Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime

Very high cycle fatigue tests under axial loading at frequencies of 95 Hz and 20 kHz were performed to clarify the effect of loading type on fatigue properties of a high strength bearing steel in combination with experimental result of this steel under rotating bending. As a result, this steel represents the single P-S-N (probabilistic-stress-life) curve characteristics for surface-induced fracture and interior inclusion-induced fracture, just like that under rotating bending. However, fatigue strength is lower, where the run-out stress at 10⁹ cycles is evaluated to be 588 MPa, less than that under rotating bending with about 858 MPa. Occurrence probability of larger and deeper inclusion-induced fracture is much higher than that under rotating bending. Furthermore, the formation process of fine granular area (FGA) is independent of the type and frequency of loading, which is very slow and is explained as the crack nucleation process under the special dislocation mechanism. The stress intensity factor range at the front of FGA, ΔK_FGA, is approximately regarded as the threshold value controlling the stable propagation of interior crack. For the control volume of specimen under axial loading, the estimated value of fatigue limit by FGA is similar to experimental run-out stress value at 10⁹ cycles, but that by inclusion is larger. However, the corresponding estimated results under rotating bending are all conservative.     

Mechanical and degradation characteristics of natural silk fiber reinforced gelatin composites

Silk reinforced gelatin based composites were prepared by compression molding. The fiber content in the composite was 20 wt.%. Tensile strength (TS), tensile modulus (TM), bending strength (BS), bending modulus (BM), impact strength (IS) and hardness of the composites were found 44.5 MPa, 0.65 GPa, 63 MPa, 3.7 GPa, 5.1 kJ/m² and 96 shore A respectively. The environmental effect on composite was observed by simulating weathering test and the composite lost 15.2% TS at the end of 30 h of the weathering testing period. The biodegradation test shows that the composite degrades very quickly and losses 52.1% weight at the end of 24 h. Morphological analysis was carried out to observe fracture behaviour and fiber pullout of the samples.     

Effect of NiCrAlY platelets inclusion on the mechanical and thermal shock properties of glass matrix composites

NiCrAlY platelets modified glass matrix composites were prepared. Their microstructures were characterized, their Young's modulus, fracture strength in bending, Vickers hardness, and indentation toughness were measured, and their thermal shock resistance was studied using quenching-strength and indentation-quench methods. With increasing NiCrAlY content, evident enhancements of the Young's modulus and indentation toughness were obtained. The NiCrAlY alloy inclusion could exert significant influences on the retained bending strength of the samples after quench tests, from 9.6 MPa for NiCrAlY-free glass to 32.0 MPa for 30 wt.% NiCrAlY-containing composites. The indentation-quench tests showed that NiCrAlY alloy inclusion elevated the critical quenching temperatures for propagation of pre-crack, from 150 °C for NiCrAlY-free glass to 225 °C for 30 wt.% NiCrAlY-containing composites. Inclusion debonding and intersection, crack deflection and bridging were observed, and are likely the micromechanisms accounted for the improvement of fracture resistance.     

Design of a textile composite bone plate using 3D-finite element method

Bone plates are the most common devices used for long bone fracture fixations. Metallic bone plates are conventionally used for load bearing regions suffer the disadvantages that they usually needs to be removed 1–2 years after surgery due to stress shielding and ion releasing effects. One solution to overcome these problems is to use bone plates made of composite materials with desirable mechanical properties as substitutes for the metallic types. In this research, a partially resorbable composite bone plate consisting of a poly L-Lactic acid matrix and textile bioglass fibers used as reinforcement was modeled and analyzed using the ANSYS software V. 9.0. Micromechanical study of a representative volume element (REV) was carried out using the 3D-finite element method to optimized volume fraction of the reinforcement. In this stage, ultimate tensile strength of the composite was determined. In the macromechanical analysis, a three dimensional, quarter symmetric finite element model was developed for a plate with five holes. Bending analysis was performed to determine the bending strength and the bending modulus of the plate. Results showed that for a volume fraction equal to 45%, the longitudinal modulus of elasticity and the ultimate tensile strength would be 23 GPa and 230 MPa, respectively. The bending strength and bending modulus of the plate were calculated to be about 55 MPa and 16.6 GPa, respectively. Compared to the data available on forearm bones in which the longitudinal modulus of elasticity is about 18 GPa, the tensile and bending strength are about 150 MPa and 40 MPa and the bending modulus is 7 GPa, it is concluded that the composite plate system is suitable for forearm region and it is capable of reducing stress shielding effects at the fracture site.     

