Interlaminar shear strength ILSS

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. 

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. 

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

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... 

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). 

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. 

D Almeida [40] has reported interlaminar shear strength (ILSS) results for 65v/o Kevlar 49 fibres in DER 383 epoxy resin cured with 27phr DEH 50 aromatic polyamine with a void content below 0.5%. The ILSS values decreased as a function of immersion time, with specimens immersed in distilled water declining faster than those in saline solution, which is absorbed more slowly. When the results were normalized to water content the results were coincident, suggesting that the degradation mechanism was the same in both cases (Fig. 7.7). 

Carbon fibers are surface treated by the manufacturers using proprietary processes to improve the bond between resin and fiber. For control purposes, it is usual to measure the three point interlaminar shear strength (ILSS) and the choice of resin, volume fraction, span/depth ratio, mode of failure and time of treatment (Table 9.1) are important parameters. As the fiber modulus increases, longer treatment times will be required to obtain the same level of ILSS and overtreatment can result in loss of properties, particularly impact strength (Figure 20.15). 

Measurement of the interlaminar shear strength (ILSS) by short beam... 

In FIG. 14 the interlaminar shear strength (ILSS) of the two types of composites is illustrated in dependence of the deposition time of the plasma polymer. Applying the mixture of S seem acetylene/IS seem air it is found that the ILSS is increase by about 25 % relative to that of the virgin fibre. This result is explained by the improvement of adhesion between fibre and matrix resulting from chemical bonds between qx)xy resin and the functional groups of the plasmapolymerized acetylene. 

Under three point bend loading of a composite (beam), cracks may be developed due to tensile stresses at the lower stratus of the specimen as well as compression stresses at the upper one, or due to interlaminar shear. The type of failure depends on the ratio of span to depth (L/D). Short beam specimens usually fail in shear and long ones by tensile or compression stresses. For interlaminar shear strength (ILSS) tests, a L/D = 5 was chosen (ASTM-D-2344-76). In case of flexural strength tests, this ratio was fixed to 40 (DIN 29971). 

Stress concentrations for linear elastic matrices and fibres have been calculated in [5] and [6]. The stresses are maximum at or near the interface. The composite interlaminar shear strength (ILSS) is SCF times lower than the interface strength x. when fracture initiates at the interface. The composite strength predicted by such calculations decreases with increasing fibre volume fraction. The decrease is very strong near the maximum packing density. These predictions may be too pessimistic because any yield behaviour will reduce the stress concentrations. 

Gibson and coworkers [2] emphasized that the preheating section of their thermoplastic pultrusion process was of crucial importance to a successful operation. Besides the results achieved with CF/PEEK tapes, these authors presented a wide range of interlaminar shear strength (ILSS) data for pultrudates from glass fiber (GF)/polyamide 12 (PA12) tapes. 

The most widespread method for testing interlaminar shear strength (ILSS) is the short-span flexural test. If, despite the short span to thickness ratio, the specimen fails in a flexural rather than a shear mode, the result of the test should not be reported as ILSS. 

Interlaminar shear strength (ILSS) depends primarily on the matrix properties and fiber-matrix interfacial adhesion rather than on the fiber properties [141]. The ILSS, measured by the short-beam shear test, can be enhanced by increasing the tensile strength and volume fraction of the matrix as well as by increasing the interfacial adhesion in the laminates. 

Surface modification of Kevlar fabrics with reactive functional groups can enhance the interlaminar shear strength (ILSS) of the Kevlar fibre/2,2-bis[4-(4-maleimidophenoxy)phenyl] propane (BMPP) composities. Chlorosulfonation of the fibres, by treatment with dilute chlorosufonic acid, improved the ILSS, but the use of higher concentrations of the reagent for a longer reaction times decreased ILSS. The optimum chlorosulfonation conditions involved the use of 0.2% chlorosulfonic acid for 150 seconds (Chapter 8, Section 6). 

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