压痕法测定Mo_5SiB_2合金断裂韧性的经验方程确定

分析了载荷对Mo_5SiB_2合金压痕裂纹影响规律,观察了压痕裂纹形貌特征,表征了裂纹长度与载荷的关系,确定了压痕裂纹系统,选取不同的压痕方程计算了室温断裂韧性KIC值,探讨了压痕载荷以及压痕方程对KIC值的影响关系。结果表明,Mo_5SiB_2合金压痕裂纹系统属于中位裂纹,只要选择合适的压痕方程,载荷并不影响断裂韧性值。压痕法测定Mo_5SiB_2合金断裂韧性合适的经验方程为KIC=0.8193H~(0.5)E~(0.5)a~2c~(-1.5),测得的其KIC值为2.63 MPa·m~(1/2)。     

Al_（65）Cu_（20）Co_（15）准晶体的力学性能SCIEI

用压痕法测量退火前的Ａｌ_（６５）Ｃｕ_（２０）Ｃｏ_（１５）十边形准晶体的室温Ｖｉｃｋｅｒｓ硬度为１１ＧＰａ，利用压痕裂纹长度算出了断裂韧性Ｋ_（１ｃ）约为１．０ＭＰａ·，抗拉强度为４５０ＭＰａ．在８５０℃退火３６ｈ后，显微硬度值、断裂韧性无明显变化，但抗拉强度提高到５５０ＭＰａ．     

原位合成MoSi_2-SiC复合材料的室温增韧

原位合成 ＭＯＳｉ２－ＳｉＣ复合材料的断裂韧性明显高于单一ＭｏＳｉ２的断裂韧性.组织结构的ＴＥＭ与ＨＲＥＭ研究结果表明：原位合成 ＭｏＳｉ２／ＳｉＣ界面为直接的原子结合,无ＳｉＯ２非晶层存在结合对该复合材料的ＫＩｃ断口形貌及压痕裂纹连续扩展路径的观察分析表明,其室温增韧机制为 ＭｏＳｉ２－ＳｉＣ界面间较高的结合力、ＭｏＳｉ２基体晶粒细化及裂纹偏转和桥接．     

非晶态Fe_(46)Ni_(32)V_2Si_(14)B_6合金的分形断裂

测定了结构弛豫阶段退火温度ＴＲ对非晶态Ｆｅ４６Ｎｉ３２Ｖ２Ｓｉ１４　Ｂ６合金条带室温断裂韧性（Ｋｃ）的影响．ＴＲ小于２００℃时,ＫＣ大致不变,平均值约８ＭＰａｍ１／２;随着ＴＲ由２００℃增加至３５０℃,ＫＣ值逐渐下降至２ＭＰａｍ１／２;相应地断裂剖面形貌由不规则特征（分形的）逐渐变为平直的或光滑的特征．用垂直截面法测定断裂剖面裂纹轮廓线的分形维数Ｄｆ．实验结果表明,在准二维脆性断裂条件下,Ｋｃ值与分形维数增量的平方根成正比,并讨论不同结构弛豫阶段断裂特征长度ａｃ与断裂机制     

表面残余应力对玻璃压痕参数的影响

通过研究化学钢化前、钢化后钠钙玻璃的压痕形貌、几何尺寸和表面力学性能，分析和探讨了表面残余应力对玻璃压痕参数以及表面力学性能变化的影响机理。结果表明，普通玻璃的压痕与钢化玻璃的压痕有明显的差别，钢化玻璃的压痕周围呈圆形放射状光斑，压痕裂纹很小，其硬度值比相同载荷下钢化前玻璃硬度值增加了约15．8％，压痕断裂韧性有明显的提高，是钢化前玻璃断裂韧性的2．75倍，显示了钢化后表面残余压应力对玻璃的压痕参数和力学性能的影响。     

单晶SnO2纳米带裂纹形核的临界应力和断裂韧性

使用纳米压痕法测量了单晶SnO2纳米带的硬度、断裂韧性以及裂纹形核的临界应力。结果表明，当载荷大于临界值后微裂纹就从压痕顶端形核、扩展；与此相应，在载荷一位移曲线上出现位移突变平台，根据平台载荷计算出压痕裂纹形核的临界应力σc=3．4GPa；利用裂纹的长度计算出SnO2纳米带的断裂韧性为0．028—0．066MPa-m^1／2，其平均值为KIc=0．044MPa-rn^1／2，它比其它块体脆性材料的断裂韧性小一个数量级,实验测出SnO2纳米带的硬度H=6．25GPa和弹性模量E=86．7GPa。     

PZT-5铁电陶瓷恒载荷压痕裂纹在空气和水中的扩展过程

用Vickers硬度计作为加载装置,研究了极化锆钛酸铅(PZT-5)铁电陶瓷压痕裂纹恒载荷下在室温空气和水中的扩展规律.结果表明,恒载荷下,压痕裂纹在空气和水中不断扩展, 120 h后趋于稳定,从而就可获得裂纹扩展速率和裂纹止裂的门槛应力强度因子KISCC,它们均显示各向异性.研究表明,裂纹扩展速率和KISCC的各向异性与铁电陶瓷断裂韧性的各向异性有关,即裂纹扩展速率随KIC的增加而降低,而KISCC随KIC的增加而升高.平行极化方向裂纹的断裂韧性比垂直极化方向高,即KCICdn/dt;与在空气中相比,在水中应力腐蚀更敏感,即裂纹扩展速率更高、门槛值更低.     

