I think that blades made out of wood or fibre glass and foam are too soft to resist the impact of large hail balls, especially when the rotor is running fast and the balls hit a blade with a large speed. Some of my smaller VIRYA windmills have blades made out of 7.14 % cambered stainless steel sheet and these blades may resist big hail balls because the blade works a knife and simple cuts a ball in two halves. The VIRYA-2.2S is equipped with these blades and has turned for about ten years. There must have been hail in this period but the blades still look new. But the maximum diameter of hail which we have had here is about 1 cm.
The same problem arises if airplanes fly in hail storms. The blades in front of the yet engine must also be strong enough if they come in contact with hail or birds. I think that these blades are made of a very strong metal and also have a sharp leading edge.
My personal observations on different material choices for wind turbine rotor blades, ranked in order of durability:
{lowest durability}
<5 years - Fiberglass shell with hollow core /1
~5 years - Fiberglass shell with foam/balsa core (hobbyist skill) /1
~5 years - Hollow plastic extrusion /1
~5 years - Wood single-board /2
<10 years - Fiberglass with foam/balsa core (professional skill) /3
<10 years - Carbon fiber with foam/balsa core (hobbyist skill) /1
<10 years - Carbon fiber solid pultrusion /1
~10 years - Wood laminated board /2/3
~10 years - Carbon fiber w/ foam/balsa core (professional skill) /4
~25 years - Wood laminated board with fiberglass wrapping and leading edge shield /5
~25 years - Carbon fiber shell w/ foam/balsa core (professional skill) /7/8
>25 years - Hollow aluminum extrusion /7
>25 years - Flat steel plate /6
>50 years - Solid aluminum /5
{highest durability}
Notes
/1 Limited by material variability; usually poor process control
/2 Limited by grade/species of wood selected
/3 Limited by resin and surface finish
/4 Typical of hobbyist aircraft propellors
/5 Typical of type-certificated aircraft propellors, which are actually limited by flight hours
/6 Typical of fan blades
/7 Typical of helicopter blades, which are actually limited by flight hours
/8 Typical of large-scale commercial wind turbines
The list is based on various sources:
- published overhaul life of common aircraft propellors and helicopter rotors
- observations of condition and operation of building air conditioning fans of varying ages
- examination of damage from fiberglass wind turbine blades post-failure
- published specifications of numerous types and sizes of WT blades for sale on e-bay
- published specifications of numerous types and sizes of WT blades used on commercial turbines
- failure reports published on Fieldlines over 15 years
- durability reports published by NREL and other testing agencies
- load tests personally conducted on a variety of carbon fiber and fiberglass aircraft parts in my workplace
- fabrication, direct observation and use of a variety of laminated and wood wind turbine blades
- examination of fiberglass parts found on homebuilt and type-certified aircraft of various ages
- etc.
The list is long because there are lots of practical choices. This is good; you can choose materials that suit any budget or performance level you want.
I have omitted a few, such as sailcloth, knowing very little about these materials.
