摘要:

Abstract—Corrosion fatigue crack growth rates in high strength steel are often increased when a large cathodic polarization is applied. The corrosion fatigue mechanism in this case is generally considered to be due to hydrogen embrittlement. In the present study the crack growth process was carefully monitored by taking replicas from initially smooth specimens of a high strength steel under fully reversed push‐pull loading while: (1) exposed to laboratory air, (2) immersed in a 0.6 M sodium chloride (NaCl) solution at open circuit potential (OCP) and (3) with an applied cathodic potential of —1250 mV (SCE). It is shown that the effect of cathodic polarization is dependent on the applied stress level and the nature of the cracking process, which in turn, is related to the sue of the crack. For stress levels at or below the in‐air fatigue limit, failure did not occur for cathodically polarised specimens despite the number of loading cycles being 10 times that of the lifetime of identical tests in solution at the open circuit potential. At stress levels above the in‐air fatigue limit the reduction in fatigue endurance caused by the presence of the corrosive environment can be partially recovered through cathodic polarization. The role of non‐metallic inclusions in the cracking process under various exposure conditions is discussed, and a cracking mechanism

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00876.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—Crack growth through a spatially non‐uniform microstructure of 1045 steel edge‐notched and pre‐cracked panel specimens was investigated experimentally and computationally. The microstructural gradient was produced by heat treatment and differential quenching. A fractographic examination was performed on the crack growth specimens to correlate fracture mode and local toughness. Tensile specimens, corresponding to different sites in the edge‐notched panel, were used to characterize the constitutive response and fracture resistance of the as‐treated material. The Rockwell B hardness was used to identify and map material constitutive responses to corresponding locations in a finite element model of the panel. A debonding algorithm was used in the finite element simulations to model stable crack growth using a local fractur

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00877.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—The application of combined low and high humidity environments during the early stages of fatigue crack propagation causes the formation of markings on the fracture surface. These markings form at the initiation zone and in the threshold region. The markings show the position of the crack front on a micro level and also give information about local crack growth rates. A comparison is made between the topography of the striations and the beach marks. Etch pitting is used to determine the orientation of crystallographic planes for crack growth in both beach mark and striation regions. The tests were carried out on two A1 alloys (7075‐T651 and 8090‐T851) at two stress ratios (R= 0.1 and

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00878.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—Since the Aloha accident the multiple site damage (MSD) problem of riveted lap joints in aircraft fuselages has drawn much attention. The failure scenarios for a lead crack and more small MSD cracks as discussed by Broek and Swift are summarized, including recent results of a relevant test series by Broek. It shows that small MSD cracks can significantly reduce the load for unstable crack extension. Prevention of catastrophic consequences requires crack arresting capability of the structure. Related aspects of the problem are discussed with reference to failure criteria for ligament failure, the MSD problem for existing and new aircraft, and different options for crack stopper band

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00879.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—Statistical fatigue tests have been conducted on a structural medium carbon steel, S45C, in room air and in 3%NaCl solution, using five cantilever‐type rotary bending fatigue testing machines which were specially manufactured for the purpose of the present study. Fatigue life distribution was examined at three and five stress levels in air and in 3%NaCl solution, respectively, and twenty specimens were allocated to each stress level. In room air, it was found that fatigue life distributions followed the three‐parameter Weibull distribution, which were closely related to fracture morphology. In 3%NaCl solution, they also followed the Weibull distribution, but the scatter in fatigue life was smaller in comparison to that in air. It is suggested that the decrease in the scatter of fatigue life may be attributed to a smaller fraction of crack initiation life in 3%NaCl solution. The growth of corrosion pits was investigated using a laser microscope. The distribution of corrosion pit depths followed the log‐normal distribution, and the corrosion pit depths increased with increasing time or the number of cycles. It was found that the growth of corrosion pits was accelerated by stress cycling and the depths increased with increasing stress level. Based on these results, a growth law of corrosion pits, including the effect of stress cycling, is p

