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1.
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 JQ 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 .  相似文献   

2.
Single edge-notched bend (SENB) specimens containing shallow cracks (a/W < 0.2) are commonly employed for fracture testing of ferritic materials in the lower-transition region where extensive plasticity (but no significant ductile crack growth) precedes unstable fracture. Critical J-values J c ) for shallow crack specimens are significantly larger (factor of 2–3) than the J c )-values for corresponding deep crack specimens at identical temperatures. The increase of fracture toughness arises from the loss of constraint that occurs when the gross plastic zones of bending impinge on the otherwise autonomous crack-tip plastic zones. Consequently, SENB specimens with small and large a/W ratios loaded to the same J-value have markedly different crack-tip stresses under large-scale plasticity. Detailed, plane-strain finite-element analyses and a local stress-based criterion for cleavage fracture are combined to establish specimen size requirements (deformation limits) for testing in the transition region which assure a single parameter characterization of the crack-tip stress field. Moreover, these analyses provide a framework to correlate J c )-values with a/W ratio once the deformation limits are exceeded. The correlation procedure is shown to remove the geometry dependence of fracture toughness values for an A36 steel in the transition region across a/W ratios and to reduce the scatter of toughness values for nominally identical specimens.  相似文献   

3.
A new methodology for predicting the location of maximum crack extension along a surface crack front in ductile materials is presented. Three-dimensional elastic-plastic finite element analyses were used to determine the variations of a constraint parameter (αh) based on the average opening stress in the crack tip plastic zone and the J-integral distributions along the crack front for many surface crack configurations. Monotonic tension and bending loads are considered. The crack front constraint parameter is combined with the J-integral to characterize fracture, the critical fracture location being the location for which the product h is a maximum. The criterion is verified with test results from surface cracked specimens.  相似文献   

4.
This study describes a computational framework to quantify the influence of constraint loss and ductile tearing on the cleavage fracture process, as reflected by the pronounced effects on macroscopic toughness (J c , c). Our approach adopts the Weibull stress w as a suitable near-tip parameter to describe the coupling of remote loading with a micromechanics model incorporating the statistics of microcracks (weakest link philosophy). Unstable crack propagation (cleavage) occurs at a critical value of w which may be attained prior to, or following, some amount of stable, ductile crack extension. A central feature of our framework focuses on the realistic numerical modeling of ductile crack growth using the computational cell methodology to define the evolution of near-tip stress fields during crack extension. Under increased remote loading (J), development of the Weibull stress reflects the potentially strong variations of near-tip stress fields due to the interacting effects of constraint loss and ductile crack extension. Computational results are discussed for well-contained plasticity, where the near-tip fields for a stationary and a growing crack are generated with a modified boundary layer (MBL) formulation (in the form of different levels of applied T-stress). These analyses demonstrate clearly the dependence of w on crack-tip stress triaxiality and crack growth. The paper concludes with an application of the micromechanics model to predict the measured geometry and ductile tearing effects on the cleavage fracture toughness J c of an HSLA steel. Here, we employ the concept of the Dodds-Anderson scaling model, but replace their original local criterion based on the equivalence of near-tip stressed volumes by attainment of a critical value of the Weibull stress. For this application, the proposed approach successfully predicts the combined effects of loss of constraint and crack growth on measured J c -values.  相似文献   

5.
The application of the J and the I-integrals to ductile fracture are discussed. It is shown that, because of the finite size of the fracture process zone (FPZ), the initiation value of the J-integral is specimen dependent even if the plastic constraint conditions are constant. The paradox that the I-integral for steady state elasto-plastic crack growth is apparently zero is examined. It is shown that, if the FPZ at the crack tip is modelled, the I-integral is equal to the work performed in its fracture. Thus it is essential to model the fracture process zone in ductile fracture. The I-integral is then used to demonstrate that the breakdown in applicability of the J-integral to crack growth in ductile fracture is as much due to the inclusion in the J-integral of progressively more work performed in the plastic zone as it is to non-proportional deformation during unloading behind the crack tip. Thus J R -curves combine the essential work of fracture performed in the FPZ with the plastic work performed outside of the FPZ. These two work terms scale differently and produce size and geometry dependence. It is suggested that the future direction of modelling in ductile fracture should be to include the FPZ. Strides have already been made in this direction.  相似文献   

