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。     

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。     

Thermal Activation Analyses of Dynamic Fracture Toughness of High Strength Low Alloy Steels

A formula is derived for determining the influence of temperature and loading rate on dynamic fracture toughness of a high strength low alloy steel (HQ785C) from thermal activation analysis of the experimental results of three-point bend specimens as well as introducing an Arrhenius formula. It is shown that the results obtained by the given formula are in good agreement with the experimental ones in the thermal activation region. The present method is also valuable to describe the relationship between dynamic fracture toughness and temperature and loading rate of other high strength low alloy steels.     

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.     

温度与加载速率对X70级管线钢韧脆转化现象的影响

在不同温度下测试了X70级管线钢的动态断裂韧度。研究表明:加载速率对动态断裂韧度的影响与温度对其的影响同样重要;在恒温下,增大加载速率可以诱发韧脆断裂转变;当温度由298 K向193 K逐渐降低,或加载速率从0.01 mm/s向1 000 mm/s增大时,均将导致材料的韧脆断裂转变。     

Analysis of dynamic conditions during thermal transient events for the structural assessment of a nuclear vessel

The possibility of dynamic loading conditions in a reactor pressure vessel (RPV) has been investigated in this paper. For this purpose, finite element (FE) numerical simulations of several thermal transients were performed including a normal shut down and two accidental thermal shocks, namely a loss of coolant accident (LOCA) and an extreme postulated pressurised thermal shock (PTS). The aim of the present contribution is to evaluate the influence of the loading rate on the fracture properties of the vessel steel of the Santa María de Garoña Spanish nuclear power plant (NPP) in the ductile to brittle transition (DBT) region. To describe the fracture behaviour of the steel in the DBT region, the master curve (MC) reference temperature, T₀, was used. This temperature is normally used for quasi-static conditions; however, it has been recently extended to the determination of dynamic fracture toughness by means of a phenomenological model proposed by Wallin. The dynamic reference temperature, T_0,dyn, was obtained for the loading rates corresponding to the three studied situations numerically simulated and compared with the quasi-static reference temperature, T_0,sta. From these results, conclusions about the importance of loading rates in nuclear vessels were established.     

钢结构防动载断裂选材的定量判据

分析了目前钢结构防断选材中应用的脆性转变温度方法的局限性,扼要介绍了材料断裂韧度的温度效应和加载速率效应.在钢材断裂特性分析的基础上提出了平面应变型断裂临界温度的概念,构建了断裂特征分析图,从而为钢结构防动载断裂选材提出了一个新的定量的判据,并以船只设计选材为例,阐明了该判据的应用.     

Experimental and analytical studies in crack initiation under fatigue: Part II — Analysis

The experimental results on damage evolution before crack initiation reported in Part I are analyzed. An energy release rate due to damage growth before crack initiation is evaluated with the use of a semi-empirical method for energy calculations and experimental measurements. Correlation of the elementary movements of the damage zone with the energy release rate shows that the rate of damage growth decreases monotonically. This is consistent with the growth behavior of an average damage density within the zone. The experimental data have shown that crack initiation occurred when damage density within a core zone ahead of the notch tip reached a critical level, independent of loading conditions. Assuming that damage evolution is a stress-temperature driven process, it is shown that a first order reaction equation describes reasonably well damage growth within the core zone. The rate constants are evaluated by invoking principles of kinetic theories of fracture and experimental results on crack initiation times. An activation energy for defect nucleation is found to be half the energy for thermal destruction and twice the enthalpy of activation for secondary chain motions below glass transition temperature. This result indicates that chain motions and chain scission are possible rate determining processes leading to crack initiation. This is consistent with the observation that dense crazing precedes crack initiation.     

Bruchverhalten der Stähle

Fracture Behaviour of Steels Crack formation during production or by monotonic, cyclic, chemical and thermal loading during service. Stable and unstable crack propagation. Clivage, dimple, intercristalline and fatigue fracture. Influence of temperature, loading conditions and microstructure on the transition from tough to brittle fracture behaviour.     

