PA6/POE-g-MAH共混物在缺口冲击与缺口拉伸实验中的脆韧转变

用缺口冲击和缺口拉伸实验研究PA6/POE-g-MAH共混物的脆韧转变。结果显示,POE-g-MAH含量对共混物脆韧转变的影响主要是POE-g-MAH含量对裂尖局部应变速率的影响。在缺口冲击和缺口拉伸实验中,随着POE-g-MAH含量增加,裂尖附近参与变形的范围增大,导致局部应变速率降低。当局部应变速率降低至某临界值时,材料的断裂发生脆韧转变。在缺口拉伸实验中,随着拉伸速度增加,PA6/POE-g-MAH共混物发生脆韧转变的POE-g-MAH含量增加。这可能是拉伸速度与POE-g-MAH含量对PA6/POE-g-MAH共混物裂尖局部应变速率共同影响的结果。     

不同加载速率下高性能水泥基复合材料断裂性能研究

为研究加载速率对高性能水泥基复合材料(High-performance cement-based composites,HPCC)断裂性能的影响,本研究对带预制裂缝的HPCC矩形梁进行了三点弯曲测试.以裂缝嘴张开位移(Crack mouth opening displacement,CMOD)为加载控制参数,加载速率分别为0.001 mm/s、0.01 mm/s和0.1 mm/s,试件内钢纤维体积掺量分别为0％和2％(均为质量分数).基于荷载-裂缝嘴张开位移(P-CMOD)曲线分析了第一裂纹应力、弹性模量、弯曲强度、失稳韧度以及断裂能等一系列断裂特征参数随加载速率的变化规律.试验结果表明:(i)第一裂纹应力和弹性模量几乎不受钢纤维含量和加载速率的影响;(ii)HPCC弯曲强度与应变率比呈对数关系,且含钢纤维HPCC的弯曲强度率效应更明显;(iii)含钢纤维HPCC的失稳韧度和断裂能有很大程度的提升,但其对加载速率的敏感性较低.掺钢纤维能够有效提高HPCC材料抵抗冲击荷载的能力.     

聚丙烯材料的动态断裂

聚丙烯材料存在着由加载速率变化引起的韧脆转化现象，增加速率，该材料由韧性断裂向脆性断裂方式转化，提高试验温度，使这一转化向商加载速率方向移动，引入Ａｒｒｈｅｎｉｕｓ关系，通过热激活分析，可以获得定量化描述断裂能量，加载速率以及试验温度之间的关系。     

有机玻璃纯Ⅰ型和纯Ⅱ型动态断裂行为的实验研究

采用尖槽式中心切口圆盘试件,在分离式霍布金森压杆试验装置上对有机玻璃纯Ⅰ型和纯Ⅱ型加载条件下的动态断裂行为进行了实验研究。结果表明,加载速率对有机玻璃的断裂行为有显著的影响,有机玻璃纯Ⅰ型和纯Ⅱ型断裂韧度的测试结果均表现出明显的加载速率相关性,且随着加载速率的增加而增大。     

有机玻璃纯Ⅰ型和纯Ⅱ型动态断裂行为的实验研究EI北大核心

采用尖槽式中心切口圆盘试件,在分离式霍布金森压杆试验装置上对有机玻璃纯Ⅰ型和纯Ⅱ型加载条件下的动态断裂行为进行了实验研究。结果表明,加载速率对有机玻璃的断裂行为有显著的影响,有机玻璃纯Ⅰ型和纯Ⅱ型断裂韧度的测试结果均表现出明显的加载速率相关性,且随着加载速率的增加而增大。     

聚氯乙烯的热激活断裂

展示了聚氯乙烯（ＰＶＣ）在不同温度下由试验速率增加而引起的韧脆性转化。随加载速率升高，材料断裂吸附的能量不断降低，低速范围加载的断裂方式以塑性机制为主，而高速加载导致材料脆化。通过热激活分析，获得了该转化过程的定量化关系。该关系初步描述了由于速率和温度引起的韧脆性转化过程。     

