Dynamic fracture toughness of 4340 VAR steel under conditions of plane strain

Plate impact experiments are conducted to study the dynamic fracture processes in 4340 VAR steel which occur on submicrosecond timescales. These experiments involve the plane strain loading of a planar crack by a plane tensile pulse with a duration of approximately 1 μs. The loading is achieved by impacting a precracked, disk-shaped specimen by a thin flyer plate. Motion of the rear surface of the specimen, caused by waves diffracted from the stationary crack and by waves emitted from the running crack, is monitored at four points ahead of the crack tip using a laser interferometric system. The measured rear surface motion is compared with the calculated motion using the finite element method to gain understanding of the dynamic fields that occur near the crack tip during crack initiation and propagation. For low temperature experiments, the measured rear surface particle velocity fields are in good agreement with the computed profiles obtained for a constant velocity crack propagation model. For the room temperature experiments, the experimental free surface particle velocityvs time profiles show a sharp spike, with a duration of less than 100 ns at the moment of crack initiation. The spike, which is not predicted by the inverse square root singular stress fields of linear elastic fracture mechanics, is understood to be related to the onset of crack growth. Critical values of the fracture toughness are estimated from the crack initiation times determined both from the velocity time profiles and the elastodynamic modeling of crack advance. The toughness values obtained increase with increasing impact velocity and are as large as 170 MPa√m at the highest impact velocity. Such relatively high values appear to be consistent with the ductile mode of crack initiation observed at all impact velocities used in the present study. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

冲击荷载下含层理介质动态裂纹扩展特性研究

利用数字激光动态焦散线实验系统（DLDC）,对含不同层理角度（30°,45°和60°）的3组有机玻璃板（Polymethyl methacrylate, PMMA）试件进行三点弯落锤冲击试验,借助高速相机记录了试件的断裂过程和裂纹尖端的动态焦散斑形状变化过程,得到了其Ⅰ、Ⅱ型动态应力强度因子的变化特征,并分析了其裂纹尖端位移及速度曲线。结合离散格子弹簧模型（DLSM）,对比分析了试件的断裂形态,得到了裂纹尖端的应力场和运动场的变化规律,研究了应力波在层理处的透射和反射特征,最后利用DLSM分析了层理参数对介质断裂特性的影响。结果表明,试件的断裂特征,裂纹的起裂时间等都随层理角度的变化而不同,裂纹在不同角度层理内扩展速度不同;试件的断裂表现出拉剪复合特征;裂纹在抵达层理前速度在某一数值上下波动;层理的弹性模量和厚度都会对试件的动态断裂特性产生影响。      

充填体室内单轴压缩试验声发射指标特性分析

随着浅部矿产资源开采殆尽,采矿活动逐渐转向深部.充填法开采作为现阶段维持矿山稳定的最有效方法,在国内外各大矿山得到了广泛应用.随着充填法的不断完善,充填体的力学行为特点引起了广泛关注.以充填体力学行为为研究对象,通过室内岩石力学试验并结合声发射监测手段,对不同倾角的预制裂隙影响下,强度在35 MPa以内的充填体试块变形...     

X70管线钢DWTT试样的分层裂纹及其断口评价

通过对管线钢不同温度的落锤撕裂试验和微观断口分析,研究了DWTT试样断口的分层裂纹及其对断裂的影响.结果表明,分层裂纹是受力变形时管线钢内部的薄弱界面受到三维应力作用的结果.分层裂纹出现于主裂纹起裂或加速之前,裂纹稳定扩展或减速时不会产生新的分层裂纹.分层裂纹的数量、张开程度和分层裂纹间距与主裂纹起裂或加速时的应力状态有关,而分层裂纹的长度与裂纹扩展时裂尖的应力状态有关.分层裂纹表面为解理形貌,解理面较大.韧脆转变温度以下的脆性断裂断口或韧脆转变温度附近混合型断口的脆性断裂区,不出现分层裂纹,仅在韧性断口或断口的韧性区出现宏观分层裂纹.研究表明,产生分层裂纹处的断口,分层裂纹无论是三角形分布或是其它形状分布,该区域均应评价为韧性.     

