FATIGUE LIFE PREDICTION METHOD FOR IMPACTED LAMINATES

The existing models are established based on the fatigue behavior of impacted laminates.It makes them unsuitable for the general use.So,a general 3-D progressive damage fatigue life prediction method for impacted laminates is developed based on the progressive damage theory and the fatigue behavior of unimpacted unidirectional plies.The model can predict the fatigue life of laminated composites with different ply parameters,geometry,impact damage,and fatigue loading conditions.In order to obtain the impact damage information in the case that no impact test data is available,a whole damage process analysis method for laminated composites under the impact loading and the fatigue loading is analyzed.The predicted damage statuses of composite laminates can be used to analyze the post-impact fatigue life.A parametric modeling program is developed to predict the impact damage process and the fatigue life of impacted laminates based on the whole damage process analysis method.The most relative error between the prediction and the test results is 7.78%.

Effect of Microstructure on Impact Fatigue Resistance and Impact Wear Resistance of Medium Cr-Si Cast Iron

A great amount of iron grinding balls in tube mills have been consumed. Under this impact abrasive wear working condition, the failure of wear resistant alloying white irons grinding balls is mainly caused by fatigue spalling. The impact wear resistance of martensitic high chromium cast iron （Cr of 15%） is not high sometimes, but its cost is not low. Thus, medium Cr-Si wear resistant cast iron is recommended. The influence of the iron on impact fatigue resistance and impact wear resistance is pronounced. Ball-on-ball impact fatigue test and high stress impact wear test of the grinding balls have been carried out. The results show that the impact fatigue resistance （IFR） and impact wear resistance （IWR） of medium Cr-Si cast iron are superior to those of martensitic high chromium cast iron （Cr of 15%）. The main reasons are that （1） the stress in medium Cr-Si cast iron is released in the as-cast state; （2） the matrix is fine pearlite with better toughness and plasticity; （3） the pearlite is more stable compared with a retained austenite under repeated impact load and less phase transformation can take place; （4） high silicon content im- proves the morphology of eutectic carbide; （5） there is no seeondary carbide which results in less crack sources. All these factors are beneficial to improvement of impact fatigue spalling resistance. The eutectic carbide M7C3 is the main constituent to resist wear.

Effect of RE Modification and Heat Treatment on Impact Fatigue Property of a Wear Resistant White Cast Iron

The morphology of carbides, as well as the generation and propagation of fatigue cracks in a wear resistant white cast iron after impact fatigue test were observed by means of optical microscope and SEM, and the relationship among the content of RE (rare earths) in the wear resistant white cast iron and the heating temperature as well as the length and propagation speed of the fatigue cracks were determined. Based on the obtained results, the effect of RE modification and heat treatment on the impact fatigue property was further studied. Experimental results show that addition of RE can defer the time required for the generation of fatigue cracks, reduce their propagation speed and increase the impact fatigue resistance. The aforesaid effect is more noticeable in case of combined RE modification with heat treatment, which can be attributed to the variation in morphology and the distribution of the eutectic carbide network.

Fatigue Behavior of High Speed Steel Roll Materials for Hot Rolling by Laser Impacting

The fatigue behavior of high speed steel （HSS） roll materials for hot rolling was researched under watercooling conditions by laser impacting. The microstructure of HSS sample and the morphologies of fatigue samples were observed by scanning electron microscope. The phase structure was detected by XRD. The morphology of situ oxide scale was observed by optical microscope, and the expansion coefficient was measured by TGA. The experiment results indicate that the cracks come into being at the carbide-matrix interface, but there are no cracks in the matrix after many times of laser impacting treatment, for the situ sample taken from the fractured roll surface, big carbides are more sensitive to the fatigue, and peel off prior to small ones. The relevant fatigue mechanisms are also discussed.

