Calculations of theoretical strength: State of the art and history

Current state and historical evolution of theoretical strength calculations is presented as a brief overview completed by a database of selected theoretical and experimental results. Principles of a sophisticated analysis of mechanical stability of crystals are elucidated by means of a schematic example. Stability conditions and Jacobian matrixes are presented for selected crystalline symmetries and deformation paths. The importance of this analysis for understanding micromechanics of fracture is shown against the background of the influence of crystal defects. Differences between theoretical and experimental theoretical strength (TS) values are discussed and some challenging tasks are outlined for the near future.     

Determination of directional strength properties in face-centered single crystals

A procedure is given for evaluating the stress-rupture strength of structures made of single-crystal materials from values for stress-rupture strength in characteristic crystal directions [100], [110], [111].Translated from Problemy Prochnosti, No. 11, pp. 47–49, November, 1990.     

残余应力对固体氧化物燃料电池弹塑性性能的影响

半电池结构NiO-YSZ/YSZ由于弹性模量不同和热膨胀系数不匹配,导致烧结过程中产生残余应力. 残余应力对于燃料电池的性能和使用带来一定的影响, 本文把残余应力引入到计算薄膜性能的逆向分析模型中, 建立了考虑残余应力影响的薄膜的纳米压痕分析模型. 利用纳米压痕方法测试了离双层材料界面不同距离处的载荷-位移曲线和相应的材料性能, 用本文建立的模型计算了不同点的性能的变化, 发现离界面越远, 电解质YSZ薄膜的硬度越大. 将热力分析得到的残余应力场作为压痕模拟的初始应力场, 计算三棱锥压头下压痕载荷-位移曲线, 结果显示考虑残余应力时的载荷-位移曲线更接近实验曲线, 并给出了残余应力下压痕形貌图, 发现有残余应力时的压痕形貌更深更大.     

High-temperature strength of bulk single crystals of III-V nitrides

A Vickers indentation method was used to determine the hardness of AlN and GaN, grown by the hydride vapor phase epitaxy technique, in the temperature range 20–1400 °C. At room temperature, the hardnesses of GaN and AlN are 10.2 and 17.7 GPa, respectively. The hardness of GaN and AlN shows a gradual decrease from RT and then a steep decrease from around 1000 °C. AlN is harder than GaN but softer than SiC. The steep decrease of the hardness means the beginning of macroscopic dislocation motion and plastic deformation. The mechanical strength of bulk single-crystal GaN is investigated at elevated temperatures directly by means of compressive deformation. The yield stress of GaN in the temperature range 900–1000 °C is around 100–200 MPa, i.e., similar to that of 6H-SiC and much higher than those of Si, Ge, GaAs.     

测定材料屈服强度的应变计法试验研究

对应变计测量屈服强度的可能性做了讨论，选择适合弹塑性区测试的应变片，进行与引伸计的对比试验以考察测试结果的异同试验表明两种方法测试同一试样所得值之差最大为７ＭＰａ，测试相同状态不同试样所得平均值之差不到７ＭＰａ，由于差别很小，故应变计法可用于屈服强度的工程测试，且与引伸计法具有相同的测量精确度。     

Measurement of ultimate tensile strength and Young modulus in LYSO scintillating crystals

Scintillating crystals are employed in high energy physics, in medical imaging, diagnostic and security. Two mechanical properties of lutetium-yttrium oxyorthosilicate cerium-doped Lu_2(1−x)Y_2xSiO₅:Ce with x=0.1 (LYSO) crystals have been measured: the ultimate tensile stress (σ_UTS) and the Young elastic modulus (E). Measurements are made by means of a 4-points loading device and the experimental results account for an elastic-brittle stress-strain relation, which depends heavily on the specimen preparation and the material defects. σ_UTS along the [0 1 0] tensile direction ranges within 68.14 and 115.61 MPa, which, in the lowest case, is more than twice with respect to those measured for PbWO₄ (PWO), exhibiting a marked difference between the annealed and the not-annealed samples. The mean elastic modulus (E), along the same direction, is E=1.80×10¹¹ (±2.15×10¹⁰) N/m², with lower dispersion respect to UTS data. This type of analysis and study can be included into quality control procedures of crystals, based on samples taken out of production; such procedures can be established for industrial processing of crystals aimed to the high energy physics (calorimeters) and medical imaging (PET, etc.) applications.     

