ملخص: Most thermoplasticp olymers exhibit a nonlinear stress-strainb ehaviour that intensifies with the increase of temperature. When a polymeric or mineral phase is added into these materials, the matrix nonlinearity combines with that of damage accumulationto producet he nonlinearb ehaviouro f the composite. The discernment between the different dissipative forms of energy responsible for this nonlinear behaviouri s neededf or the predictiono f somep rocessin ducedm echanicapl roperties of sheett hermoplasticc ompositesa nd for the accuratee stimateo f residuals tiffnessa nd fracturep ropertieso f structuralt hermoplastic omposites This work deals with the non-linears tress-strainb ehaviouro f thermoplastic composites, particularly glass beads, CaCO3 and talc filled PP composites. We investigatedth e effect of concentrations, hape,s ize and orientationo f the filler on the nonlinearb ehaviouro f filled PP. The inspectiono f the microstructuracl hangesc aused by the tensile loading provides an idea on the damagep rocessesin volved in each composites ystem. The analysist echniquea doptedin this studyi s basedo n the secant compliance-strainfo rm of expressiono f the true mechanicabl ehaviouro f materialsa nd exploits somec onceptso f micromechanicasn dc ontinuumd amagem echanicsth eories. This methodi s usedt o dissociateb etweent he nonlinearity due to the matrix material and that due to damagea ccumulationa nd thereforet o establishlo ss of stiffness-strain relationshipsfo r eacho f the composites ystemsin vestigatedin this work. First of all we have validated behaviour of the polyolefins, HDPE x model by which the nonlinear stress-strain PP, is approximated using a linear secant the and compliance-strairne lationship.S ubsequentltyh et rue stress-straibne haviouro f PP have been divided in three srages: a linearly elastic stage followed by a nonlinear stage, however having a linear secantc ompliance-strairne lationship,a nd finally a perfectly plastic stage. Having regardt o the innovativec haractero f the secantc ompliance-strain form of expressiono f the true mechanicabl ehaviouro f materialsn ew dehnitions of elastica nd yieldingl imits arep roposedin this thesis. The additived ecompositiono f the secantc omplianceh asl ed to a viscoelasticd issipativela w of energyf or PP. The stress-strainb ehavioura nalysiso f glassb eadsa nd CaCO, filled PP has revealeda n additionals tagei n the behaviouro f filled thermoplasticc ompositesn, amed here as the stage of filler/matrix debonding saturation. Accordingly the stress-strain behaviouro f filled PP hasb eend ividedi nto four stagesa: linear elastics tagef ollowed by a nonlinears tageo f continuousfi ller/matrix debondingth en by a anothern onlinear stagea ssociatedto the saturationo f cavitationa nd finally a stageo f perfectly plastic behaviour. Debonding and saturation stages are similar to the stage of nonlinear viscoelasticb ehaviouro f pureP P. It hasb eenf oundi n this studyt hat the saturationa nd yielding compliancies as well as the debonding rate can be considered as intrinsic propertiesf or every filler/PP systemin dependentloyf the filler volumef raction. Some rules from micromechanicsth eory have been useds uccessfullyto predict the elastic xi properties of the PP composite systems examined in this research. The damage mechanicsc oncepto f effectives tressa llowedu s to associateth e nonlinearv iscoelastic and plastic behaviours of the composite, after debonding saturation, to the nonlinear behaviour of the pure matrix. However, this concept when used alone failed in the interpretationo f the cavitations tagey et an empiricale quations howst hat the behaviour of the composited uring this stagem ustb e governedb y a singlel aw. It hasb eenf ound in this study that the saturationo utsetc orrespondsto the intersectiono f the secant compliance-strain curve of the matrix beyond its elastic limit with the secant compliance-straincu rveso f the differentc omposites ystemsa fter the onseto f damage. According to this original definition of the saturation threshold a constitutive law describingt he saturations tageh asb eene stablished.T his law reflectst he simultaneity and the linear independencree latively to deformationo f the matrix relaxation and filler/matrix debondingp rocesses;t he kinetic of this phenomenah as been explained using a pseudo-materiaml odel. The yieldingc ompliancew as found to be an intrinsic property of filled PP independentlyth e filler shapeo r size as long as this filler is isometric and the matrix integrity is intact. The effect of the filler size can be noticed only in the debondingra te. The analysis techniqueo f nonlinear stress-strainb ehaviour of thermoplastic composite,d evelopedin this work, haveb eena pplieds uccessfullyto characterizeth e nonlinearb ehaviouro f talc reinforcedP P. This techniqueh asb eeni mproved in order xii to considert he effecto f the skin/coreo rientationd istributiono n the nonlinearb ehaviour of suchc omposites-. T he saturationth resholdw as locatedi n the plastic domain of this materialf ollowing a correctiono f the matrixn onlinearityp arametear fter we noticedt hat the core breaks instantly at the start of damage. As a consequenceo f these considerationsw e confirmedt he absenceo f the saturations tagea nd for this reasont he stress-strainb ehaviouro f talc reinforcedP P wasd escribeda s it follows: a linear elastic stagef ollowed by a talc/PPd ebondings tage,w heret he debondingis instantaneouisn the core and progressive in the skin, which extends into the plastic stage. This observationh asb eenu sedt o illustratet he moderationo f the rate of debondingb etween orientedt alc flakesa ndP P relativelyt o the debondingb etweenis ometricf illers and PP. An anisotropicd amagem odelw as built accordingt o the compositem icrostructurea nd used to describeq uite accuratelyt he mechanicabl ehaviouro f the composited uring progressived ebondinga ndy ieldings tages

طبعة: Canada : Université de Montréal
لغة: فرنسية
الوصف المادي: 185 p. ill. ;30 cm.
الشهادة: Doctorat
مؤسسة مناقشة الرسالة: Ecole Polytechnique

ملاحظة: Graphe ; Bibliogre p(183-185)