TiAl基合金双态组织平板拉伸连续卸载试验的研究

通过多次拉伸卸载试验对TiAl基合金在经历多次拉伸卸载以后宏观性能和微裂纹面密度的变化,以及前一阶段的损伤对随后阶段损伤产生的影响进行了详细的研究.研究表明:在载荷控制下的试验中,随着卸载应力的增加,裂纹面密度并没有增大,即用微裂纹表征的损伤程度并没有增加的趋势;整个多次拉伸卸载过程并不影响材料的弹性模量E;当拉伸到某一个应力下,宏观表现为断裂应力以,断裂应变‘f和单位面积断裂功W'开始减小.材料在载荷控制的拉伸中产生损伤的程度并不能用弹性模量E和裂纹面密度.来衡量.     

全层TiAl基合金缺口试样原位拉伸卸载试验的研究

通过对TiAl基合金不同类型的缺口试样进行原位拉伸卸载实验和相应的断裂表面观察,研究了TiAl基合金全层组织的断裂机理。研究发现:对于缺口试样,裂纹起裂于缺口根部,其断裂过程主要是主裂纹首先起裂、扩展并最后断裂。在整个断裂过程中断裂是穿层断裂和沿层断裂的混合体,裂纹路径较曲折。在拉伸过程中,试样产生微裂纹导致材料发生损伤,随后卸载再加载时,与先前相比,裂纹更易扩展。预损伤加快了裂纹的产生和扩展,使损伤进一步加重,促使材料抵抗裂纹产生、扩展的能力下降。     

γ-TiAl基合金压缩损伤与断裂行为的研究

通过对γ-TiAl基合金压缩断裂及压缩卸载试验和试样断口与表面的扫描电镜(SEM)观察,分析压缩应力对裂纹产生、扩展及裂纹形态的影响,进而对该材料的压缩损伤与断裂行为进行较为深入的研究。压缩试验是室温下在Instron 1341试验机上进行的。结果表明,损伤起始于材料的塑性区载荷下降阶段,材料在断裂前发生很大的塑性变形,其压缩时有较大的塑性缓冲;随着压缩卸载应力的增大,观察到的试样表面裂纹依次增多或扩展增长,材料损伤的程度与压缩应力成正比。在压缩试样断口的中部发现存在的一个纵向韧带,当外加载荷增加,两个由压缩接触端面起裂的倾斜剪切裂纹扩展到试样中部,然后通过剪切穿过纵向韧带而连接,并诱发试样的完全脆性断裂。两个端面的切应力是裂纹形成的主要控制因素。该材料的压缩性能比拉伸性能更佳的主要原因是由于压缩时材料的损伤起始于塑性阶段,产生沿45°方向剪应力最大方向的剪切断裂和沿着压缩轴方向的准解理断裂的混合形式,而普通拉伸时材料损伤起始于弹性阶段,发生完全脆性解理断裂,在低应力下试样就会断裂。     

基于剪切修正GTN损伤模型的辊冲工艺数值模拟

在GTN模型基础上,考虑到微孔洞剪切变形对材料劣化的影响,建立适用于压应力状态的剪切修正模型.通过用户子程序接口VUMAT将与损伤耦合的弹塑性本构模型嵌入具有ALE法的有限元软件Abaqus/Explicit中.利用模拟拉伸与纯剪切试验拟合载荷-位移曲线以确定模型参数.将修正模型应用到辊冲工艺有限元模拟中预测断面质量,并进行试验验证.结果表明:前刃口为小间隙冲裁,塌角较小,光亮带较大但带有一定的倾角;后刃口为大间隙冲裁,塌角与断裂带较大,光亮带较小;裂纹会同时在前刃口凸模与凹模侧面萌发,而对于后刃口,会首先在凹模侧面产生.      

真空热压烧结制备SiCp/2024Al复合材料动态力学性能北大核心

通过真空热压烧结制备30%(体积分数)SiCp/2024Al复合材料,采用分离式霍普金森压杆(SHPB)对其进行动态压缩实验,得到应变速率为1600~3800 s-1的动态应力-应变曲线。结果表明:在一定的高应变率范围内承受动态载荷时,30%SiCp/2024Al复合材料在不同应变率下,应力-应变曲线趋势变化不大,基本表现为应变率不敏感材料。试样在动态冲击下均未出现宏观剪切破坏,采用扫描电镜(SEM)对压缩试样微观组织进行表征,高应变率试样内部出现孔洞和微裂纹等损伤,试样边界出现增强体颗粒脱粘现象,材料表现出良好的塑韧性。还讨论了动态载荷压缩变形机制。     