Fatigue crack propagation in a helicopter component subjected to impact damage

Damage tolerant methodology is increasingly used in aeronautical components,especially due the fact that the Aviation Regulation requires such an assessment in case an accidental damage occurs.At present,there is a strong and actual interest in applying such procedures to helicopter components that are subjected to high frequency cyclic loads.In this paper,an investigation on a damaged transmission shaft for a tail rotor transmission of an actual helicopter has been carried out focusing on the fatigue crack propagation.A complete sequence of experimental tests was performed in order to create an actual ballistic damage and to subsequently check the damage tolerant behaviour.The shaft was damaged by oblique ballistic impact and was subsequently subjected to torsional fatigue loading.During the fatigue cycles several cracks propagated from the ballistic damages.Both of these steps(impact and fatigue loading)were also simulated by a complex modelling approach based on Finite Element Models and fracture mechanics theory.The comparison between the experimental and numerical results shows a good agreement but it underlines the need for a very refined modelling technique capable to replicate all the features associated with the damage in order to reliably simulate the subsequent propagation phase.

Impact Fatigue Behavior of ZrN/Zr-N/Zr Coatings on H13 Steel by CFUBMSIP

ZrN/Zr-N/Zr coatings were deposited on H13 steel by close field unbalanced magnetron sputtering ion plating （CFUBMSIP） technique. The effect of two main parameters such as OEM and bias voltage for the CFUBMSIP process on the microstructure, mechanical properties and impact fatigue behavior of the coatings was investigated. The results indicate that with OEM increasing from 55% to 65% the surface particles size of the coatings increases while it remains almost similar when the bias voltage changes from 60 to 75 V. An aggregation of the particles occurs on the coatings surface, with further increasing the OEM and bias voltage to 75% and 90 V, respectively. The coatings show a columnar grain structure and are mainly composed of two phases of ZrN and Zr. The coating hardness decreases with OEM value increasing and both the coating hardness and modulus go up with bias voltage. The coating deposited under OEM of 65% and bias voltage of 75 V shows the best impact fatigue property.

Effect of RE Elements on the Morphology of Carbides and the Impact Fatigue Properties of the Low Cr White Cast Iron

An apparatus of zone melting unidirectional solidification with a steep temperature gradient and an impact fatigue tester were used.The effect of RE elements on the morphology of carbides and the impact fatigue properties of the low Cr white cast iron were investigated.Experimental results showed that the modi- fying effect of RE elements was remarkable on the M3C carbides.With the addition of RE elements in the low Cr iron,a lot of plate-like carbides were transformed into lath-like and rod-like ones.The higher the content of RE elements was,the more was the fraction of the lath-like and rod-like carbides.In the low Cr white cast iron,the RE modifying agent can efficiently increase the impact fatigue resistance and decrease the crack growth rate and delay the time of incipient cracking.With the increase of the content of RE elements in the low Cr cast iron,the impact fatigue resistance increased greatly and the crack growth rate decreased rap- idly.

STATIC AND FATIGUE BEHAVIOR OF IMPACTED AS4/PEEK THERMOPLASTIC COMPOSITES UNDER COMPRESSION LOAD

Static and fatigue tests under compression load were made on impacted AS4/PEEK and T300/913C graphite/epoxy with [45/90/-45/0]5S stacking sequence. The comparison of the damage tolerance assessment for thermosetting and thermoplastic composites shows that thermoplastics are more damage tolerant under compression. Impacted thermoplastic composites have excellent compression-compression fatigue behavior. The damage growth life is only a few percent of their total fatigue life and no regular damage growth can be found. Some design principles for thermosetting composite structures may still be used.

