A NOVEL METHOD FOR MEASURING AND CHARACTERIZING DYNAMIC FRACTURE-INITIATION TOUGHNESS OF ELASTIC-PLASTIC MATERIALS

The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a novel test method for studying dynamicfracture-initiation ahs been proposed based on the one-di- mensionaltest principle. The curve of average load v. s. displacement (P-δ)is smooth until unstable crack propagation, and the kinetic energywhich does not contribute to the crack growth can be removed fromtotal work done by external-force to the specimen.

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.

Determination of dynamic modeⅠfracture toughness of rock at ambient high temperatures using notched semi-circular bend method

To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock,dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature conditions based on the split Hopkinson pressure bar system.Experimental and analytical methods were applied to investigating the effect of temperature gradient on the stress waves.A high-speed camera was used to check the fracture characteristics of the specimens.The results demonstrate that the temperature gradient on the bars will not significantly distort the shape of the stress wave.The dynamic force balance is achieved even when the specimens are at a temperature of 400°C.The dynamic fracture toughness linearly develops with the increase of loading rate within the temperature range of 25-400°C,and high temperature has a strengthening effect on the dynamic fracture toughness.

Dynamic fracture toughness of high strength metals under impact loading:increase or decrease

An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.

DYNAMIC FRACTURE TOUGHNESS OF ULTRASTRENGTH STEELS FOR AIRCRAFT

The plots of load vs deflection(P-D)for two ultra-strength steels 30CrMnSiNi2A and 300M has been examined on a Charpy impact tester with digital memory and microcomputer. A criterion was found to availably represent the dynamic fracture toughness,K_(Ic)~D ,of the steels under impact loading.The K_(Ic)~D,.can be calculated with suggesting that the turning point of change in compliance on the P-D plot is the point of destabilized crack propagation.The K_(Ic)~D of steel 300M,quenched at 870℃ ,separately tempered at 300℃ and 450℃,was esti- mated to be 55—70 MN/m^(3/2)and 54—67 MN/m^(3/2)respectively.The K_(Ic)calculated with accuracy about 20% seems to be available to the engineering evaluation.

CRACK INITIATION POINT DETERMINATION AND DYNAMIC FRACTURE TOUGHNESS FOR CHARPY PRE-CRACKED SPECIMEN

The method to detect the crack initiation point of Charpy pre-cracked specimen under dy- namie loading was studied using strain gauge.The load-time curve and nominal strain-time curve at the crack tip for impact testing specinens may be shnultaneously measured by twin-channel oscilloscope with high speed sampling and diskette storing.Based on the dynam- ic finite element simulation of impact response of Charpy specimen,the measuring method of dynamic fracture toughness was analysed and some problems in previous dynamic fracture toughness measurement were discussed.

Fracture toughness and fracture behavior of SA508-Ⅲ steel at different temperatures

The fracture toughness of SA508-Ⅲ steel was studied in the temperature range from room temperature to 320℃ using the J-integral method. The fracture behavior of the steel was also investigated. It was found that the conditional fracture toughness （JQ） of the steel first decreased and then increased with increasing test temperature. The maximum and minimum values of do were 517.4 kJ/m^2 at 25℃ and 304.5 kJ/m^2 at 180℃, respectively. Dynamic strain aging （DSA） was also observed to occur when the temperature exceeded 260℃ with a certain strain rate. Both the dislocation density and the number of small dislocation cells effectively increased because of the occurrence of DSA; as a consequence, crack propagation was more strongly inhibited in the steel. Simultaneously, an increasing number of fine carbides precipitated under high stress at temperatures greater than 260℃. Thus, the deformation resistance of the steel was improved and the Jo was enhanced.