Evaluating strength of thermally sprayed Al2O3 on aluminum

We evaluated the strength of thermally sprayed Al₂O₃ on aluminum. The thermally sprayed Al₂O₃ films were processed using low-pressure plasma spraying. The thickness of the thermally sprayed Al₂O₃ was 0.3 mm and 0.7 mm. We arranged a 4-point bending test and a heating test to evaluate the strength of the thermally sprayed Al₂O₃. We also investigated the effect of residual stress on the strength by measuring deformation of the thermally sprayed Al₂O₃ after removing the aluminum substrates. The bending strength was 120 MPa, regardless of thickness. We assumed that the bending strength would be equal to the tensile strength because the thermally sprayed Al₂O₃ films were very thin. A crack was generated at 433 K, regardless of thickness. The thermal stress was 160 MPa when the crack was generated. It was 40 MPa higher than we estimated. We found that the residual stress was compression stress that measured 40 MPa, which contributed to the prevention of the crack generation. We presume that the tensile strength was lower than the thermal stress because the residual stress was reduced by stress-relaxing of the aluminum near the interface in the bending test. The influence of heat-resisting strength is dominant over residual stress. Therefore, strength design should take into account residual stress.     

Influence of nano-ZrO2 additive on the bending strength and fracture toughness of fluoro-silicic mica glass–ceramics

Fluoro-silicic mica glass–ceramics were prepared by a sintering process and different proportions of nano-ZrO₂ particles (3Y-TZP) were integrated during the process. Bending strength and fracture toughness were evaluated using a three-point bending test and a Vickers indenter, respectively. The bending strength and fracture toughness improved in significantly with the increase in the quantity of nano-ZrO₂ additives. The highest bending strength of 324.3 ± 12.3 MPa and fracture toughness of 4.2 ± 0.11 MPa m^1/2 were obtained with 30% (wt.) nano-ZrO₂. Good results were also obtained in morphological observations. The glass–ceramic is homogenous and the ZrO₂ grains embed in the lamellar structures of the fluoro-silicic mica homogenously and completely and array well and compactly. On the fracture surface, both the transgranular fracture and the intergranular fracture can be observed clearly.     

Measurement of the critical aspect ratio and interfacial shear strength in MWNT/polymer composites

This paper presents a bulk composite method for determining the critical aspect ratio and relative interfacial shear stress (ISS) for multiwalled carbon nanotube (MWNT)/polymer composites. Through a modified pullout test and fragmentation test, it was found that the critical aspect ratio was 300 and decreased by a factor of 3 due to surface modification, and that MWNTs at an angle of greater than 60° to the loading direction failed in bending instead of pulling out of the matrix. Finite element analysis was used to determine the critical bending shear strength and MWNT modulus. The obtained bending shear strength was used in a mechanics model developed to provide bounds for the ISS in the experimental composite system. The calculated ISS for as-received nanotube falls between 4.8 and 13.7 MPa, and for surface treated nanotube falls in the range of 11.1 and 38.3 MPa. These values are consistent with the ISS reported for carbon fiber/polymer composites and also show that the ISS almost triples due to chemical modification of the MWNT surface.     

Improved mechanical property of foam glass composites toughened by glass fiber

The mechanically improved foam glass composite toughened by glass fiber was prepared by sintering technique, using waste sodium-calcium silicon flat glass powder as main raw materials. In this study, the preparation and properties of the samples were characterized by differential thermal analysis (DTA), field-emission scanning electron microscopy (FESEM) and mechanical property test. The specific strength of the composite was defined for the first time, and applied into the investigation of mechanical property. The results show that the specific improved bending strength of 10.45-22.26 MPa/(g cm^− 3), and the specific compressive strength of 30.45-34.34 MPa/(g cm^− 3) can be displayed when sintered at 790-815 °C with the addition of 5-25 wt.% glass fiber. Good correlations between the microstructure (in particular the fiber distribution), the high specific strength and the high modulus of elasticity of glass fibers.     

