材料疲劳与断裂试验中的若干问题(1)

材料疲劳与断裂试验结果的准确性与有效性受多种因素的影响。结合试验数据,从试样加工(包括试样缺口加工形式及试样加工质量)和试验控制参量两个方面,分析讨论这些因素对材料疲劳与断裂试验结果的影响,为开展材料疲劳与断裂试验、获得准确有效的试验数据提供参考,从而为工程设计及可靠性服役提供依据。     

风电渗碳齿轮内部疲劳断裂可靠性研究

齿轮内部疲劳断裂作为风电渗碳齿轮典型失效形式，是限制风电齿轮箱服役性能提升的瓶颈之一。基于应力强度模糊干涉函数和齿轮材料强度退化理论，结合风电LDD载荷与Dang Van多轴疲劳准则建立渗碳齿轮内部疲劳断裂可靠度分析模型，通过与某2 MW风电齿轮失效样本进行对比验证了模型的适用性。采用因子试验设计方法分析齿轮硬度梯度和微观修形对内部疲劳断裂失效的影响，通过材料暴露系数回归方程进行望小优化设计获得主因子最佳参数匹配。研究结果表明心部硬度、齿向鼓形对内部疲劳断裂失效影响权重最大，通过优化设计将该齿轮副内部疲劳断裂可靠度由0.968 399提高至0.972 678。     

预制疲劳裂纹应力强度因子幅对超高强度钢断裂韧度的影响

采用多试样法对D406A超高强度钢进行了准静态断裂韧度KⅠC试验,分析了不同应力强度因子幅预制疲劳裂纹对疲劳预裂纹扩展周期、疲劳预裂纹扩展速率、试样断口形貌以及最终断裂韧度试验结果的影响。结果表明:疲劳预裂纹扩展周期和扩展速率均与应力强度因子幅呈指数变化规律,断口上的疲劳裂纹间距及最终断裂韧度试验结果均随应力强度因子幅的增大而增大,在材料断裂韧度KⅠC的20%~30%选择最大应力强度因子进行KⅠC试验结果较为稳定。     

“金属材料疲劳试验和断裂试验”专题报道征稿启事

<正>据统计表明,在机械断裂失效总数中疲劳失效约占80%以上,因此疲劳问题引起了材料研究者和产品设计者的极大关注,越来越多的试验室开始开展疲劳试验和断裂试验。目前,金属材料疲劳试验和断裂试验方法国家标准主要有GB/T 12443-2007《金属材料扭应力疲劳试验方法》、GB/T 3075-2008《金属材料疲劳试验轴向力控制方法》、     

金属非对称循环下的匀变速加载问题

由损伤力学理论可知,疲劳是由材料内部的损伤演化导致的,但其损伤演化的机理并不清楚。为此,我们将高分子物理中断裂的分子理论推广应用于金属的疲劳损伤,认为金属的断裂是一个松驰过程,宏观断裂是微观原子键断裂热活化的结果。以Q235钢为例,在CMT5105万能电子试验机上进行拉压非对称循环疲劳试验。从原子键离解的视角出发并结合试验数据,讨论并推算诸多因素影响下匀、变速加载时非对称循环疲劳损伤演化律的具体形式。再将速率作为重点考虑因素提出了新的疲劳损伤演化律,分别讨论了匀、变速情况下损伤演化律的基本形式,并对速率相关参数因子进行了修正。结果表明,新的疲劳损伤演化律形式简单、参数少、应用广泛且与试验结果贴合较好。     

“金属材料疲劳试验和断裂试验”专题报道征稿启事

<正>据统计表明,在机械断裂失效总数中疲劳失效约占80%以上,因此疲劳问题引起了材料研究者和产品设计者的极大关注,越来越多的试验室开始开展疲劳试验和断裂试验。目前,金属材料疲劳试验和断裂试验方法国家标准主要有GB/T 12443-2007《金属材料扭应力疲劳试验方法》、GB/T 3075-2008《金属材料疲劳试验轴向力控制方法》、GB/T 4337-2008《金属材料疲劳试验旋转弯曲方法》、GB/T 15248-2008《金属材料轴向等幅低循环疲劳试验方     

“金属材料疲劳试验和断裂试验”专题报道征稿启事

<正>据统计表明,在机械断裂失效总数中疲劳失效约占80%以上,因此疲劳问题引起了材料研究者和产品设计者的极大关注,越来越多的试验室开始开展疲劳试验和断裂试验。目前,金属材料疲劳试验和断裂试验方法国家标准主要有GB/T 12443-2007《金属材料扭应力疲劳试验方法》、GB/T 3075-2008《金属材料疲劳试验轴向力控制方法》、GB/T 4337-2008《金属材料疲劳试验旋转弯曲方法》、GB/T 15248-2008《金属材料轴向等幅低循环疲劳试验方法》、GB/T 26077-2010《金属材料疲劳试验轴向应变控制法》、GB/T 24176-2009《金属材料疲劳试验数据统计方案与分     

