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1.
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
2.
A compact model II fracture specimen was previously analyzed and employed to determine the mode II fracture toughness K
IIc
, of perspex. In employing this specimen for a more ductile material such as aluminium, it was observed that the load vs. crack sliding displacement record becomes nonlinear for small loads. Thus, concepts of linear elastic fracture mechanics cannot be employed. To this end, the specimen was calibrated for J-integral testing, so that J
IIc
mesurements can be performed.In this study, mode I and II tests are carried out on an aircraft aluminium alloy, AI 7075-T7351. First, standard K
Ic
tests are performed leading to a value of 27.9 15-1 which would be equivalent to a J
Ic
of 10.7 kN/m. Then standard J
Ic
tests are carried out on this material with specimen thicknesses, of 5, 7.5 and 9.9 mm, leading to an average J
Ic
value of 10.5 kN/m. Methods for J
II testing are proposed; a series of specimens of six thicknesses between 5 and 16 mm are employed for testing. An average J
IIc
value was found to be 40.2 kN/m which yields a K
IIc
value of 54.1 15-2. Thus, K
IIc
is seen to be approximately twice that of K
Ic
for this material. 相似文献
3.
Bahador Bahrami Majid R. Ayatollahi Iman Sedighi Mohd Yazid Yahya 《Fatigue & Fracture of Engineering Materials & Structures》2019,42(9):1991-1999
The effect of friction forces between the test specimen and its bottom supports on the mode II fracture toughness values obtained using the semicircular bend (SCB) specimen is investigated. First, a number of experiments were conducted on SCB specimen in order to determine the mode II fracture toughness of polymethyl methacrylate (PMMA) according to the conventional approaches available in the literature. Three different types of supports that have been frequently employed by researchers in recent years were used to evaluate the effect of support type on the fracture loads. It was found that the friction forces between the supports and the SCB specimen have a significant effect on the value of mode II fracture toughness measured using the SCB samples. Then, the specimen was simulated using finite element method for more detailed investigation on the near crack tip stress field evolution when friction forces increase between the supports and the SCB specimen. The finite element results confirmed that the type of support affects not only the stress intensity factors KI and KII but also the T‐stress. The experimental and numerical results showed that the use of the crack tip parameters available in literature for frictionless contact between the supports and the SCB specimen can result in significant errors when the mode II experiments are performed by using the fixed or roller‐in‐grove types of supports. 相似文献
4.
In this paper a mode II fracture testing method has been developed for wood from analytical, experimental and numerical investigations. Analytical results obtained by other researchers showed that the specimen geometry and loading type used for the proposed mode II testing method results in only mode II stress intensity and no mode I stress intensity at the crack tip. Experiments have been carried out to determine mode II fracture toughness K
IIC and fracture energy G
IIF from the test data collected from both spruce (pice abies) and poplar (populus nigra) specimens. It was found that there existed a very good relation between fracture toughness KIIC and fracture energy G
IIF when the influence of orthotropic stiffness E
II
* in mode II was taken into account. It verified that for this mode II testing method the formula of LEFM can be employed for calculating mode II fracture toughness even for highly orthotropic materials like wood. In the numerical studies for the tested spruce specimen, the crack propagation process, stress and strain fields in front of crack tips and the stress distributions along the ligament have been investigated in detail. It can be seen that the simulated crack propagating process along the ligament is a typical shear cracking pattern and the development of cracks along the ligament is due to shear stress concentrations at the crack tips of the specimen. It has been shown that this mode II fracture testing method is suitable for measuring mode II fracture toughness K
IIC for highly orthotropic materials like wood. 相似文献
5.
Abstract: In the traditional formulation of the stress field near a crack tip, the presence of the T‐stress is generally considered only under mode I or mixed mode I and II conditions. In this paper its presence in almost pure Mode II is experimentally investigated by mean of photoelasticity and its effects on the isochromatic fringe patterns are discussed. The test specimens are Brazilian discs containing sharp central cracks. After crack generation, all residual stresses are removed with thermal treatment of the specimens. Then, a compressive load is applied in a specific angle to induce mode II deformation. The observed isochromatic fringes show very good consistency with theoretical predictions. Experimental results indicate that this specimen has a negative T‐stress in mode II condition. The results calculated for KII and T from photoelastic experiments agree well with numerical results available from finite element method. 相似文献
6.
