The effect of the stress ratio on fatigue crack growth in a 3% NaCl solution

Corrosion fatigue crack growth tests have been carried out at various stress ratios for a low alloy steel SNCM 2 and type 304 stainless steel.

Measurements of the effective stress intensity factor range ratio U were performed to explain the effect of stress ratio R.

The corrosive environment decreased da/dN at R = 0.1, 0.4 and little affected da/dN at R = 0.9 for SNCM 2 and increased da/dN at all R ratios for SUS 304.

It was confirmed that there exists a threshold stress intensity factor ΔK_thCF in 3% NaCl solution for both materials tested.

The corrosive environment decreased ΔK_thCF for all conditions tested except at R = 0.1 and 0.4 for SNCM 2, where ΔK_thCF-values were nearly equal to ΔK_th-values in air. ΔK_thCF/ΔK_th was 0.6 at R = 0.9 for SNCM 2 and 0.8, 0.5 and 0.7 at R = 0.1, 0.7 and 0.9 for SUS 304, respectively.

It was shown that the complicated effect of stress ratios on crack growth for SNCM 2 can be explained using effective stress intensity factor ΔK_eff.     

INFLUENCE OF RETAINED AUSTENITE ON FATIGUE CRACK PROPAGATION IN HP 9-4-20 HIGH STRENGTH ALLOY STEEL

Abstract— Industrial multi-pass TIG weldments of HP 9-4-20 high strength alloy steel have been found to contain significant volume fractions (around 10%) of retained austenite which are not readily transformed after stress relieving and subsequent refrigeration procedures. To determine whether the presence of such retained austenite in tempered martensitic structures could be detrimental to fatigue resistance in HP 9-4-20 steel, fatigue crack propagation behavior was examined over six orders of magnitude in growth rate, in commercially heat-treated material (containing less than 3% austenite) and in intercritically heat-treated and tempered material (containing approx. 14% austenite) in an environment of moist, ambient temperature air. Whereas crack propagation rates were unchanged at growth rates exceeding 10⁻⁶ mm/cycle, structures containing 14% austenite showed somewhat superior resistance to near-threshold crack propagation at growth rates less than 10 ⁻⁶ mm/cycle, the threshold for crack growth (Δ K ₀) being over 20% higher than in commercially heat-treated material. The presence of retained austenite further appeared to inhibit the occurrence of intergranular fracture at near-threshold levels. It was concluded that significant proportions of retained austenite are not detrimental to fatigue crack propagation resistance in HP 9-4-20 steel, and may indeed have some beneficial effect at very low, near-threshold growth rates by increasing resistance to environmentally-assisted cracking.     

Fatigue crack growth behavior of X2095 Al–Li alloy

Microstructures and micro-textures of X2095 Al–Li alloy in as-received/superplastic state were characterized by means of SEM/BDS, X-ray diffraction and orientation imaging microscopy (OIM). It was observed that the microstructure of the alloy was typical of a particulate-reinforced composite material, consisting of aluminum matrix and homogeneously distributed T_B(Al₇Cu₄Li) particles with a volume fraction of about 10%. Brass-type texture was the dominant texture component. Both constant amplitude and near-threshold fatigue crack growth rates of the alloy in the L–T and T–L orientations were determined at different stress ratios. Particular attention was paid to the role of the T_B phase in the fatigue crack growth. When a fatigue crack approached a T_B particle, the crack basically meandered to avoid the particle. The T_B particles thus provided a strong resistance to the propagation of fatigue crack by promoting crack deflection and the related crack closure effects. The fatigue crack propagation behavior has been explained by the microstructural features, micro-textures, cracking characteristics and crack closure effects.     

Growth mechanism of stress corrosion cracking in high strength steel

Stress corrosion crack growth rates are measured at sveral stress intensity levels for low-tempered 4340 steel in 0.1N H₂SO₄ solution. The characteristics of the growth rates are divided into three regions of stress intensity factors: Region I near K_1SCC; Region III near unstable fracture toughness, K_1SC; and Region II, which lies between the two. K_1SCC is the value of K at which no crack growth can be detected after 240 hr.

In order to explain these experimental results, the crack initiation analysis reported in a previous paper is extended to the growth rates. A detached crack initiates and grows at the tip of an already existing crack. When the detached crack reaches the tip of the main crack, the process repeats as a new existing crack.

