Ti6Al4V基体及其改性试样的冲击磨损行为分析

在Ti6Al4V合金基表面制备了N+注入与氮化层、TiN膜及DLC膜,进行小载荷反复冲击,分析不同冲击周次下磨痕形貌及磨坑深度,比较磨损性能。结果表明疲劳磨损是反复冲击条件下实验试样的共同失效机制,主要表现为疲劳剥落。改性后试样的抗冲击磨损能力较基体材料均有提高,其中N+注入与氮化的"长程强化效应"使试样在较高冲击周次保持良好的抗磨性能;晶态结构TiN膜层由于较高的表层硬度,提高了试样的耐磨性,尤其是较低的冲击周次下;DLC膜的特殊非晶态结构,使其在整个冲击过程中具有小的磨损深度,高的失效周次。     

Impact and subsequent fatigue damage growth in carbon fibre laminates

The influence of the ply stacking sequence on the impact resistance and subsequent O-tension fatigue performance of carbon fibre laminates has been investigated. Drop-weight impact tests were conducted on a range of 16 ply carbon fibre laminates with either all non-woven plies or mixtures of woven and non-woven plies. Damaged coupons were tested in O-tension fatigue for up to 10⁶ cycles, scanned using an ultrasonic probe and then loaded in tension until failure.The impact resistance and subsequent fatigue performance have been found to be sensitive to the ply stacking sequence. The non-woven composites showed a marked sensitivity to impact loading, but increases in residual static strength were noted after cycling. The inclusion of a woven fabric served to improve the impact resistance of the laminates. Fatigue cycling resulted in considerably improved residual static strengths; by 10⁶ cycles any effect of the impact damage had been removed.     

Reformed austenite transformation during fatigue crack propagation of 13%Cr-4%Ni stainless steel

In the as-quenched state, 13%Cr-4%Ni martensitic stainless steels are essentially 100% martensitic. However, a certain amount of austenite is formed during the tempering of this alloy. This reformed austenite is thermally stable at room temperature but can transform to martensite under stress. This transformation is known to happen during impact testing but it has never been established if it occurs during fatigue crack propagation. This study presents the results of X-ray diffraction measurements of reformed austenite before and after crack growth testing. It has been found that reformed austenite does transform to martensite at the crack tip and that this transformation occurs even at a low stress intensity factor. Low-cycle fatigue tests were conducted to verify austenite transformation under cyclic straining. It was found that reformed austenite transforms only partially during the first strain reversal but that essentially all austenite has disappeared after 100 cycles. The relation between austenite transformation under low-cycle fatigue and its transformation during crack growth is also discussed.     

Effect of fatigue loading on the bond behaviour between UHM CFRP plates and steel plates

Extensive research has been conducted on static bond behaviour between CFRP and steel. However, very limited research is available on the effect of fatigue loading on the bond behaviour between CFRP and steel. This paper attempts to fill the knowledge gap in this area. A series of static and fatigue tests on UHM (ultra high modulus) CFRP plate and steel plate double strap joints were conducted. Five specimens were tensioned to failure under static loading as control specimens. The other 12 specimens were tested under fatigue loading with load ratios ranging from 0.2 to 0.6 (defined as the ratio of the maximum fatigue load to the average static bond strength of control specimens). After going through pre-set number of fatigue cycles, the specimens were tensioned to failure under static loading. The failure modes, residual bond strength and residual bond stiffness of such specimens were compared with those of control specimens, to facilitate the investigation of the effect of fatigue loading on the bond behaviour. Microscopic investigation was also performed to reveal the underlying failure mechanism.     

Fatigue life and plastic deformation behavior of electrodeposited copper thin film under variable amplitude loading

As an exploratory study to investigate fatigue behavior of an elecrodeposited copper thin film under variable amplitude loading, repeated two-block loading tests were performed for various loading conditions. Plastic deformation behavior was continuously measured during the tests. Due to the load interaction effect under repeated two-block loading, fatigue life is significantly reduced. A concept is introduced to evaluate the load interaction effect and successfully utilized to predict fatigue life through simulation of the monotonic plastic strain (ratcheting strain) behavior under repeated two-block loading. Effects of load parameters of repeated two-block loading on fatigue life were also discussed.     