编者按

<正>据统计表明,在机械断裂失效总数中疲劳失效约占80%以上,因此疲劳问题引起了材料研究者和产品设计者的极大关注,越来越多的试验室开始开展疲劳试验和断裂试验。目前,金属材料疲劳试验和断裂试验方法国家标准主要有GB/T 12443-2007《金属材料扭应力疲劳试验方法》、GB/T 3075-2008《金属材料疲劳试验轴向力控制方法》、GB/T 4337-2008《金属材料疲劳试验旋转弯曲方法》、GB/T 15248-2008     

“金属材料疲劳试验和断裂试验”专题报道征稿启事

<正>据统计表明,在机械断裂失效总数中疲劳失效约占80%以上,因此疲劳问题引起了材料研究者和产品设计者的极大关注,越来越多的试验室开始开展疲劳试验和断裂试验。目前,金属材料疲劳试验和断裂试验方法国家标准主要有GB/T 12443-2007《金属材料扭应力疲劳试验方法》、GB/T 3075-2008《金属材料疲劳试验轴向力控制方法》、GB/T 4337-2008《金属材料疲劳试验旋转弯曲方法》、GB/T 15248-2008《金属材料轴向等幅低循环疲劳试验方法》、GB/T 26077-2010《金属材     

“金属材料疲劳试验和断裂试验”专题报道征稿启事

<正>据统计表明,在机械断裂失效总数中疲劳失效约占80%以上,因此疲劳问题引起了材料研究者和产品设计者的极大关注,越来越多的试验室开始开展疲劳试验和断裂试验。目前,金属材料疲劳试验和断裂试验方法国家标准主要有GB/T 12443-2007《金属材料扭应力疲劳试验方法》、GB/T 3075-2008《金属材料疲劳试验轴向力控制方法》、GB/T 4337-2008《金属材料疲劳试验旋转弯曲方法》、GB/T 15248-2008《金属材料轴向等幅低循环疲劳试验方法》、GB/T 26077-2010《金属材料疲劳试验轴向应变控制     

Prediction of Fatigue Characteristics of Metals at Different Loading Frequences

The procedure for predicting fatigue characteristics by high-frequency test results over lifetime ranges up to 10¹⁰ cycles is proposed. The procedure is based on the fatigue fracture model accounting for the loading frequency and stress ratio. The potentials of the method are exemplified by the tests of smooth specimens and specimens with a stress concentrator from different materials (nickel-, aluminum-, and titanium-base alloys). The prediction results for different loading frequencies (35–10,000 Hz) and stress ratios (from -1 to 0.5) are shown to vary by about 10% from experimentally obtained data.     

Creep fatigue behaviour of Type 321 stainless steel at 650°C

Abstract

Type 321 austenitic stainless steel has been used in the UK’s advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650°C and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650°C. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650°C in conjunction with the materials properties that were determined from the uniaxial test data.     

Fracture of pearlitic ductile cast iron under different loading conditions

In this study, fracture surfaces of pearlitic ductile iron generated under impact, bending and fatigue tests were characterised and compared. The fracture mechanisms in each case were identified from the observation of scanning electron microscopy images, and the characteristic topographic features were quantitatively and qualitatively evaluated. Cleavage was the predominant fracture mechanism for impact loading at all test temperatures and for slow bending testing. On the other hand, under fatigue loading, the fracture surfaces showed a mix of cleavage and ductile striation areas. A complex but reliable methodology for the determination of the direction of propagation of the main crack was also proposed, based on a careful analysis of the river patterns along the cleavage planes.     

Description of creep and creep fatigue crack growth in 1% and 9% Cr steels

Creep crack growth tests on a 1CrMoV steel are presented, covering the aspects of specimen size, geometry and service-like stresses. To consider nonstationary loading in modern plants creep fatigue crack growth tests have been started. As test materials a 1CrMoV steel and a modern 9%Cr-steel were used. By means of a comparison of creep crack and creep fatigue crack results the effectiveness of the fracture mechanics parameters K₁, ΔK₁, and C* could be evaluated.     

COMPUTER AIDED MULTIAXIAL FATIGUE TESTING

Abstract— This paper describes a microcomputer system that controls a multiaxial fatigue test facility. The system involves interface hardware, command control units, data acquisition units, and associated software for data processing, setting-up of test procedures and system monitoring. Without such a microcomputer system it would be impossible to acquire and analyse the vast amount of data generated by multiaxial fatigue tests, but perhaps more important, it would be impossible to control and monitor the important parameters that govern fatigue fracture behaviour of metals. This is particularly true for complex tests in which waveform shapes and out-of-phase angles can be changed during the course of a test. Finally, such a facility permits for the first time a detailed investigation of the effects of anisotropy on the deformation and fracture behaviour of metals.     