A relationship between crack growth rate, v, and stress intensity factor, K, is obtained as a function of b/a and a = b + d, where b is the distance from the tip of the main crack to the detached crack, and d is the ydrogen atom saturated domain.

The experimental data are in good agreement with the theoretical values in Region II when a = 0.02 mm, b/a = 0.8, c₁/c₀ = 2.8 for 200°C tempered specimens and a = 0.015 mm, b/a = 0.7, c₁/c₀ = 3.0, ρ_b = 0.055 mm for 400°C tempered specimens, where ρ_b is a fictitious notch radius. The plateau part in Region II for 400°C tempered specimens is also successfully explained by the present theory. For Region III, the value of b/a will be almost equal to 1 because v → ∞ for b/a → 1. On the other hand, for Region I, b/a will be zero, since the value of v becomes negligibly small and no crack growth is observable.     

基于α'组织设计适于激光立体成形的新型高塑性Ti-4.13Al-9.36V合金

为了提高Ti-6Al-4V合金的加工硬化率和塑性,基于其团簇成分式12[Al-Ti₁₂](AlTi₂)+5[Al-Ti₁₄](V₂Ti)设计成分式为4[Al-Ti₁₂](AlTi₂)+12[Al-Ti₁₄](V₂Ti)的(Ti-4.13Al-9.36V, %)合金,采用激光立体成形工艺制备Ti-4.13Al-9.36V和Ti-6.05Al-3.94V(对比合金),研究了沉积态和固溶温度对其显微组织和力学性能的影响。结果表明,沉积态Ti-4.13Al-9.36V和Ti-6.05Al-3.94V合金的显微组织均由基体外延生长的初生β柱状晶和晶内细小的网篮α板条组成。Ti-6.05Al-3.94V合金的初生β柱状晶的宽度约为770 μm,α板条的宽度约为0.71 μm;而Ti-4.13Al-9.36V合金的初生β柱状晶的宽度显著减小到606 μm,α板条的宽度约为0.48 μm。经920℃固溶-淬火处理后Ti-6.05Al-3.94V样品的显微组织为α'+α相,其室温拉伸屈服强度约为893 MPa,抗拉强度约为1071 MPa,延伸率约为3%。经750℃固溶-淬火处理后Ti-4.13Al-9.36V样品的显微组织为α'+α相,与α'马氏体相比,应力诱发的α'马氏体能显著地提高合金的加工硬化能力,其室温拉伸屈服强度约为383 MPa,抗拉强度约为 989 MPa,延伸率达到了17%。这表明,根据团簇理论模型调控α'+α的显微组织能有效提高激光立体成形Ti合金的加工硬化能力和塑性。     

A new microcomputer-aided system for measuring fatigue crack propagation threshold and selecting testing parameters

A new mathematical model is proposed for measuring fatigue crack propagation (FCP) threshold by using K-decreasing and K-increasing method. The formulae for efficiently selecting load-variation coefficients and crack growth increments (Δa) , (Δa) , in the case of K-decreasing and K-increasing tests are given and a correct method for determining FCP rate in the near-threshold region is recommended. On the basis of the above-mentioned work, a personal microcomputer-aided system is set up and successfully used for the FCP rate measurement in Lc9 aluminum alloy in 3.5% NaCI salt water environment. Compared with other systems, this system can increase the measurement accuracy, shorten testing time and obtain more information. Furthermore, the hardware is inexpensive.     

含盐量对冻结粉土单轴抗压强度影响的试验研究

采用取自甘肃省白银市平川区的典型天然盐渍粉土,洗去土中的盐分,在含水量为20%的情况下,以NaCl和Na₂SO₄为研究对象,研究了含盐量对冻结粉土单轴抗压强度和破坏应变的影响。试验结果表明:当含盐量介于0～3%时,随着NaCl含量的增加,冻土中的未冻水含量增加,土颗粒间的距离增大,凝聚力和内摩擦角减小,冻结粉土的单轴抗压强度以指数规律减小,破坏应变则呈现出先增加后减小的变化趋势。由于Na₂SO₄易于结晶析出,当含盐量小于3%时,盐分含量不同,盐分在土体中发挥着不同的作用,致使含盐粉土的单轴抗压强度随着含盐量的增加呈先减小后增大的变化趋势;破坏应变则呈现出先增大后减小的趋势。     

