Engineering property comparisons of 7050-T73651, 7010-T7651 and 7010-T73651 aluminium alloy plate

A comparison of some engineering properties of 7050-T73651, 7010-T7651 and 7010-T73651 plate has been made. The properties investigated were strength, stress corrosion resistance, fracture toughness and fatigue crack propagation resistance under flight simulation loading.

It was found that both 7050 and 7010 are high strength deep hardenable alloys with only minor differences in crack tolerance properties. The fracture toughness of both alloys is equivalent, while 7050 possesses slightly better resistances to stress corrosion cracking and fatigue crack propagation under flight simulation loading.     

Toughness and fatigue of encapsulation-processed silicon carbide-polymer matrix particulate composites

A new process for composite fabrication was developed which improves distribution of the particulate reinforcing phase by polymer encapsulation of the particulate prior to consolidation. The effect of such processing on the fatigue-crack propagation and fracture toughness behaviour of particulate thermoplastic composites was investigated. Composites of several particulate size ranges were fabricated into disc-shaped, compact tension specimens and tested under cyclic and monotonie loading conditions. For comparison, a composite was also fabricated using a standard casting technique. The observed fatigue-crack growth rates spanned three orders of magnitude (10^–11 to 10^–9 m per cycle) over an applied stress intensity range, K, of 0.3 to 1.1 MPa m^1/2. The measured fracture toughness values ranged from 0.69 to 2.95 MPa m^1/2. Comparison of the two processing techniques indicated that encapsulation processing increased the fracture toughness of the composite by approximately 33%; however, the fatigue-crack growth behaviour was unaffected. In addition, a trend of increasing crack growth resistance (toughness) with increasing reinforcement particle size was observed. These results are discussed in the light of crack shielding and bridging models for composite toughening.     

Comparison of shear and tensile fracture in high strength aluminum alloys

A comparison was made between tensile (mode I) and shear (mode II) fracture characteristics in high strength aluminium alloys (7075-T6 and 6061-T651) using a relatively new mode II fracture specimen to evaluate the critical stress intensity factor. The enlarged plastic zone during mode II fracture required that an increased specimen thickness be used for determining K _Hc under a purely plane strain condition. Plane stress conditions prevailed in the mode II fracture of 7075-T6 with a specimen thickness less than 10 mm, while plane strain controlled mode II fracture at a thickness of 10 mm or greater. Fractographic analysis revealed a distinctive difference in the micromechanisms responsible for crack extension. Small dimples were observed only on the mode II fracture surfaces, resulting from a microvoid nucleation fracture mechanism. The mode I fracture surfaces showed a mixed distribution of dimple sizes resulting from a void growth fracture mechanism. Comparing the critical stress intensity factors, the shear mode of failure exhibited a substantially higher value than the tensile mode, resulting from the effect of the sign and magnitude of the hydrostatic stress state on the microvoid nucleation event. Zero hydrostatic tension in the mode II loading configuration helps delay microvoid nucleation, increasing the apparent toughness. The high hydrostatic tension resulting from a mode I loading configuration enhances microvoid nucleation which promotes crack propagation at relatively lower stress intensity factors.     

Fatigue-Crack Propagation in Heat-Resistant Alloys under Thermomechanical Loading. Part 2. A Method of Accounting for the Influence of Thermomechanical Loading on the Material Resistance to Fatigue-Crack Growth

A method is proposed for a comprehensive consideration of the influence of thermomechanical-loading components on the variation of the fatigue-crack propagation rate. The method takes into account the variability of the values of the calculated stress intensity factors over the crack front and with the time of a thermal cycle. Experimental results obtained for alloys KhN70VMTYu (Eacute;I617) and KhN73MBTYu (Eacute;I698) are described. For two regimes of thermal cycling, near-threshold and medium sections of kinetic diagrams of fracture of these alloys are constructed.     