The Role of Hydrogen on the Local Fracture Toughness Properties of 7XXX Aluminum Alloys

High-resolution synchrotron X-ray microtomography has been successfully used to evaluate the local crack driving force at arbitrary crack tip locations as a form of CTOD. This is to our knowledge the first experimental evidence in supporting a correlation between the local fracture toughness associated with the corresponding hydrogen-assisted fracture mode including quasi-cleavage, intergranular, and dimple. Our results have revealed that very small CTOD, of about 1.26 μm, is observed when the crack tip is located in the quasi-cleavage fracture. Compared to quasi-cleavage fracture, the CTOD values increase by a factor of 5 when the crack tip is located in intergranular fracture mode and even greater increase in CTOD (of about 18 times) is observed when the crack tip is located in dimple fracture mode. We also observed that the crack propagation process under the influence of hydrogen deviates greatly from that of standard behavior, where stable crack growth is accompanied by a change in crack tip singularity from the HRR to the RDS. It was concluded that the presence of high concentration of hydrogen ahead of the crack tip increases the slip localization, and thereby reduces crack tip blunting. Hence crack continues to grow before the crack tip becomes fully blunt.     

板条马氏体钢变形与断裂过程的原位观察

 在备有拉伸装置的扫描电镜上,原位观察了低碳板条马氏体钢的变形和断裂过程。结果表明,板条马氏体的变形是以滑移方式进行的,位错沿滑移面的滑移受阻,在试样表面留下呈波纹状的变形带。在应力峰值前后,主裂纹开始起裂;在主裂纹扩展过程中,在主裂纹前面的薄弱区域如夹杂等会先起裂形成小裂纹或空洞,随应力加大相邻的微孔聚合、连接长大成新裂纹;在断裂过程中,裂纹在板条束界发生转折。尽管原奥氏体晶粒尺寸小的试样起裂载荷大,不同晶粒尺寸马氏体组织的变形和断裂过程没有本质差别。     

锌锅用热轧钢板解理断裂行为分析简

 通过断裂试样断口的宏观和显微分析、显微组织表征、拉伸和冲击试验以及解理断裂应力条件,讨论分析了锌锅用低强度级别钢板弯曲成形断裂的微观解理断裂行为。结果表明,钢板发生解理断裂的微观机制与冲击试样断裂相同,即晶粒尺寸控制的穿过晶界的裂纹扩展是解理断裂的临界事件。粗大的铁素体晶粒的面积分数过高显著降低了裂纹扩展阶段所需的局部解理断裂应力σf。断口宏观分析判断在钢板边部应存在导致应力集中的初始裂纹源,这极大降低了启动解理断裂的断裂应力并同时提高裂纹源前端的正应力σyy,扩大了解理断裂活跃区至初始裂纹前端,从而不可避免地发生脆性解理断裂。     

Crack path in single crystals

A two-dimisnsional analysis of crack propagation in anisotropic single crystal has revealed that the cleavage path depends on the shape of the γ-plot. Consequently, for a given notch plane the fracture path has been predicted to depend on the notch front orientation. The results of this study suggest that the fracture path depends not only on the level of resistance to fracture but also on the change of fracture energy with orientation. Furthermore, when a crack kinks into a minimum energy plane, it does not necessarily exhibit a lower fracture toughness than the case where crack follows a higher energy plane.     

SiC纳米及晶须增强Si_3N_4基复相陶瓷断裂行为的研究

用扫描电镜、透射电镜及努氏压痕法研究了添加SiC晶须、纳米颗粒及晶须和纳米颗粒的三种Si3N4基复相陶瓷在外力作用下的断裂行为。这三种材料断裂的主要方式是沿晶断裂 ,偶尔可见穿晶断裂。在裂纹发展的路径上当裂纹尖端遇到了晶须、集聚的纳米颗粒及类晶须时 ,会产生扭转、偏转、断裂、拔出和终止 ,从而使裂纹能量消耗 ,抑制和阻碍了裂纹的扩展和传播 ,起到了增韧补强的作用。     

A study of the dynamics of high temperature brittle intergranular fracture by acoustic emission

This paper presents a study of high temperature brittle intergranular fracture (HTBIGF) in which acoustic emission (AE) data and fractography were used together to make deductions about the dynamics of fracture. The AE event rate was proportional to the rate of overall crack growth, and the average crack increment per AE event correlated well with spacing of striations observed on the fracture surface. The AE from HTBIGF was also compared with ambient temperature brittle intergranular fracture and high temperature ductile fracture. The HTBIGF generated about 50% of the emission of fully brittle fracture, and at least an order of magnitude more than that from ductile fracture. The observation of discrete bursts of AE of this magnitude, coupled with the striations on the fracture surface, indicates that HTBIGF takes place in discrete brittle steps. This is consistent with the segregation of sulphur as an embrittling species to the region of the crack tip, promoting stepwise decohesion, rather than a continuous crack growth process.     

