Stress Amplitude Level Research Articles

The Gradient Effect on Cyclic Behavior of 316L Stainless Steel in the Ultrasonic Bending Test.

Nanoindentation measurements were conducted to investigate the high-cycle response of 316L stainless steel in bending fatigue. Hardness variation owing to the gradient flexure stress amplitude for different curvatures was plotted along with the thickness and length, respectively. Scanning electron microscopy (SEM) was subsequently conducted to explore the deformation characteristics in multiple layers, which had cyclic gradient stress, on the cross-section of specimens. The nanoindentation results indicated that the cyclic hardening response of 316L stainless steel is correlated with the level of stress amplitude in the high-cycle fatigue (HCF) regime. Furthermore, an analytical model was proposed to clarify the relationship between nanohardness and stress amplitude. Finally, the evolution of damage accumulation due to irreversible plastic deformation is continuous during stress reduction up to the neighboring zone at the neutral surface of the flexure beam in some individual grains.

Features of the experimental evaluation of the fatigue resistance characteristics of power lugs

The features of the fatigue strength of the lugs in different zones of the cycle asymmetry coefficient are considered. It is shown that the most severe conditions of variable loading of the lugs correspond to a cycle asymmetry coefficient from 0.1 to 0.2. Formulas have been obtained to clarify the statistical assessment of the durability of symmetrical lug samples when, during their fatigue testing, destruction occurs on only one side. Recommendations are given for the selection of accelerated test modes with forced loading and for comparative tests at the same stress amplitude level.

The effect of heat treatment on fatigue strength of additively manufactured AlSi10Mg

This research aims to analyze how varying heat treatment conditions affect the fatigue life performance of additively manufactured specimens from AlSi10Mg. The study focuses on the transition from High Cycle Fatigue (HCF) to the Fatigue Limit (FL) domain. To eliminate inconsistencies, four sets of hourglass specimens were created through Laser Power Bed Fusion (L-PBF) technology within one build cycle. All fatigue specimens were of the same geometry and with as-built surface, but four different heat treatments were applied. Fatigue testing was conducted in the uniaxial tension domain using a resonant pulsator with the load ratio of 0.1. To further characterize the different heat treatments of the specimens, a thermal camera was utilized to measure the temperature response during cyclic loading. The observed increase of temperature, referred to as the Self-Heating effect (S-H), is directly related to heat dissipation during cyclic loading. A test with blocks of subsequently increasing load amplitude was conducted to establish a stabilized temperature response in each block for further analysis. The pairs of the stress amplitude levels, and stabilized temperatures can be used as a characteristics of heat dissipation during cyclic loading related to different heat treatments.

Fatigue tests and design of Q460 CHS-CHS T-joints under axial loading and in-plane bending

Previous research has primarily focused on stress concentration factors (SCFs) and fatigue life of tubular joints made from low-strength steel (nominal yield strength of 235–360 MPa). The fatigue behavior of high-strength tubular joints remains insufficiently studied. In this study, a total of sixteen CHS-CHS T-joints fabricated from Q460 steel were designed, eight of which were subjected to axial load and eight of which were under in-plane bending. Static and fatigue tests were conducted to analyze the distribution of hot spot stress, fatigue crack propagation, and fatigue failure modes. The experimental results showed that the crack initiation consistently occurred at location of maximum hot spot stress for all specimens. The maximum SCFs were concentrated at the saddle (90°) for specimens under axial loads and around the position of 22.5° for those under in-plane bending. The modified SCF design formula for in-plane bending was proposed based on regression analysis. Prior to crack initiation, all specimens exhibited a relatively slow stiffness degradation due to cumulative damage from fatigue loading. Through least squares analysis of the test data, the fatigue design Srhs-N curve for Q460 CHS-CHS T-joints was developed. The proposed Srhs-N curve is capable of accurately predicting the fatigue life of Q460 CHS-CHS T-joint subject to axial loads and in-plane bending. Comparison with previous research and existing design guidelines shows that under the same stress amplitude level, the fatigue lives of CHS-CHS T-joints subjected to axial loading of Q460 steel are comparable to that of low-strength steel (WFG36Z). Under in-plane bending, the fatigue lives for Q460 steel are higher than that for low-strength steels (C350L0, t < 4 mm).

