LD10铝合金疲劳裂纹扩展速率的研究

用三点弯曲多试样方法研究了LD10铝合金的疲劳裂纹扩展速率规律,从而为装备的安全可靠性提供数据依据,利用这些数据可以评估在役装备的剩余寿命。     

EFFECT OF LASER BEAM RADIATION ON FATIGUE CRACK PROPAGATION IN AISI 4150 STEEL

The effect of laser beam radiation on fatigue crack growth in AISI 4150 steel was performed on compact-tension (CT) specimens, in which a composite region (CR) comprised of the hardened zones (HZs) on the top and bottom surfaces and the base metal (BM) in the interior, was aligned either along or normal to the crack growth direction. The microstructure of the HZs consisted of martensite, while lower bainite was present in the 300 °C preheated laser-hardened specimens. When the crack propagated along the laser tracks (LTs), the fatigue crack growth rates (FCGRs) of the laser-hardened specimen were lower than those of the base plate, particularly at low ΔK ranges. On the other hand, for a crack propagating normally to the LTs, decelerated FCGRs in the regions preceding the CR and accelerated FCGRs within the CR itself were found. However, enhanced FCGRs in the CR were not found in preheated specimens with a bainite structure in the HZs as the crack grew normal to the LTs. The enhancement of FCGRs in the CR, which became more accentuated at high ΔK values, was closely related to an embrittled microstructure (martensite) in the HZs.     

液体介质对聚氯乙烯疲劳裂纹扩展的影响

本文研究了环境介质对聚氯乙烯疲劳裂纹扩展的影响。结果表明:1.溶剂可使硬聚氯乙烯的疲劳裂纹扩展速率下降至空气中的1/3—1/10;2.硬聚氯乙烯在溶剂中的疲劳裂纹扩展速率下降的幅度随着其与溶剂之间的溶解度参数差值变小而增大;3.硬聚氯乙烯在溶剂中的疲劳裂纹扩展断口比在空气中的表现出较大的塑性。本文还从液体压力所致裂纹闭合效应和裂纹-银纹应力场这两个方面讨论了环境介质降低硬聚氯乙烯的疲劳裂纹扩展速率的机理。     

应变片位置对测定裂纹闭合灵敏度的影响

从理论上分析了在裂纹尖端附近区域内，侧贴电阻应变片的位置对测定疲劳裂纹闭合时灵敏度的影响，并用实验加以验证；得出了一些有益的结论。     

FATIGUE CRACK GROWTH BEHAVIOUR OF METALLIC BIOMATERIALS IN A PHYSIOLOGICAL ENVIRONMENT

Fatigue crack propagation (FCP) in Ringer's solution has been studied on four metallic biomaterials, i.e. pure titanium (TP35H), stainless steel (SUS304), dual-phase stainless steel (SUS329J4L), and titanium alloy (Ti–6Al–4V). For TP35H and SUS329J4L, the crack growth characteristics in Ringer's solution were almost the same as those in room air in the entire ΔK region examined, and thus the effect of Ringer's solution was not observed. The crack growth rates of SUS304 were enhanced in Ringer's solution in the region of ΔK ≥ 25  MPa m as compared to room air, while below this ΔK level they were similar in both environments. Also in Ti–6Al–4V, similar crack growth behaviour in Ringer's solution was found. From the comparison among the da/dN–ΔK relationships of the four materials in Ringer's solution, TP35H exhibited the fastest growth rates and then Ti–6Al–4V, SUS304, SUS329J4L in the order of decreasing growth rate. On the contrary, if the data were plotted in terms of ΔK_eff /E, Ti–6Al–4V and TP35H showed superior crack growth resistance to the stainless steels.     

EQUIVALENT CONSTANT AMPLITUDE CONCEPTS EXAMINED UNDER FATIGUE CRACK PROPAGATION BY BLOCK LOADING

Abstract— Fatigue crack growth behaviour under block loading sequences has been studied on the aluminium alloy 2024 T351, using four different equivalent constant amplitude concepts. The root mean squared method gives acceptable results only for relatively long blocks. The equivalent method based on the Paris law (modified to take into account crack closure) gives good predictions for the observed growth rates as does Elbers' method. Finally, a new method based on energy considerations gives excellent results for the studied test conditions.     

