Types Of Fracture Modes Research Articles

쇼트피닝 처리를 한 베어링강의 고사이클 피로수명 및 파괴거동

Shot-peening effects on the fatigue behavior of bearing steel were investigated under the high cyclic loading. Hourglass shape specimens were made of bearing steel(JIS-SUJ2) for rotary bending fatigue tests. Two kinds of treatments were performed : a heat treatment and a shot-peened surface treatment after the heat treatment. The fracture surfaces of specimens were classified into two types of fracture mode : the surface fracture mode induced by a surface defect and the internal fracture mode induced by a nonmetallic inclusion. Inclusion depth and shape affected considerably the fatigue life. Shot-peening treatment improved much the fatigue life of the bearing steel under low and high levels of cyclic loads. Probabilistic-stress-life (P-S-N) curves were suggested for the reliable fatigue life estimation of the improved bearing steel.

Effects of Isothermal Aging on Tensile Properties and Fracture Behavior of 316L Austenitic Stainless Steel Weld Metal

In this research, variations of tensile properties and fracture behavior of 316L austenitic stainless steel weld metal as a function of aging temperature and time have been investigated. Stainless steel plates were butt-welded using GTAW process. Weld metal tensile specimens were subjected to various aging heat treatments at temperatures of 750 and 850°C for periods of 1 to 100 hours prior to tensile tests at 25 and 500°C. Dissolution of delta-ferrite and formation of sigma phase network during aging resulted in a mild increase in tensile strength and significant reduction in ductility, particularly at 25°C. Although fracture surfaces exhibited ductile features, the dimple morphology and the macroscopic fracture mode were found to be affected by aging. The unaged weld metal exhibited a classic mode of cup and cone fracture, whereas slant and flat types of fracture modes were observed in the aged weld metals, with the slant mode being dominant at 500°C. The slant mode was associated with deformation localization along arrays of primary voids, nucleated at cracked sigma phase particles, oriented at about 45° to loading direction. The transition in the fracture mode is further discussed based on variations in the dimple morphologies and strain hardening exponent.

Very high cycle fatigue properties of bearing steel under axial loading condition

It has been observed to appear a step-wise or duplex S– N curve under the test of rotary bending fatigue using high strength steel. This behavior was caused by the transition of fracture mode from surface-induced fracture to subsurface inclusion-induced fracture. The aim of this study is to clarify the S– N characteristic under an axial loading fatigue in the very high cycle fatigue regime. In order to investigate the mean-stress effects, fatigue tests were carried out in air at room temperature under three applied stress ratios of −1, 0 and 0.5 using a hour-glass shaped specimen of high carbon–chromium bearing steel, JIS SUJ2. From the results, three types of fracture mode were observed on the fracture surface, such as surface-induced fracture, subsurface inclusion-induced fracture without granular bright facet (GBF) area and that with GBF area around an inclusion. Fatigue lifetime for transition in the fracture mode depended on the applied stress ratio. Shape of the S– N curve was a smooth and continuous under three testing conditions in spite of the occurrence on the three types of fracture. Detail discussion for fatigue fracture behavior was made through the observation of fracture surface and from point in view of the fracture mechanics. In addition, an effect of residual stress in the specimen surface layer on the transition of fracture mode was discussed and compared with the experimental results.

Study of Delamination of Thin Film Coating on Cyclic Nano-Indentation Test

The aim of this paper is to evaluate the cyclic interfacial strength between thin film and its substrate by cyclic nano-indentation tests. The specimen used in this study is PET substrate/ITO coatings layered specimen. From the indentation load and displacement curve, we proposed an evaluation method for the interfacial strength. The results are good agreement with the interfacial strength evaluated by peel test. After cyclic indentations, the surface profile was observed by atomic force microscope. The number of elongates increased with indentation cycles when the indentation load is low, whereas elongates number is almost constant under high load cases. These phenomena can be explained by simple models. In this study, two types of fracture modes are proposed. They are “subsidiary fracture mode” and “buckling mode”.