Processing and mechanical properties evaluation of glass fiber-reinforced PTFE laminates

A specific manufacturing process to obtain continuous glass fiber-reinforced PTFE laminates was studied and some of their mechanical properties were evaluated. Young’s modulus and maximum strength were measured by three-point bending test and tensile test using the Digital Image Correlation (DIC) technique. Adhesion tests, thermal analysis and microscopy were used to evaluate the fiber–matrix adhesion, which is very dependent on the sintering time. The composite material obtained had a Young’s modulus of 14.2 GPa and ultimate strength of 165 MPa, which corresponds to approximately 24 times the modulus and six times the ultimate strength of pure PTFE. These results show that the PTFE composite, manufactured under specific conditions, has great potential to provide structural parts with a performance suitable for application in structural components.     

Uniaxial tensile and shear deformation tests of gold-tin eutectic solder film

This paper describes a novel experimental technique for measuring mechanical properties of gold-tin (Au-Sn) eutectic solder film used for soldering package in microelectromechanical systems (MEMS). Dual-source DC magnetron sputtering was employed to deposit Au-20 weight % (wt%) Sn film. The tensile test with in situ X-ray diffraction (XRD) measurement evaluates the Young's modulus and Poisson's ratio at intermediate temperatures. The Young's modulus and Poisson's ratio at room temperature were found to be 51.3 GPa and 0.288, lower than bulk values. The Young's modulus decreased with increasing temperature, whereas the Poisson's ratio did not depend on temperature. The XRD tensile test also showed creep deformation behavior of Au-Sn film. We have developed a shear deformation test technique, which is performed by using Au-Sn film sandwiched by two single crystal silicon (Si) cantilever structures, to characterize the shear properties of the film. The shear moduli obtained from the shear deformation tests ranged from 11.5 to 13.3 GPa, about 38% lower than those from the XRD tensile tests. The measured shear strength from 12 to 17 MPa exhibited a temperature dependency. Information about the tensile and shear characteristics would likely to be of great use in designing Au-Sn soldering packages for MEMS.     

Inclusion-controlled high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel

Axial loading fatigue tests were carried out to study the influence of inclusion on high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel (AISI 11). The fatigue strength of 1538 MPa with endurance life of 10⁷ cycles were obtained by stair-case method. The fatigue specimens were also subjected to a constant maximum stress of 1650 MPa to investigate the relationship among inclusion origin size (10-30 μm), fish-eye size (70-130 μm) and fatigue life (10⁵-10⁷ cycles). The fatigue life was found to be dependent on the inclusion size and the crack propagating length. A compressive residual stress of 300-450 MPa turned out to be present at the specimen surface, and finally induced the interior failure mode. Further investigation into the correlation between stress intensity factors of inclusion origin and corresponding stages of fatigue crack growth and fatigue life revealed that the high cycle fatigue behavior was controlled by crack propagation. According to the fractographic investigation, two distinct zones were observed in fish-eye, representing Paris-Law and fast fatigue crack growth stage, respectively. Threshold stress intensity for crack propagation of 3.9 MPa√m was obtained from the well correlated line on the ΔK_I-log N_? graph. The fracture toughness can also be estimated by the mean value of stress intensity factor ranges for fish-eye.     

