Strength interlaminar shear

It is critical that surface treatment conditions be optimized to composite properties since overtreatment as well as undertreatment will degrade composite properties. Typically composite interlaminar shear strength (ILSS), in-plane shear, and transverse tension ate used to assess the effectiveness of surface treatment. More recently damage tolerance properties such as edge delamination strength, open hole compression, and compression after impact have become more important in evaluating the toughness of composite parts. 

As already mentioned, the first pyrolysis cycle leads to an open porosity of about 20-30 %. This high porosity limits the interlaminar shear strength (ILS). A reduction in the porosity is possible by reimpregnation with polymers and subsequent pyrolysis or CVI (Chemical Vapour Infiltration). 

MPa. At the stress maximum or in the region where the stresses remain nearly constant a strong delamination occurred, which probably reduces the interlaminar shear strength dramatically. Therefore the high strain-values cannot really be used for design. 

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. 

ISO 14130 1997 Fibre-reinforced plastic composites - Determination of apparent interlaminar shear strength by short-beam method ISO 15024 2001 Fibre-reinforced plastic composites - Determination of mode I interlaminar fracture toughness, GIC, for unidirectionally reinforced materials... 

ISO 3597-4 2003 Textile-glass-reinforced plastics - Determination of mechanical properties on rods made of roving-reinforced resin - Part 4 Determination of apparent interlaminar shear strength... 

Interlaminar shear strength (ILSS), AES atomic percent, contact angle, 0, and surface energy, y, data for untreated and electrochemically oxidized pitch-based carbon hber"... 

Gilbert, A.H., Goldstein, B. and Marom, G. (1990), A liquid droplet measurement technique as a means of assessing the interlaminar shear strength of fiber reinforced composites. Composites 21. 408-414. 

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... 

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

Fisher, S., Rosensaft, M. and Marom, G. (1986). Dependence of the interlaminar shear strength on the loading span-to-depth ratio in aramid fiber-reinforced beams. Composites Sci. Technol. 25, 69-73. 

Fig. 7.8. (a) Normalized impact fracture toughness and (b) interlaminar shear strength (ILSS) of carbon fiber-epoxy matrix composites as a function of glycidyl acrylate/methyl acrylate (GA/MA) interlayer... 

Fig, 7.16, Impact fracture toughness (O) and interlaminar shear strength (ILSS, ) of carbon fiber-epoxy matrix composites with varying number of nylon sheets as delamination promoters. After Havre (1977). 

Fig. 8.25. Interlaminar shear strength as a function of stitch density for seven layer off-loom stitched glass fiber-epoxy matrix composites. After Adanur ct al. (1995).

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 function of the resin matrix material in filament-wound structures is to help distribute the load, maintain proper fiber position, control composite mechanical and chemical properties, and provide interlaminar shear strength. Either a thermosetting or a thermoplastic resin material may be selected. Thermosetting resins may be selected for application in a wetwinding process or as part of a prepreg resin system. 

Figure 14.18 Interlaminar shear strength results on predried, formed laminates (third data set from left) compared with unformed laminates (two leftmost sets) and manufacturer s data (right)...

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... 

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