Effect of Stress Ratio on Axial-Loading Fatigue Characteristics of High Carbon-Chromium Bearing Steel in Very High Cycle Fatigue Regime

Axial-reversed loading fatigue tests under three applied stress ratios of –1, 0 and 0.5 were carried out in air at room temperature using hour-glass shaped specimen of high carbon-chromium bearing steel, JIS SUJ2, to investigate fatigue fracture characteristic in very high cycle regime. A granular bright facet (GBF) was observed around subsurface inclusion in all specimens ruptured at long-life fatigue over 106cycles. Three types of fracture mode were observed from the fracture surface, such as surface-induced fracture, subsurface inclusion-induced fracture without GBF area and that with GBF area. Number of cycles to transition in the fracture mode depended on the applied stress ratio. Shape of the S–N curve obtained from the axial loading fatigue was a smooth and continuous in spite of the occurrence on three types of fracture mode, and was different from that of rotary bending fatigue. The characteristic of fatigue fracture behavior was discussed from point of view in fracture mechanics and also residual stress distribution on the specimen surface.

Experimental and Numerical Investigation of the Behaviour and Strength of Composite Steel-CFRP Members

The use of advanced composite materials to strengthen and repair existing structures is increasing rapidly. One specific area in which the technique has recently been introduced is the strengthening of metallic structures with bonded carbon-fibre laminates. In this paper, the behaviour of composite steel-CFRP members is studied experimentally and using FE analysis. A new type of test specimen has been developed to examine the effect of various material parameters on the behaviour and strength of bonded steel-CFRP elements. Finite element analysis of the tested elements was also employed to examine the type and magnitude of interfacial stresses in the bond line of these elements. Moreover, different types of fracture mode could be examined by testing composite elements with different combinations of CFRP-laminates and adhesives. The tested composite elements displayed different behaviour and a large difference in strength and ductility could be observed.

Characteristics of Low Temperature Brittle Fracture Surface of High Nitrogen Austenitic Steel

Fracture surface and crack propagation in low temperature brittle fracture (LTBF) of an 18Cr-18Mn-0.7N high nitrogen austenitic steel (HNAS) were examined by means of scanning electronic microscopy, and compared with behaviours of LTBF of low carbon steel. Similar to BCC low carbon steel, the HNAS experienced a typical ductile-to-brittle transition (DBT) with decreasing temperature, and the appearance of the fracture surface transited from fibrous to granular. Dual-surface observation revealed that there were three types of fracture modes in LTBF of the HNAS: annealing twin boundary fracture, intergranular fracture, and transgranular fracture. The annealing twin boundary fracture facets were parallel to {111} planes, and were fairly flat and smooth, with a pattern of three sets of parallel straight-lines intersecting at 60. There were also bent steps that originated and terminated at grain boundaries. The transgranular fracture facets were coarse and uneven, with uniformly distributed small pits and partially river pattern on them. The intergranular fracture facets were smoothly curved ones on which more than three sets of parallel deformation structure trace lines were observed. Careful observation on crack propagation demonstrated that during LTBF of the HNAS, microcracks formed firstly at grain boundary and annealing twin boundary, and then these microcracks came together and coalesced to induce crack propagation through grains, resulting in a fracture appearance with shiny facets distributing in dull facets.

Plane bending fatigue behavior of interstitial-free steel at room temperature

Abstract Interstitial-free steels are widely used as thin sheet for automobile external panels. For this type of application, essential properties of the steel are deep-drawability and stretch-formability. However, low-strength interstitial-free steel is nearly pure iron and work on fatigue fracture behavior of this steel might be adopted as a basic study to understand the behavior of high-strength steels used for structural components. In this study, the fatigue fracture behavior of low-strength interstitial-free steel has been studied at room temperature in air and in an argon atmosphere. Fractographic observations on fatigue fracture surfaces reveal a mixed type of fracture mode (intergranular and transgranular cracking) for all specimens. The major portion of intergranular fracture has been found at the regions of low and medium crack lengths. Further increase in the crack length causes a decrease in the proportion of intergranular cracking and, finally, the fracture mode becomes completely transgranular. At the same test temperature, the proportion of intergranular fracture drastically decreased when the test was carried out in the argon atmosphere.

Fatigue Strength and Fracture Mechanisms of IF28 Steels

Interstitial free (IF) steels are widely used as thin sheet in the automobile industries because of their many favorable properties. Although, fatigue properties of IF steels do not have significant importance to auto body makers, however, they are very concerned about the tensile and fatigue strength of the steels used for structural purposes to ensure safety of passengers. So, fatigue results of this steel might help researchers to understand the behaviors of high strength steels. In this study cyclic and static properties have been studied at room temperature in the air. Initiative has also been taken to observe the fatigue fracture morphology of this steel. Experimental results show that the fatigue limit is corresponding to about 40% of tensile strength and 80% of the yield strength of this steel. Fractographic observations reveal a mixed type of fracture mode (intergranular and transgranular cracking) fractures.

