Monotonic Tensile Strength Research Articles

The Influence of Dimensions and Powder Recycling on the Roughness and Mechanical Properties of Ti-6Al-4V Parts Fabricated by Laser Powder Bed Fusion.

Powder bed fusion technology has undergone a remarkable amount of development in recent years in the field of medical implants due to the advantages associated with it. In many implant applications that demand loads in parts with a high degree of roughness and small dimensions, the mechanical properties, especially fatigue properties, play a key role in the success of the implants. One of the most used materials in this field is Ti-6Al-4V. On the other hand, the high cost of titanium powders makes it necessary to search for suitable powder recycling strategies. In this work, the effects of dimensions and powder recycling on the roughness and the mechanical properties of cylinder specimens were obtained from tensile static and fatigue tests of Ti-6Al4V Extra-Low Interstitial (ELI) parts. Four types of specimens were fabricated by laser powder bed fusion (two dimensions (section diameters of 2 mm and 5 mm) with new powder and with recycled powder). Results show that the oxygen concentration increased with recycling. No significant effects of recycling were observed on the monotonic tensile strength specimens. However, specimens fabricated with recycled powder showed greater roughness, lower ductility, and lower fatigue strength than those fabricated with new powder. On the other hand, the 5-mm-diameter specimens showed slightly better fatigue behavior than the 2-mm-diameter ones.

Flaw sensitivity and tensile fatigue of a high-strength hydrogel

This work describes mechanical behavior of a high-strength hydrogel composite relative to costal cartilage. The materials are analyzed for monotonic ultimate tensile strength (UTS) with differing flaws that represent extreme differences in local stress concentration and distribution. Selected hydrogel and cartilage samples are investigated for fatigue analysis via cyclic loading. Synthetic hydrogel samples are more sensitive toward sharp cracks, while cartilage samples are more sensitive toward cylindrical holes in fatigue. This work represents a unique tensile fatigue approach in studying a high-strength soft material and porcine costal cartilage with and without extreme defects.

Impact of annealing treatment on the fatigue behavior of Inconel 625 produced by laser‐based powder bed fusion

AbstractLaser‐based powder bed fusion (LPBF) is attractive to build complex parts in spite of the low machinability of Inconel 625. The LPBF microstructures and tensile properties are well documented in literature. But fatigue properties and impact of the LPBF intrinsic defects and microstructures on the crack initiation and propagation mechanisms are uncommon. Low and high‐cycle fatigue tests at room and service temperature presented in this paper prove that the LPBF defects shorten the crack initiation stage. After annealing heat treatment, the microstructure adapts to the defects, and the fatigue response becomes similar to a free defect material. The fatigue life of as‐built Inconel 625 is lower than 50% of its monotonic yield tensile strength (YTS) while the annealed material reaches 100% of its YTS. The propagation rate is slowed by the homogeneous annealed microstructure and improved ductility resulting from removal of dislocation cells and changes in grain morphology.

Energy Dissipation and Damage Evolution Characteristics of Salt Rock under Uniaxial Cyclic Loading and Unloading Tension

Salt rock has been regarded as the optimal surrounding rock for underground gas storage (UGS), and it is occasionally subjected to cyclic tension because of the gas injection and production of salt cavern, which leads to the change in mechanical properties of salt rock. In this paper, a laboratory study is conducted to investigate the energy dissipation and damage evolution characteristics of salt rock under uniaxial cyclic tension monitored by acoustic emission (AE) machine. Compared to monotonic tension, both tensile strength and deformation capacity of salt rock are enhanced under cyclic tension. The fracture crack is approximately a single linear crack with large elliptical plastic deformation zone, which is consistent with the spatial distribution of AE events. In yield stage, the proportion of dissipative energy increases first but decreases subsequently. The relationship between AE energy-based damage variable and displacement is established. It is concluded that the damage variable is a piecewise power correlation with displacement while the growth rate of damage variable increases in the pre-peak stage but decreases in post-peak stage.

