Characterisation of fatigue fracture surfaces of friction stir channelling specimens tested at different temperatures

The fatigue fracture surfaces of friction stir channelling specimens tested at room temperature, 120 °C and 200 °C were observed in a scanning electron microscope (SEM) in order to analyse their morphology and the crack propagation mechanisms. Three different friction stir channelling conditions were tested and analysed. For all specimens tested the developing fatigue-crack has always initiated at the advancing side, namely on the boundary between the nugget and the thermo mechanically affected zone (TMAZ) into the interior of the specimen. The crack has propagated through the channel nugget with a path tangential to the advancing side. After the crack has reached the processed surface, a second crack initiated at the channel bottom. The fracture surfaces have shown a semi-elliptical shape crack front. This second crack has propagated uniformly through the base material. Fatigue crack propagation on the TMAZ was mainly characterised by fatigue striations. It was found, on most of the surfaces observed, a clear coexistence of the intergranular fracture mode and the transgranular fracture mode. A relationship between the fatigue testing temperature and the roughness of the fracture surfaces was found. The fracture surfaces roughness was considerably lower at a testing temperature of 200 °C for the three friction stir channelling conditions analysed.     

Estimating the importance of cyclic thermal loads in thermo-mechanical fatigue

A method for evaluating the effect of cyclic thermal loading on crack tip stress fields is developed. In its development, advantage is taken of the periodic nature of fatigue loading and only harmonic loadings are evaluated. Formulating the problem in this way permits the extraction of time as an explicit variable and replaces its role with a dependence on the frequency of the thermal loading. The means for evaluating the effect of periodic loadings on crack tip stress fields is the stress intensity factor which is calculated from numerically defined stress and displacement fields using a path independent integral. Results obtained indicate that stress intensity factors of cracked components exposed to thermal fatigue conditions have a significant dependence on the frequency of the thermal cycle and the crack geometry. Numerical estimates for mode I thermal stress intensity have been obtained using thermal fatigue test data for a titanium alloy and can be as high as 25 percent of the critical mode I mechanical stress intensity.     

Annular specimens with wedge-shaped section for the investigation of thermal fatigue of materials

Institute of the Strength of Materials, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 2, pp. 60–64, February, 1989.     

A strain-energy-density-based lifetime prediction model for notched specimens: Polycarbonate under thermal fatigue

Polycarbonate is more and more extensively used in engineering because of its good mechanical properties. Pieces of polycarbonate are used in environments with variable temperature, especially in electronic devices. Thermal stresses could become important and, for this reason, the effects of thermal stresses must be taken into account in designing these pieces. We propose a method for the prediction of the life of notched specimens based on the density of dissipated strain energy. The laws of behavior of polycarbonate at various temperatures are determined, and the fatigue tests performed on smooth specimens give the laws of thermal fatigue of the material. The fatigue tests on notched specimens and finite-element-method computations enable us to establish the relationship between the stress concentration factor, the density of strain energy dissipated at the notch roots, and the density of nominal strain energy. A life-prediction model is proposed and discussed. Laboratory of Mechanical Reliability, Metz University, Metz, France. Published in Problemy Prochnosti, No. 6, pp. 32–42, November–December, 1998.     

Measurement of temperature fields in specimens of quartz ceramic during surface ablation

The authors propose a method of mounting thermocouples and have obtained temperature fields within specimens of pure and doped quartz ceramic. The linearity of the dependence for deep isotherms has been proved experimentally.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 2, pp. 313–318, August, 1989.     

Calculation of temperature fields in thermal drilling of glacial deposits

The method of moving heat sources [1, 2] is applied to the calculation of temperature fields in drilling glacial deposits with a hot disc-shaped drill.     

Calculation of temperature fields and thermal conductivity in structurized systems

We suggest a new numerical method to calculate temperature fields and thermal conductivity coefficients in structurized systems. It allows one to describe the processes of heat conduction in deformation and transformation of the structure of the system. Calculations were carried out for an organomineral system and for a system consisting of sand, water, and air.     

