Influence of notch radius and microstructure on the fracture behavior of Al–Zn–Mg–Cu alloys of different purity

The influence of notch radius on the fracture behavior of two high-strength Al–Zn–Mg–Cu alloys with different Fe content in the T73 condition was investigated. The fracture toughness tests were performed on non-fatigue-precracked notched bending specimens with different notch radii ranged from 0.15 mm to 1.0 mm. The obtained data were interpreted using the concept of Notch Fracture Mechanics combined with finite-element method (FEM) calculations. It was found that both alloys are very sensitive to the notch radius. The fracture toughness increases with increasing notch radius. For a given notch radii, the increase in fracture toughness is more significant for the more pure alloy. The fracture behavior of investigated alloys with respect to microstructural features and their relation with the fracture micromechanisms were analyzed.     

Notch fracture toughness of high-strength Al alloys

In this study, the notch fracture toughness (NFT) of high-strength Al alloys was examined by a non-standardized procedure. The NFT is defined as the critical notch stress-intensity factor (NSIF) K_ρ,c, which is determined by using several methods of analysis and computing. A set of specimens with different notch root radii made from overaged 7xxx alloy forging was selected. The influence of the notch radius on the fracture toughness of the material was considered. It was found that the notch radius strongly affects the fracture behavior of forged 7xxx alloy in overaged condition. The notch fracture toughness was higher than the fracture toughness of a cracked specimen and increased linearly with notch radius. The critical notch radius was related to the spacing of intermetallic (IM) particles which promote an intergranular or transgranular fracture mechanism according to their size. It appeared that ductile transgranular fracture generated by the formation of dimples around dispersoids and matrix precipitates was predominant which indicates that intense strains are limited to a much smaller zone than the coarse IM particles spacing. This double mechanism is also operate for crack propagation of ductile fatigue. The nature and morphology of IM particles exert significant effects on the rate of fatigue crack growth and fracture toughness properties.     

The fracture behaviour of notched specimens of polymethylmethacrylate

The fracture behaviour of notched specimens of polymethylmethacrylate has been examined for a wide range of geometries in Charpy impact tests, and in tensile and slow bend fracture tests. It was found that the failure of the very sharply notched specimens was consistent with linear elastic fracture mechanics and defined a constant fracture toughness K _IC for a constant notch tip radius, whereas the blunt notched specimens failed at a constant critical stress at the root of the notch.     

Effect of notch root radius on fracture toughness of polycrystalline cubic boron nitride

The fracture toughness of five grades of polycrystalline cubic boron nitride (PCBN) has been determined using Single Edge V-Notched Beam specimens. Both coarse and fine grade materials were considered, containing CBN grain sizes of between 1 μm and 22 μm. The influence of notch root radius on the measured fracture toughness was examined. The notch root radius was found to have a major effect for materials with smaller CBN grain sizes while only a small effect was noted for the material with large CBN grain sizes. A simple analytical model was developed to explain the effect of the notch root radius on the fracture toughness and was found to agree well with experiment for all the materials tested. It was shown that the effect of notch root radius is directly linked to the size of the CBN grain. It is proposed that this effect results from the interaction between the microstructure and the stress field around the notch tip.     

Numerical prediction of notch bluntness effect on fracture resistance of SM490A carbon steel

The present work investigates the notch radius effect on fracture resistance using the finite element (FE) damage analysis based on the multiaxial fracture strain model. The damage model was determined from experimental data of notched bar tensile and fracture toughness test data using a sharp‐cracked compact tension specimen. Then, the FE damage analysis was applied to simulate fracture resistance tests of SM490A carbon steel specimens with different notch radii. Comparison of simulated results with experimental data showed good agreement. Further simulation was then performed to see effects of the specimen size, thickness, and side groove on J‐R curves for different notch radii. It was found that effects of the specimen size and thickness became more pronounced for the larger notch radius. Furthermore, it was found that without side groove, tearing modulus for notched specimens was similar to that for cracked specimens, regardless of the notch radius.     

Notch strength of maraging steel

A series of tensile tests was carried out on steel cylinders, on which had been machined circumferential notches of known root radii.

