Experimental and numerical investigation of cracked chevron notched Brazilian disc specimen for fracture toughness testing of rock

The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to quantify mode I fracture toughness (K_Ic) of rock, and it has also been applied to mode II fracture toughness (K_IIc) testing in some research on the basis of some assumptions about the crack growth process in the specimen. However, the K_Ic value measured using the CCNBD specimen is usually conservative, and the assumptions made in the mode II test are rarely assessed. In this study, both laboratory experiments and numerical modeling are performed to study the modes I and II CCNBD tests, and an acoustic emission technique is used to monitor the fracture processes of the specimens. A large fracture process zone and a length of subcritical crack growth are found to be key factors affecting the K_Ic measurement using the CCNBD specimen. For the mode II CCNBD test, the crack growth process is actually quite different from the assumptions often made for determining the fracture toughness. The experimental and numerical results call for more attention on the realistic crack growth processes in rock fracture toughness specimens.     

Fracture toughness measurement by cylindrical specimen with ring-shaped crack

A method for measuring the plane strain fracture toughness of metals by means of cylindrical specimen in tension with axi-symmetrical ring-shaped crack is discussed. Owing to the fact that the crack tip of such a specimen is closer to ideal plane strain state, the K_1c value measured is effective and reliable. This investigation has fairly satisfactorily solved the problems of crack prefabrication, experimental technique, data processing and requirements for specimen dimensions.In both safety evaluation and life estimation of engineering components by linear elastic fracture mechanics, it is necessary to measure the fracture resisting parameter—fracture toughness under plane strain. According to the ASTM-E399-74 standard[1], when measuring the fracture toughness K_1c values of medium and low strength steels with a standard compact tension specimen or three-point bending specimen, it is necessary to use specimens of large dimensions, great tonnage fatigue testing machine and universal testing mechine. Naturally, this presents great difficulties to the investigation and application of fracture mechanics and it is precisely for the purpose of overcoming these difficulties that we have studied the method of measuring the plane strain fracture toughness by a cylindrical specimen in tension with axi-symmetrical ring-shaped crack. This method has fairly satisfactorily solved the problems of crack prefabrication, experimental technique, data processing and requirements for specimen size. Owing to the fact that the field around the crack tip of such a specimen is closer to ideal plane strain state, the results obtained are values smaller than those by using compact tension and three-point bending specimens and are more reliable fracture resisting constants for materials in linear elastic fracture mechanics analysis. Moreover, this method is more practical and economical because no expensive large fatigue testing machine is needed and the specimen size is small.     

Fracture behavior of as-cast pearlitic nodular iron

The fatigue crack propagation and fracture toughness behavior of an ASTM A536, as-cast, pearlitic nodular iron with a bull's-eye structure was studied. The material had an ultimate strength of 76 ksi, a yield strength of 59 ksi, and an elongation of 1.6%. Fracture toughness tests were conducted on compact tension specimens with thicknesses of 0.30, 0.50, 0.70, and 1.00 inches. The conditional fracture toughness, K_Q, was found to be insensitive to the specimen thickness in the above range and to have a value of approximately 30 ksi-in^1/2. The dependence of the fatigue crack growth rate, da/dN, on the stress intensity factor, δK, was determined for stress ratios of 0.1, 0.3, 0.5, and 0.7. At the same °K level, the da/dN rates were higher for the higher stress ratios. The parameters, C and n, of the Paris equation, da/dN=C(δK)ⁿ were determined for each stress ratio. Near-threshold tests were also conduced for the stress ratio of 0.1. The threshold stress intensity factor, δK_TH, was found to be in the range of 6.3 to 7.7 ksi-in^1/2. A statistical model was used to calculate the agreement of the results of two duplicate fatigue crack growth tests.     

