Interlaminar shear tests

Figure 4 Interlaminar shear test by short beam flexure failure modes.

In general, the notched Charpy (ISO 179) [26] and Izod (ISO 180) [27] tests are not meaningful for composites, and ISO 179 recommends that only unnotched specimens should be tested. The difficulty with these tests is that in the notched condition the majority of specimens tested perpendicular to the plane of the test panel delaminate at the root of the notch. This reduces the specimen to a thinner version of the unnotched specimen, which as described above for the interlaminar shear test (see Section 5.4) is susceptible to compression-initiated failures under complex local loads. Specimens cut in the plane of the laminate or sheet will be less susceptible to delamination at the notch tip, and crack growth will be possible from the notch tip. However, other compression shear failure modes are still possible in some composites, and they will not be loaded in this direction in most applications. 

Interlaminar shear testing was carried out using the short beam shear technique in 3-point bending at a displacement rate of 0.02 /minute from room temperature to 1300 C. Results of the interlaminar shear strength [31] forthe 0/90° cross-ply Nicalon/BN/SiC/BS AS composites, containing 38-40 volume per cent of fibers are given in Table 5. The failure mode for all the test specimens was shear. 

The interlaminar shear strength (ILSS) is the value of the shear strength producing a delamination between two composite layers along the plane of their interface. The measurement is made by a three-point deflection test with the supports very close together. 

In addition to the direct measurements of fiber-matrix interface properties discussed in Section 3.2, a number of testing techniques have been devised to assess the fiber-matrix interface bond quality by inference from the gross mechanical properties such as interlaminar shear strength (ILSS), translaminar or in-plane shear strength, and transverse tensile strength. These testing techniques invariably employ... 

Apart from the short beam shear test, which measures the interlaminar shear properties, many different specimen geometry and loading configurations are available in the literature for the translaminar or in-plane strength measurements. These include the losipescu shear test, the 45°]5 tensile test, the [10°] off-axis tensile test, the rail-shear tests, the cross-beam sandwich test and the thin-walled tube torsion test. Since the state of shear stress in the test areas of the specimens is seldom pure or uniform in most of these techniques, the results obtained are likely to be inconsistent. In addition to the above shear tests, the transverse tension test is another simple popular method to assess the bond quality of bulk composites. Some of these methods are more widely used than others due to their simplicity in specimen preparation and data reduction methodology. 

ASTM D 2344 (1989). Test method for apparent interlaminar shear strength of parallel fiber composites by short-beam method. 

As with tensile properties, both compressive strength and modulus depend on the fiber content and hber orientation (see Table 5.8). The interlaminar shear strength reported in Table 5.8 is a measure of the shear strength in the thickness direction of the SMC sheet. It is determined by three-point flexural testing of beams with short span-to-depth ratios and is considered to be a quality-control test for molded composites. 

The degree of bonding analysis has been verified for both compression molding and online consolidation of thermoplastic composites. In these studies, composite test specimens were consolidated under controlled processing conditions. The most common types of tests performed to measure the interply bond strength were the interlaminar (short beam) shear test [21,25] or the lap shear test [12,21,26]. 

Both fiber-matrix interphase-sensitive mechanical tests (interlaminar shear strength, 90° flexure) and interphase-insensitive tests (0° flexure) were conducted on high volume composite samples fabricated from the same materials and in the same manner as discussed above to see if the interphase and its properties altered the composite mechanical properties and in what manner. A summary of the data is plotted as a bar graph in Fig. 7. The first set of bars represents the difference in fiber-matrix adhesion measured between the bare fibers and the sized fibers by the ITS. The composite properties plotted on the figure also show increased values for the epoxy-sized material over the bare fiber composite. 

Short beam tests provided information on the apparent interlaminar shear strength (ILSS) of laminates made with bare and sized E-glass fibers. In all cases, the specimens failed in shear at or near the midplane, allowing comparisons between fiber types. The ILSS, SD, and number of specimens tested are given in Table 5. 

Fiber type Interlaminar shear strength (MPa) SD (MPa) No. of specimens tested... 

Recently Nishijima et al. investigated the radiation effects of three-dimensional glass-fabric reinforced plastics (3DFRP) mentioned in the preceding section, since the interlaminar shear strength of composites was expected to be greatly enhanced by the presence of Z-axis reinforcement [78]. Two kinds of 3DFRP were newly developed and named as ZI-003 and ZI-005 of which the matrices were epoxy and BT resins, respectively [28]. The compressive tests... 

Including 4-bromophenol in the phenol-formaldehyde resol system impacts the cross-link density of the cured product. In a systematic study of this copolymer, a comparison was made among the polymers obtained using phenol only, a 9 1 mole ratio of phenol to 4-bromophenol and a 1 1 mole ratio of phenol to 4-bromophenol. Comparisons included measurement of interlaminar shear strength and cone calorimetry tests of composites prepared using these phenolic resins and S2-glass fiber plain weave. 

The reinforcing fabrics used were aramid (Kevlar DuPont) and poly(p-pheny-lene-2,6-benzobisoxazole) (PBO) (Zylon Toyobo). The fabrics were first treated with a 1.5% solution of HB PAMAM (AD-102), dried, then interleaved with an epoxy film structural adhesive (epoxy stage B on polyester net), and compression-molded for 90 min at 120 °C. For the aramid-based composite, FM-73 (Cy-tec) was used as the matrix. For the PBO-based composite, AF-191 (3M) was used as the matrix. The laminates were cut into strips and tested for interlaminar shear strength (ASTM D-2344) using a three-point bending instrament. 

The mechanical results of interlaminar shear strength for the two kinds of laminates prepared using hyperbranches impregnated in the reinforced fibers are presented in Table 15.8. The two laminates tested included Kevlar fabric primed with 1.5 wt.% HMW PAMAM (AD-102) and bonded with FM-73, and Zylon fabric primed with 1.5 wt.% HMW PAMAM (AD-102) and bonded with AF-191. 

The variety of materials possible results in specimens having a range of behaviors. Thus, even in a single test, such as interlaminar shear, unacceptable failure modes may occur depending on the material characteristics. 

In the broad sense, interlaminar adhesion means the adhesion between any laminations of similar or dissimilar materials and hence could be taken to include just about any adhesion test. One particular type for fiber-reinforced plastics, which is known as interlaminar shear strength, is normally considered as a short beam flexural test (sec Chapters 9 and 18). A method peculiar to laminated plastics tube is given in BS 2782, Method 346A [59] and called cohesion between layers of laminated tube. A sample of tube is subjected to compression to induce shear forces between layers and delamination observed by eye,... 

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