Decrease Of Stress Amplitude Research Articles

Effect of electromagnetic force installation method on critically reinforced interference sizes and fatigue behavior of CFRP bolted joints

AbstractThe electromagnetic force dynamic installation (DI) method of interference fit fasteners has significant advantages in improving the quality of the assembly interface of composite structure. This study aims to investigate how improving assembly interface quality may help in achieving high‐precision interference‐fit joints in composites. Single‐lap joint specimens were prepared using electromagnetic installation technique, the fatigue life and failure behaviors of the joints under different bolt‐hole clearances were measured and analyzed experimentally. In addition, a three‐dimensional finite element model was developed to predict the radial pre‐compression stresses around the hole and the progressive failure behavior of various typical damage modes in carbon fiber‐reinforced polymer induced by the interference. The results show that the critical interference range for the DI‐composite bolted structure is 1.0%–1.2%. The fatigue life of the best interference (1.0%) and the worst interference (0%, neat fit) have a difference of 36 times in amplitude. The residual stress with uniform distribution and high amplitude reduces the stress concentration of the hole wall and improves the failure evolution of the loaded hole. The contribution of this study provides vital guidance for improving the fatigue performance of CFRP bolted structures using a novel DI method.Highlights I = 1.0% is the critical damage‐enhancing interference size for dynamically installation interference fit bolts with electromagnetic forces. The residual stress amplitude introduced by the interference does not increase uniformly as the interference size increases. Excessive interference significantly reduces the uniformity and magnitude of residual stresses. Under subsequent cyclic loading, the fifth layer near the inlet and outlet of the extruded lower laminate appears to be the critical ply of extrusion deformation, and severe damage is concentrated in the critical ply of deformation. The decrease of stress amplitude and the variation of mean stress are the mechanisms of fatigue life enhancement in interference fit joints under external cyclic loading.

Numerical investigation to the effect of wear on fretting fatigue life using CDM and fracture mechanics

Fretting fatigue is a common type of problem in the engineering fields. Due to its multiaxial characteristics, it leads to a shorter overall fatigue life compared to plain fatigue. Fretting fatigue is caused by the micro slip at the contact surface, which is always accompanied by interface wear. This paper presents a refined elastic numerical simulation method combining interfacial wear with continuum damage mechanics (CDM) and fracture mechanics to predict fatigue life. The crack initiation life is determined by damage evolution law and the initiation position is determined by the total dissipated energy. Crack propagation life and path can be predicted using stress intensity factor. The stress redistribution and geometric change of contact surface caused by wear are considered in the whole fatigue process. Results indicate that the fretting fatigue simulation method incorporating wear has higher accuracy compared with the experimental results in the literature. It is noteworthy that wear retards nucleation damage process and crack propagation. As stress amplitude decreases, fatigue life increases, and delay effect of wear becomes more obvious. Additionally, wear causes the crack initiation position to move out, but crack initiation region is still near the slipping zone and propagation path does not change.

The Effect of Defect Characteristics on Prediction of Fatigue Life of TC4 Titanium Alloy Welded Joints

The size and depth of defects significantly affect fatigue performance; the main purpose of this article is to clarify the effect of stress correction factors caused by defects on the fatigue life of α + β Type titanium alloy welded joints. In order to investigate the fatigue characteristics of α + β Type titanium alloy welded joints, axial constant amplitude loading fatigue tests were performed with a stress ratio of −1. The test results show that the fatigue life continues to increase as the stress amplitude decreases, and the failure modes can be classified into two types: interior failure and surface failure. A fatigue parameter (λ) determined by both defect depth and size was proposed which allowed for a good generalization of the data point distribution in short and long-life regions. The stress correction factor (W) related to the S–N characteristics was constructed by combining the λ and average defect size, and it effectively improved the dispersion of the test data. Continuing, the fatigue life prediction model was established under the condition that the defect type and size can be estimated or detected on the basis of the dislocation energy method. The results indicated that the evaluated values of the new life model associated with W are in good agreement with the test results.