基于应力三轴度的TA18钛合金损伤失效研究

对TA18钛合金材料设计不同缺口尺寸的拉伸试样,进行不同应力状态下的室温拉伸试验及断口形貌观察,通过试验和数值计算结合的方法研究TA18钛合金的韧性断裂特性,分析了应力状态对微观断裂机制的影响规律.利用Bridgman正向计算法修正颈缩失稳后的应力数据,建立了TA18钛合金的Johnson-Cook(J-C)本构模型,计算了拉伸试样的平均应力三轴度和断裂应变,回归确定了TA18钛合金损伤失效模型.结果 表明:不同应力状态下的TA18钛合金断裂应变各不相同,断裂应变随着应力三轴度的增大而减小,断口韧窝尺寸与应力三轴度呈正相关关系,所建立的损伤失效模型能够描述该材料的断裂特性.     

断裂仿真中的网格尺寸效应与断裂曲线修正

有限元仿真模拟方法可实现对结构材料从变形到破坏全过程的有效预测，而仿真模型建立的精度对于预测的准确性至关重要。应用有限元对标优化方法建立DP780材料的光滑拉伸、缺口拉伸及剪切拉伸试样的准静态仿真模型。划分不同网格尺寸，逆推得到材料的大变形本构曲线，以及各试样的应力三轴度、罗德角参数，系统研究了网格尺寸对材料拉伸本构曲线及韧性断裂参数逆推结果的影响。基于Lou-Huh(2012)断裂准则模型建立了不同网格尺寸下的断裂曲线。采用GISSMO理论建立断裂应变的网格尺寸效应修正函数，并对平面应力条件下的断裂曲线进行修正。研究结果表明：随着网格的细化，塑性应变-真应力曲线的对标结果向大应变方向延伸并趋于收敛，相同试样所标定的应力三轴度值与临界断裂应变值增大。随着网格尺寸的增大，临界断裂应变呈现指数形式递减，网格尺寸大于2 mm后趋于平稳。基于网格尺寸效应修正的韧性断裂模型可有效降低材料断裂仿真分析对网格尺寸的敏感性。     

工艺参数对铝合金微槽道挤压成形的影响

将热处理后的1050铝合金拉伸试样进行等温拉伸试验,获得真实应力-应变曲线,使用Deform-3D软件模拟1050铝合金微槽道的挤压成形过程。分析挤压速度、摩擦因数以及槽道宽高比这些关键工艺参数对材料等效应力-应变曲线分布的影响。结果表明,随着挤压速度和摩擦因数的增大,材料等效应力和应变均变大,变形不均匀性增大;随着槽道宽高比的变大,材料的等效应力和应变整体呈现上升趋势,微槽道板筋处出现了明显的应力集中现象,变形不均匀。根据模拟结果,选取最优参数进行1050铝合金微槽道挤压成形模拟试验,结果显示材料的流动均匀性更好,成形过程更加稳定,所得零件表面精度显著提高。     

微观组织对高速车轮钢解理断裂应力的影响

利用不同奥氏体化温度和冷却速率对碳质量分数为0.54%高速车轮钢进行热处理,得到具有不同晶粒尺寸和珠光体片间距微观组织的试样.在-120~20℃温度下对具有不同微观组织的拉伸试样和三点弯曲（3PB）缺口试样进行测试;采用二维平面应变有限元计算三点弯曲缺口试样缺口前的应力分布;利用扫描电镜对3PB试样断口进行观察并测量解理起裂源的位置;测定不同微观组织车轮钢试样的解理断裂应力.在扩展控制断裂机制下,微观组织对车轮钢的解理断裂应力具有明显影响,晶粒尺寸和珠光体片间距越小解理断裂应力越高.细化晶粒使未扩展微裂纹的特征长度减小,细化珠光体片间距有助于提高珠光体的有效表面能,从而使得解理断裂应力提高.      