Effect of loading rates on crack propagating speed,fracture toughness and energy release rate using single-cleavage trapezoidal open specimen under impact loads

The former studies indicate that loading rates significantly affect dynamic behavior of brittle materials,for instance,the dynamic compressive and tensile strength increase with loading rates.However,there still are many unknown or partially unknown aspects.For example,whether loading rates have effect on crack dynamic propagating behavior(propagation toughness,velocity and arrest,etc).To further explore the effect of loading rates on crack dynamic responses,a large-size single-cleavage trapezoidal open(SCTO)specimen was proposed,and impacting tests using the SCTO specimen under drop plate impact were conducted.Crack propagation gauges(CPGs)were employed in measuring impact loads,crack propagation time and velocities.In order to verify the testing result,the corresponding numerical model was established using explicit dynamic software AUTODYN,and the simulation result is basically consistent with the experimental results.The ABAQUS software was used to calculate the dynamic SIFs.The universal function was calculated by fractal method.The experimental-numerical method was employed in determining initiation toughness and propagation toughness.The results indicate that crack propagating velocities,dynamic fracture toughness and energy release rates increase with loading rates;crack delayed initiation time decreases with loading rates.

Application of split Hopkinson tension bar technique to the study of dynamic fracture properties of materials

A novel approach is proposed in determining dy- namic fracture toughness （DFT） of high strength steel, using the split Hopkinson tension bar （SHTB） apparatus, com- bined with a hybrid experimental-numerical method. The center-cracked tension specimen is connected between the bars with a specially designed fixture device. The fracture initiation time is measured by the strain gage method, and dynamic stress intensity factors （DSIF） are obtained with the aid of 3D finite element analysis （FEA）. In this approach, the dimensions of the specimen are not restricted by the connec- tion strength or the stress-state equilibrium conditions, and hence plane strain state can be attained conveniently at the crack tip. Through comparison between the obtained results and those in open publication, it is concluded that the ex- perimental data are valid, and the method proposed here is reliable. The validity of the obtained DFT is checked with the ASTM criteria, and fracture surfaces are examined at the end of paper.

Study on dynamic fracture behavior of TA15ELI alloy under mode-Ⅱ loading by caustics method

The dynamic fracture behavior of TA 15ELI alloy with lath-like microstructure was studied by caustics method. Specimens with double-side pre-notch were tested under the plane-stress condition at mode-II loading with a drop hammer system. Caustics information recorded in films illustrated the histories of both crack length and stress intensity factor. The dynamic fracture toughness and crack growth velocity of TA15ELI with lath-like microstructure were determined to be 279 MPa.m1/2 and 32.6 m/s, respectively. SEM fractograph analysis showed a mixed feature of mainly plastic mode for TA 15ELI alloy in dynamic mode-II fracture. Shear localization was observed in the vicinity of the crack initiation area.

EFFECT OF DYNAMIC LOADING ON FRACTURE BEHAVIOR OF WELDED JOINTS OF STRUCTURAL STEEL

To investigate the effect of dynamic loading on fracture behavior of weldedjoints of structural steel Q235B and 16Mn in common use and compare the earthquake resistances ofthe two kinds of materials, dynamic tension and fracture toughness tests are carried out at roomtemperature. On the basis of the tests, the stress-strain fields near the crack tip of the compactspecimens are analyzed by three-dimensional finite element model. The test results and finiteelement analysis results show that, the fracture toughness of welds and base metal of 16Mn steelincreases with the increment of loading rate. Compared with 16Mn steel, Q235B steel is moresensitive to dynamic loading. The fracture toughness of welds of Q235B is comparatively low understatic loading and dynamic loading at room temperature. Compared with the static loading, thefracture toughness of Q235B parent metal under dynamic loading decreases by about four times.Therefore, it can be concluded that compared with 16Mn steel, the earthquake resistances of weld andparent metal of Q235B are rather poor.

Size Effect on the Fracture Strength and Toughness of Nano-cracked CoSb_(3):A Molecular Dynamics Study

Molecular dynamics simulations are implemented to study the mechanical fracture of CoSb_(3) with penetrated nanocracks under the mode-Ⅰ stress.The crack surface and crack front direction are(100)and[001],respectively.It is found that,at a fixed initial crack length,the fracture strength varies with the sample size,but the calculated value of fracture toughness KIC,by employing the classical formula of linear elastic fracture mechanics,maintains constant.When the crack is short in length relative to the sample,the variation of the fracture strength with the initial crack length is well fitted mathematically,and the extrapolation shows rationality even up to the macroscale.More general analyses reveal that,the fracture toughness increases monotonically with increasing the initial crack length until reaching the limit,and the increment is particularly noticeable below 36 nm.Furthermore,different atomic configurations at the crack tip are considered,which show an evident influence on the strength of nano-cracked CoSb_(3).

