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1.
In this paper, the principle and method for determining fracture toughness indexes KIC, δ C and JIC of structural steel by side-cut SE (B) are introduced. This method has many advantages: clear critical point, simple operation, stable date, less cost and will be valuable to use more frequently. 相似文献
2.
The fracture toughness of a 30 CrMnSiA steel plate of three thicknesses (10,8 and 5 mm) and three widths (110,80 and 56 mm) has been investigated by using surface-flaw method under room temperature. It is not easy to compute the value of KIE by the maximum applied load. But the values of KIE and KIC could be obtained easily, if the computation of the conditional applied load P10 and P5 based on the relative effective extension Δ a/ a0 = 10% and 5% were adopted, together with the conditions of Pmax/ P10 1.2 and Pmax/ P5 1.3. The KR — Δ a curve, i.e. the resistance-curve described by the parameter K, has been plotted. The values of KIC and KIE are then the resistances corresponding to the real extensions of flaws of Δ/ a0 = 2 and 7%, respectively. These values so obtained are in good agreement with the computed values of KIC and KIE by using the conditional applied loads. The values of KIC and KIE so obtained are also in agreement with the value of KIC converted from the J-integral and the effective value of KIE computed by the maximum applied load, respectively. An approximate relation between KIC and KIE has been found to be: KIC = (0.85˜0.95)KIE. The requirements for the dimensions of specimens are: Thickness of plate: B 1.0(KIC/σ0.2)2 or 1.25(KICσ0.2)2]; Width of plate: 8 W/B 10, 4 W/2c 5; Effective length: l 2W. 相似文献
3.
The effect of the ratio a/ W on the fracture behaviour of ductile and brittle materials has been studied by measuring the crack-opening displacement and J-integral for ductile material and the stress intensity factor for brittle material in three-point bend specimens with shallow and deep notches. It is shown that, for ductile material, the values of δ i and Ji, for specimens with shallow notches are larger than those of deep notches. On the contrary, for brittle material, the values of KIC for specimens with shallow notches are smaller than those of deep notches. The reason for this is explained. 相似文献
4.
Evaluation of Mode I interlaminar fracture toughness for unidirectional hybrid composites fabricated with a bead filled epoxies was carried out. The two important fracture toughness parameters, GIC and KIC values of hybrid composites, were reviewed in accordance with the orthotropic fracture model. The deviation of measured GIC and KIC values from predicted values were explained based on the critical review of the basic assumption of orthotropic fracture model and characteristic material properties of hybrid composites. It can be said that, basically, the orthotropic fracture model can be used for evaluation of hybrid composite materials. However, careful analysis for GIC and KIC values which were derived from different source and some correction factor for KIC values are necessary. 相似文献
5.
A comprehensive program is being conducted relative to applying fracture mechanics technology to large turbine-generator rotors. One facet of this program involves the determination of plane-strain fracture toughness ( KIc) over a range of temperatures for various types of rotor steels. Data have been obtained for ten large production forgings, representing three alloys, using various types of compact KIC and spin burst test specimens. These results demonstrate that valid KIC data can be obtained in these types of intermediate-strength, high-toughness steels in the temperature range of practical interest. Data indicate that the plane-strain fracture toughness of these steels increases rapidly with increasing temperature and is rather high ( Ktc/σ YS > 1 in1/2), in the application range. As a result, the critical defect sizes for catastrophic failure upon a single cycle of loading are relatively large. The plane-strain fracture toughness measurements, as well as the application of these data, are presented and discussed. 相似文献
6.
The main purpose of this investigation is to evaluate the effect of the thickness, width, aspect ratio and geometry of the fracture toughness specimen on the resultant displacement due to growth of plastic zone and crack. The analytical part evaluates the effect of the width, aspect ratio, geometry and flow properties on the displacement due to the growth of plastic zone as well as the crack. The experimental part evaluates the effect of thickness and width of a compact tension specimen on the displacement and on the thickness direction contraction due to the growth of the plastic zone. The main result of the investigation is that the plastic zone size decreases and the constraint to yielding increases as the width of a CT specimen increases. Based on this and other analytical and experimental result, a new procedure for the determination of KIC has been proposed. The procedure is verified by experimental data obtained by other workers. The procedure overcomes the limitation of ASTM E399 for the determination of KIC. 相似文献
7.