渗硼层脆性判据K的物理本质

从实验和理论上对显微硬度压痕结合声发射测量渗硼层脆性的合理性及及其脆性判据Ｋ的物理意义进行了讨论，Ｋ值可以反映渗硼层显微压痕裂纹长度随负荷增加的变化率，因而可以表征渗硼层的脆性。根据断裂力学可以导出Ｋ与ＧＩＣ之间存在如下关系：Ｋ＝β（ＧＩ－ＧＩＣ）。Ｋ值间接反映了渗硼层的断裂韧性，可以作为定量评价渗硼层本质脆性的力学参量。     

渗硼层脆性判据K的物理本质

从实验和理论上对显微硬度压痕结合声发射测量渗硼层脆性的合理性及其脆性判据Ｋ的物理意义进行了讨论。Ｋ值可以反映渗硼层显微压痕裂纹长度随负荷增加的变化率，因而可以表征渗硼层的脆性。根据断裂力学可以导出入与Ｇ＿（ＩＣ）之间存在如下关系：Ｋ＝β（Ｇ＿Ｉ－Ｇ＿（ＩＣ））。Ｋ值间接反映了渗硼层的断裂韧性，可以作为定量评价渗硼层本质脆性的力学参量。     

氧化铝基陶瓷材料断裂韧性的测量与评价

用压痕法和单边切口梁法测量了氧化铝基陶瓷材料的断裂韧性,对这两种方法所测得的结果进行了分析对比,确定了最适用的压痕法计算公式。研究发现,原料粉末的起始粒度对用压痕法计算材料KIC的最优公式选择并无影响,但对材料的断裂韧性及抗弯强度有不同程度的影响,亚微米第二相增强微米基体的材料综合力学性能最佳;TiN的加入使Al2O3-TiC基体材料的断裂韧性有所增加;在较小的压痕载荷下计算出的材料的断裂韧性值更接近实际;在用不同的公式计算同一材料的断裂韧性值时,断裂韧性值与压痕载荷之间有相似的关系曲线。     

MEASUREMENT OF K_(IC)OF MEDIAN OR LOW STRENGTH STEEL USING ROUND BARS WITH CIRCUMFERENCIAL CRACK

The size effect of tension round bars with circumferential crack was described and used tomeasure the plane-strain fracture toughness K_(1c) of median or low strength steel.The size ofcylindrical specimen required for K_(1c) measurement is much smaller than that of standardspecimen.The J-integral values of the cylindrical sqecimens were calculated to characterizethe size effect.The finite element calculution and fracture morphology analysis have beenconducted to eyplain the fact that smaller cylindrical specimen could be used to measureK_(1c) of median or low strength steel.     

FRACTURE TOUGHNESS TEST BY IMPACT-FATIGUE METHOD

多次冲击条件下裂纹尖端应力强度因子的表达式为K_I=(AE/BW)~(1/2)·F(a/W)。经柔度法标定,其临界值可用以度量材料的断裂韧性。所得结果相当于加载速率为k=10~5公斤/毫米~(3/2)/秒下的动态断裂韧性K_(Id)。断口上临界裂纹尺寸在(a/W)=0.36-0.6范围内,结果有效。多次冲击法较为简便,只需测出断口上的临界裂纹尺寸,便可在已绘好的曲线上直接查出材料的断裂韧性。在实际生产中可以作为一种筛选方法应用。     

The formation of cuprous oxide films on α-brass stress-corrosion fracture surfaces

A study has been made of the nature of the fracture surface of α-brass specimens after failure by stress-corrosion cracking in pH 7·2 Mattsson's solution. The yellow colour of the stress-corrosion fracture surface points to a possible inadequacy in the “tarnish-rupture” model of the mechanism of this instance of stress-corrosion. The techniques of cathodic reduction and chemical dissolution have been used to remove any oxide present on the original fracture surface. On subsequent re-immersion in pH 7·2 Mattsson's solution, the stress-corrosion fracture surface remains untarnished, whereas the mechanical fracture surface rapidly acquires a black tarnish film. Comparison of plateau lengths obtained on further cathodic reduction has shown that for equivalent times of re-immersion, a much thicker film forms on the mechanical fracture region than on the stress-corrosion region of the fracture surface. The experimental results are interpreted as being inconsistent with the “tarnish-rupture” model but consistent with a stress-corrosion mechanism involving dezincification at the crack tip.     

