FAILURE MECHANISMS IN IMPACT FATIGUE OF METALS

Abstract Impact fatigue tests were performed with smooth and notched specimens of low carbon steels under various impact loading conditions. The characteristic failure mechanisms in impact fatigue was discriminated by comparison with those in non-impact, ordinary fatigue. The fatigue life of smooth specimens was uniquely related to the range of plastic strain at the middle of the fatigue life in both impact and non-impact fatigue, although the characteristics of micro-structural deformation and cyclic stress-strain relationships were markedly different. The growth rate of a fatigue crack in impact fatigue of notched specimens was higher than that in non-impact fatigue when compared at the same stress intensity factor. Fractographic observations with scanning electron microscopy and the X-ray diffraction technique revealed more abundant cleavage facets and a smaller spread of the plastic zone beneath the fracture surface made by impact fatigue. Both nucleation and propagation lives in notched specimens were much shorter in impact fatigue than in non impact fatigue when compared at the same values of nominal stress and stress concentration factor.     

THE INFLUENCE OF TEST TEMPERATURE ON IMPACT FATIGUE CRACK GROWTH BEHAVIOR IN QUENCHED AND TEMPERED Cr-Mo ALLOY STEELS WITH DIFFERENT PRIOR AUSTENITE GRAIN SIZES

Abstract— Impact fatigue tests were carried out using a rotating-disk type impact fatigue testing machine. The influence of prior austenite grain size, ductile-brittle transition temperature and test temperature on impact fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel in which the prior austenite grain size was varied from 8–3 to 25-4 μm. The results in impact fatigue tests were compared to those under non-impact conditions. The crack growth rates associated with striation formation were insensitive to the change in prior austenite grain size, ductile-brittle transition temperature and test temperature regardless of impact and non-impact fatigue. When the material was in the brittle condition, impact fatigue gave rise to a transition from striation formation to intergranular and cleavage cracking. Such a transition will result in the acceleration of crack growth rate. The Paris Law exponent values in impact fatigue were reasonably expressed by the ratio of test temperature to ductile-brittle transition temperature.     

Effects of Constituents and Lay-up Configuration on Drop-Weight Tests of Fiber-Metal Laminates

Impact responses and damage of various fiber-metal laminates were studied using a drop-weight instrument with the post-impact damage characteristics being evaluated through ultrasonic and mechanical sectioning techniques. The first severe failure induced by the low-velocity drop-weight impact occurred as delamination between the aluminum and fiber-epoxy layers at the non-impact side. It was followed by a visible shear crack in the outer aluminum layer on the non-impact face. Through-thickness shear cracks in the aluminum sheets and severe damage in the fiber laminated layers (including delamination between adjacent fiber-epoxy laminae with different fiber orientations) developed under higher energy impacts. The impact properties of fiber-metal laminates varied with different constituent materials and fiber orientations. Since it was punched through easily, the aramid-fiber reinforced fiber-metal laminates (ARALL) offered poorer impact resistance than the glass-fiber reinforced fiber-metal laminates (GLARE). Tougher and more ductile aluminum alloys improved the impact resistance. GLARE made of cross-ply prepregs provided better impact resistance than GLARE with unidirectional plies.     

THE GROWTH OF SMALL FATIGUE CRACKS IN A LOW CARBON STEEL; THE EFFECT OF MICROSTRUCTURE AND LIMITATIONS OF LINEAR ELASTIC FRACTURE MECHANICS

The growth characteristics of small fatigue cracks were studied under rotary bending in a low carbon steel prepared with two ferrite grain sizes of 24 and 84 μm, and were compared with the growth characteristics of large through cracks in fracture mechanics type specimens. The effect of microstructure on crack growth rates and the interaction in growth behaviour between two neighboring small cracks were examined experimentally, and also the critical crack lengths above which linear elastic fracture mechanics (LEFM) is applicable were evaluated for small crack growth and for fatigue crack thresholds. It is found that small cracks grow much faster than large ones and also show growth rate perturbations due to grain boundaries. It is indicated that the critical crack lengths for fatigue crack thresholds are significantly shorter than those for small crack growth.     

