Fiber composites since 1985

Fiber composite

Fiber com­pos­ites offer excep­tion­al prop­er­ties that are dif­fi­cult or impos­si­ble to match by tra­di­tion­al mate­ri­als such as steel, alu­minum or wood. Today, com­pos­ites are used in near­ly every high-per­for­mance dynam­ic com­po­nent on land, sea and air. Fiber com­pos­ites are what make it pos­si­ble to devel­op and pro­duce ultra-light­weight com­po­nents that are pro­tect­ed against cor­ro­sion while being able to with­stand the high­est loads. The need to pro­duce ever lighter and more effec­tive com­po­nents increas­ing­ly requires the use of these materials.

Benefits of fiber composite

In order to grasp and under­stand the full range of pos­si­bil­i­ties offered by mod­ern com­pos­ites, a good knowl­edge of the wide vari­ety of indi­vid­ual mate­ri­als and their prop­er pro­cess­ing is required. Here we pro­vide an insight into the the­o­ret­i­cal prin­ci­ples of com­pos­ites tech­nol­o­gy, intro­duce the var­i­ous mate­ri­als and their prop­er­ties, and describe the com­mon pro­cess­ing tech­niques used to man­u­fac­ture a wide vari­ety of com­po­nents from these materials.

Theory of composite materials

In its sim¬≠plest form, a com¬≠pos¬≠ite mate¬≠r¬≠i¬≠al is a com¬≠po¬≠si¬≠tion of at least two mate¬≠r¬≠i¬≠al com¬≠po¬≠nents whose clever com¬≠bi¬≠na¬≠tion results in a new mate¬≠r¬≠i¬≠al with mechan¬≠i¬≠cal prop¬≠er¬≠ties that are fun¬≠da¬≠men¬≠tal¬≠ly dif¬≠fer¬≠ent from those of the two com¬≠po¬≠nents it con¬≠tains. In prac¬≠tice, these are usu¬≠al¬≠ly a sur¬≠round¬≠ing mate¬≠r¬≠i¬≠al (the so-called ‚Äúmatrix‚ÄĚ) and a rein¬≠force¬≠ment in the form of a fiber that gives the matrix greater strength and stiff¬≠ness. Com¬≠pos¬≠ites assem¬≠bled in this way can be divid¬≠ed into three main groups:

Poly¬≠mer-matrix com¬≠pos¬≠ites are the most wide¬≠ly used. They are also known under the gener¬≠ic term FRP (fiber-rein¬≠forced plas¬≠tics) and con¬≠sist of a matrix, always based on poly¬≠mers, in which var¬≠i¬≠ous fibers ‚ÄĒ such as glass, car¬≠bon or aramid ‚ÄĒ are embedded.

In the auto¬≠mo¬≠tive indus¬≠try and in air¬≠craft con¬≠struc¬≠tion, met¬≠al-matrix com¬≠pos¬≠ites are increas¬≠ing¬≠ly being used, in which most¬≠ly sil¬≠i¬≠cone car¬≠bide fibers are embed¬≠ded in an alu¬≠minum matrix. Com¬≠pos¬≠ites with a ceram¬≠ic matrix are used wher¬≠ev¬≠er very high tem¬≠per¬≠a¬≠tures are expect¬≠ed, such as in aero¬≠space and mil¬≠i¬≠tary appli¬≠ca¬≠tions for jet engines. Short fibers (so-called ‚Äúwhiskers‚ÄĚ) of sil¬≠i¬≠con car¬≠bide or boron nitride are embed¬≠ded in the ceram¬≠ic matrix.

Polymer Matrix Composites

Resin sys¬≠tems such as epoxy or poly¬≠ester have ‚ÄĒ on their own ‚ÄĒ lim¬≠it¬≠ed uses in the man¬≠u¬≠fac¬≠ture of struc¬≠tur¬≠al com¬≠po¬≠nents. Their mechan¬≠i¬≠cal prop¬≠er¬≠ties are quite low, for exam¬≠ple com¬≠pared to most met¬≠als. How¬≠ev¬≠er, they also have some advan¬≠tages over met¬≠als; in par¬≠tic¬≠u¬≠lar, their abil¬≠i¬≠ty to be used to cre¬≠ate almost any shape, even high¬≠ly com¬≠plex ones.