This paper presents a study on fracture of materials at microscale (∼1 μm) by the strain gradient theory (Fleck and Hutchinson,
1993; Fleck et al., 1994). For remotely imposed classical K fields, the full-field solutions are obtained analytically or numerically for elastic and elastic-plastic materials with
strain gradient effects. The analytical elastic full-field solution shows that stresses ahead of a crack tip are significantly
higher than their counterparts in the classical K fields. The sizes of dominance zones for mode I and mode II near-tip asymptotic fields are 0.3l and 0.5l,while strain gradient effects are observed within land 2l to the crack tip, respectively, where l is the intrinsic material length in strain gradient theory and is on the order of microns in strain gradient plasticity (Fleck
et al., 1994; Nix and Gao, 1998; Stolken and Evans, 1997). The Dugdale–Barenblatt type plasticity model is obtained to provide
an estimation of plastic zone size for mode II fracture in materials with strain grain effects. The finite element method
is used to investigate the small-scale-yielding solution for an elastic-power law hardening solid. It is found that the size
of the dominance zone for the near-tip asymptotic field is the intrinsic material lengthl. For mode II fracture under the small-scale-yielding condition, transition from the remote classical K
IIfield to the near-tip asymptotic field in strain gradient plasticity goes through the HRR field only when K
IIis relatively large such that the plastic zone size is much larger than the intrinsic material length l. For mode I fracture under small-scale-yielding condition, however, transition from the remote classical K
I field to the near-tip asymptotic field in strain gradient plasticity does not go through the HRR field, but via a plastic
zone.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
Experimental and numerical investigation of cracked chevron notched Brazilian disc specimen for fracture toughness testing of rock
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M.D. Wei F. Dai N.W. Xu T. Zhao 《Fatigue & Fracture of Engineering Materials & Structures》2018,41(1):197-211
The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to quantify mode I fracture toughness (KIc) of rock, and it has also been applied to mode II fracture toughness (KIIc) testing in some research on the basis of some assumptions about the crack growth process in the specimen. However, the KIc 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 KIc 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. 相似文献
8.
Jamal Bidadi Javad Akbardoost Mohammad Reza Mohammad Aliha 《Fatigue & Fracture of Engineering Materials & Structures》2020,43(2):277-291
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 KIc and KIIIc 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 KIIIc/KIc 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 KIIIc/KIc . 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. 相似文献
9.
Srgio M.O. Tavares Luis Reis Manuel de Freitas Paulo M.S.T. de Castro 《Fatigue & Fracture of Engineering Materials & Structures》2019,42(11):2441-2456
The mixed mode I‐II fatigue and fracture is briefly reviewed, addressing experimental and numerical modelling aspects, and focusing on planar specimens. One major challenge concerns the determination of equivalent stress intensity factor (Keq) in mixed mode situations. Several approaches were compared through the determination of Keq/KI over a wide range of values of KI/KII or KII/KI. Whereas all different approaches converge to the same value as KI/KII increases, the same does not happen for large KII/KI, where differences between values of Keq persist. In the regions of 0 < KI/KII < 2 and 0 < KII/KI < 2, no stable trend of results can be defined. Experimental fatigue crack growth results are presented for Al alloy AA6082‐T6. Compact tension specimens, modified with holes, and four‐point bending specimens under asymmetrical loading promoting mixed mode situations, were subjected to fatigue crack growth tests, where crack path and crack growth rate were measured. The presentation of the fatigue crack growth data was made using a Paris law based upon Keq. Differences in the Paris law constants were found for the different Keq criteria. Recent developments in numerical techniques, as the implementation of the extended finite element method (XFEM) in finite element software packages allows to determine accurately crack paths in mixed mode fracture. This article highlights concepts for mixed‐mode fatigue and fracture and supporting data, identifying challenges still to be overcome. 相似文献
10.
In the previous article a new methodology is proposed for the study on fracture criterion for the notched or cracked specimens under mixed Modes I and II. In this methodology, any value of KII/KI can be applied to the thin-walled hollow cylindrical specimen with notch or crack, the length of which is perpendicular to the specimen axis. Thus the data can be obtained covering all values of KII/KI ranging from ∝ to 0, that is, from mode II to mode I.Using this method, the experimental studies were carried out on the effects of ferrite grain diameter upon the fracture of low carbon steel under mixed Modes I and II. The experimental results were compared with various fracture criteria hitherto proposed in literatures. The fracture criterion experimentally obtained is a function of ferrite grain diameter and the value of KII/KI, at fracture increases with increase of ferrite grain diameter. On the other hand, the overall direction of the crack growth obeys approximately maximum stress criterion or energy momentum tensor criterion, independent of ferrite grain diameter. Furthermore, this also shows that the fracture criterion does not reveal directly overall direction of crack growth. These characteristics are quite similar in trend to those of the unnotched specimens. 相似文献
11.