Nonhomogeneous microstructure formation and its role on tensile and fatigue performance of duplex stainless steel 2205 multi-pass weld joints

In this study, the microstructure characteristics and formation mechanism of DSS (duplex stainless steel) multi-pass weld joints were firstly investigated in terms of the OM (optical microscope) and EBSD (electron back-scattered diffraction) observation, and a series mechanical tests, including the monotonic tensile tests assisted by the DIC (digital image correlation) observation and stress-controlled fatigue tests, were subsequently performed to clarify the intrinsic correlation between mechanical behavior and microstructure. The fatigue fracture morphologies and cracks propagation behavior were analyzed by the OM and SEM (scanning electron microscope) observation. The results show that the significant ferritiszation and determined precipitations were found at the HAZ (heat-affected zone), whereas the austenite was dominating at weld filler zone due to the more formation of the IGA (intragranular austenite). A nearly balance phase ratio was obtained at weld back and cap zones, but the strip GBA (grain boundary austenites) was connected into piece along thickness direction. The monotonic tensile strength of weld joints was higher than that of the based materials due to the refined-grains structures, and the final rupture occurred at the BM (based material) zone. However, the fatigue lifetimes of weld joints were much shorter than that of based materials for the same cyclic amplitude. The fatigue cracks preferred to nucleate at weld back or cap passes although where phase ratio was approximately balance. The austenitic grains which were connected into pieces accelerated the initiation of fatigue cracks. Apart from maintaining phases balance and avoiding detrimental precipitation, the alternative distribution of austenite and ferrite should also be obtained in the engineering.

Tensile Fatigue of Poly(Vinyl Alcohol) Hydrogels with Bio‐Friendly Toughening Agents

AbstractInspired by the avoidance of toxic chemical crosslinkers and harsh reaction conditions, this work describes a poly(vinyl alcohol)‐based (PVA) double‐network (DN) hydrogel aimed at maintaining biocompatibility through the combined use of bio‐friendly additives and freezing–thawing cyclic processing for the application of synthetic soft‐polymer implants. This DN hydrogel is studied using techniques that characterize both its chemical and mechanical behavior. A variety of bio‐friendly additives are screened for their effectiveness at improving the toughness of the PVA hydrogel system in monotonic tension. Starch is selected as the best additive for further tensile testing as it brings about a near 30% increase in ultimate tensile strength and maintains ease of processing. This PVA–starch DN sample is then studied for its tensile fatigue properties through cyclic, strain‐controlled testing to develop a fatigue life curve. Though an increase in monotonic tensile strength is observed, the PVA–starch DN hydrogel does not bring about an improvement in the fatigue behavior as compared to the control. Although synthetic hydrogel reinforcement is widely researched, this work presents the first fatigue analysis of its kind and it is intended to serve as a guide for future fatigue studies of reinforced hydrogels.

Tensile and fatigue properties of Jute-Glass hybrid fibre reinforced epoxy composites

In this study, the tensile properties and fatigue life of woven jute-glass hybrid-reinforced epoxy composite were investigated experimentally. Various types of composites with different constituents (jute, glass and hybrid composites) were prepared and tested according to ASTM (D3039 and D3479) standards. Tests included: tensile test for determining the monotonic tensile strength and modulus, and tension-tension fluctuating cyclic loading with a frequency of 5 Hz and stress ratio of 0.1 for measuring the fatigue life and stiffness degradation. The results showed that jute fibre could partially replace glass fibre within the polymeric hybrid composites towards the production of more eco-friendly composites although there was a relative decline in composite’s tensile properties and fatigue life. Incorporation of jute fibre within the hybrid composite reduced the fatigue sensitivity (stiffness degradation) of the composite. A comparison between glass and hybrid composites indicates that composite’s stiffness after 10 000 cycles reduced 21% (glass composites), 16% (30:70 of jute:glass hybrid composites), and 12% (45:55 of jute:glass hybrid composites). Results suggested that the eco-friendly and cheap natural fibres such as jute could be successfully hybridised with glass fibre for structures subjected to cyclic loading.

Fatigue Life Prediction of Vortex Reducer Based on Stress Gradient

The effects of stress gradient and size effect on fatigue life are investigated based on the distribution of stress at the notch root of notched specimens of GH4169 alloy. The relationship between the life of notched specimens and smooth specimens is correlated by introducing the stress gradient impact coefficient, and a new life model of predicting notched specimens based on the Walker modification for the mean stress effect is established. In order to improve the prediction precision of life model with the equation parameters having a definite physical significance, the relationships among fatigue parameters, monotonic ultimate tensile strength, and reduction of area are established. Three-dimensional elastic finite element (FE) analysis of a vortex reducer is carried out to obtain the data of stress and strain for predicting its life. The results show that there is a high-stress gradient at the edge of the air holes of the vortex reducer, and it is thus a dangerous point for fatigue crack initiation. The prediction result of the vortex reducer is more reasonable if the mean stress, the stress gradient, and the size effect are considered comprehensively. The developed life model can reflect the effects of many factors well, especially the stress concentration. The life of notched specimens predicted by this model give a high estimation precision, and the prediction life data mainly fall into the scatter band of factor 2.