Research on the Fatigue Crack Growth Behavior of a Zr/Ti/Steel Composite Plate with a Crack Normal to the Interface.

The current work reveals the influence of loading parameters on the crack growth behavior of a Zr/Ti/steel composite plate with a crack normal to the interface by using an experiment and the finite element method. The Chaboche model was first used to study cyclic plastic evolution in composite materials. The results reveal that an increase in Fmax, Fm, and Fa can promote da/dN; meanwhile, an increase in R will reduce da/dN. The plastic strain accumulation results indicate that Fm mainly contributes to the tensile strain and compressive stress after the first cycle. Additionally, Fa increases the stress range and compression stress and greatly improves the plastic strain accumulation degree in subsequent loading cycles. The Fmax can significantly increase the stress amplitude and plastic strain accumulation level. When R increases, the plastic strain accumulation increases a little, but the stress amplitude and compression stress decrease greatly. Furthermore, it is also found that the elastic-plastic mismatch also affects the plastic evolution, that is, strengthening or weakening the effect of the loading parameters.

Reliability modeling of the fatigue life of lead-free solder joints at different testing temperatures and load levels using the Arrhenius model

Reliability of the microelectronic interconnection materials for electronic packages has a significant impact on the fatigue properties of the electronic assemblies. This is due to the correlation between solder joints reliability and the most frequent failure modes seen in electronic devices. Due to their superior mechanical and fatigue properties, SAC alloys have supplanted Pb-solder alloys as one of the most commonly used solder materials used as interconnection joints on electronic packages. The main aim of this study is to develop a prediction model of the fatigue life of the solder joints as a function of the experimental conditions. Using a customized experimental setup, an accelerated fatigue shear test is applied to examine the fatigue life of the individual SAC305 solder joints at actual setting conditions. OSP surface finish and solder mask defined are used in the studied test vehicle. The fatigue test includes three levels of stress amplitude and four levels of testing temperature. A two-parameter Weibull distribution is used for the reliability analysis for the fatigue life of the solder joints. A stress–strain curve is plotted for each cycle to construct the hysteresis loop at each cyclic load and testing temperature. The acquired hysteresis loop is used to estimate the inelastic work per cycle and plastic strain. The Morrow energy and Coffin Manson models are employed to describe the effects of the fatigue properties on the fatigue life of the solder joints. The Arrhenius model is implemented to illustrate the evolutions in the stress life, Morrow, and Coffin Manson equations at various testing temperatures. The fatigue life of SAC305 solder joints is then predicted using a general reliability model as a function of the stress amplitude and testing temperature.

Effect of the stress concentration factor on the final fracture zone of aluminium AW 6063 T6 for rotating bending specimens

The paper presents fracture surfaces of the polycrystalline material - AW 6063 T6 aluminium alloy - under a rotating bending load. The fatigue fracture surfaces were analysed for high-cycle fatigue at approximately 1 × 105, 8 × 105 cycles and stress amplitude level equal to 140 MPa. The specimens with four different stress concentration factors Kt (with values ~1, 1.4, 2 and 2.6) were made from a drawn bar. The fatigue tests were performed by rotating bending stand with a frequency of 50 Hz. Thirty (30) tests were carried out for each geometry of the specimen. Final fracture zones for unnotched specimens were 39.9%, 35.8% and 43.3% of the total cross-sectional area for 1 × 105, 8 × 105 cycles and 140 MPa stress amplitude, respectively, However, the final fracture zone for the notched specimen with Kt equal to 2.6 was less and had a value of 13.7%, 13.8% and 18.8% of the total cross-sectional area for 1 × 105, 8 × 105 cycles and 140 MPa stress amplitude, respectively. The research aimed to compare the ratio of the area of the final fracture zone to the total cross-sectional area. The authors have found a relationship that could be used to estimate the stress concentration factor based on the area of the final fracture zone. Based on this, the stress concentration factor can be determined. The estimates calculated using the equation and the test results show a good correlation between the stress concentration factor and the ratio of the area of the final fracture zone to the total cross-sectional area. The proposed method of determining the stress concentration factor can be used to verify different manufacturing processes (e.g. surface finish, notch radius) and the loading history (e.g. existing overload conditions).