Fatigue Resistance of Fillet Welds of Traction Rod Brackets on a Locomotive Bogie Based on International Union of Railways Standards and Improvement Measures Adopted

To solve the problem of fatigue failure in fillet welds of traction rod brackets on locomotive bogies of a given model, the cause for failure and the improvement method were studied. The results show that when there is maximum clearance at weld roots, maximum incomplete fusion of sidewalls, maximum incomplete fusion at weld roots, and maximum pores allowable in the ISO 5817 standard, the stress amplitude separately increases by 70~97%, 53~55%, 40~46%, and 19~34%. Despite this, when various types of defects of the maximum size are present in the weld alone, the static and fatigue strengths of fillet welds with a throat depth of 6 mm on the traction rod bracket can still meet the requirements in the UIC615-4 standard. In practical fillet welds, defects including clearance at weld roots, incomplete fusion, and pores are very likely to occur at the same time, which may induce fatigue failure in fillet welds of traction rod brackets within the original design life. According to the size of the frame and the traction rod brackets, a strengthening scheme for increasing the throat depth of fillet welds of traction rod brackets to 8 mm was designed. Calculation results of the strengthening scheme show that for new structures subjected to overall post-weld stress-relief thermal treatments, the maximum stress amplitude decreases by 5~29% when increasing the throat depth of fillet welds from 6 to 8 mm. For structures in service with the throat depth of fillet welds increased from 6 to 8 mm through repair welding, peak residual stress at the weld root after repair welding can reach 383 MPa. Because overall stress-relief thermal treatments cannot be performed on repair-welded structures, the fatigue strength of repair-welded fillet welds cannot meet the requirements of UIC615-4; therefore, local stress-relief treatments have to be performed in the welded zone. The results are of guiding significance for the treatment of locomotives in service and performance improvement of new locomotives and suggest that the current standard is relatively conservative.

A new model for stress analysis in pin–Plate interference fit joints under tension loading

The decrease of stress amplitude is generally believed as the mechanism of fatigue life enhancement in interference fit joints under cyclic loading for each load cycle. In order to exactly estimate it, a new analytical model was presented. In this model, influences of mass, geometry, and plastic deformation are considered to modify and improve the spring model, and, moreover, the stress amplitude at the smallest crossing section is investigated. Taking these factors into account, stress amplitude decrease of various interference values can be calculated more accurately; therefore, the analytical solutions have good agreement with the finite element numerical and experimental results under different interference values and load levels. The analytical model indicates that the decrease ratio of stress amplitude is of a certain value for a specific interference value with the external load under the critical value. Furthermore, the decrease ratio of stress amplitude mainly depends on the stiffness of the pin and plate, which is mostly affected by the geometric shapes and plastic deformation ranges of the pin and plate. In addition, raising the interference value leads to larger stress amplitude reduction since the plastic deformation areas expand with the increment of interference values.

The influence of nanosized precipitates on Portevin-Le Chatelier bands and surface roughness in AlMgScZr alloy

The influence of different precipitate-dislocation interactions, namely dislocation shearing and bypassing mechanisms, on PLC bands and the resultant surface roughness in AlMgScZr alloy was investigated. Three-dimensional surface roughness was quantitatively measured by confocal microscopy. We find that the introduction of shearable precipitates increases the stress amplitude, decreases the PLC bands number and surface roughness. However, the stress amplitude decreases, the PLC bands number and surface roughness increase with shearable precipitates turning to nonshearable precipitates. By analyzing the precipitation strengthening mechanisms quantitatively, the influence of precipitates on PLC bands and the resultant surface roughness was explored. Furthermore, our study demonstrates that the shearable precipitates can decrease the surface roughness by decreasing the number of PLC bands, which is instructive for designing structural materials with desirable mechanical property and surface quality.