Analysis on the fatigue damage evolution of notched specimens with consideration of cyclic plasticity

This paper presents a damage mechanics method applied successfully to assess fatigue life of notched specimens with plastic deformation at the notch tip. A damage‐coupled elasto‐plastic constitutive model is employed in which nonlinear kinematic hardening is considered. The accumulated damage is described by a stress‐based damage model and a plastic strain‐based damage model, which depend on the cyclic stress and accumulated plastic strain, respectively. A three‐dimensional finite element implementation of these models is developed to predict the crack initiation life of notched specimens. Two cases, a notched plate under tension‐compression loadings and an SAE notched shaft under bending‐torsion loadings including non‐proportional loadings, are studied and the predicted results are compared with experimental data.     

Tensile and fatigue behavior of thin-walled cylindrical specimens under temperature gradient condition

In the present study, a temperature gradient system was designed with the aim of carrying out the tensile and fatigue test of thin-walled cylindrical alloyed steel (30CrMnSi). The tensile test under different temperatures was first carried out to obtain the static mechanical parameters. And then a three-dimensional (3D) finite element model was constructed to further study the deformation behavior under temperature gradient by the finite element analysis (FEA). The FE result was in good agreement with that of the experiment. Following this, the tensile fatigue test was performed under cooled air and no-cooled air to investigate the influence of the temperature gradient on fatigue life-time, respectively. The influence of cooled air under the lower nominal stress on fatigue life was not obvious than that of higher nominal stress. Finally, the scanning electron microscopy (SEM) was employed to investigate the fracture mechanism. The microstructure revealed that the fracture first occurred at the zone where there was a lower temperature.     

Relation between fatigue threshold and fatigue limit for surface-rolled specimens

After cylinder notch fatigue specimens of 40 CrNiMo steel were rolled, their fatigue limit increased by 41%. The rolled specimens did not fracture, even though they had been loaded for 10⁷ cycles under fatigue limit stress, but a non-propagating fatigue crack was generated. Thus the value of the fatigue limit depends on the fatigue threshold value ΔK_th of the metal of the rolled layer. Plastic deformation increased ΔK_th in these experiments. It can be inferred that ΔK_th of the rolled layer increases from the occurrence of plastic deformation and microvoids on the layer. Calculation of the effect of residual stress in the crack wake on the stress intensity factor ΔK indicates that residual compression stress decreases ΔK by 21.5 MPa √M. It was calculated that rolling induced both the length of the non-propagating crack and the increase of fatigue limit. The calculated values are in accord with experiment. Analysis and calculations indicate that the non-propagating crack is generated on the rolled layer. Thus the fatigue limit is improved because rolling produces residual compression stress in the layer (which decreases the stress intensity factor), and increases ΔK_th of the layer.     

Energy approach to fatigue under combined cyclic bending with torsion of smooth and notched specimens

We present the results of fatigue tests of smooth and notched round specimens made of 10HNAP steel under the combined action of cyclic bending and torsion. The experimental results are analyzed by using the well-known energy models proposed by Neuber and Molski-Glinka and a new model proposed by the authors and based on the analysis of the amplitude of the strain-energy density. The accumulation of fatigue damage in the stage of crack initiation was observed only in the active part of the cross section of the specimen where the level of stresses is higher than the fatigue limit. The proposed model enables one to determine the fatigue life of smooth and notched specimens under the combined action of cyclic bending and torsion by using standard characteristics of uniaxial fatigue and the relationship between cyclic stresses and strains. Department of Mechanics and Machine Design, Technical University of Opole, Opole, Poland. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 5, pp. 34–42, September–October, 1998.     

Rapid determination for fatigue parameters of AZ31B magnesium alloy based on evolution of temperature under high cyclic fatigue

Abstract

Based on the infrared thermography method, experiments are carried out to investigate the evolution of temperature field of the extruded AZ31B magnesium alloy specimens under high cyclic fatigue load. The experimental results show that the superficial temperature of specimen under cyclic fatigue load changes with the number of cycles. According to the characteristics of surface temperature change, we propose a formula to calculate the residual fatigue life using energy approach. The proposed formula to assess the fatigue parameters (fatigue limit, residual fatigue life, fatigue life and S–N curve) achieves good results for AZ31B magnesium alloy. Furthermore, the fatigue limits (Δσ_eSN?=?90·3 MPa) derived from the traditional method through 10⁷ cycles were compared with the values predicted by the infrared thermographic method (Δσ_eTM?=?87·3 MPa) and the energy approach (Δσ_eΦ?=?86·2 MPa), and the comparison results of percentage differences are 3·3 and 4·5% respectively.