1.1
本试验方法包括通过恒定应力速率弯曲试验确定高级陶瓷的缓慢裂纹扩展(SCG)参数,其中弯曲强度是在高温下给定环境中施加应力速率的函数。在特定环境中,随着施加应力速率的降低而表现出的强度退化是该试验方法的基础,该方法能够评估材料的缓慢裂纹扩展参数。
注1:
该试验方法通常被称为“动态疲劳”试验
(
1-
3.
)
2.
其中术语“疲劳”与术语“缓慢裂纹扩展”互换使用避免与术语中定义的仅在循环载荷下发生的材料“疲劳”现象混淆
E1823
,该试验方法使用术语“恒定应力速率试验”而不是“动态疲劳”试验。
注2:
在玻璃和陶瓷技术中,相当长时间的静态试验称为“静态疲劳”试验,这是一种被指定为应力断裂(术语)的试验
E1823
).
1.2
本试验方法主要用于
可以忽略不计的
试样的蠕变,本试验方法中施加了蠕变的特定限制。
1.3
本试验方法主要适用于宏观均匀和各向同性的高级陶瓷。该试验方法也适用于某些晶须或颗粒增强陶瓷,这些陶瓷表现出宏观均匀的行为。
1.4
本试验方法适用于各种试验环境,如空气、真空、惰性气体和任何其他气体环境。
1.5
本标准测试中表示的数值符合国际单位制(SI)和
IEEE/ASTM 硅 10
.
1.6
本标准并非旨在解决与其使用相关的所有安全问题(如有)。本标准的用户有责任在使用前制定适当的安全、健康和环境实践,并确定监管限制的适用性。
1.7
本国际标准是根据世界贸易组织技术性贸易壁垒(TBT)委员会发布的《关于制定国际标准、指南和建议的原则的决定》中确立的国际公认标准化原则制定的。
====意义和用途======
4.1
对于在役的许多结构陶瓷部件,其使用寿命通常受到缓慢裂纹扩展过程控制的寿命的限制。该试验方法为评估陶瓷材料在高温特定环境下的相对缓慢裂纹扩展敏感性提供了经验参数。该试验方法与试验方法类似
C1368
但给出了高温试验的规定除外。此外,该试验方法可以确定加工变量和成分对新开发或现有材料的缓慢裂纹扩展以及强度行为的影响,从而允许裁剪和优化材料加工以进行进一步修改。
总之,该试验方法可用于材料开发、质量控制、表征和有限设计数据生成目的。
注3:
本试验方法产生的数据不一定对应于在使用条件下可能遇到的裂纹速度。出于设计目的使用本试验方法生成的数据可能会导致大量外推和精度损失。
4.2
在本试验方法中,弯曲应力计算基于简单梁理论,假设材料各向同性且均匀,拉伸和压缩弹性模量相同,且材料具有线性弹性。平均晶粒度不应大于梁厚度的五十分之一(1/50)。
4.3
在本试验方法中,根据试验方法选择了试样尺寸和试验夹具
C1211
,它在实际配置和产生的误差之间提供了平衡,如参考文献中所述
(
5.
,
6.
)
. 本测试方法中仅使用四点测试配置。
4.4
在本试验方法中,缓慢裂纹扩展参数(
n
和
D
)根据弯曲强度和施加应力速率logσ之间的数学关系确定
f
= [1/(
n
+1)]日志
σ
˙
+日志
D
,并结合实测实验数据。推导该关系的基本假设是,缓慢的裂纹扩展由经验幂律裂纹速度控制,
v=A
[
K
我
/
K
集成电路
]
n
(参见
附录X1
).
注4:
还有各种其他形式的裂纹速度定律,通常在数学上更复杂或不太方便,或两者兼有,但在物理上可能更现实
(
7.
)
. 本试验方法中的数学分析不包括此类替代裂纹速度公式。
4.5
在该试验方法中,基于慢速裂纹扩展参数至少为的假设,推导了弯曲强度和应力速率之间的数学关系
n
≥
5.
(
1.
,
8.
)
. 因此,如果一种材料表现出非常高的慢裂纹扩展敏感性,即,
n
<5,在解释结果时应特别小心。
4.6
根据中的方法对测试结果进行数学分析
4.4
假设材质不显示上升
R
-曲线行为,即断裂阻力(或裂纹扩展阻力)不随裂纹长度增加而增加。应注意的是,这种行为的存在无法通过该试验方法确定。分析进一步假设相同的缺陷类型控制整个测试范围的强度。也就是说,不会产生新的缺陷,并且在最高应力速率下控制强度的缺陷在最低应力速率下控制强度。
4.7
陶瓷材料的缓慢裂纹扩展行为随机械、材料、热和环境变量的变化而变化。因此,测试结果准确反映研究中特定变量的影响至关重要。
只有这样,才能在有效的基础上将一项调查的数据与另一项调查的数据进行比较,或作为表征材料和评估结构行为的有效基础。
4.8
高级陶瓷的强度本质上是概率的。因此,由陶瓷材料的弯曲强度确定的缓慢裂纹扩展也是一种概率现象。因此,统计再现性和设计需要适当的测试速率范围和数量以及每个测试速率下适当数量的样本
(
2.
)
. 本试验方法提供了指导。
注5:
对于给定的陶瓷材料/环境系统,SCG参数
n
尽管其再现性取决于前面提到的变量,但与样本尺寸无关。
相比之下,SCG参数
D
很大程度上取决于强度,因此取决于试样尺寸(参见
等式X1.7
).
4.9
给定试样和测试夹具配置的陶瓷材料的高温强度取决于其固有的抗断裂能力、缺陷的存在、测试速率和环境影响。尽管不在本试验方法的范围内,但强烈建议对断口进行分析,尤其是为了验证与故障相关的机制(参考实践)
C1322
).
