An improved semi-circular bend specimen for investigating mixed mode brittle fracture (original) (raw)
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Mixed mode fracture analysis using new semi-circular specimens
Computers & Structures, 1988
A new semi-circular specimen is proposed by the senior author. The specimen, which is derivable from cores, consists of an edge crack cut at different angles. Three-point bending is used for loading. Modes I, II and mixed modes can be investigated depending on the inclination of the edge crack. Data derived from the proposed specimen compare favorably with published data from mode I using conventional specimens.. Size effects are discussed.
Mixed mode fracture: numerical evaluation and experimental validation using PMMA specimens
Frattura ed Integrità Strutturale
The analysis of mixed mode crack propagation has been more accessible with the development of new numerical tools, allowing to evaluate the influence of non-mode I loadings in the crack path and in the growth rate. The aim of this work is to analyse the propagation of pre-existing cracks under pure mode I, pure mode II and unstable mixed mode I-II loading conditions. Numerical simulations with two-dimensional models created in Abaqus software were performed, and the results obtained were compared with the analytical and experimental results. To determine T-stress and stress intensity factors, the conventional finite element method was used, post-processed with the modified virtual crack closure technique (mVCCT) and / or the J-integral. The extended finite element method (XFEM) is also used to predict the crack trajectory under different loading conditions. The experimental procedures were performed in three-and four-point bending tests on single edge notch specimens of PMMA, in which a natural pre-crack was created by small impacts on a blade located over the pre-existing machined notch. The four-point bending test was performed with an asymmetrical configuration. Varying the locations of supports and loading points with respect to the crack plane, this configuration allows to create pure mode II and mixed-mode I-II situations. The three-point bending test was performed in the conventional manner, with only the distance from the load line to the crack plane being varied.
Arabian Journal of Geosciences, 2015
The semicircular specimen under three-point bending (SCB) has been widely used to investigate mode I, mode II, and mixed mode I/II fracture behavior in brittle rocks. Compared to the other numerical methods, the extended finite element method (X-FEM) models a crack independently of the finite element mesh without any remeshing step in fracture propagation. In this regard, a numerical code called MEX-FEM, based on X-FEM, has been developed for modeling crack propagation in rock material. Since dimensionless stress intensity factors (i.e., Y I and Y II) in crack modeling of SCB specimen depend on crack length, crack angle, and span ratio, these factors and crack propagation trajectory are determined for different combinations of crack angle, crack length ratio, and span ratio. A very good agreement exists between the calculated factors Y I and Y II in this research work and those reported in the literature. The results show that the values of Y I and Y II increase by increasing the span ratio. The angle in which the pure mode II occurs decreases with increasing of the crack length ratio and it increases when the span ratio increases. The crack growth is along the initial crack when the specimen is subjected to pure mode I (i.e., the crack angle is 0), whereas in mixed mode, the crack is deviated toward the upper loading point. The results of this study demonstrate the utility and robustness of the X-FEM to simulate the crack growth in rock materials.
Analysis of a new specimen for mixed mode fracture tests on brittle materials
Engineering Fracture Mechanics, 2009
Numerical and experimental studies were performed on a new fracture test configuration called the diagonally loaded square plate (DLSP) specimen. The mode I and mode II stress intensity factors were computed for different crack lengths and crack orientation angles using finite element analysis. The numerical results show that the DLSP specimen is able to provide pure mode I, pure mode II and any mixed mode loading conditions in between. Fracture experiments were also conducted on Plexiglas using the DLSP specimen. It is shown that the results obtained from the fracture tests are consistent very well with mixed mode fracture theories.
A New Fracture Toughness Test Covering Mixed-Mode Conditions and Positive and Negative T-Stresses
International Journal of Fracture, 2010
It is known that sign of T-stress in cracked specimens affects fracture toughness under mixed mode conditions. We suggest a new test involving an inclined edge cracked semi circular specimen subjected to asymmetric three-point bend loading (IASCB specimen) that covers a broad range of modes I and II SIFs and T-stress values. It can provide both positive and negative T-stresses. This is illustrated by FEM computations.