Quantitative evaluation of fracture toughness-microstructural relationships in alpha-beta titanium alloys

The fracture toughness of two alpha-beta titanium alloys containing an alpha platelet in a transformed beta matrix has been examined in terms of the microstructural parameters controlling the fracture initiation and propagation in the alloys. Equations have been formulated that show that the highest toughness values of both alloys were associated with the finest platelet spacings and the thickest alpha platelets. It is proposed that the fracture initiation process in both alloys is controlled by the distance between the platelets, the fracture toughness of the alloys being dependent on the distance between active centers of void nucleation, i.e., as a function of the alpha platelet thickness and spacing between the platelets. Seven models of ductile fracture relating fracture toughness to mechanical property and microstructural parameters have been compared in their ability to predict the toughness of the alloys after solution treatments, which produce varying platelet thickness and inter-platelet spacings. The principle has been adopted following Rice and Rosengren and Hutchinson (HRR)^[1,2] that there must be a 1/x energy singularity at the crack tip, which also prescribes the stress and strain distribution ahead of a crack tip. Any model not incorporating these requirements should be rejected.     

Q690CFD高强钢焊接热影响区的断裂韧性

选用Gleeble-1500试验机模拟了两种成分Q690CFD高强钢的焊接粗晶热影响区(CGHAZ),测试了CGHAZ的冲击韧性,并通过三点弯曲试样,测试了CGHAZ的CTOD特征值. 利用SEM对CTOD试样断口及近断口区裂纹扩展行为进行了分析. 结果表明,示波冲击止裂功及CTOD特征值δm/c/u(8)随温度降低呈明显下降趋势;将CTOD特征值δm用Boltzmann函数拟合后所得脆韧转变温度与示波冲击变化规律相对应;两种成分钢的CTOD失稳裂纹扩展路径与近断口区组织亚结构中大角度晶界和M/A分布位置有关.     

镁合金AZ31B在不同pH值NaCl溶液中的应力腐蚀行为研究

通过慢拉伸应力腐蚀试验机、电化学工作站和扫描电子显微镜(SEM)等手段,研究挤压态镁合金AZ31B在pH=2、7和12的NaCl溶液中的应力腐蚀行为。结果表明,随pH值增大,AZ31B静态腐蚀速率和应力腐蚀敏感性均降低,这是由于碱性溶液更有利于Mg(OH)2表面保护膜的形成。应力腐蚀试验中点蚀形成主要裂纹源,拉伸断口分析表明应力腐蚀具有多裂纹源特征,应力加速腐蚀进程,断裂方式属于穿晶解理断裂。     

Fracture mechanics tests for measuring the adhesion of magnetron-sputtered TiN coatings

Experiments for determining the adhesion strength between TiN coatings and high speed steel substrates have been performed by using the three-point bending test and a modified shear test. Sample preparation is easier for the modified shear test; however, the interpretation of results is more complicated. The three-point bending test yields fracture mechanics data such as the interface fracture energy G_c and the fracture toughness K_c. The most critical problem is to prepare a sample with a notch which initiates crack propagation at the film-substrate interface. Various sublayers with weak adhesion, such as carbon, oxide and metals, have been tested as a notch, as well as thin mechanical slits. The results obtained so far demonstrate that the adhesion strength depends on the substrate cleaning treatment and on the impurity gas content in the receiver. Typical values of the fracture toughness K_c for substrates that have not been sputter cleaned are in the range 1–3 MN m^-3/2. Advantages and disadvantages of the various methods used are analysed and critically discussed with respect to sample preparation, reproducibility and data evaluation.     

Mechanical properties evaluation of hot-pressed Si3N4–SiC(w) composites

Silicon nitride based ceramic composites have been prepared using an alternative development route, aiming to reach properties for structural applications. The prepared compositions were silicon nitride based matrix with aluminum nitride and yttrium oxide, as sintering aids, reinforced with silicon carbide whiskers. The powders were grounded/homogenized directly as received. Samples were obtained by uniaxial hot-pressing and, physical (specific mass, X-ray diffraction and microstructural analysis) and mechanical (microhardness, fracture toughness and flexural strength) properties were determined. The resulting high fracture toughness values were related to crack deflection and crack bridging toughening mechanisms, observed by SEM.     

Microstructural Modeling of Intergranular Fracture in Tricrystals With Random Low- and High-Angle Grain Boundaries

Intergranular (IG) fracture behavior near triple junctions (TJs) in f.c.c. tricrystals with a variety of grain boundary (GB) misorientations has been investigated. Based on a dislocation-density GB interaction scheme, critical fracture conditions were coupled to evolving dislocation-density pileups and local stresses by using a dislocation-density-based crystalline plasticity formulation within a nonlinear finite-element framework to elucidate the effects of local GB structure, dislocation–GB interactions, and misorientations on IG crack propagation in f.c.c. crystalline materials. Tricrystals with low-angle GBs had higher fracture toughness than tricrystals with high-angle GBs. In TJs with a combination of random low- and high-angle GBs, the formation of dislocation-density pileups in the high-angle GB led to IG crack propagation along the high-angle GB rather than along the low-angle GB. These predictions, which are consistent with experimental observations, indicate that fracture behavior near TJs is controlled by highly local, evolving, and interrelated events, such as dislocation-density pileups and GB misorientations.