缺口拉伸试样断裂应力与温度的相关性

无缺口拉伸试样的断裂应力表现出多种温度相关性。本文揭示了缺口试样拉伸试验中也表现出各种温度相关性。为此在缺口根部半径7．0mm、2．0mm和0．075mm的试样上以-140℃～＋82℃的温度范围进行了拉伸试验。测得的数据显示出7个温度范围，代表断裂应力（和面缩率）的不同温度关系。这些温度特性由7个不同的拟合曲线描述。这些拟合曲线的交点引出过渡温度解理断裂-混合断裂及混合断裂-纤维断裂。就缺口根部半径7．0mm和2．0mm的试样而言，这些试验得出了与无缺口拉伸试样相同的温度特性。而缺口根部半径0．075mm的试样在温度特性方面则表现出一些不同，这在文中将得到详细论述。     

Influence of strain rate on the stress-strain curve in the range of Lüders strain

The influence of strain rate on the stress-strain curve of materials showing yield point phenomenon are determined in the range of small strains and high strain rates by means of the indirect impact tension test. With increasing deformation velocity, the Lüders strain is found to increase considerably. The stress reduction after reaching the upper yield point, is not abrupt and takes place gradually with increasing strain. These phenomena are investigated on Armco iron, on the tempered steel 42 CrV 6, on pure tantalum and tungsten alloys. A simple mechanical model is introduced which shows that the Lüders front velocity increases with strain rate approaching asymptotically the value of the plastic wave velocity as an upper limit. With this restriction, the local strain within the Lüders band must increase by increasing the rate of elongation of the tensile specimen. The Lüders strain could be estimated quantitatively as a function of strain rate. The model describes also the stress-strain function after reaching the upper yield point considering the limited stiffness of the test arrangement.     

含齿形裂隙类岩石材料单轴压缩试验研究

为研究含齿形裂隙岩石在单轴压缩下的破坏特征及强度特性,制作了含不同裂隙倾角和起伏角的齿形裂隙类岩石材料试件,并采用岩石力学伺服试验机进行单轴压缩试验。试验结果表明：（1）试件主要产生拉伸、剪切和拉剪复合裂纹,且根据裂纹的扩展路径可划分为A型（拉伸破坏）、B型（剪切破坏）、C型（复合破坏）3种破坏模式,裂隙倾角对试件最终破坏模式影响显著;（2）当裂隙倾角较小时,试件应力—应变曲线为多峰曲线,随着裂隙倾角的增大,曲线呈单峰形式,表现为延性减弱,脆性增强,而裂隙倾角相同但起伏角不同的试件应力—应变曲线大致相同;（3）当裂隙起伏角相同时,试件当量峰值强度随裂隙倾角的增大呈先减小后增大的规律,且裂隙起伏角对试件当量峰值强度的影响小于裂隙倾角。     

Stress concentration at a notch tip in unidirectional metal matrix composites

A method to calculate static stress concentration factors in the fibers at the tip of a rectangular notch in unidirectional metal matrix composites was presented based on the shear lag analysis for a two-dimensional (2-D) model. By using this method, the influences of matrix yielding, width and size of notch, and size of specimen on the stress concentration factors were described. Furthermore, this method was applied successfully to describe the load-COD curve and tensile strength of notched-boron/aluminum and alumina/aluminum composites.     

全层TiAl基合金室温断裂机制的研究

通过拉伸、压缩、弯曲实验分析研究了全层(FL)组织TiAl基合金的断裂机制。研究发现:拉伸和压缩时材料抵抗裂纹的扩展能力不同,抗压强度远高于抗拉强度,这是由于两者的变形及断裂机制不同。TiAl基合金拉伸断裂机制为脆性解理断裂,压缩变形断裂是剪应力和正应力共同作用下的断裂,是准解理断裂。TiAl基合金的缺口弯曲断裂方式也为解理断裂,其断裂过程是先在缺口处产生微裂纹,一旦裂纹在缺口根部产生,由于材料已积累足够的能量使得材料快速失稳解理断裂。     

不同组织状态TC4 ELI钛合金动态力学性能研究

利用分离式霍普金森压杆装置(SHPB)对低间隙Ti-6Al-4V(TC4 ELI)合金的等轴组织、双态组织和魏氏组织试样进行了动态压缩试验。应变率分别为ε=2000,3000,4000 s-1,得到了动态压缩真应力-应变(σ-ε)曲线,并对试验后发生剪切失效破坏的试样沿纵剖面切开,利用金相显微镜(OM)进行显微组织观察。结果表明:动态压缩条件下TC4 ELI合金3种组织试样的真应力-应变曲线大致分为弹性阶段和塑性阶段,没有明显的屈服平台,3种组织状态下的试样在高应变率下应变强化效应不明显,表现出一定的应变率强化效应;在4000 s-1应变率加载条件下,平均动态流变应力(σ)、均匀动态塑性应变(ε)以及冲击吸收功(E)按等轴组织、双态组织和魏氏组织顺序依次减小,等轴组织试样的σ,ε和E分别达到了1400 MPa,0.34%和470 kJ.m-3,具有较好的动态力学性能;在4000 s-1应变率加载条件下3种组织状态的试样均发生了剪切失效破坏,并在其纵剖面上都观察到了一条白亮的绝热剪切带(ASB),裂纹沿着绝热剪切带由圆柱试样的圆柱面向中心扩展,与ASB形成和扩展的方向一致,剪切带与导致断裂的裂纹密切相关。     