Crack propagation in mixed-mode specimens described via multi-parameter fracture mechanics
IOP Conference Series: Materials Science and Engineering, 2019
The main objective of the paper is to employ a multi-parameter fracture mechanics concept to describe crack propagation through a specimen loaded in mixed-mode. This concept in particular was used because it has been shown that application of the generalized fracture mechanics concept can play a key role for materials with specific fracture behaviour, i.e. when fracture processes occur not only in the very vicinity of the crack tip, but also at larger distances from it. Two mixed-mode (I+II) geometries for the investigation of crack behaviour are presented here. The Williams series expansion is used for crack-tip stress field approximation. Then, considering the higher-order terms of the Williams expansion with regard to maximum tangential stress criterion can provide better estimates of the crack deflection angle. The coefficients of the Williams expansion were determined by means of the over-deterministic method for the purposes of this work. This analysis was performed for each c...
A mixed-mode fracture specimen: analysis and testing
International Journal of Fracture, 1986
A mixed-mode fracture specimen which is employed to measure K I and KII failure values for 0 ~< KI//KII ~ 0.28 is analyzed. In order to perform experiments, specimen calibration formulas relating K 1 and Kll to the applied load and specimen geometric parameters must be obtained.
Mixed mode brittle fracture in PMMA—An experimental study using SCB specimens
Materials Science and Engineering: A, 2006
A series of mixed mode I/II fracture tests is conducted on polymethylmethacrylate (PMMA) in the full range from pure mode I to pure mode II using a semi-circular bend (SCB) specimen containing an edge crack. The fracture load and the path of crack growth are obtained from experiments for various crack angles. It is shown that the conventional mixed mode I/II fracture criteria such as the maximum tangential stress (MTS) criterion overestimate the fracture strength of PMMA when the SCB specimen is used for fracture tests, particularly for mode II dominant loading conditions. However, improved predictions of fracture load are achieved when a generalized MTS criterion is employed. While the path of crack growth is straight for pure mode I, it deviates significantly from the angle of fracture initiation for pure mode II and mode II dominant loading conditions. It is shown that the path of crack growth predicted by the generalized MTS criterion is also in a good agreement with the observed fracture path in the fractured SCB samples.
A mixed mode fracture specimen for mode II dominant deformation
Engineering Fracture Mechanics, 1984
theories have been proposed for the failure of metals, as well as for the angle of crack propagation in mixed mode loading. In order to demonstrate the validity of these theories, the majority of tests have been carried out with an oblique crack placed in a uniaxial stress field. Better testing conditions may be achieved by placing a crack in a uniform bidimensional stress field. A specimen which was recently developed for K,c measurement may be readily adapted to achieve a bidimensional stress field and be used for mixed mode testing for the case in which shear deformation is dominant. The main aims of this study are to examine both the cracked and untracked specimen by means of photoelasticity and finite elements in order to analyze the capabilities and limitations of this specimen for mixed mode testing. It will be demonstrated that there exists a nearly uniform biaxial field in the untracked specimen. Moreover, calibration formulas will be presented for K, and K,,.
2016
In this paper, a new disc-shaped specimen containing a tilted crack was proposed so as to conduct fracture tests under mixed mode I/III loading. This specimen was able to produce complete mode mixities, ranging from pure mode I to pure mode III. Many finite element analyses were performed to obtain crack parameters (i.e. stress intensity factors at the crack tip) and geometry factors. It was shown that the mode III was added to the mode I loading as the crack angle changed. Moreover, the crack length as well as position of the lower supports was varied to study loading type at the crack tip. Finally, applicability of the proposed specimen in experimental point of view was considered by performing fracture experiments on the asphalt concrete. The results showed that fracture strength of the asphalt concrete decreases as the proportion of mode III at the crack tip enhances.