6061-T6 铝合金激光焊接接头腐蚀疲劳裂纹扩展

目的研究6061-T6铝合金激光焊接接头的腐蚀疲劳裂纹扩展特性,并分析裂纹扩展的影响因素。方法利用光纤激光器,焊接尺寸为150 mm×100 mm×4 mm(焊接方向、横向、熔深方向)的6061-T6铝合金,采用SE(B)三点弯曲疲劳裂纹扩展试验并利用连续降K法,分别在空气和人工海水中进行疲劳裂纹扩展试验,通过使用金相显微镜(OE)和扫描电子显微镜(SEM),对金相结构进行观测分析。结果同样工艺参数的焊接接头,在海水中疲劳裂纹门槛值(4.063 016 MPa·m~(0.5))大于空气中的门槛值(3.479 166 MPa·m~(0.5));在疲劳裂纹扩展中速区(da/dN10~(-5) mm/cycle)时,海水焊接接头疲劳裂纹扩展速率大于空气中的,低速区(da/d N10~(-5) mm/cycle)则小于在空气中的。结论成形良好的焊缝、晶粒细小的焊缝组织有助于接头疲劳裂纹扩展性能的提高;中速区,海水中疲劳裂纹扩展速率偏大,主要是由腐蚀条件下焊缝裂纹尖端阳极溶解和交变载荷共同作用导致;低速区,海水中疲劳裂纹扩展速率偏小,主要原因是腐蚀产物堆积于疲劳裂纹扩展尖端,产生较强裂纹闭合效应。     

Fatigue crack growth behaviour of gas tungsten arc,electron beam and laser beam welded Ti–6Al–4V alloy

The present investigation is aimed to evaluate fatigue crack growth parameters of gas tungsten arc, electron beam and laser beam welded Ti–6Al–4V titanium alloy for assessing the remaining service lives of existing structure by fracture mechanics approach. Center cracked tensile specimens were tested using a 100 kN servo hydraulic controlled fatigue testing machine under constant amplitude uniaxial tensile load. Crack growth curves were plotted and crack growth parameters (exponent and intercept) were evaluated. Fatigue crack growth behavior of welds was correlated with mechanical properties and microstructural characteristics of welds. Of the three joints, the joint fabricated by laser beam welding exhibited higher fatigue crack growth resistance due to the presence of fine lamellar microstructure in the weld metal.     

Cyclic fatigue crack growth behaviour in β-(Si–Al–O–N) at ambient and elevated temperatures

Four-point bending fatigue tests on a hot-pressed sintered Sm–-(Si–Al–O–N) ceramic were conducted at room temperature, 900 °C and 1000 °C in air under different load ratios and cyclic frequencies. The growth of indentation cracks was measured during the fatigue tests. The results indicate that the cyclic fatigue crack growth threshold is lower and crack growth rates are higher, for given values of K_max, at 1000 °C than those at room temperature. The cyclic fatigue crack growth behaviour at 900 °C is similar to that at room temperature. It was found that the crack growth retardation due to cyclic fatigue loading is much more pronounced at higher frequencies. An increase in cyclic frequency from 1 to 10 Hz cause a reduction of up to two orders of magnitude in crack propagation rates. High-temperature cyclic fatigue crack growth rates increased and threshold stress intensity factor ranges decreased with increasing load ratio. Possible mechanisms for cyclic crack growth are discussed.     

WNQ570桥梁钢及其对接焊缝疲劳裂纹扩展性能试验研究

对WNQ570桥梁钢及其对接焊缝进行了疲劳裂纹扩展速率和疲劳裂纹扩展门槛值测定试验,采用两种不同数据拟合方法分别得到具有95%保证率的疲劳裂纹扩展参数。结果表明:本批次的WNQ570钢材具有良好的疲劳裂纹扩展性能,其中对接焊缝疲劳裂纹扩展速率高于母材;在应力强度因子幅值处于10 MPa·m1/2～70 MPa·m1/2的常规区间时,基于单试件数据点的处理结果对应的裂纹扩展速率明显高于基于成组数据点的处理结果;WNQ570的疲劳裂纹扩展速率随应力比增加而增加,疲劳裂纹扩展门槛值随应力比增加而降低。     

Environmental fatigue crack growth in titanium aluminides and hydrogen evolution behaviour

The influence of environment on fatigue crack growth behaviour was investigated both in nearly lamellar and in duplex titanium aluminides, and the hydrogen evolution kinetics was analysed by thermal desorption spectroscopy. The tensile strength of the duplex material decreases in the order of the extent of the water molecule content in the environment: the strength in vacuum is the highest, and decreases in the order of laboratory air and finally in water. In the case of the lamellar material, the fatigue crack growth rate in dry air is higher in the R–C crack plane orientation than that in the L–C crack plane orientation. The crack growth rate becomes higher when the crack grows as the lamellae tear. However, in the case of the duplex material, the crack growth rate in the R–C crack plane orientation is smaller in the low Δ K (Δ K _eff ) region. When cathodic charging is applied, the fatigue crack growth rate becomes higher than in dry air, particularly in the higher stress intensity factor range. The hydrogen evolution rate is increased by cathodic charging, with lower temperature peaks and higher ones. The peaks at lower temperatures are correlated with the decomposition of hydrides and de-training of hydrogen from microstructural imperfections such as microvoids. As-received materials also show an evolution peak at a higher temperature, and the evolution rate is almost independent of cathodic charging. In addition, the evolution rate at a high temperature (above 800 °C) is increased by cathodic charging. The hydrogen is considered to have an important role on fatigue crack growth acceleration.     