6.
Fracture criterion of the J-integral finds wide application in the integrity evaluation of welded components, but there exist some confused problems such as the dependence of the fracture toughness on the strength mis-matching and specimen geometry which need to be clarified. It is rough and unsuitable to attribute the variation of J-integral fracture parameter simply to the effect of mechanical heterogeneity. In the present paper, a two-dimensional finite element method is employed to analyze the distribution and variation of crack tip field of welded joints with different strength mis-matching in four kinds of specimen geometry, and then the validity of J-dominance in welded joints is investigated. It is found that the crack tip field of mis-matched joint is different from that of either the weld metal or base metal of which the joint is composed, but it is situated between those of weld metal and base metal. Under the plane strain, there is obvious difference in stress triaxiality for different strength mis-matched joints. The validity of J-dominance in welded joint can not be obtained by comparing whether the stress triaxiality meets that required by the HRR solution because of the existence of mechanical inhomogeneity. By ascertaining if the stress triaxiality of welded joint near the crack tip is dependent of specimen geometry, the conclusion can be arrived at: for plane stress the validity of J-dominance is valid, whilst for plane strain the validity of J-dominance is lost. Based on the above, attempt has been made to point out that the influence of mechanical heterogeneity on the fracture toughness of weldment arises from the variation of constraint intensity-crack tip stress triaxiality. Compared with the effect of mechanical heterogeneity on the stress triaxiality, the losing of validity of J-dominance in mis-matched joint under plane strain may play a more critical role in the variation of J-integral fracture parameter of weldment.  相似文献   

7.
Conventional theories of fracture assume that the state of stress and strain in the vicinity of a crack tip, and so the onset of failure, is characterised by a single parameter. The physical extent of these single-parameter fields is determined by the geometry, size and mode of loading of the engineering structure or test specimen containing the crack. It is now recognised that fracture toughness is a material property characterised by a single parameter J only in special circumstances which involve a high degree of constraint at the crack-tip. In general the apparent toughness of a material changes according to the shape and size of the cracked configuration and the mode of loading imposed. Recent analytical, numerical and experimental studies have attempted to describe fracture in terms of both J and a second parameter. The reason for the second parameter is to provide further information, which J on its own is unable to convey, concerning how the structural and loading configuration affects the constraint conditions at the crack-tip. One particular candidate parameter is the elastic T-stress which is directly proportional to the load applied to the cracked geometry. This paper brings together published solutions for the T-stress for a range of two and three-dimensional cracked geometries and presents some new results calculated at AEA Technology. The application of two-parameter fracture mechanics is a subject of ongoing development and users of the data in this paper are recommended to seek expert advice regarding applications to specific structural integrity assessments.  相似文献   

8.
In the present work, mode I and mode II tests were carried out on two low alloyed high strength steels. An asymmetrical four point bend specimen and J II-integral vs. crack growth resistance curve technique were used for determining the mode II elastic-plastic fracture toughness, J IIc · J II-integral expression of the specimen was calibrated by finite element method. The results indicate that the present procedure for determining the J IIc values is easy to use. Moreover, the mode I fracture toughness J Ic is very sensitive to the rolling direction of the test steels, but the mode II fracture toughness J IIc is completely insensitive to the rolling direction of the steels, and the J IIc /J Ic ratio is not a constant for the two steels, including the same steel with different orientations. Finally, the difference of the fracture toughness between the mode I and mode II is discussed with consideration of the different fracture mechanisms.  相似文献   