Experimental investigation on low cycle fatigue and fracture behaviour of a notched Ni‐based superalloy at elevated temperature

In order to investigate the effects of stress concentration on low cycle fatigue properties and fracture behaviour of a nickel‐based powder metallurgy superalloy, FGH97, at elevated temperature, the low cycle fatigue tests have been conducted with semi‐circular and semi‐elliptical single‐edge notched plate specimens at 550 and 700 °C. The results show that the fatigue life of the notched specimen decreases with the increase of stress concentration factor and the fatigue crack initiation life evidently decreases because of the defect located in the stress concentration zone. Moreover, the plastic deformation induced by notch stress concentration affects the initial crack occurrence zone. The angle α of the crack occurrence zone is within ±10° of notch bisector for semi‐circular notched specimens and ±20° for semi‐elliptical notched specimens. The crack propagation rate decreases to a minimum at a certain length, D, and then increases with the growth of the crack. The crack propagation rate of the semi‐elliptical notched specimen decelerates at a faster rate than that of the semi‐circular notched specimen because of the increase of the notch plasticity gradient. The crack length, D, is affected by both the applied load and the notch plasticity gradient. In addition, the fracture mechanism is shown to transition from transgranular to intergranular as temperature increases from 550 to 700 °C, which would accelerate crack propagation and reduce the fatigue life.     

Constraint Loss under Dynamic Loading in Rate Independent Plastic Solids

The objectives of this paper are to examine the loss of crack tip constraint in dynamically loaded fracture specimens and to assess whether it can lead to enhancement in the fracture toughness at high loading rates which has been observed in several experimental studies. To this end, 2-D plane strain finite element analyses of single edge notched (tension) specimen and three point bend specimen subjected to time varying loads are performed. The material is assumed to obey the small strain J ₂ flow theory of plasticity with rate independent behaviour. The results demonstrate that a valid J–Q field exists under dynamic loading irrespective of the crack length and specimen geometry. Further, the constraint parameter Q becomes strongly negative at high loading rates, particularly in deeply cracked specimens. The variation of dynamic fracture toughness K _dc with stress intensity rate K for cleavage cracking is predicted using a simple critical stress criterion. It is found that inertia-driven constraint loss can substantially enhance K _dc for .     

Structure,fracture toughness,and fatigue of two aluminium matrix compositesproduced by vertical squeeze casting technique

Abstract

Composite materials produced from ceramic reinforcement of aluminium alloys have some properties that are better (higher modulus and strength, lower thermal expansion coefficient and density, and good creep and wear resistance) than those of the conventional monolithic aluminium alloys. However, they have a poor fracture toughness. The aim of the present work was to characterise the structure and mechanical properties of two different aluminium matrix composites (AS9C1G/20%(Al₂O₃-SiO₂) and 2014/20%(Al₂O₃-SiO₂)) manufactured using the vertical squeeze casting technique. Tensile, plane strain fracture toughness, and fatigue crack growth rate tests were carried out. In particular, the influence of specimen geometry on the toughness tests was examined. It was found that chevron notched short bar specimens gave toughness values ～ 40% higher than other types of specimens. Fatigue crack growth rate data were interpolated using some semiempirical models. An accurate metallographic investigation of both the structures and the fatigue fracture surfaces was carried out using optical microscopy and energy dispersive spectroscopy with SEM.     

Short-fiber/particle hybrid reinforcement: Effects on fracture toughness and fatigue crack growth of metal matrix composites

We have studied the effects of short-fiber/particle hybrid reinforcement on fracture toughness and fatigue crack growth in metal matrix composites. Reinforcement hybridization was achieved by a hybrid preform process, and composites were fabricated by the squeeze casting method. Al6061 matrix alloy and four composites having different short-fiber/particle ratio were tested. The fracture toughness (K_IC) and the fatigue threshold (ΔK_th) increased with increasing particle contents, whereas the Paris’ exponent (m) was insensitive to the short-fiber:particle ratio. These results emerged as a shift of the crack growth curve which implies on enhanced crack resistance over the entire stress intensity factor range. The positive aspect of particulate reinforcement is advocated by comparison of microstructural variables, and by observation of the crack path and surfaces. The characteristics of hybrid composites in damage tolerance are emphasized.     