3013 繰り返しナノインデンテーションによるコーティング薄膜のはく離(S38 皮膜の磨耗および残留応力特性(2),S38 コーティング材料の皮膜特性とその評価)

The aim of this paper is to evaluate the cyclic interfacial strength by cyclic nanoindentation tests. The specimen used in this study is PET substrate/ITO coatings layered specimen. In this study mono cycle nanoindentation test and 10 cycle nanoindentation were conducted on each load, and these results were compared. From indentation load and displacement curves, we classified those results into 3 types (A-C). These three types relate the maximum loads. After the cyclic indentation, we observed the surface profile by the function of atomic force microscope. On Type A, the deviations were observed on the edge on indentations. On Type B, some elongates were observed in the indentations. In addition the number of elongates increased by repeating nanoindentation. These phenomena can be explained by simple models. In this study, two types of fracture modes are proposed. They are "subsidiary fracture mode" and "buckling mode". From the deviation of indentation load and displacement curve, we proposed the method to estimate the energy used for thin film delamination and the interfacial strength was evaluated. The results are good agreement with the interfacial strength evaluated by peel test.

Tensile test behavior of the eutectic Sn-Ag solder joint in ball grid array assemblies

In this study, the mechanical behavior of a eutectic Sn-3.5Ag ball-grid array (BGA) assembly was evaluated by tensile testing at strain rates of 10−4 and 10−5 s−1 at various homologous temperatures. The maximum strength of the eutectic Sn-3.5Ag ball-grid array assembly decreased as the testing temperatures increased and the testing strain rates decreased. Two types of fracture modes were found during the tensile tests depending on the strain rates and homologous temperatures. At a strain rate of 10−4 s−1 and at homologous temperatures lower than 0.65, the crack propagation occurred at the angle 45 deg with the tensile axis and along the Ag3Sn/Sn interfaces. However, at strain rates lower than 10−4 s−1 or at homologous temperatures higher than 0.65, the tensile crack formed by linking the creep voids that propagate normal to the tensile axis. A transgranular creep fracture was found on the failed surfaces of Sn-3.5Ag ball-grid array assemblies.

Gigacycle Fatigue Behavior and Fracture Morphology of High Speed Tool Steel, JIS SKH 51.

Gigacycle fatigue characteristics of high speed tool steel, JIS SKH 51, was investigated under cantilever-type rotary bending fatigue tests in an open environment at room temperature. As a result of fatigue tests, S-N curve was clearly classified into two types of fracture modes by the different crack origins. One fracture mode occurred at the surface inclusion in the region of short fatigue life and high stress amplitude level. The other was at the subsurface inclusion in the region of long fatigue life and low stress amplitude level. The transition of crack initiation site from surface to subsurface appeared at about 2×104 cycles and the stress amplitude for the transition was affected by the tempering temperature of the specimen. On the other hand, difference on fatigue life for internal fracture mode between specimens treated by different tempering temperature did not observed. Fatigue fracture mechanisms were discussed through the detailed observation of subsurface crack initiation site.

923 低速衝撃を受ける合わせガラスの破壊挙動

Impact fracture tests of laminated glass were performed using an instrumented drop weight testing apparatus. Different types of fracture mode were obtained by changing input energy and fracture absorbed energy was evaluated for each of the fracture modes. The experimental results clearly exhibited that large amount of energy absorbed through the deformation and fracture of the middle polymeric layer results in higher impact resistance of laminated glass than that of single glass. The result of strain measurement on the glass surfaces showed that the free glass layer fractured prior to the impact layer under this impact condition.

211 スポット溶接部材の強度と破断様式に及ぼす因子の検討(O.S.5 : 溶接・接合材の強度および力学的特性の評価)

In order to clarify the relation between strength and fracture mode, the tensile-shear test of spot welds and stress analysis has been done. The results showed that the bending moment and axial force induced by tensile-shear test affected 3 types of fracture modes (shear type fracture, plug type fracture and fracture across base metal), which depended upon the material properties, the specimen geometry, weld size, position and numbers of spot welds. As the results, the strength of spot weld changes with fracture mode.