Mate¬≠ri¬≠als such as glass, aramid and boron do have extreme¬≠ly high ten¬≠sile and com¬≠pres¬≠sive strength. How¬≠ev¬≠er, these prop¬≠er¬≠ties can hard¬≠ly be used in their ‚Äúpure form‚ÄĚ. If these mate¬≠ri¬≠als are sub¬≠ject¬≠ed to a ten¬≠sile load, fail¬≠ure occurs far below the the¬≠o¬≠ret¬≠i¬≠cal load lim¬≠it due to minute defects on the mate¬≠r¬≠i¬≠al sur¬≠face. Fibers are there¬≠fore made from these mate¬≠ri¬≠als; in this way, the sur¬≠face defects, which are equal in num¬≠ber, are dis¬≠trib¬≠uted over a lim¬≠it¬≠ed num¬≠ber of indi¬≠vid¬≠ual fibers. When these iso¬≠lat¬≠ed fibers then fail under a ten¬≠sile load, the rest remains intact and the mate¬≠r¬≠i¬≠al can much bet¬≠ter dis¬≠play its prop¬≠er¬≠ties. Nev¬≠er¬≠the¬≠less, the appli¬≠ca¬≠tion of such fibers remains lim¬≠it¬≠ed to absorb¬≠ing ten¬≠sile loads along the fiber direc¬≠tion, rough¬≠ly com¬≠pa¬≠ra¬≠ble to the indi¬≠vid¬≠ual fibers of a rope.

Only when the fibers are embed¬≠ded in a resin matrix does the full range of mechan¬≠i¬≠cal prop¬≠er¬≠ties become avail¬≠able. The resin intro¬≠duces the applied forces into the mate¬≠r¬≠i¬≠al and dis¬≠trib¬≠utes them even¬≠ly to adja¬≠cent fibers. Sur¬≠face dam¬≠age caused by indi¬≠vid¬≠ual fibers rub¬≠bing against each oth¬≠er or impact on the work¬≠piece sur¬≠face is pre¬≠vent¬≠ed, because each indi¬≠vid¬≠ual fiber is now ful¬≠ly embed¬≠ded in the resin and fixed in posi¬≠tion. High strength and stiff¬≠ness, ease of shap¬≠ing com¬≠plex shapes, and the best resis¬≠tance to weath¬≠er¬≠ing ‚ÄĒ all com¬≠bined with a low, spe¬≠cif¬≠ic weight; cre¬≠ate com¬≠pos¬≠ites that are far supe¬≠ri¬≠or to met¬≠als in many appli¬≠ca¬≠tions. Because poly¬≠mer-matrix com¬≠pos¬≠ites always com¬≠bine a resin with one or more types of fibers, the result¬≠ing prop¬≠er¬≠ties of the mate¬≠r¬≠i¬≠al are a result of the prop¬≠er¬≠ties of the fibers and the resins as indi¬≠vid¬≠ual materials.

The fiber vol¬≠ume frac¬≠tion depends pri¬≠mar¬≠i¬≠ly on the man¬≠u¬≠fac¬≠tur¬≠ing process. Since the prop¬≠er¬≠ties of the fiber are supe¬≠ri¬≠or to those of the resin in most respects, the gen¬≠er¬≠al rule is that the high¬≠er the fiber con¬≠tent in a lam¬≠i¬≠nate, the bet¬≠ter the mechan¬≠i¬≠cal prop¬≠er¬≠ties. In prac¬≠tice, this rule is lim¬≠it¬≠ed by the require¬≠ment that real¬≠ly all fibers must be com¬≠plete¬≠ly enclosed by resin; if this is not the case (‚Äúdry lam¬≠i¬≠nates‚ÄĚ), defects occur in the form of extreme¬≠ly fine air pock¬≠ets that can lead to fail¬≠ure under load. Depend¬≠ing on the pro¬≠cess¬≠ing method select¬≠ed, such flaws vary in severity.

