Mode II Brittle Fracture Assessment Using ASFPB Specimen

The four-point bend specimen subjected to anti-symmetric loading (ASFPB) is frequently used for determining pure mode II fracture resistance of rock materials. It is shown in this paper that, when the applied loads are close to the crack plane, the ASFPB specimen does not provide pure mode II condition, since the effect of mode I also appears in crack tip deformation. A set of fracture test were also conducted on a type of marble using ASFPB configuration. The test results showed that fracture resistance is strongly dependent on the loading distance from the crack plane. The effective fracture toughness increases when the distance between the loading points and the crack plane decreases. It is shown that the enhanced fracture resistance of marble samples could be mainly because of very large negative T-stresses that exist for the mentioned loading situations.     

Brittle Fracture Analysis Using a Ring-Shape Specimen Containing Two Angled Cracks

A ring shaped specimen is used for studying mixed mode fracture in brittle materials. The ring specimen is subjected to a compressive diametral load and contains two angled central cracks. A series of fracture tests are conducted under various mode mixities using the ring specimen. It is shown that the obtained experimental results are in a very good agreement with theoretical predictions of the modified maximum tangential stress criterion.     

On Suitability of the Averaged Strain Energy Density Criterion in Predicting Mixed Mode I/Ii Brittle Fracture of Blunt V-Notches with Negative Mode I Contributions

Strength of Materials - The main goal of the present research is to check the suitability of the well-known brittle fracture criterion, namely the averaged strain energy density (ASED), in...     

Sudden Fracture from U-Notches in Fine-Grained Isostatic Graphite Under Mixed Mode I/II Loading

The U-notched maximum tangential stress (UMTS) criterion, proposed originally and utilized previously by the author and his co-researcher for predicting mixed mode I/II fracture in plexi-glass (PMMA) and also pure mode II fracture in PMMA and soda-lime glass, was employed to estimate the experimental results reported in literature dealing with brittle fracture of many U-notched fine-grained isostatic graphite plates under combined tensile/shear loading conditions. By using the fracture curves of the UMTS criterion, which can predict the onset of brittle fracture in terms of the notch stress intensity factors (NSIFs) in the entire domain from pure mode I to pure mode II, the mixed mode fracture toughness (i.e. the load-bearing capacity) of U-notched graphite plates was successfully estimated.     

Fracture Roughness Evolution During Mode I Dynamic Crack Propagation in Brittle Materials

The evolution of roughness RMS during dynamic fracture in thin plates of epoxy (Araldite B) was studied in relation to important fracture parameters such as crack velocity a, acceleration and deceleration ä and mode I stress intensity factors ( KID ). Dual-focus high speed photography was carried out to evaluate values of KID by the caustic ( KID ( C ) ) and photoelastic ( KID ( P ) ) methods simultaneously. A specially designed jig applied successive tensile loadings to specimens which resulted in cyclic change of a and ä. The changes of a, RMS and KID are shown to be qualitatively similar with respect to the value of a. Quantitatively, a typical result showed that the position of the first maximum value of a came 7 mm behind the first loading axis, while that of KID ( P ), RMS and KID, ( C ) appeared on the fracture surface 7 mm, 15 mm, and 25 mm behind the axis, respectively. The agreement between KID ( C ) and KID ( P ) was acceptable except for data around the first loading axis. Discussions are given on the uniqueness problem of a and KID.     

脆性材料断裂韧性测定的山形缺口试件和方法

本文扼要地介绍了测定新型结构陶瓷、高强度金属、以及其他脆性材料断裂韧性的山形缺口试件与方法。概述了这个方法的发展史、优缺点、目前研究概况、主要理论分析与实验研究成果,以及存在的问题和发展动向。     

Mixed Mode Brittle and Ductile Fracture of a High Strength Rotor Steel at Room Temperature

The mixed mode fracture of a high strength rotor steel has been investigated at room temperature using single edge notched specimens. In mode I, and for limited amounts of shear loading, the steel exhibited cleavage fracture. For conditions near mode~II ductile fracture occurred. A transition from brittle to ductile fracture occurred for mixed mode loading. Finite element analysis provided estimates of the extent of near crack tip yielding and elastic-plastic stress intensity factors. Test results agreed with the maximum tensile stress (MTS) criterion for small scale yielding for limited amounts of shear loading. The load for mode II fracture was lower than predicted from the MTS criterion, but higher than predicted from plastic collapse predictions. Observed fracture angles where in broad agreement with the predicted fracture mechanisms. The load for the transition from brittle to ductile fracture was found to agree approximately with the predicted load. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Brittle Versus the Ductile Nature of Fracture Modes I and II

