Distribution of dimple sizes on the fracture surface of spheroidized steel in the transition region

Fractography on fractured K_IC specimens of spheroidized steel has revealed a statistical distribution of dimple sizes (in the ductile failure region) dependent on the temperature and distance from the pre-crack tip. The statistical distributions of carbide sizes and dimple sizes conform closely to a Weibull distribution. The size parameter of the statistical distribution of dimples increases with the distance from the pre-crack tip, but diminishes as the temperature rises. The shape parameter declines as the distance from the pre-crack tip increases, but grows with the test temperature. The experimental findings have confirmed a probabilistic theory of void nucleation ahead of the pre-crack tip. The dimple density on the fracture surface at a certain distance from the pre-crack tip corresponds to the density per unit area of carbides multiplied by the probability of void nucleation.     

Micromechanism of fracture in Al/SiC composites

An SEM study was made of the micromechanism of fracture in SiC particle-reinforced 6061 aluminium composites. The fracture toughness tests on the composites with SiC volume fractions of 0%, 10% and 20% were conducted on single-edge notched sheet specimens. Both qualitative observations of void nucleation at the notch root of the composite samples and quantitative measurements of crack profiles are made to assess the special role of the particle effects in these composites. The results are discussed with respect to the micromechanism of particle breakage and interface debonding and their effect on the nucleation and propagation of microcracks. Two kinds of void are defined to explain the facts that Al/SiC is brittle macroscopically and ductile microscopically. The direction of crack propagation in Al/SiC and the microstructure in the tip region of the crack are also studied with these results.     

Stop drilling procedure for fatigue life improvement

This paper investigated how the stop drilling procedure improved the crack initiation life and the total fatigue life in specimens of 6061-T651 aluminium alloy and AISI 304 stainless steel. The crack initiation life was the number of fatigue cycles initiating a 0.2 mm crack at a stophole edge. The larger the stophole diameter, the longer the crack initiation and total fatigue lives. The 2, 2.5, and 3 mm diameter stopholes improved the nonstop-drilled total fatigue life of the 6061-T651 specimen by 187%, 321%, and 443% and that of the AISI 304 specimen by 72%, 121%, and 174%. At each stophole diameter, the 6061-T651 crack initiation life improved over the AISI 304 counterpart because of lower values of the K_f factor and the ΔJ/ρ_c ratio.     

Progressive failure monitoring and analysis in aluminium by in situ nondestructive evaluation

Damage initiation and progression in precipitate hardened alloys are typically linked to the failure of second phase particles that result from the precipitation process. These particles have been shown to be stress concentrators and crack starters as a result of both particle debonding and fracture. In this investigation, a precipitate hardened aluminium alloy (Al 2024‐T3) is loaded monotonically to investigate the role the particles have in the progressive failure process. The damage process was monitored continuously by combining the acoustic emission method either with in situ scanning electron microscopy or X‐ray microcomputed tomography to obtain both surface and volume microstructural information. Particles were observed to fracture only in the elastic regime of the material response, while void growth at locations predominantly near particles were found to be associated with progressive failure in the plastic region of the macroscopic response. Experimental findings were validated by fracture simulations at the scale of particle‐matrix interface.     

Study of crack tip opening under cyclic loading taking into account the environment and R ratio

This study involves the R effect and environment effect on crack closure mode, in 7175 T 651 aluminium alloy. To obtain one of the selected objectives, it was necessary to use a clip gauge located at the notch of the compact specimen and a C.T.O.D. gauge located at the crack tip. The crack opening phenomena observed in our tests depends on the applied method which accounts for the differences in ΔK_eff found in the literature. The systematic use of the two methods allowed us to bring to light common features permitting the calculation of ΔK_eff according to Elber's criteria. The concept of ΔK_eff does not fully explain the influence of R ratio and the environment effect.     