Experimental study on acoustic emission frequency characteristics of granite under different cyclic loading and unloading

In Granite acoustic emission tests were performed during three different cyclic loading and unloading processes. The relationship between the acoustic emission frequency distribution pattern and the loading and unloading mode of granite during the cyclic loading and unloading process was discussed. The distribution pattern of the peak frequency with the stress evolution is analyzed qualitatively and quantitatively. Studies have shown that: in the first cycle of loading and unloading at each stage of the three cyclic loading and unloading processes, the acoustic emission ringing count and peak frequency are significantly increased, which has a significant Kaiser effect. It proves that the Kaiser effect essentially remembers the extent of the damage inside the rock previously. In the process of loading and unloading in the fourth stage cycle, the AE ring count showed a “U”-shaped change law, showing a significant Felicity effect. Comparing the three different cyclic loading and unloading modes, with the increase of the lower limit stress, the stress amplitude decreases, and the AE ringing count corresponding to the lower limit stress value of the unloading gradually decreases, and the peak frequency distribution shows more dense or concentrated characteristics. At the beginning of the third stage cycle, the proportion of high frequency band (140∼162kHz) gradually decreased, and the proportion of lower frequency band (11-18kHz) gradually increased. With the increase of the lower limit of cyclic loading and unloading, the lower frequency band (11-18kHz) in the fourth-stage cyclic loading and unloading process takes up a higher proportion, even as high as 95%. It indicates that the rock samples at this stage mainly undergo large-scale failure, and the rock samples are in a critical failure state. These provide the basis for exploring the evolution mechanism of rock mechanical behavior and the prediction method of instability and damage.

Evaluation of the low-cycle fatigue strength of Sn3.0Ag0.5Cu solder at 313 and 353 K using a small specimen

Abstract This study performs fatigue tests by using a small specimen of Sn3.0Ag0.5Cu solder at 313 and 353 K. The specimen has a 3 mm-diameter gage section and a gage length and total length of 6 and 55 mm The specimen is heated in an electric furnace to control the temperature within ±1 K from the target temperature. The strain ranges employed are 0.5 %, 0.7 %, 1.0 %, and 1.5 %. A triangular waveform with 0.5 % × s−1 strain rate is adopted. The number of cycles to failure (failure life) Nf is defined as the number of cycles that causes a 25 % decrease of stress amplitude from that at the middle of the lifespan or at specimen breakage. The relationship between the strain range and failure life at 313 and 353 K coincides well, and no variation of the failure life with the test temperature is identified. Stress amplitudes decrease with increasing cycles and decrease faster for higher strain ranges because of higher crack propagation rates. The degree of relaxation in the stress amplitude is larger at 353 K than at 313 K. Crack observation of the fatigued specimens is performed to discuss the relationship between crack propagation and the failure life of solders. The relationship among slips, microcracks, and main cracks is discussed, and the mechanisms for fatigue failure are proposed. Almost no change in the crack morphology is observed at 313 and 353 K, and this result is related to the lack of changes in the failure mechanism because of the change in the test temperature.

Experimental and theoretical studies on thermo‐mechanical fatigue test for aluminium cast alloy

AbstractEvaluation of the thermo‐mechanical behaviour and prediction of the service life of cast aluminium alloys are important for the design of automobile engine cylinder heads. In this study, cast Al alloy specimens are extracted from cylinder heads and subjected to in‐phase thermo‐mechanical cyclic loading. The hysteresis curves related to stress and strain were recorded under the individual thermo‐mechanical loading conditions. The number cycles to failure corresponding to multiple mechanical strain and temperature ranges were obtained. It is found that the cyclic stress amplitude decreases and the cyclic softening rate increases with increasing maximum temperature rise. A modified fatigue‐creep model based on energy conservation has been developed for prediction of the fatigue life of cylinder heads. The proposed method shows good agreement with the well‐established Ostergren model and low standard deviations. In summary, the proposed method described in this study provides an option for prediction of the thermo‐mechanical behaviour of metals.

