A study of crack closure under constant amplitude loading for 6063-T6 Al-alloy

Crack closure experiments were performed on 6063-T6 Al-alloy, using a COD gauge for various load ranges (Δp) and stress ratios, R. On the basis of the experimental results a model for effective stress intensity range ratio U was developed. This model was found to be a function of the stress ratio, R, and was fitted to existing constant amplitude crack propagation data for 6063-T6 Al-alloy. The crack closure load stabilized after 1 mm initial crack growth.     

Investigation of yield strength and single cycle overload on crack closure

Crack closure experiments were performed on 6063-T6 and 6061-T6 aluminium alloys, using a crack opening displacement gauge, for various overload ratios (1·67, 1·88 and 2·06). On the basis of these experiments some relationships are developed. The delay period after application of a single overload increases with increase in the magnitude of overload. The retardation is decreased with increase in prestrain for the same overload ratio. After the overload cycle the increase in U values is less in prestrained material than in the as-received material. Crack growth rate also decreases after application of an overload cycle; it attains a constant amplitude crack growth rate and crack closure value after a certain number of cycles. For the same stress ratio, the delay period in the 6063-T6 alloy was found to be more than in the 6061-T6 alloy. The experimental results when plotted on log-log graph paper, show that N_D/N_CAL vs overload ratio and crack length (when U = U_min) vs overload ratio fit a straight line, from which the power laws are developed.     

Effect of stress ratio on effective stress range ratio and crack growth in 6061-T6 Al-alloy

Crack closure experiments using a COD gauge were performed on 6061-T6 Al-alloy, for various stress ratios. Experimental results show that for a given stress ratio, R, the crack length increases up to 1 mm after which it is stabilized. On the basis of the experimental results a model for effective stress range ratio U, which was found to be a function of stress ratio R, was developed. This model was fitted to existing constant amplitude crack propagation data for the 6061-T6 Al-alloy.     

Effect of variable single cycle peak overload on fatigue life

Crack propagation experiments were conducted on 6061-T6 Al-alloy, for various overload ratios (1·75, 2·00, 2·15, 2·25 and 2·5). On the basis of these experiments a power law is developed to predict the delay period. The delay period after the application of a single overload increases as the magnitude of overload increases. Crack growth decreases after the application of an overload cycle and after a certain number of cycles it tends to attain its CAL crack propagation rate.     

Study on the influence of magnitude and instant of overload on fatigue

Experiments on single tensile overload applied at different crack length were performed on 6063-T6 Al alloy. Experimental results show that increasing the magnitude of overload increases crack growth retardation. It was also found that crack growth retardation decreases when overload is applied at larger crack length. A model for finding the number of delay cycles by considering overload ratio and instant of overload is developed.     

Experimental observation of crack propagation in 6063-T6 al-alloy under constant amplitude loading

Crack propagation experiments were performed on 6063-T6 Al-alloy for various load ranges and stress ratios. Experimental results show that for a constant load range, the life of specimens decreased as stress ratio increased. At constant maximum load, the life of the specimen increased as the load ratio increased. The crack growth data were analysed in terms of ΔK_eff as a function of stress ratio R. The data covered R values from 0 to 0·5 and a good relationship was obtained for K_eff/K = 0·55 + 0·12 R². A crack growth rate equation was developed.     

Experimental observations on mixed mode fatigue crack propagation

Fatigue crack propagation experiments were conducted on D16AT Al alloy to study the effect of load ratio on mixed mode fatigue crack propagation. The experiments were conducted on a Compact Tension Shear specimen at different crack angles in both mode I and mixed mode conditions. At each angle tests were conducted at three different load ratios of 0·1, 0·25 and 0·5. It has been observed that the crack growth rate increases with decrease in load ratio. The fracture surfaces were analysed under the scanning electron microscope. The effect of load ratio was studied in terms of striation spacing.     

Retardation of fatigue crack propagation by indentation technique

This paper used indentations to retard crack development and thus to prolong the crack growth life. The growth retardation resulting from the indentation-induced strain hardening and the crack closure due to indentation-induced residual stresses were explored. The retardation tests using 3–10 kN indentation loads were performed on different thickness specimens of AISI 4130 low alloy steel and AISI 304 stainless steel. These loads were applied using a hemispherical indenter to both sides of the expected crack path on the specimen surfaces. Loads of 4.5 kN and greater increased the microhardness at and around the indentation position, indicating that at those loads the hardness or the strain hardening contributed to retarding crack growth. In addition, all the loads caused different levels of crack closure. The greater the loads, the stronger were the crack closure effect and the accompanying growth retardation. In the 3.5 mm thick AISI 4130 specimens, a 10 kN load exerted the strongest growth retardation to arrest the post-indentation crack propagation.     