劈裂荷载下的岩石声发射及微观破裂特性

通过开展花岗岩和大理岩巴西圆盘声发射试验，结合扫描电镜进行破裂面微观形貌分析，探讨了劈裂荷载下岩石声发射特性与微观破裂机制的关系。结果表明:基于RA（上升时间与幅值的比值）和AF（平均频率）的变化趋势，不同裂纹模式（拉伸裂纹、剪切裂纹以及复合裂纹）的分布和破坏强度受岩石结构影响，但岩石裂纹演化过程不受其影响。相应地，两种岩样破裂信号均以400～499 kHz为主，100～199 kHz的信号次之，但不同破裂阶段的峰值频率变化趋势显著不同。在微观形貌上，花岗岩劈裂面的微观形貌以层叠状、台阶状及平坦状为主；而大理岩以光滑多面体状为主。此外，结合频率?尺度缩放关系可推测，400～499 kHz的信号应主要来自钾长石、大理岩矿物颗粒内部的破裂；100～199 kHz的信号应主要来自石英矿物颗粒内部不连续分离以及压密阶段矿物颗粒之间的滑移。      

Hydrogen-Induced subcritical crack growth of a 12 Cr-1 Mo ferritic stainless steel

Subcritical crack growth and tensile ductility measurements have been made on a 12 Cr-1 Mo ferritic stainless steel at cathodic potentials in a 1 N H₂SO₄ solution at 25 °C. The tensile ductility was found to be a minimum at −600 mV (SCE) and both the subcritical crack growth behavior and tensile ductility were similar for material in the tempered (760 °C/2.5 h) or tempered-plus-segregated (540 °C/240 h) condition. A rising-load crack growth threshold of 20 MPa √m was measured and a rising-load fracture toughness of 110 MPa √m was determined from extrapolation of the stage III crack growth curve. A K-independent stage II was observed and a stage II crack growth rate of about 1 × 10⁻⁵ mm/s was measured. The fracture mode was a mixture of intergranular and quasi-cleavage for both heat treatments and for subcritical and tensile fracture tests. Impact fracture properties were independent of heat treatment and grain boundary composition with the fracture mode predominantly transgranular. The difference in the fracture mode for hydrogen-induced crack growth and dynamic crack growth was explained by a difference in the relationship between their stress profiles and the maximum grain boundary segregation distribution.     

A kinetic criterion for quasi-brittle fracture

A new criterion is proposed for quasi-brittle fracture based on the recognition that quasi-static fracture toughness measurements are merely a limiting case of dynamic fracture. It is found that a unique relationship exists between the energy stored ahead of a loaded crack or blunt notch and the velocity of a crack injected into the material by the failure of a microstructural element lying in the zone of elevated stress. The criterion establishes a minimum velocity required for the continued propagation of the injected crack as a function of the crack tip opening displacement at the instant of crack nucleation. Comparison of the predictions of the model with existing data on various steels and an aluminum alloy show excellent correlation. The implications of the model for phenomena such as the ductile-to-brittle transition, microstructural effects on the resistance to unstable crack propagation and grain boundary embrittlement/strengthening by solute segregation are also explored.     

TC4-DT钛合金高周疲劳行为研究

对TC4-DT钛合金的高周疲劳性能及断口形貌进行了研究。结果表明：TC4-DT钛合金的S-Ⅳ曲线（应力比R=-1）不出现呈水平线的疲劳极限,10^7次不被破坏的条件疲劳极限为550MPa,置信度为95％;疲劳裂纹源均出现在试样的表面,疲劳裂纹扩展区较大说明材料具有较高的断裂韧度;疲劳裂纹扩展区由许多解理小刻面组成,解理面上可见疲劳条带及二次裂纹,以解理断裂为主;断裂区断口表面由许多互相连接的凹坑所组成,主要表现为韧窝断裂。     

A geometric model for fatigue crack closure induced by fracture surface roughness

Mechanisms for fatigue crack closure under plane strain conditions have recently been identified at very low (near-threshold) stress intensities in terms of effects of excess corrosion deposits or fracture surface roughness in promoting premature closure of the crack. In the present paper, a geometric model is presented for crack closure induced by fracture surface roughness. This model specifically addresses the contribution from both Mode I and Mode II crack tip displacements in addition to considering the nature of the fracture surface morphology. The implications of this model are briefly discussed in light of the roles of grain size, yield strength, microstructure, and crack size in influencing near-threshold fatigue behavior in engineering alloys.     