平面应变断裂韧度的试验原理与过程控制

阐述了平面应变断裂韧度试验的理论模型和GB/T 4161-2007中平面应变断裂韧度KIC有效性判定依据的构建原理,在此基础上分析了预置裂纹应力强度因子和拉伸速率两个影响试验结果的因素,明确说明增大试样厚度是有效提高试验获得KIC值的方法,并以TC4和TA15两种钛合金材料的平面应变断裂韧度试验数据加以证明。     

EFFECT OF TEMPERATURE ON THE FATIGUE-CRACK PROPAGATION BEHAVIOUR OF INCONEL X-750

Abstract— Linear-elastic fracture mechanics techniques were used to characterize the effect of temperature on the fatigue-crack propagation behaviour of precipitation heat-treated. Inconel X-750 in an air environment over the temperature range 24 to 649°C. In general, crack growth rates were found to increase with increasing temperature, particularly at the highest test temperature (649°C). The effect of stress ratio on the fatigue-crack growth behaviour of Inconel X-750 was examined at 538°C, and results indicated that the elevated temperature fatigue response of this nickel-base superalloy was relatively insensitive to stress ratio level at the growth rate levels studied. Metallographic and electron fractographic examination of Inconel X-750 fatigue fracture surfaces revealed operative crack growth mechanisms to be a function of temperature and prevailing stress intensity factor. Under room temperature and intermediate temperature conditions (up to 538°C), all fatigue fracture surfaces exhibited a faceted crystallographic morphology at low crack growth rates followed by striations in the higher growth rate regime. At the highest test temperature (649°C), the fatigue crack was found to propagate by an intergranular mechanism.     

Fatigue behavior and plane-strain fracture toughness of sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy

In this study, the tensile properties, high cycle fatigue behavior and plane-strain fracture toughness of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy were investigated, comparison to that of sand-cast plus T6 heat treated magnesium alloy which named after sand-cast-T6. The results showed that the tensile properties of the sand-cast alloy are greatly improved after T6 heat treatment, and the fatigue strength (at 10⁷ cycles) of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy increases from 95 to 120 MPa after T6 heat treatment, i.e. the improvement of 26% in fatigue strength has been achieved. The plane-strain fracture toughnesses K_IC of the sand-cast and sand-cast-T6 alloys are about 12.1 and 16.3 MPa m^1/2, respectively. In addition, crack initiation, crack propagation and fracture behavior of the studied alloys after tensile test, high cycle fatigue test and plane-strain fracture toughness test were also investigated systematically.     

超高强结构钢AF1410的断裂韧度试验研究

平面应变断裂韧度是材料进行损伤容限设计时用到的重要力学性能指标,因此如何通过试验手段获得该指标显得格外重要。本文对超高强结构钢AF1410进行了平面应变断裂韧度和延性断裂韧度的试验研究,结果表明:由于该材料的断裂韧度值较高,使得确保处于平面应变状态的试样尺寸过大,难以在普通低吨位的试验机上完成,而选用合适的试样尺寸,通过延性断裂韧度试验和相关的公式计算,则可以间接地获得超高强结构钢材料的有效平面应变断裂韧度值。     

Some engineering property comparisons for 7050 and AZ 74.61 die forgings

A comparison of some of the engineering properties of 7050-T736 and AZ 74.61 die forgings is made. The properties investigated were strength, fracture toughness, stress corrosion resistance and fatigue life and crack propagation resistance. Some of the properties of these alloys are also compared with those of 7079 and DTD 5024 die forgings. It is concluded that 7050 possessed a superior combination of properties. However, the susceptibility of 7050 to general corrosion warrants extra consideration of corrosion protection systems applied to this alloy.     