Microstructural evolution and thermal fatigue resistance of grey cast iron

In the present work, thermal fatigue in grey cast iron has been investigated by means of a numerical and an experimental approach. Temperature gradients were generated within the material by means of a testing rig specifically designed for the experiments. The temperature gradients were responsible for the formation of severe stress fields that led to the failure of the specimens after a fairly low number of cycles. Crack growth was monitored during the tests, and the microstructure and hardness of samples were analysed after failure and compared with those of untested alloy. The repeated thermal cycles at peak temperatures of 600, 700, and 800°C led to important microstructural alterations of cast iron and to a drop in material hardness. The pearlite lamellae lost their original shape and became more fragmented. Oxygen‐rich regions surrounding the graphite flakes were produced by microgalvanic corrosion mechanism.     

In situ monitoring in real time of fatigue-induced damage growth in composite materials by acoustography

There are growing concerns about the effects of accidental impact damage on the structural integrity of aerospace composites and about the possible growth of the damage due to in-service fatigue. There has been some success in the use of established methods (ultrasonic C-scan, thermography, X-rays) to monitor damage development during fatigue experiments by interrupting a test and removing the specimen for damage inspection but this stop-and-restart test procedure is far from satisfactory. Real-time damage monitoring in composite materials during fatigue has now become possible by the emergence of a new ultrasonic imaging technology, acoustography. The successful integration of acoustography and a servo-hydraulic fatigue test machine has resulted in a new measurement system which can be used for the in situ monitoring in real time of damage growth in composite specimens during long-term fatigue tests. Results are presented which show damage-area growth during fatigue cycling under high compressive loads. After an initial small enlargement (stage 1), damage grows at a constant rate (stage 2) until the third stage is reached when there is further growth at an increasing rate to final failure. However, a ‘fatigue limit’ has also been observed. At stresses below this fatigue limit, a zero damage-growth régime has been found in studies of >10⁶ fatigue cycles. The results obtained have important implications for the understanding of the effects of damage on fatigue life and for the design of ‘safe’ damage-tolerant structures.     

Influence of rest time on recovery and damage during fatigue tests on bituminous composites

Introducing rest periods during fatigue tests can significantly improve the remaining lifetime of bituminous mixes. An experimental protocol has been designed such as to study the range and kinetics of loss of stiffness and recovering of properties during rest periods. The range of the maximum recovery depends on the number of cycles previously applied. Recovery is not permanent and the loss of stiffness speeds up under repeated fatigue sequences. The speed of loss of stiffness seems to be a damage indicator. It evolves differently according to the fact that the specimen has (or not) previously been subjected to more aggressive solicitations. This points out the fact that the mix “keeps in memory” previous loadings.     

A method for evaluating the thermal fatigue resistance of ceramics and its application to a fibre reinforced refractory material

An original method of characterizing thermal fatigue of ceramic materials has been proposed. This method is based on after-shock measurements of the degree of damage through a compliance calibration using compact tension (CT) test pieces. This method has been applied to a fibre-reinforced refractory material subjected to repeated thermal shock between 20 and 800° C. It has been demonstrated from both experiments and finite element analysis that the CT specimen is a convenient shape for the evaluation of thermal fatigue behaviour. In these specimens it has been established that the damage primarily affects the notch tip. The thermal fatigue behaviour of the CT specimens depends on notch length: when the notch length is greater than 30 mm, catastrophic failure occurs after a few cycles. When the notch length is less than 30 mm, the crack formed at the notch tip during the first cycle grows slowly during subsequent cycles. This behaviour has been explained by the variation of the stress intensity factor K _I.     

Prediction of low-cycle fatigue-life by acoustic emission—2: 7075-T6 aluminum alloy

Low cycle high stress fatigue tests were conducted by tension-tension on an Alclad 7075-T6 aluminum sheet alloy, until rupture. Initial crack sizes and orientations in the fatigue specimens were randomly distributed. Acoustic emission was continuously monitored during the tests. Extremal peak-amplitudes, equivalent to extremal crack-propagation rates, are shown to be extremally Weibull distributed. The prediction of the number of cycles left until failure is made possible, using an ordered statistics treatment and an experimental equipment parameter obtained in previous experiments (Part 1). The predicted life-times are in good agreement with the actual fatigue lives. The amplitude distribution analysis of the acoustic signals emitted during cyclic stress has been proven to be a feasible nondestructive method of predicting fatigue life.     

Modelling of damage and failure of glass/epoxy composite plates subject to impact fatigue

An investigation has been carried out to study the impact fatigue damage of glass/epoxy laminated composites. Accumulation of damage, such as matrix cracking, delamination and fibre breakage, with repeated impact of the composite material may reduce the overall stiffness. These damage modes have been combined in a very complicated way to describe damage growth and fracture. A model is proposed for characterising the damage as a function of the normalised impact number. The scalar variable D, which characterises the material damage, is written as a function of the life duration β, using a modified form of the Mankowsky empirical law [Int J Solids Struct 32(11) (1995) 1607]. The macroscopic failure mode and the internal damage in laminated specimens of glass/epoxy as a consequence of impact fatigue are analysed at different levels of incident impact energy. The impact fatigue tests have been conducted on an apparatus built in our laboratory.     

