Fatigue 测试 of Composite Material Verifies Component Durability for Long-Term Applications

当一种材料被用于部件或产品时, 疲劳寿命在实际应用中是一个重要的考虑因素. 在循环加载过程中，大多数材料最终都会失效. The longevity of a material (before failure) is dependent on the force and application of stress.

为什么复合材料疲劳试验很重要

It is important to predict fatigue life of structures in order to anticipate potential failures, understand the useful life and diminishing returns of a product or material. Analyzing fatigue properties of materials through real-life simulation testing is often an effective process for avoiding loss in revenue in all parts of the product life cycle. There is a long history and theoretical basis for understanding metal fatigue, 但是复合具有独特的属性, so 疲劳测试 is imperative to bring these attributes into consideration.

复合材料的疲劳性能

While some composites have the same properties in all directions, for example .作为一种环氧成型化合物，大多数复合材料是非各向同性的. Designing with these composites requires consideration of both static and fatigue loads in all directions.

在面内纤维方向上，复合材料具有较强的刚性.  在这个方向上，静态属性通常会限制设计. 求平面内方向, 疲劳性能优于单调材料, 因为裂缝可以被纤维阻挡住.  As a result, in-plane designs are usually limited by strength, rather than fatigue.  

大多数复合结构都设计成具有较低的面外载荷. 然而, 也有材料应用与大弯曲应力, 比如在叶片中发现的那些, 喷气发动机风扇, 直升机旋翼和风力涡轮机. These blades may experience extremely high fatigue during application and require testing.

在疲劳试验期间, stresses are most often applied with a constant amplitude and sinusoidal waveform. Other waveforms may be applied, but a sinusoidal waveform is most common in industry. In spectrum fatigue, the loads are used to simulate the load regime materials see in use. Amplitudes are varied in a way that simulates what has been observed in actual parts. This may be important if the area around a fatigue crack tip is work hardening.  This is not usually the case with continuous fiber polymeric composites, 因此，谱疲劳不是复合材料疲劳试验的常规方法.

ASTM D3479测试

ASTM D3479是张力的标准 复合材料疲劳试验. The test approach is to take a standard ASTM D3039 tensile coupon and apply cyclic loads in tension. 高负荷与低负荷的比例为10:1. 这是用R = 0来描述的.1. R比是低应力除以高应力. The test is generally conducted in the elastic region of the stress strain curve. 试验速率的选择使材料不会自热. 通常使用5到10赫兹. A thermocouple is often attached to the specimen to monitor for self-heating. 主动冷却, 比如扇子, may be utilized to increase the test rate and the allowed self-heating should be specified by the test requester.

复合材料疲劳试验是如何进行的

和大多数人一样 机械测试，标本准备是至关重要的. 用于张力测试, load introduction tabs should be used to prevent wear and uneven loading in the gripping area. 边缘的表面处理应尽可能光滑. 试样的角度应与纤维方向正确.

CMH17, 该设计手册适用于复合材料和飞机的使用, recommends characterizing open-hole and bearing fatigue to help understand joint behavior. 这些测试可以在不同的R比下进行，例如0.2或完全相反，在R=-0时.1. 在完全相反的情况下, specimens are usually supported with a side support fixture per ASTM D6484.  The specific details for open-hole 疲劳测试 are described in ASTM D7615. 在im体育APP, we take considerations from the CMH17 handbook into consideration for our testing and composites are evaluated thoroughly.

有时，定义故障标准是很重要的.  When the sample will no longer carry a load and winds up in two pieces, it has clearly failed. 然而, 有些试样只是逐渐变形, so sometimes a maximum deformation or displacement needs to be defined in the failure criterion.

冲击损伤层压板的疲劳试验

冲击损伤层压板的疲劳试验也很有趣. Designers need to know if damage from an event like an impact can propagate during use. 为了帮助考虑这一点，标本受到了小的冲击. 这些造成的损伤被称为BVID，几乎看不见的冲击损伤. This level of damage is selected because it can be observed by maintenance personnel on the aircraft, 尽管它很小. By 疲劳测试 这个损坏区域，可以判断损坏是否会在使用中增加. 这些疲劳试验通常是在弯曲夹层梁上进行的.

The 疲劳测试 of this material may continue without specimen failure. A preset maximum number of cycles, often called “run out”, will be specified by the test requester. 对于复合材料来说，这通常是100万次循环. 当样品达到耗尽时，测试就结束了, and a residual strength test or other nondestructive evaluation may be performed.

了解疲劳裂纹如何扩展

了解疲劳裂纹如何扩展 in materials and structures is part of the science of fracture mechanics. 来帮助理解疲劳行为, a suite of tests is used to characterize cracking in different opening modes. Double cantilever beam specimens have a delamination at one end and are gradually pulled apart. The static test described in ASTM D5568 provides a value for strain energy release rate in the tensile mode, 模式1或GiC. 模态一的疲劳试验方法是ASTM 6115. This tests the number of cycles to onset of crack growth at various levels of crack energy. 通过在不同的G值下进行初始化测试 断裂韧性，专家们创造了一个G - dash - N曲线. The strain energy crack growth rate or D a / d N can then be calculated.

通过弯曲有缺口的试样, 称为末端缺口弯曲或ENF, 裂纹可能在剪切中开始扩展, 给出一种模态二应变能释放率或Giic. 静态模式2试验是ASTM D7905. 模态2测试的疲劳版本尚未标准化, 但通常遵循模式1方法.

复合材料疲劳测试最佳实践

In most cases, 疲劳测试 is performed to generate an S dash N curve. 这是循环次数与应力的曲线. When plotted logarithmically this is a straight line to obtain an SN curve and at least 12 specimens should be tested, 至少有三到四种不同的压力水平. 更多的标本将为这条曲线提供更好的分辨率.

Fatigue testing is an important step in assessing the durability of materials. This process should be performed at the environmental extremes that may occur when the structure is in service.. This includes high and low temperature testing and moisture content in composites. 如果主负荷是热负荷，则可以进行热循环.

由于大量的测试可能性, it is recommended to utilize experts when testing for fatigue on products and materials. 在im体育APP, we have the global capacity and expertise to assist with your 疲劳测试 needs across all major industries to meet advanced quality standards.

im体育APP 今天安排你的疲劳测试.