Efectos de la irradiación solar concentrada en las transformaciones alotrópicas del acero inoxidable AISI 316

Se investiga la transformación de fases que pueden ocurrir en un acero austenítico (AISI 316) mediante la demostración de la aparición de ferrita-δ que se obtiene en ciclos iniciales de calentamiento usando irradiación solar concentrada (ISC) a magnitudes necesitadas para obtener temperaturas de operación de sistemas de tipo torre central. Cuatro especímenes de acero inoxidable AISI 316 cortados de una misma pieza, fueron expuestos a ISC en el Horno Solar de Alto Flujo Radiativo de la Universidad Nacional Autónoma de México para desarrollar el ciclado térmico. El acero AISI 316 fue seleccionado por ser reportado entre los materiales más baratos usado en receptores de ISC. Pruebas de resistencia a la tensión monotónica demostraron que no existe un efecto relevante del ciclado térmico en las propiedades mecánicas resultantes. Las transformaciones de fase fueron caracterizadas usando microscopía óptica, difracción de rayos X, y microscopía electrónica de barrido con espectroscopía de dispersión de energía de rayos X. La aparición de la fase de ferrita-δ fue la principal diferencia entre los especímenes tratados con ISC y dos especímenes de referencia. La fracción UV de la ISC demostró tener el potencial de lograr estas transformaciones de fase a una temperatura cercana a los 630 °C.

Effect of Overaging on the Cyclic Deformation Behavior of an AA6061 Aluminum Alloy

The present work encompasses the effect of overaging on the strain-controlled low-cycle fatigue (LCF) behavior of an extruded AA6061 aluminum alloy at varying strain amplitudes. During the T7 aging treatment, the size of precipitates increased from 60 nm under T6 conditions to 220 nm after aging for 48 h at 200 °C, leading to a decrease in the monotonic tensile strength. During the LCF tests, nearly symmetrical hysteresis loops can be observed in the mid-life cycle under all test conditions, whereas the first-cycle hysteresis loops were moderately inflected under long-aging conditions. With increasing aging time, the cyclic peak stresses decreased and the plastic strain increased. Nearly ideal Masing behavior was exhibited under T6 conditions, while it was lost under T7 overaging conditions. The cyclic stress responses were similar under all tested conditions, involving stabilization at low strain amplitudes and softening at high strain amplitudes, with initial hardening for the first few cycles. Compared to the T6 condition, the fatigue life increased with increasing T7 aging time. Various LCF parameters were estimated based on the Coffin-Manson and Basquin relationships and on the LCF experimental results. The relationship between the fatigue life, strength, and microstructure of the investigated AA6061 aluminum alloy under various aging conditions was discussed.

Effects of cold working and corrosion on fatigue properties and fracture behaviors of precipitate strengthened Cu-Ni-Si alloy

In this study, the tensile and fatigue tests were conducted on precipitate strengthened Cu-Ni-Si alloy. The fatigue properties of specimens with cold working and without cold working in air, as well as that of the specimen with cold working in salt atmosphere were investigated. The results show that the monotonic tensile strength and the yield strength are improved obviously, while the elongation decreases by cold working. The S-N curves of all the types of specimens show a continuous decrease. The fatigue strength of specimens in air is slightly reduced by cold working, however, that of the specimen with cold working at 107 cycles is largely decreased from 190 MPa to 120 MPa in salt atmosphere. All the fatigue cracks initiate from the specimen surface by slip deformation. In air condition, the specimens with cold working show shear mode fracture with transgranular crack propagation and it is caused by the obvious texture, while that without cold working show normal mode fracture with both transgranular and intergranular crack propagation. However, in salt atmosphere condition, the specimens with cold working show normal mode fracture with totally intergranular crack propagation.

MECHANICAL CHARACTERIZATION OF ANCIENT PORTUGUESE RIVETED BRIDGES STEELS

In repairing and retrofitting processes of ancient riveted steel bridges is crucial to assess the structural state of old metals to guaranty structural safety. Metals under long-time operations (mild, rimmed low carbon steels <0.1% C, puddle irons) and cyclic loading present a tendency for degradation processes. The case-studies of this work are five metallic bridges existing in Portugal (Luiz I, Eiffel, Fao, Pinhao and Trezoi). This work presents the study of some characteristics of materials extracted from the five case-studies, such as: monotonic tensile strength, chemical composition, microstructures, hardness, notch toughness and fatigue crack propagation. In terms of monotonic tensile tests, the materials from Luiz I, Eiffel and Fao bridges are similar to puddle steel and the materials from Pinhao and Trezoi bridges are similar to mild steel. In terms of toughness only the material from the Pinhao bridge exhibits acceptable toughness properties, considering current design requirements. The materials from the other bridges exhibit relatively low toughness properties. The fatigue crack propagation data from the old Portuguese riveted steel bridges were correlated using the Paris’s law and the possibility for a design crack growth rate was discussed.

