Introduction to Engineering Materials ENGR2000 Chapter 16: Composites. Dr. Coates

Transcription

1 Introdution to ngineering Materials NGR2000 Chapter 16: Coposites Dr. Coates

2 16.1 Introdution Many odern tehnologies require aterials with unusual obinations o properties that annot be et by onventional etals, erais and polyers. Airrat engineers seek strutural aterials with low density, high strength, high stiness, abrasion & ipat resistant, not easily orroded. However Strong aterials have high densities Inreasing strength or stiness results in derease in ipat resistane

3 Priniple o obined ation Better property obinations are ashioned by the obination o two or ore distint aterials. Metal alloys Pearliti steels have alternating layers o errite and eentite

4 Classiiation o Coposites Natural oposites Those that our in nature Syntheti oposites (ous o this hapter) Man-ade

5 Natural oposites - wood & bone Fibrous aterial (ellulose) Woody ell wall (natural polyer) Ceenting aterial (reinoreent) Cross-setion o a bone

6 Two phases in a oposite Matrix phase Continuous and surrounds the other phase Dispersed phase Fibers Partiles

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8 Classiiation o oposites

9 16.2 Large-Partile Coposites

10 16.2 Large-Partile Coposites

11 lasti odulus o partile-reinored oposites

12 l u l where p p is is is is lasti odulus o partile-reinored oposites is p u the elasti odulus the elasti odulus the elasti odulus p o o the volue ration o the volue ration o p p p o the oposite, the atrix phase the partile phase the atrix phaseand the partile phase.

13 Conrete - both atrix & dispersed phases are erais partile reinoreent

14 Two ain types o onrete Portland eent onrete Strutural building aterials Asphalti onrete Paing aterial

15 Portland eent onrete Coposite ade o Portland eent Fine aggregate (sand) Coarse aggregate (gravel) Water

16 Portland eent onrete Sand & gravel at as iller aterial to redue the overall ost Ceent is relatively expensive Sand & gravel is heap Fine sand partiles eetively ill the void spaes between the gravel partiles Bonding between the eent & aggregate ours when the right aount o water is added

17 Portland eent onrete Advantages Can be poured in plae Hardens at roo teperature (even when suberged in water) Disadvantages xtreely brittle Considerable theral expansion & ontration with teperature lutuations Water penetrates into external pores & auses raking at lower teperatures

18 Reinored onrete The strength o Portland eent onrete is inreased by additional reinoreent Steel rods, wires, bars, et.

19 Reinored onrete - using steel rods Advantages o steel as a reinoreent aterial oeiient o theral expansion siilar to that o onrete Not easily orroded in the eent Strong adhesive bonds between the steel & the onrete Adhesion is enhaned by the ontours on the surae o the steel rods, whih perits ehanial interloking between the steel & onrete.

20 Classiiation o oposites

21 Fiber-Reinored Coposites Mehanial harateristis o the oposite depend on Properties o the iber & atrix Degree to whih an applied load is transitted to the ibers by the atrix

22 16.4 Inluene o Fiber Length Critial iber length neessary or eetive strengthening o the oposite: l where d is the iber diaeter, uts d 2 uts is the ultiate tensile strength o the iber, is the iber - atrix bond strength or the shear yield strength o the atrix, whihever is saller.

23 Load Transer to Fiber

24 Classiiation o oposites

25 16.4 Inluene o Fiber Length Continuous ibers: l l Disontinuous or short ibers: l l Matrix deors around the iber suh that there is virtually no stress transerene and little reinoreent by the iber

26 Classiiation o oposites

27 16.5 Inluene o iber orientation and onentration

28 Classiiation o oposites

29 Tensile stress-strain behavior longitudinal loading Assue the iber to be brittle and the atrix to be dutile

30 Brittle iber Dutile atrix * *

31 Fibers & atrix deor elastially The atrix yields The atrix deors plastially while the ibers ontinue to deor elastially

32 Frature Behavior o Coposites Begins with the raking o the brittle phase The stress is then transerred to the surrounding dutile phase I the surrounding phase is not able to withstand the stress, the aterial ratures.