2024-T3铝合金的动态断裂韧性

目的实现2024-T3铝合金动态断裂韧性的测量,揭示加载速率对动态断裂韧性的影响机理。方法采用屏蔽措施避免电磁干扰,测量2024-T3铝合金在不同加载速率下的动态断裂韧性,采用扫描电子显微镜(SEM)观察断口形貌,理论分析加载速率对动态断裂韧性的影响机理。结果当加载速率小于103MPa·m~(1/2)·s~(-1)时,2024-T3铝合金的动态断裂韧性约为35 MPa·m~(1/2);当加载速率高于105 MPa·m~(1/2)·s~(-1)时,动态断裂韧性超过40 MPa·m~(1/2),且随加载速率的增加而不断增大至101 MPa·m~(1/2)。断口分析表明,加载速率较低时,断口形貌为微孔聚集型;当加载速率超过105 MPa·m~(1/2)·s~(-1)时,断口特征由延性韧窝向准解理形态转变。理论分析表明,上述现象主要是由于裂纹尖端的无位错区域尺寸随加载速率的增大而减小,位错对裂纹尖端应力场的屏蔽效应增大,从而导致裂纹起裂后迅速由韧窝状态向准解理状态转变。结论电磁屏蔽后的电阻应变片法,能够准确测量电磁环境下2024-T3铝合金的动态断裂韧性,且动态断裂韧性表现出明显的应变率敏感性;2024-T3铝合金的微观断裂机制在准静态下为微孔聚集型,加载速率超过105MPa·m~(1/2)·s~(-1)时,材料的断裂表现为由延性韧窝形态向准解理形态转变。     

不同缺口尺寸试样断裂行为的加载速率敏感性

对三种不同缺口尺寸的16MnR钢试样(3V，3I和3C)在-110℃和1～500mm／min的加载速率范围内进行了三点弯曲断裂试验。发现应力集中和塑性拘束度按3V，3I和3C依次增大的三种试样，其断裂行为敏感的加载速率范围按1～120mm／min，1～60mm／min和1～30mm／min依次降低。在敏感的加载速率范围内，断裂模式随加载速率的增加由延-脆转变断裂迅速转变为全脆性解理断裂，韧性随之迅速降低。在超出敏感速率范围的较高速率范围内，三种试样的断裂行为对加载速率不敏感，断裂模式保持为全脆性解理断裂，韧性较低，并基本不随加载速率变化。     

AA5052薄板磁脉冲胶焊复合接头动态力学性能研究

目的 研究AA5052铝合金薄板在高速冲击载荷下的磁脉冲胶焊复合接头的动态力学性能,探究不同载荷速率对该胶焊复合接头力学和失效行为的影响规律.方法 利用磁脉冲焊接系统成功制备了胶焊复合连接试件.采用万能拉伸试验机、高速拉伸试验系统,结合全场应变测量系统,获得胶焊复合接头的力学性能规律,以及渐进失效过程和搭接区应变变化.通过扫描电镜观察焊缝断口,对其显微形貌进行表征.结果 当剪切速率从2 mm/min提升到10 m/s时,接头的峰值载荷从6086.5 N增大到6592.5 N,吸收的能量从41.1 J增大到96.4 J.相比于2 mm/min准静态剪切断口,1 m/s高速剪切断口中等轴韧窝和胶粘剂薄层的比例均提高.当剪切速率在1 m/s以下时,接头发生胶层和焊缝断裂失效;当剪切速率在1 m/s以上时,胶层和焊缝得到的强化程度高于母材,胶层开裂程度减小,接头搭接区的刚度提高,所以拉伸端母材发生严重的颈缩直至断裂失效.结论 随着剪切速率的增加,接头的峰值载荷和吸收能量均增大,胶层的开裂程度减小,失效形式由焊缝和胶层断裂转变为母材断裂.     

不同冲击断裂方式下橡胶粒子形态对HIPS脆韧转变的影响

核-壳型/蜂窝型高抗冲聚苯乙烯(HIPS)树脂在Izod冲击和落锤冲击断裂过程中的脆-韧转变行为不同。透射电镜(TEM)对亚断裂形变区的观察表明,蜂窝型HIPS对落锤冲击作用下的脆韧转变敏感,应力集中能有效引发基体的塑性形变,呈韧性断裂,但在v-notched Izod冲击断裂过程中,却呈脆性;而核-壳型HIPS的冲击断裂性能与之相反。将LDO假说引入冲击破坏研究并结合损伤竞争准数Da和Vincent图探讨了分散相特性及冲击方式对脆韧转变行为的影响。     

Dynamic fracture toughness of X70 pipeline steel and its relationship with arrest toughness and CVN

The dynamic fracture toughness K_1d and J_1d, arrest toughness K_1a and Charpy V-notched impact toughness (CVN) of a pipeline steel, X70, were studied at different temperatures. It was found that fracture toughness was strongly affected by temperature and loading rate. The fracture toughness decreases with decreasing temperature from 213 to193 K and increasing loading rate from to . At constant temperatures, only increasing loading rate can induce the transition from ductile to brittle. There exists a fracture transition caused by loading rate. Through thermal activation analysis, a quantitative relationship has been derived:

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. It can describe the fracture process at different temperatures and loading rates. At a loading rate of , the relationship can predict arrest toughness well. It provides the possibility of measuring arrest toughness with small size specimen. An empirical equation has been derived: CVN=4.84×10⁶T^−2.8K_1d(K_1a), which correlates K_1d and K_1a with CVN in one equation. This means that we can calculate K_1d and K_1a when we get CVN.     