Improvement of Toughness of Ultrahigh Strength Steel Aermet 100

The influence of double aging on the microstructure and mechanical properties of ultrahigh strength steel Aermet 100 was analyzed. Under the double aging, there is no apparent decrease in the strength of steel. However, the impact fatigue life can be prolonged by 35.5% and dynamic fracture toughness be raised by 22.6% respectively, as compared with the normal aging. Based on the observation of microscopic structure, the physical mechanism of the prolongation of impact fatigue life and the enhancement of stability of the reverted austenite, AR, is analyzed further. The results show that this new technique is a breakthrough of combination optimization between strength and toughness for Aermet 100 steel. In the light of the current understanding on this subject, the volume fracture of soften and tough AR formed in process of heat preservation at higher temperature of double aging increases drastically. Moreover, during the treatment of lower temperature of double aging, the carbon separating from the martensitic ferrite will diffuse into AR, resulting that the martensitic brittleness decreases and the stability of AR increases.

Effect of SrCO_3 addition on the dynamic compressive strength of ZTA

Ceramic parts usually experience dynamic load in armor applications. Therefore, studying the dynamic behaviors of ceramics is important. Limited data are available on the dynamic behaviors of ceramics; thus, it is helpful to predict the dynamic strength of ceramics on the basis of their mechanical properties. In this paper, the addition of SrCO3 into zirconia-toughened alumina（ZTA） was demonstrated to improve the fracture toughness of ZTA due to the formation of the SrAl12O19（SA6） phase. The porosity of ZTA was found to be increased by the addition of SrCO3. These newly formed pores served as the nucleation sites of cracks under dynamic load; these cracks eventually coalesced to form damaged zones in the samples. Although the KIC values of the samples were improved, the dynamic strength was not enhanced because of the increase in porosity; in fact, the dynamic strength of ZTA ceramics decreased with the addition of SrCO3.

Numerical study of the semi-circular bend dynamic fracture toughness test using discrete element models

The semi-circular bend(SCB) dynamic fracture toughness test is simulated using discrete element models. The influence of the frictional boundary condition, constitutive law and specimen thickness on the test measurements is investigated. It is found that friction between loading plates and the rock specimen affects the test results. Therefore, friction must be carefully considered to obtain accurate measurements. The simulation results also show that in contrast to the 2D model in which a rate-dependent cohesive law must be introduced, 3D models with a rate-independent law can produce good results. Furthermore, the study suggests that test measurements are seriously affected by specimen thickness; thus, full 3D modeling is required for simulation of the SCB test.

Dynamic Fracture Toughness of Armour Grade Quenched and Tempered Steel Joints Fabricated Using Low Hydrogen Ferritic Fillers

The armour grade quenched and tempered steel joints fabricated using low hydrogen ferritic steel （LHF） filler exhibited superior joint efficiency owing to preferential ferrite microstructure in the welds and also they offered required resistance to HIC. However, the combat vehicles used in military operations will be required to operate under a wide range of road conditions ranging from first class to cross country. Structural components in combat vehicles are subjected to dynamic loading with high strain rates during operation. Stress loadings within the vehicle hull of these vehicles are expected to fluctuate considerably and structural cracking especially in welds during the service life of these vehicles can lead to catastrophic failures. Under these conditions, fracture behaviour of high strain rate sensitive structural steels can be better understood by dynamic fracture toughness （K1d）. Hence, an attempt was made to study dynamic fracture toughness of the armour grade quenched and tempered steel and their welds fabricated using LHF consumables. The experimental results indicate that the K1d values of the joints fabricated by shielded metal arc welding （SMAW） are higher than those of the joints fabricated using flux cored arc welding （FCAW） process.