Fatigue and fracture of a Ni–P amorphous alloy thin film on the micrometer scale

Fracture and fatigue tests have been performed on micro‐sized specimens for microelectromechanical systems (MEMS) or micro system technology (MST) applications. Cantilever beam type specimens with dimensions of 10 × 12 × 50 μm³, approximately 1/1000th the size of ordinary‐sized specimens, were prepared from a Ni–P amorphous thin film by focused ion beam machining. Fatigue crack growth and fracture toughness tests were carried out in air at room temperature, using a mechanical testing machine developed for micro‐sized specimens. In fracture toughness tests, fatigue pre‐cracks were introduced ahead of the notches. Fatigue crack growth resistance curves were obtained from the measurement of striation spacing on the fatigue surface, with closure effects on the fatigue crack growth also being observed for micro‐sized specimens. Once fatigue crack growth occurs, the specimens fail within one thousand cycles. This indicates that the fatigue life of micro‐sized specimens is mainly dominated by a crack initiation process, also suggesting that even a micro‐sized surface flaw may be an initiation site for fatigue cracks which will shorten the fatigue life of micro‐sized specimens. As a result of fracture toughness tests, the values of plane strain fracture toughness, K_IC, were not obtained because the criteria of plane strain were not satisfied by this specimen size. As the plane strain requirements are determined by the stress intensity, K, and by the yield stress of the material, it is difficult for micro‐sized specimens to satisfy these requirements. Plane‐stress‐ and plane‐strain‐dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This indicates that fracture mechanics is still valid for such micro‐sized specimens. The results obtained in this investigation should be considered when designing actual MEMS/MST devices.     

Der Mischbruch im Zugversuch

Mixed fracture in the tension test If a tensile test specimen does not break before by cleavage, voids are nucleated at second-phase particles and inclusions during plastic deformation. At the center of the necked region these voids coalesce by internal necking or shearing of the material between them forming a fibrous crack which expands radially. In a temperature range which is dependent on the material cleavage fracture is initiated by the fibrous crack, resulting in a mixed fracture. If no cleavage fracture is initiated a completely fibrous fracture is formed. Mixed fracture surfaces consist of a cleavage fracture zone surrounding the central fibrous fracture zone and the tensile specimen behaves like a fracture mechanics specimen. Fracture toughness can be calculated by equations for tensile specimens with a central penny shaped crack. A comparison of fracture toughness values obtained by the use of unnotched tensile specimens and of fracture mechanics specimens show good agreement inbetween the temperature range of valid K_lc values according to standards of linear-elastic fracture mechanics.     

Fracture toughness and fatigue crack growth behaviour of an Al2O3-SiC composite

The fracture toughness and fatigue crack growth characteristics of an Al₂O₃-SiC whisker composite were investigated. Quasi static fracture experiments were conducted on double edge-notched tension specimens and on four-point bend specimens containing a through-thickness Mode I crack which was introduced under uniaxial cyclic compression. The toughness results obtained using this procedure are more reproducible than those derived from the indentation technique and the notched bend bar method. The fracture toughness of the composite is about 60% higher than that of the unreinforced matrix material. Crack growth characteristics at room temperature were also investigated in notched plates of Al₂O₃-SiC subjected to fully compressive far-field cyclic loads. In the presence of a stress concentrator, this composite is found to be highly susceptible to fatigue crack growth under cyclic compressive loads.     

Relation between fatigue crack initiation and propagation,toughness and microstructure in large steel blooms for automotive plastic molds

Molds for plastic automotive components such as bumpers and dashboards are usually machined from large pre-hardened steel blocks. Due to their dimensions, the heat treatment produces mixed microstructures, continuously varying with the distance from the quenched surface, at which fracture toughness and fatigue behavior are not well known; fracture toughness is generally lower than that corresponding to a fully quenched and tempered condition. The response of the mold to defects and stresses applied during service depends on steel properties, that in turn depend upon the heat treatment and the microstructure.A survey of the mechanical properties of some commercial blooms was carried out by using three point fatigue bending tests on notched samples to evaluate the threshold behavior and the crack growth behavior by ΔK-decreasing and ΔK-increasing methods. The samples were obtained from different depths of the blooms. The relationship between mechanical properties, fracture surfaces and microstructure was also investigated.     