Failure surface orientations are determined for states of uniaxail tensile stress and shiar stree appropriate to fracture Modes I and II. The method uses a newly developed failure criterion along with the associated flow rule. The failure angles show a sharp demarcation between ductile and brittle material types. Only the brittle materials class show consistency and likely represents a shear localization effect rether than explicit fracture.     

Mixed Mode I/II fatigue crack growth in adhesive joints

Fatigue crack growth (FCG) tests have been carried out on adhesively bonded compact tension-shear (CTS) specimens to assess the behaviour of a structural adhesive under Mixed Mode I/II conditions. The fractographic analysis revealed that energy dissipation mechanisms due to inelastic phenomena like bulk plastic deformation and crazing are more pronounced in Mode I than in Mixed Mode and Mode II. This is reflected by a FCG rate that increases going from Mode I to Mode II for a given value of the range of strain energy release rate, ΔG.     

基于细分粒子的刚体破裂动画

为了模拟固体在外力作用下产生的破裂现象,提出一种采用细分粒子的刚体破裂的模拟算法.该算法首先将固体的四面体网格绑定到一系列离散的粒子上;再利用光滑粒子流体动力学（SPH）对线弹性力学方程进行离散求解;并采用粒子细分的算法来进行开裂面的生成和延展.最后实现了多个固体现象的模拟,如砖块碎落、砖墙受力倒塌等.文中算法可适用于刚体脆性破碎的动画应用.     

An Efficient Mixed-Mode Brittle Fracture Experiment Using Paper

Mixed-mode fracture is a common failure phenomenon of materials, while its experiment requires at least a material test machine, test materials and special machining procedures. In this paper, a simple but effective mixed-mode fracture experiment using office paper is proposed, so one can easily conduct the mixed-mode fracture experiment and understand its fracture criteria such as the maximum tensile stress (MTS) criterion.     

Fracture behaviour of model toughened composites under Mode I and Mode II delaminations

The fracture behaviour of two toughened epoxy composite systems was investigated using various microscopy techniques. The Mode I delamination fracture toughness,G _IC, Mode II delamination fracture toughness;G _IIC, and compression after impact (CAI) strength of these model composite systems were also measured. Under Mode I fracture, it was found that these composites exhibit nearly identical toughening mechanisms to those of the rubber-modified neat resins. The composites differ primarily in having smaller damage zones than the neat resin equivalents. Under Mode II fracture, the typical hackles were found to initiate from inside the resin-rich interlaminar region due to the presence of the toughener particles. The CAI strength, based on the present study as well as the work conducted by others, appeared to be related to, but not necessarily strongly dependent on, the interlaminarG _IC andG _IIC, the thickness of the interlaminar resin-rich region, and the type of the interlaminar toughener particles. Approaches for improving theG _IC,G _IIC, and CAI strength of high-performance toughened composites are discussed.     

J-Dominance in Mixed Mode Ductile Fracture Specimens

The asymptotic mixed mode crack tip fields in elastic-plastic solids are scaled by the J-integral and parameterized by a near-tip mixity parameter, M _{_p} . In this paper, the validity and range of dominance of these fields are investigated. To this end, small strain elastic-plastic finite element analyses of mixed mode fracture are first performed using a modified boundary layer formulation. Here, a two term expansion of the elastic crack tip field involving the stress intensity factor |K| the elastic mixity parameter M _{_e} as well as the T-stress is prescribed as remote boundary conditions. The analyses are conducted for different values of M _{_e} and the T-stress. Next, several commonly used mixed mode fracture specimens such as Compact Tension Shear (CTS), Four Point Bend (4PB), and modified Compact Tension specimen are considered. Here, the complete range of loading from contained yielding to large scale yielding is analyzed. Further, different crack to width ratios and strain hardening exponents are considered. The results obtained establish that the mixed mode asymptotic fields dominate over physically relevant length scales in the above geometries, except for predominantly mode I loading and under large scale yielding conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Role of Interface Modification on the Mode I and Mode II Fracture Behavior of Glass Fabric / Vinyl Ester Composites