Statistical analysis of fatigue crack growth for 16MnR steel under constant amplitude loads

A statistical investigation of the fatigue crack propagation process has been carried out. Twenty-one pre-cracked specimens obtained from the same sheet of Chinese steel 16MnR were subjected to identical load and environmental conditions. Tests were conducted under constant amplitude loads in order to study the effect of material property randomness. From the experimental data, the distribution of the crack size at any given number of cumulative load cycles, or the distribution of the number of load cycles to reach any given crack size, is determined. The statistical distribution of the crack growth rate at any given number of load cycles or at any given crack size is also discussed. The statistical investigation methods of the fatigue crack propagation process, proposed by this article, were also found to be suitable for other materials.     

Tearing–fatigue interactions in 316L(N) austenitic stainless steel

This paper presents the results from a programme of tearing, fatigue and tearing–fatigue tests performed on specimens from a 316L(N) stainless steel plate. All tests were carried out at ambient temperature. The experimental results have been compared with assessments performed using current guidance within the R6 defect assessment method. The work has shown that there is some evidence that fatigue cycling modifies the JR-curve behaviour of this material. In most cases, the data lie approximately 20–30% above the base-line JR-curve. However, whilst there may be a modest influence of fatigue crack growth on the ductile tearing characteristics, it is difficult to separate this from experimental scatter. In tearing–fatigue tests performed at a stress ratio, R=0.2, ductile tearing reduces the fatigue crack growth rates by up to 50%. This is likely to result from the presence of a residual compressive zone at the crack-tip, and increased crack closure due to the irregular and non-matching fracture surfaces generated by the ductile crack growth mechanisms. For R=0.1 tearing–fatigue tests, fatigue crack growth rates are apparently enhanced by a factor up to of 10, particularly during the latter stages of the tests when ΔK>60 MPam. This is likely to result from: (i) loading being in the elastic–plastic regime where the J-integral (rather than K) characterises the crack-tip fields, (ii) increments of ductile tearing which may occur during each fatigue cycle, and (iii) crack blunting which reduces crack closure effects. For the R=0.2 tearing–fatigue tests, the linear summation approach described in R6 provides a consistently conservative prediction of ductile, fatigue and total crack growth during the tests. However, for the R=0.1 tearing–fatigue tests, the Paris law under-predicts fatigue crack growth rates. This may be corrected by using the Kaiser equation, which acknowledges loading in the elastic–plastic regime and incorporates incremental growth due to tearing as well as fatigue. R6 provides conservative predictions of instability for the CT specimen geometry tested in the current programme, both in terms of the critical crack growth and load required for instability to occur.     

冻融损伤混凝土梁抗力性能演化的裂缝分形特征

为分析冻融循环作用下钢筋混凝土梁受压区的表面裂缝分布特征,采用自主设计的混凝土梁反力试验架对受压区受冻融循环作用的梁进行抗弯试验,取得各级荷载作用下梁表面的裂缝图像。依据分形理论计算表面裂缝分布的分形维数,并讨论分形维数与梁的荷载、跨中挠度、屈服荷载和冻融循环次数之间的关系。研究表明,受压区冻融损伤的钢筋混凝土梁表面裂缝的分布符合分形特征,其分形维数与荷载、跨中挠度、屈服荷载和冻融循环次数之间均有一定关系,裂缝的分形维数可作为钢筋混凝土构件安全性能预测的指标。研究成果可为冻融损伤混凝土梁安全预测提供参考。     

Some formulae for the effective stress range ratio used in crack growth of 6063-T6 aluminium alloy

The influence of stress ratio R and stress intensity range (ΔK) on fatigue crack growth experiments were determined for 6063-T6 aluminium alloy and crack growth data were analysed with different formulae for the effective stress intensity range ratio U. The data covered R values from 0 to 0·3. A good correlation was obtained from da/dN and ΔK_eff using the equation for U as a function of R.     

基于分形理论的改良膨胀土龟裂发展规律探究

为探究改良膨胀土龟裂发展规律,通过石灰改良膨胀土的室内龟裂试验,基于分形理论,采用Matlab编程技术对膨胀土干缩裂隙图像进行数字化,并对裂隙表面结构特征进行定量统计与分析。结果表明,膨胀土的龟裂发展规律主要由基质吸力引起的应力场控制;不同石灰掺量的改良膨胀土在脱湿过程中裂隙率均先迅速增大,在塑限含水率附近达到峰值之后开始缓慢减小,最终在缩限含水率附近趋于稳定;经过石灰改良后的膨胀土裂隙率峰值出现较早,随着石灰掺量的增加峰值对应的含水率逐步减小,同时裂隙率峰值和残余值有明显的减小,当石灰掺量超过6%时改良效果不再随石灰掺量的增多而增强;可利用膨胀土裂隙率与表面的分形维数的关系估算膨胀土的裂隙率。     

柴油机燃用柴油/甲醇混合燃料时的燃烧特性研究

通过添加助溶剂形成一种稳定的柴油／甲醇混合燃料，并开展了柴油机燃用此混合燃料的燃烧特性研究。研究结果表明：随着混合燃料中甲醇含量的增加，预混燃烧阶段的放热率增加，扩散燃烧时间缩短。滞燃期随甲醇含量的增加而增加，此现象在低负荷和高转速下更为明显。甲醇含量对快燃期长短影响较小，总燃烧期随甲醇含量的增加而缩短。低转速下放热率曲线中心随甲醇含量的增加而移近上止点，最大压力升高率和最高放热率随甲醇含量的增加而增加。高转速高负荷下放热率曲线中心随甲醇含量的增加而移近上止点，高转速低负荷下放热率曲线中心随甲醇含量的增加偏离上止点；高转速下最大压力升高率和最高放热率随甲醇含量的增加而增加，而进一步增加甲醇含量反而使最大压力升高率和最高放热率降低。当混合燃料中含氧量小于6％时，缸内最高压力随甲醇含量的增加而增加；进一步增加含氧量时缸内最高压力保持不变或略有降低。缸内最高平均气体温度基本上不随甲醇含量而变化。     

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.     

