Cyclic Loading In Water Research Articles

Corrosion fatigue investigations on cast, forged and welded nickel-martensitic steels for hydro turbines

Facing an increased demand for flexible operation of hydro power plants (HPPs), Francis runners are substantially subjected to operating conditions beyond the traditional operating ranges. In order to ensure a reliable service of major HPP components covering their complete service life, a fundamental understanding of both load and material fatigue behavior is essential. Due to the cyclic loading in water, reliable corrosion fatigue data is required for the runner fatigue assessments. In previous publications, a state-of-the-art corrosion fatigue testing campaign within a collaborative German research project was described and results for a common cast steel used for the manufacturing of hydro runners were presented. The material testing program for the 13 % Chromium and 4 % Nickel steel has advanced since then and important aspects beyond varying water qualities and different stress ratios have been investigated. Within this conference contribution, corrosion fatigue investigations that were carried out on cast, forged and welded nickel-martensitic steels are described and recently gained results are presented.

The influence of FRC base and bonded CAD/CAM resin composite endocrowns on fatigue behavior of cracked endodontically-treated molars

Cracked endodontically treated molars (ETMs) are commonly treated with full crowns. Less invasive techniques could include fiber-reinforced composite (FRC) base and adhesively bonded endocrowns. The purpose of this study was to assess the fatigue resistance of ETMs restored with CAD/CAM resin composite endocrowns and reinforced with different FRC bases. Cracks were simulated in fifty mandibular molars by cutting the teeth longitudinally and reassembling the two parts. After performing endodontic treatments and preparing cavities, teeth were restored in one of the following methods (n = 10). In Group 1, cavity floors were lined with 0.5 mm of flowable composite (Universal Injectable; GUI, shade A2). In Groups 2 and 3, the cavity floors were covered with one and three layers of FRC-net (ES Net) respectively, whereas in Groups 4 and 5 with 1 mm and 2 mm of flowable FRC-resin (EverX Flow Dentin; EXFD) respectively. Endocrowns (Cerasmart 270; CE, A2 HT) with 5 mm thickness were adhesively luted with preheated composite and all specimens were subjected to cyclic loading in water at 5 Hz, starting with a load of 600 N (5000 cycles), followed by stages of 1000, 1300, 1600, 1900, 2200 and 2500 at a maximum of 20 000 cycles each. Results were analyzed using Kaplan-Meier survival analysis and the Log-Rank test (p = 0.05). The fatigue failure load of ES Net applied in three layers (1617 N ± 371) or in one layer (1499 N ± 306), as well as EXFD applied in 2 mm (1496 N ± 264) or in 1 mm (1434 N ± 372) did not differ significantly (p > 0.05). Control teeth fractured at 1255 N ± 350 (p > 0.05). In the fiber-reinforced groups, restorable fractures were observed in 50%–80%, with only 30% deemed restorable in the control group. The fractures originated mainly at the occlusal contact points and the main crack propagated in the corono-apical direction. No crack origin (primary or secondary) starting from the artificial pre-existing crack was observed in fractured specimens. FRC bases did not improve the fatigue resistance of cracked ETMs, but favorably influenced the failure mode.

Damage sensitivity of dental zirconias to simulated occlusal contact

ObjectiveMechanical damages can occur from dental restoration processing and fitting, or while it is in-service. This study evaluates the damage sensitivity of translucent zirconia (5Y-PSZ) relative to conventional 3Y-PSZ following mouth-motion simulations at various loads. Methods5Y-PSZ and 3Y-PSZ discs were adhesively bonded to a dentin-like substrate and divided into groups according to the load (50 N or 200 N) and number of cycles (up to 106) used in the chewing simulation. Specimens were mounted with 30° inclination in an electrodynamic mouth-motion simulator, and subjected to contact-slide-liftoff cyclic loading in water. Surface and sub-surface damages were analyzed using a sectioning technique. After the simulation, specimens were removed from the substrate and loaded with the damaged surface in tension for biaxial strength testing to assess their damage tolerance. ResultsThe strength of both ceramics underwent significant degradation after mouth-motion simulations. For 5Y-PSZ, the strength degradation was greater (∼60%) and occurred at a lower number of cycles than 3Y-PSZ. Herringbone cracks emerged on 3Y-PSZ and 5Y-PSZ surfaces under a 200-N load after 50 and 10 cycles, respectively. Meanwhile at a 50-N load, cracks formed at ∼1000 cycles in both ceramics. Further increasing the number of cycles only had moderate effects on the strength of both ceramics, despite an increase in surface and sub-surface damage. More significantly, a 50-N occlusal load can debase the zirconia strengths as much as a 200-N load. SignificanceSurface flaws produced during the chewing simulation are capable of significant strength degradation in zirconia, even after a small number of low-load cycles.