Effect of strontium modification on near- threshold fatigue crack growth in an Al-Si-Cu die cast alloy

The effects of strontium modification on microstructure and fatigue properties in a die cast com-mercial aluminum-silicon alloy are demonstrated. Strontium additions of 0.010 and 0.018 wt pct drastically change the morphology of the eutectic silicon. The influence of these microstructural changes on fatigue properties is evaluated through fatigue crack growth testing. Examination of the fracture surfaces and the crack path establish distinct fatigue fracture modes for the modified and unmodified eutectic structures. Changes in fracture mode and crack path are correlated to the mi-crostructure changes. A higher energy fracture mode and increased crack path tortuosity explain the observed improvement in fatigue properties for the modified alloys. Strontium modified alloys exhibit a 10 to 20 pct higher fatigue crack growth threshold compared to an unmodified alloy for testing at a load ratio of 0.5. No difference was observed for testing at a load ratio of 0.1. Formerly Research Project Engineer, Briggs & Stratton Corporation, Milwaukee, WI 53222     

Crack Growth Resistance of Hybrid Fiber-Reinforced Cement Matrix Composites

The effect of hybrid fiber reinforcement on fracture energy and crack propagation in cement matrix composites is examined. The crack in cement matrix composites is allowed to fracture under mode-I loading with three-point bending beam specimens. The influence of fiber types and their combination is quantified by using the toughness index and fracture energy. A proper hybrid combination of steel fibers and polyvinyl alcohol microfibers enhances the resistance to both the nucleation and growth of the crack. The micromechanical model of hybrid composites by using a fiber bridging law is emphasized, and the numerical model prediction closely matches the behavior obtained from the experiment. The influencing role of the material parameters in the fracture tests (e.g., the fracture toughness index and fracture energy) becomes more apparent than ones used in some conventional strength-based or fiber pullout tests, and these fracture parameters could screen the effect of fiber/microfiber reinforcement in enhancing the crack growth resistance of cementitious composites. This study demonstrates that fundamental fracture tests are effective to characterize and develop high-performance hybrid fiber–reinforced cement matrix composites.     

Further study on the mechanism of the ductile-to-brittle fracture transition in C-Mn base and weld steel

In the present study, the crack opening displacement (COD) tests of specimens of C-Mn base and weld steel were carried out in the ductile-brittle transition temperature region. The majority of the specimens were fractured and others were unloaded prior to fracture after ductile fracture initiated and extended. The cavities and cleavage microcracks located in the vicinities of tips of fibrous cracks of the unloaded specimens were observed in detail. The finite element method (FEM) calculations of the stress and strain distribution ahead of the tip of an extending fibrous crack were completed. The mechanism of the ductile-to-brittle fracture transition was further investigated. It was revealed that in the ductile-brittle transition temperature region, the ductile fracture process was independent of temperature. The ductile-to-brittle fracture transition was triggered by initiating a catastrophic extension of a cleavage crack ahead of the fibrous crack tip, which occurred in a condition satisfying a combined criterion composed of three items, i.e., ε _p ≥ ε _pc for initiating a crack nucleus; σ _m √σ ≥ T _c for preventing the crack nucleus from blunting; and σ _yy ≥ σ _f for propagating the crack nucleus. For a specimen in which a fibrous crack occurred and propagated, the critical event for initiating a brittle cleavage fracture was the propagation of a ferrite grain-sized crack into neighboring grains. With extension of a fibrous crack, the behavior of the ductile-to-brittle fracture transition could be analyzed by the effect of the size of an “active zone” on the initiation of the brittle cleavage fracture.     

Fatigue and fracture toughness of a Nb-Ti-Cr-Al-X single-phase alloy at ambient temperature

The fatigue and fracture resistance of a commercially made, single-phase Nb-base alloy with 35 at. pct Ti, 5 at. pct Cr, 6 at. pct Al, and several elements to increase solid solution strengthening have been investigated. The threshold for fatigue crack growth was determined to be ≈7 MPa√m and fracture toughness ≈35 MPa√m. Crack growth was intermittent and sporadic; the fracture path was tortuous, crystallographic, and appeared to favor the {100} and {112} planes. Fatigue crack closure was measured directly at the crack tip. The fatigue and fracture properties of the commercial alloy are compared against those of Nb-Cr-Ti and Nb-Cr-Ti-Al alloys. The comparison indicated that Ti addition is beneficial for, but Al addition is detrimental to, both fracture toughness and fatigue crack resistance.