Fatigue Life of Austenitic Steel 304 Bolts Strengthened by Surface Treatment with Graphene Oxide Layer and Surface Shot Peening.

This paper presents the results of investigations of the effect of graphene oxide and surface shot peening on the mechanical properties and fatigue life of bolts made of austenitic 304 steel. An innovative method for the uniform deposition of graphene oxide on screws is presented. The process involved activating the surface using plasma and then performing graphene oxide deposition using centrifugal force and vacuum drying. The screw specimens prepared in this way were subjected to a surface peening process. Comparative studies have shown that the combination of graphene oxide deposition and shot peening processes results in an increase in fatigue life of approximately 42 ÷ 275% (depending on the stress amplitude level) compared to the as-delivered samples. The results presented are promising and may provide a basis for further research on the application of graphene and its derivatives to increase fatigue life and improve the mechanical properties of machine components.

Effect of Fine Particle Peening Using Hydroxyapatite Particles on Rotating Bending Fatigue Properties of β-Type Titanium Alloy

Fine particle peening (FPP) using hydroxyapatite (HAp) shot particles was performed to improve the fatigue strength and form a HAp transfer layer on a beta titanium alloy (Ti–22V–4Al). The surface microstructures of the FPP-treated specimen were characterized using scanning electron microscopy, micro-Vickers hardness testing, energy dispersive X-ray spectrometry, X-ray diffraction, and electron backscattered diffraction. A HAp transfer layer with a thickness of 5.5 μm was formed on the surface of the Ti–22V–4Al specimen by FPP. In addition, the surface hardness of the Ti–22V–4Al was increased, and high compressive residual stress was generated on the specimen surface by FPP. Rotating bending fatigue tests were performed at room temperature in laboratory air over a wide cycle-life region (103–109 cycles). In the long cycle-life regime, the fatigue strength at 107 cycles of the FPP-treated specimen became higher than that of the untreated specimen. This result is attributed to the formation of a work-hardened layer with high compressive residual stress by FPP. However, the fatigue strength was not improved by FPP in the short cycle-life regime, because fatigue cracks were initiated at surface defects formed during the FPP process. The fatigue fracture mode of the FPP-treated specimens shifted from surface-initiated fracture to subsurface-initiated fracture at a stress amplitude level of 600 MPa.

Comparative study of tensile and high-cycle fatigue properties of extruded AZ91 and AZ91–0.3Ca–0.2Y alloys