Research of dynamic mechanical performance of cement rock

As Daqing Oilfield is developing oil layer with a big potential, the requirement for the quality of well cementation is higher than ever before. Cement rock is a brittle material containing a great number of microcracks and defects. In order to reduce the damage to cement ring and improve sealed cementing property at the interface, it is necessary to conduct research on the modification of the cement rock available. According to the principle of super mixed composite materials, various fillers are added to the ingredients of cement rock.Dynamic fracture toughness of cement rock will be changed under the influence of filler. In order to study the damage mechanism of the cement circle during perforation and carry out comprehensive experiments on preventing and resisting connection, a kind of comprehensive experiment equipment used to simulate perforation and multi-functional equipment for testing the dynamic properties of the material are designed. Experimental study of the dynamical mechanical performance of original and some improved cement rock and experiment used to simulate the well cementation and perforation are carried out. Standard for dynamical mechanical performance of the cement rock with fine impact resistance and mechanical properties of some improved cement rock are also given.

冲击荷载作用下纤维混凝土动态劈裂试验与分析

为研究不同纤维材料对混凝土动态劈裂力学性能的影响以及作用效果的差异,采用直径为100 mm的分离式霍普金森压杆试验装置和V2512超高速数字摄像机,分别对素混凝土、棕榈纤维混凝土以及钢纤维混凝土试件进行了巴西圆盘动态劈裂试验研究。通过DIC软件技术分析不同纤维混凝土试件的破裂过程和裂纹扩展规律,获得不同纤维混凝土试件的动态抗拉性能与动态断裂韧性等。试验与分析结果表明:在相同冲击荷载作用下,纤维的掺入能够提高混凝土的抗冲击韧性,减少试件加载端部因应力集中而产生的崩坏现象,延缓试件的破坏时间并有效抑制裂纹的扩展,其中钢纤维对混凝土的阻裂作用时间长于棕榈纤维。在相同荷载作用时间情况下,棕榈纤维混凝土和钢纤维混凝土的峰值应变都低于素混凝土。钢纤维的掺入能够显著阻滞混凝土试件内部的破裂过程,损伤变量变化相对缓慢,试件开裂后在相同裂纹宽度下,钢纤维混凝土具有更大的残余应力。

基于分子动力学研究纳米孔洞对多晶FeO/Fe中FeO断裂的影响

孔洞缺陷是影响表面氧化皮断裂失效的重要因素.本文结合分子动力学软件Lammps和Atomsk软件建立了半径相同位置不同(模型1)和孔洞位置相同半径不同(模型2)两种模型,为了探究两者对FeO/Fe多晶模型拉伸失效的影响,文章从抗拉强度、断裂韧性、中心对称参数(CSP)三方面进行了分析,模拟结果表明:1.当孔洞位于晶界和三叉点时,模型1和2的抗拉强度随着孔洞半径的增大而减小,断裂韧性在抗拉强度降低的同时增强.2.孔洞位于晶内时,模型2的抗拉强度呈现“先下降-后上升-再下降”的变化趋势,断裂韧性整体降低;模型1既增强了模型的抗拉强度,又降低了断裂韧性.3.裂纹扩展受孔洞半径影响较大,半径达到1 nm以上时,模型2的裂纹扩展速度加快,模型1的裂纹扩展速度减慢.本研究为进一步认识氧化皮的断裂过程提供了帮助.

Incubation time approach in rock fracture dynamics

The paper is summarizing latest results connected with application of the incubation time approach to problems of dynamic fracture of rock materials. Incubation time based fracture criteria for intact media and media with cracks are discussed. Available experimental data on high rate fracture of different rock materials and incubation time based fracture criteria are used in order to evaluate critical parameters of causing fracture in these materials. Previously discovered possibility to optimize (minimize) energy input for fracture is discussed in connection to industrial rock fracture processes. It is shown that optimal value of momentum associated with critical load in order to initialize fracture in rock media does strongly depend on the incubation time and the impact duration. Existence of optimal load shapes minimizing momentum for a single fracturing impact or a sequence of periodic fracturing impacts is demonstrated.