Effect of strength mis-match and dynamic loading on ductile fracture initiation

It has been well known that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using a two-parameter criterion based on equivalent plastic strain and stress triaxiality.The present study focuses on the effects of geometrical discontinuity, strength mis-match, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on the critical condition for ductile fracture initiation using a two-parameter criterion. Fracture initiation testing has been conducted under static and dynamic loading using circumferentially notched round-bar specimens. In order to evaluate the stress/strain state in the specimens, especially under dynamic loading, a thermal elastic-plastic dynamic finite element (FE) analysis considering the temperature rise due to plastic deformation has been carried out.The tensile tests on specimens with an undermatching interlayer showed that the relationship between the critical equivalent plastic strain to initiate ductile fracture and stress triaxiality was equivalent to that obtained on homogeneous specimens under static loading. Moreover, the two-parameter criterion for ductile fracture initiation is shown to be independent of the loading rate. It was demonstrated that the critical global strain to initiate ductile fracture in specimens with strength mis-match under various loading rate can be estimated based on the local criterion, that is two-parameter criterion obtained on homogeneous specimens under static tension, by mean of FE-analysis taken into account accurately both strength mis-match and dynamic loading effects on stress/strain behaviors.     

Analysis of interfacial cracks in a TBC/superalloy system under thermal loading

Thermal barrier coatings (TBCs) provide thermal insulation to high temperature superalloys. Residual stresses develop in TBCs during cool down from processing temperatures and subsequent thermal cyclic loading due to the thermal expansion mismatch between the different layers (substrate, bond coat, and TBC). These residual stresses can initiate microcracks at the bond coat/TBC interface and can lead to debonding at the bond coat/TBC interface. The highest residual stresses occur at the interfaces. The effect of voids or crack like flaws at the interface can be responsible for initiating debonding and accelerate the oxidation process. The effect of interfacial microcracks has been investigated using the fracture mechanics approach. In particular, J-integral and the energy release rate G, for both mode I and mode II using the virtual crack extension method were evaluated. Two types of specimens were studied. The specimens were cooled down from processing temperature of 1000°C to 0°C. The variation of the properties as a function of temperature were used for the analysis. It was found that the use of temperature dependent properties in contrast to constant properties provide significantly different values of J-integral and G. For the stepped-disc specimen with an edge crack, crack growth is only due to mode II, while for the cylinder specimen with an internal crack, crack growth is due to mixed-mode loading. An important implication of this result is that edge delaminations in a disk specimen may only grow due to mode II conditions under pure thermal loading. Shear fracture characteristics of interfacial crack thus become important in the failure of the TBC.     

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.     

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.     

热力载荷下多材料构件连接区的界面断裂分析

应用改进的虚裂纹闭合技术对热、力载荷作用下多材料构件连接区界面进行断裂分析。首先,通过对含橡胶夹层的复合材料层合板单腿弯曲（SLB）试件断裂分析,研究了在不同温度载荷作用下,橡胶夹层对试件能量释放率及其各型分量的影响。其次,对具有热流边界下,典型复合材料-橡胶-金属组成的多材料圆柱壳体连接裙结构进行了热力耦合断裂分析,结果表明裂纹总能量释放率随温度升高而增大。最后,针对该连接裙结构讨论了裂纹位置和橡胶层厚度对裂纹能量释放率的影响,指出适当增加橡胶层厚度可以降低裂纹能量释放率,但橡胶厚层度与界面韧性之间存在尺寸效应。     

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.     

Numerical study of spalling in an aluminum alloy 7020-T6

Planar impact experiment is frequently used to investigate dynamic fracture of materials, particularly the spall phenomenon. Spalling is caused by the superposition of rarefaction waves reflected from free surfaces and the spall zone is found in the interior of the target. Behavior of materials in this kind of experiment is strongly affected by the stress level, time of loading and temperature. The rate and temperature effects are closely related to the thermally activated micromechanical processes [1]. Thus, in a stressed body the creation of new fracture surfaces frequently occurs with the assistance of thermal activation. For a more detailed study, it is therefore necessary to take into account the physical aspects of spalling, including dynamic plasticity and temperature coupling. This paper reports the numerical analysis performed using a finite element FE code by implementation of a cumulative fracture criterion proposed in [2] where the apparent energy of activation for spalling depends on stress, temperature and load history. Initially, a series of calculations have been run for the purely elastic case to analyze the minimum critical impact velocity needed to obtain the spall stress and it has been determined to be a function of the critical time of loading. Such analysis is of great value in designing experiments that are relatively expensive. Next, a viscoplastic constitutive relation together with the cumulative criterion, and the equation of heat conduction have been implemented in a FE code. The set of relations takes into account strain hardening, strain rate sensitivity and temperature. This series of FE calculations have been performed in order to take into account, changes of temperature due to volume dilatation as well as conversion of plastic work into heat. In addition to spalling, the free surface velocity–time profiles have been calculated for a number of impact velocities. Specific variations of the free surface velocity indicates the creation of a new fracture surface inside the target plate. The two sets of FE calculations reported in this paper led to some discussion on the influence of physical parameters on spall mechanics.     