Investigation of high cycle and Very-High-Cycle Fatigue behaviors for a structural steel with smooth and notched specimens

The high cycle and Very-High-Cycle Fatigue (VHCF) properties of a structural steel with smooth and notched specimens were studied by employing a rotary bending machine with frequency of 52.5 Hz. For smooth specimens, VHCF failure did occur at fatigue cycles of 7.1 × 10⁸ with the related S–N curve of stepwise tendency. Scanning Electron Microscopy (SEM) was used for the observations of the fracture surfaces. It shows that for smooth specimens the crack origination is surface mode in the failure regime of less than 10⁷ cycles. While at VHCF regime, the material failed from the nonmetallic inclusion lies in the interior of material, leading to the formation of fisheye pattern. The dimensions of crack initiation region were measured and discussed with respect to the number of cycles to failure. The mechanism analysis by means of low temperature fracture technique shows that the nonmetallic inclusion in the interior of specimen tends to debond from surrounding matrix and form a crack. The crack propagates and results to the final failure. The stress intensity factor and fatigue strength were calculated to investigate the crack initiation properties. VHCF study on the notched specimens shows that the obtained S–N curve decreases continuously. SEM analysis reveals that multiple crack origins are dominant on specimen surface and that fatigue crack tends to initiate from the surface of the specimen. Based on the fatigue tests and observations, a model of crack initiation was used to describe the transition of fatigue initiation site from subsurface to surface for smooth and notched specimens. The model reveals the influences of load, grain size, inclusion size and surface notch on the crack initiation transition.     

Thickness effect on the mode III fracture resistance and fracture path of rock using ENDB specimens

Study of the thickness effect in predicting the crack growth behavior and load bearing capacity of rock‐type structures is an important issue for obtaining a relation between the experimental fracture toughness of laboratory subsized samples and the real rock structures with large thickness. The fracture of rock masses or underground rock structures at deep strata may be dominantly governed by the tensile or tear crack growth mechanism. Therefore, in this research, a number of mode I and mode III fracture toughness experiments are conducted on edge notch disc bend (ENDB) specimen made of a kind of marble rock to investigate the effect of specimen thickness on the corresponding K_Ic and K_IIIc values. It is observed that the fracture toughness of both modes I and III are increased by increasing the height of the ENDB specimen. Also, the ratio of K_IIIc/K_Ic obtained from each thickness of the ENDB specimens is compared with those predicted by some fracture criteria, and it was shown that the minimum plastic radius (MPR) criterion is the main suitable criterion for investigating the fracture toughness ratio K_IIIc/K_Ic . Also, the effect of ENDB height on fracture trajectory of tested samples is assessed. It is shown that the crack grows curvilinearly in thicker ENDB samples and cannot extend along the crack front in small specimens.     

Experimental determination of KIIC of normal strength concrete

The double-edge notched specimen made of normal strength concrete has been subjected to compressive loads. It is shown that a shear crack develops at the notch tip. Visual observation and evaluation of displacement measurements have proven that this specimen allows pure mode II testing. It was possible to determine fracture toughness K_IIc. Recommendations for the testing are made.     

Fracture mode identification of low alloy steels and cast irons by electron back-scattered diffraction misorientation analysis

The fracture modes of low alloy steels and cast irons under tensile and fatigue conditions were identified by electron back-scattered diffraction(EBSD) misorientation analysis in this research. The curves of grain reference orientation deviation(GROD) distribution perpendicular to the fracture surface were obtained by EBSD observation, and the characteristics of each fracture mode were identified. The GROD value of the specimen fractured in tension decreases to a constant related to the elongation of corresponding specimen in the far field(farther than 5 mm away from the fracture surface). The peak exhibits in GROD curves of two smooth specimens and a notched specimen near the fracture surface(within 5 mm away from the fracture surface), and the formation mechanisms were discussed in detail based on the influences of specimen geometries(smooth or notched) and material toughness. The GROD value of fatigue fractured specimen is close to that at undeformed condition in the whole field, except the small area near the crack path. The loading conditions(constant stress amplitude loading or constant stress intensity factor range K loading) and the EBSD striation formation during fatigue crack propagation were also studied by EBSD observation parallel to the crack path.     

