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 sim­plest form, a com­pos­ite mate­r­i­al is a com­po­si­tion of at least two mate­r­i­al com­po­nents whose clever com­bi­na­tion results in a new mate­r­i­al with mechan­i­cal prop­er­ties that are fun­da­men­tal­ly dif­fer­ent from those of the two com­po­nents it con­tains. In prac­tice, these are usu­al­ly a sur­round­ing mate­r­i­al (the so-called “matrix”) and a rein­force­ment in the form of a fiber that gives the matrix greater strength and stiff­ness. Com­pos­ites assem­bled in this way can be divid­ed into three main groups:

Poly­mer-matrix com­pos­ites are the most wide­ly used. They are also known under the gener­ic term FRP (fiber-rein­forced plas­tics) and con­sist of a matrix, always based on poly­mers, in which var­i­ous fibers — such as glass, car­bon or aramid — are embedded.

In the auto­mo­tive indus­try and in air­craft con­struc­tion, met­al-matrix com­pos­ites are increas­ing­ly being used, in which most­ly sil­i­cone car­bide fibers are embed­ded in an alu­minum matrix. Com­pos­ites with a ceram­ic matrix are used wher­ev­er very high tem­per­a­tures are expect­ed, such as in aero­space and mil­i­tary appli­ca­tions for jet engines. Short fibers (so-called “whiskers”) of sil­i­con car­bide or boron nitride are embed­ded in the ceram­ic matrix.

Polymer Matrix Composites

Resin sys­tems such as epoxy or poly­ester have — on their own — lim­it­ed uses in the man­u­fac­ture of struc­tur­al com­po­nents. Their mechan­i­cal prop­er­ties are quite low, for exam­ple com­pared to most met­als. How­ev­er, they also have some advan­tages over met­als; in par­tic­u­lar, their abil­i­ty to be used to cre­ate almost any shape, even high­ly com­plex ones.

Mate­ri­als such as glass, aramid and boron do have extreme­ly high ten­sile and com­pres­sive strength. How­ev­er, these prop­er­ties can hard­ly be used in their “pure form”. If these mate­ri­als are sub­ject­ed to a ten­sile load, fail­ure occurs far below the the­o­ret­i­cal load lim­it due to minute defects on the mate­r­i­al sur­face. Fibers are there­fore made from these mate­ri­als; in this way, the sur­face defects, which are equal in num­ber, are dis­trib­uted over a lim­it­ed num­ber of indi­vid­ual fibers. When these iso­lat­ed fibers then fail under a ten­sile load, the rest remains intact and the mate­r­i­al can much bet­ter dis­play its prop­er­ties. Nev­er­the­less, the appli­ca­tion of such fibers remains lim­it­ed to absorb­ing ten­sile loads along the fiber direc­tion, rough­ly com­pa­ra­ble to the indi­vid­ual fibers of a rope.

Only when the fibers are embed­ded in a resin matrix does the full range of mechan­i­cal prop­er­ties become avail­able. The resin intro­duces the applied forces into the mate­r­i­al and dis­trib­utes them even­ly to adja­cent fibers. Sur­face dam­age caused by indi­vid­ual fibers rub­bing against each oth­er or impact on the work­piece sur­face is pre­vent­ed, because each indi­vid­ual fiber is now ful­ly embed­ded in the resin and fixed in posi­tion. High strength and stiff­ness, ease of shap­ing com­plex shapes, and the best resis­tance to weath­er­ing — all com­bined with a low, spe­cif­ic weight; cre­ate com­pos­ites that are far supe­ri­or to met­als in many appli­ca­tions. Because poly­mer-matrix com­pos­ites always com­bine a resin with one or more types of fibers, the result­ing prop­er­ties of the mate­r­i­al are a result of the prop­er­ties of the fibers and the resins as indi­vid­ual materials.

The fiber vol­ume frac­tion depends pri­mar­i­ly on the man­u­fac­tur­ing process. Since the prop­er­ties of the fiber are supe­ri­or to those of the resin in most respects, the gen­er­al rule is that the high­er the fiber con­tent in a lam­i­nate, the bet­ter the mechan­i­cal prop­er­ties. In prac­tice, this rule is lim­it­ed by the require­ment that real­ly all fibers must be com­plete­ly enclosed by resin; if this is not the case (“dry lam­i­nates”), defects occur in the form of extreme­ly fine air pock­ets that can lead to fail­ure under load. Depend­ing on the pro­cess­ing method select­ed, such flaws vary in severity.

In a typ­i­cal hand lam­i­nate for boat­build­ing, fiber vol­ume frac­tions of about 30–40 % are found. In the high-end sec­tor, e.g. in the man­u­fac­ture of air­craft parts, val­ues of up to approx. 70 % can be achieved. The arrange­ment of the fibers in the com­pos­ite is also of great impor­tance, since all fibers exhib­it their high­est strength par­al­lel to the fiber direc­tion (0°). Quite unlike isotrop­ic met­als, the prop­er­ties of a com­pos­ite mate­r­i­al are there­fore high­ly anisotrop­ic, i.e. par­tic­u­lar­ly pro­nounced in one direc­tion. Frac­ture tests of a work­piece with dif­fer­ent­ly ori­ent­ed force appli­ca­tion lead to com­plete­ly dif­fer­ent results. It is there­fore of utmost impor­tance for the suc­cess­ful uti­liza­tion of the mate­r­i­al-typ­i­cal advan­tages that the direc­tion and inten­si­ty of the expect­ed loads are already known in the design phase and are tak­en into account accord­ing­ly. In this way, lam­i­nates can be opti­mized accord­ing to the load sit­u­a­tion; sim­ply put, fibers are placed only where loads are expect­ed to occur, thus avoid­ing unnec­es­sary over­siz­ing in areas or direc­tions sub­ject 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 mate­r­i­al is sub­ject­ed to four dif­fer­ent loads in a fin­ished struc­ture: Ten­sile, com­pres­sive, shear and bend­ing loads.

Ten­sile 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.

Com­pres­sive 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.

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.

Bend­ing 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.