ARTIFICIAL RETARDATION OF FATIGUE CRACK GROWTH BY THE INFILTRATION OF CRACKS BY FOREIGN MATERIALS

The effects of inducing artificial crack closure into fatigue cracks in AISI 304 stainless steel by infiltrating foreign materials have been investigated. The foreign materials used include pure epoxy resin and resin mixed with 0.3  μm and 4  μm TiO2 , 4  μm Fe, as well as 18  μm AISI 316L stainless steel. In all the cases studied, different degrees of crack growth retardation have been achieved. When the particle size was small enough or when the prop-opening load for infiltration was large enough, crack arrest occurred. Crack retardation and arrest were mainly caused by the infiltrated material rather than the propping load. A rigid-wedge model was found to have limited value in predicting the possible outcome of an infiltration. On the other hand, the degree of crack closure immediately on resumption of a test after infiltration could tell whether the treatment was going to be successful or not.     

疲劳裂纹扩展的降载停滞

用恒应力强度因子幅法研究了AISI409和18Cr-Nb铁素体不锈钢疲劳裂纹扩展的降载停滞特点。结果表明,停滞效应取决于降载幅值、材料强度和应力比。这与裂纹尖端的塑性变形及其对裂纹的闭合作用有关。     

CRACK CLOSURE AND THE FATIGUE-CRACK PROPAGATION THRESHOLD AS A FUNCTION OF LOAD RATIO

The phenomenon of crack closure, which involves the premature closing of fatigue cracks during the unloading portion of a fatigue cycle resulting in the development of crack-tip shielding due to crack wedging, has become widely accepted as a critical mechanism influencing many aspects of the behaviour of fatigue cracks in metallic materials; these include effects of load ratio, variable-amplitude loading, crack size, microstructure, environment and the magnitude of the fatigue threshold. Recently, however, the significance of crack closure has been questioned and alternative suggestions made for many of these phenomena, e.g. the effect of the load ratio (i.e. the ratio R of the minimum to maximum loads) on threshold behaviour. In the light of this, the present paper provides evidence to rebut the assertion that crack closure is an insignificant process. Particular attention is given to the effect of crack closure on the threshold level as a function of load ratio.     

COMPRESSION FATIGUE CRACK GROWTH BEHAVIOUR OF AN ALUMINIUM ALLOY: EFFECT OF OVERLOADING

Abstract— The behaviour of fatigue cracks in an Al-alloy under cyclic compression, either with or without overloads, was studied. For constant-amplitude compressive cycling, a non-catastrophic (saturation) character of the fatigue crack behaviour was confirmed, with the final depth of a crack depending on the applied load level. Single (tensile or compressive) intermittent overloads were shown to re-activate a previously arrested crack while reversed (tensile—compressive or compressive—tensile) ones were also shown to maintain continual fatigue crack extension under otherwise fully compressive cycling.     

PREDICTING THE EFFECTS OF LOAD RATIO ON THE FATIGUE CRACK GROWTH RATE AND FATIGUE THRESHOLD

The effect of the load ratio, R, on fatigue crack growth behaviour is analysed on the basis of the recently proposed inelastic discrete asperities model. A wide range of load ratios, both positive and negative, are examined. Particular emphasis is placed on compressive excursions, i.e. negative R loadings. The inelastic discrete asperities model is a micro-mechanical analysis based on the plastic crushing of a single asperity (or multiple asperities) located on the crack face close to the crack tip and under dominantly plane strain conditions. Experimental data have indicated that the primary crack face contacts which obstruct closure are immediately adjacent to the crack tip, although segments of the crack face more distant from the crack tip are not neglected. However, the more distant asperities are a part of the past crack advance history which does not influence current behaviour. By use of this model, it is shown that the effect of the load ratio can be adequately predicted once some baseline information on mechanical material properties and surface roughness is provided. The model also provides useful trend information and explains many of the observed phenomena, e.g. the ‘saturation’ of the compressive underload effects. For a constant applied nominal stress intensity factor range, ΔK_nom , it is shown that the effective stress intensity factor range, ΔK_eff , initially decreases as the positive R decreases (corresponding to the increasing influence of closure), reaches a minimum around R = 0, and then starts increasing with negative R (corresponding to the plastic crushing of the asperities which reduces closure), eventually reaching a saturation level below ΔK_nom . Conversely, for an assumption of a constant ΔK_eff , the applied ΔK_nom increases as the positive load ratio decreases, reaching a maximum around R = 0, and then decreases with more negative R values, eventually reaching again a saturation level (above ΔK_eff ). It is also shown that the effect of material hardness can be directly analysed based on this model.     