Effect of crack-closure treatment on fatigue durability of cracked rib-to-deck welded joints in steel bridge decks

To evaluate the effect of treating long cracks with the impact crack-closure retrofit(ICR)technique,three rib-to-deck welded specimens with a crack length of about 100 mm were tested.The metallographic structure,crack section,crack propagation life,and stress variation were analyzed.Finite-element models were also developed,and some optimal values of certain parameters are suggested according to the simulated results.The results show that new crack sources are generated on both sides of the ICR-treated region because of the stress distribution.The fatigue lives of cracked specimens with long cracks are significantly improved by the technique.Considerable residual compressive stress is also induced,and so it is suggested that the optimal impact angle to be applied to real bridges should be 70°.The stress at the weld root is distributed uniformly with the crack closed,and the optimal crack-closure depth is 4 mm.To evaluate the effect of different crack-closure depths in tests,it is recommended that a hot-spot stress method which is extrapolated by three reference points should be adopted.

Improving fatigue performance of welded joints of X65 pipeline steel by UIT

Treating weld toes properly can improve the fatigue performance. Ultrasonic impact treatment （UIT） is a more effective and convenient method to enhance the fatigue strength of welded joints and suchlike structures. Fatigue tests were conducted on the specimens made of X65 pipeline steel. The test specimens were investigated on the fatigue strength and the fatigue life at the same stress range level by comparing the ones peened by UIT with the others without the treatment： the fatigue strength of the specimens as UIT, 90% of the fatigue strength of the base mental, is increased by 38% compared with that of as welded only; the fatigue life of the ones as UIT is prolonged by 11 multiples of the ones as welded only.

Accurate diagnosis of myalgic encephalomyelitis and chronic fatigue syndrome based upon objective test methods for characteristic symptoms

Although myalgic encephalomyelitis(ME) and chronic fatigue syndrome(CFS) are considered to be synonymous,the definitional criteria for ME and CFS define two distinct,partially overlapping,clinical entities.ME,whether defined by the original criteria or by the recently proposed criteria,is not equivalent to CFS,let alone a severe variant of incapacitating chronic fatigue.Distinctive features of ME are:muscle weaknessand easy muscle fatigability,cognitive impairment,circulatory deficits,a marked variability of the symptoms in presence and severity,but above all,post-exertional "malaise":a(delayed) prolonged aggravation of symptoms after a minor exertion.In contrast,CFS is primarily defined by(unexplained) chronic fatigue,which should be accompanied by four out of a list of 8 symptoms,e.g.,headaches.Due to the subjective nature of several symptoms of ME and CFS,researchers and clinicians have questioned the physiological origin of these symptoms and qualified ME and CFS as functional somatic syndromes.However,various characteristic symptoms,e.g.,post-exertional "malaise" and muscle weakness,can be assessed objectively using wellaccepted methods,e.g.,cardiopulmonary exercise tests and cognitive tests.The objective measures acquired by these methods should be used to accurately diagnose patients,to evaluate the severity and impact of the illness objectively and to assess the positive and negative effects of proposed therapies impartially.

Muscle fatigue in women with primary biliary cirrhosis:Spectral analysis of surface electromyography

AIM： To evaluate the myoelectric manifestations of peripheral fatigability in patients with primary biliary cirrhosis in comparison to healthy subjects. METHODS： Sixteen women with primary biliary cirrhosis without comorbidity and 13 healthy women matched for age and body mass index （BMI） completed the selfreported questionnaire fatigue impact scale. All subjects underwent surface electromyography assessment of peripheral fatigability. Anterior tibial muscle isometric voluntary contraction was executed for 20 s at 80% of maximal voluntary isometric contraction. During the exercise electromyographic signal series were recorded and root mean square （expression of central drive） as well as mean and median of electromyographic signal frequency spectrum （estimates of muscle fatigability） were com- puted. Each subject executed the trial two times. EMG parameters were normalized, then linear regression was applied and slopes were calculated. RESULTS： Seven patients were fatigued （median fatigue impact scale score： 38, range： 26-66） and 9 were not fatigued （median fatigue impact scale score： 7, range： 0-17）. The maximal voluntary isometric contraction was similar in patients （82, 54-115 N） and controls （87, 74-101 N）, and in patients with high （81, 54-115 N） and low fatigue impact scale scores （86, 65-106 N）. Root mean square as well as mean and median of frequency spectrum slopes were compared with the Mann-Whitney U test, and no significant difference was found between fatigued and non-fatigued patients and controls. CONCLUSION： No instrumental evidence of peripheral fatigability can be found in women with primary biliary cirrhosis but no comorbidity, suggesting that fatigue in such patients may be of central origin.

Performances of fissured red sandstone after thermal treatment with constant-amplitude and low-cycle impacts

In the engineering practices,it is increasingly common to encounter fractured rocks perturbed by temperatures and frequent dynamic loads.In this paper,the dynamic behaviors and fracture characteristics of red sandstone considering temperatures(25℃,200℃,400℃,600℃,and 800℃)and fissure angles(0°,30°,60°,and 90°)were evaluated under constant-amplitude and low-cycle(CALC)impacts actuated by a modified split Hopkinson pressure bar(SHPB)system.Subsequently,fracture morphology and second-order statistics within the grey-level co-occurrence matrix(GLCM)were examined using scanning electron microscopy(SEM).Meanwhile,the deep analysis and discussion of the mechanical response were conducted through the synchronous thermal analyzer(STA)test,numerical simulations,one-dimensional stress wave theory,and material structure.The multiple regression models between response variables and interactive effects of independent variables were established using the response surface method(RSM).The results demonstrate the fatigue strength and life diminish as temperatures rise and increase with increasing fissure angles,while the strain rate exhibits an inverse behavior.Furthermore,the peak stress intensification and strain rate softening observed during CALC impact exhibit greater prominence at increased fissure angles.The failure is dominated by tensile damage with concise evolution paths and intergranular cracks as well as the compressor-crushed zone which may affect the failure mode after 400℃.The second-order statistics of GLCM in SEM images exhibit a considerable dependence on the temperatures.Also,thermal damage dominated by thermal properties controls the material structure and wave impedance and eventually affects the incident wave intensity.The tensile wave reflected from the fissure surface is the inherent mechanism responsible for the angle effect exhibited by the fatigue strength and life.Ultimately,the peak stress intensification and strain rate softening during impact are determined by both the material structure and compaction governed by thermal damage and tensile wave.

Influence of railway wheel tread damage on wheel-rail impact loads and the durability of wheelsets

Dynamic wheel-rail contact forces induced by a severe form of wheel tread damage have been measured by a wheel impact load detector during full-scale field tests at different vehicle speeds.Based on laser scanning,the measured three-dimensional damage geometry is employed in simulations of dynamic vehicle-track interaction to calibrate and verify a simulation model.The relation between the magnitude of the impact load and various operational parameters,such as vehicle speed,lateral position of wheel-rail contact,track stiffness and position of impact within a sleeper bay,is investigated.The calibrated model is later employed in simulations featuring other forms of tread damage;their effects on impact load and subsequent fatigue impact on bearings,wheel webs and subsurface initiated rolling contact fatigue of the wheel tread are assessed.The results quantify the effects of wheel tread defects and are valuable in a shift towards condition-based maintenance of running gear,and for general assessment of the severity of different types of railway wheel tread damage.

Nanotwining induced by tensile fatigue and dynamic impact of laser powder bed fusion additively manufactured CoCrFeNi high-entropy alloy

The laser powder bed fusion(L-PBF)additively manufactured CoCrFeNi high-entropy alloy(HEA),with face-centered cubic(FCC)crystal structure,demonstrates better comprehensive mechanical properties in the building direction(BD).Loading quasi-static,dynamic fatigue,and dynamic separated Hopkinson press bar(SHPB)impact stress conditions along the BD of the L-PBF processed HEA exhibit intriguing mi-crostructural evolution characteristics.The L-PBF generates hierarchical dislocation grids containing nu-merous cell substructures within the HEA FCC grains,impeding dislocation motion during deformation and improving the strength.When subjected to dynamic fatigue loading,the dislocation grids restrict the mean free path of dislocations and thus trigger the activation of abundant stacking faults.Hence,nu-merous nanotwins form near the end of the fatigue life.Multiple twinning systems can also be activated under dynamic high-speed impact loading.Especially at a low temperature of 77 K,the stacking fault energy of the CoCrFeNi HEA decreases,resulting in increased activation of nanotwins,exhibiting excep-tional toughness and resistance to dynamic loads.Additional twin boundaries also impede dislocation movement for the strain hardening.These findings hold valuable implications for the study of additively manufactured HEA parts working in extreme environments.

某迫榴炮击针疲劳寿命仿真与提升研究

击针大多采用细长及变截面结构,在循环撞击力作用下,易引起结构应力集中及交变应力,经常出现疲劳断裂故障,严重影响自动武器的可靠性,甚至威胁到作战人员安全。某迫榴炮击针在试验中偶发断裂现象,疲劳寿命约200发左右,为了提升该击针疲劳寿命,基于载荷传递过程,建立了击针在火药气体反冲力下高速运动与限位块的瞬态撞击模型,获取击针撞击过程交变应力,通过循环载荷作用下击针疲劳寿命评估,计算击针疲劳寿命与试验现象基本一致,验证了计算模型的准确性。在此基础上,开展了不同材料击针疲劳寿命提升研究,研究表明,将击针材料由PCrNi3Mo更换为30CrNi2MoVE后,击针疲劳寿命由215发提升到1670发,提升效果显著,改进后的击针经实弹射击试验验证在规定寿命周期内未发生断裂现象。

基于车桥耦合的钢-混凝土组合连续梁桥局部疲劳分析

钢-混凝土组合梁桥在满足使用性能的同时也会发生疲劳损伤,在桥梁设计和使用阶段应保证桥梁不发生疲劳破坏。为研究不同因素对桥梁疲劳损伤的影响,选取一座双工字钢-混凝土组合连续梁桥,采用车桥耦合分析程序对不同的车速、车重、桥面不平整度、桥面板厚度和主梁腹板高度下主梁的疲劳损伤进行分析。首先计算不同影响因素下桥梁的动应力,分析应力幅和等效应力幅个数的变化规律;然后分析应力幅冲击系数随各影响因素的变化规律;最后分析不同影响因素下主梁疲劳细节位置的疲劳损伤。研究结果表明:车重、桥面不平整度和主梁腹板高度对应力幅的影响较大,桥面不平整度对等效应力幅个数和应力幅冲击系数的影响较大;桥面不平整度通过改变应力幅冲击系数以影响应力幅,当桥面不平整度为C级及以下时,应力幅冲击系数偏离规范值;车重、桥面不平整度、主梁腹板高度对主梁疲劳损伤的影响较大,在超载和刚度储备不足的情况下主梁会发生疲劳破坏,应通过限载、及时维修桥面和提高主梁刚度的方法减小主梁的疲劳损伤。

热冲击高强钢焊接接头的低周疲劳及断裂机理研究

为了研究热冲击下高强钢焊接接头的低周疲劳性能及断裂机理,用电弧焊焊接Q345结构钢,分别在-20~400℃、-40~650℃、-60~900℃下对焊接接头进行循环冲击,用疲劳试验机加载低周疲劳载荷,分析热冲击温度对低周疲劳性能及疲劳强度的影响,并用扫描电子显微镜等分析焊接接头的断裂机理。结果表明:相同疲劳加载次数下,焊接接头的总应变幅值随热冲击循环次数的增加匀速下降;相同热冲击循环次数下,疲劳加载次数越多,焊接接头的应变幅值越小;相同疲劳加载次数和相同热冲击循环次数下,随冲击温度范围的扩大,焊接接头的总应变幅值逐渐减小,但总应变幅值整体变化与冲击温度范围正相关。