Elucidating the Relations Between Monotonic and Fatigue Properties of Laser Powder Bed Fusion Stainless Steel 316L

The laser powder bed fusion (L-PBF) technique builds parts with higher static strength than the conventional manufacturing processes through the formation of ultrafine grains. However, its fatigue endurance strength σ f does not match the increased monotonic tensile strength σ b. This work examines the monotonic and fatigue properties of as-built and heat-treated L-PBF stainless steel 316L. It was found that the general linear relation σ f = mσ b for describing conventional ferrous materials is not applicable to L-PBF parts because of the influence of porosity. Instead, the ductility parameter correlated linearly with fatigue strength and was proposed as the new fatigue assessment criterion for porous L-PBF parts. Annealed parts conformed to the strength-ductility trade-off. Fatigue resistance was reduced at short lives, but the effect was partially offset by the higher ductility such that comparing with an as-built part of equivalent monotonic strength, the heat-treated parts were more fatigue resistant.

Fatigue behavior of double-edge notched oxide/oxide ceramic matrix composite in a combustion environment

Tension–tension fatigue tests in a combustion environment were performed on double-edge notched oxide/oxide ceramic matrix composite specimens. The composite, designated as N720/A, constituted woven 0°/90° Nextel™720 fibers in alumina matrix. Monotonic tensile and cyclic loads at a frequency of 1 Hz and a stress ratio of 0.05 were applied on the specimens in a combustion environment. The maximum specimen temperature due to combustion flame impingement in the notch region was 1250 ± 50℃. A stiffness reduction of less than 10% evaluated for the run-out specimens showed the harsh combustion environment had a minimal effect on specimen degradation. The residual strength was evaluated to be ∼75%–85% the strength of non-fatigued (virgin) double-edge notch specimens in room temperature. The monotonic tensile strength and the fatigue limit for 90,000 cycles (run-out) were found to be ∼40 MPa less in the combustion environment when compared to published results for 1200℃ laboratory air environment. The damage mechanisms were also the same in the two environments. Finite element analyses showed that the reduction in strength and fatigue limit in the combustion environment (as compared to the laboratory air environment) was due to the presence of thermal gradient stresses because of non-uniform specimen temperature distribution.

Influence of moisture uptake on the static, cyclic and dynamic behaviour of unidirectional flax fibre-reinforced epoxy laminates

This papers aims to characterize the influence of moisture uptake on the mechanical behaviour of unidirectional flax fibre-reinforced epoxy laminates. Monotonic and cyclic tensile tests and free vibration characterization are carried out. Results show that UD flax-epoxy composites, when exposed to hygrothermal conditioning at 70°C and 85% RH, exhibit a diffusion kinetic which follows a one dimensional Fickian behaviour. The mass uptake at equilibrium is approximately 3.3% and the diffusion coefficient 6.5×10−6m2s−1. Water vapour sorption is shown to induce a significant change in the shape of the tensile stress-strain curve, a decrease in the dynamic elastic modulus of about 20% and a 50% increase in the damping ratio. Contrary to all expectations, water saturation does not degrade the monotonic tensile strength of such a flax-epoxy composites and leads to an increase in the fatigue strength for a high number of cycles.

Fatigue Testing of Titanium Alloy Ti6Al4V Used in Medical Devices

Because titanium alloy Ti6Al4V features low density, good corrosion resistance and biocompatibility, it a metal of choice for medicine, used in orthopedics, accident surgery and dentistry. The insertion of elements such as nitrogen, carbon, hydrogen, iron or oxygen improve fatigue strength of this material. This paper presents certain basic fatigue and monotonic tests performed on the alloy. We have proposed a unique sample geometry and sample dimensions smaller than the reference ones. The dimensions of the working part of the sample correspond to the geometry of a dental implant connector. This part of the implant has to withstand stresses that are both intricate and variable in time. We performed monotonic tensile strength tests and uniaxial fatigue tests using the Instron 8874 machine. Then, we examined the samples under a stereoscopic microscope and measured the fracture development angle.