Stress-corrosion crack growth in aluminum alloys

A macroscopic study of the growth of a stress corrosion crack in an aluminum alloy was undertaken by using the optical method of caustics. The dependence of the crack-tip stress intensity factors K_I on the applied stress, as well as on the molarity of a NaCl aqueous solution was studied in detail. The calculation of these quantities was made by taking into account the thickness variation of the specimen due to the corrosive environment. A criterion of the crack growth in terms of the caustics obtained by illuminating the cracked specimen by a light beam was formulated and the corresponding values of the threshold stress intensity factor K_ISCC were accurately determined. Finally, the creation and the evolution of the pits developed at the vicinity of the crack tip was studied and their significance on the mechanisms of fracture of the specimens was examined.     

Creep failure mechanisms in a particulate-filled epoxy resin

The deformation-induced volume damage in a series of creep specimens is examined in this investigation in order to improve the basic understanding of creep failure in particulate-reinforced epoxy resins. The results are correlated with the fracture surface morphology reported elsewhere. Volume damage was found to consist of matrix shear yielding, silica-particle debonding and matrix cracking. Fracture is shown to be initiated by shear yielding and debonding which is followed by sub-critical crack growth, demonstrating the importance of volume damage in fracture. Sub-critical crack growth occurs by debonding or by void coalescence depending on the temperature and loading conditions. The temperature and loading dependence of volume damage and the above crack propagation mechanisms are examined and presented graphically in a damage mechanism map.     

Effect of stress triaxiality levels in crack tip regions on the characteristics of void growth and fracture criteria

The behaviour of void growth in the crack tip regions of four specimen geometries with different stress triaxiality levels have been investigated by the FEM method and experimental observations in plane strain and plane stress cases respectively. It was found that the shape change of growing voids, the configurations of a blunting crack tip and the sizes of decreasing ligament between the void and the crack tip are strongly dependent upon the stress triaxiality levels. Under the condition of plane stress, the stress triaxiality on the ligaments of cracks are nearly the same for different specimen geometries, also the void growth, crack tip blunting asnd decreasing of ligament size are identical for various specimens with increasing load levels, which lead to the conclusion that the J_i-value is independent of specimen geometries. However, in the plain strain case, different void growth, crack tip blunting and decreasing in ligament size for various stress triaxiality levels directly caused the J_i-value to be dependent on the specimen geometries. It was found that when the void is linked to the blunting crack tip by the extrapolation to the zero ligament from FEM calculations, the J_i values, measured experimentally, are underestimated slightly.     

Crack initiation condition in the specimens of ductile materials

In this paper, the elasto-plastic Finite Element Method was used to simulate the ductile fracture initiation of four kinds of specimens made of low-carbon steel No. 20 (Chinese steel), and the damage parameter V_D was calculated for these specimens. The results showed that the damage mechanics criterion V_D proposed by the author and B. X. Yang [Engng Fracture Mech. 27, 371–386 (1987)], could be applied to predict the crack initiation. Also, the relationship between the damage parameter V_Dc and the crack tip opening displacement was discussed in this study.     

Temperature effect on fracture behaviour of an alumina particulate-reinforced 6061-aluminium composite

The effect of temperature on the fracture behaviour of a peak-aged alumina particulate 6061 aluminium composite was studied in the range of 25 to 180° C. The fracture toughness was found to be independent of test temperature. The role of the reinforcement phase was examined in detail at 180° C, and compared to observations at room temperature, by using an interrupted test methodology. Ductile fracture occurred at all temperatures. At room temperature the fractured particles acted as void nucleation sites and at 180° C both debonded and fractured particles were responsible for void nucleation. Large particles were found to be susceptible to fracture and nucleate microvoids earlier than small particles. A decrease in the range and size of the reinforcement phase would increase the fracture resistance for this MMC material.On leave at the Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.     

