Thermal striping fatigue damage in multiple edge-cracked geometries

Thermal fatigue striping damage may be caused when incompletely mixed hot and cold fluid streams pass over the surface of a component or structure containing a defect. Stress intensity factor (SIF) fluctuations are developed in response to the surface temperature fluctuations. An existing methodology for the analysis of striping damage in geometries containing a single edge‐crack geometry is extended to such an analysis of multiple edge cracks. SIFs are calculated as functions of crack depth, when an edge‐cracked plate and semi‐infinite solid, each containing multiple cracks, are subjected to thermal striping. The effect of various restraint conditions and striping frequencies on the SIF values for a stainless steel plate is examined. The degree of conservatism is shown when an assessment of thermal fatigue striping damage is based on a single, rather than multiple, crack analysis. Accurate curve fits are developed resulting in practical weight functions for an edge‐cracked plate and semi‐infinite solid.     

Surface damage evolution of engineering steel

The surface damage evolution of ordinary low carbon steel under mechanical fatigue loading was observed using a long‐distance microscope. Fatigue cracks initiated at pits, sulfide inclusions and in the soft ferrite phase. Damage evolution was not only pronounced by the initiation of well‐defined cracks. Areas of high plastic activity interspersed with microcracks exist. Coalescence of propagating well‐defined cracks with these areas of high plastic activity shows clearly the contribution of such plastified areas to the damage accumulation. Therefore, well‐known damage parameters such as line counts (i.e. maximum crack length) are unsuitable to describe the damage evolution. In this paper, a new damage parameter is defined based on the observation that cracks and plastified areas appear as dark regions. It is shown that this parameter satisfies the basic conditions for a damage parameter. It is used to evaluate the damage accumulation quantitatively based on surface observations even for complex damage patterns.     

Thermoelasticity for the analysis of crack tip stress fields — a review

A comprehensive review is given of methods to determine the stress intensity factor at crack tips using thermoelastic stress analysis. In order to obtain accurate results a number of areas of experimental procedure need to be considered and these are discussed in detail. The paper concludes with a discussion on the future potential of the use of thermoelasticity for the analysis of cracks.     

Microstructural effects on the short crack behaviour of a stainless steel weld metal during low-cycle fatigue

An experimental study into microstructural effects on short fatigue crack behaviour of 19 stainless steel weld metal smooth specimens during low-cycle fatigue is performed by a so-called ‘effective short fatigue crack criterion’. This material has a mixed microstructure in which it is difficult to distinguish the grains and measure the grain diameter. The columnar grain structure is made up of matrix-rich δ ferrite bands, and the distance between the neighbouring rich δ ferrite bands is an appropriate measurement for characterizing this structure. Particularly, the effective short fatigue cracks (ESFCs) always initiate from the bands of δ ferrite in the matrix in the weakest zone on one of the specimen surface zones which is orientated in accordance with the inner or outer surface of welded pipe from which the specimens were machined. These cracks exhibit characteristics of the microstructural short crack (MSC) and the physically small crack (PSC) stages. The average length of the ESFCs at the transition between MSC and PSC behaviour is ≈40 μm, while the corresponding fatigue life fraction is ≈0.3 at this transition. Different from previous test observations, the growth rate of the dominant effective short fatigue crack in the MSC stage still shows a decrease with fatigue cycling under the present low-cycle fatigue loading levels. A statistical evolution analysis of the growth rates reveals that the short fatigue crack growth is a damage process that gradually evolves from a non-ordered (chaotic) to a perfectly independent stochastic process, and then to an ordered (history-dependent) stochastic state. Correspondingly, the microstructural effects gradually evolve from a weak effect to a strong one in the MSC stage, which maximizes at the transition point. In the PSC stage, the effects gradually evolve from a strong to weak state. This improves our understanding that the short crack behaviour in the PSC stage is mainly related to the loading levels rather than microstructural effects.     

Transient analysis of multiple parallel cracks under anti-plane dynamic loading

The problem of a homogeneous linear elastic body containing multiple non-collinear cracks under anti-plane dynamic loading is considered in this work. The cracks are simulated by distributions of dislocations and an integral equation relating tractions on the crack planes and the dislocation densities is derived. The integral equation in the Laplace transform domain is solved by the Gaussian–Chebyshev integration quadrature. The dynamic stress intensity factor associated with each crack tip is calculated by a numerical inverse Laplace scheme. Numerical results are given for one crack and two or three parallel cracks under normal incidence of a plane horizontally shear stress wave.     