Mechanical Characterization of Sheet Moulding Composites for the Automotive Industry

The mechanical properties of Sheet Moulding Composite (SMC) have been analyzed by means of static and fatigue tests in tension. SMCs show a substantial in-plane anisotropy either in terms of stiffness or strength. Fatigue data were modeled adopting a fatigue model with two parameters, represented by the cyclic number and the mean stress. The statistical implementation of such model was based on the hypothesis that the monotonic tensile strength follows a twoparameter Weibull distribution. The model has the potential to be predictive indicating that the fatigue characterization of a given laminate can be achieved with a minimum number of experimental tests. The reliability of such procedure and its applicability limits are discussed in the light of the model parameters obtained for different glass fiber reinforced composites.

Effect of strain rate on elastic-plastic deformation in high cycle fatigue properties

The aim of the present study was to investigate the contribution of the strain rate to the transition of elastic-plastic deformation behavior and the difference between the fatigue strength coefficient and monotonic tensile strength in terms of the modified Basquin’s relation, which considers microporosity variation. The transition phenomena of elastic-plastic deformation were evaluated through comparison of the overall contour of the hysteresis loops measured in high cycle fatigue tests of a low-pressure die-cast A356 alloy. The increase of the alternating stress amplitude at a given excitation frequency causes a variation in the strain rate per unit time interval that depends fundamentally on the excitation frequency. In addition, the transition of elastic-plastic deformation behavior is induced by variation of strain rate in high cycle fatigue test, i.e., typically, a variation of the elastic modulus and extension of the elastic deformation region, compared with monotonic deformation under a very slow strain rate. The dependence of the elastic modulus on the strain rate due to the variation of the stress amplitude can be described in an exponential form of strain rate. The modified Basquin’s equation which includes the contribution of the strain rate and microporosity to the fatigue strength coefficient and fatigue life was re-established, including the dependence of the elastic modulus and transition of elastic-plastic deformation on the variation of the strain rate.

Dependence of fatigue life of low-pressure die-cast A356 aluminum alloy on microporosity variation

The aim of the present study was to investigate the dependence of the high cycle fatigue property on the microporosity variation of a low-pressure die-cast A356 alloy. Also, it aimed to describe quantitatively the relationship between the fatigue property and monotonic tensile strength using modified Basquin’s equation which takes into account the microporosity variation. The fatigue life of the A356 alloy can be described by an exponential dependence on the variation of the fractographic porosity, in terms of the modified Basquin’s equation which is composed of the defect susceptibility of fatigue life to microporosity variation and the maximum tensile strength achievable in the defect-free condition. Using a modified form of Basquin’s equation, the maximum values of the fatigue strength coefficient and exponent in the defect-free condition are 341.5 MPa and −0.076, respectively, even though the nominal values of fatigue strength coefficient and exponent without consideration of microporosity variation are 237.6MPa and −0.048, respectively. Also, the difference between the maximum tensile strength and the fatigue strength coefficient on modified Basquin’s equation is about 120MPa, and it arises from variation in the deformation behavior due to the difference of loading condition between the monotonic and cyclic test modes such as the strain rate, Bauschinger effect, cyclic work hardening and damage accumulation on loading condition.

Comparison of the fatigue and impact fracture behaviour of five different steel grades used in the frog of a turnout

The material requirements for the frog of a turnout are quite complex. The crossing nose region of the frog represents a discontinuity in the track as the wheel needs to change from one rail to the other. This causes increased dynamic contact forces and slip between wheel and frog. The high contact pressures and slip can cause damage such as wear, rolling contact fatigue and severe plastic deformation in the frog. The material from which the frog is constructed plays a crucial role in the development of damage. On the one hand, a plastic material response can reduce the contact pressures by an adaption of the frog’s geometry. On the other hand, materials have a different resistance to wear, crack initiation and growth. To clarify the requirements for materials for the manufacture of frogs, five different steel grades used to create frogs that are in service in various railway turnouts were investigated. The chosen materials were ‘Hadfield steel’ (an Mn-13, abrasion-resistant grain-refined steel with 400 HV), bainitic railway steel, tempered steel 51CrV4 (DIN 1.8159) and maraging steel. They were investigated in the same conditions as they would experience in service, covering a variety of steel microstructures and macroscopic hardness values. To compare the materials’ responses to the high impact forces, their impact fracture toughness was measured and selected fracture surfaces were investigated with scanning electron microscopy. To compare their response to the cyclic loading, the materials were tested with respect to their monotonic tensile strength as well as fatigue strength by means of low cycle fatigue tests. Measurements of the fracture toughness KIC were performed on all materials to compare their fracture resistance. The achieved results can be used as input data for finite-element-based calculations of the local loading conditions in railway turnouts. Additionally, the comparison can help the choice of the best material for specific loading conditions, i.e. specific crossing geometries and track loading conditions.