Fracture toughness of turbine-generator rotor forgings

A comprehensive program is being conducted relative to applying fracture mechanics technology to large turbine-generator rotors. One facet of this program involves the determination of plane-strain fracture toughness (K_Ic) over a range of temperatures for various types of rotor steels. Data have been obtained for ten large production forgings, representing three alloys, using various types of compact K_IC and spin burst test specimens. These results demonstrate that valid K_IC data can be obtained in these types of intermediate-strength, high-toughness steels in the temperature range of practical interest. Data indicate that the plane-strain fracture toughness of these steels increases rapidly with increasing temperature and is rather high (K_tc/σ_YS > 1 in^1/2), in the application range. As a result, the critical defect sizes for catastrophic failure upon a single cycle of loading are relatively large. The plane-strain fracture toughness measurements, as well as the application of these data, are presented and discussed.     

Flight simulation fatigue crack propagation in 7010 and 7075 aluminium plate

The fatigue crack propagation resistances of 7010-T7651, 7010-T73651 and 7075-T7351 thick plate were compared for flight simulation (gust spectrum) loading conditions. The alloy 7075-T7351 was found to be slightly superior, and this superiority was not essentially due to its higher fracture toughness. The influence of specimen thickness on the crack propagation resistance was significant. Thinner specimens gave longer crack propagation lives and, generally, lower crack propagation rates and longer delays in crack growth following severe flights.     

全层状TiAl基合金断裂中晶界的双重作用

通过SEM原位拉抻技术和双晶体压缩实验研究了全片层TiAI基合金晶界断裂行为。研究表明,在全层状组织的断裂行为中,晶界具有双重作用。一方面,微裂纹首先萌发于晶界区,其扩展方式取决于晶界两侧片层的取向。另一方面,不同类型的晶界对裂纹扩展的阻力不同,因而对全层状TiAI基合金韧性的作用不同,纵向晶界有助于断裂韧性的提高,而横向晶界对合金韧性不利。     

Fatigue-crack growth in 2090 AlLi alloy

The fatigue-crack growth kinetics of the 12.7 mm thick 2090-T8E41 alloy plate vary significantly with plate thickness location. The apparent fatigue-crack growth rates in the midthickness location are substantially lower than those in the surface region at intermediate and high applied stress intensity ranges. The lower crack growth rates are related to the extraordinarily tortuous fatigue profile which induces high crack closure levels. Such a tortuous crack profile appears to be primarily a consequence of the alloy's intense crystallographic texture and its great propensity for planar slip.     

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys undergoing stress-induced martensitic transformation

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys in β solution-treated condition was investigated. The fracture toughness of the alloys was found to increase with an increase in grain size initially, reach a maximum and subsequently decrease with further increase in grain size. This trend was attributed primarily to the effect of grain size on the enhancement of fracture toughness due to stress-induced martensitic transformation (SIMT) at the crack tip, which in turn can be related to the effect of grain size on trigger stress for SIMT. Alloys containing higher Al and Nb showed a higher toughness for the same grain size, which was also explained in terms of effect of composition on the trigger stress.     

Fatigue behavior of Zr-based bulk-metallic glasses

The objective of this paper was to study the fatigue characteristics of zirconium (Zr)-based bulk-metallic glasses (BMGs) and to investigate the mechanisms of fatigue-crack initiation, crack propagation, and fracture in BMGs. The fatigue ratios (fatigue limit/tensile strength) (0.30–0.55) of Zr-based BMGs were found to be generally comparable with those of crystalline alloys, such as steel and titanium alloys. Fatigue cracks typically initiate from shear bands, inclusions, and/or porosities. The striations resulting from the blunting and resharpening of the fatigue-crack tip formed in the fatigue-crack-growth region. The fine striation spacing seems to be comparable to that of the crystalline alloys.     

NOTCH SENSITIVITY OF ALUMINUM-LITHIUM ALLOYS IN FATIGUE

In order to evaluate the notch fatigue strength and notch sensitivity of aluminum-lithium, 2090 and 8090, alloys, rotary bending fatigue tests have been carried out using circumferentially notched specimens with different stress concentration factors. The results were compared with those of traditional aluminum, 2024T4 and 7075-T6511, alloys. It was found that 2090 and 8090 alloys showed superior notch fatigue strength in comparison to the conventional aluminum alloys. The notch sensitivities to the crack initiation limit of the aluminum-lithium alloys were lower than those of 7075-T6511, while they were nearly equal to those of 2024T4 for blunt notches. The notch sensitivities to the crack propagation limit were also lower in aluminum-lithium alloys, in particular the 8090 alloy, than in the conventional aluminum alloys. It was suggested that the decreased notch sensitivities of the aluminum-lithium alloys were attributed to both the crack propagation mode and the excellent propagation resistance related to their microstructures.     