Evaluation of distributed building thermal energy storage in conjunction with wind and solar electric power generation

Energy storage is often seen as necessary for the electric utility systems with large amounts of solar or wind power generation to compensate for the inability to schedule these facilities to match power demand. This study looks at the potential to use building thermal energy storage as a load shifting technology rather than traditional electric energy storage. Analyses are conducted using hourly electric load, temperature, wind speed, and solar radiation data for a 5-state central U.S. region in conjunction with simple computer simulations and economic models to evaluate the economic benefit of distributed building thermal energy storage (TES). The value of the TES is investigated as wind and solar power generation penetration increases. In addition, building side and smart grid enabled utility side storage management strategies are explored and compared. For a relative point of comparison, batteries are simulated and compared to TES. It is found that cooling TES value remains approximately constant as wind penetration increases, but generally decreases with increasing solar penetration. It is also clearly shown that the storage management strategy is vitally important to the economic value of TES; utility side operating methods perform with at least 75% greater value as compared to building side management strategies. In addition, TES compares fairly well against batteries, obtaining nearly 90% of the battery value in the base case; this result is significant considering TES can only impact building thermal loads, whereas batteries can impact any electrical load. Surprisingly, the value of energy storage does not increase substantially with increased wind and solar penetration and in some cases it decreases. This result is true for both TES and batteries and suggests that the tie between load shifting energy storage and renewable electric power generation may not be nearly as strong as typically thought.     

Crack propagation rate as a function of crack tip characteristics

The dependence of crack growth rate on various crack tip parameters was studied. Experiments were performed on thin sheets of 6063-T6 Al-alloy having a central notch, to find crack tip opening displacement, total strain range, plastic strain range, crack opening stress and crack growth rate. Crack tip opening displacement and crack opening stress were measured, using a surface measurement technique, with small crack opening displacement gauges. The theoretical predictions of crack tip opening displacement compare fairly well with the experimental values. It is found that crack propagation rate vs total strain range-plastic strain range gives a straight-line fit on a log-log graph and, for positive stress ratios, the fatigue crack growth rates are found to be independent of R.

Experimental results show that the crack opening stress is not affected by the position of the gauge when it is mounted behind and near the crack tip.

The effect of mechanical properties and loading on crack growth were also studied. The specimens were fatigue cracked to a predetermined length and some specimens were annealed and again loaded cyclically. The application of cyclic loads to annealed specimens caused significant increase in crack propagation rates in comparison with the specimens having no heat-treatment. The load-displacement record was found to stabilize in about 10 cycles; the crack then extended slowly as a fatigue crack. Crack propagation rates for different values of R for annealed and work-hardened material were plotted against a crack tip parameter, ΔK*, based on notional crack lengths. Since the results of da/dN vs ΔK* for both states of material (as-received and annealed) seem to lie on the same straight line on a log-log graph, the study provides a hope that the results for a material tested in any state (annealed or work-hardened) for positive values of R (0·0≤R≤0·3) will lie on this line, thus eliminating fatigue tests on the same material under different work-hardening conditions for different values of R. Models for da/dN have been developed using various crack tip parameters.     

Green's function approach for crack propagation problem subjected to high cycle thermal fatigue loading

An efficient numerical approach using Green's function for the analysis of crack propagation under thermal transient load has been presented. The present approach based on multi-Green's functions pre-determined for each stage of the incremental crack growth substantially shortens the calculation time of the stress intensity factor (SIF) ranges. It was shown that the Green's function method (GFM) can be efficiently used to evaluate not only thermal stresses for fatigue analysis but also the SIF for crack propagation analysis. The crack propagation analysis results have been compared with those of the actual observation for the piping structure subjected to thermal striping load in a liquid metal fast breeder reactor. It was shown that the function determined at a fixed temperature can be applied to a relatively wide range of temperatures because of the compensation effect of the material properties, that is, some properties increase while the others decrease as the temperature increases.     

主轴承盖结构优化设计

在发动机气缸体的框架设计开发过程中,由于主轴承盖结构承载着曲柄连杆机构往复运动所施加的交变载荷,主轴承盖结构的疲劳强度直接影响着气缸体的使用寿命。本文阐述设计阶段通过模拟发动机实际工作状态,对零件进行疲劳安全性能的计算,以便有效地降低框架实际生产后裂纹、断裂等失效形式的发生。