Fracture energies of Zircaloy-4

Fracture properties of Zircaloy-4 were determined as a function of the energies required for failure. Charpy V-notch fracture samples were prepared from nuclear grade Zircaloy-4. The fracture mechanism was studied as a function of orientation, loading rate, and temperature. Yield stress, tensile streets, and integrated energy values were determined from the resulting fracture force-displacement curves. Fracture toughness values were found to be independent of orientation but strongly dependent upon temperature and loading rate; K-values decreased with increasing temperature, dynamic values were higher than static. Structure of the force-displacement curves revealed four distinct segments: (a) elastic deformation, (b) plastic deformation, (c) fracture, and (d) tearing after crack arrest. Elastic energy (a) values decreased with temperature, while plastic energy (b) values increased with temperature. Cracking energy (c) values decreased with increasing temperature for statically loaded specimens but increased with increasing temperature for dynamically loaded specimens. Tearing energy (d) values were strongly dependent upon temperature and orientation. Two orientations (T-S and L-S) exhibited a gradual increase in tearing energy with increasing temperatures very characteristic of ductile materials. The other two orientations (T-L and L-T) had a temperature transition curve reminiscent of a ductile-brittle transition material. Sharp temperature transition resulted from the onset of crack arrest. A sudden transition dip was observed in the tearing energy which was a function of loading rate and orientation.     

Some studies on the impact behavior of banded microalloyed steel

Microalloyed steels are used in automobile industries, offshore platforms and in structural applications. It is essential to establish a relation between service consition such as temperature, loading rate and fracture behavior of the steel. Impact study on new material is very handy to understand the mechanicl properties in a rapid and inexpensive way. The present investigation aims to assess impact toughness (CVN), ductile brittle transition temperature (DBTT, 25J), and initiation dynamic fracture toughness (J_ld*) of the indigenously developed microallayed seel. The steel has shown banding with alternate layers of ferrite and pearlite. The banding concentration (ferrite bands per mm) has been altered by heat treatment. Presence of banding has given spikes and splits in impact fracture. Change in banding concentration has affected DBTT of the steel, upper shelf energy and the extent of splitting. A model of crack divider with respect to the present microstructure has been analyzed. Banding in divider orientation improves the impact as well as initiation dynamic fracture toughness of the steel. The effect of temperature on splitting is also discussed. Splits in fractured surface disappear with decreasing temperature and higher numbers of splits yield lower toughness. Further, initiation dynamic fracture toughness is calculated for all temperatures and correlated with impact toughness.     

Ductile to brittle fracture transition of a plain carbon steel and its thermal activation model

An investigation has been made on the ductile to brittle fracture transition of a C-Mn steel due to variation of loading rates at constant temperatures. The transition takes place with increasing loading rate, which is similar to that caused by decreasing temperature. An equation has been derived from the thermal activation analysis which correlates the fracture toughness with temperature and loading rate. A model of fracture transition has been proposed from the thermal activation movement of atoms.     

单晶Si中(10)面沿[11]方向裂纹的脆韧性转变

利用改进压轮法预制出与以往不同的（１０）面［１１］方向的平直裂纹．采用三点弯曲法测定裂纹临界应力强度因子 Ｋ＿ｃ,用扫描电镜分析裂纹面断口的形貌,研究了硅单晶中的脆韧性转变（ＢＤＴ）行为 结果表明,随着加载速率从 ４μｍ／ｓ增加到 １６μｍ／ｓ,脆韧性转变温度向高温方向移动,转变区间由３５Ｋ减至狭小的８Ｋ,在这一区间内临界应力强度因子突然上升．在脆韧性转变过程中当裂纹扩展越过塑性饱和区后出现（１）和（１）面的交滑移,说明脆韧性转变与滑移系的启动有密切的关系     

Investigation of loading rate and plate thickness effects on dynamic fracture toughness of PMMA

To investigate the effects of loading rate and plate thickness on the fracture toughness of PMMA (polymethyl methacrylate) under impact loading, two methods, A method and B method, are applied as follows. In the A method, a dynamic finite element method and a strain gage method are applied to measure the dynamic fracture toughness in the fracture test using an air gun. In the B method, a single axis strain gage method is applied to measure the critical dynamic stress intensity factor, namely dynamic fracture toughness, in the fracture test using a weight dropping type apparatus. The dimensions of the PMMA specimen are L = 140 mm length and W = 30 mm width. Three values of the plate thickness B, 15.0 mm, 10.0 mm and 5.0 mm, are selected to investigate the plate thickness effect in the fracture test. Both results by the A and B methods precisely indicated the minimum value and the loading rate effect on the dynamic fracture toughness.     

