Meet the WTS Board - Martin Ansell and Windfarms

The Wood Technology Society
,
17 Dec 2019

Martin Ansell with his wife Frances, enjoying fell walking on Hellvellyn, Lake District.

Martin and Frances Ansell enjoying fell walking on Hellvellyn, Lake District.

 

Martin Ansell is a long-standing Board Member of the Wood Technology Society and former President of the Institute of Wood Science from 1994 to 1996. He was asked to explain why laminated wood became a material of choice for the manufacture of commercial wind turbine blades.

Today, windfarms are a common feature of the UK landscape and in 2017 15% of the UK’s electricity was derived from the wind. Back in the late 1970s and 1980s the fledgling wind turbine industry was slowly emerging, seeking credibility as a viable source of alternative energy. The British Wind Energy Association was formed in 1978 and held annual conferences where news of new developments was eagerly awaited. Early designs for turbine blades in the UK and USA were based on high density (approx. 7,500 kg.m-3) steels but the dynamic loads created by aerodynamic forces (producing lift and rotation) together with gravity self-loading led to the development of fatigue cracks in the steel and catastrophic blade failures. Many of the early Californian wind farms were built in haste to meet the deadline for the award of tax credits and the small capacity (100kW) turbines relied on medium density (approx. 2,500 kg.m-3) glass fibre-reinforced plastics for blade fabrication and these blades frequently failed by delamination and fracture of the hub end fixings. This is where Martin became involved in the fatigue evaluation of low density (approx. 500 kg.m-3) laminated wood for turbine blades. Wind turbine blades are hollow aerofoil structures and multi-layer thick D-spar walls are able to withstand buckling loads much better than thinner, stiffer materials such as carbon-reinforced plastics. Furthermore, steel studs can be directly bonded into the roots of the wooden blades to make bolted connections to the turbine hub.

Working with Jim Platts and Mark Hancock at Gifford Technology, Southampton, Martin was responsible for the fatigue testing of wood laminated from 4mm veneers, funded by a series of research grants from the EPSRC and the UK Department of Energy (later DTI). The bonded wood technology was developed originally for boat-building by Gougeon Brothers in the USA using the WEST epoxy system for bonding veneers. Back in the first half of the 20th century the aircraft designer Fokker stated that “fatigue in properly seasoned wood is unknown”. However, all engineering materials subjected to cyclic loads accumulate progressive damage ultimately leading to catastrophic failure. Martin managed a long-term fatigue testing programme of laminated wood which, as well as establishing design data for fatigue lives under cyclic and complex loads, allowed the mechanism of fatigue damage to be understood. In conjunction with BRE, very thin longitudinal sections of wood were made with a wood microtome at various stages of fatigue loading and examined in an optical microscope. Progressive fatigue failure was initiated by the formation of compression kinks in single wood cell walls. This micro-damage spread to double wood cell walls and eventually formed micro-buckles which with time became major macro-scale compression creases leading to ultimate failure of the wood laminate. As a result of the fatigue work the safe design loads permissible for a laminated wood blade subjected to complex loads with a 25 year design life could be determined.

Images of fatigue damaged cells, (a) compression kinks (5 microns wide), (b) compression crease

Images of fatigue damaged cells, (a) compression kinks (5 microns wide), (b) compression crease

 

Gifford Technology expertise was transferred to several companies including the Wind Energy Group (Taylor Woodrow) in a series of takeovers and finally became the property of Aerolaminates. Laminated wood-epoxy wind turbine blades were manufactured by Aerolaminates on the Isle of Wight. In 1998, following the takeover of Aerolaminates, NEG Micon built a new facility for the manufacture of hybrid laminated wood/carbon fibre-reinforced hybrid blades and in 2003 developed a commercial 110 meter diameter rotor for the Vestas Wind Systems A/S NM 110/4200 wind turbine with a rated power of 4.2 MW.

Wind Energy Group turbine with laminated wood blades

Wind Energy Group turbine with laminated wood blades

 

As well as his track record in wood science and timber engineering Martin is currently involved in research on other natural materials in his role as Honorary Reader at the University of Bath. Following the failure of several fibrous plaster ceilings in London theatres Martin is part of a team working with Historic England to determine the mechanisms of failure in these ceilings some of which are well over 100 years old. Fibrous plaster in its basic form is comprised of hessian fibre and wooden laths in a matrix of hydrated gypsum plaster. Another preoccupation is writing articles on beer and pubs for a beer magazine covering the Bath and Bristol region. Martin also volunteers for the National Trust and is part of the team eradicating American Signal crayfish from the Prior Park Lakes in Bath and is also a member of a group evaluating historic features of the Bath landscape and performing geophysical surveys to discover hidden treasures including Roman roads, Iron Age field boundaries and the foundations of demolished historic buildings. His favourite occupation is walking the Lake District fells.

 

 

 

Martin Ansell, 15th December 2019