A method of fracture parameters determination at yield point and fracture initiation is derived from theoretical analysis of bending load-time response of materials during CHARPY impact testing. It allows to overcome the difficulties due to inertial effect and specimen vibrations. Furthermore, the method enables to determine the time at which fracture initiates, that is when the specimen crack begins to extend. The procedure has been applied to various model materials such as steel, aluminum alloy, PMMA and soda-lime glass in order to specify the fracture parameters which can be carried out by the method. Depending on the type of materials Kid and JId can be determined. 相似文献
8.
A general formulation of the equivalent domain integral (EDI) method for mixed-mode fracture problems in cracked solids is presented. The method is discussed in the context of a 3-D finite-element analysis. The J-integral consists of two parts: the volume integral of the crack front potential over a torus enclosing the crack front and the crack surface integral due to the crack front potential plus the crack-face loading. In mixed-mode crack problems the total J-integral is split into JI, JII, and JIII, representing the severity of the crack front in three modes of deformation. The direct and decomposition methods are used to separate the modes. These two methods were applied to several mixed-mode fracture problems in isotropic materials. Several pure and mixed-mode fracture problems were analyzed and results found to agree well with those available in the literature. The method lends itself to be used as a post-processing subroutine in a general purpose finite-element program. 相似文献
9.
The viability of the instrumented Charpy impact testing for studying dynamic fracture behaviour of rotor steels is investigated. This encompasses determination of dynamic fracture toughness ( KId) and dynamic J-integral ( JId), establishing correlation between oscilloscope profiles and fracture morphology of the ruptured samples and identifying fracture mechanisms involved. The predicted oscilloscope profiles for common fracture modes, their experimental counterparts, and the inferences drawn from these concerning operating fracture mechanisms are in good accord with the fractographic observations made on broken samples. Thus, the respective oscillographs vividly manifest the observed variations in the fracture processes. Fracture mechanics analysis of load-time and energy-time records of pre-cracked Charpy samples gave dynamic fracture toughness ( KId) values of 43, 74 and 124 MN/ m3/2, and dynamic J-integral ( JId) values of 0.008, 0.03 and 0.06 MJ/ m2 at −180°, 26° and 96°C respectively. It is possible that the deduced JId values correspond to a small but finite amount of crack extension instead of Zero Crack extension, in line with the emerging trends of JId estimation. Apart from increasing with temperature, both parameters recorded a true transition around 35°C which is attributed to the combined influence of a change in the fracture mode and relaxation of crack tip constraint. Another significant outcome of this investigation concerns about the existence of a minimum crack depth ratio for valid JId determination which, based on a detailed fractographic study, is interpreted in terms of the collective influence of crack tip plasticity and notch constraint. 相似文献
10.
The Dynamic Tear (DT) test permits the measurement of fracture propagation energy across the toughness spectrum for metals which are definable by linear elastic analyses to those requiring gross plastic strains for fracture. The linear elastic fracture mechanics parameter Kic provides a relationship between critical flaw size and stress level at which crack instability will occur. Unlike the DT test, the Kic toughness test cannot be utilized for fracture under conditions of elastic-plastic or gross plastic strain. A correlation has been developed between the DT test and the KIC parameter for ahuminum alloys. The relationship may also be expressed in terms of βic-DT and ic-DT. The Kic values were determined with several specimen types and a comparison of the values for different specimens is provided. The correspondence between Kic and DT serves several purposes. It provides a frame of reference for DT values obtained from frangible metals that fracture under linear elastic conditions. Accordingly, it permits utilization of the inexpensive DT test to approximate the flaw size-stress instability conditions which otherwise must be determined by the more expensive Kic test. Furthermore, through extrapolation, it is possible to utilize the DT test to estimate the critical flaw size under an elastic-plastic strain field. 相似文献
11.
