AMPTIAC - Advanced Materials, Manufacturing and Testing ...

Figure 1: Uniaxial loading applied to moiré interferometry.shown in Figure 1. However, the question remained as towhether the calculated displacements, hence strains, in eachply were accurate. This is where the second half of the experimentalwork came into play. Incrementally testing the specimensto failure was a lengthy process that ultimately revealedthe initial failure as a single transverse crack isolated in a 45°ply. X-rays were used to observe the failure initiation as wellas its evolution after additional increments of load wereapplied. Since radiography is primarily a 2-D analysis and thespecimens in question were actual structural composite laminatescontaining many 45° plies, it was not possible to specificallyidentify the exact ply that cracked. What the tests didprovide was the initial mode, its relative location, and theload required to induce the crack. This load was then inputinto the model and a ply-by-ply analysis of predicted transversestress (the stress component perpendicular to the fibers)was conducted. The results from this analysis indicated aregion of transverse stress sufficient to initiate a crack in oneof the 45° plies. It also showed that the transverse stresses inall other plies were of insufficient magnitude to initiate failure.It is worth mentioning that the load level at which thefailure initiated was approximately 40% of the ultimatestrength. While the criterion used in this analysis was simplistic,it did provide anecdotal evidence that SVELT accuratelypredicted stresses in the individual plies within the laminate.Coupled together with the moiré results, there was adequatejustification to warrant additional investigation anddevelopment of SVELT.Once the preliminary results were in, chief engineers fromthe aerospace prime contractors were invited to Dayton, Ohioto be briefed about the ongoing program and to solicit theirsupport in continuing to refine the model. The goal was toformulate SVELT into a tool that could easily be transitionedinto existing design practices. In 1995 a formal partnershipreferred to as an Alliance was signed by the Air Force and theindustrial participants including Boeing, Lockheed Martin,McDonnell Douglas, Northrop Grumman, Bell Helicopter-Textron, and the United Technologies Research Center. Thepurpose of the Alliance was to preserve the momentum of theprogram and to ensure that industrial participation would beavailable to help guide future development efforts. Since thattime, many refinements and modifications have been madethat have expanded the capabilities of SVELT. Progress hasbeen reported to the Alliance participants at yearly programreviews and at other times throughout the year. Feedback isprovided back to the SVELT development team to help guidetheir efforts.Enhancements to SVELT (1995-2000)In the early days of composites it was commonly assumed thestresses that exist within a laminated structure would be 2-Din nature. Once real structures were built and tested, it wasobserved that delaminations would sometimes spontaneouslyform and grow. Independent experimental and theoreticalanalyses revealed that 3-D stresses exist in the area adjacent toa free edge such as the side of a plate [5,6]. These stresses canbe either tensile or compressive in nature and their magnitudecan be extremely high. They are commonly referred to as singularities.The nature or “order” of the singularity is extremelydependent upon the lay-up of the composite. In some laminatesthe singularity decays quite quickly as one retreats fromthe free edge while in others there is a considerably largeregion of high stress. Like the edges of a plate, hole boundariesare also free edges but, since they also possess a curvedgeometry, the nature of the singularity becomes dependentupon the angle as measured from a coordinate system transfixedat the center of the hole. As such, the prediction of theirmagnitude and order does not lend itself to exact elasticitysolutions.The original formulation of SVELT could accurately predictthe detailed stress distributions adjacent to holes but accuratemodeling required a very fine mesh and as a result, longsolution times. To improve the accuracy and speed of the predictions,an asymptotic solution was developed and superimposedover the spline approximations [7,8]. Verifying whetherthe new method accurately predicted the angularly dependentstresses resulted in the decision to pursue a new experimentalmethod to measure through-the-thickness displacements. Thecurrent Air Force SVELT Program Manager, DavidMollenhauer, created a new verification method while workingtoward his doctorate at Virginia Tech [9,10]. He developeda technique that allowed him to replicate a diffraction gratingon the inside of a hole drilled through a composite specimen.continues, page 6The AMPTIAC Newsletter, Volume 4, Number 1 3