Influence of ferrite-martensite microstructural morphology on the low cycle fatigue of a dual-phase steel

A CMnSi dual-phase steel has been heat-treated to yield three different microstructures — a dispersion of martensite in fine-grained ferrite (Type I), a continuous network of martensite around ferrite grains (Type II) and martensite islands encapsulated in a ferrite matrix (Type III) — and low cycle fatigue tests have been conducted. All three conditions have been found to exhibit cyclic hardening. The lower hardening in the Type I microstructure and its cyclic softening at lower strains are attributed to a more uniform initial distribution of dislocations because of the dispersion of the martensite. The Type I microstructure displays the best fatigue performance and is shown to obey the Coffin-Manson law. The deviation of the other two microstructures from this law is shown to be due to a transition from a ductile mode of fracture at lower strains to a brittle type of fracture at higher strains.     

层间颗粒增韧HT3/QY8911的损伤阻抗和损伤容限

采用热塑性颗粒对双马基体复合材料层合板HT3/ Q Y8911 进行层间增韧, 制备未加入增韧颗粒的对比件和加入增韧颗粒的改性件。通过准静态压痕(QSI) 实验和冲击后压缩实验, 研究了层间增韧后HT3/ Q Y8911的损伤阻抗和损伤容限特性。结果和分析表明, 改性件在静压痕力下具有较高的分层起始载荷, 或在相同载荷水平下具有较小的损伤面积。提出从分层起始点载荷大小和载荷下降程度两方面来衡量试件对准静压痕力的响应。在相同的静压痕力或单位厚度冲击能量下, 层间增韧后的HT3/ Q Y8911 具有较深的凹坑, 表明塑性变形较大的层间区域以凹坑为代价换取了较小的损伤面积, 具有较高的损伤阻抗。在冲击后压缩面积相差不大的情况下, 层间增韧件的冲击后压缩强度和压缩破坏应变显著增加。      

Experimental and finite element analyses on stress intensity factors of an elliptical surface crack in a circular shaft under tension and bending

Experimental backtracking technique and finite element analysis have been employed to evaluate the stress intensities along the front of an elliptical surface crack in a cylindrical rod. The finite element solution covers a wide range of crack shapes loaded under end-free and end-constrained axial tension and pure bending. Convenient closed form stress intensity expressions along the whole crack front for each of the loading cases have been given in terms of the crack aspect ratio, crack depth ratio and place ratio.The closed form solutions have been compared against a number of representative solutions collected from the literature. It has been found that different finite element results for the interior points are generally in good mutual agreement, while solutions derived from other methods may sometimes indicate different trends. At the surface interception point agreement is less good because of a complication in the interpretation of stress intensity there.Experimental backtracking results on the end-constrained axial tension case corroborate well with the closed form solution presented. It suggests that the current closed form solution is adequate in describing the stress intensities along the whole crack front of real surface cracks in cylindrical rods.     

Fatigue strength of steel and aluminium welded joints based on generalised stress intensity factors and local strain energy values

Weld bead geometry cannot, by its nature, be precisely defined. Parameters such as bead shape and toe radius vary from joint to joint even in well-controlled manufacturing operations. In the present paper the weld toe region is modelled as a sharp, zero radius, V-shaped notch and the intensity of asymptotic stress distributions obeying Williams’ solution are quantified by means of the Notch Stress Intensity Factors (NSIFs). When the constancy of the angle included between weld flanks and main plates is assured and the angle is large enough to make mode II contribution non-singular, mode I NSIF can be directly used to summarise the fatigue strength of welded joints having very different geometry. By using a large amount of experimental data taken from the literature and related to a V-notch angle of 135°, two NSIF-based bands are reported for steel and aluminium welded joints under a nominal load ratio about equal to zero. A third band is reported for steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch and units for NSIFs are the same as conventional SIF used in LEFM. Afterwards, in order to overcome the problem related to the variability of the V-notch opening angle, the synthesis is made by simply using a scalar quantity, i.e. the mean value of the strain energy averaged in the structural volume surrounding the notch tips. This energy is given in closed form on the basis of the relevant NSIFs for modes I and II and the radius R_C of the averaging zone is carefully identified with reference to conventional arc welding processes. R_C for welded joints made of steel and aluminium considered here is 0.28 mm and 0.12 mm, respectively. Different values of R_C might characterise welded joints obtained from high-power processes, in particular from automated laser beam welding. The local-energy based criterion is applied to steel welded joints under prevailing mode I (with failures both at the weld root and toe) and to aluminium welded joints under mode I and mixed load modes (with mode II contribution prevailing on that ascribable to mode I). Surprising, the mean value of ΔW related to the two groups of welded materials was found practically coincident at 2 million cycles. More than 750 fatigue data have been considered in the analyses reported herein.     