33 Frature Behavior o Coposites Progress o daage in dutile oposites Frature ours in isolated ibers Stresses transerred to surrounding dutile atrix Frature is loally stabilized Overall toughness o the oposite is high

34 Coposite ailure is not atastrophi Not all ibers rature at the sae tie ven ater iber rature, the atrix is still intat Fratured ibers are shorter ibers, but are still ebedded within the atrix Capable o sustaining a diinished load

35 lasti behavior - longitudinal loading Assue no sliding ours Δl l Δl l Δl l - isostrain ondition between the iber and atrix :

36 A A A A A A A A F F F F their volue rations : thesae as are rations the iber and atrix area For ontinuous - iber oposites, : Using The totalore :

37 l l l F F or & atrix : by theibers the load arried Ratio o the oposite. o elasti odulus the longitudin al, is where 1 xpression or stress: Using the isostrain ondition :

38 xaple 16.1 A ontinuous and aligned glass iber-reinored oposite onsists o 40 % vol o glass ibers having an elasti odulus o 69 GPa and 60 % vol o a polyester resin that, when hardened, displays a odulus o 3.4 GPa. a. Copute the elasti odulus o the oposite in the longitudinal diretion b. I the ross-setional area is 250 2, and a stress o 50 MPa is applied in this longitudinal diretion, opute the agnitude o the load arried by eah o the iber and atrix phases.. Deterine the strain that is sustained by eah phase when the stress in part (b) is applied.

39 ???? ? al longitudin l F F MPa A Gpa Gpa oposite reinored - iber glass aligned and ontinuous - : Proble stateent

40 kn F kn F F and Using the two equations in F GPa Solution : A F F F F F : Theory l l : 30

41 Solution : / / / / Theory : A F A F A F A F

42 The ore is F And A Totalstressin the transverse diretion : F A F A - isostress ondition perpendiular to the iber diretion : lasti behavior - transverse loading

43 the oposite. o is the transverse elasti odulus where or strainin the transverse diretion : Total t t t t 1

44 lasti odulus o partile-reinored and iber-reinored oposites t l p p p p p l u u l oposites: - reinored Fiber oposites: Partile - reinored

45 xaple 16.2 Copute the elasti odulus o the oposite aterial in exaple 16.1, but assue that the stress is applied perpendiular to the diretion o iber alignent.

46 Proble stateent : - ontinuous and aligned glass iber - reinored oposite t Gpa 3.4Gpa? Theory : t t Solution : 5.5Gpa

47 Longitudinal tensile strength I Longitudin al strength o oposite: * l * ' ' where, ibers will ail beore the atrix. * * is thestressin the atrix at iber ailure.

48 Transverse tensile strength Oten, the reinoring eet o the ibers is a negative one on the transverse tensile strength.

49 Pratie probles , 16.4, 16.8, 16.10, 16.14, 16.16

50 16.6 The Fiber Phase A sall diaeter iber is uh stronger than the bulk aterial The probability o the presene o a law is less in a sall diaeter iber Classiied as whiskers, ibers, wires on basis o diaeter and harater

51 Whiskers ery thin single rystals xtreely large length-to-diaeter ratios irtually law ree Aong strongest known aterials ery expensive

52 Fibers Polyrystalline or aorphous Sall diaeters Polyers or erais

53 Wires Relatively large diaeters Steel, olybdenu, tungsten -eg. autoobile tire reinoreent, ilaent wound roket asings, wire wound high pressure hoses

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56 Classiiation o oposites

57 Strutural oposites Coposed o hoogenous and oposite aterials Properties depend on Constituent aterials Geoetrial design o the oposite

58 Classiiation o oposites

59 16.14 Lainar Coposites Sheets or panels with a preerred high-strength diretion are staked or eented together

60 Lainate Coposites Fibers arranged along dierent diretions

61 Lainate Coposites Cross-plied lainate oposites Good shear resistane Transversely isotropi

62 Two-diensional abri weave arhiteture Plain weave Five harness satin weave

63 16.15 Sandwih Panels ore

64 Are oposites anisotropi or isotropi aterials?