屈服准则用于高强度热轧钢板的适用性分析

为确定适合描述高强度热轧钢板变形行为的屈服准则,采用Hollomon流动应力方程和三种屈服准则对几类高强度热轧钢板在不同应变路径下达到成形极限的成形过程进行了模拟.比较了Barlat(1989)、Hill(1948)、Barlat六参数3种屈服准则,对热轧酸洗板QStE340TM、SAPH370和热轧镀锌板ZStE260P在单向拉伸、平面应变和双向等拉3种应变路径下的变形过程进行了比较.结果表明,Barlat(1989)屈服准则能较好地描述单元的变形行为,且在平面应变路径下的模拟结果最符合实验结果.     

沉淀强化镍基合金屈服强度的理论计算EI

本文提供了依据合金的化学成分、显微组织参数定量计算合金屈服强度性能的理论方法。具体计算了一组镍基合金的室温屈服强度，计算值与实测值很好地符合。     

管线钢管屈服强度的稳定性试验

针对目前API Spec 5L-2009《管线钢管规范》中采用冷压平方法测试管线钢管屈服强度的不稳定性问题,采用双肩圆试样拉伸、展平率法板试样拉伸、预拉伸至弹性极限Rp0.02板试样拉伸和预拉伸至弹性极限Rp0.01板试样拉伸四种方法对不同取样方向的管线钢管试样进行了拉伸试验,并对四种方法测得的屈服强度结果进行分析比较。结果表明:双肩圆试样拉伸法由于不是全厚度试样,因而不能真实地反映管线钢管的屈服强度,且试样加工比矩形平板状试样更为繁琐;展平率法板试样拉伸方法由于同一取样角度只计算一个试样的展平率,因而试验结果比较分散,且需要采用滞后环法对试验数据进行处理,工作量较大;预拉伸至弹性极限板试样拉伸方法可以方便、准确地计算出管线钢管的屈服强度,但预拉伸至弹性极限Rp0.02时的非比例伸长延伸率稍大,因此建议采用预拉伸至弹性极限Rp0.01板试样拉伸的方法测试管线钢管的屈服强度。     

钢管静水压爆破试验中屈服强度的测定

依据AS 2885.5:2002标准的要求进行钢管静水压爆破试验,通过对试验的压力-进水量曲线进行分析,表明钢管静水压爆破试验屈服强度的测定可以在应用数学统计方法和数值计算软件的基础上,使得屈服点的求取定量化.采用Matlab软件进行曲线拟合和函数微分,定量求取了爆破试验的屈服强度.     

Bending strength of piezoelectric ceramics and single crystals for multifunctional load-bearing applications

The topic of multifunctional material systems using active or smart materials has recently gained attention in the research community. Multifunctional piezoelectric systems present the ability to combine multiple functions into a single active piezoelectric element, namely, combining sensing, actuation, or energy conversion ability with load-bearing capacity. Quantification of the bending strength of various piezoelectric materials is, therefore, critical in the development of load-bearing piezoelectric systems. Three-point bend tests are carried out on a variety of piezoelectric ceramics including soft monolithic piezoceramics (PZT-5A and PZT-5H), hard monolithic ceramics (PZT-4 and PZT-8), single-crystal piezoelectrics (PMN-PT and PMN-PZT), and commercially packaged composite devices (which contain active PZT-5A layers). A common 3-point bend test procedure is used throughout the experimental tests. The bending strengths of these materials are found using Euler-Bernoulli beam theory to be 44.9 MPa for PMN-PZT, 60.6 MPa for PMN-PT, 114.8 MPa for PZT- 5H, 123.2 MPa for PZT-4, 127.5 MPa for PZT-8, 140.4 MPa for PZT-5A, and 186.6 MPa for the commercial composite. The high strength of the commercial configuration is a result of the composite structure that allows for shear stresses on the surfaces of the piezoelectric layers, whereas the low strength of the single-crystal materials is due to their unique crystal structure, which allows for rapid propagation of cracks initiating at flaw sites. The experimental bending strength results reported, which are linear estimates without nonlinear ferroelastic considerations, are intended for use in the design of multifunctional piezoelectric systems in which the active device is subjected to bending loads.     

