5.1 Advanced ceramics usually display a linear stress-strain behavior to failure. Lack of ductility combined with flaws that have various sizes and orientations leads to scatter in failure strength. Strength is not a deterministic property but instead reflects an intrinsic fracture toughness and a distribution (size and orientation) of flaws present in the material. This practice is applicable to brittle monolithic ceramics that fail as a result of catastrophic propagation of flaws present in the material. This practice is also applicable to composite ceramics that do not exhibit any appreciable bilinear or nonlinear deformation behavior. In addition, the composite must contain a sufficient quantity of uniformly distributed reinforcements such that the material is effectively homogeneous. Whisker-toughened ceramic composites may be representative of this type of material.
5.2 Two- and three-parameter formulations exist for the Weibull distribution. This practice is restricted to the two-parameter formulation. An objective of this practice is to obtain point estimates of the unknown parameters by using well-defined functions that incorporate the failure data. These functions are referred to as estimators. It is desirable that an estimator be consistent and efficient. In addition, the estimator should produce unique, unbiased estimates of the distribution parameters (6). Different types of estimators exist, including moment estimators, least-squares estimators, and maximum likelihood estimators. This practice details the use of maximum likelihood estimators due to the efficiency and the ease of application when censored failure populations are encountered.
5.3 Tensile and flexural test specimens are the most commonly used test configurations for advanced ceramics. The observed strength values are dependent on test specimen size and geometry. Parameter estimates can be computed for a given test specimen geometry ( m^, σ^θ), but it is suggested that the parameter estimates be transformed and reported as material-specific parameters ( m^, σ^0). In addition, different flaw distributions (for example, failures due to inclusions or machining damage) may be observed, and each will have its own strength distribution parameters. The procedure for transforming parameter estimates for typical test specimen geometries and flaw distributions is outlined in 8.6.
5.4 Many factors affect the estimates of the distribution parameters. The total number of test specimens plays a significant role. Initially, the uncertainty associated with parameter estimates decreases significantly as the number of test specimens increases. However, a point of diminishing returns is reached when the cost of performing additional strength tests may not be justified. This suggests that a practical number of strength tests should be performed to obtain a desired l......<......
由于剪切强度受有效应力的影响,通过在不同约束压力下测试一组三个试样,可以根据库仑模型定义失效包络,定义参数c''和φ'',并评估不排水的剪切强度。固结排水(CD)用这种方法,以有效应力来测量剪切强度。试样饱和,允许在所需的围压下固结。一旦固结,试样就被施加了一定的荷载,在此期间允许水分排出,而不会产生孔隙压力。...
如今,现代计算机显微镜以及图像分析系统,为国际或行业标准所涵盖的大多数评估和评估方法的自动化提供了快速、准确的手段。 测量通常是在一系列二维图像上进行的,可分为两大类:用于量化离散颗粒的大小、形状和分布的测量(特征测量)和与基体微观结构相关的测量(现场测量)。 第一组中的几个例子,钢的夹杂物含量的测定,铸铁中石墨分类,以及热喷涂涂层或烧结零件中孔隙率的评估。 ...
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