The properties of the notch ductility of Maraging steel were compared to similar properties of XC 10, a mild steel possessing a great ductility.

Two types of relationships were obtained: Firstly, it can be shown that the algebraic relationship between the elastic stress concentration factor and the notch strength ratio is also dependant on the root radius of the notch, and secondly, a connection was established between an extension parameter of the plastic zone around the notch and the ultimate tensile strength.

These expressions can be used to predict the ultimate tensile strength of a specimen of specific geometry.     

Fracture investigation of U‐notch made of tungsten–copper functionally graded materials by means of strain energy density

The averaged strain energy density over a well‐defined control volume was employed to assess the fracture of U‐notched specimens made of tungsten–copper functionally graded materials under prevalent mode II loading. The boundary of control volume was evaluated by using a numerical method. Power law function was employed to describe the mechanical properties (elasticity modulus, Poisson's ratio, fracture toughness and ultimate tensile stress) through the specimen width. The effect of notch tip radius and notch depth on notch stress intensity factors and mode mixity parameter χ were assessed. In addition, a comparison based on fracture load between functionally graded and homogeneous W–Cu was made. Furthermore, in this research, it was shown that the mean value of the strain energy density over the control volume can be accurately determined using coarse meshes for functionally graded materials.     

Effect of notch root radius on fracture toughness of Ti–18Al–8Nb alloy

Abstract

The effect of notch root radius on the mode I fracture toughness of Ti–18Al–8Nb alloy in beta solution treated and water quenched condition was investigated. The apparent fracture toughness K _IA was found to be independent of the notch root radius below a critical notch root radius ρ ₀ and subsequently increase linearly with the square root of notch root radius ρ^1/2 beyond ρ ₀. The critical notch root radius in this alloy was found to be ～50 μm. The results were explained on the basis of strain controlled fracture model.     

Coupling effect of primary voids and secondary voids on the ductile fracture of heat-treatable aluminum alloys

Ductile fracture of commercial aluminum alloys is controlled not only by the primary voids but also by the secondary voids, which are respectively nucleated at cracked constituents and at decohered dispersoid. In this paper, experiment and modeling were carried out to study the combined effect of the two populations of voids on the ductile fracture in two kinds of heat-treatable aluminum alloys, i.e., Al-Cu-Mg alloys and Al-Mg-Si alloys. Different heat treatments were applied to the alloys to achieve various combinations of the two voids, which were subsequently related to the mechanical properties. A multiscale fracture model was proposed to describe quantitatively the relationships between parameters of the two voids and the ductility and fracture toughness of heat-treatable aluminum alloys. It is revealed experimentally and theoretically that the presence of secondary voids will reduce the ductile properties especially when the intervoid spacing is less than about 0.5 μm. All calculations are in good agreement with experimental results.     

Effects of ferrite grain size, notch acuity and notch length on brittle fracture stress of notched specimens of low carbon steel

The effects of ferrite grain size, notch acuity and notch length on brittle fracture stress and fracture toughness of notched specimens were experimentally studied at −196°C for a low-carbon steel.

For the case of smaller notch root radius, fracture stress and fracture toughness are not so much conspicuously affected by ferrite grain size. The effect of ferrite grain size will increase with increase of notch root radius. Fracture stress and fracture toughness will decrease with increase of d^−1/2 (d = grain size diameter) a smaller range of d^−1/2, and increase nearly linearly with increase of d^−1/2 in larger range of d^−1/2, and, thus have minimum at some value of d^−1/2.     

EFFECTS OF CRACK LENGTH, NOTCH ROOT RADIUS AND GRAIN SIZE ON FRACTURE TOUGHNESS OF FINE CERAMICS

Generally, fracture toughness and fracture stress of ceramics depend on crack length, notch root radius and grain size. These three parameters are most important when assessing the integrity of structural ceramic members and developing high-performance ceramics. A new failure criterion called the process zone size failure criterion, has been proposed based on the existence of a crack-tip process zone. Using this criterion, it is shown that theoretical values are in good agreement with many test results quoted from many papers. It is concluded that this failure criterion is useful when evaluating crack length and notch root radius problems. The effect of grain size on both the fracture toughness and on the toughening mechanism is also considered.     