Mg–Al–Zn–Ca–Y alloys with excellent ignition and corrosion resistances—termed SEN alloys (where the letters “S,” “E,” and “N” stand for stainless, environmentally friendly, and non-flammable, respectively)—have been developed recently. In this study, the microstructure, tensile properties, and high-cycle fatigue properties of an extruded Mg–9.0Al–0.8Zn–0.1Mn–0.3Ca–0.2Y (SEN9) alloy are investigated and compared with those of a commercial Mg–9.0Al–0.8Zn–0.1Mn (AZ91) alloy extruded under the same conditions. Both the extruded SEN9 alloy and the extruded AZ91 alloy have a fully recrystallized structure comprising equiaxed grains, but the former has a smaller average grain size owing to the promoted dynamic recrystallization during extrusion. The extruded AZ91 alloy contains coarse Mg17Al12 discontinuous precipitate (DP) bands parallel to the extrusion direction, which are formed during its cool down after extrusion. In contrast, the extruded SEN9 alloy contains relatively fine undissolved Al2Ca, Al8Mn4Y, and Al2Y second-phase particles, which are formed during the solidification stage of the casting process. The tensile strength of the extruded SEN9 alloy, which has finer grains and more abundant particles, is slightly higher than that of the extruded AZ91 alloy. However, the difference in their strengths is relatively small because the stronger solid-solution hardening and precipitation hardening effects in the extruded AZ91 alloy offset the stronger grain-boundary hardening and dispersion hardening effects in the extruded SEN9 alloy to some extent. The tensile elongation of the extruded AZ91 alloy is significantly lower than that of the extruded SEN9 alloy because the large cracks formed in the DP bands in the former cause its premature fracture. Although the extruded SEN9 alloy has higher tensile properties than the extruded AZ91 alloy, the high-cycle fatigue life and fatigue strength of the former are shorter and lower, respectively, than those of the latter. The DP bands in the extruded AZ91 alloy do not act as fatigue crack initiation sites, and therefore, fatigue cracks initiate on the specimen surface at all stress amplitude levels. In contrast, in most of the fatigue-fractured specimens of the extruded SEN9 alloy, fatigue cracks initiate on the undissolved Al2Ca and Al2Y particles present on the surface or subsurface of the specimens because of the high local stress concentration on the particles during cyclic loading. This particle-initiated fatigue fracture eventually decreases the high-cycle fatigue resistance of the extruded SEN9 alloy.

Estimating of the Size of the Maximum Non-Metallic Inclusions in Martensitic Steels and their Influence on Fatigue Properties

Fatigue cracks can start from non-metallic inclusions in the subsurface zone at low stress amplitude level and high number of cycles. This phenomenon was observed mainly at high strength steels and parts with hardened surface zone. Fatigue strength of steel can be then estimated on the basis of hardness and maximum size of defect/inclusions measurements. Many inclusions rating methods exist, some of which have been adopted as the standards. However, “standard” methods for characterization of inclusion content are not convenient for assessment of their influence on fatigue strength of materials. This contribution deals with assessment of maximum sizes of inclusions in three types of martensitic steels used for production of rotating blades of steam turbines (Böhler T552, Böhler T671, and MLX 17) using extreme value statistics methods. Murakami´s concept of √AREAmax parameter and procedure according to ASTM E2283-08 were applied. Results were compared one another and also with the result of standard methods, too. Founded parameters were used for calculation of expected limit of the fatigue strength and compared with results of fatigue test of these steels.

Mechanical Behavior Evolution and Damage Characterization of Coal under Different Cyclic Engineering Loading

The cyclic loading causes the strength change of porous rocks, which can be frequently encountered in underground coal mining. In order to quantify the cumulative damage of porous coal, multilevel uniaxial cyclic compressive tests were carried out considering different loading frequencies. The results show that under cyclic loading, Young’s modulus of coal shows a first drastic increase and then decrease trend in terms of the number of loading-unloading cycles, while the Poisson’s ratio gradually increased at lower peak stress amplitudes and then increased sharply with peak cyclic stress amplitude until losing load bearing capacity, following a stepwise rising trend. At a higher loading frequency, volumetric compressibility-dilatancy transition is shifted to an earlier time. The loading/unloading response ratio (LURR) was used to evaluate the damage of coal under cyclic loading. It is found that LURR is an effective parameter to evaluate the damage of coaly rock under cyclic loading and is also useful in distinguishing different deformation mechanism at different loading stages. The results also show that the damage of coal can initiate at the early stage of cyclic loading at a low loading frequency; however, the increase in frequency of cyclic loading can delay the damage time, requiring a relatively higher level of peak cyclic stress amplitude.

