Bell Helicopter in Hurst, Texas (basically a suburb of Fort Worth) NDI'd their V-22 Grip in the early days of the program using MRI Magnetic Resonance Imaging, around 1985 to 1989. The Grip puts the angle into the rotor blades as they rotate, and is a thick-walled, hollow, dog-bone-like composite structure, probably 5 foot long or so, that surrounds the centrifugal, load-bearing yoke which is attached to the rotor blades. Initially Bell used the medical MRI resources of the local HEB Hurst Euless Bedford Hospital, but I remember there was a budget proposed before I left in 1989 for Bell to buy their own machine, as the MRI fees apparently pretty much paid for the local Hospital's machine. I do not know if Bell purchased the proposed MRI. It was apparently the only way they could effectively examine the Grip structure, which an Aviation Weekly report at the time described as the single most complex composite structure being built. As the slightly earlier composite Beech Starship was having certification problems with the FAA, the MRI was Bell's approach to getting the FAA to understand and accept the relatively new Graphite Epoxy composite materials. I remember conversations that related porosity issues with the Grip, but when destructively tested, it proved fantastically strong, exceeding the anticipated life by half again as I recall - even with the MRI-observed "defects". The MRI inspection was a major factor in understanding and accepting this highly complex and thick-walled composite part. Obviously, composites could not be treated like metals, where any porosity would significantly reduce their strength. Even with very minor, and now acceptable porosity issues, the V-22 is a great aircraft. Despite a few early technical issues, resulting in a couple of highly unfortunate crashes, which had NOTHING to do with the integrity of the structural composites nor with the basic airworthiness of the tilt rotor design, and which is quite typical for new and highly innovative aircraft designs if anyone reviews similar programs, it is currently serving extremely well with the US armed forces. Maurice Grandsoult ex-Bell Helicopter Rotor Blade Manufacturing Engineer Jan 2009
This is a very interesting post. Can you comment on the ability of MRI to resolve small defects within the laminate such as individual pores in groups of porosity, fibre waviness (also called marcelling) and resin rich areas ? I have seen very impressive X-ray and Microfocus X-ray images on a number carbon fibre laminates using computed tomography (CT) which can image very small features within the laminate and wondered how it compared to the MRI systems that you have worked with. For example see:
My interest here is in the use of CT methods to 'see' what is actually in the composite material in order to validate and qualify measurements made using ultrasound techniques, much in the same way that standard X-ray is used to verify and size weld defects in test specimens that are to be inspected and sized using ultrasound. My understanding is that standard X-ray imaging is pretty difficult on composites, and although impressive, high resolution Microfocus X-ray CT can usually only image a small volume of material.