Effect of test frequency on fatigue strength of low carbon steel

Ultrasonic fatigue tests (test frequency: 20 kHz) and conventional tension–compression fatigue tests (10 Hz) have been conducted on annealed and 10% pre-strained specimens of 0.13% carbon steel. Small holes were introduced on the specimen surface to investigate the effect of test frequency on small crack growth. The dynamic stress concentration factor and the stress intensity factor under ultrasonic fatigue tests were checked to be almost the same as those of conventional tension–compression fatigue tests. However, the fatigue properties were dependent on the test frequency. Ultrasonic fatigue tests showed longer fatigue life and lower fatigue crack growth rate for the annealed and 10% pre-strained specimens. Slip bands were scarce in the neighbourhood of cracks under ultrasonic fatigue tests, while many slip bands were observed in a wide area around the crack under conventional fatigue tests. In order to explain the effect of test frequency on fatigue strength, dynamic compression tests with Split Hopkinson bars were carried out. The stress level increases substantially with the strain rate. Thus, the increase in fatigue strength might be, to a large extent, due to a reduction in crack tip cyclic plasticity during ultrasonic fatigue tests.     

Creep rupture behaviour of modified 9Cr-1Mo heat-resistant steel strengthened with different mechanisms

The creep rupture behaviours and microstructural changes of a modified 9Cr-1Mo heat-resistant steel were investigated at 853 K. Analysis of creep results suggests that dislocation climb is the dominant deformation mechanism with true stress exponent of 5 under the present conditions. Based on the microstructural analysis, strengthening contributions from M₂₃C₆ carbides and MX carbonitrides were clarified. The M₂₃C₆ carbides can promote grain boundary strengthening by exerting Zener pinning forces, whereas MX carbonitrides can enhance the creep strength by interacting with mobile dislocations to induce threshold stress. Besides, softening of the steel is related not only to the decrease of dislocations, but also the coarsening of precipitates and substructures. The value of creep damage tolerance factor is close to 6.6, which further confirms that the creep damage is mainly attributed to the microstructural degradations, such as the coarsening of precipitates and substructures and decrease of dislocations.     

Modelling the residual strength of fatigue damage at a single semicircular edge notch: Semielliptical crack and through‐the‐thickness crack

In the present paper, computational framework for fatigue performance analysis of a semicircular edge notch with a through‐the‐thickness crack or a semielliptical crack is discussed. The failure behaviour of such configurations is theoretically examined through the stress‐intensity analysis and residual life estimation. The stress field of a damaged notch configuration is herein investigated by employing analytical and numerical approaches. Further, a fracture mechanics–based methodology, developed for fatigue life assessment, is taking into account the crack growth model proposed by Huang and Moan in which the stress ratio is involved. The efficiency of the obtained fatigue damage assessments, related to the edge notch configurations, is verified through appropriate experimental observations.     

Powder metallurgy of Al-based composites reinforced with Fe-based glassy particles: Effect of microstructural modification

Al metal matrix composites reinforced with high volume fraction of Fe_50.1Co_35.1Nb_7.7B_4.3Si_2.8 glassy particles were fabricated by mechanical milling followed by hot pressing. Elemental Al powders blended with 60 vol.% of glassy particles were mechanically milled for 1, 5, 10, 15, and 20?h, respectively. Selected samples were sintered by uniaxial hot pressing under Ar atmosphere. The changes in the microstructure along with their mechanical properties were investigated. Structural and microstructural characterization followed by microhardness and compression test results of the bulk composite material is reported. The use of high volume fraction of Fe-based glassy particles as reinforcement led to significant hardening of the Al matrix while leading to a remarkable combination of high strength and plasticity.     