The influence of cyclic stress on surface properties of soft liners.

This paper evaluates the changes in surface properties and softness of soft relining materials after cyclic loading in water. 3 polysiloxane (Silagum AM Comfort, Molloplast B, Mollosil Plus) and 2 acrylic-based (Vertex Soft, Astron LC Soft) proprietary soft relining materials and a vinyl polysiloxane (Imprint 2 Garant) as the reference impression material evaluated. A stainless steel block for detail reproduction was used on the basis of the apparatus recommended by the ISO 4823:1992 test method for preparing a standardized surface. A control group from each material was only subjected to water immersion. Non-destructive cyclic loading was carried out with a strain of 16.6% and a frequency of 1.6Hz 200,000 times in distilled water at 37°C. The specimens were then duplicated and compared with the controls using roughness measurements, detail reproduction and Scanning Electron Microscope. Shore A hardness values before and after cycling were also measured. After degradation in the form of mechanical cycling in water, loss of substance and loss of surface detail was observed. Surface properties and Shore A hardness values of acrylic-based soft liners changed significantly (p<0.05). The chemical composition of the soft relining materials seemed to affect their potential of preserving the surface texture and softness after mechanical cycling. Mechanical cycling influences the surface degradation process leading to changes of the surface texture. Polysiloxane-based materials preserved their softness, surface texture and surface smoothness better under cyclic loading compared to acrylic resin -based plasticized materials.

Comparison of fatigue resistance and failure modes between metal-ceramic and all-ceramic crowns by cyclic loading in water

ObjectivesTo compare fatigue resistance and fracture mode of metal-ceramic crowns with all-ceramic crowns containing yttria tetragonal zirconia polycrystal (Y-TZP) frameworks under compressive cycling loading in water. MethodsTwenty specimens of ivory were randomized and individually prepared to receive anatomically shaped metal-ceramic (n=10) or veneered Y-TZP all-ceramic crowns (n=10). All steps in production were equivalent to clinical situations. Resistance to fatigue fracture was tested under compressive cyclic loading using a universal testing machine, with a loading frequency of 12Hz using a spherical tungsten carbide indenter (6mm diameter) in distilled water. The maximum compressive load was increased as the number of cycles increased (600,000 cycles at 400N, 200,000 cycles at 600N, 200,000 cycles at 800N and 200,000 cycles at 1000N). The specimens were inspected after each loading sequence for initial failures such as infractions. Final failure was considered as any loss of material which automatically ended the test and the number of cycles until final failure was recorded. Fractographic analysis of the fractured specimens was performed with scanning electron microscopy (SEM). ResultsThe two types of crowns exhibit similar fatigue resistance (P=0.87) to compressive cycling loading under wet conditions. The failure modes as observed with SEM were similar in the two groups and were found in the veneer ceramic, except that three veneered Y-TZP all-ceramic crowns displayed a complete framework fracture. ConclusionsWithin the limitation of this study using simulated oral masticatory function, the results revealed that the fatigue resistance was similar for the two crown types. Clinical significanceIn this study metal-ceramic crowns and veneered Y-TZP all-ceramic crowns showed similar fracture resistance to compressive cycling loading in water. The test conditions were simulating clinical conditions. Thus, the result may predict the long-term clinical performance of these types of crowns.

Air–particle abrasion on zirconia ceramic using different protocols: Effects on biaxial flexural strength after cyclic loading, phase transformation and surface topography

This study evaluated the effect of different air–particle abrasion protocols on the biaxial flexural strength and structural stability of zirconia ceramics. Zirconia ceramic specimens (ISO 6872) (Lava, 3M ESPE) were obtained (N=336). The specimens (N=118, n=20 per group) were randomly assigned to one of the air-abrasion protocols: Gr1: Control (as-sintered); Gr2: 50 µm Al2O3 (2.5 bar); Gr3: 50 µm Al2O3 (3.5 bar); Gr4: 110 µm Al2O3(2.5 bar); Gr5: 110 µm Al2O3 (3.5 bar); Gr6: 30 µm SiO2 (2.5 bar) (CoJet); Gr7: 30 µm SiO2(3.5 bar); Gr8: 110 µm SiO2 (2.5 bar) (Rocatec Plus); and Gr9: 110 µm SiO2 (3.5 bar) (duration: 20 s, distance: 10 mm). While half of the specimens were tested immediately, the other half was subjected to cyclic loading in water (100,000 cycles; 50 N, 4 Hz, 37 °°C) prior to biaxial flexural strength test (ISO 6872). Phase transformation (t→m), relative amount of transformed monoclinic zirconia (FM), transformed zone depth (TZD) and surface roughness were measured. Particle type (p=0.2746), pressure (p=0.5084) and cyclic loading (p=0.1610) did not influence the flexural strength. Except for the air-abraded group with 110 µm Al2O3 at 3.5 bar, all air-abrasion protocols increased the biaxial flexural strength (MPa) (Controlnon-aged: 1030±153, Controlaged: 1138±138; Experimentalnon-aged: 1307±184–1554±124; Experimentalaged: 1308±118–1451±135) in both non-aged and aged conditions, respectively. Surface roughness (Ra) was the highest with 110 µm Al2O3(0.84 µm. FM values ranged from 0% to 27.21%, higher value for the Rocatec Plus (110 µm SiO2) and 110 µm Al2O3 groups at 3.5 bar pressure. TZD ranged between 0 and 1.43 µm, with the highest values for Rocatec Plus and 110 µm Al2O3 groups at 3.5 bar pressure.

Effect of water temperature on cyclic fatigue properties of glass-fiber-reinforced hybrid composite resin and its fracture pattern after flexural testing.

To evaluate in vitro the influence of dynamic loading applied to a glass-fiber-reinforced hybrid composite resin on its flexural strength in a moist, simulated oral environment. Three-point flexural strength specimens were subjected to cyclic loading in water at 37°C and 55°C to investigate the influence of immersion temperature on impact fatigue properties. Specimens were subjected to cyclic impact loading at 1 Hz for up to 5 × 105 cycles to obtain the number of cycles to failure, the number of unbroken specimens after 5 × 105 cycles, and the residual flexural strength of unbroken specimens. Maximum loads of 100, 200, and 300 N were chosen for both the non-reinforced and the glass-fiber reinforced hybrid composite resins. The mean residual flexural strength for 100 N impact loading at temperatures of 37°C and 55°C was 634 and 636 MPa, respectively. All specimens fractured at fewer than 5 × 105 cycles for loads of 200 and 300 N. Reduced numbers of cycles to fracture and lower fatigue values were observed as both the maximum load and immersion temperature increased.