硅酸镱环境障涂层抗熔盐腐蚀行为与机制研究

稀土硅酸盐环境障涂层(EBC)是应用于新一代高推重比航空发动机热端部件的重要材料, 但其在高温熔盐环境的腐蚀行为与机制尚不明晰。本工作采用真空等离子喷涂技术(VPS)制备了Yb₂SiO₅/Yb₂Si₂O₇/Si环境障涂层, 并研究了该涂层体系在900 ℃、Na₂SO₄+25% NaCl(质量分数)熔盐环境中的腐蚀行为与机制。研究发现, 所制备的Yb₂SiO₅/Yb₂Si₂O₇/Si涂层体系结构致密, 各层之间结合良好; 涂层体系腐蚀240 h, 熔盐组分渗透Yb₂SiO₅涂层, 在Yb₂Si₂O₇中间层发生富集。涂层中Yb₂SiO₅相具有良好的稳定性, Yb₂O₃第二相与熔盐发生反应, 且随腐蚀时间延长, Yb₂O₃含量减少。中间层Yb₂Si₂O₇相与熔盐反应生成磷灰石相NaYb₉Si₆O₂₆和钠硅酸盐, 并产生Cl₂和SO₂等挥发性物质, 从而影响服役寿命。硅黏结层中未发现熔盐渗透现象, 保持完整。该涂层体系具有良好的抗熔盐腐蚀性能。     

Fractal characteristics of J-R resistance curves of Ti-6A1-4V alloys

J-R resistance curves of slow crack growth in Ti-6Al-4V alloys can be expressed in terms of fractal dimensions. The lineal fractal dimension D_f of crack growth profile obtained by the J-R technique correlates well with SEM observation of the crack surface.     

Near-tip dynamic fields for a crack advancing in a power-law elastic-plastic material: Modes I, II and III

Near-tip dynamic asymptotic stress fields of a crack advancing in an incompressible power-law elastic-plastic material are presented. It is shown that the stress- and strain-singularity are, respectively, of the order (In(R₀/r))^1/(n−1) and (In(R₀/r))^n/(n−1), where R₀ is a length parameter, r measures distance from the crack tip, and n is the power-law exponent. The angular variations of these fields are identical with those corresponding to dynamic crack growth in an elastic-perfectly-plastic material (Gao and Nemat-Nasser, 1983a,b).     

共压共烧法制备La0.8Sr0.2(Ga0.8Mg0.2)0.1Fe0.9O3-δ致密扩散障碍层极限电流型氧传感器

采用固相合成法制备了La_0.8Sr_0.2(Ga_0.8Mg_0.2)_0.1Fe_0.9O_3-δ(LSGMF)混合导体和La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ(LSGM)固体电解质, 利用XRD、TGA、范德堡直流四探针法和热膨胀仪等对试样进行了分析。以LSGMF为致密扩散障碍层, 以LSGM为氧泵层, 采用共压共烧结法制备了极限电流型氧传感器, 利用SEM和EDS对LSGMF/LSGM陶瓷体横截面的微观形貌和成分进行了分析。结果表明: LSGMF具有菱方钙钛矿结构(R-3c空间群), 它在650℃失重速率最快, 其电导率随温度的升高而增大; 300~1000℃范围, LSGM与LSGMF的热膨胀系数分别为12.51×10^-6/℃和12.80×10^-6/℃。650~850℃范围, 氧传感器具有良好的极限电流平台, lgI_L(极限电流I_L)与1000/T呈线性关系, LSGMF中氧离子的扩散激活能为0.4008 eV。800℃、0.3mol%<x(O₂)<21.0mol%时, 极限电流I_L与氧含量x(O₂)间的关系为: I_L(mA)=10.285x(O₂)(mol%), R=0.9982。LSGMF和LSGM结合牢固, 未产生裂纹, EDS分析基本符合各化合物的化学计量比。     

The effect of second principal stress on the fatigue propagation of mode I surface crack in Al2O3/Al alloy composites

Using a plate made of A2017-T6 metal matrix composites reinforced with 10 volume % and 20 volume % Al₂O₃ particles and Al alloy possesses the same composition as matrix alloy, the crack propagation rate da/dN of a mode I surface crack by the simultaneous action of plane bending and cyclic torsion are studied. And the effects of crack tip opening stress σ_top, crack opening displacement COD, biaxial stress ratio C (=second principal stress/first principal stress) and the surface roughness of crack section are examined. When stress intensity factor range ΔK is lower than the specific level, da/dN decreases with the increase of volume fraction of Al₂O₃ in C=0 and C=−0.55. But, da/dN of Al alloy becomes minimum in C=−1 and the effect of Al₂O₃ particles disappears. σ_top rises with the increase of volume fraction of Al₂O₃ particles and the decline of C. On the other hand, COD doesn’t always rise with the decline of C. These phenomena can be explained by the residual compressive stress formed at the surface layer of the specimen by the fatigue test and the surface roughness of crack section.     