Fatigue analysis of unidirectional GFRP composites under combined bending and torsional loads

Fatigue behavior of unidirectional glass fiber reinforced polyester (GFRP) composites at room temperature under in-phase combined torsion/bending loading was investigated. All fatigue tests were carried out on constant-deflection fatigue machine with frequency of 25 Hz. A 30% reduction from the initial applied moments was taken as a failure criterion in the combined torsion/bending fatigue tests of the composite materials. A series of pure torsional fatigue tests were conducted to construct the failure contour of GFRP composites using different failure theories. The obtained S–N curves from combined torsion/bending tests were compared with both, pure torsion fatigue test results and published results of pure bending fatigue tests of GFRP rods. Pictures by scanning electron microscope were used to closely examine the failure mode of the tested specimens under combined torsion/bending loading.

The results showed that, the unidirectional glass fiber reinforced polyester composites have poor torsional fatigue strength compared with the published results of pure bending fatigue strength. Endurance limit value (calculated from S–N equation at N = 10⁷ cycles) of GFRP specimens tested under combined torsion/bending loading equals 8.5 times the endurance limit of pure torsion fatigue. On the other hand the endurance limit of combined torsion/bending fatigue strength approximately half the fatigue limit of pure bending fatigue strength. The predicted values of combined torsion/bending fatigue strength at different number of cycles, using the published failure theory are in good agreement with the experimental data. For the investigated range of fiber volume fractions (V_f) it was found that higher stress levels are needed to produce fatigue failure after the same number of cycles as V_f increases.     

Behaviour ofE-glass fibre reinforced vinylester resin composites under impact fatigue

An impact fatigue study has been made for the first time on 63.5% glass fibre reinforced vinylester resin notched composites. The study was conducted in a pendulum type repeated impact apparatus especially designed and fabricated for determining single and repeated impact strengths. A well-defined impact fatigue (S-N) behaviour, having a progressive endurance below the threshold single cycle impact fracture stress with decreasing applied stress has been demonstrated. Fractographic analysis revealed fracture by primary debonding having fibre breakage and pullout at the tensile zone, but a shear fracture of fibre bundles at the compressive zone of the specimen. The residual strength, modulus and toughness showed retention of the properties at high impact stress levels up to 1000 impacts followed by a sharp drop. Cumulative residual stresses with each number of impacts not withstanding the static fatigue failure at long endurances have been ascribed for the composite failures under the repeated impact stresses.     

Gigacycle fatigue data sheets for advanced engineering materials

Gigacycle fatigue data sheets have been published since 1997 by the National Institute for Materials Science. They cover several areas such as high-cycle-number fatigue for high-strength steels and titanium alloys, the fatigue of welded joints, and high-temperature fatigue for advanced ferritic heat-resistant steels. Some unique testing machines are used to run the tests up to an extremely high number of cycles such as 10¹⁰ cycles. A characteristic of gigacycle fatigue failure is that it is initiated inside smooth specimens; the fatigue strength decreases with increasing cycle number and the fatigue limit disappears, although ordinary fatigue failure initiates from the surface of a smooth specimen and a fatigue limit appears. For welded joints, fatigue failure initiates from the notch root of the weld, because a large amount of stress is concentrated at the weld toe. The fatigue strength of welded joints has been obtained for up to 10⁸ cycles, which is an extremely high number of cycles for large welded joints. The project of producing gigacycle fatigue data sheets is still continuing and will take a few more years to complete.     

The effect of solution heat-treatment time on the fatigue properties of an Al-Si-Mg casting alloy

Abstract Experiments have been carried out to investigate whether reducing the solution heat-treatment time of Al-7Si-0.6Mg castings from the currently recommended values adversely affects their fatigue properties. Fatigue endurance tests have been carried out in zero-tension ( R =0) and measurements made of the casting defects that initiated the fatigue cracks. The work has been limited to stresses that produce a fatigue life of ~10⁵ cycles and to two solution–treatment times (8 and 4 h). Two statistical techniques have been applied to the fatigue life data and no effect of solution heat-treatment time was detected at a confidence level of better than 95%. Similarly, no effect of cyclic test frequency could be detected for tests carried out at 1 and 60 Hz. The conclusions are confirmed by an analysis of the relation between fatigue life and the size of the casting defects that initiated fatigue failure. The scatter in fatigue lives is related to the scatter in the sizes of casting defects in the specimens. It is clear that there is a potential for considerable savings in heat-treatment costs for castings of the size and shape chosen for the study.     