Optimization and experimental research on process parameters of the softness consolidation abrasive particles

SCA (softness consolidation abrasive) finishing is a new type of machining method. The dilatancy effect of abrasive particles during processing causes the local accumulation of abrasives particles and leads to scratches on the surface of the workpiece, which in the meantime decreases finishing efficiency. So, microscopic contact mechanic model of abrasive particles is established to analyze the force chain transfer phenomenon. A microscope constitutive model of abrasive particles is established to reflect the microscopic force-displacement relationship between abrasive particles. The force chains net evolution of the abrasive particles with different porosity and the surface stress distribution of workpieces are simulated by PFC3D.The results show that when the porosity exceeds 44%, the internal force chain transmission path of the abrasive particles disappears; the surface stress of workpieces exhibits a periodic distribution. As the porosity increases, the stress amplitude decreases. Finally, the test platform is set up and the laws of porosity and abrasive’s size on surface roughness of workpieces can be concluded after a series of polishing experiments by pneumatic wheels with softness consolidation abrasive. The experiments results show that when the abrasive’s size is 800 and the porosity is 24%, the surface roughness of workpieces is small, and the surface average roughness of workpieces can decrease from 313.744 to 67.11 nm.

Effect of Phase Angle on Fatigue Failure of 30CrMnSiA Steel under Tension-torsion Fatigue Loading

Tension-torsion high-cycle fatigue experiments with solid cylindrical bar specimens of 30CrMnSiA steel were carried out under different phase angles. And the experiment data, characteristics of stresses with plane orientation and fracture appearance under different loading conditions were analyzed and investigated. Experiment results show that fatigue life increases gradually with the increase of phase angle. The fatigue life is shortest for 0° phase angle, while the fatigue life is longest for 90° out-of-phase angle. It could be found from analysis of stress parameters that as the phase angle increases, the shear stress amplitude decreases and the maximum normal stress increases gradually, which are on the maximum shear stress amplitude plane close to the axial of specimen. Macro-analysis of the fracture appearances show that when the phase angle increases, the angel between crack initiation plane and specimen axial decreases, and cracks are inclined to initial and propagate on or near the maximum shear stress amplitude plane with larger maximum normal stress under proportional loading as well as non-proportional loading.

Interior crack initiation and growth behaviors and life prediction of a carburized gear steel under high cycle fatigue and very high cycle fatigue

Abstract

Effects of a Single Open Joint on Energy Transmission Coefficients of Stress Waves with Different Waveforms

Open joints have significantly different effects on stress wave propagation across them as compared to closed and filled joints. In the present study, a theoretical model is developed based on the analysis of interaction process between stress waves and a single open joint. The analytical solutions to the energy transmission coefficient are mathematically derived for stress waves across an open joint. Parametric studies are conducted to evaluate the effects of various parameters on the energy transmission coefficient. It is found that the energy transmission coefficient follows a similar trend for all types of stress waves, but stress waves with different waveforms have different values. The energy transmission coefficient increases with the increase in the wave amplitude and duration, but decreases with the gap width. It also increases at the outset, and then decreases gradually with the increase in the incident angle for rectangle, symmetric triangle and ascending triangle waves, but it decreases with the incident angle for sine and descending triangle waves. Furthermore, different-shaped stress waves have different critical gap widths for wave transmission. The optimal incident angle and the critical gap width increase as the wave duration increases, but the critical stress amplitude decreases.