AGEING EFFECTS ON FATIGUE CRACK CLOSURE OF AN Al–Li–Zr ALLOY IN Na2SO4, SOLUTION

Abstract— Fatigue of an Al–Li–Zr alloy has been studied as a function of ageing stage by measuring crack closure in Na₂SO₄ solution 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 Na₂SO₄ solution has been investigated by potentiodynamic polarisation and potentiostatic current transient experiments. The Na₂SO₄ solution increased the intrinsic fatigue crack propagation (FCP) rate for the overaged specimen in dry air compared to that for the peak-aged sample. The Na₂SO₄ solution 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 Na₂SO₄ solution when compared to that of the peak-aged specimen. The oxygen dissolved in the Na₂SO₄ solution 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 Na₂SO₄ solution 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 involving anodic dissolution.     

Use of thiourea to inhibit the incorporation of hydrogen in Ti and Ti-6Al-4V alloy

Commercially pure titanium and a Ti-6Al-4V alloy covered with a thin thermal oxide film derived from hot rolling were electrolytically hydrogenated in 1N aqueous sulfuric acid solution (H₂SO_4(aq)) to see the influence of thiourea (H₂N-CS-NH₂) on the hydrogen uptake of both metals under atmospheric conditions. The inhibitive effect of thiourea was evaluated through quantitative composition analyses (by using a glow discharge spectrometer, GDS) and qualitative microstructural examinations (by using XRD). Thiourea acts as a hydrogenation inhibitor irrespective of the applied current densities. However, the inhibitive effect is more evident at lower charging current densities for longer operating periods.     

Influence of stress ratio at baseline loading on delayed retardation phenomena of fatigue crack growth in A553 steel and A5083 aluminium alloy

The delayed retardation phenomena of fatigue crack growth following a single application of tensile overload were investigated under the baseline loading with the stress ratio, R = σ_min/σ_max, ranging from −1 to 0.5 for A553 steel and A5083 aluminium alloy. Two different overload cycles were applied; the one is the case that the ratio of peak stress range to baseline stress range, r = Δσ₂/Δσ₁, is equal to two and the other is the case that the ratio of maximum peak stress to maximum baseline stress, σ_2max/σ_1max, is equal to two. The retardation took place stronger in aluminium than in steel. Under the condition of r = 2 the normalized number of cycles, N_D/N_C, (N_D: the number of cycles during retardation, N_C: the number of cycles required for propagation through the overload-affected-zone size) decreased slightly as the R ratio increased from −1 to 0.5, while under the condition of σ_2max/σ_1max = 2 the N_D/N_C-values increased drastically as the R ratio increased from −1 to 0 (or the overload ratio, r, increased from 1.5 to 2) in both the materials. These retardation behaviors were expressed theoretically according to the model proposed by Matsuoka and Tanaka [1, 3] by using four parameters: the overload ratio, r, the exponent in Paris equation, m, the overload-affected-zone size, ω_D, and the distance at the inflection point, ω_B.     

Modeling the fatigue crack growth behavior of Ti-6Al-4V by considering grain size and stress ratio

Ti-6Al-4V is a commonly used titanium base alloy in aerospace applications. The increasing demand for damage-tolerant designs of such components necessitates a detailed knowledge of its crack growth behavior. The aim of this research was the characterization and phenomenological modeling of long crack growth behavior with respect to microstructure and stress ratio. Therefore, the long crack propagation was characterized for eight different heat treatment conditions and four stress ratios. For comparison, physically short crack growth tests were also performed. The long crack growth threshold was found to be dominated by roughness-induced crack closure, and the fracture surface roughness is controlled by the primary α-grain size. The reason for this correlation is a near-threshold crack propagation mode, which is dominated by the transcrystalline fracture of α-grains. This correlation was used to model the crack growth threshold with respect to microstructure. A linear relation was determined between the stress ratio and the threshold value, which was also found in this approach. Further presented models cover the crack growth behavior in the near-threshold (Stage I) and mid-growth rate regions (Stage II).     

