Unidirectional specimens

The data shown in Figure 7.15 were obtained from two-ply T300/P1700 unidirectional specimens that were compression molded in a 76.2 mm (3 in.) square steel mold. The cross-sections of the consolidated specimens were examined by optical microscopy and the degree of intimate contact was determined as the amount of the interply region that was in contact divided by the total area of the cross-section. Additional details of the experimental procedures are given in Reference 22. 

The commercial composite materials being marketed today are optimized in order to make the interfacial properties acceptable in the sense that they will not fail at such low levels as to detract from the overall composite behavior. Considering a unidirectional specimen, where the fibers are all aligned parallel to each other, commercial systems can be described by a rule of mixtures661 relationship (Fig. 10). Properties of the matrix and fiber can be linearly combined based on the volume fraction of each constituent. For example, the longitudinal tensile modulus is the sum of the proportion of each component. The interface in these systems is considered ideal in that it efficiently transmits forces between fiber and matrix without failure. Using this model as a basis for micromechanical analysis and discussion, the magnitude of the forces present at the interface can be predicted. 

Some fibre-bridging by fibre bundles from the 90°-plies perpendicular to the direction of delamination propagation in most specimens for the symmetric and the non-symmetric lay-up, varying in amount from one specimen to the other (Fig. 1) as well as fibre bridging in the unidirectional lay-up. The amount in the unidirectional specimens, if quantified, was low or medium. 

The other difficulty concerns the tendency for different failure modes to occur. For example, materials with mat reinforcement will fail in tension, while a fully unidirectional specimen will often fail on the compression face through local microbuckling assisted by the local compression and shear loads at the central loading roller(s). 

Fracture and impact properties should be specially mentioned since they are probably the subject of more argument than any other composite property. This arises because of the extreme complexity of composite failure under impact conditions. To use information generated on unidirectional specimens to predict the behaviour of laminates and structures is extremely difficult, if not impossible, but the reader should study Chapter 6 for more information. 

Unidirectional specimen or nonunidirectional laminates (ASTM D 3410), chopped fiber reinforced specimens (ASTM D 695). 

The Tg s taken as the maximum of the tanS curve and as the onset (i.e. ttie intersection of interpolated lines) of the E drop are compared in Table 4 for test carried out on unidirectional specimens and in Table 5 for test carried out on [ 45°]i4 specimens. 

Inflated Diaphragm Method (ASTM D3886). This method is appHcable both to woven and knitted fabrics. The specimen is abraded by mbbing either unidirectionally or multidirectionally against an abradant having specified surface characteristics. The specimen is supported by an inflated mbber diaphragm under a constant pressure. Evaluation of abrasion resistance can be either by determination of the number of cycles required to wear through the center of the fabric completely or by visual examination of the specimens after a specified number of cycles. 

First, consider uniaxial tension loading in the 1-direction on a flat piece of unidirectionally reinforced lamina where only the gage section is shown in Figure 2-20. The specimen thickness is not just one lamina, but several laminae all of which are at the same orientation (a single lamina would be too fragile to handle). The strains and E2 are measured so, by definition,... 

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

Typical tension stress-strain curves of baseline and irradiated unidirectional T300/934 composites tested in [0] and [90] orientations at three different temperatures (121 are shown in Figures 11 and 12. Irradiation had essentially no effect on the fiber-dominated tensile modulus of the [0] specimen and caused only a small (10-15%) reduction in strength at the low and elevated temperatures. For the matrix-dominated [90] laminates, irradiation caused a very substantial decrease in strength at three test temperatures (-38% at -157°C, -26% R.T., -13% 121°C). Irradiation increased the modulus at -157°C and R.T. (10 - 15%), but lowered it at 121°C (-15%). These results are consistent with results obtained on the neat resin specimens discussed above. 

The [10°] off axis tension specimen shown in Fig 3.23 is another simple specimen similar in geometry to that of the [ 45 ]s tensile test. This test uses a unidirectional laminate with fibers oriented at 10° to the loading direction and the biaxial stress state (i.e. longitudinal, transverse and in-plane shear stresses on the 10° plane) occurs when it is subjected to a uniaxial tension. When this specimen fails under tension, the in-plane shear stress, which is almost uniform through the thickness, is near its critical value and gives the shear strength of the unidirectional fiber composites based on a procedure (Chamis and Sinclair, 1977) similar to the [ 45°]s tensile test. 

The Li-Loos intimate contact model was verified for compression molded unidirectional graphite-polysulfone and graphite-PEEK (APC-2) laminae and graphite-PEEK (APC-2) cross-ply laminates. The degrees of intimate contact of the unidirectional and cross-ply specimens were measured by optical microscopy and scanning acoustic microscopy, respectively. The predicted degrees of intimate contact agreed well with the measured values for both the unidirectional and cross-ply specimens processed at different temperature and pressures. 

Figure 13.12 Fiber strength test articles, (a) the impregnated fiber strand specimen, unidirectional lamina tensile specimen, and NOL ring (b) 4-in. biaxial tube test specimen...

In-Plane Shear Properties. The basic lamina in-plane shear stiffness and strength is characterized using a unidirectional hoop-wound (90°) 0.1 -m nominal internal diameter tube that is loaded in torsion. The test method has been standardized under the ASTM D5448 test method for in-plane shear properties of unidirectional fiber-resin composite cylinders. D5448 provides the specimen and hardware geometry necessary to conduct the test. The lamina in-plane shear curve is typically very nonlinear [51]. The test yields the lamina s in-plane shear strength, t12, in-plane shear strain at failure, y12, and in-plane chord shear modulus, G12. 

The mode I delamination fracture toughness is measured on flat coupons using the Double Cantilever Beam (DCB) test method. The test method utilizes a unidirectional rectangular composite specimen of uniform thickness with nonadhesive insert (as a delamination initiator) at the midplane. This test method is currently in the process of standardization by ASTM. 

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