SHORT AND LONG FATIGUE CRACK GROWTH IN A SiC REINFORCED ALUMINIUM ALLOY

Fatigue crack growth behaviour in a 15 wt% SiC particulate reinforced 6061 aluminium alloy has been examined using pre-cracked specimens. Crack initiation and early growth of fatigue cracks in smooth specimens has also been investigated using the technique of periodic replication. The composite contained a bimodal distribution of SiC particle sizes, and detailed attention was paid to interactions between the SiC particles and the growing fatigue-crack tip. At low stress intensity levels, the proportion of coarse SiC particles on the fatigue surfaces was much smaller than that on the metallographic sections, indicating that the fatigue crack tends to run through the matrix avoiding SiC particles. As the stress intensity level increases, the SiC particles ahead of the growing fatigue crack tip are fractured and the fatigue crack then links the fractured particles. The contribution of this monotonic fracture mode resulted in a higher growth rate for the composite than for the unreinforced alloy. An increase in the proportion of cracked, coarse SiC particles on the fatigue surface was observed for specimens tested at a higher stress ratio.     

Crack surface relative displacement analysis of mixed-mode fracture mechanics problems

In this paper a unique criteria, crack surface relative displacement, is used to evaluate mixed-mode (mode I and mode II) fracture mechanics problems. Using a conic-section simulation of a crack surface, relationships among the energy release rate G, the stress intensity factors (K₁ and K₂), and crack surface relative displacement are developed. Because the crack surface relative displacement criterion makes direct use of the displacements on the crack surface, instead of the stress field in the region of the crack tip, it simplifies numerical analysis of crack problems. A finite element model of a slant-center-cracked plate is employed to demonstrate the applicability of crack surface relative displacement to mixed-mode problems. The numerical results obtained agree well with analytical solutions. In addition, it is illustrated that similar to K₁, K₂, and G (J in LEFM), crack surface, relative displacement can serve as a fracture criterion for general mixed-mode I and II fracture mechanics problems.     

Nanoscale void nucleation and growth and crack tip stress evolution ahead of a growing crack in a single crystal

A constrained three-dimensional atomistic model of a cracked aluminum single crystal has been employed to investigate the growth behavior of a nanoscale crack in a single crystal using molecular dynamics simulations with the EAM potential. This study is focused on the stress field around the crack tip and its evolution during fast crack growth. Simulation results of the observed nanoscale fracture behavior are presented in terms of atomistic stresses. Major findings from the simulation results are the following: (a) crack growth is in the form of void nucleation, growth and coalescence ahead of the crack tip, thus resembling that of ductile fracture at the continuum scale; (b) void nucleation occurs at a certain distance ahead of the current crack tip or the forward edge of the leading void ahead of the crack tip; (c) just before void nucleation the mean atomic stress (or equivalently its ratio to the von Mises effective stress, which is called the stress constraint or triaxiality) has a high concentration at the site of void nucleation; and (d) the stress field ahead of the current crack tip or the forward edge of the leading void is more or less self-similar (so that the forward edge of the leading void can be viewed as the effective crack tip).     

不同应力比下Cu/WCp复合材料疲劳裂纹扩展行为及影响机制分析

通过粉末冶金工艺制备了一种高压电触头用Cu/WCp颗粒增强复合材料。研究了不同应力比下Cu/WCp颗粒增强复合材料的疲劳裂纹扩展行为,并结合裂纹闭合模型和两参数驱动力模型分析了应力比对Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率的影响机制。研究结果表明:随着应力比R的增大裂纹扩展速率增大,尤其在近门槛值附近裂纹扩展速率差别最明显。裂纹闭合模型和两参数驱动力模型均可以较好地将不同应力比R下(da/d N-ΔK)关系曲线关联起来,且两参数驱动力模型的相关性更好。这说明导致不同应力比R下Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率差异的原因主要是Kmax引起裂纹尖端单调损伤,其次是裂纹闭合效应。根据SEM断口分析发现高应力比的断面较低应力比的粗糙,低应力比时断口以基体撕裂为主而高应力比时以颗粒基体脱粘为主。     

Strain energy density fracture criterion in elastodynamic mixed mode crack propagation

Sih's fracture criterion based on strain energy density, S, for mixed mode crack extension under static loading is extended to dynamic mixed mode, K_I and K_II, crack propagation. Influence of the second order term, σ_ox, which represents the non-singular constant stress acting parallel to the direction of crack propagation, on the S distribution surrounding the crack tip, is demonstrated. Numerical studies show that positive σ_ox enhances the fracture angle and negative σ_oxreduces the fracture angle irrespective of the sign of K_II/K_I, when S is measured at a critical distance r_c from the crack tip. This fracture criterion is verified by the crack curving results of dynamic photoelastic fracture specimens. Omission of σ_ox term leads to predicted fracture angles which are at variance with experimental data.     