High-cycle fatigue of a metal matrix composite based on an aluminum alloy 7075-T1 reinforced with silicon carbide particles

The article presents results of the fatigue strength study of a metal matrix composites based on an aluminum alloy of the 7075-T1 grade, containing 20 vol.% silicon carbide obtained by powder technology using mechanical alloying followed by hot pressing. The high-cycle fatigue (HCF) study was carried out at five levels of stress amplitude at room temperature and at four levels of stress amplitude at high temperature (100°C). Smooth samples of corset type with a circular cross section were manufactured in accordance with the requirements of GOST 25.502–79. Synthesized samples went through the heat treatment stage characteristic of the matrix aluminum alloy 7075. Spherical particles within the range from 5 to 70 μm served as the raw material for the initial matrix of the composite material, and the raw material for the reinforcing component were silicon carbide powder particles, which after a long mechanical alloying with matrix particles take the form of granules from 400 to 600 microns.The study of fatigue characteristics led to the conclusion that the introduction of a 20% reinforcing phase into the 7075-T1 matrix alloy based on silicon carbide particles made it possible to achieve a 1.3 times increase in endurance limits based on 2·107 cycles. An increase in the test temperature (100°C) leads to a decrease of 8% in the endurance limit. The change in the values of progressive deformation and resonance frequency during testing at room and high temperatures (100°C) is considered. At room temperature, it was found that with increasing durability, progressive deformation accumulates. At high temperature (100°C), it was found that with a decrease in durability, resonance frequency decreases more pronouncedly at each stress level.

Effects of fines content and plasticity on liquefaction resistance of sands

A detailed experimental programme was conducted to evaluate the liquefaction susceptibility of non-plastic and low-plasticity soils subjected to cyclic loading under undrained triaxial loading conditions. The study mainly focused on examining the influence of the amount of fines and plasticity indices on the liquefaction resistance of sands. After mixing silt and clay fractions into fine sand, 16 soil combinations were prepared. The silty sands contained up to 40% non-plastic fines and the low-plasticity soils contained a clay fraction of 5–40%. Each cylindrical soil specimen was constituted to a medium relative density and saturated specimens were subjected to a confinement pressure of 100 kPa. The consolidated specimens were then subjected to various levels of cyclic stress amplitudes using a sinusoidal wave load form at a frequency of 1 Hz. The results showed that both the non-plastic and low-plasticity clay soils were less resistant to liquefaction than the fine sand. The soils belonging to categories SM and SC (silty sand and clayey sand, respectively, as per Indian standard soil classifications) were susceptible to liquefaction if the fines passing through a 75 μm sieve were ≤ 40%, the liquid limit was ≤ 40%, the plasticity index was &lt; 15 and the saturated water content was about 0.86 times the LL.

Strain-Detecting properties of hybrid PE and steel fibers reinforced cement composite (Hy-FRCC) with Multi-Walled carbon nanotube (MWCNT) under repeated compression

This experimental research investigates the feasibility of using hybrid polyethylene (PE) and steel fibers reinforced cement composite (Hy-FRCC) with multi-walled carbon nanotube (MWCNT) as strain sensors. Smart construction material presented in the study is a new type of multifunctional cement composite with both structural material and self-sensing functionalities. The effect of MWCNT dosage (1 and 2% by weight of cement) and the magnitude of repeated compressive stress on the strain-detecting function and elastic modulus was studied. The history of applied compressive loading consists of five successive levels (30, 50, 60, 70 and 80% of compressive strength) of stress amplitude to evaluate the repeated strain-sensing ability of Hy-FRCC. Test results revealed that the incorporation of MWCNT within 2% has little effect on the compressive strength of Hy-FRCC at curing ages of 28-day while under repeated compression, MWCNT is effective to decrease the reduction of the elastic modulus in the Hy-FRCC. Piezoresistivity experiments indicated that the change in resistivity of Hy-FRCC-2 (with 2% MWCNT) specimen can simulate the changes in the measured strain during repeated compressive loading at increasing stress amplitudes up to 80% of compressive strength. It is found that the Hy-FRCC-2 mixture proposed in the present study is likely to be used as strain-sensors for structural health monitoring (SHM) of civil infrastructures.

Physical based parameter of statistical strength distributions

The aim of the work was the search for criteria that characterize the statistical properties of materials and are related to their mechanical properties, fracture mechanisms, and loading conditions. The statistical distributions of microhardness of mild steel under tension measured at different distances from the fracture surface of the specimen were obtained, and the relationships between the slope parameter of cumulative distributions and non-destructive testing parameters were established. It was shown that the cumulative distributions of fatigue life, taken from the literature, can also be described by an exponential functions, and the slope exponent is related to the level of stress amplitude, also being sensitive parameter to change in fracture mechanism.