混凝土I型裂缝扩展准则比较研究

针对混凝土I型裂缝扩展问题,分别采用以起裂韧度为参数的裂缝扩展准则、最大拉应力准则以及裂尖处应力强度因子为零的裂缝扩展准则,数值模拟了强度等级C20、C40、C60、C80和C100的混凝土三点弯曲梁裂缝扩展全过程,获取了试件的荷载-裂缝口张开位移(P-CMOD)曲线并与试验结果进行了比较。结果表明,三种准则中以起裂韧度为参数的裂缝扩展准则计算得到的峰值荷载及P-CMOD全曲线与试验结果差别最小。随着混凝土强度等级的提高,最大拉应力准则以及裂尖处应力强度因子为零的裂缝扩展准则计算出的P-CMOD曲线与试验结果相比均有较为明显的偏离,但以起裂韧度为参数的裂缝扩展准则计算结果与试验曲线更为吻合。试验与计算结果表明,以起裂韧度为参数的裂缝扩展准则更适用于不同强度混凝土材料的断裂分析。     

Microstructural influences on fatigue and fracture resistance in high strength structural materials

A concise review is given of microstructural influences on fatigue strength, fatigue crack propagation, fracture toughness, and stress corrosion in high strength aluminium alloys, titanium alloys, and steels.     

The effect of D2O on fatigue-crack propagation in a high-strength aluminum alloy

The rates of fatigue-crack propagation for a high-strength aluminum alloy (7075-T651) in an environment of D₂O (99.98% purity) at room temperature were determined and compared with data obtained in high-purity argon and distilled water. The results showed that D₂O caused a ten-fold increase in the rate of fatigue-crack propagation (up to 10^–4 inch per cycle), which is equal to the increase caused by distilled water. These results lend further support to the previous observation that the rate controlling process for fatigue-crack propagation in this alloy (at rates below 10^–4 inch per cycle) is the mechanical process of creating new crack surfaces, instead of either the transport of aggressive environment to the crack-tip or diffusion of hydrogen ions into the material ahead of the crack tip.     

Plane-strain propagation of a fluid-driven fracture during injection and shut-in: Asymptotics of large toughness

This paper considers the problem of plane-strain fluid-driven fracture propagating in an impermeable elastic medium under condition of large toughness or, equivalently, of low fracturing fluid viscosity. We construct an explicit solution for a fracture propagating in the toughness-dominated regime when the energy dissipated in the viscous fluid flow inside the fracture is negligibly small compared to the energy expended in fracturing the solid medium. The next order corrections in viscosity to this limiting solution are then derived, allowing the range of problem parameters corresponding to the toughness-dominated regime to be established. The first-order small viscosity (large toughness) solution is shown to provide an excellent approximation of the solution for the crack length in the wide range of the viscosity parameter. Furthermore, this solution, when combined with the first-order small-toughness solution of Garagash and Detournay [Journal of Applied Mechanics, 2005], provides a simple analytical approximation of the crack length solution in practically the entire range of viscosity (toughness). It is also shown that the established method of asymptotic expansion in small parameter is equally applicable to study other small effects (e.g., fluid inertia) on the otherwise toughness-dominated solution. A solution for the fracture evolution during shut-in (i.e., after fluid injection rate is suddenly stopped) is also obtained. This solution, which corresponds to a slowing fracture evolving towards the toughness-dominated steady state, draws attention to the possibility of substantial fracture growth after fluid injection is ceased especially under conditions when the fracture propagation during injection phase is dominated by viscous dissipation.