In a typ¬≠i¬≠cal hand lam¬≠i¬≠nate for boat¬≠build¬≠ing, fiber vol¬≠ume frac¬≠tions of about 30‚Äď40 % are found. In the high-end sec¬≠tor, e.g. in the man¬≠u¬≠fac¬≠ture of air¬≠craft parts, val¬≠ues of up to approx. 70 % can be achieved. The arrange¬≠ment of the fibers in the com¬≠pos¬≠ite is also of great impor¬≠tance, since all fibers exhib¬≠it their high¬≠est strength par¬≠al¬≠lel to the fiber direc¬≠tion (0¬į). Quite unlike isotrop¬≠ic met¬≠als, the prop¬≠er¬≠ties of a com¬≠pos¬≠ite mate¬≠r¬≠i¬≠al are there¬≠fore high¬≠ly anisotrop¬≠ic, i.e. par¬≠tic¬≠u¬≠lar¬≠ly pro¬≠nounced in one direc¬≠tion. Frac¬≠ture tests of a work¬≠piece with dif¬≠fer¬≠ent¬≠ly ori¬≠ent¬≠ed force appli¬≠ca¬≠tion lead to com¬≠plete¬≠ly dif¬≠fer¬≠ent results. It is there¬≠fore of utmost impor¬≠tance for the suc¬≠cess¬≠ful uti¬≠liza¬≠tion of the mate¬≠r¬≠i¬≠al-typ¬≠i¬≠cal advan¬≠tages that the direc¬≠tion and inten¬≠si¬≠ty of the expect¬≠ed loads are already known in the design phase and are tak¬≠en into account accord¬≠ing¬≠ly. In this way, lam¬≠i¬≠nates can be opti¬≠mized accord¬≠ing to the load sit¬≠u¬≠a¬≠tion; sim¬≠ply put, fibers are placed only where loads are expect¬≠ed to occur, thus avoid¬≠ing unnec¬≠es¬≠sary over¬≠siz¬≠ing in areas or direc¬≠tions sub¬≠ject to less or no load.

Anoth­er impor­tant dif­fer­ence to met­als is the def­i­n­i­tion of the mate­r­i­al prop­er­ties by the proces­sor. Any­one who process­es met­als can no longer change their prop­er­ties, once giv­en, to the advan­tage or dis­ad­van­tage of the prod­uct to be man­u­fac­tured. The sit­u­a­tion is quite dif­fer­ent with com­pos­ites: Here, the mate­r­i­al is cre­at­ed only in the course of man­u­fac­tur­ing a com­po­nent; its qual­i­ty depends to a large extent on the qual­i­ty of the work of the indi­vid­ual proces­sor. By act­ing care­ful­ly and respon­si­bly, he can make a deci­sive con­tri­bu­tion to ensur­ing that the prop­er­ties expect­ed in the­o­ry are actu­al­ly achieved. On the oth­er hand, his neg­li­gence or igno­rance can also lead to the fail­ure of entire assemblies!

Strains & Loads:

Each mate¬≠r¬≠i¬≠al is sub¬≠ject¬≠ed to four dif¬≠fer¬≠ent loads in a fin¬≠ished struc¬≠ture: Ten¬≠sile, com¬≠pres¬≠sive, shear and bend¬≠ing loads.

Ten¬≠sile load

The fig­ure shows a ten­sile load act­ing on a com­pos­ite com­po­nent. Its resis­tance to such a load depends essen­tial­ly on the stiff­ness and ten­sile strength of the fibers used.