Dynamic ductile crack growth and transition to cleavage – a cell model approach

The fracture behavior of ferritic steel in the transition regime is controlled by the competition between ductile tearing and cleavage. Many test specimens that failed by catastrophic cleavage showed significant amounts of ductile tearing prior to cleavage fracture. The transition from ductile tearing to cleavage has been attributed to the increase in constraint and sampling volume associated with ductile crack growth. This work examines the role of dynamic ductile crack growth on the fracture mode transition by way of a cell model of the material. The cell model incorporates the effects of stress triaxiality and strain rate on material failure characteristics of hole growth and coalescence. Loading rate and microstructure effects on the stress fields that evolve with rapid (ductile) crack growth are systematically studied. The stress fields are employed to compute the Weibull stress which provides probability estimates for the susceptibility to cleavage fracture. A center-cracked panel subjected to remote tension is the model problem under study. The computational model uses an elastic-viscoplastic constitutive relation which incorporates enhanced strain rate hardening at high strain rates. Adiabatic heating due to plastic dissipation and the resulting thermal softening are also accounted for. Under dynamically high loading rate, our model shows the crack speed achieves its peak value soon after crack initiation and quickly falls off to slower speeds with further crack growth. Remarkably, the Weibull stress follows a similar pattern which suggests that the transition to the cleavage fracture is most likely to occur, if at all, at the peak speed of ductile crack growth. Key words: Dynamic fracture, ductile tearing, crack growth, transition regime, cleavage fracture, cell model, finite element.     

Influence of temperature on fracture initiation in PET and PA66 fibres under cyclic loading

The fatigue of single thermoplastic fibres has been well documented to occur in a reproducible manner when they are subjected to certain cyclic loading conditions. The fatigue fracture morphologies of these fibres are very distinctive and differ markedly from other types of failure. This type of behaviour, which is clearly seen with the unambiguous tests on single fibres, must reflect behaviour of fibres in more complex structures which are subjected to cyclic loading. Only limited numbers of reports have, however, shown similar fracture morphologies with fibres extracted from fibre bundles embedded in a matrix material such as rubber. Usually the fractured ends of fibres taken from structures are seen to be shorter than those obtained in single fibre tests and also they show more complex and confused crack growth. The present study reveals that the low thermal conductivity of the fibres, exacerbated when they are embedded in a rubber matrix, leads to very high temperature rises, which is not the case in single fibre tests and under these conditions, crack initiation occurs across the fibre section instead of being restricted to the near surface region. Tests on single fibres at temperatures up to and beyond the glass transition temperature have shown how the fracture morphologies become modified. The fatigue process has been seen to become generalised throughout the fibre and failure occurs due to the coalescence of several cracks, some of which are initiated in the core of the fibre. In all cases, the cracks can be seen to have been initiated by solid inclusions in the fibres.     

Mixed Mode Brittle and Ductile Fracture of a High Strength Rotor Steel at Room Temperature

The mixed mode fracture of a high strength rotor steel has been investigated at room temperature using single edge notched specimens. In mode I, and for limited amounts of shear loading, the steel exhibited cleavage fracture. For conditions near mode~II ductile fracture occurred. A transition from brittle to ductile fracture occurred for mixed mode loading. Finite element analysis provided estimates of the extent of near crack tip yielding and elastic-plastic stress intensity factors. Test results agreed with the maximum tensile stress (MTS) criterion for small scale yielding for limited amounts of shear loading. The load for mode II fracture was lower than predicted from the MTS criterion, but higher than predicted from plastic collapse predictions. Observed fracture angles where in broad agreement with the predicted fracture mechanisms. The load for the transition from brittle to ductile fracture was found to agree approximately with the predicted load. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation on Fracture Toughness at Dynamic Loading for Welded Joint Based on the Local Approach

The changes in mechanical properties and fracture toughness by dynamic loading were investigated with experiments. The parameter R, which can reflect the effect of the loading rate and the temperature rising during the high loading rate, could be employed to describe the constituent relation for the typical structure steel and its weld metal. The dynamic loading effect on the stress/strain fields and the temperature variation in the vicinity of the crack tip was analyzed by the finite element method, the dynamic fracture behavior was evaluated based on the local approach. It has been found that the Weibull stress is an effective fracture parameter, independent of the temperature and the loading rate.