9.
To evaluate the elastic-plastic fracture toughness parameter of nuclear pressure-vessel steel A533B-1, a newly developed technique (the recrystallization-etch technique) for plastic strain measurement was applied to different sizes of compact tension specimens with a crack length/specimen width of 0.6–0.5 that were tested to generate resistance curves for stable crack extensions. By means of the recrystallization-etch technique, the plastic energy dissipation or work done within an intense strain region at the crack tip during crack initiation and extension was measured experimentally. Furthermore, the thickness effects on this crack tip energy dissipation rate were examined in comparison with other fracture-parameter J integrals. Thickness effects on critical energy dissipation and energy dissipation rate during crack extension were obtained and the energy dissipation rate dW p/da in the mid-section shows a constant value irrespective of specimen geometry and size, which can be used as a fracture parameter or crack resistance property.  相似文献   

10.
The constraint effect on J–resistance curves of ductile crack growth is considered under the condition of two-parameter JQ* controlled crack growth, where Q* is a modified parameter of Q in the JQ theory. Both J and Q* are used to characterize the JR curves with J as the loading level and Q* as a constraint parameter. It is shown that Q* is independent of applied loading under large-scale yielding or fully plastic deformation, and so Q* is a proper constraint parameter during crack growth. An approach to correct constraint effects on the JR curve is developed, and a procedure of transferring the JR curves determined from standard ASTM procedure to nonstandard specimens or real cracked structures is outlined.The test data of fracture toughness, JIC, and tearing modulus, TR, by Joyce and Link (Engng. Fract. Mech. 57(4) (1997) 431) for a single-edge notched bend specimen with various depth cracks are employed to demonstrate the efficiency of the present approach. The variation of JIC and TR with the constraint parameter Q* is obtained, and then a constraint-corrected JR curve is constructed for the test material of HY80 steel. Comparisons show that the predicted JR curves can match well with the experimental data for both deep and shallow cracked specimens over a reasonably large amount of crack extension.Finally, the present approach is applied to predict the JR curves of ductile crack growth for five conventional fracture specimens. The results show that the effect of specimen geometry on the JR curves is generally much larger than the effect of specimen sizes, and larger specimens tend to have lower crack growth resistance curves.  相似文献   

11.
Valid plane-stress fracture toughness evaluation of short fibre reinforced composites relies essentially on the successful separation of the energy absorbed in the localized crack-tip region out of the total energy absorbed by the cracked material body at large. Three different experimental techniques, all stemming from the energetic interpretation of theJ integral, are utilized and their relative merits in the characterization of fracture initiation in short glass fibre reinforced injection-moulded nylon 6.6 examined. Various theoretical aspects concerning these experimental methods are outlined. The rationale behind using a single-edge-notched tension type specimen for theJ c test is presented. TheJ c value obtained from the compliance calibration method and the quasistatic energy method agree closely and can be considered to be independent of pre-crack length and specimen geometry when the pre-crack length to specimen width ratio (a/w) is larger than 0.45. The extrapolation method fails nevertheless to yield a physically consistentJ c value, possibly due to its questionable theoretical representation. As no constraint on boundary conditions is necessitated during the course of crack extension, the quasistatic energy is physically more appealing.  相似文献   

12.
Previous work by Dodds and Anderson provides a framework to quantify finite size and crack depth effects on cleavage fracture toughness when failure occurs at deformation levels where J no longer uniquely describes the state of stresses and strains in the vicinity of the crack tip. Size effects on cleavage fracture are quantified by defining a value termed J SSY: the J to which an infinite body must be loaded to achieve the same likelihood of cleavage fracture as in a finite body. In weld metal fracture toughness testing, mismatch between weld metal and baseplate strength can alter deformation patterns, which complicate size and crack depth effects on cleavage fracture toughness. This study demonstrates that there is virtually no effect of ±20 percent mismatch on J SSYif the distance from the crack tip to the weld/plate interface (L min) exceeds 5 mm. At higher levels of overmatch (50 to 100%), it is no longer possible to parameterize the departure of J SSYfor a weldment from that for a homogeneous SE(B) based on L min alone. Weld geometry significantly influences the accuracy with which J SSYfor a welded SE(B) can be approximated by J SSYfor a homogeneous specimen at these extreme overmatch levels.  相似文献   