HIDA activity on 2¼ Cr1Mo steel

Historically, the 2¼CrlMo steel is one of the first type of low alloy steels used in high temperature plant. The objective of the present study is to test uniaxial, standard fracture mechanics and feature specimens from the same batch of material as well as test welded and ex-service materials and samples with mechanically induced residual stress. The creep and creep/fatigue crack initiation and growth results using fracture mechanics modelling techniques will then be used to validate the ‘HIDA’ procedure for high-temperature defect assessment. This paper presents an outline of the testing programme. The initial results for this steel from both static and cyclic loading conditions are presented for the test temperature of 565°C. The feature tests, simulating actual components, consist of three industrially relevant pipe types which are pre-notched and internally pressurised. In addition some of these pipes are being tested under four point bending. Early results of the X-ray and magnetic measurements to characterise creep damage are also presented. Initial results of short term laboratory data derived from uniaxial tests as well as compact tension specimens are presented.     

Experimental analysis of bending fatigue strength of plain and notched case-hardened gear steels

Fatigue tests were performed on two commercial case-hardened steels using plain and notched specimens and results were compared with data available in the literature for similar steels. The purpose of the paper was to compare performances of different materials under the same testing conditions. Moreover, the results obtained may form the basis to apply ISO Standard methods mentioned in the Annex A, which estimate the fatigue life of gears starting from experimental data generated from specimens. Even though ISO Standard suggests primarily the use of experimental results generated from testing gears, specimens’ testing is faster and cheaper to evaluate different materials during the design process. From this point of view, specimens’ based approaches seem promising. Plane bending as well as axial fatigue tests were carried out, along with static tensile tests. Fatigue design coefficients relevant to the notch support effect, surface finish, mean stress effect in bending and load type (axial or bending) were derived from the experimental test results and, whenever possible, were compared with ISO Standard recommendations. Microstructures, hardness profiles, residual stresses and fracture surfaces were analyzed as well. To evaluate the accuracy of specimens-based approaches mentioned in the Annex of ISO Standard, additional work is needed in order to compare theoretical estimations with experimental results obtained from gears.     

CRACK GROWTH RATES DURING CREEP, FATIGUE AND CREEP-FATIGUE IN AN AUSTENITIC FEATURE WELD SPECIMEN

Abstract— Static creep crack growth tests and displacement controlled fatigue and creep-fatigue crack growth tests have been performed on austenitic feature weld specimens at 650°C. The creep-fatigue tests incorporated hold times of up to 96 h. During these tests, crack growth appeared to comprise cyclic and dwell components. Cyclic crack growth components were characterised by the fracture mechanics parameter K whilst creep crack growth contributions were correlated with C *. In order to determine K and C * for the non-standard feature weld specimen, elastic and elastic-plastic creep finite element analyses were conducted. Good correspondence is shown between the feature weld data and comparable data from compact tension specimen tests on similar materials. Equations obtained from the compact tension specimen results, which describe total crack growth rates as the sum of the cyclic and dwell contributions, are shown to adequately describe the features test results also. Furthermore, it is demonstrated that a reference stress approach can be used to estimate C * for the features specimens.     

A generalized grain-size dependence of the fatigue limit

A generalized model is developed for calculating the fatigue limit of the material based on the data on its microstructure. The experimental results of fatigue and cyclic fracture toughness tests and the data on the analysis of the microstructure of two-phase (α + β) titanium VT3-1 alloy with a different microstructure (globular, bimodal and fine lamellar microstructure) have shown that the dependence of the fatigue limit on the structural parameter, which is responsible for the material fatigue strength, has a nonlinear S-like nature. The formula for calculating the fatigue limit is justified by the data on the microstructural parameters and by the results of static tensile tests of the material, with a fair agreement obtained between the calculated and experimental results. The model has been validated for materials with another type of the crystal lattice [steel containing 0.16% C with a body-centered cubic lattice (bcc) and brass 70/30 with a face-centered cubic lattice (fcc)] and has shown a satisfactory agreement between the experimental and calculation results.     

Werkstoffverhalten eines metallischen Faserverbundes und seiner Komponenten bei LCF‐Wechselbeanspruchung

Behaviour of a metallic fibre composite material and its components in LCF‐test The mechanical behaviour of unidirectional metallic fiber composite materials is investigated in the low cycle fatigue tests. The material consists of a copper matrix reinforced by continuous unidirectional fibers of austenitic steel with two different volume fractions. A comparative investigation carried out on the component materials. In addiction to the result of the strain controlled tension‐compression‐tests, the fatigue life is estimated using the Manson‐Coffin relation. The fracture characteristics are studies metallographically.