Fracture behavior and mechanism of 2D C/SiC-BCx composite at room temperature

2D C/SiC composite was modified with partial BC_x matrix by low pressure chemical vapor infiltration technique (LPCVI), which was named as 2D C/SiC-BC_x composite. The flexural fracture behavior, mechanism, and strength distribution of 2D C/SiC-BC_x composite are investigated. The results indicate that the flexural strength, fracture toughness, and fracture work are 442.1 MPa, 22.84 MPa m^1/2, and 19.2 kJ m⁻², respectively. The flexural strength of C/SiC-BC_x composite decrease about 20% than that of C/SiC composite. However, the fracture toughness and fracture work increase about 19% and 18.5%, respectively. The properties varieties between C/SiC-BC_x composite and C/SiC composite can be attributed to the weak-bonding interface between BC_x/SiC matrices according to the results of detailed microstructure analysis. The strength distribution of 2D C/SiC-BC_x composite follows as Normal distribution or Weibull distribution with σ_u = 0, and m = 8.1393. The mean value of flexural strength for 2D C/SiC-BC_x composite is 443 MPa obtained by theory calculation, which is consistent with experiment result (442.1 MPa) very well.     

Crack arrest testing of high strength structural steels for naval applications

The crack arrest fracture toughness of two high strength steel alloys used in naval construction, HSLA-100, Composition 3 and HY-100, was characterized in this investigation. A greatly scaled-down version of the wide-plate crack arrest test was developed to characterize the crack arrest performance of these tough steel alloys in the upper region of the ductile-brittle transition. The specimen is a single edge-notched, 152 mm wide by 19 mm thick by 910 mm long plate subjected to a strong thermal gradient and a tensile loading. The thermal gradient is required to arrest the crack at temperatures high in the transition region, close to the expected service temperature for crack arrest applications in surface ships. Strain gages were placed along the crack path to obtain crack position and crack velocity data, and this data, along with the applied loading is combined in a “generation mode” analysis using finite element analysis to obtain a dynamic analysis of the crack arrest event. Detailed finite element analyses were conducted to understand the effect of various modeling assumptions on the results and to validate the methodology compared with more conventional crack arrest tests.Brittle cracks initiation, significant cleavage crack propagation and subsequent crack arrest was achieved in all 15 of the tests conducted in this investigation. A crack arrest master curve approach was used to characterize and compare the crack arrest fracture toughness. The HSLA-100, Comp. 3 steel alloy had superior performance to the HY-100 steel alloy. The crack arrest reference temperature was T_KIA = −136 °C for the HSLA-100 plate and T_KIA = −64 °C for the HY-100 plate.     

Synthesis, characterization and mechanical properties of nano alumina particulate reinforced magnesium based bulk metallic glass composites

Mg₆₇Zn₂₈Ca₅ bulk metallic glass reinforced with 0.66-1.5 vol% of nano alumina particulates were successfully synthesized using disintegrated melt deposition technique. Microstructural characterization revealed reasonably uniform distribution of alumina particulates in a metallic glass matrix. The reinforced particles have no significant effect on the glass forming ability of the monolithic glass matrix. Mechanical characterization under compressive loading showed improved micro hardness, fracture strength and failure strain with increase in nano alumina particulate reinforcement. The best combination of strength, hardness and ductility was observed in Mg/1.5 vol% alumina composite with fracture strength of 780 MPa and 2.6% failure strain.     

Mechanical properties of 3D fiber reinforced C/SiC composites

High toughness and reliable three dimensional textile carbon fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. Mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.0–2.1 g cm⁻³ after the three dimensional carbon preform was infiltrated for 30 h. The values of flexural strength were 441 MPa at room temperature, 450 MPa at 1300°C, and 447 MPa at 1600°C. At elevated temperatures (1300 and 1600°C), the failure behavior of the composites became some brittle because of the strong interfacial bonding caused by the mis-match of thermal expansion coefficients between fiber and matrix. The shear strength was 30.5 MPa. The fracture toughness and work of fracture were as high as 20.3 MPa m^1/2 and 12.0 kJ·m⁻², respectively. The composites exhibited excellent uniformity of strength and the Weibull modulus, m, was 23.3. The value of dynamic fracture toughness was 62 kJ·m⁻² measured by Charpy impact tests.     