Effect of zirconia addition on the fatigue behaviour of fine grained alumina

The fatigue behaviour of fine grained Al₂O₃ and ZrO₂ toughened Al₂O₃ (ZTA) compositions with 15 vol% ZrO₂ (3 mol% Y₂O₃ stabilized: 3Y-TZP) have been investigated by using three different techniques. Primarily 4-point bending load was employed to generate tension-tension fatigue data under both static and cyclic conditions. The results clearly showed that the materials were susceptible to both the static and cyclic fatigue and the time to failure under cyclic loading was considerably shorter than the equivalent static loads. The repeated indentations at the same spot with varying loads showed a typical fatigue behaviour. In addition, both the materials were subjected to the repeated impact cycles at varying subcritical loads simulating impact fatigue. In all the cases typical fatigue curves were obtained having a progressive endurance at subcritical loads having an endurance limit. The fatigue behaviour of Al₂O₃ was much improved by the addition of 15 vol% 3Y-TZP, having micro-plasticity contributing towards the cyclic fatigue phenomena of these materials.     

Multiaxial fatigue criterion for a high‐density polyethylene thermoplastic

The multiaxial fatigue behaviour of a high‐density polyethylene was investigated at room temperature and constant frequency. As a consequence of the mode of failure, an end‐of‐life criterion for fatigue tests is discussed in the first part of the work, in order to define the number of cycles to failure. Based on force controlled fatigue tests under tension, compression and torsion at two stress ratio, a multiaxial fatigue criterion including the stress‐ratio effect is proposed for the fatigue design of this polymer. This criterion is based on the maximum and mean values of the second invariant of the stress tensor.     

Flexural fatigue of glass-fibre-reinforced cement

Results of a large number of tests on glass-fibre-reinforced cement (GRC) subjected to repeated flexure on a prototype multiple specimen fatigue rig are presented in the form of probability-stress-number of cycles to failure (PSN) diagrams. The fatigue limit at 10⁶ cycles was found to be approximately 90% of the limit of proportionality determined in static bending. The effect of age, storage conditions and overstressing are reported and some comparisons are drawn between the behaviour of GRC and asbestos cement.     

Gigacycle fatigue data sheets for advanced engineering materials

Gigacycle fatigue data sheets have been published since 1997 by the National Institute for Materials Science. They cover several areas such as high-cycle-number fatigue for high-strength steels and titanium alloys, the fatigue of welded joints, and high-temperature fatigue for advanced ferritic heat-resistant steels. Some unique testing machines are used to run the tests up to an extremely high number of cycles such as 10¹⁰ cycles. A characteristic of gigacycle fatigue failure is that it is initiated inside smooth specimens; the fatigue strength decreases with increasing cycle number and the fatigue limit disappears, although ordinary fatigue failure initiates from the surface of a smooth specimen and a fatigue limit appears. For welded joints, fatigue failure initiates from the notch root of the weld, because a large amount of stress is concentrated at the weld toe. The fatigue strength of welded joints has been obtained for up to 10⁸ cycles, which is an extremely high number of cycles for large welded joints. The project of producing gigacycle fatigue data sheets is still continuing and will take a few more years to complete. r 2007 Published by Elsevier Ltd.     

EFFECT OF NON-PROPORTIONAL OVERLOADING ON FATIGUE LIFE

Abstract— The effect of non-proportional overloading on both low cycle and high cycle fatigue life has been studied. Low cycle multiaxial fatigue tests were performed on EN 15R (a low alloy steel) using sequential loading blocks which comprised uniaxial "ordinary" cycles and torsion "overload" cycles, and vice versa. In high cycle fatigue, the behaviour of mode I crack growth in a medium carbon steel subjected to mixed (I and II) mode overloading was examined.
Under tension-torsion sequential overloading, crack growth behaviour shows an earlier transition from Stage I to Stage II with a pronounced reduction in accumulated fatigue life. Tensile overloading on torsion cycles was found to be more damaging compared to torsion overloading on repeated tensile cycles. The crack-load interaction in sequential overloading and its influence on crack growth and fatigue life is discussed. In low strain fatigue, Stage II crack growth retardation closely relates to the overload plastic zone size, crack tip blunting and crack surface shielding. Mixed mode overloading is shown to have a significant effect only if the mode I component of overloading is large enough to keep the crack open. Under both low cycle and high cycle fatigue conditions non-proportional overloading is shown to be more damaging than proportional overloading.