Fractographic examinations of fracture surfaces of single edge crack plate tension fracture toughness test specimens of some new Fe---Mn base maraging alloys have been conducted. The interrelations between the fractographic features, fracture toughness and other mechanical properties of these alloys have been studied. It is observed that the width of the stretched zone between fatigue and rapid fracture is related to K/σ ys of the material where K is either KIC, KQ or the stress intensity for onset of microscopic slow crack growth. The stretched zone width is approximately equal to the average dimple size. Also it is of the order of the process zone size (calculated by modified Krafft's model) and the critical crack opening displacement in plane strain condition. Hahn and Rosenfield's model to estimate KIc was found to show much higher values in those cases where the fracture mode was predominantly cleavage, quasicleavage or intergranular. 相似文献
12.
A critical evaluation of the feasibility of obtaining crack growth parameters from quasi-static bend tests is presented. First derived are the governing differential equations which characterize the time-history of bend test parameters for a given elastic material exhibiting power law crack growth behavior v = vmax( KI/ KIC)N. A numerical solution scheme is then developed which is capable of solving the initial value problem, thus quantitatively assessing the influence of crack growth on the load-displacement output. The results of this analysis indicate that in order to calculate KIC accurately based on the peak load data, the loading rate has to be set sufficiently fast compared to vmax (but below dynamic rates), otherwise, it will generally overestimate its true value whenever slow crack growth occurs during the test; and that for high N materials the flexural test method gives a broad error band in N prediction and hence is not a reliable technique. However, it can be used by a designer to quickly screen the new materials with high Nvalues which are potential candidates for structural application. 相似文献
13.
The authors wish to present an all-fracture mode specimen with which it is possible to conduct fracture mechanics tests for pure mode I, pure mode II, pure mode III, as well as for all possible combinations of the above-mentioned. By means of a finite element analysis of this specimen, the stress intensity factors KI, KII, and KIII were computed. It was discovered that KII and KIII are coupled for in-plane shear and anti-plane shear loading, i.e. a mixed state occurs locally. The integral mean along the crack front yields however only to a KII factor for in-plane shear and to a KIII factor for anti-plane shear loading. Fracture experiments under mixed-mode loading, using this new specimen, demonstrate the influence of the loading type on the orientation and on the structure of the fracture surface. 相似文献
14.
A survey is given on the effect of microstructure on crack propagation mechanisms and fracture toughness. The influence of inclusions and of the material's matrix are treated separately. An attempt was made to correlate some simple, but typical microstructures with corresponding crack propagation mechanisms and to establish a qualitative sequence of these microstructures with respect to their effect on KIC. Because of the lack of sufficient experimental evidence this attempt is necessarily incomplete. Finally, some KIC-calculations are compared with measured values. 相似文献
15.
Scatter in KIC-results can often be quite extensive, and to make reliable interpretations of the results it is of great importance to understand the nature of it. Cleavage fracture in steels is of a statistical nature and therefore the scatter in KIC-results will behave similarly. Two different approaches, one based on a microstructural statistical model and an other based on the Weibull distribution are applied to evaluate the theoretical scatter in KIC-results. With both methods it is shown that the theoretical value of the relative scatter described through the Weibull slope factor is constant and equal to four. The reason for the discrepancy between the theoretical value and the experimentally determined values of the slope factor is shown to be caused by inadequate number of experimental KIC-measurements. The existence of a lower limiting Kmin value is verified and a simple procedure for conservative estimation of the KIc-mean and lower bound values is presented. 相似文献
16.
The application of the fracture mechanics approach to time-dependent high temperature crack growth has been reviewed. Available data on several structural alloys indicate that depending on the environmental sensitivity and creep ductility of the material, creep crack growth can be characterized by either linear elastic parameter, K, non-linear elastic-plastic parameter, J*-integral, or reference stress, σ ref. In particular for materials that are significantly sensitive to environment, K can adequately characterize the growth rate, and for materials that are significantly creep ductile, σ ref can be used to predict creep life of a cracked body. Finally, for materials that are relatively ductile and wherein crack growth occurs predominantly by a deformation process, J* integral appears to be the characterizing parameter for the growth rate. Data for several materials indicate that under steady state crack growth conditions, there may be a unique growth rate- J* relation independent of temperature and material. This would have a profound impact in terms of the utility of fracture mechanics approach to predict creep crack growth rate and needs to be examined further. Conditions under which K, J* or σ ref is applicable are discussed in detail. 相似文献
17.