Microstructure and mechanical properties of silicon nitride-titanium nitride composites prepared by spark plasma sintering

Si₃N₄-TiN composites were prepared by spark plasma sintering (conventional sintering (SPS1) and in situ reaction sintering (SPS2)). Homogeneous distribution of equiaxed TiN grains in Si₃N₄ matrix results in the highest microhardness (21.7 GPa) and bending strength (621 MPa) of sample SPS1 sintered at 1550 °C. Dispersion of elongated TiN grains in Si₃N₄ matrix results in the highest fracture toughness (8.39 MPa m^1/2) of sample SPS2 sintered at 1300 °C.     

Failure criteria for mixed mode delamination in glass fibre epoxy composites

Critical strain energy release rate of glass/epoxy laminates using the virtual crack closure technique for mode I, mode II, mixed-mode I + II and mode III were determined. Mode I, mode II, mode III and mixed-mode I + II fracture toughness were obtained using the double cantilever beam test, the end notch flexure test, the edge crack torsion test and the mixed-mode bending test respectively. Results were analysed through the most widely used criteria to predict delamination propagation under mixed-mode loading: the Power Law and the Benzeggagh and Kenane criteria. Mixed-mode fracture toughness results seem to represent the data with reasonable accuracy.     

A study of the effect of the electro-mechanical loading history on the fracture strength of piezoelectric material

Many studies to grasp and describe the fracture behavior of piezoelectric materials under electro-mechanical loading have been done. Although the crack energy density (CED) theory predicts that the mechanical and electrical CEDs can depend on the loading history, the effect of electro-mechanical loading history on the fracture strength of piezoelectric materials has not been studied. Therefore, in this paper, a fracture criterion based on the mechanical contribution of CED (CEDM) is introduced. Its applicability is studied by analyzing the results of three-point bending test regarding the loading path dependence of the fracture strength of piezoelectric ceramic. From the results of (E → M) and (M → E) tests for a C-21 piezoelectric ceramic specimen, it was found that the fracture behavior of piezoelectric ceramics depend on the loading history. The results further showed that the effect of the electric field on the fracture strength of piezoelectric ceramic in the (M → E) test was larger than that in the (E → M) test. Results from linear FE analyses, which assumed that the fracture-initiating load was the same, indicated that the CEDM values increased linearly from negative to positive, and the slopes of CEDM in descending order were: linear (M → E) analysis > linear (M, E) analysis > linear (E → M) analysis.     

The thermal shock resistance of the ZrB2–SiC–ZrC ceramic

In the present work, the thermal shock resistance of the ZrB₂–SiC–ZrC ceramic was estimated by the water quenching method and the flexural strength of the quenched specimen was measured. The measured critical temperature difference of the ZrB₂–SiC–ZrC ceramic was significantly greater than that of the ZrB₂–15 vol.% SiC ceramic. The improvement in thermal shock resistance was attributed to its higher fracture toughness (6.7 MPa m^1/2) and lower flexural strength (526 MPa) relative to the ZrB₂–15 vol.% SiC ceramic (4.1 MPa m^1/2 and 795 MPa) based on Griffith fracture criterion. Furthermore, the temperature and thermal stress distributions in the specimen during instantaneous water quenching were simulated by Finite element analysis.     

Effect of particle size and weight fraction on the flexural strength and failure mode of TiO2 particles reinforced epoxy

Effect of inclusions size and weight fraction on flexural strength and failure mode of composite containing SC-15 epoxy resin and TiO₂ particles has been studied in this investigation. The sizes of particles varied from macro (0.02 mm) to nano (5 nm) scale, and these particles were infused into the part-A of SC-15 through sonic cavitations and then mixed with part-B of SC-15 by using a high speed mechanical agitator. Three-point bending tests were performed on unfilled, 0.5 wt.%, 1.0 wt.% and 1.5 wt.% particles filled SC-15 epoxy to identify the loading effect on mechanical properties of the composites. Results show that 1.0 wt.% nanoparticles reinforced epoxy exhibit the highest mechanical performance. Higher than 1.0%, strength of composite decreased because of poor dispersion. Experimental results also shown that micro-sized particles have little effect on strength of epoxy at such low loading, and strength of composite increased as the size of particles decreased to nano scale. However, degradation in strength was found in 5 nm TiO₂/epoxy system due to agglomeration.