Fatigue behavior and damage characteristic of ultra-fine grain low-purity copper processed by equal-channel angular pressing (ECAP)

The S–N and Coffin–Manson plot, cyclic stress–strain response, changes of microstructure, and the surface morphology of ultra-fine grain (UFG) low-purity copper processed by ECAP were tested and observed in present study. And the formation mechanism of shear bands was discussed in detail. The results show that the UFG Cu represents longer lifetime under stress-controlled fatigue, but lower fatigue resistance under strain-controlled fatigue when compared with the coarse grain counterpart. Cyclic stress–strain responses of UFG Cu under stress-controlled fatigue alter from cyclic softening to cyclic hardening as stress amplitude decreases. But the responses always show cyclic softening under strain-controlled fatigue in present testing. By electron back scattering diffraction and transmission electron microscope technique, the shear bands were discovered on the surface of all cycled samples and no grain coarsening was discovered near the shear bands, which indicated that there was no inevitable relationship between formation of SBs and cyclic softening/grain coarsening. The discovery should be related to impurities in copper. The oriented distribution of defects along the shear plane in the last ECAP processing is one of the major mechanisms of SBs formation.

岩石の圧縮疲労破壊についてのＣｏｓｔｉｎモデルによる考察

In order to investigate the length of cracks developed by stress corrosion and cyclic fatigue and the time to failure under cyclic loading in compression, the equations proposed by Costin et al. is estimated from the results of creep and fatigue tests for OGINO tuff, KIMACHI sandstone and AKIYOSHI marble. Under the fatigue tests at cyclic frequency of 1 Hz, the ratio of the length of crack due to stress corrosion to the total one increases with increasing upper stress ratio. The length of crack due to cyclic fatigue is proportional to the exponent of cyclic frequency of stress, and the time to failure is affected by it. Also, in case that the length of crack due to stress corrosion occupies the most part of the total crack length, the increase in the time to failure is smaller with decreasing frequency. The time up to failure is longer as the stress amplitude decreases, but below a given ratio of stress amplitude, it is smaller with decreasing the ratio of stress amplitude. Also, the ratio of stress amplitude, at which the time to failure takes a maximum value, and the length of crack due to cyclic fatigue are smaller as the ration of stress amplitude decreases.

Influence of electroless Ni-P deposits on the corrosion-fatigue properties of an AISI 1045 steel

Electroless nickel (EN) deposits containing approximately 10% phosphorous and about 20 μm thickness are shown to increase the corrosion-fatigue properties of the AISI 1045 steel in the stress amplitude range of 221–331 MPa, in the presence of an aqueous solution of 3% NaCl. Such an increase is enhanced if the coating is in the as-deposited condition rather than post-heat treated at 473 K for 4 h, despite the fact that such a treatment produces an increase in hardness. The improvement observed in corrosion-fatigue properties is also more pronounced as the stress amplitude decreases. The monotonic mechanical properties determined under tensile conditions are not affected by the presence of the coating. The inadequate agitation of the bath has given rise to microporosities and `orange-peel' type effects associated with the deposit which are believed to be related to the trapping of hydrogen bubbles.

封圧下における稲田花こう岩の疲労特性

Cyclic loading tests on Inada Granite were carried out under 0, 5 and 25 MPa of confining pressure to examine the next two effects: the effect of confining pressure on the characteristics of strength and deformation; and the effect of stress amplitude on fatigue life. The main results are as follows.1) The fatigue strength rises as the confining pressure is increased. However, the rate of the increase of the fatigue life Nf to the decrease of the maximum differential stress σDmax is almost identical despite of the different level of confining pressure. A same curve can represent the relationship between σDcσDmax (difference between static strength σDc and σDmax) and log Nf under the two levels of confining pressure (5, 25 MPa). That is, this curve can be used to predict the fatigue strength from the static strength.2) Three stages, namely. transient, steady and tertiary stage, are observed in fatigue deformation. Under the 25 MPa of confining pressure, the transient stage is shorter and the steady stage is relatively longer. As the confining pressure is increased, dilatant strain is restrained and the rate of the increase of volumatric strain per one cycle decreases.3) When the maximum differential stress is fixed at a constant value, the fatigue life increases as the stress amplitude decreases. This tendency becomes more evident as the value of the maximum differential stress becomes lower.4) When the maximum differential stress in fatigue tests and creep tests are the same, time to failure in fatigue is shorter than that in creep in the range of the maximum differential stress lower than a certain point.