Flexural fatigue characteristics of sandwich structures at different loading frequencies

The effects of frequency on the fatigue behavior of S2 glass fiber–vinylester reinforced sandwich composites with two different PVC cores have been investigated. Flexural fatigue tests were performed on sandwich beams with core densities of 130 and 260 kg/m³ at frequencies of 3 and 15 Hz, at a stress ratio, R=0.1 and at four different load levels viz. 90%, 85%, 80% and 75% of the ultimate load. S–N diagrams were generated and it was observed that the fatigue strength increased with core density, and the number of cycles to failure, N_f, increased with increase in frequency. In all cases failure was dominated by a primary shear crack in the core however, the crack path and crack propagation rates varied with frequency. The fatigue crack growth rate (FCGR) in the core of the H130 sandwich beams was subsequently investigated and the relationship between the crack growth rate, da/dN, and the cyclic stress intensity range, ΔK, was determined. It was found that crack growth rate decreased with increase in loading frequency.     

水相溶胶路线制备钇掺杂ZrO2纳滤膜及其分离性能研究

以氧氯化锆为前驱体, 采用环境友好的水相溶胶路线制备出完整无缺陷的管式钇掺杂的ZrO₂(YSZ)纳滤膜。并考察了溶胶粒径, 材料晶型与孔结构的关系。结果表明: 前驱体浓度越低, 所制备的溶胶粒径越小, 经400℃煅烧得到的粉末中单斜相含量越高, 堆积孔径增大; 向ZrOC₂O₄溶胶中添加8mol%钇, 有效抑制了ZrO₂四方相向单斜相的转变, 延缓晶粒生长, 同时防止膜层开裂。经纳滤测试表明, 实验制备的YSZ纳滤膜对PEG的截留分子量为860 Da, 纯水渗透率为200 L/(m²·h·MPa)。实验还详细考察了YSZ纳滤膜在pH=6, 压力0.8 MPa的条件下对NaCl、Na₂SO₄、CaCl₂和MgCl₂ 4种溶液的截留性能, 结果显示YSZ纳滤膜对0.005 mol/L CaCl₂和MgCl₂溶液的离子截留率分别达到65%和78%。     

EFFECT OF WAVEFORM ON CORROSION FATIGUE CRACK GROWTH

Abstract A study was made on the effects of stress rise time T ₁, maximum stress holding time T ₂, stress decreasing time T ₃ and minimum stress holding time T ₄ of a cycle on fatigue crack growth for a low alloy carbon steel in 3% NaCl solution. Measurements of the effective stress intensity range ratio U and observations of crack tip response were performed to clarify the causes of waveform effects.
The results were summarized as follows; T ₁ had a strong accelerating effect due to corrosive dissolution of the fresh surfaces of the crack which were formed during T ₁. The crack growth rate was enhanced as T ₁ increased and reached a constant value (about 3 times that in air) after T ₁= 10s. The crack growth rate at low Δ K , however, decreased as T ₁ increased more than T ₁= 1 s. T ₂, T ₃ and T ₄ decreased the crack growth rate. The extent of decrease not only depended on the period of T ₂ (or T ₃, T ₄), but also on Δ K and T ₁. A previously derived crack growth law which considered waveform and frequency effects, is also valid to a first approximation for the present results.     

Crack growth resistance characterized by the strain energy density function

Stable ductile fracture of a typical metal alloy is found to be governed by the condition dS/da = const., i.e. the rate change of the strain energy density S with crack length 2a (or a) remained constant. Since fracture and/or yielding are load rate dependent, the incremental theory of plasticity is employed for analyzing crack growth where unloading in the material near the crack can take place. Attention is focused on the energy per unit volume, dW/dV, stored along the prospective path of crack growth. The nearest neighbor continuum element must necessarily be at a finite distance r from the crack front. This leads to the general relation dW/dV = S/R. The critical value (dW/dV)_c representing the area under the uniaxial true stress and strain curve is assumed to correspond with failure of material elements. If yielding and unloading occurred locally, a certain amount of irrecoverable energy will not be available for dissipation during macrocracking. Hence, the threshold energy density must be modified to read as (dW/dV)_c^* < (dW/dV)_c. The quantity (dW/dV)_c may be regarded as the crack growth resistance whose magnitude decreases with increasing distance from the crack tip at which point yielding is most intensified.

The results are displayed graphically and shown that the condition dS/da = const. provides a rational means of collating and interpreting ductile fracture data.