Unconstrained and constrained tensile flow and fracture behavior of an Nb-1.24 At. Pct Si alloy

The unnotched and notched tensile behavior of the β-phase constituent (Nb with Si in solid solution) of the (Nb)/Nb₅Si₃ composite has been investigated at room temperature and -196 °C. At room temperature, the unnotched tensile behavior comprises significant strengthening due to Si, low strain-rate sensitivity, low strain hardening, extensive ductility, and ductile microvoid coalescence fracture, even at strain rates as high as 1.1 s⁻¹. At −196 °C, the unnotched alloy exhibited much higher strength, good ductility, and cleavage fracture. At room temperature, the notched specimens exhibited cleavagelike fracture with significant plasticity, and at −-196 °C, they exhibited cleavagelike fracture with much lower plasticity at the notch. A finite-element analysis (FEA) of stress and strain fields in the vicinity of the notch root, together with un-notched tensile behavior, indicates that plasticity plays an important role in nucleating cracks, while the high-axial tensile stress component governs crack propagation. These results are used to rationalize the observed toughening and fracture behavior of a (Nb)/Nb₅Si₃ composite.     

Experimental and numerical study of ball indentation for evaluation of mechanical properties and fracture toughness of structural steel

Determination of mechanical properties of materials using non-conventional techniques has been an active area of research for a long time. Among various small specimen techniques to determine mechanical properties of materials, the ball indentation technique (BIT) has proved to be advantageous. BIT is used to measure material’s mechanical properties including fracture toughness when a tensile test cannot be performed: on welded joints or critical locations of components under service. The present work highlights the applicability of BIT for evaluating flow behaviour of engineering structural steels En24. Standard mechanical properties like ultimate tensile strength, yield stress, strain hardening coefficient evaluated for steels with varying microstructures generated through heat treatment. To determine fracture toughness from the flow behaviour, non linear damage models have been utilized. Using this model, fracture toughness of the En24 steel at different heat treated conditions has been computed using the results generated by the BIT. These results are verified with the already established correlation in literature. Numerical validation of the results generated by the BIT has been carried out by Finite Element Modelling (FEM) using standard ABAQUS software package. The FE model of the BI simulation helps to compute sub indenter stress-strain fields which determine the extent of pile-up in highly ductile materials.     

A critical-strain criterion for hydrogen embrittlement of cold-drawn, ultrafine pearlitic steel

A stress-modified, critical-strain model of fracture-initiation toughness has been adapted to the case of hydrogen-affected pearlite shear cracking, which is a critical event in transverse fracture of cold-drawn, pearlitic steel wire. This shear cracking occurs via a process of cementite lamellae failure, followed by microvoid nucleation, growth, and linkage to create shear bands that form across pearlite colonies. The key model feature is that the intrinsic resistance to shear-band cracking at a transverse notch or crack is related to the effective fracture strain at the notch root. This fracture strain decreases with the logarithm of the diffusible hydrogen concentration (C _H). Good agreement with experimental transverse fracture-initiation-toughness values was obtained when the sole adjustable parameter of the model, the critical microstructural dimension (l*), was set to the mean dimension of shearable pearlite colonies within this steel. The effect of hydrogen was incorporated through the relationship between local effective plastic strain (ɛ _eff ^f ) and C _H, obtained from sharply and bluntly notched tensile specimens analyzed by finite-element analysis (FEA) to define stress and strain fields. No transition in the transverse fracture-initiation morphology was observed with increasing constraint or hydrogen concentration. Instead, shear cracking from transverse notches and precracks was enabled at lower global applied stresses when C _H increased. This shear-cracking process is assisted by absorbed and trapped hydrogen, which is rationalized to either reduce the cohesive strength of the Fe/Fe₃C interface, localize slip in ferrite lamellae so as to more readily enable shearing of Fe₃C by dislocation pileups, or assist subsequent void growth and link-up. The role of hydrogen at these sites is consistent with the detected hydrogen trapping. Large hydrogen-trap coverages at carbides can be demonstrated using trap-binding-energy analysis when hydrogen-assisted shear cracking is observed at low applied strains.