13.
The multiple specimen J 0.2/BL initiation fracture toughness test procedure from the ISO standard, ISO 12135:2002, is evaluated using the EURO fracture toughness data set. This standard is also compared with the ASTM standard, ASTM E 1820, multiple specimen J Ic procedure. The EURO round robin data set was generated to evaluate the transition fracture toughness methods for steels. However, many of the tests resulted in ductile fracture behavior giving final J versus ductile crack extension points. This is the information that is measured in a multiple specimen J initiation fracture toughness test. The data set has more than 300 individual points of J versus crack extension with four different specimen sizes. It may be the largest data set of that type produced for one material. Therefore, its use to determine J initiation values can provide an important evaluation of the standard procedures. The results showed that a J 0.2/BL value could be determined from the ISO standard for three of the four specimen sizes, the smallest size did not meet the specimen size requirement on J. The construction line slopes in this method are very steep compared with the ASTM construction line slopes. This resulted in low J initiation values, about a factor of two lower than the one from the ASTM method. Of the various criteria imposed to determine a valid J 0.2/BL value, the one limiting the maximum J value was the most questionable. It had an effect of eliminating small specimen data that was identical to acceptable large specimen data.  相似文献   

14.
A theoretical model relating fracture toughness expressed as J IC and bulge ductility {ie71-1} for a material exhibiting linear elastic behavior at low temperature and elastic-plastic behavior at higher temperatures is proposed. This model shows a variation of J IC with {ie71-2} for linear elastic behavior and J IC with {ie71-3} for elastic-plastic behavior. The model contains three constants to be determined experimentally for a given material, specimen geometry and testing conditions. A case study on 1045 steel in the temperature range ?60 to 25°C confirms the validity of the model. The experimental results help in determining the size of the fracture zone ahead of the crack as well as the mechanisms for crack blunting and crack growth.  相似文献   

15.
In this work, the constraint parameter Ap based on crack‐tip equivalent plastic strain was calculated by finite element analyses for the cracks located at different locations in two interface regions in a dissimilar metal weld joint (DMWJ). The capabilities of the parameter Ap for characterizing material constraint and establishing correlation of material constraint with fracture toughness of the interface region cracks have been examined. The results show that the parameter Ap can characterize material constraint effect caused by material mismatch and initial crack positions in the interface regions. Based on the Ap, the correlation lines and formulae of material constraint with fracture toughness of the interface region cracks in the DMWJ can be established, and they may be used for obtaining material constraint‐dependent fracture toughness for the interface region cracks. The results in this work combining with those in the previous studies indicate that the parameter Ap may be a unified constraint parameter that can characterize both geometry constraint (including in‐plane and out‐of‐plane constraints) and material constraint, and it may be used in accurate fracture assessments of welded components with different geometry and material constraints.  相似文献   

16.
Application of elastic-plastic fracture mechanics techniques to the problems of welded structure toughness evaluation is relatively new. In recent times, a subject of increasing interest is the correlation of Rice's path independent J-integral and its extended formulations to crack growth initiation in materials under plastic deformation. The circumferentially notched implant weldability test specimen is now widely used to evaluate the fracture characteristics of weld-base metal composites due to its advantages of small size and associated low cost. In this paper, an incremental finite element analysis, suitably modelled to admit linear work hardening of materials, has been carried out for this specimen under monotonic load histories.As a sequel to an earlier work, the domain-independent property of suitable J solutions available in the literature have been critically examined from the viewpoint of applying J-criterion to this specimen. Some new results about J and the associated notch tip deformation patterns are obtained here. Also, the role of surface and line contributions under the integral signs in axisymmetric J has been discussed in the context of the present problem. Solutions presented demonstrate that the J-integral can be used as a useful notch-tip characterizing parameter for this test specimen.  相似文献   