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00880.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—Fatigue of an Al–Li–Zr alloy has been studied as a function of ageing stage by measuring crack closure in Na2SO4solution with an unloading elastic compliance technique and comparing results in dry air and oxygenated solution. Anodic behaviour of peak‐aged and overaged alloy specimens in the Na2SO4solution has been investigated by potentiodynamic polarisation and potentiostatic current transient experiments. The Na2SO4solution increased the intrinsic fatigue crack propagation (FCP) rate for the overaged specimen in dry air compared to that for the peak‐aged sample. The Na2SO4solution inhibited the development of crack closure for the peak‐aged specimen in dry air, but aided it in the overaged condition. The result of the environmental crack closure study is discussed in terms of the more enhanced through‐thickness tortuosity of the overaged specimen in the Na2SO4solution when compared to that of the peak‐aged specimen. The oxygen dissolved in the Na2SO4solution slightly increased the environmental intrinsic FCP rate, which seems to be due to the reduced repassivation rate as compared to that in the N2‐purged solution. The anodic dissolution rate from the bare surface of the overaged specimen in the Na2SO4solution was higher than that from the peak‐aged sample. The difference between environmental FCP rates and crack closures for the peak‐aged and overaged specimens is discussed in terms of environment‐assisted crack‐tip damage processes inv

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00881.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—In this paper, the fatigue threshold ΔKthof a cracked body is studied. Unlike other approaches given in the literature, the shakedown theory is used for predicting ΔKth. A crack is considered as a sharp notch, the radius of which, at the threshold stress level, is a material constant. The threshold of crack propagation is explained as being due to shakedown of the cracked body, and a simple but reasonable model is derived. The value of ΔKthis found to be proportional to the yield stress multiplied by the square root of the effective crack tip radius. Using this model, ΔKthis calculated for some materials. Comparison of the predicted fatigue thresholds with those obtained by experiments, or by using other approaches, indicates that our model provides satisfying re

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00882.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—A local simulation principle is presented which allows the transfer of geometry‐dependent fracture parameters from a specimen to a component if the local constraint conditions of the crack tip in the component are simulated by the specimen. This can be achieved by adapting the specimen to the component with respect to the ligament length, the ligament width, the crack length and the loading geometry. If these requirements are met, then the relationship between the load and the driving force in the component can be estimated from the specimen by a simple scaling procedure. The application of the local simulation principle is illustrated by an exam

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00883.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—The fracture behaviour of high strength steels under the influence of hydrogen was studied, with special emphasis on the critical condition for the formation of intergranular (IG) cracks. Mechanical tests were carried out on cathodically charged specimens subjected to both a constant load and a constant displacement under a variety of hydrogen‐charging conditions. Experimental analyses show that a high local hydrogen concentration plus a high stress intensity at a quasi‐cleavage (QC) crack tip are required to initiate intergranular cracking. The condition for continued intergranular crack propagation, leading to micro‐void coalescence (MVC), is determined by the combined effect of the speed of crack propagation together with the rate of hydrogen diffusion. A quasi‐cleavage crack triggers the onset of intergranular crack growth irrespective of the test control parameters, i.e. constant loading or constant displacement conditions.A fracture map is proposed relating to the boundary conditions between QC, IG and MVC cracking, which will assist further

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00884.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—A systematic study of the fatigue crack growth characteristics and mechanisms in Al–Si–Mg and A356 casting alloys was carried out. Compact tension specimens, prepared from modified and unmodified alloys were tested at different stress ratios and stress intensity factor range values, and a study of the mechanistic role of the silicon particles in influencing the fracture behaviour during fatigue crack propagation was made, employing both optical and scanning electron microscopy. The results indicated that the fatigue crack growth behaviour of the alloys is affected by the stress ratio, stress intensity level and the size, shape and distribution of the eutectic silicon particles. The particle characteristics also determine the fracture mode of the alloy. Fracture Characteristics observed include decohesion of the silicon particles from the aluminum matrix; silicon particle cleavage/cracking; and striations in the aluminum phase, particularly at high stress r

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00885.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY