大初始缝高比混凝土试件双K断裂参数的试验研究

双K 断裂准则能够定量描述混凝土裂缝的起裂、稳定扩展和失稳断裂。该文采用混凝土三点弯曲梁试件,通过在试验中测得的起裂荷载P_ini、最大荷载P_max及临界裂缝口张开位移CMOD_C计算了初始缝高比为0.3~0.9共7组试件的起裂断裂韧度K_ICini 和失稳断裂韧度K_ICun 。结果表明,当初始缝高比为0.3~0.7时,混凝土裂缝扩展经历起裂、稳定扩展和失稳破坏3 个阶段,双K 参数均是与初始缝高比无关的材料参数;当初始缝高比大于或等于0.8 时,混凝土裂缝起裂后便进入失稳扩展阶段,起裂荷载即为最大荷载,且计算得到的K_ICini 仍与初始缝高比无关。因此,在确定K_ICini 时,仅需测得初始缝高比大于或等于0.8试件的P_max,将P_max作为P_ini直接计算得到K_ICini。同以往的试验方法相比,其结果更为准确且试验方法简单。     

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.     

FATIGUE AND FRACTURE IN POLYACETAL RESINS

Abstract— The fatigue behaviour of polyoxymethylene (POM) of different molar mass (6.1 × 10^4–1.5 × 10⁵) was studied at stress levels from 35 to 50 MPa (about 55 and 75% of the yield strength, respectively). It was found that cyclic loadings promoted bond breaking of the polymeric chains which considerably reduced the molar mass both in the crack tip region of cracked specimens and in the bulk in the case of uncracked samples. This process probably included the formation of reactive radicals which subsequently favoured the formation of microvoids that weakened the material. The static fracture toughness of fatigued samples decreased on increasing the stress level, the process accelerating as the cyclic loads approached the yield strength. As a result of material modifications during the tests the low molar mass resins did not exhibit stable crack propagation.     

45钢拉杆断裂失效分析

45钢拉杆在使用过程中发生断裂。采用化学成分分析、显微组织检验、硬度测试及断口的宏、微观形貌观察等方法对断裂件进行了分析。结果表明,在正常的交变载荷作用下,拉杆因材料强度级别偏低而导致其早期疲劳断裂;另外机加工不当造成的应力集中加速了拉杆的断裂。     

ANALYSIS OF CAST STEEL FRACTURE MECHANISMS FOR DIFFERENT STATES OF STRESS

The process of fracture in a low-carbon cast steel was studied for different states of stress. As a result of heat treatment, two different microstructures have been obtained: ferritic-pearlitic and bainitic. The triaxial states of stress were realised by tensile tests on specimens with various notch configurations and on smooth specimens subjected to different hydrostatic pressures.
During tensile tests carried out under triaxial stress states, the following quantities at fracture were determined: the effective strain, effective stress, stress state components, mean stress and stress triaxiality factor. Fractography of the specimens was carried out to observe the fracture mechanisms and relate them to the state of stress. The fracture mechanism depended on the state of stress and microstructure. With a decreasing stress triaxiality factor, the failure mechanism changed from ductile to shear. The fracture mechanism changed across the diameter of the sample and also depended on the microstructure. The small, smooth samples fractured at a higher stress than the larger samples. Ductile fracture in the ferritic-pearlitic microstructure was controlled by cracking of the matrix–precipitate boundary. Samples with the bainitic microstructure fractured by shear, and fracture depended mainly on the effective stress, although void growth (which is controlled by stress triaxiality) reduced the critical effective stress at positive values of mean stress.     

油田气阀弹簧断裂分析

采用扫描电镜、光镜、化学成分分析和显微硬度测试等方法对断裂件进行了分析,结果表明,弹簧的断裂属于早期疲劳断裂,弹簧材质不符合设计要求,其组织中存在粗大氧化物夹杂及原始裂纹是导致弹簧断裂的主要原因。     

高氮奥氏体钢低温脆断中的三种断裂刻面

用扫描电镜对18Cr-18Mn-0.7N高氮奥氏体钢低温脆断中裂纹的形成、扩展及断面进行了分析.结果表明,低温脆断中,首先沿退火孪晶界及晶界形成微裂纹,微裂纹穿晶连接使裂纹扩展,其结果导致断口上形成三种断裂刻面,即光滑平面状退火孪晶界断裂刻面、光滑曲面状沿晶断裂刻面及粗糙不平的穿晶断裂刻面.     

Fatigue assessment using an integrated threshold curve method - applications

This paper deals with the analysis and prediction of a high-cycle fatigue behaviour in notched and damaged specimens, as well as butt-welded joints by using a threshold curve for fatigue crack propagation that includes the short crack regime (a function of crack length, a). The approach regards the effective driving force applied to the crack as the difference between the total applied driving force defined by the applied stress distribution corresponding to a given geometrical and loading configuration, ΔK(a), and the threshold for crack propagation, ΔK_th(a). Chapetti’s model is used to estimate the threshold for crack propagation by using the plain fatigue limit, Δσ_eR, the threshold for long cracks, ΔK_thR, and the microstructural characteristic dimension (e.g. grain size). Applications, predictions and results, in good agreement with experimental results from the literature, demonstrate the ability of the method to carry out quantitative analyses of the high cycle fatigue propagation behavior (near threshold) of short cracks in different geometrical, mechanical and microstructural configurations.     

疲劳强度的发展及任务

疲劳破坏很早就被人们注意。经过二百多年的工作,已形成了由材料、力学和机械等组成的边缘学科——疲劳强度,疲劳一般指室温下的高周疲劳,考虑了温度、腐蚀介质和受载方式等的环境因素,有不同机理的疲劳。疲劳强度的研究应微观和宏观结合,从标准式样试验到整机试验,从常规疲劳设计到现代疲劳设计,并随科学技术的发展,不断深入。     

混凝土裂缝扩展的断裂过程准则与解析

该研究工作对混凝土这一多相的复合材料,通过实验和理论相结合的科学手段,建立了一套完整的描述混凝土裂缝发展的断裂理论以及分析方法。根据实验观测结果提出了双K断裂参数,可以反映混凝土裂缝发展特性。在线形渐进叠加假定基础上,给出了双K断裂参数的解析表达式。根据分布于断裂过程区上粘聚力对裂缝扩展阻力的增强作用,得到了双K断裂参数适用的解析解,并通过实验分析了各种可能因素对双K断裂参数的影响。在考虑粘聚力影响条件下,提出了裂缝扩展阻力的新KR曲线,并将双K断裂参数与之对应起来。研究工作又通过能量的观点提出了与双K断裂参数相对应的以能量释放率为参数的双G断裂参数。通过数值计算和实验分析证实了能量法和应力场法在描述混凝土断裂性能方面的等效性。     

淬火回火低碳合金钢的疲劳形变与断裂的电镜观察

应用透射电子显微术研究了淬火并400℃和600℃回火钢中疲劳位错结构随循环加载周数的增加所发生的变化。钢中出现具有循环显微硬化与循环显微软化的位错结构形式。前者主要是形成位错缠结,后者主要是粗大疲劳变形带的萌生和扩展,在扩展过程中萌生显微疲劳裂纹。     

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.     

An examination of load shedding during fatigue fracture

The existence of fatigue markings across the entire fracture surface of certain failed components implies that load shedding had occurred during the course of the fracture process. In order to examine the role of load shedding, a sample containing three parallel load paths with the central path being cracked was tested in fatigue using fixed grips. Loads in the three load paths, measured with strain gauges, were observed to be in good agreement with values predicted by theoretical analysis, based upon compliance calculations. Measured fatigue crack propagation rates also agreed well with those predicted from theoretical calculations.     

金属疲劳断裂的声发射检测技术

疲劳断裂是金属结构的主要失效形式,通过金属疲劳断裂时声发射特征参数的提取,建立了声发射特征参数和裂纹扩展速率之间的关系,由试样的三点弯曲疲劳试验,证明采用声发射技术监测疲劳裂纹的扩展,不仅与疲劳裂纹扩展的变化规律相似,而且能实时的捕捉到疲劳裂纹的产生。     

吐丝机螺栓断裂的失效分析

采用化学成分分析、宏观扣微观检验、金相组织分析和力学性能测定等方法分析了吐丝机用高强度螺栓发生断裂的原因。结果表明,由于机加工不当而在螺纹根部产生表面缺陷,并且这些表面缺陷周围在随后的热处理过程中发生全脱碳,大大降低了螺栓的疲劳强度,导致螺栓在使用过程中早期疲劳断裂。     

A model for short crack propagation in polycrystalline materials

A model for microstructurally short crack propagation in a grain structure of a polycrystalline material is developed. The crack propagation model is based on a crystal plasticity model and a microstructurally short crack propagation model in the spirit of the model by Navarro and de los Rios [A model for short fatigue crack propagation with an interpretation of the short-long crack transition. Fatigue Fract Eng Mater Struct 1987;10:169-86]. Numerical examples, where the combined crystal plasticity and crack propagation model is implemented in a model of a microstructure representing a duplex stainless steel, concludes the paper. Results showing how the misorientation of the crack- and slip-directions between two adjacent austenitic grains influences the crack propagation rate, as the crack propagates across their common grain boundary, are given.     

地铁列车转向架断裂原因分析

地铁列车转向架是由钢板焊接而成，在服役一年后断裂，对断裂件进行了断口形貌、金相组织、化学尬发分析及硬度测定。结果表明，断口显示出疲劳断裂痕迹，焊接缺陷是引起疲劳裂纹萌生及扩展的主要原因，而热影响区粗大的贝氏体及低碳马氏体组织的存在加速了疲劳断裂过程，最终导致转向架早期疲劳断裂。     

FD2815X型农用车转向节臂断裂分析

采用光学显微镜和扫描电镜对断裂转向节臂的宏，微观组织进行了分析。检验结果表明，表面碳层的存在及表面粗糙是导致构件断裂的主要原因。断裂性质属疲劳断裂。     

CRACK PROPAGATION AND FRACTURE IN CONTACTING BODIES

When a body subject to contact loads develops a crack, the origin, orientation, and tendency to propagate or not depend on fabrication details and maintenance history as well as the service environment. Research over the past twenty years has led to better characterization of crack behavior in some contacting bodies (railroad rail and wheels). Computational methods in solid mechanics provide the basis for quantitative results, but true understanding also requires application of materials science principles. The two disciplines are being brought to bear in combination; quantitative assessments of potential manufacturing improvements and prediction of effects on performance in service are now within reach.     

XM—19不锈钢锻造开裂的研究

研究了ＸＭ－１９不钢锻造开裂的原因,实验结果表明,钢锭加热温度过高,从而极大地降低了晶界塑性,晶界强度低于晶内强度,造成锻造开裂。锻造时,裂纹起地晶界和表面初始缺陷处,并沿晶界扩展,向纵深发展,断口显示出典型的沿晶断裂。