C. Wang Z.M. Zhu H.J. Liu 《Fatigue & Fracture of Engineering Materials & Structures》2016,39(10):1193-1203
Four‐point bend experiments on black granite are conducted. The fracture behaviours of granite under pure mode I, pure mode II and I–II mixed mode are investigated, and the corresponding stress intensity factors KI , KII and the non‐singular term T‐stress are obtained through numerical–experimental method. The results are compared with the theoretical predictions of generalized maximum tangential stress criterion and other conventional criteria. It shows that generalized maximum tangential stress criterion fits the experimental results better for considering the effect of T‐stress. Contrasting with other loading configurations, the values of T‐stress for asymmetric four‐point bend specimens are much smaller, especially for pure mode II specimens, which provide an asymmetric deformation field where the T‐stress is approaching zero. 相似文献
12.
Calculation of the crack tip parameters in the holed‐cracked flattened Brazilian disk (HCFBD) specimens under wide range of mixed mode I/II loading
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J. Akbardoost H. R. Ghadirian M. Sangsefidi 《Fatigue & Fracture of Engineering Materials & Structures》2017,40(9):1416-1427
In this paper, the crack tip parameters including the stress intensity factors (KI and KII), T‐stress and the third terms of the stress field (A3 and B3) are determined comprehensively for a disk‐type sample named holed‐cracked flattened Brazilian disk (HCFBD) under various combinations of mode I and mode II loading. The HCFBD specimen is a circular disk containing a central hole in which the initial cracks are created radially from the hole circumference. Moreover, the ends of HCFBD are flattened for the sake of convenient loading. Performing enormous finite element analyses and calculating the stress intensity factors KI and KII, the states of pure mode II are determined for different configurations of HCFBD. Furthermore, the sign and magnitude of parameter A3 which plays an important role to justify the geometry and size effects on the fracture toughness of quasi‐brittle materials are also determined for HCFBD with different geometrical ratios. 相似文献
13.
A. P. Kfouri M. W. Brown 《Fatigue & Fracture of Engineering Materials & Structures》1995,18(9):959-969
Abstract A fracture criterion is proposed, based on maximum energy release rates at the tips of short kinks when the main cracks are subjected to mixed mode loading. The criterion differs from existing energy based criteria in that the fracture toughness, gc, is not independent of the stress mode prevailing in the region of the tip of the kink but is a function of the ratio of the mode II to mode I stress intensity factors at the tip of the kink, i.e., gc is determined directionally by an elliptical region with major and minor axes equal to the fracture resistances of the material, KIr and KIIr, for pure mode I and pure mode II, respectively. Points inside the elliptical region are considered safe. When KIIr is equal to KIr the ellipse degenerates into a circle and the fracture criterion reverts to the existing familiar maximum energy release rate criterion based on a single value of the fracture toughness, irrespective of the active mode prevailing in the region at the tip of the kink. In this case, under pure shear (mode II) applied load, KII, the angle of inclination of the fracture crack extension to the main crack, α, is in the region of ?76°, in general agreement with previous well established results. However, when the ratio r (=KIIrKIr) is less than r′ (=0.82, approximately) a different pattern emerges and, in particular, under pure mode II load, the crack advance is co-planar with the main crack, i.e., in mode II. A lower transition value r″ (=0.582, approximately) was also detected under pure mode I applied load. Thus for values of r≥r″, the crack extension is in pure mode I and is co-planar with the main crack but when r < r″, the crack branches out at an angle (which can be positive or negative) in mixed modes I/II crack extension. Some implications of these results are discussed. 相似文献
14.
The centrally cracked Brazilian disc specimen has been used by many researchers to study mode I and mode II brittle fracture in different materials. However, the experimental results obtained in the past from this specimen indicate that the fracture toughness ratio (KIIc/KIc) is always significantly higher than the theoretical predictions. It is shown in this paper that the increase in the ratio KIIc/KIc can be predicted if a modified maximum tangential stress (MTS) criterion is used. The modified criterion takes into account the effect of T-stress in addition to the conventional singular stresses. The fracture toughness ratio KIIc/KIc is calculated for two brittle materials using the modified criterion and is compared with the relevant published experimental results obtained from fracture tests on the cracked Brazilian disc specimen. A very good agreement is shown to exist between the theoretical predictions and the experimental results. 相似文献
15.