Reliability modeling for aged SAC305 solder joints cycled in accelerated shear fatigue test

Solder joints reliability is a determinant factor for the life of the electronic assemblies. In this study, the reliability of actual SAC305 solder joints is investigated using accelerated shear fatigue tests. The effect of aging time and stress amplitude are studied. Five levels of aging time at 100 °C and three levels of stress amplitudes are applied in the experiments. Two-parameters Weibull distribution is used to describe the fatigue life of the solder joints at each test condition. A numerical model for the solder joints reliability is developed as a function of stress amplitude and aging time. The hysteresis loops at different aging times and stress amplitudes are demonstrated. The relationships between the inelastic work, plastic strain, and fatigue life are identified by exploiting the Coffin-Manson and Morrow Energy models. The results indicate a reduction in fatigue life and an increase in inelastic work and plastic strain when the cyclic stress amplitude is increased. Increasing the aging time has a negative impact on the fatigue life of the solder joint. The Morrow Energy model significantly outperforms the Coffin-Manson model in describing the fatigue behavior of the solder joints regardless of the aging conditions. Finally, a general reliability model as a function of the inelastic work per cycle is introduced.

In situ X-ray computed tomography study on the effect of rock blocks on fatigue damage evolution of a subgrade SRM

This study examines the effects of the rock block percentage (RBP) on the cyclic behaviors in soil rock mixture (SRM) by conducting cyclic triaxial tests at a constant stress amplitude and low strain level. The SRM samples were prepared using compaction testing with different RBPs. For the evaluation of the mesoscopic damage and stress dilatancy behaviors, the in-situ X-ray computed tomography (CT) technique was employed. It was revealed that the hysteresis loop presents a sparse-dense-sparse pattern that is caused by the plastic strain accumulation. Adding rock block to SRM influences the shape of the hysteresis loop, and the area of the hysteresis loop decreases with increasing RBP for samples at the same stress amplitude, the energy loss becomes smaller for SRM samples with a high RBP. On the other hand, after extracting cracks from the original CT images, it shows that damage of the SRM with different RBPs is different under the same loading cycle, the geometric parameters of crack decrease as the RBP increases. Localized deformation in the SRM sample results in the differential stress dilatancy behavior, there is interlocking among rock blocks and restriction of localized bands, which is affected by the different RBP in the SRM sample.

Cyclic response and fatigue failure of Nitinol under tension–tension loading

Abstract

Effects of Fine Particle Shot Peening Treatment on Fatigue Properties of Al-7Si-0.3Mg Alloy

In this study, two different fine particle shot peening (FPSP) treatments were performed on Al-7Si-0.3Mg alloy specimens to evaluate their effects on fatigue properties. After the treatment, the microstructure morphology of the specimens was observed, and the surface roughness and residual stress distribution were measured. Rotating bending fatigue tests were conducted, and the fatigue fracture surfaces were observed using scanning electron microscopy (SEM). The crack initiation and propagation behavior were investigated using the replica method. The results indicate that FPSP can introduce residual stress on the surface layer of the material and refine the surface microstructure, and also change the surface roughness. At low stress amplitude levels, FPSP can improve fatigue life significantly. At 5 × 107 cycles, the fatigue strength of shot-peened specimens increased by 33% compared to unpeened specimens. In unpeened and steel shot-peened (SSP) specimens, both crystal slip and casting defects can induce fatigue failure, while only casting defects can cause failure in the case of ceramic shot-peened (CSP) specimens. Moreover, FPSP can move the crack initiation site to the subsurface from the surface. The range of threshold stress intensity factor of the material can be improved significantly by this treatment. Casting defects have a significant influence on fatigue strength; the effect on fatigue strength can be well evaluated by the modified Murakami equation.