双真空熔炼(VIM+VAR)30CrMnSiNi2A钢屈服强度的影响因素分析

对影响双真空(VIM VAR)熔炼30CrMnSiNi2A钢屈服强度(σ0-2)的因素进行了分析。结果表明，C，Si对该钢σ0-2影响较大，为了确保σ满足技术规范的要求，C，Si含量宜控制在上限；在规定200～300C回火温度范围内，较高的温度回火对提高σ0-2有利。     

较高应变速率下结构钢屈服强度的估计

本文给出了一种考虑应变速率效应时材料屈服强度近似求法，它依据两组低应变速率的拉伸试验数据，对其它应变速率下的屈服强度进行估计，并具有良好的精度。     

Large oscillator strength excitons in PbGa2S4 crystals

Exciton states with a large oscillator strength (transverse-longitudinal splitting of 50 meV) and binding energy of 290 meV have been observed in PbGa₂S₄ crystals. The ground exciton states are stable up to the room temperature. The ground (n = 1) exciton state and two excited (n = 2 and n = 3) states of the short-wavelength B-excitons series were observed at low temperature (T = 10 K). The ground (n = 1) and the excited (n = 2) states of the long-wavelength A-exciton series were also revealed. The exciton Bohr radius of the ground exciton state was determined to be 70 Å for the A-exciton and 10 Å for the B-exciton. The band splitting and the main parameters of excitons have been determined from the calculation of the reflectivity spectra contours. The group theory analysis of the band symmetry was performed and the scheme of bands responsible for exciton transitions in the center of the Brillouin zone was proposed.     

Comparison of mode-resolved fracture strength of silicon with mixed-mode failure of diamond crystals

Currently ab initio theory provides the ideal tensile and shear strength of the {111} cleavage plane in single-crystal silicon and diamond only for few selected planes and directions (‘geometries’). These values can be compared with the real strength of nano-, micro-, and single-crystalline devices to roughly judge their mechanical quality. A novel contact-free and notch-free optical laser method allows the measurement of the fracture strength with plane nonlinear surface acoustic waves (SAWs), providing a unique way to discriminate between tensile and shear stresses for well-defined crystallographic planes and directions in anisotropic materials. Calibration yields the critical fracture stress or strength for the geometries, which can be compared with theory. In the case of diamond mostly mechanistically unresolved mixed-mode failure has been studied for complex geometries. Nevertheless, the comparison of these critical failure stresses with the strength of the ideal lattice and the mode- and geometry-resolved fracture behavior of silicon provides new insight into the mechanical stability and failure behavior of diamond materials.     

Lattice-Distortion-Enhanced Yield Strength in a Refractory High-Entropy Alloy

Severe distortion is one of the four core effects in single-phase high-entropy alloys (HEAs) and contributes significantly to the yield strength. However, the connection between the atomic-scale lattice distortion and macro-scale mechanical properties through experimental verification has yet to be fully achieved, owing to two critical challenges: 1) the difficulty in the development of homogeneous single-phase solid-solution HEAs and 2) the ambiguity in describing the lattice distortion and related measurements and calculations. A single-phase body-centered-cubic (BCC) refractory HEA, NbTaTiVZr, using thermodynamic modeling coupled with experimental verifications, is developed. Compared to the previously developed single-phase NbTaTiV HEA, the NbTaTiVZr HEA shows a higher yield strength and comparable plasticity. The increase in yield strength is systematically and quantitatively studied in terms of lattice distortion using a theoretical model, first-principles calculations, synchrotron X-ray/neutron diffraction, atom-probe tomography, and scanning transmission electron microscopy techniques. These results demonstrate that severe lattice distortion is a core factor for developing high strengths in refractory HEAs.     

冲击速度对20Cr和40Cr钢应变速率、动态屈服强度作用的研究

研究了不同热处理状态的２０Ｃｒ和４０Ｃｒ钢在不同冲击速度（ｖ）下，应变速率（ε）和动态屈服强度（σα）的变化规律，研究表明：σα＝σ０ｅｘｐ（Ａｌｎε），ε＝ε０ｅｘｐ（Ｂｖ），σα＝σ０ｅｘｐ（Ｃｖ）．其中只有两个是独立的，并用粘塑性本构理论分析了微观作用机制。