Cyclic loading of notched dental composite specimens

Objective The purpose of this study was to examine the fracture toughness ( K IC) of three direct dental composites and one indirect dental composite subject to cyclic loading. Methods The composites were a micro-filled (Micronew, Bisco INC., Schaumburg, IL, USA), a hybrid (Renew, Bisco INC.), a nano-filled composite (Filtek Supreme Plus, 3M ESPE, St. Paul, MN, USA) and an indirect dental composite (BelleGlass HP, SDS-Kerr, Orange, CA, USA). Rectangular bar specimens (3 mm × 3 mm × 25 mm) were fabricated, notched, aged (5 months) and cyclic loaded in four different environments, air, water, artificial saliva, and a 50/50 by volume mixture of ethanol and water. Specimens were cyclic loaded for 1, 1000, 10,000, and 100,000 cycles. Results A 3-way ANOVA (non-aged and aged group, four aging media, four loading cycles) showed a significant difference between non-aged and aged, aging media, and loading cycles. For the control groups as the number of cycles increased, there were no significant differences on the number of cycles completed and fracture toughness, except for Micronew, which showed an increased specimen failure rate and decreased fracture toughness. In the aged groups, cyclic loading in water and artificial saliva did not have a significant effect on BelleGlass HP, Filtek Supreme Plus and Renew for fracture toughness and the number of cycles completed, However for Renew in the 50/50 mixture at 100,000 cycles, Filtek Supreme Plus in air and the 50/50 mixture, and Micronew, there was an increased specimen failure rate and a decreased fracture toughness during cyclic fatigue loading as the number of cycles increased. Significance BelleGlass HP displayed the best overall resistance to cyclic loading, followed by Renew and Filtek Supreme Plus, and Micronew.

Fatigue and fracture properties of yttria partially stabilized zirconia crown systems

Objectives The purpose of this study was to test in vitro the fatigue and fracture properties of the Zirconia Everest ® core material after being veneered with a sintered or a heat-pressed veneer material. Methods Fifty zirconium copings were made using Kavo Everest ZS-blanks and the CAD/CAM technology. These were divided equally into two groups. Group one was veneered by a heat-pressed material (IPS e.max ® ZirPress) and group two was veneered by a sintered material (IPS e.max ® Ceram). All the crown shapes were subjected to 50,000 cycles of cyclic loading in water between 20 and 200 N, at a rate of 1 Hz and then loaded dynamically at a crosshead speed of 1 mm/min, until failure. Specimens of the core and the veneering materials were characterized using X-ray diffraction (XRD) and secondary electron imaging (SEI). Results There was no statistically significant difference in the mean failure load ( N ± S.D.) between group 1: 2135.6 ± 330.1 and group 2: 2189.9.1 ± 317.6 ( p > 0.05). No difference was found in the Weibull modulus and characteristic failure load values between the two groups ( p > 0.05). The specimens displayed mainly cohesive failure within the veneering material. XRD of the zirconium powder before sintering revealed tetragonal and monoclinic phases while after sintering only tetragonal zirconia could be identified. Significance There was no difference in the fatigue properties of the Zirconia Everest ® core material following sintering or heat pressing of the veneering material ( p > 0.05).

In vitro lifetime of dental ceramics under cyclic loading in water

All-ceramic dental restorations exhibit enhanced esthetics and biocompatibility as compared to traditional metal-based prosthesis. However, long-term fatigue and subcritical crack growth in the presence of water and cyclic loading can decrease the strength of ceramic components over time. We investigated the cyclic fatigue in water of three dental materials currently used as frameworks in all-ceramic restorations: a 3 mol%-yttria partially stabilized zirconia (3Y-TZP, Cercon, Degudent GmbH), a Al 2O 3–ZrO 2–Glass composite (Inceram-Zirconia, Vita Zahnfabrik GmbH) and a Li 2O·2SiO 2 glass ceramic (Empress 2, Ivoclar Vivadent AG). Fatigue and fast fracture tests were performed to determine the Weibull distribution of lifetime and initial mechanical strength for each framework component. In spite of its noticeable susceptibility to fatigue in water, the 3Y-TZP material was found to be particularly suitable for the preparation of posterior all-ceramic bridges due to its high initial mechanical strength. Guidelines are provided for the selection of materials and the design of all-ceramic posterior bridges exhibiting lifetime longer than 20 years under severe wet and cyclic loading conditions.

