analyzed by a scan electron microscope. On the basis of the investigation on the Kindil–Brown–Miller and Fatemi–Socie’s critical plane approaches, a shear strain based multiaxial fatigue damage parameter was proposed by von Mises criterion based on combining the maximum shear strain and the normal strain excursion between adjacent turning points of the maximum shear strain on the critical plane. The proposed multiaxial fatigue damage parameter does not include the weight constants. According to the proposed multiaxial fatigue damage parameter, the multiaxial fatigue life prediction model was established with the Coffin–Manson equation, which is used to predict the multiaxial fatigue life of medium-carbon steel. The results showed that the proposed multiaxial fatigue damage parameter could be used under either multiaxial proportional or non-proportional loading.     

In-body corrosion fatigue failure of a stainless steel orthopaedic implant with a rare collection of different damage mechanisms

A stainless steel orthopaedic (femoral) implant was examined, which had fractured inside a patient’s thigh. It had been in his leg for almost two years, with no problem in the bone reconstruction process. The plate had apparently fractured during the first few months, when the bone had not been reconstructed completely. Different investigations such as visual assessments, hardness testing, stereoscopy, metallography, quantometry, SEM fractography and EDS microanalysis were performed in order to analyze the failure mechanism and its cause(s). Different damages were observed such as crevice corrosion pitting, initiation of cracks from these pits, intergranular surface cracking inside the crevice, and also SCC-like branched cracks. But, the main failure mechanism was determined to be corrosion fatigue assisted by crevice corrosion.     

The choice of cyclic plasticity models in fatigue life assessment of 304 and 1045 steel alloys based on the critical plane-energy fatigue damage approach

The present study intends to examine various cyclic plasticity models in fatigue assessment of 304 and 1045 steels based on the critical plane-energy damage approach developed earlier. Cyclic plasticity models of linear hardening, nonlinear, multi-surface, and two-surface were chosen to study fatigue damage and life of materials under proportional and non-proportional loading conditions. The effect of additional hardening induced due to non-proportional loading in 1045 steel and particularly in 304 steel was further evaluated as different constitutive models were employed. In the present study, the plasticity models were calibrated by the equivalent cyclic stress–strain curves. The merits of the models were then investigated to assess materials deformation under proportional and non-proportional loading conditions. Under non-proportional loading, the cyclic plasticity models were found to be highly dependent upon the employed hardening rule as well as the materials properties/coefficients.The stress and strain components calculated through constitutive laws were then used as input parameters to evaluate fatigue damage and assess the fatigue life of materials based on the critical plane-energy approach.The calculated values of stress components based on constitutive laws resulted in a good agreement with those of experimentally obtained under various loading paths of proportional and non-proportional conditions in 1045 steels. In 304 steel, the calculated stress components were however found in good agreement when plasticity models were employed for proportional loading conditions. Under non-proportional loading, the application of the multi-surface plasticity model in conjunction with the fatigue damage approach resulted in more reasonable results as compared with other plasticity models. This can be attributed to the motion of the yield surface in deviatoric stress space in the multi-surface model encountering additional hardening effect through estimated higher stress values under non-proportional loading conditions.Predicted fatigue lives based on the critical plane-energy damage approach showed such range of agreements as ±1.05–±3.0 factors in 1045 and 304 steels as compared with experimental life data when various constitutive plasticity models were employed.     

Mean stress effects on fatigue crack growth and failure in a rail steel

Over a limited range, the effect of mean stress has been studied on fatigue crack propagation and on the critical fatigue crack size associated with sudden fast fracture in centre-notched plate specimens of a rail steel under pulsating loading. The results have been presented in terms of the stress intensity factor range $\text{ΔK}$ and the ratio $\text{R}$ of the minimum to maximum stress. Increasing $\text{R}$ was found to both accelerate cracking and reduce the critical crack size at instability. The data have been correlated with three crack growth equations currently used in the literature and it was found that the equation of Forman et al. relating crack growth rate to $\text{ΔK}$ and $\text{R}$ gave the best fit. This equation was used to predict life in the finite range of the S-N curve. Fractographic examination revealed that the fracture surfaces were complex and a number of fracture modes contributed to cracking.     