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material under thermo-mechanical loading

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material (FGM) is investigated using a three point bend specimen. The modified split Hopkinson pressure bar (SHPB) apparatus in conjunction with induction coil heating system is used during elevated temperature dynamic loading experiments. A simple and accurate technique has been developed to identify the time corresponding to the load at which the fracture initiates. A series of experiments are conducted at different temperatures ranging from room temperature to 800 °C, and the effect of temperature and loading rate on the fracture initiation toughness is investigated. The material fracture toughness is found to be sensitive to temperature and the fracture initiation toughness increases as the temperature increases. Furthermore, the fracture initiation toughness is strain rate sensitive and is higher for dynamic loading as compared to quasi-static loading.     

Experimental and numerical study on dynamic fracture behaviour of AISI 1045 steel for compressor crankshaft

Dynamic fracture behaviour of AISI 1045 steel for compressor crankshaft was studied by experimental and numerical methods. True stress–strain relations of the material under different strain rates were measured, and dynamic constitutive model with consideration to strain‐hardening and strain‐rate hardening was proposed. Dynamic fracture tests loaded by Hopkinson pressure bar were carried out, and fracture toughness was determined using a finite element method with the combination of ABAQUS and Zencrack software. Loading states of the specimen and determination methods of the dynamic fracture toughness were discussed. By comparing the fracture behaviours under quasi‐static and dynamic conditions, it was found that the fracture modes exhibited a transition from ductile to brittle fracture with the increasing loading rate, and the dynamic fracture toughness value was less than the quasi‐static one.     

CYCLIC LOADING AND FRACTURE TOUGHNESS OF STEELS

Abstract— The paper considers the effect of cyclic loading and loading rate upon fracture toughness characteristics of steels at room and low temperatures. It is shown that fracture toughness of a low-alloy ferrite-pearlite steel with 0·1% C (steel 1) and for 15G2AFDps steel of the same class (steel 2) are 2 to 2·5 times lower under cyclic loading (50 and 0·5 Hz) and dynamic loading (= 1·5 × 10⁶MPa √m s⁻¹) than under static loading (= 0·6 to 9 MPa √m s⁻¹). For quenched and low-tempered 45 steel at 293 K and for armco-iron at 77 K fracture toughness characteristics do not depend on the loading condition. Macro- and micro-fractographic investigations revealed a correlation between the plastic zone size and the length of brittle fracture areas which are formed in steels 1 and 2, and in armco-iron during unstable propagation of the fatigue crack. Dependence of the decrease of the critical stress intensity factor under cyclic loading on the number of load cycles are obtained for repeating ( R = 0) and alternating bending ( R =−1) of specimens with a crack. A model for the transition from stable to unstable crack propagation is proposed involving crack velocity in the zone ahead of the crack tip damaged by cyclic plastic deformation. A new approach is suggested to the classification of materials on the basis of the sensitivity of fracture toughness characteristics to cyclic conditions of loading.     

Mechanical behaviour of poly(methyl methacrylate)

A series of tensile and three-point bending studies was conducted at various temperatures and loading rates using a commercial poly(methyl methacrylate) (PMMA). Tensile properties and fracture toughness data were obtained for the various conditions. In general, both tensile strength and fracture toughness increase with increasing loading rate and decreasing temperatur E. However, when the temperature reaches the glass transition region, the relationships between fracture toughness, loading rate, and temperature become very complex. This behaviour is due to the simultaneous interaction of viscoelasticity and localized plastic deformation. In the glass transition region, the fracture mechanism changes from a brittle to a ductile mode of failure. A failure envelope constructed from tensile tests suggests that the maximum elongation that the glassy PMMA can withstand without failure is about 130%. The calculated apparent activation energies suggest that the failure process of thermoplastic polymers (at least PMMA) follows a viscoelastic process, either glass or transition. The former is the case if crack initiation is required.Deceased.