Fatigue crack initiation and microcrack propagation in X7091 type aluminum P/M alloys

Fatigu crack initiation in extruded X7091 RSP-P/M aluminum type alloys o°Curs at grain boundaries at both low and high stresses. By a process of elimination this grain boundary embrittlement was attributed to Al₂O₃ particles formed mainly during atomization and segregated to some grain boundaries. It is not due to the small grain size, to Co₂Al₉, to η precipitates at grain boundaries, nor to a precipitate free zone. Thermomechanical processing after extrusion of X7091 with 0.8 pct Co was done by Alcoa to produce large recrystallized grains. This resulted in initiation of fatigue cracks at slip bands, and the resistance to initiation of fatigue cracks at low stresses was much greater. Microcrack growth is, however, much faster in the thermomechanically treated samples, as well as in ingot alloys, than in extruded and aged X7091.     

Superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy

The superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy was assessed in the temperature range of 573 to 773 K. The thermomechanical treatment (TMT) comprised of three steps of solution treatment, overaging, and warm rolling. There are large η-phase (MgZn₂) precipitate particles of average size of 1.30 μm in the overaged condition. The warm-rolled alloy undergoes continuous recrystallization at the test temperatures of 573 and 623 K, exhibiting a maximum tensile elongation of 450 pct at 573 K and a strain rate of 8 × 10⁻⁵ s⁻¹. The precipitate particles play a major role in the process of continuous recrystallization. For a given volume fraction of precipitate particles and constant amount of warm rolling (in the course of TMT), an optimum precipitate particle size is expected to maximize the rate of continuous recrystallization and render the finest recrystallized grain size. The warm-rolled alloy undergoes static recrystallization at temperatures above 673 K. The grain growth accompanying the deformation at these test temperatures limits the tensile ductility to a lower value. Irrespective of the test temperature and strain rate, the specimens undergo extensive cavitation when deformed at elevated temperatures.     

钴磷镀层表面热疲劳裂纹的萌生及扩展机理

在自约束型热疲劳试验机上对化学镀钴磷合金镀层进行了热疲劳试验,采用光学显微镜、扫描电子显微镜研究了在热应力作用下镀层表面热疲劳裂纹萌生、扩展的方式和形态。结果表明:200次冷热循环后,V型缺口处发生塑性变形,并且随冷热循环的进行,热疲劳裂纹由缺口底部萌生并沿着循环方向扩展。重点分析了钴磷合金镀层表面热疲劳裂纹的萌生及扩展机理。     

航空用超高强度钢中夹杂物导致疲劳裂纹萌生与扩展的微观行为

采用扫描电镜原位观测的方法,跟踪观察了低周疲劳载荷下航空用超高强度钢MA250中夹杂物导致裂纹萌生与扩展的微观行为,得到了MA250钢低周疲劳裂纹萌生与扩展的基本特性,讨论了精确测定MA250钢疲劳裂纹扩展速率的方法.     

Crack Propagation in Flexural Fatigue of Concrete

In this paper the behavior of concrete subjected to flexural fatigue loading is studied. Notched concrete beams were tested in a three-point bending configuration. Specimens were subjected to quasi-static cyclic and constant amplitude fatigue loading. The cyclic tests were performed by unloading the specimen at different points in the postpeak part of the quasi-static loading response. Low cycle, high amplitude fatigue tests were performed to failure using four different load ranges. The crack mouth opening displacement was continuously monitored throughout the loading process. Crack propagation caused by quasi-static and fatigue loads is described in terms of fracture mechanics. It is shown that the crack propagation in the postpeak part of the quasi-static load response is predicted using the critical value of the mode I stress intensity factor (KIC). The ultimate deformation of the specimen during the fatigue test is compared with that from the quasi-static test; it is demonstrated that the quasi-static deformation is insufficient as a fatigue failure criterion. It is observed that crack growth owing to constant-amplitude fatigue loading comprises two phases: a deceleration stage when there is a decrease in crack growth rate with increasing crack length, followed by an acceleration stage where the rate of crack growth increases at a steady rate. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the peak load of the quasi-static response. Analytical expressions for crack growth in the deceleration and acceleration stages are developed, wherein the expressions for crack growth rate in the deceleration stage are developed using the R-curve concept, and the acceleration stage is shown to follow the Paris law. It is observed that the crack length at failure for constant amplitude fatigue loading is comparable to that of the corresponding load in the postpeak part of the quasi-static response. Finally, a fracture-based fatigue failure criterion is proposed.     