Microstructural effect on tensile and fatigue behaviour of C–Mn steel

Stress–strain behaviour in tension and in torsion was studied in ferrite–pearlite and ferrite–bainite microstructures of C–Mn steel. The fatigue tests were performed under reverse torsional loading on hour-glass shape specimens. The cyclic plasticity in torsion was found at about 37% of the monotonic yield stress in both ferrite–pearlite and ferrite–bainite microstructures. The ferrite phase in the direction of maximum shear stress was the preferable site for crack nucleation. The ferrite–bainite microstructure showed better fatigue properties than the ferrite–pearlite microstructure.     

Low cycle fatigue behavior and failure mechanism of a dual-phase steel

Low cycle fatigue behavior and failure mechanism of a dual-phase steel was investigated by using LCF tests, interrupted LCF tests, SEM, TEM and XRD peak broadening. LCF tests were performed at constant strain amplitudes of ±0.002, ±0.004, ±0.006 and ±0.1. Microscopic investigations were carried out on gauge surfaces of the interrupted LCF specimens at various stages of their fatigue life. It was observed that at high strain amplitudes damage was started at fractured martensite particles and passed through areas with high density of martensite. At low strain amplitudes damage was started at separated ferrite/martensite interface and passed through areas with low density of martensite. All specimens showed fatigue hardening during LCF tests. It was also observed that the rate of hardening was affected by strain amplitude. The results show that XRD peak broadening is sensitive to the strain amplitude and the stage of damage in the specimens.     

A simplified approach to predict the failure of steel members under interaction effect of fracture and fatigue

This paper presents a simple approach to predict the failure of steel structures under the interaction effect of fracture and fatigue which is termed as ultra-low cycle fatigue (ULCF) failure. The approach mainly consists of simple fracture criterion, which is obtained from further simplification of available cyclic void growth model (CVGM). As for alternative component of ULCF analysis, a simple non-linear hardening model is proposed to employ in elasto-plastic analysis for situations where parameters of available mix hardening models (combined isotropic and kinematic hardening models) are difficult to determine. Initially the simplified ULCF fracture criterion is clearly presented and associated ULCF fracture prediction methodology is also indicated. Then the elasto-plastic hardening model is proposed and model behavior is compared with experimental behavior of few materials. The procedure to determine hardening constants and finite element (FE) formulation are also clearly mentioned. The simplified approach is then employed to determine ULCF fracture of some structural models. Hence verification of the simplified approach is confirmed by comparing the results with previous approaches-based estimations. Finally, study tends to conclude that the simplified approach produces reasonable accurate prediction to ULCF fracture of steel structures where magnitude of triaxiality remains relatively constant. Further, it was able to prove that commonly available elasto-plastic FEM packages can be reasonably employed to estimate ULCF fracture of steel structure when simplified ULCF fracture criterion governs the failure.     

Influence of surface defects and corrosive medium on the fatigue resistance of ST17G1S steel

We study the influence of surface defects and corrosive medium on the characteristics of fatigue resistance of specimens of ST17G1S steel extensively used for the production of pipes in the oil-and-gas industry. The accumulated results enable us to estimate the degree of decrease in the limited fatigue strength for specimens of ST17G1S steel in the presence of surface defects and under the action of corrosive medium. __________ Translated from Problemy Prochnosti, No. 4, pp. 99–107, July–August, 2007.     

A two-parameter damage criterion and high-temperature fatigue crack growth in a corrosion-resistant steel

Summary The paper is designed to develop the two-parameter failure criterion for describing the growth of fatigue cracks under conditions of high-temperature fatigue. The criterion can be used with laboratory tests on specimens to construct kinetic diagrams for fatigue failure such as would be given if the requirements of linear failure mechanics were met. The initial data are the v-K_eff fatigue failure diagrams, which are effective ones incorporating crack closure, so one can derive v-K_v theoretical diagrams as conservative characteristics for a material for crack growth under conditions of planar strain.The two-parameter criterion enables one to incorporate the effects of frequency and cycle asymmetry at high K, where there are changes in crack growth rate due to changes in the rate of plastic strain at the crack vertex. At low K, the effects of loading frequency are additionally determined by the crack closure, the blunting, and the physicochemical action of the medium, which restricts the scope for using the two-parameter criterion to construct fatigue-failure kinetic diagrams invariant with respect to frequency.Translated from Fiziko-khimicheskaya Mekhanika Materialov., Vol. 26, No. 5, pp. 9–19, September–October, 1990.     