Effects of graphene nanoplatelets and graphene nanosheets on fracture toughness of epoxy nanocomposites

The effects of graphene nanoplatelets (GPLs) and graphene nanosheets (GNSs) on fracture toughness and tensile properties of epoxy resin have been studied. A new technique for synthesis of GPLs based on changing magnetic field is developed. The transmission‐electron microscopy and the Raman spectroscopy were employed to characterize the size and chemical structure of the synthesized graphene platelets. The critical stress intensity factor and tensile properties of epoxy matrix filled with GPL and GNS particles were measured. Influence of filler content, filler size and dispersion state was examined. It was found that the GPLs have greater impact on both fracture toughness and tensile strength of nanocomposites compared with the GNSs. For instance, fracture toughness increased by 39% using 0.5 wt% GPLs and 16% for 0.5 wt% GNSs.     

陶瓷材料断裂韧性与缺口半径 Ⅱ 断裂韧性估算方法

在陶瓷材料裂纹尖端存在一个断裂过程区，当断裂过程区内平均应力达到断裂强度时，裂纹扩展。本文由理论推导结合实验数据，得到了新断裂过程区的大小是平均晶粒直径的四倍。并由平均应力断裂模型，给出了陶瓷材料断裂韧性和缺口半径及平均晶粒直径之间的关系式，由此关系式可以用宽缺口试件测出的断裂韧性去估算陶瓷材料的本质断裂韧性。     

Investigation of blunting line and evaluation of fracture toughness under mixed mode I/III loading in commercially pure titanium

Abstract

The blunting line and fracture toughness in commercially pure titanium under mode I and mixed mode I/III loading was studied. A modified compact tension geometry was used for determining the blunting line as well as mixed mode I/III fracture toughness. The results showed that the constraint factor m in the blunting line equation under mode I loading was 1.84. Also, there was no effect of notch root radius on the slope of the blunting line. The blunting line slope under mixed mode I/III loading was found to be lower than that under mode I loading and agreed with empirical correlations. The fracture toughness under mode I loading was found to be higher for specimens with larger notch root radius. However, notch root independent fracture toughness could be obtained from blunt notch specimen tests using stretch zone width measurements. The fracture toughness was found to decrease with increasing mode III loading.     

The effect of fibre length on fracture toughness and notched strength of short carbon fibre/epoxy composites

The effect of radius of curvature on the tensile notched strength of random short carbon fibre/epoxy composites containing 1, 5 and 15 mm length fibres is studied. The strength of all laminates showed a sensitivity to the radius of curvature, with the tensile strength decreasing at smaller radii of curvature. A model is developed to predict notched strength based on assumed evolution and propagation of damage from the tip of the notch. The predictions of the model depend principally on two material properties: the unnotched tensile strength and fracture toughness. Reasonable agreement is achieved between the predicted notched strength and experimental data.     

Tensile and fracture behaviour of PP/wood flour composites

Polypropylene/wood flour composites with different fibre content were prepared. The effect of composition and the incorporation of maleinated polypropylene on the materials tensile and fracture and failure behaviour was investigated. Reliable fracture toughness data that will be useful for structural applications were obtained. In unmodified composites an increase in Young´s modulus was found with the addition of wood flour to PP, whereas tensile strength, strain at break and fracture toughness were observed to decrease as fibre content increased. The presence of MAPP was beneficial to tensile strength and ductility and had no significant effect on fracture toughness, as a result of enhanced fibre dispersion within the matrix and improved interfacial adhesion. Although reduced ductility and toughness were observed for the composites respect to the matrix, in the case of modified composites, environmentally friendly stiffer materials were obtained with cost saving without sacrificing strength.     

