MODELLING DELAMINATION CAUSED BY DEFECTS IN LARGE DIAMETERCOMPOSITE CYLINDERS

摘要

In the framework of the VEGA European smalllauncher programme it was necessary to assess thedamage tolerance of composite solid rocket motorcasings. Advanced manufacturing techniques haveallowed the development of large diameter cylindersleading to mass and cost savings, but as for anycomposite structure defects may occur due to themanufacturing process or due to impact damage. Inparticular delamination at defect locations was animportant failure criterion for the structure.The Virtual Crack Closure Technique (VCCT) based onthe fracture mechanics approach was adopted to modeland predict the onset of delamination. The VCCTmethod has been around for a quarter of a century, butthe application to large diameter cylinders withrelatively thick walls had to be studied in detail. TheVCCT is a method that calculates three strain energyrelease rates for interlaminar tension, interlaminarsliding shear and interlaminar scissoring shear at thedelamination front.An important aspect of the VCCT method was derivingthe correct element size at the delamination front toensure convergence of the strain energy release rates.Once an appropriate element size was ascertained andconvergence verified it was possible to carry outsensitivity studies. Apart from the length and depth ofthe defect, the initial thickness or void can alter theresults in terms of energy release rates.Specimen testing of the specific layup is required toderive allowable values for the interlaminar tension andsliding. In addition a mixed mode failure criterionknown as the power law is derived to predict the onsetof delamination. Besides determining the allowablevalues, the specimen tests will be used to correlate FEmodels and the associated post-processing techniquesapplied.This paper will outline the analytical approach adoptedfor the analysis of the VEGA solid rocket motor caseswith the emphasis on modelling defects and predicting delamination onset using finite element techniques. Theresults will allow design improvements with respect tothe layup to be studied and provide valuable input fordetermining an allowable defect size.