K-0517 高強度鋼SNCM439における超高サイクル疲労特性に関する研究(S04-2 高強度鋼の超高サイクル疲労(1))(S04 金属材料の組織と疲労強度信頼性)

The present study investigates giga-cycle fatigue properties of a high strength steel SNCM 439, a Ni-Cr-Mo steel known as a bearing steel. Cantilever rotating bending fatigue tests have been made on the SNCM 439 steel at room temperature in air. The characteristic duplex S-N diagram has been obtained for the SNCM 439 steel. Two types of fracture modes were observed according to the applied stress level; i.e., surface fracture was observed at stress levels higher than 1200 MPa, whereas internal fracture at those lower than 1200 MPa. At the lower stress levels, fish-eye cracks were observed as the origin of fatigue fracture and were always initiated at Al_2O_3 inclusions in the present study. Statistical distributions of fatigue lives of the present high strength steel were determined at five stress levels by the use of Weibull probability paper. It was found that fatigue life followed the composite Weibull distribution at the five stress levels investigated, indicating that the fracture mode is different at higher and lower stress levels.

The effects of strain rate on the mechanical behaviour of kevlar fibre bundles: an experimental and theoretical study

Abstract The stress–strain curves of kevlar49 aramid fibre bundles under both quasi-static loading and high rate loading were obtained. The strain rate ranges from 10−4/s to 103/s. Based on a bimodal Weibull distribution statistical model of strain-rate dependence of fibres and a test method of determining mechanical properties and Weibull parameters of fibres from fibre bundle test, the rate dependence of kevlar49 fibre strength is studied. Experimental results show that kevlar49 fibre has sensitivity to strain rate. The peculiar `skin-core' structure of kevlar aramid fibre and experimental Weibull data confirm that there are two types of fracture mode in kevlar fibre and the application of bimodal Weibull distribution is correct. Consistence of simulated and experimental results indicates that the model is valid to explain the mechanical behaviour of kevlar49 fibre and the test method is feasible and reliable.

Trial Fabrication of Particle Impact Test Rig and Evaluation of Impact Fracture Strength of Ceramics.

The application of structural ceramics to gas turbine components is currently expected as a means of improving peformance. Due to its low toughness, however, an assessment of the resistance against damage caused by foreign objects is requied. In order to investigate the damage caused by impact from a projectile, we tentatively fabricate a particle impact test rig, which is a gas-gun-type accelerator operated with air or helium gas. It can accelerate projectiles up to a velocity that is almost equal to the speed of the tip of a gas tubine moving-blade. In this study, the single -particle fracture test for four kinds of materials was made on this test rig. Two types of fracture modes were obsreved. One is the fracture with cracks propagating conically from the contact point and the other is that with the cracks propagating radially from the opposite side of the impact location. The critical velocity of each fracture mode is explained as a function of projectile size, target thickness and mechanical properties such as flexural strength or fracture toughness. Furthermore, from the viewpoint of practical operation, the influence of compressive stress field on critical velocity is discussed.

Fabrication of an Al-4.5Cu/15SiC composite: II. Effects of the processing parameters on the fracture properties

The mechanical properties of Al-based composites obtained from mechanical alloying Al-4.5 wt% Cu with 15 wt% SiC reinforcement (part I of this two-part paper) have been investigated. Test samples in the form of 67 mm diameter disks were employed to investigate the fracture properties of the composite using the fracture-mechanics approach. It was found that the fracture toughness of the composite decreased with mechanical alloying time due to the existence of porosity. From fractographic and metallographic studies using scanning electron microscopy, three types of fracture modes were observed, namely: (i) fracture of the SiC; (ii) fracture due to the coalescence of micro-voids in the matrix; and (iii) fracture due to debonding between the matrix and the SiC particles. Fracture due to debonding was found to be the dominating feature for the range of mechanical alloying times investigated.