Effects of test procedures and lithology on estimating the mode I fracture toughness of rocks using empirical relations
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Empirical estimation is a common method for getting mode I fracture toughness KIC of rock. By collecting data from tests in this study and literature, 204 sets of KIC and tensile strength σt test data are obtained for new empirical KIC‐σt relations regression. The empirical relations make the estimation of KIC values from σt conveniently, but test procedures and lithology will influence its reasonability and reliability. Results indicate that the empirical KIC‐σt relations obtained from the four different suggested KIC test methods are all in good but obviously different linear relationship. The analyses show that cracked chevron notch Brazilian disc specimen (CCNBD) test‐based empirical relation is more accurate for estimating KIC than the other three test‐based empirical relations. As to different lithology, isotropic rocks such as sandstone and carbonatite may be more appropriate for the application of empirical estimation method. However, for coarse grained or anisotropic rocks such as granite and marble, estimation method should be applied carefully because of possibly weak KIC‐σt relations. 相似文献
16.
This paper proposes a hybrid approach to determine the fracture resistance for mode I and mixed‐mode I and II fracture specimens, combining both numerically computed and experimentally measured load (P) versus load‐line displacement (LLD or Δ) relationships for metallic fracture specimens. The hybrid approach predicates on the same principle as the conventional, multiple‐specimen experimental method in determining the energy release rate. The hybrid method computes the P–Δ curves from multiple finite element (FE) models, each with a different crack depth. The experimental procedure measures the P–Δ curve from a standard fracture specimen with a growing crack. The intersections between the experimental P–Δ curve and the numerical P–Δ curves from multiple FE models dictate the LLD levels to compute the strain energy (U) using the area under the numerical P–Δ curves. This method provides accurate estimates of the J resistance data for both SE(B) specimen under mode I loading and single‐edge notched specimens under mixed‐mode I and II loading. 相似文献
17.
Continuing the experiments on the double-edge notched specimens on which the mode II fracture toughness K
IIc of concrete was measured, a practical testing approach to determine mode II fracture energy G
IIF is studied using the same geometry. 相似文献
18.
Fracture toughness and creep crack growth characteristics under combined mode I and II loadings were studied using the compact
tension shear (CTS) specimens of polypropylene. The K
I
- K
II
envelope for crack initiation was obtained under various combined mode loadings. The creep crack growth rates da/dt under combined mode I and mode II loadings can be correlated with a single effective stress intensity factor K
Ieff
based on the combined mode fracture envelope. 相似文献
19.
Hossein Darban Mohammad Haghpanahi Abbas Assadi 《Computational Materials Science》2011,50(5):1667-1674
In this paper, a new asymmetric semicircular bend specimen (ASCB) is presented. Having more geometric parameters in asymmetric bend elements gives the opportunity of covering a wider range of K?, K?? and T-stress in comparison with classical SCB specimens. Finite element method is used to obtain these parameters from pure mode I to pure mode II. Extensive numerical calculations are made to get a wide range data for crack tip parameters of this specimen. It is observed that for ASCB specimens with specified geometries under pure mode II loading, one of the bottom supports can move horizontally without significant variation in YI. The complete sets of numerical results are obtained and can be used for verification and interpretation of future experimental results. 相似文献
20.
The method of Coherent Gradient Sensing (CGS) in transmission, in conjunction with two and three dimensional finite element methods, is used to study the effect of mode mixity on crack tip stress fields. Using a two dimensional finite element analysis the outer bounds of the region of K-dominance were determined. A three dimensional finite analysis was utilized to study the effect of mode mixity on the three dimensional nature of the stress field in the immediate vicinity of the crack tip and to obtain an inner bound of the region of K-dominance. It was noted that increasing mode mixity leads to an increased rotation of the three dimensional zone, keeping its shape and size unchanged. In contrast, the region of K-dominance is seen to dramatically depend on mode mixity, both in shape and size. In addition, an analysis of the CGS interferograms was conducted to obtain an estimate of the regions of K-dominance experimentally. A least squares fit data analysis technique was used to extract fracture parameters, namely the stress intensity factors K
I, K
II and subsequently the crack tip phase angle, . The data points used for the least square fitting were obtained from the determined regions of K-dominance. The same fracture parameters were also evaluated from the finite element analysis, and good agreement was found between experimental measurements and finite element predictions. 相似文献