1.1
This test method covers the determination of slow crack growth (SCG) parameters of advanced ceramics by using constant stress-rate flexural testing in which flexural strength is determined as a function of applied stress rate in a given environment at elevated temperatures. The strength degradation exhibited with decreasing applied stress rate in a specified environment is the basis of this test method which enables the evaluation of slow crack growth parameters of a material.
Note 1:
This test method is frequently referred to as “dynamic fatigue” testing
(
1-
3
)
2
in which the term “fatigue” is used interchangeably with the term “slow crack growth.” To avoid possible confusion with the “fatigue” phenomenon of a material which occurs exclusively under cyclic loading, as defined in Terminology
E1823
, this test method uses the term “constant stress-rate testing” rather than “dynamic fatigue” testing.
Note 2:
In glass and ceramics technology, static tests of considerable duration are called “static fatigue” tests, a type of test designated as stress-rupture (Terminology
E1823
).
1.2
This test method is intended primarily to be used for
negligible
creep of test specimens, with specific limits on creep imposed in this test method.
1.3
This test method applies primarily to advanced ceramics that are macroscopically homogeneous and isotropic. This test method may also be applied to certain whisker- or particle-reinforced ceramics that exhibit macroscopically homogeneous behavior.
1.4
This test method is intended for use with various test environments such as air, vacuum, inert, and any other gaseous environments.
1.5
Values expressed in this standard test are in accordance with the International System of Units (SI) and
IEEE/ASTM SI 10
.
1.6
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.7
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
====== Significance And Use ======
4.1
For many structural ceramic components in service, their use is often limited by lifetimes that are controlled by a process of slow crack growth. This test method provides the empirical parameters for appraising the relative slow crack growth susceptibility of ceramic materials under specified environments at elevated temperatures. This test method is similar to Test Method
C1368
with the exception that provisions for testing at elevated temperatures are given. Furthermore, this test method may establish the influences of processing variables and composition on slow crack growth as well as on strength behavior of newly developed or existing materials, thus allowing tailoring and optimizing material processing for further modification. In summary, this test method may be used for material development, quality control, characterization, and limited design data generation purposes.
Note 3:
Data generated by this test method do not necessarily correspond to crack velocities that may be encountered in service conditions. The use of data generated by this test method for design purposes may entail considerable extrapolation and loss of accuracy.
4.2
In this test method, the flexural stress computation is based on simple beam theory, with the assumptions that the material is isotropic and homogeneous, the moduli of elasticity in tension and compression are identical, and the material is linearly elastic. The average grain size should be no greater than one fiftieth (1/50) of the beam thickness.
4.3
In this test method, the test specimen sizes and test fixtures were chosen in accordance with Test Method
C1211
, which provides a balance between practical configurations and resulting errors, as discussed in Refs
(
5
,
6
)
. Only the four-point test configuration is used in this test method.
4.4
In this test method, the slow crack growth parameters (
n
and
D
) are determined based on the mathematical relationship between flexural strength and applied stress rate, log σ
f
= [1/(
n
+ 1)] log
σ
˙
+ log
D
, together with the measured experimental data. The basic underlying assumption on the derivation of this relationship is that slow crack growth is governed by an empirical power-law crack velocity,
v = A
[
K
I
/
K
IC
]
n
(see
Appendix X1
).
Note 4:
There are various other forms of crack velocity laws which are usually more complex or less convenient mathematically, or both, but may be physically more realistic
(
7
)
. The mathematical analysis in this test method does not cover such alternative crack velocity formulations.
4.5
In this test method, the mathematical relationship between flexural strength and stress rate was derived based on the assumption that the slow crack growth parameter is at least
n
≥
5
(
1
,
8
)
. Therefore, if a material exhibits a very high susceptibility to slow crack growth, that is,
n
< 5, special care should be taken when interpreting the results.
4.6
The mathematical analysis of test results according to the method in
4.4
assumes that the material displays no rising
R
-curve behavior, that is, no increasing fracture resistance (or crack-extension resistance) with increasing crack length. It should be noted that the existence of such behavior cannot be determined from this test method. The analysis further assumes that the same flaw types control strength over the entire test range. That is, no new flaws are created, and the flaws that control the strength at the highest stress rate control the strength at the lowest stress rate.
4.7
Slow crack growth behavior of ceramic materials can vary as a function of mechanical, material, thermal, and environmental variables. Therefore, it is essential that test results accurately reflect the effects of specific variables under study. Only then can data be compared from one investigation to another on a valid basis, or serve as a valid basis for characterizing materials and assessing structural behavior.
4.8
The strength of advanced ceramics is probabilistic in nature. Therefore, slow crack growth that is determined from the flexural strengths of a ceramic material is also a probabilistic phenomenon. Hence, a proper range and number of test rates in conjunction with an appropriate number of specimens at each test rate are required for statistical reproducibility and design
(
2
)
. Guidance is provided in this test method.
Note 5:
For a given ceramic material/environment system, the SCG parameter
n
is independent of specimen size, although its reproducibility is dependent on the variables previously mentioned. By contrast, the SCG parameter
D
depends significantly on strength, and thus on specimen size (see
Eq X1.7
).
4.9
The elevated-temperature strength of a ceramic material for a given test specimen and test fixture configuration is dependent on its inherent resistance to fracture, the presence of flaws, test rate, and environmental effects. Analysis of a fracture surface, fractography, though beyond the scope of this test method, is highly recommended for all purposes, especially to verify the mechanism(s) associated with failure (refer to Practice
C1322
).