Fracture toughness and fatigue crack growth behaviour of an Al2O3-SiC composite

The fracture toughness and fatigue crack growth characteristics of an Al₂O₃-SiC whisker composite were investigated. Quasi static fracture experiments were conducted on double edge-notched tension specimens and on four-point bend specimens containing a through-thickness Mode I crack which was introduced under uniaxial cyclic compression. The toughness results obtained using this procedure are more reproducible than those derived from the indentation technique and the notched bend bar method. The fracture toughness of the composite is about 60% higher than that of the unreinforced matrix material. Crack growth characteristics at room temperature were also investigated in notched plates of Al₂O₃-SiC subjected to fully compressive far-field cyclic loads. In the presence of a stress concentrator, this composite is found to be highly susceptible to fatigue crack growth under cyclic compressive loads.     

Effect of pearlite interlamellar spacing on fatigue property of a wheel steel

Different pearlite interlamellar spacings of 0.54 % carbon (C) wheel steel were obtained by heat treatment, and the influence of interlamellar spacing on fatigue behavior of the steel was studied through conducting the staircase method fatigue testing and decreasing/increasing stress intensity factor range (ΔK) fatigue crack growth tests. The results indicate that the fatigue endurance limit and fatigue threshold with smaller interlamellar spacing are higher than those with larger one, which can be well explained by dislocation slipping theory. Furthermore, the fatigue crack growth rates in the near-threshold and Paris regions were found to reduce with decreasing the interlamellar spacing. The decreased growth rate is attributed to the deflected crack path induced crack closure effect, as evidenced by fatigue steps on the fatigue fracture surface. The present results show how to enhance the fatigue property of wheel steel through refining the pearlite interlamellar spacing.     

New Concept on Fatigue Crack Growth Characteristics under Combined Stress Conditions

Using thin walled cylindrical specimens subjected cyclic tension and torsion, fatigue crack growth tests under combined stress conditions were carried out. The characteristics of the fatigue crack growth rate (FCGR) and morphology of the fatigue crack were identified.By comparing the experimental results with the results of finite element analyses, it was found that the experimental characteristics of FCGR were related to fatigue damage accumulation before initial crack growth. Damage mechanics approach is indispensable to characterize the fatigue crack growth under combined stress conditions.     

THE GROWTH OF SMALL CORROSION FATIGUE CRACKS IN ALLOY 2024

Abstract— The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminium alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by: (1) crack initiation at constituent particle pits, (2) intergranular microcracking for a≤100μm, and (3) transgranular small crack growth for a≥100μm. In aqueous 1% NaCl and at a constant anodic potential of −700 mV_SCE, small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of Δ K (< 1 MPa√m) below the long crack Δ K _th value. When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Similar small and long crack growth behavior at various levels of R suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks for a≥100 μm. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for alloy 2024 exposed to salt water.     

The effect of short time post-weld heat treatment on the fatigue crack growth of 2205 duplex stainless steel welds

The influence of γ content and its morphology on the impact and fatigue crack growth behavior of 2205 duplex stainless steel (DSS) welds were studied in this work. Short time post-heating was able to effectively raise the γ content and the impact toughness of the weld. The variation in microstructures showed less influence on the fatigue crack growth rate (FCGR) of the steel plate and weld except in the low ΔK regime. In contrast, residual welding stresses played a more significant affection on the FCGR of the DSS weld than microstructural factors did. Plastic deformation induced martensitic transformation within a definitely thin layer was responsible for the difference in crack growth behavior between specimens in the low ΔK range. Coarse columnar structure was more likely to have tortuous crack path in comparison with the steel plate.     

FATIGUE BEHAVIOUR OF A RAIL STEEL UNDER LOW AND HIGH LOADING RATES

Abstract— In this study, both sinusoidal wave loading and rectangular pulse (impact) loading are applied to a sample of rail steel in order to compare the growth rates for a surface fatigue crack. The results show that the rectangular pulse (impact) loading gives a crack growth rate on average 5 times that of the sinusoidal wave loading but can be almost 100 times in some instances. There are distinct differences between the fractograph patterns under the two modes of loading. The fractograph under sinusoidal wave loading clearly exhibits the pearlite structure of the material but under impact loading this microstructure is not revealed; only a quasicleavage pattern is observed. Analysis of the experimental fatigue crack growth data indicated that the surface fatigue crack growth rate behaviour could be described by the Paris-type relationship.     