Cyclic fatigue in water of veneer–framework composites for all-ceramic dental bridges

Objectives Ceramic materials applied in dentistry may exhibit significant subcritical crack growth due to the severe cyclic loading in the aqueous environment encountered in the mouth during mastication. The authors report on the subcritical crack growth behavior of three dental restoration systems (Empress 2/IPS Eris, TZP/Cercon S and Inceram-Zirconia/Vita VM7) under cyclic loading in water, in order to establish guidelines for the use and design of long-lifetime all-ceramic posterior bridges. Methods Inert strength and lifetime tests under cyclic loading in an aqueous environment were performed in a mechanical bending apparatus and evaluated with Weibull statistics. Results Subcritical crack growth occurred predominantly in the outer veneer layer of the veneer–framework composites. The apatite-based veneer (IPS Eris) was more susceptible to subcritical crack propagation compared to the feldspathic glass veneers (Cercon S and Vita VM7). Significance Dental restoration systems containing apatite-based veneers and weak frameworks (Empress 2/IPS Eris) are not recommended for the fabrication of all-ceramic bridges in the molar region. Conversely, veneer–framework systems consisting of feldspathic glass veneers and tough zirconia-based frameworks (TZP/Cercon S and Inceram-Zirconia/Vita VM7) may exhibit lifetimes longer than 20 years if the bridge connector is properly designed.

Fatigue of zirconia under cyclic loading in water and its implications for the design of dental bridges

Objectives The aim of this study was to evaluate the cyclic fatigue behavior of zirconia (3Y-TZP) in water and derive guidelines for the design of zirconia-based dental bridges with extended lifetime. Methods The subcritical crack growth parameters under aqueous and cyclic loading conditions were determined from Weibull distributions of the initial mechanical strength and of the lifetime of TZP specimens. The strength and lifetime data were obtained using a specially designed bending machine under simple and oscillatory loading conditions, respectively. Results The TZP components submitted to cyclic loading in water exhibited subcritical crack propagation at stress levels significantly (∼50%) lower than the critical stress intensity factor ( K IC = 5.6 MPa m 1/2). In spite of this susceptibility to subcritical crack growth, calculations based on the fatigue parameters and on the stress applied on the prosthesis indicate that posterior bridges with zirconia frameworks can exhibit lifetimes longer than 20 years if the diameter of the bridge connector is properly designed. Significance This in vitro study indicates that partially stabilized zirconia (3Y-TZP) can withstand the severe cyclic loading and wet conditions typically applied in the molar region of the mouth and is therefore an appropriate material for the fabrication of all-ceramic multi-unit posterior bridges.

Competing Fracture Modes in Brittle Materials Subject to Concentrated Cyclic Loading in Liquid Environments: Monoliths

The competition between fracture modes in monolithic brittle materials loaded in cyclic contact in aqueous environments with curved indenters is examined. Three main modes are identified: conventional outer cone cracks, which form outside the maximum contact; inner cone cracks, which form within the contact; and median–radial cracking, which form below the contact. Relations describing short-crack initiation and long-crack propagation stages as a function of number of cycles, based on slow crack growth within the Hertzian field, are presented. Superposed mechanical driving forces—hydraulic pumping in the case of inner cone cracks and quasiplasticity in the case of median–radials—are recognized as critically important modifying elements in the initial and intermediate crack growth. Ultimately, at large numbers of cycles, the cracks enter the far field and tend asymptotically to a simple, common relation for center-loaded pennylike configurations driven by slow crack growth. Crack growth data illustrating each mode are obtained for thick soda-lime glass plates indented with tungsten carbide spheres in cyclic loading in water, for a range of maximum contact loads and sphere radii. Generally in the glass, outer cone cracks form first but are subsequently outgrown in depth as cycling proceeds by inner cones and, especially, radial cracks. The latter two crack types are considered especially dangerous in biomechanical applications (dental crowns, hip replacements) where ceramic layers of finite thickness are used as load-bearing components. The roles of test variables (contact load, sphere radius) and material properties (hardness, modulus, toughness) in determining the relative importance of each fracture mode are discussed.