Creep deformation of polycrystalline mullite

Creep tests for polycrystalline mullites (3Al2O3.2SiO2; average grain sizes, d=1·4 and 2·1 μm) were carried out in air at temperatures between 1365 and 1480°C. The dependence of steady-state strain rates, ϵ, on stress and grain size was given by ϵασ/d2·5 at lower temperatures (<1460°C) and lower stresses (<100 MPa), suggesting the deformation mechanism is diffusional creep. Significant cavitation was not observed for deformed specimens. The effective diffusion coefficients were calculated using the rate equation for diffusional creep of mixed oxides, showing very high activation energy (810 kJ mol−1). The strain rate apparently increased at higher stresses where fracture often occurred during creep testing within small beam deflections. The SEM examination of fractured specimens revealed two types of fracture mode: slow crack growth pattern observed on the fracture surface of specimens with the small grain size, and cavitation at grain boundaries (intergranular separation) observed on the tensile surface of specimens with the larger grain size. An polykristallinem Mullit (3Al2O3.2SiO2 mit einer durchschnittlichen Korngröße von 1·4-2·1 μm) wurden zwischen 1365 und 1480°C an Luft Kriechversuche durchgeführt. Die Abhängigkeit der stationären Kriechgeschwindigkeit ϵ von der Spannung und der Korngröße bei tieferen Temperaturen (<1460°C) und geringeren Spannungen (<100 MPa) ergab sich zu ϵασ/d2·5 was auf Kriechen durch Diffusion als Verformungsmechanismus schließen läßt. Nennenswerte Porenbildung an Korngrenzen wurde für die verformten Proben nicht beobachtet. Die tatsächlichen Diffusionskoeffizienten wurden aus der Gleichung für Kriechen durch Diffusion gemischter Oxide berechnet und zeigten sehr hohe Aktivierungsenergien (810 kJmol−1). Die Verformungsgeschwindigkeit nahm offensichtlich bei höherer Spannung, wo der Bruch oft während des Kriechversuchs bei kleinen Auslenkungen stattfand, zu. Die rasterelektronen-mikroskopische Untersuchung gebrochener Proben zeigte zwei verschiedene Bruchmuster: an der Bruchfläche der Proben mit der kleinen Korngröße wurden Merkmale langsamen Rißwachstums beobachtet, an den Proben mit der größeren Korngröße wurde an der Zugseite der Proben Porenbildung an der Korngrenze (Separation der Körner) beobachtet. On a effectué des essais de fluage sur des mullites polycristallines (3Al2O3.2SiO2; tailles moyennes des grains d = 1·4 et 2·1 μm) dans l'air à des températures comprises entre 1365 et 1480°C. La vitesse de déformation en régime permanent ϵ en fonction de la contrainte et de la taille des grains est donnée par la relation ϵασ/d2·5 à basses températures (<1460°C) et pour des contraintes faibles (<100 MPa), suggérant que le mécanisme de déformation est un fluage diffusionnel. On n'a pas observé de cavitation notable dans les échantillons déformés. Les coefficients de diffusion effectifs ont été calculés à partir de l'équation de vitesse de fluage diffusionnel pour les mélanges d'oxydes, et présentent une énergie d'activation très élevée (810 kJ mol−1). La vitesse de déformation croît à l'évidence pour des contraintes élevées où la rupture se produit souvent durant l'essai de fluage à faible allongement de l'éprouvette. L'examen des échantillons révèle deux modes de rupture: propagation lente des fissures observée sur la surface de rupture des échantillons à grains fins et cavitation aux joints de grains (décohésion intergranulaire) sur la surface en traction des éprouvettes à gros grains.

The evaluation of the strength distribution of silicon carbide and alumina fibres by a multi-modal Weibull distribution

The strengh distributions of silicon carbide and alumina fibres have been evaluated by a multimodal Weibull distribution function. This treatment is based on the concept that the fracture of the fibre is determined by competition among the strength distributions of several kinds of the defect sub-population. Since those fibres were observed to have two types of fracture mode, the evaluation of a bi-modal Weibull distribution was performed in comparison with the single Weibull distribution usually employed. The accuracy of the fit for these two distributions was judged from maximum logarithm likelihoods and cumulative distribution curves. The result showed that the logarithm likelihood calculated using the bi-modal Weibull distribution function gave a larger value, as compared with those using the single Weibull distribution function. The curve predicted from the former function was also in good agreement with the data points. In addition, the strength distribution and the average value at a different gauge length were extrapolated from the Weibull parameters estimated at the original gauge length. In this case, also, the bi-modal Weibull distribution gave a more accurate prediction of the data points.