Figure 1. Blade cross-section schematic showing the locations where biaxial laminates are used.
Blade designers continue to push existing composite material capabilities as they look to capture more wind with longer rotors. Recent industry observations of damage occurring earlier than expected in the blade panel regions (Figure 1) were brought to the attention of researchers at Sandia National Laboratories and Montana State University (MSU).
Blade panel regions typically use biaxial laminates to handle the torsional blade loads as well as the edgewise loading which is driven by the gravity loads of the blade itself. Biaxial laminates are composites where the fibers are typically aligned in a ±45° pattern with respect to the blade axis. Damage typically initiates in the matrix material which can be seen in the cross-hatch patterns in Figure 2.
Figure 2. Matrix cracking in biaxial laminates observed under three different fatigue loading conditions (R values) in reflected light (top row) and transmitted light (bottom row).
Our team conducted extensive tensile and compression testing of biaxial laminate coupon samples at MSU’s composite testing facilities with both constant and cyclic loading. A key finding was that the main parameter determining the fatigue lifetime for these laminates is the cumulative time under load—not the number of cycles, the conventionally used metric. So, given two samples that are both subjected to the same maximum load and same number of cycles, the one with the lower frequency of cycles will experience damage sooner. This is the opposite of uniaxial laminates that are used in the spar cap. In uniaxial laminates, the conventional design guidance is that the fatigue life depends on the number of cycles, regardless of the frequency of the applied loads.
Figure 3. Biaxial laminates subjected to a constant load (creep test) or an oscillating load (fatigue test) show similar time to failure depending only on the cumulative time under load rather than number of cycles.
As turbine blades get larger and heavier, they will experience increased edgewise fatigue loading. This loading is supported by the panel sections with biaxial laminates. The new data shows damage could develop faster if a conventional design approach is still used. Also, if the observed types of damage are allowed to develop, this further introduces extra off-axis loads into other primary structures, and could lead to early severe blade failure.