The Effect of ITMT’s and P/M Processing on the Microstructure and Mechanical Properties of the X7091 Alloy

The influence of microstructure and texture on the monotonic and cyclic properties of X7091-T651 was investigated. The various structures were developed from conventional ingot metallurgy (I/M), powder metallurgy (P/M) and intermediate thermal mechanical treatments (ITMT). Powder metallurgy produced a finer grain structure and particle distribution than I/M. Intermediate thermomechanical treatment produced a recrystallized, coarse grain structure with a weak texture, compared to the unrecrystallized grain structure and sharp texture obtained with conventional processing (CP). All materials had comparable monotonic properties. The resistance to fatigue crack initiation (FCI) increased with both a reduction in grain size and a finer particle distribution. Smaller grain sizes and finer particle distributions reduced the degree of cyclic strain localization. The CP-P/M alloy had the poorest ductility and FCI resistance of all the materials, although the slip was fairly homogeneous. This may be due to the presence of oxides at the grain boundaries and a sharp texture. The threshold stress intensity, ΔK_th, and the fatigue crack growth rate (FCGR) roughly follow a grain size dependence with the resistance of fatigue crack propagation (FCP) increasing with increasing grain size. It appears that large grains allow more reversible slip and reduce the amount of accumulated plastic strain within the reverse plastic zone. It is also believed that a greater degree of fatigue crack closure, which may be associated with large grains and a rough FCP surface, results in a lower FCGR in the lowΔK region. The intermediate thermomechanical treatment of P/M X7091 produced the optimum microstructure giving the best combination of mechanical properties. The important features include a small recrystallized grain structure, a fine particle distribution, a weak texture, and a low concentration of oxides at grain boundaries. Formerly Director, Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA.     

Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment

The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios. Threshold stress intensity range, ΔK₀, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95 pct relative humidity air, with ΔK ₀ decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained in humid air with the data forvacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK₀ for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental and microstructural contributions.     

Mechanisms for Solidification Crack Initiation and Growth in Aluminum Welding

In the present work, mechanisms are proposed for solidification crack initiation and growth in aluminum alloy 6060 arc welds. Calculations for an interdendritic liquid pressure drop, made using the Rappaz–Drezet–Gremaud (RDG) model, demonstrate that cavitation as a liquid fracture mechanism is not likely to occur except at elevated levels of hydrogen content. Instead, a porosity-based crack initiation model has been developed based upon pore stability criteria, assuming that gas pores expand from pre-existing nuclei. Crack initiation is taken to occur when stable pores form within the coherent dendrite region, depending upon hydrogen content. Following initiation, crack growth is modeled using a mass balance approach, controlled by local strain rate conditions. The critical grain boundary liquid deformation rate needed for solidification crack growth has been determined for a weld made with a 16 pct 4043 filler addition, based upon the local strain rate measurement and a simplified strain rate partitioning model. Combined models show that hydrogen and strain rate control crack initiation and growth, respectively. A hypothetical hydrogen strain rate map is presented, defining conceptually the combined conditions needed for cracking and porosity.     

Analytical Prediction of Fatigue Crack Propagation in Metals

An analytical model for fatigue crack propagation of long cracks in metals and metal alloys is presented. The key features of the model are an extension of Griffith’s theory of fracture to include fatigue, a dislocation model for the crack tip opening displacement, and cyclic plasticity-induced closure. The net cyclic stretch of the process zone at the crack tip plays a major role in the fatigue crack propagation under cyclic loading. Only constant amplitude loading is considered in this paper. The model predictions utilize only the readily available material properties, such as Young’s modulus, yield strength, threshold stress intensity factor, and the fracture toughness. There are no empirical fitting constants. The model predictions are validated by an extensive amount of published fatigue crack growth studies. The agreement between the model predictions and the experimental data is good.     