INFLUENCE OF LOW-INTENSITY SHOT PEENING ON THE FATIGUE STRENGTH OF AN Al/Al

Metal matrix composites are increasingly utilized in engineering applications because of their favourable strength/weight ratio. However, their applications are still limited because of their relatively poor fatigue resistance, which could be improved by using some appropriate surface treatment. In this paper, an Al/Al₂ O₃ (20%) composite material is considered. In particular its fatigue strength is examined to verify if, how and when shot peening can be used to improve the performances of MMC components that are cyclically loaded. Fatigue tests allow one to evaluate the influence of shot peening on the fatigue limit of both smooth and notched specimens. Residual stress measurements, fracture surface analyses and metallographic examinations allow one to evaluate the role of residual stresses, hardening and grain distortion on the improvement that can be achieved with this treatment. On the basis of these results, some guidelines are drawn on how to optimize the choice of peening parameters.     

THE EFFECT OF REGULAR AND IRREGULAR SHEAR LIPS ON FATIGUE CRACK GROWTH IN Al 2024

Abstract— Multiple underloads and overloads with constant Δ K were carried out on centre-cracked tension specimens. It was found that when shear lips develop, underloads or overloads affect the crack growth rate d a /d N and the subsequent retardation. The appearance of the shear lip fracture surfaces depends on the frequency. At higher frequencies a greater number of rough shear lip fracture surfaces will develop, while at lower frequencies there is a tendency towards smooth shear lips. The amount of crack closure differs in each case. It was found that the type of shear lip, rough or smooth, can be related to the effect on d a /d N during and after underloads. The effect of rough and smooth shear lip growth was investigated in constant Δ K tests, performing these tests with and without crack closure.     

FATIGUE CRACK GROWTH BEHAVIOUR OF FIBRE-METAL LAMINATE GLARE-1 AND METAL LAMINATE 7475 WITH DIFFERENT BLUNT NOTCHES

Abstract— The fatigue crack growth behaviour of the fibre metal laminate "GLARE-1" has been investigated for different blunt notches in Constant Amplitude (CA) tests. In order to investigate the influence of the fibres, the same laminate material but containing no fibres (Laminate 7475) was also tested. The fatigue crack growth properties of GLARE-1 are superior to those of Laminate 7475. GLARE-1 shows lower crack growth velocities at the same K_nom values and in addition the crack growth rates decrease with increasing crack length. The Laminate 7475 shows typical metal behaviour for single crack propagation and accelerating crack growth with increasing crack length. In GLARE-1, multiple crack propagation takes place. The cracks propagate independent of each other and have similar crack growth rates, in part due to closure effects caused by the unbroken fibre layers.
The crack growth rates of specimens having a small root radius are higher in both materials than in specimens with a large notch radius. In GLARE-1, the superiority of a larger notch radius is more pronounced than in the Laminate 7475 and is attributed to a stronger crack closure effect owing to fibre bridging. The reason for the higher bridging capability in specimens containing larger notches is that less fibres are broken or damaged in the notch vicinity.     

CRO试样的疲劳裂纹扩展行为试验方法研究

考虑到如紧凑拉伸和三点弯曲等标准试样的大尺寸要求以及高试验成本等突出问题,该文发展了含外侧径向裂纹C形环小试样（C-ring with an outer radial crack,CRO）的疲劳裂纹扩展行为试验方法。利用有限元分析建立了CRO小试样的高精度应力强度因子算式以及基于柔度法理论的裂纹长度预测公式。采用5083-H112铝合金分别完成了CRO试样和标准CT试样的疲劳裂纹扩展速率试验,获得了相应的Paris方程中的幂指数。通过对比发现,CRO和CT试样的疲劳裂纹扩展规律基本一致,验证了新方法的有效性。基于上述方法对C250钢两种厚度CRO试样的疲劳裂纹扩展行为进行了应用研究。     