This work presents an experimental investigation of the fracture properties of three different short-fiber-reinforced composites [one chopped strand mat (CSM) and two sheet molding compound (SMC) materials]. Fracture tests are performed on double-cantilever beam (DCB) specimens loaded with pure bending moments. In this experimental configuration, the bridging law for the material can be derived directly from measurements. No significant dependency on specimen height was observed in our results. The bridging laws determined can, therefore, be considered as material properties. The coupling between microstructure and fracture behaviour is discussed through the measured bridging laws. The beneficial effect (in terms of fracture energy) of increasing tendency for pull-out is confirmed for one SMC, referred to as Flex-SMC, which shows remarkably high fracture energy, Jc=56.0 kJ/m 2, compared to a standard SMC, termed Std-SMC, Jc=25.9 kJ/m 2. This increasing tendency for pull-out is observed to shift the bridging law towards larger crack openings. On the basis of our observations we find the concept of characterising the failure behaviour in terms of bridging laws attractive since it can be used as a tool for the tailoring of the microstructure towards desired fracture behaviour. 相似文献
18.
The behaviour of void growth in the crack tip regions of four specimen geometries with different stress triaxiality levels have been investigated by the FEM method and experimental observations in plane strain and plane stress cases respectively. It was found that the shape change of growing voids, the configurations of a blunting crack tip and the sizes of decreasing ligament between the void and the crack tip are strongly dependent upon the stress triaxiality levels. Under the condition of plane stress, the stress triaxiality on the ligaments of cracks are nearly the same for different specimen geometries, also the void growth, crack tip blunting asnd decreasing of ligament size are identical for various specimens with increasing load levels, which lead to the conclusion that the Ji-value is independent of specimen geometries. However, in the plain strain case, different void growth, crack tip blunting and decreasing in ligament size for various stress triaxiality levels directly caused the Ji-value to be dependent on the specimen geometries. It was found that when the void is linked to the blunting crack tip by the extrapolation to the zero ligament from FEM calculations, the Ji values, measured experimentally, are underestimated slightly. 相似文献
19.
The fracture behaviour of a glass-fabric-reinforced epoxy composite has been investigated experimentally. Load-displacement curves for single-edge-notched specimens were obtained on an MTS system and the J-integral evaluated through its energy rate interpretation. Jc, the critical value of the J-integral, obtained directly for a0/ w > 0·4 and that obtained through an extrapolation procedure for a0/ w < 0·4 compare quite well. Jc appears to be independent of crack length for specimen widths between 15 and 45 mm. Jc for ±45° specimens is less than half that for 0/90 specimens. 相似文献
20.
The constraint based fracture mechanics methodology, J– A2 method, has been used to interpret cleavage fracture recently. In all previous studies, the constraint parameter A2 was determined by stresses analytically calculated from finite element analyses (FEA). In the current paper, it is first demonstrated that A2 can be measured during a fracture test using the crack tip opening displacement (CTOD). A single-edge-notched specimen under bending (SENB) is used to compare the A2 values determined from δ5 displacement and the stress components. Finally, cleavage fracture toughness values for A533-B reactor pressure vessel (RPV) steel at −40°C obtained from test programs at Oak Ridge National Laboratory (ORNL) and the University of Kansas (KU) are interpreted using the J– A2 analytical model. Particular emphasis is placed on using the A2 determined from CTOD to characterize the fracture event. It is demonstrated that the effects of crack depth (shallow vs deep) and specimen size (small vs large) on the fracture toughness from the test programs can be interpreted and predicted using J and the constraint level A2 measured from the displacement. 相似文献
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