Experimental approach to dimple fracture mechanisms under short pulse loading

Dimple fracture mechanisms are discussed for three kinds of aluminum alloys on the basis of an experimental approach and a finite element (FEM) analysis. The void growth and coalescence process was observed by an optical microscope and a scanning electron microscope. The fractographic observation for aluminum alloys 7075-T651 and 6061-T651 showed that several large voids called a dominant void are nucleated at inclusion sites or the second-phase particles ahead of the crack tip and followed by fine voids initiation leading coalescence of the dominant voids with the crack tip. On the other hand, in aluminum alloy 2017-T3, voids are nucleated very close to the crack tip and directly coalesce with the crack tip. FEM computation results suggested that the void nucleation and growth process is closely related to the triaxial stress state ahead of the crack tip.     

Modelling of ductile fracture initiation in strength mismatched welded joint

In this paper, the effect of strength mismatch and width of the welded joints on the stress–strain distribution in the crack tip region has been discussed. The single-edge notched bend (SENB) specimens (precrack length a₀/W = 0.32) were experimentally and numerically analysed. The model of local approach to fracture, proposed by Gurson, Tvergaard and Needleman, was used. High-strength low-alloyed (HSLA) steel was used as a base metal in quenched and tempered condition. The flux-cored arc-welding process in shielding gas was used. Two different fillers were selected to make over- and undermatched weld metal. The experimental analysis of fracture behaviour of the over- and undermatched welded joints was followed by numerical computations of void volume fraction in front of the crack tip. The critical void volume fraction, f_c, used in prediction of the crack growth initiation on the SENB specimen had been previously determined on a round smooth specimen. Three widths of weld metal were considered: 6, 12 and 18 mm. A comparison of the crack tip opening displacement (CTOD) values corresponding to crack initiation in the SENB specimens is given, as determined both experimentally and using the GTN model.     

Theoretical and experimental study of superimposed fracture modes I, II and III

The authors wish to present an all-fracture mode specimen with which it is possible to conduct fracture mechanics tests for pure mode I, pure mode II, pure mode III, as well as for all possible combinations of the above-mentioned. By means of a finite element analysis of this specimen, the stress intensity factors K_I, K_II, and K_III were computed. It was discovered that K_II and K_III are coupled for in-plane shear and anti-plane shear loading, i.e. a mixed state occurs locally. The integral mean along the crack front yields however only to a K_II factor for in-plane shear and to a K_III factor for anti-plane shear loading. Fracture experiments under mixed-mode loading, using this new specimen, demonstrate the influence of the loading type on the orientation and on the structure of the fracture surface.     

风力机叶片复合材料裂尖温度场及微观损伤研究

依据热力耦合建立含微缺陷叶片的裂尖温度场数值模型,并研究了微缺陷叶片断裂微观损伤方式。首先,建立裂尖温度场数学模型需要确定塑性区范围和塑性区内的内热流密度函数。基于正交各向异性复合材料裂纹尖端应力场和Tsai-Wu屈服准则理论推导,得到含微缺陷风电叶片Ⅰ/Ⅱ复合型裂纹的塑性区范围;内热流密度函数按照裂纹扩散规律构造。其次,利用电子扫描电镜技术对叶片试件的断口失效微观结构进行检测。通过红外热像仪监测微缺陷叶片试件表面温度实验,验证了裂尖温度场计算模型的准确性;确定计算温度场模型中内热流密度函数幂数为2;通过显微技术发现含气泡缺陷的叶片试件有纤维断裂、基体开裂损伤方式。