The objective of this study was to determine the influence of five different combinations of silane coupling agents on the Mode I and Mode II fracture toughness and strength of woven glass fabric / vinyl ester composite laminates. Double Cantilever Beam (DCB) and End Notch Flexure (ENF) tests were conducted to estimate the influence of surface treatment of glass fabric wilh methacryl silane and methanol washing, on the Mode I and Mode II fracture toughness, respectively. The results were compared with those of a conventional epoxy silane coated composite. It was found that an increasing content of methacryl silane (0.01 wt.% to 1.0 wt.%) reduced the Model fracture toughness and enhanced the Mode II fracture toughness values. Methanol washing after treatment with melhacryl sitane resulted in an increase of both Mode I and Mode II fracture toughness values.     

Study of Fatigue Behavior of Epoxy‐Carbon Composites under Mixed Mode I/II Loading

This paper presents an experimental assessment of the initiation and propagation of interlaminar cracks under mixed mode I/II dynamic fracture loading of a composite material with an MTM45‐1 epoxy matrix and unidirectional IM7 carbon‐fiber reinforcement. The aims of the experimental program developed for this purpose are to determine, on the one hand, the initiation curves of the fatigue delamination process, understood as the number of load cycles needed to generate a fatigue crack, and on the other, the crack growth rate (delamination rate) for different percentages of static Gc, in both cases for two mode mixities (0.2 and 0.4) and for a tensile ratio R = 0.1. All this with the goal of quantifying the influence of the degree of mode mixity on the overall behavior of the laminate under fatigue loading. The results show that the energy release rate increases with increasing loading levels for both degrees of mode mixity and that the fatigue limit is located around the same percentages. However, crack growth rate behavior differs from one degree of mode mixity to the other. This difference in the behavior of the material may be due to the varying influence of mode I loading on the delamination process.

     

Milling-Related Brittle Fracture Mechanisms of a SiCp/Al Composite

Strength of Materials - SiCp/Al composites were machined with carbide cutting tools, the surface morphology of the materials was examined with SEM and EDXS. The SiCp removal in the aluminum matrix...     

Brittle Fracture of Hot-Forging Cast Dies

The site applications of cast hot-forging dies made from a new type cast steel were performed and several of the dies experienced brittle fracture. Analysis of the operating modes and evaluation of mechanical properties were used to establish relations among failure patterns and lives of dies. The results show that the dies experience elevated-temperature wear, plastic deformation and thermal fatigue and that brittle fracture was the predominant failure pattern. Brittle fracture of cast dies has two patterns: early brittle fracture and impact fatigue fracture. The early brittle fractures seldom occur but, when this failure mode occurs, the failure severely shortens the life of the cast dies. The early brittle fracture is attributed to abnormal heat-treatment and operation. Impact fatigue fracture is the dominant failure pattern in the cast dies. This failure mode usually initiates from sand inclusions, micro-cracks and/or thermal fatigue cracks. The specifications of the die material should include a requirement for impact toughness to assure that the dies are resistant to multiple impact loads. This means that strength-dominant toughness must be controlled and optimization of the heat-treatment processes must focus on this requirement.     

Mixed Mode Fracture of Concrete under Proportional and Nonproportional Loading

A novel testing procedure for mixed mode crack propagation in concrete is presented: four point bend of notched beams under the action of two independent force actuators. In contrast to classical procedures, this method allows nonproportional loading and crack trajectory modifications by changing the action of one actuator. Different experimental crack trajectories, under mixed mode and nonproportional loading, are presented together with the corresponding curves of load-CMOD and load-displacement. The tests were performed for three homotetic specimen sizes and two mixed mode loading conditions. The results are useful for checking the accuracy of mixed mode fracture analytical and numerical models. The models should predict the crack trajectory and a complete group of experimental records of load and displacements on several control points in the specimen. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamic Fracture of Nominally Brittle Materials

Current understanding of dynamic fracture mechanisms and the methods of modeling are reviewed critically. Experimental methods used in dynamic fracture investigations and key experimental observations are reviewed. This is followed by a critical review of the dynamic fracture models. Mechanistic and phenomenological models as well as discrete and continuum models and their ability to reproduce experimental results are discussed.