Deep‐penetrating conical cracks in brittle layers from hydraulic cyclic contact

A study is made of fracture from cyclic loading of WC spheres on the top surfaces of thick (1 mm) brittle layers on polymeric substrates, as representative of repetitive occlusal contact on dental crown structures. The advantage of glass layers is that internal cracks can be followed in situ during the entire cyclic loading process. The glass surfaces are first given a surface-abrasion treatment to control the flaw state, such that the strengths match those of dental porcelains. Cyclic contact tests are carried out at prescribed maximum loads and frequencies, in water. In addition to conventional cone cracks that form outside the contact circle, additional, inner cone cracks form within the contact in the water environment. These inner cones are observed only in cyclic loading in water and are accelerated at higher frequencies, indicating a strong mechanical driving force. They tend to initiate after the outer cones, but subsequently catch up and penetrate much more rapidly and deeply, ultimately intersecting the underlying coating/substrate interface. Comparative tests on glass/polymer bilayers versus monolithic glass, in cyclic versus static loading, in water versus air environment, on abraded versus etched surfaces, and with glass instead of WC indenters, confirm the existence of a dominant mechanical element in the inner-cone crack evolution. It is suggested that the source of the mechanical driving force is hydraulic pressure from intrusion and entrapment of liquid in surface fissures at the closing contact interface. This new type of cone cracking may limit dental crown veneer lifetimes under occlusal fatigue conditions, especially in thicker layers, where competing modes-such as undersurface radial cracks-are suppressed.

Static and Dynamic Fatigue of Glass–Carbon Hybrid Composites in Fluid Environment

Unidirectional glass fiber reinforced and glass-carbon fiber reinforced epoxy matrix composite samples (% glass to carbon fiber volume ratio 90: 10 and 75: 25)were subject to two-point bend rupture test in distilled water at 75 C and tension-tension cyclic test in both air and distilled water at 25 C. In the rupture test, while no obvious difference in rupture time was observed at larger deflections, glass– carbon hybrid samples showed prolonged rupture time in comparison to all-glass samples when loaded at 10% maximum deflection. When compared to samples tests in air, cyclic loading in water resulted in shorter fatigue lives for both all-glass and hybrid composite samples. However, hybrid samples showed better environmental fatigue resistance in water than all-glass samples, to 10 7 cycles. By incorporating appropriate amount of carbon fibers in glass fiber composite, long-term performance of glass–carbon hybrid composites under static and dynamic fatigue is substantially enhanced. It is demonstrated by the present study that the intra-ply hybrid approach is effective and economical in improving the durability of fiber reinforced polymer composites for long term applications.

Influence of core buildup material on the fatigue strength of an all-ceramic crown

Statement of Problem. Clinically relevant variables and testing methods have not been used to investigate the effects of core buildup materials on the strength of all-ceramic restorations. Purpose. The aim of this study was to evaluate the compressive strength of Optimal Pressable Ceramic (OPC) all-ceramic crowns supported by 1 of 3 different core materials and subjected to static or cyclic loading in air or water. Material and Methods. A total of 135 human extracted third molar teeth were divided into 3 equal groups, and cores of amalgam, composite, and dentin (control) were fabricated. Complete crown preparations with a circumferential 90° shoulder margin 1 mm deep were prepared to support complete crown restorations. Testing in compression was performed at a loading rate of 2.0 mm/min. Sixty teeth (20 with each type of core) were subjected to static loading, with equal numbers tested in air and water. The remaining 75 teeth (25 with each type of core) were subjected to cyclic loading in water. The compressive and diametral tensile strengths of 6 × 12-mm OPC cylinders also were tested in both air and water. The data were analyzed with 1-way analysis of variance followed by the Tukey multiple means comparison test (P<.05). Results. Analysis of the static fracture stress of OPC crowns indicated no significant differences among the core materials (values from testing in air and water, respectively: dentin core 22.7 ± 4.5 MPa and 19.7 ± 4.7 MPa; amalgam core 25.2 ± 5.2 MPa and 21.6 ± 6.3 MPa; and composite core 20.9 ± 4.1 MPa and 18.7 ± 4.9 MPa). Under wet conditions, static versus cyclic loading produced significantly different results for each of the core materials (dentin core 19.7 ± 4.7 MPa vs. 9.5 ± 1.5 MPa; amalgam core 21.6 ± 6.3 MPa vs. 10.3 ± 1.6 MPa; composite core 18.7 ± 4.9 MPa vs. 13.3 ± 3.2 MPa). A significant difference was observed for the solid compressive cylinders tested in water (793.8 ± 280.1 MPa) versus in air (1473.6 ± 527.4 MPa). Conclusion. Within the limitations of this study, the mean compressive strength of OPC crowns tested in water was significantly different under cyclic versus static loading. No significant difference among the 3 core materials was found with respect to crown compressive strength. (J Prosthet Dent 2001;86:624-31.)