17.
Constraint can be divided into two conditions of in‐plane and out‐of‐plane, and each of them has its own parameter to characterize. However, in most cases, there exists a compound change of both in‐plane and out‐of‐plane constraint in structures, a unified measure that can reflect both of them is needed. In this paper, the finite element method (FEM) was used to calculate the equivalent plastic strain (ɛp) distribution ahead of crack tips for specimens with different in‐plane and out‐of‐plane constraints, and the FEM simulations based on Gurson–Tvergaard–Needleman (GTN) damage model and a small number of tests were used to obtain fracture toughness for the specimens with different constraints. Unified measure and characterisation parameter of in‐plane and out‐of‐plane constraints based on crack‐tip equivalent plastic strain has been investigated. The results show that the area APEEQ surrounded by the ɛp isoline ahead of crack tips can characterize both in‐plane and out‐of‐plane constraints. Based on the area APEEQ, a unified constraint characterisation parameter Ap was defined. It was found that there exists a sole linear relation between the normalised fracture toughness JIC/Jref and regardless of the in‐plane constraint, out‐of‐plane constraint and the selection of the ɛp isolines. The unified JIC/Jref−reference line can be used to determine constraint‐dependent fracture toughness of materials. The FEM simulations with the GTN damage model (local approach) can be used in obtaining the unified JIC/Jref−reference line for materials with ductile fracture.  相似文献   

18.
In the paper an alternative formulation of the RKR local fracture criterion is proposed. It is based on the features of the stress distribution in front of a blunted crack in an elastic-plastic material. The stress distribution is computed using the finite strain option in the finite element method. It is postulated that the opening stress in front of the crack should be greater than the critical one, σc, over the distance l ? lc, where lc is considered as a material parameter. The hypothesis is applied to estimate the influence of the in-plane constraint on fracture toughness. New formulas to compute the critical value of the J-integral are derived both for the small scale yielding and large plastic deformations in front of the crack. The results obtained are compared with the Sumpter and Forbes experimental results and with the O’Dowd analytical formula concerning the Jc = Jc(JIC,Q) relation.  相似文献   

19.
This paper presents a new simplified approach for ductile failure analysis of semi-elliptical surface cracks under uniform tension by representing the load as a multiplication of two separate functions; a crack geometry function and a material deformation function. Experimental data of specimens with wide range of crack depth, crack width, and thickness are used in the implementation of this approach. The semi-elliptical surface crack is treated as an equivalent single edge crack. An appropriate form for such a treatment is carefully selected and studied. Both the geometry and deformation functions for the studied data are developed. For the first time, an pl value is developed for such a complex geometry. The existence of load separation, an equivalent one-dimensional crack length, and an pl value may allow the elastic-plastic fracture parameter J to be evaluated using a single specimen for this geometry. Both the geometry function pl and are also developed using Mattheck et al. limit load formula which is based on the Dugdale model. They agree well with those developed from the experimental data within the tested range. Finally, a key curve is developed and used in the prediction of the load-displacement records.  相似文献   

20.
This study examines crack front length and constraint loss effects on cleavage fracture toughness in ferritic steels at temperatures in the ductile-to-brittle transition region. A local approach for fracture at the micro-scale of the material based on the Weibull stress is coupled with very detailed three-dimensional models of deep-notch bend specimens. A new non-dimensional function g(M) derived from the Weibull stress density describes the overall constraint level in a specimen. This function remains identical for all geometrically similar specimens regardless of their absolute sizes, and thus provides a computationally simple approach to construct (three-dimensional) fracture driving force curves w vs. J, for each absolute size of interest. Proposed modifications of the conventional, two-parameter Weibull stress expression for cumulative failure probability introduce a new threshold parameter w–min. This parameter has a simple calibration procedure requiring no additional experimental data. The use of a toughness scaling model including w–min>0 increases the deformation level at which the CVN size specimen loses constraint compared to a 1TSE(B) specimen, which improves the agreement of computational predictions and experimental estimations. Finally the effects of specimen size and constraint loss on the cleavage fracture reference temperature T 0 as determined using the new standard ASTM E1921 are investigated using Monte Carlo simulation together with the new toughness scaling model.  相似文献   

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