Verbundwerkstoffe Zuglast

Com¬≠pres¬≠sive load

Here, too, the strength of the com­pos­ite depends on the prop­er­ties of the fibers, with the dif­fer­ence that the resin com­po­nent resists com­pres­sion much bet­ter than ten­sion. The most impor­tant task of the resin in this case, then, is to hold the fibers togeth­er in their arrangement.

Verbundwerkstoffe Drucklast

Shear load

This force attempts to dis­place the indi­vid­ual fibers against each oth­er. This is where the resin plays the essen­tial role; dis­trib­ut­ing the force over as large an area as pos­si­ble. The resin must there­fore not only have very good mechan­i­cal prop­er­ties, but also good adhe­sion to the enclosed fibers. ILSS (Inter­lam­i­nar Shear Strength) refers to the strength in a mul­ti­lay­er laminate.

Verbundwerkstoffe Scherlast

Bend¬≠ing load

Bend­ing forces rep­re­sent a com­bi­na­tion of the three afore­men­tioned loads; the upper region of the com­po­nent is under com­pres­sion, the cen­ter expe­ri­ences shear load­ing, and the low­er region is pulled.

Verbundwerkstoffe Biegelast

Com­par­i­son with oth­er struc­tur­al materials

Due to the fac¬≠tors described above, there is a wide range of pos¬≠si¬≠ble mechan¬≠i¬≠cal prop¬≠er¬≠ties that a fiber com¬≠pos¬≠ite mate¬≠r¬≠i¬≠al can exhib¬≠it. Even if only one type of fiber is con¬≠sid¬≠ered, the prop¬≠er¬≠ties can vary by a fac¬≠tor of 10 as a result of dif¬≠fer¬≠ent fiber pro¬≠por¬≠tions and ori¬≠en¬≠ta¬≠tions! In the fol¬≠low¬≠ing, these prop¬≠er¬≠ties are com¬≠pared with each oth¬≠er; the low¬≠est val¬≠ues in each case are based on sim¬≠ple man¬≠u¬≠fac¬≠tur¬≠ing process¬≠es and mate¬≠r¬≠i¬≠al forms (e.g. fiber injec¬≠tion process¬≠es), the high¬≠er ones on high-end process¬≠es (e.g. uni¬≠di¬≠rec¬≠tion¬≠al lam¬≠i¬≠nates from auto¬≠claves) from aero¬≠space. Val¬≠ues such as strength and stiff¬≠ness of oth¬≠er mate¬≠ri¬≠als, such as alu¬≠minum alloys, are also list¬≠ed for comparison.

Zugfestigkeit

Zugfestigkeit 2

Spezifisches Gewicht

The above graph¬≠ics impres¬≠sive¬≠ly show the range of mechan¬≠i¬≠cal prop¬≠er¬≠ties that fiber com¬≠pos¬≠ites can exhib¬≠it. They can be sum¬≠ma¬≠rized with the term: ‚ÄúHigh strength and stiff¬≠ness at low weight‚ÄĚ. These prop¬≠er¬≠ties are what make fiber com¬≠pos¬≠ites so extreme¬≠ly inter¬≠est¬≠ing for many uses in struc¬≠tur¬≠al appli¬≠ca¬≠tions. In par¬≠tic¬≠u¬≠lar, of course, this applies to appli¬≠ca¬≠tions in which large mass¬≠es have to be accel¬≠er¬≠at¬≠ed and brought to a stand¬≠still again from an eco¬≠nom¬≠ic point of view, i.e. in the field of trans¬≠porta¬≠tion in the broad¬≠est sense (air¬≠planes, ships, cars, trains, etc); here, mass reduc¬≠tion while ensur¬≠ing (safe!) strength plays the deci¬≠sive role.

The fol­low­ing graphs show the strength prop­er­ties of the var­i­ous mate­ri­als and mate­r­i­al com­bi­na­tions in rela­tion to their weight; these are there­fore spe­cif­ic val­ues. In con­trast, the above graphs 6 and 7 show absolute val­ues, where as a rule the strongest mate­r­i­al is also the heaviest.

Spezifische Zugfestigkeit

Zugmodul

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