Environmental fatigue behavior and life prediction of unidirectional glass–carbon/epoxy hybrid composites

Unidirectional glass fiber reinforced and glass–carbon fiber reinforced epoxy matrix composite specimens were subjected to tension–tension fatigue in air and in distilled water at 25 °C. While no significant change in fatigue life was observed for both types of specimens tested in air and in water when cyclically tested at 85% of average ultimate tensile strength (UTS), the detrimental effect of water becomes apparent at lower stress levels of 65 and 45% UTS. Compared to specimens tested in air, cyclic loading in water results in shorter fatigue lives for both glass and hybrid specimens. While all of the glass fiber specimens did not survive to 10 6 cycles when cyclically loaded in water, hybrid specimens (with 25% carbon fiber (by volume), 75% glass fiber (by volume), 30% total fiber volume fraction) showed better retention in structural integrity under environmental fatigue, for fatigue lives up to 10 7 cycles, a consequence of the corrosion resistant of carbon fiber. Thus it is shown, by incorporating appropriate amount of carbon fibers in glass fiber composite, a much better performance in fatigue can be achieved for glass–carbon hybrid composite. A simple life prediction model for the hybrid composite is proposed. Model predictions are compared with experiments results from both laminated interply and intermingled intraply hybrid composites. Results suggest that synergistic effect of the reinforcing fibers is critical in governing the fatigue behavior of intraply hybrid composite.

Environmental fatigue of unidirectional glass–carbon fiber reinforced hybrid composite

Abstract Unidirectional glass fiber reinforced and glass–carbon fiber reinforced (glass to carbon fiber volume ratio 3:1) epoxy matrix composite samples were subjected to tension–tension fatigue in air and in distilled water at 25°C. While no significant change in fatigue life was observed for both types of samples tested in air and in water when cyclically tested at 85% of average ultimate tensile strength (UTS), the detrimental effect of water becomes apparent at lower stress levels of 65 and 45% UTS. Compared to samples tested in air, cyclic loading in water results in shorter fatigue lives for both glass and hybrid samples. While all of the glass fiber composite samples did not survive to 106 cycles when loaded in water, hybrid samples showed better retention in structural integrity under environmental fatigue for lives up to 107 cycles, a consequence of the corrosion resistant of carbon fiber. Thus, by incorporating appropriate amount of carbon fibers in glass fiber composite, a much better performance in fatigue can be achieved for glass–carbon hybrid composite.

Ａｌ２Ｏ３セラミックスの疲労破壊特性に及ぼす微視組織の影響

In order to improve the fatigue fracture characteristics of Al2O3, the fracture behavior of two kinds of Al2O3, whose microstructure was controlled by sintering additives, was investigated under static and cyclic loading. It was observed that there was a significant difference in fracture behavior between the fine equiaxial grained Al2O3 (Material M) and the coarse columnar grained one (Material SC). The examination of fatigue lifetime showed that the fatigue strength of Material SC was higher than that of Material M, but simultaneously a significant decrease by cyclic loading. The crack propagation rate da/dt of Material M made no difference between in air and in water, but that of Material SC accelerated in water. According to the results of an energy dispersive X-ray spectroscopy (EDX) analysis on the fracture surface of Material SC, it was found that the sintering additive components of Si and Ca decreased markedly in SCG region under cyclic loading in water, and it was expected that hydration reaction on the glass phase of grain boundary occurred actively by water. Consequently, the reason why the fatigue strength of Material SC decreased under cyclic loading in water was considered to the significant promotion of stress corrosion cracking at the crack tip which is caused by marked decrease of particle bridging effect (i.e., increase in KItip).