Metallurgical investigation on fatigue failure of stainless steel chain in a continuous casting machine

Stainless steels strips (chains) are used for the connection of dam blocks in belt casting machines. Thermal cycling and repetitive stressing under complex loading conditions due to tension and bending are the most frequent function modes during production. Samples from fractured stainless steel strips used for the connection of dam blocks in a copper rod continuous casting line, were sent for failure investigation. Optical and scanning electron microscopy for structural and fractographic evaluation along with mechanical testing are used as the principal analytical techniques in the context of the present investigation. Failure analysis findings suggest strongly that the failure was caused by bending fatigue which assisted also by thermal cycling, initiated from the strip surface and followed by ductile final overload fracture. Final fracture occurred via ductile failure, when the remaining strip cross sectional area reaches a critical size, becoming unable to sustain the operating load. Review of the service history (operating conditions, e.g. process design, applied loads, thermal cycles), in combination to the examination of a potential substitution of the material to a more heat (and fatigue) resistant one are suggested as further fatigue damage preventive actions.     

Cumulative fatigue damage of stress below the fatigue limit in weldment steel under block loading

To investigate the cumulative fatigue damage below the fatigue limit of multipass weldment martensitic stainless steel, and to clarify the effect of cycle ratios and high‐stress level in the statement, fatigue tests were conducted under constant and combined high‐ and low‐stress amplitude relative to stress above and below the fatigue limit. The outcomes indicate that neither modified Miner's nor Haibach's approach provided accurate evaluation under repeated two‐step amplitude loading. Moreover, effect of cycle ratios has been determined. Additionally, the cumulative fatigue damage saturated model is established and validated. Cumulative fatigue damage contributed by low‐stress below the fatigue limit in high stress of 700 MPa is higher than that with 650 MPa at identical conditions (fatigue limit 575 MPa). Thus, high stress affects fatigue damage behaviour below the fatigue limit. A new predicted approach has been proposed based on Corten‐Dolan law, whose accuracy and applicability have been proven.     

A study of the low-cycle fatigue failure of a galvanised steel lighting column

This paper presents the results of a low-cycle fatigue test on a galvanised steel lighting column. The aim of the test was to simulate the behaviour of the column undergoing large amplitude resonant vibration caused by wind. A metallurgical study of the failure revealed the significant role of the galvanised coating in the failure process. Results from a detailed 3D finite element model are also used to explain the failure mechanism.The swage joint in the column was confirmed as a failure location by both experiment and finite element analysis. This in itself is not surprising and the position of the fatigue failure is consistent with those observed in the field. Of more importance is the fact that the experiment shows that galvanising can lead to premature failure of such columns. This is a highly significant conclusion as it implies that improving the weld detail in an effort to improve fatigue life may be ineffective for lighting columns coated in this manner.Given the detrimental effect of galvanising on fatigue performance and the fact that the most severe corrosion will be on the outside of columns, then the fatigue life of such structures may benefit if the inner surface was not galvanised in high stress regions. An alternative improvement would be the use of a galvanising coating with higher toughness and less susceptibility to cracking and damage.Attention is drawn to the need for a better understanding of the fatigue performance of galvanised steel columns resulting from large amplitude wind induced resonant vibration. The approach adopted so far for lighting column resonant vibration, has been to try and avoid it. While this is a laudable objective, clearly this has not always been possible, as designs push the limits permitted by Codes of Practice.     

Variation in the physical properties during fatigue and variation in the fatigue strength of boronized St. 20 steel

Changes in both the magnetic permeability and the electrical resistance were used to study the kinetics of fatigue fracture of boronized low-carbon steel.