Influence of pitting on the fatigue life of a turbine blade steel

The role of pits as stress raisers and their influence on fatigue life has been investigated for a 12Cr turbine blade material. A particular feature of this work was the establishment of an electrochemical procedure for generating pits with ‘controlled’ pit depth and low density. Pits grown under laboratory conditions were partially spherical in shape and simulated, in general appearance, those observed in service. In terms of the threshold stress intensity factor, the results supported the concept of pits acting as effective cracks of the same depth, provided that a short crack model based on an effective crack length is used.     

全厚度缝合复合材料泡沫芯夹层结构力学性能研究与损伤容限评定

探索了全厚度缝合复合材料闭孔泡沫芯夹层结构低成本制造的工艺可行性及其潜在的结构效益。选用3 种夹层结构形式, 即相同材料和工艺制造的未缝合泡沫芯夹层和缝合泡沫芯夹层结构及密度相近的Nomex 蜂窝夹层结构, 完成了密度测定、三点弯曲、平面拉伸和压缩、夹层剪切、结构侧压和损伤阻抗/ 损伤容限等7 项实验研究。结果表明, 泡沫芯夹层结构缝合后, 显著提高了弯曲强度/ 质量比、弯曲刚度/ 质量比、面外拉伸和压缩强度、剪切强度和模量、侧压强度和模量、冲击后压缩(CAI) 强度和破坏应变。这种新型结构形式承载能力强、结构效率高、制造维护成本低, 可以在飞机轻质机体结构设计中采用。      

On the formation of ridges and burnished debris along internal fatigue crack propagation in 42CrMo4 steel

A detailed microscopic analysis of fracture surfaces of 42CrMo4‐hardened steel after ultrasonic fatigue testing revealed globular and cylindrical particles located in ridges along the crack propagation direction. Observed particles could be easily taken for non‐metallic inclusions; however, chemical analysis showed that they are of the same composition as the steel matrix. The formation of such round‐shaped debris was found to be a result of cutting out the matrix fragments by ‘en passant’ cracks interaction and their subsequent fretting burnishing. Possible correlation of the parameters of ridges and debris with cyclic plastic zone width and martensite structure is discussed.     

A modification of UniGrow 2‐parameter driving force model for short fatigue crack growth

In this paper, a modification of the UniGrow model is proposed to predict total fatigue life with the presence of a short fatigue crack by incorporating short crack propagation into the UniGrow crack growth model. The UniGrow model is modified by 2 different methods, namely the “short crack stress intensity correction method” and the “short crack data‐fitting method” to estimate the total fatigue life including both short and long fatigue crack propagations. Predicted fatigue lives obtained from these 2 methods were compared with experimental data sets of 2024‐T3, 7075‐T56 aluminium alloys, and Ti‐6Al‐4V titanium alloy. Two proposed methods have shown good fatigue life predictions at relatively high maximum stresses; however, they provide conservative fatigue life predictions at lower stresses corresponding high cycle fatigue lives where short crack behaviour dominates total fatigue life at lower stress levels.     

Coupled behaviour of interacting dielectric cracks in piezoelectric materials

Existing studies indicate that the commonly used electrically impermeable and permeable crack models may be inadequate in evaluating the fracture behaviour of piezoelectric materials in some cases. In this paper, a dielectric crack model based on the real electric boundary condition is used to study the electromechanical behaviour of interacting cracks arbitrarily oriented in an infinite piezoelectric medium. The electric boundary condition along the crack surfaces is governed by the opening displacement of the cracks. The formulation of this nonlinear problem is based on modelling the cracks using distributed dislocations and solving the resulting nonlinear singular integral equations using Chebyshev polynomials. Numerical simulation is conducted to show the effect of crack orientation, crack interaction and electric boundary condition upon the fracture behaviour of cracked piezoelectric media.     

A precise computation of stress intensity factor on the front of a convex planar crack

In this paper, we consider a general integral expression for mode I stress intensity factor along the fronts of convex planar cracks. For this integral approximation, we develop a simple numerical quadrature formula on every convex set Ω which allows a precise estimation of the error. This permits the use of extrapolation techniques for the accurate computation of the integral. Copyright © 2002 John Wiley & Sons, Ltd.