Dynamic fracture toughness testing of epoxy resin filled with sio2 particles

The fracture behavior of epoxy resin used as one of electrical insulation materials is generally brittle compared with that of metals. Therefore, when epoxy resin is used as a structural material, the effect of impact loading must be taken into consideration in design. In the present study, the dynamic fracture toughness of epoxy resin filled with SiO₂ particles has been evaluated both by the absorbed energy method and by the impact load obtained from the instrumented Charpy type impact test. Therefore, the absorbed energy has been analysed to evaluate the real fracture toughness. Moreover, the influence of inertial loading on the impact load must be also considered; therefore, the dynamic fracture toughness has been evaluated by the formula taking the inertial loading effect into consideration. Thus both fracture toughness values evaluated from absorbed energy and from impact load have been compared; as a result, a good agreement has been ascertained.It is common to perform impact test on specimens with blunt notches since they are easy to be prepared. However, variation of fracture toughness with notch root radius in the brittle material cannot be ignored. Therefore, the influence of notch root radius on the fracture toughness has been examined. As a result, it has been ascertained that the variation of fracture toughness with notch root radius follows the formula presented by Williams.     

Effect of notch root radius on tensile behaviour of 316L(N) stainless steel

Abstract

Type 316L(N) stainless steel (SS) is used as the major structural material for high temperature components of sodium cooled fast reactors. The influence of notch root radius on the tensile behaviour of 316L(N) SS under multi-axial stress state was investigated. Double U-notches with five different kinds of notch geometry were incorporated symmetrically into the tensile testing specimens by changing the notch root radius while keeping the gross diameter, net diameter and notch depth as the same for all the notches. The notch root radius was varied as 0·25, 0·5, 1·25, 2·5 and 5 mm. Tensile tests were carried out on the notched specimens at room temperature (298 K) and at 923 K at a constant strain rate of 3×10^?3 s^?1. The tensile strength and yield strength of notched specimen of 316L(N) SS increased with decrease in notch radius at both the temperatures and the notch severity was less pronounced at high temperature. The fractured notch surface was analysed using scanning electron microscope and unfractured notch was sliced along the axis and observed under optical microscope. Finite element analysis was performed on the models of notched specimens with various notch root radii. These results showed that Von Mises equivalent stress which was derived from triaxial stresses decreased with decrease in notch radius. The shift of location of peak values of maximum principal stress and hydrostatic stress towards the axis of the specimen, leading to formation of cracks, occurred at a lower nominal stress when the notch radius was increased.     

Prediction of intrinsic fracture toughness for brittle materials from the apparent toughness of notched-crack specimen

In this investigation, fracture process zone model is used to establish a new relationship to predict the intrinsic fracture toughness from the apparent fracture toughness of a notched-crack specimen. The parameters needed in the proposed model are very rare, such as, the fracture process zone size of materials, the notch radius. Specimens made up of two kinds of polycrystalline alumina and one soda-lime glass with notch radii as small as a few micrometer are used to verify the predictions of this model. Besides, the results also show that fracture toughness of ceramics decreases with the decreasing of notch root radius. Under condition of the radius of crack tip is not greater than the averaged grain size, the apparent toughness can be approximately regarded as the fracture toughness of the materials.     

Effect of consolidation temperature on mechanical properties of rapidly solidified Al–7Mg–1 Zr alloy

Abstract

The mechanical properties achieved via the extrusion of non-degassed billets prepared from an inert gas atomised powder of nominal composition Al–7Mg–lZr are reported. The alloy was extruded over the temperature range 350–550°C and the tensile mechanical properties and plane strain fracture toughness were evaluated. It was found that the yield strength remained fairly constant over the entire temperature range, with only a small decrease in strength observed at the highest extrusion temperature. The strength could be related to microstructure using standard models for solid solution, dispersoid, and substructural strengthening mechanisms, and the last was found to make the greatest contribution. The sensitivity of strength to subgrain size was found to be nearly three times higher than that for pure Al. The optimum combination of strength and fracture toughness was obtained for extrusion at 500°C (yield strength 400 MN m^?2; T–L K_Iv 21 MN m^?3; elongation 20%). The poor values of K_lv obtained at other temperatures were attributed to coarse dispersoids (highest extrusion temperature), undeformed powder particles (lowest extrusion temperature), and inhomogeneous dispersoid distributions (intermediate temperatures). It is concluded that extrusion process control plays an important role in determining the mechanical properties of consolidated rapidly solidified powders. Considering the excellent ductility and toughness obtained, vacuum degassing before extrusion may not be essential in the processing of inert gas atomised powders of a non heat treatable composition.

MST/1721