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10 °C to 80 °C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from ~150 nm at room temperature to ~400 nm at 313 K (40 °C). A re-evaluation of published data reveals the apparent activation energy, E _a for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently ~35 kJ/mol for temperatures above ~313 K (40 °C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al₂CuMg where E _a is ~80 kJ/mol. For temperatures below ~313 K (40 °C) E _a is independent of copper content for stress intensity factors below ~14 MNm^−3/2, with a value ~80 kJ/mol but is sensitive to copper content for stress intensity factors above ~14 MNm^−3/2, with E _a , ranging from ~35 kJ/mol for copper-free alloys to ~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently ~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested, depending on the circumstances, that intergranular crack propagation in humid air and distilled water can be enhanced by the generation of aluminum hydride, AlH_3, ahead of a propagating crack and/or its decomposition after formation within the confines of the nanoscale volumes available after increments of crack growth, defined by the crack arrest markings on intergranular fracture surfaces.     

Fatigue Strength and Crack Initiation Mechanism of Very-High-Cycle Fatigue for Low Alloy Steels

The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5?Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 10⁸ cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with ??fish-eye?? pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (K _I ) at different regions of fracture surface were calculated, indicating that the K _I value of fish-eye crack is close to the value of relevant fatigue threshold (??K _th ). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations.     

Corrosion Fatigue Crack Initiation in a Mode II Notch Specimen

The stress intensityK _II of a Mode II specimen was calculated using a finite element methodvia theJ integral. The site, direction, and the threshold value for crack initiation from the notch under cyclic Mode II loading in air, in water, and under dynamic charging with hydrogen were investigated. The results showed that the Mode II fatigue crack in a high strength steel initiated at or close to the site of the maximum principal stress, rather than at the site of the maximum shear stress, and the subsequent crack growth was oriented approximately normal to the direction of the maximum principal stress. The site and direction of crack initiation in water and under dynamic charging with hydrogen were similar but different from that in air. The threshold values for crack initiation in air, in water, and under dynamic charging were 28.8, 12.3, and 10.2 MPa m^1/2, respectively. The fracture surface of a corrosion fatigue crack in water and under dynamic charging consisted of intergranular facets at low ΔK _II values but of quasi-cleavage at higher ΔK _II values and were different from those in air.     

Effects of Pore Position in Depth on Stress/Strain Concentration and Fatigue Crack Initiation

The stress field around a pore was analyzed as a function of the pore position in depth in the surface of a linear elastic solid using finite element modeling. It was found that the pore depth dominated the stress field around the pore on the surface and that the maximum stress was increased sharply when the pore intercepted with the surface at its top. Given the applied nominal stress, the magnitude of the maximum main stress only depended on the relative depth of the pore, while the pore size affected the stress distribution in the surface. An elastic-plastic model was also used to account for the yielding effect in the region where stress was over the yield strength. The results still indicated a significant maximum stress concentration when the pore was just buried underneath the surface, but with a lowered value than that of the linear elastic model. These results were consistent with the experimental observations that fatigue cracks were preferably initiated from pores and particles, which were just intercepted at their top with the sample surface or just buried beneath the surface.     

The development of two texture variants and their effect on the mechanical behavior of a high strength P/M aluminum alloy,X7091

The influence of texture on the deformation behavior and monotonic and cyclic properties of two recrystallized P/MX7091-T651 plates was investigated. Thermal mechanical treatments were used to produced two different textures which varied in intensity by a factor of four. The two plates had similar grain and precipitate structures. The deformation behavior and mechanical properties were correlated with grain orientation and grain boundary misorientation. Differences in fracture surface roughness and crack deflection frequencies were observed for the two texture variants during fatigue crack propagation studies. Deformation behavior, crack closure, and crack deflection affected the fatigue crack growth rates. A small but measurable improvement in tensile strength, fatigue strength, and fatigue crack growth resistance was obtained in the sharply textured material when compared with the weakly textured counterpart.     

Fatigue Crack Propagation in Friction Stir Welded and Parent AA6082

The fatigue crack propagation characteristics of a friction stir welded Al‐Mg‐Si alloy, 6082, have been investigated. The electrical potential drop method was used for measurements. A low and a high load ratio (R) level were tested. At low load ratio (R=0.1) and a low stress intensity δK the propagation rate in the weld was higher than in the parent material by a factor of 3 to 5. However, the propagation rates were approaching each other close to fracture. At high load ratio (R=0.8) the propagation rate was similar in the parent material and weld. The weld crack growth rate was about the same at low and high R (except close to fracture), while the parent material growth rate increased at high R. Paris law was used to describe the measured crack propagation rates in the weld. In the case of the parent material, showing an R‐dependence, Forman's law was used.