电解充氢对堆焊熔合区疲劳裂纹扩展行为的影响

研究了阴极充氢前后堆焊熔合区疲劳裂纹扩展行为,发现堆焊熔合区对疲劳裂纹的扩展有阻碍作用,充氢对裂纹扩展速率无明显影响,但使熔合区出现大量二次裂纹,随着充氢时间的延长,二次裂纹将更加严重。     

超高韧性水泥基复合材料疲劳裂缝扩展公式的理论与试验研究

该文基于Paris公式,对具有应变硬化和多缝开裂特征的超高韧性水泥基复合材料(UHTCC)的疲劳裂缝扩展理论进行了理论研究,并通过试验进行验证。研究表明,Paris公式适用于UHTCC。其中：裂缝扩展参数由裂缝面积扩展A表示;与之相应,使用复合断裂能增长量#x00394;J代替应力强度因子变化量#x00394;K。基于试验,该文求出UHTCC的疲劳裂缝扩展门槛值,即当疲劳过程中的断裂能幅值#x00394;J小于某一临界值#x00394;J_th时,疲劳裂缝不扩展。在疲劳过程中,UHTCC的裂缝覆盖面积随疲劳过程呈三阶段线性发展,与疲劳变形的发展趋势一致。UHTCC的疲劳裂缝扩展随疲劳最大荷载与荷载幅值的增大而加速发展。     

LIMITATIONS ON THE USE OF THE STRESS INTENSITY FACTOR, K, AS A FRACTURE PARAMETER IN THE FATIGUE PROPAGATION OF SHORT CRACKS

Abstract— It is well known that for very short cracks the stress intensity factor K is not a suitable parameter to estimate the stress level over the small but finite Stage II process zone activation region of size r_s near the crack tip, within which crack growth events take place. A critical appreciation of the reasons for the limitations on the applicability of ΔK as a fatigue crack propagation (FCP) parameter, when the crack length a is of the same order of magnitude or smaller than the size of the ‘fatigue-fracture activation region’, r_s is presented. As an alternative to ΔK the range Δσ_s of the cyclic normal stress at a point situated at the fixed distance s=r_s/2, ahead of the crack tip, inside the fatigue-fracture activation region, is proposed. It is observed that the limitation on the use of ΔK when the crack is short, is mathematical (and not physical) but this inconvenience is easily circumvented if the stress Δσ_s at the prescribed distance is used instead of ΔK since nowadays Δσ_s can be obtained numerically by using finite element methods (FEM). It follows that the parameter Δσ_s is not restricted by the mathematical limitations on ΔK and so it would seem that there is, a priori, no reason why the validity of the parameter Δσ_s cannot be extended to short cracks. It is shown that if the Paris law is expressed in terms of Δσ_s (πrr_s)^½ instead of ΔK the validity of the modified Paris law can be extended to short cracks. A coherent estimate of the value of the fatigue-fracture activation region r_s is derived in terms of the fatigue limit Δσ_FL obtained from S-N tests and of the threshold value ΔK_th obtained from tests on long cracks where both relate to Stage II crack growth that ends in failure, namely, r_s= (ΔK_th/Δσ_FL)²/π. An overall, threshold diagram is presented based on the simple criterion that, for sustained Stage II FCP, Δσ_s must be greater than Δσ_FL. The study is based on a simple continuum mechanics approach and its purpose is the investigation of the suitability of both ΔK and Δσ_s to characterise the crack driving force that activates complex fracture processes at the microstructure's scale. The investigation pertains to conditions that lead to the ultimate failure of the component at values of Δσ_s > Δσ_FL.     

轴承钢疲劳裂纹扩展速率的研究

研究了轴承钢碳化物及晶粒细化对轴承钢疲劳寿命的影响。结果表明;细化轴承钢中的碳化物可以使其疲劳裂纹扩展速率下降,而同时细化轴承钢中的碳化物和晶粒,会使其疲劳裂纹扩展速率下降更明显。     

中心裂纹混凝土板的裂纹扩展和延迟失稳

本文把混凝土看作是具有损坏区的粘弹性材料,并结合裂纹前缘破坏过程的观测资料,提出了带裂纹的混凝土和破坏模型,研究了混凝土裂纹的早期扩展和延迟失稳问题。用本文给出的理论,较好地解释了在工程实际中所观察到的,以及大量实验所证实了的混凝土的早期裂纹扩展问题,并与实验数据符合较好。因此,对预测长期持续载荷作用下混凝土结构的安全性具有实际的意义。