Influence of specimen size and microstructure on the fracture toughness of a martensitic stainless steel

The fracture toughness of alloy HT-9,² a martensitic stainless steel under consideration for fast reactor and fusion reactor applications, was determined from circular compact tension specimens using the multi-specimen R-curve method. Specimens with thicknesses of 11.94, 7.62 and 2.54mm and widths of 23.88 and 11.94 mm were tested to investigate the effects of specimen size on fracture toughness. The test results obtained from all specimens are in good agreement and thickness requirements for a valid J_1c test are satisfied. The experiment indicates that small specimens of HT-9 may be used for post-irradiation fracture toughness testing.Fractographic examination of the fracture surfaces reveals that fracture in HT-9 is significantly influenced by delta ferrite stringers present in the material. The fracture surface examination and crack opening displacement measurements for specimens tested at various temperatures are consistent with the temperature dependence of the J_1c results.     

Fracture Toughness Determination for Aluminium Alloy 2011-T6 using Tensile Notched Round Bar (NRB) Test Pieces

While the use of notched round bar (NRB) test pieces to determine planestrain fracture toughness K _Ic is not novel, relatively few authors have so far attempted this. This letter serves to report a series of such tests conducted on Al 2011-T6, a high mechanical strength, free-machining aluminium alloy. A total of 9 specimens was used, comprising 3 specimens each of 3 different notch root radii, ρ. No fatigue precracking was carried out. A graph of apparent fracture toughness K _ρ versus ρ / D was extrapolated to zero, corresponding to a fatigue pre-cracked configuration. The resulting K _Ic for this alloy tested using the NRB specimens was thus found to be 27.9 MPa√m. This is a valid K _Ic value, as there is only a 6.8% difference between it and the median value (26.0 MPa√m) determined previously from 67 tests using the existing ASTM E399 standard. A new geometric correlation based on ρ, D and d is proposed to linearly extrapolate K _ρ values, measured on NRB specimens with arbitrary geometries (but assuming plane-strain conditions hold), back to a single value of K _Ic.     

On thumb-nail pattern of fatigue crack front observed in standard compact tension specimen

The thumb-nail shape of a crack front has been observed in a compact tension (CT) specimen, when a fatigue crack is formed ahead of the machined notch in the course of preparation for the fracture toughness testing. Assuming that the distribution of the stress intensity factor K _I along the crack front line is equal to that corresponding to the original straight crack front, a slightly curved crack front is obtained with the aid of Paris' law for the fatigue crack growth rate. Comparing the calculated results with the experimental observation in the 10 T CT specimens, whose thickness is 247 mm, it is found that the effect of the stress singularities at the intersection of the crack front line and the free surface can be disregarded and that they show extremely good agreement with each other.

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Résumé On a procédé à l'observation de la forme en ongle que prend le front de la fissure de fatigue que l'on réalise au droit de l'entaille usinée des éprouvettes de traction compactes destinées aux essais de ténacité à la rupture. En supposant que la distribution du facteur d'intensité de contrainte K _I le long du front de la fissure est égale à celle correspondant à un front de fissure originalement droit, on obtient un front de fissure légèrement incurvé grâce au recours à la loi de Paris sur la vitesse de croissance d'une fissure de fatigue. En comparant les résultats calculés et les observations expérimentales sur des éprouvettes 10 TCT, dont l'épaisseur atteint 247 mm, on trouver que l'effet de singularités des contraintes à l'intersection de la ligne de front de la fissure et des surfaces libres peut être négligé et que l'accord est extrêmement satisfaisant.