First of all, a brief review about what is fatigue. Fatigue is the effect of resistance reduction and failure due to applying cyclic loading. Composite materials are used in the aerospace and automotive industries which the cyclic loading is present.
Causes of fatigue in composite materials
Fatigue failure in composite materials can appear due to different factors:
- Materials mechanical properties
- Loads applied
- Types of cycles and frequency
- Ambiental conditions
Apart from this, due to the anisotropy make difficult to analysis fatigue in composite materials as there are different failures modes occurring at the same time. Composite materials experiment an increase in the displacement under cyclic loading. That is why many tests are performed until the laminate has reduced its value of stiffness at a certain point without having failed the specimen.
Stages of fatigue in composite materials
Reifsnider et al has study the development of damage in composite when it has cyclic loading. It is divided into 5 stages:
- Matrix has cracks along the fiber directions. It increases proportionally to the applied stress.
- When the Characteristic damage state (CDS) is reached, due to the saturation of matrix cracking, the fiber failure begins in the adjacent plies regions of stress concentration created by the primary cracks. There are two types of cracks: the primary, which form through the thickness and run across the width of the laminate and secondary cracks, perpendicular to the primary cracks.
- The high interlaminar stresses in the region near the free edges or discontinuities are driving force behind the initiation delamination’s.
- The delamination growth at a constant rate.
- Finally, the damage development is typified in an increasing rate progression of all damage modes. The fracture appears as it cannot withstand the tensile stress.
Conclusions
Damage in composite materials is important when designing components. The designers have to understand who the components it is going to works to design according to the different types of damage and achieve a part damage-tolerant. However, fatigue failure in composite materials is a uncommon type of failure that affects composites.