Pre-strain effect on fatigue strength characteristics of SUH660 plain specimens

Precipitation particles in precipitation-strengthened materials are considered to be cut by pre-strain treatment, which affects the fatigue strength. In this study, fatigue tests were performed on precipitation-strengthened stainless steel SUH660 to investigate the effect of pre-strain on fatigue crack initiation and propagation characteristics. Fatigue test results showed that pre-strained specimens have a shorter fatigue crack propagation life compared to non-strained specimens; this is the opposite of results observed in carbon steel. The accelerated fatigue crack growth observed in pre-strained specimens was first assumed to be caused by precipitate cutting. A dislocation accumulation model for the fatigue crack tip in the precipitation-strengthened material was then suggested. Buff-polished specimens were also used for the fatigue tests, and the results showed that the work-hardened layer had a significant impact on the fatigue strength and fatigue life.     

Effects of oxide on fatigue crack growth behaviour of type 347 stainless steel in PWR water conditions

The fatigue crack growth behaviour of type 347 stainless steel in pressurized water reactor (PWR) conditions was investigated at two different temperatures, 25 and 316 °C. The fatigue crack growth rate was slightly increased at the elevated temperature in air. In the simulated PWR water environment, the fatigue crack growth rate was changed in a rather complicated manner with a change of the dissolved oxygen (DO) content. The DO content did not significantly change the fatigue crack growth rate at 25 °C. With the lower oxygen content of 5 ppb, which is the practical limit of deoxygenated water, the fatigue crack growth rate was similar at both 25 and 316 °C. The fatigue crack growth rate was significantly decreased at 316 °C with the higher oxygen level of 100 ppb. Under 316 °C water conditions, oxides were observed on the fatigue crack surface where the size of oxide particles was about 0.2 µm at 5 ppb and about 1 µm at 100 ppb. The thickness of the oxide layers also increased with the increase of DO. Moreover, the ΔK threshold (ΔK_th) also increased as the DO increased from 5 to 100 ppb. The dissolved hydrogen levels did not affect the measured crack growth rate at the given test conditions. The decrease of the fatigue crack growth rate with higher DO content is attributed to a crack closure resulting from the formation of larger oxides near the crack tip at a rather fast loading frequency of 10 Hz that was used in this study.     

Estimating durability of steels at repeated bending impacts

Many components are subjected to repeated impacts, or in some cases these impacts can appear as additional loads. Repeated impacts define a fatigue phenomenon known under the name of Impact Fatigue. Because the strain rate changes the material characteristics it is to expect that the material properties at impact fatigue to be different in regard to those obtained at non-impact fatigue. This paper presents a classification of repeated impact tests, and starting from this a series of parameters used for durability estimation will be analyzed. The high number of parameters used by different authors creates difficulties in comparison the different laboratories results. The importance of the shape and dimensions of specimens, and the stiffness of supports are highlighted. In order to avoid these influences the authors proposed an experimental technique, based on testing of Charpy specimens, in similar conditions as single impact test. A new parameter η is proposed in order to correlate the durability at repeated impacts with the Charpy V Notch (CVN) impact energy.     

Fatigue crack initiation and growth of 16MnR steel with stress ratio effects

The effects of stress ratio on the fatigue crack initiation and growth were investigated by a newly developed unified model, which is based on the cyclic plasticity property of material and a multiaxial fatigue damage criterion in incremental form. The cyclic elastic-plastic stress-strain field was analyzed using the general-purpose finite element software (ABAQUS) with the implementation of a robust cyclic plasticity theory. The fatigue damage was determined by applying the calculated stress-strain responses to the incremental fatigue criterion. The fatigue crack growth rates were then obtained by the unified model. Six compact specimens with a thickness less than 3.8 mm were used for the fatigue crack initiation and growth testing under various stress ratios (−1.0, 0.05, 0.1, 0.2, 0.3 and 0.5). Finite element results indicated that crack closure occurred for the specimen whose stress ratio was less than 0.3. The combined effects of accumulated fatigue damage induced by cyclic plastic deformation and possible contact of cracked surfaces were responsible for the fatigue crack initiation and growth. The predicted results agreed with the benchmark mode I fatigue crack growth experiments very well.     

Environment effects and surface roughness on fatigue crack growth at negative R-ratios

Fatigue crack growth has been widely studied, since it plays an important role on the damage tolerance analysis of mechanical components and structures. The environment, material properties and stress ratio significantly influence the fatigue crack growth behaviour of materials. Experimental tests were performed in M(T) specimens of a normalized DIN Ck45 steel at constant load ratios for R = 0.7, 0.5, 0, −1, −2, −3, in ambient air and vacuum conditions, using a new and patented chamber of vacuum. Special emphasis is given to the study of environment effects, stress ratios and related effects of crack roughness. Fracture surface roughness and crack closure effect were systematically measured for all tests in order to compare the influence of different environment and R-ratios. Results have shown that fatigue crack growth rates are higher in air than in vacuum and the fracture surface roughness is also higher in air than in vacuum for comparable stress ratios. The effect of the environment on fatigue crack growth rates seems to be more significant than any mechanical contributions such as plasticity, oxide and roughness which can induce the so-called crack closure.