复合材料的疲劳试验验证了部件长期使用的耐久性

When a material is used in a component or product, fatigue life is an important consideration in practical applications. During cyclic loading, most materials will eventually fail. 材料的寿命(失效前)取决于受力和施加的应力.

Why composite 疲劳测试 is important

预测结构的疲劳寿命是预测结构潜在失效的重要手段, 了解产品或材料的使用寿命和递减收益. 通过真实的模拟测试分析材料的疲劳特性通常是一个有效的过程，以避免在产品生命周期的所有部分的收入损失. 对金属疲劳的认识有着悠久的历史和理论基础, but composites have unique attributes, 因此疲劳测试必须考虑到这些属性.

Properties of fatigue of composite materials

而有些复合材料在所有方向上都具有相同的特性，例如 .an epoxy molding compound, most composites are non-isotropic. 使用这些复合材料进行设计需要考虑各个方向的静态和疲劳载荷.

In the in-plane fiber direction, composites are strong and stiff.  In this direction, static properties usually limit the design. For the in-plane direction, fatigue performance is better than on monotonic materials, because cracks may be arrested by the fibers.  因此，平面内设计通常受强度限制，而不是疲劳.  

Most composite structures are designed to have low out-of plane loads. 然而, there are also materials applications with large bending stresses, such as those found in blades, 喷气发动机风扇, helicopter rotors and wind turbines. 这些叶片在使用过程中可能会经历极高的疲劳，需要进行测试.

During 疲劳测试, 应力通常以恒定振幅和正弦波形施加. 其他波形也可以应用，但正弦波形在工业中最常见. 在谱疲劳中，载荷用来模拟材料在使用中所看到的载荷状态. 振幅以一种模拟实际部分所观察到的方式变化. 如果疲劳裂纹尖端周围的区域是加工硬化，这可能是重要的.  This is not usually the case with continuous fiber polymeric composites, so spectrum fatigue is not routine for composite 疲劳测试.

ASTM D3479 testing

ASTM D3479 is a standard for tension-tension 疲劳测试 of composites. 测试方法是采用标准的ASTM D3039拉伸券并施加循环载荷的张力. The ratio of the high load to the low load is 10:1. This is described by an R ratio of 0.1. The R ratio is the low stress divided by the high stress. 试验一般在应力应变曲线的弹性区进行. The test rate is selected such that the material will not self-heat. 5 to 10 Hertz is often used. 通常在试样上附有热电偶以监测其自热. 主动冷却, 比如扇子, 可用于提高测试速率，允许的自热应由测试请求方指定.

How composite 疲劳测试 is performed

和大多数人一样 mechanical testing, specimen preparation is critical. For tension-tension tests, 应使用负载引入卡箍来防止夹持区域的磨损和负载不均匀. Surface finish of the edges should be as smooth as practical. The angle of the specimen should be correctly oriented with the fibers.

CMH17, the design handbook for the use of composite materials and aircraft, 建议描述裸眼和轴承疲劳，以帮助理解关节行为. These tests may be performed at a differing R ratio such as 0.2 or fully reversed, at R=-0.1. In the fully reversed case, 根据ASTM D6484，通常用侧面支撑夹具支撑试样.  裸眼疲劳试验的具体细节见ASTM D7615. 在im体育APP, 我们从CMH17手册考虑到我们的测试和复合材料进行了彻底的评估.

Sometimes it is important to define failure criteria.  当样品不再承载载荷并缠绕成两部分时，它显然已经失效了. 然而, some specimens only deform gradually, 因此有时需要在破坏准则中定义最大变形或位移.

Fatigue testing of impact-damaged laminates

Fatigue testing of impact-damaged laminates is also of interest. 设计师需要知道撞击等事件造成的损害是否会在使用过程中传播. To help consider this, specimens are subjected to small impacts. These create damage known as BVID, barely visible impact damage. 选择这种程度的损坏是因为飞机上的维修人员可以观察到, even though it is small. By 疲劳测试 this damaged area, one can determine if the damage will grow in service. These fatigue tests are often performed on sandwich beams in bending.

这种材料的疲劳试验可以继续进行而不破坏试样. 测试请求者将指定预设的最大循环数，通常称为“运行耗尽”. This is often one million cycles for composites. When a specimen reaches run-out the test is concluded, 并可进行残余强度试验或其他无损评价.

Understanding how fatigue cracks propagate

了解疲劳裂纹如何在材料和结构中传播是断裂力学的一部分. To help understand fatigue behavior, 一套试验用于表征不同开孔模式下的开裂. 双悬臂梁试件一端有分层现象，并逐渐被拉开. ASTM D5568中描述的静态测试提供了拉伸模式下应变能释放率的值, 模式1或GiC. The fatigue test method for mode one is ASTM 6115. 这测试了在不同裂纹能量水平下裂纹扩展开始的循环次数. By running tests at various levels of G for the initiation fracture toughness, experts create a G dash N curve. 然后可以计算应变能裂纹扩展速率或D a / D N.

By bending a notched specimen, called the end notched flex or ENF, the crack may be initiated to propagate in shear, giving a mode two strain energy release rate or Giic. The static mode 2 test is ASTM D7905. The fatigue version of the mode 2 test has not been standardized, but generally follows the mode 1 approach.

Composite 疲劳测试 best practices

在大多数情况下，进行疲劳测试以生成S - dash - N曲线. This is a curve of the number of cycles versus stress. 当对数绘制时，这是一条直线，得到SN曲线，至少需要测试12个样品, with at least three or four different stress levels. More specimens will provide better resolution for this curve.

疲劳试验是评估材料耐久性的重要步骤. 该过程应在结构服役时可能出现的极端环境下进行.. 这包括高低温测试和复合材料中的水分含量. 如果主负荷是热负荷，则可以进行热循环.

Due to a multitude of testing possibilities, 当对产品和材料进行疲劳测试时，建议使用专家. 在im体育APP, 我们拥有全球能力和专业知识，可以帮助您满足所有主要im体育平台app下载的疲劳测试需求，以满足先进的质量标准.

im体育APP today to schedule your 疲劳测试.