Laminates interlaminar shear strength

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)...

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. 

Michelle Leali Costa, Sergio Fraseino M. de Almeida, Mirabel Cerqueira Rezende, The influence of porosity on the interlaminar shear strength of carbon/epoxy and earbon/bismaleimide fabrie laminates. Composites Science and Technology 61 (2001),... 

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 results show that composites made of primed HMW HB PAMAM ara-mid and PBl fibers in epoxy matrices demonstrate much improved interlaminar shear strength (the Kevlar laminate shear strength increased to 160% and that of the Zylon to 140%). 

Table 15.8 Interlaminar shear strength (ASTM D-2344) for laminates with and without HMW PAMAM.

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,... 

Figure 1.10 Effect of various particulate fillers on the interlaminar shear strength of glass fabric reinforced epoxy resin laminates (23]. A, no filler B, BOixm glass beads, untreated C, same but silane treated D, 7 i,m glass beads, untreated E, same but silane treated F, 15p.m glass flakes G, 8 i,m calcium carbonate H, 15p.m quartz I, 15p.m alumina trihydrate, fire retardant J, 20p.m mica K, 60p.m thin-walled hollow-glass microspheres...

The use of fillers tends, with few exceptions, to reduce the damage tolerance, tensile and flexural strength of laminates. The effect on the interlaminar shear strength of glass fabric/epoxy laminates depends on the filler and is shown in Fig. 1.10. Note that glass beads and quartz had no adverse effect whereas the extremely thin-walled hollow glass microspheres were damaging [23]. Thicker walled spheres would have improved the performance. 

Davies et al. [64] studied the influence of water absorption on the interlaminar shear strength and end notch flexure (ENF mode II shear) fracture toughness of quasi-unidirectional (88% 0°, 12% 90°) E-glass fibres in DGEBA epoxy resin cured with an amine hardener. Laminates were immersed in (a) distilled water and (b) the Atlantic Ocean ( at Boca Raton, Florida) for up to 8 months at temperatures of 20°C, 50°C and 70°C. Sea water was more slowly absorbed than distilled water. This observation has frequently been made and it provides some reassurance that laboratory tests using distilled water are useful, if slightly cautious, estimates of behaviour in the ocean. 

A unidirectional carbon fibre-epoxy laminate 3 mm thick is cut into a 10 mm wide strip and subjected to three-point bending with a span of 20 mm. Failure occurs by interlaminar splitting at a load of 2400 N. Calculate the interlaminar shear strength of the composite. 

Harding J, Dong L, Effect of strain rate on the interlaminar shear-strength of carbon-fiber-reinforced laminates, Composite Science Technology 51(3), 347-358, 1994. 

Figure 22.4 Interlaminar shear stress as a function of ply angle for Type I high modulus carbon fiber epoxy resin laminate tested in uniaxial tension in x-direction. The maximum interlaminar shear strength occurs at 0 = 35° and the stresses are zero at 0, 60 and 90°. Source Reprinted with permission from Pipes RB, Pagano NJ, Interlaminar shear stress in composite laminates under axial tension, J Composite Mater Sci, 13, 2131-2136, 1978. Copyright 1978, Sage Publications.

A 5-harness satin (5-HS) weave will have its warp yarns running over four weft yarns and under one weft yarn as shown in Fig. 9.5. Other n-HS weaves used in composite materials include 8-HS, where the warp yarn passes over seven weft yarns and under one, and 12-HS where the warp yarn passes over eleven weft yarns and under one. It can easily be understood that the more yarn passes over the other yarn, the straighter and less crimped will be the yarns present in the fabric. The straighter the segments, the more the fabric composite behaves as a lamina in a laminated composite, increasing the inplane properties at the expense of out-of-plane properties and interlaminar shear strength - and of course vice versa. 

Figure 4.12 Interlaminar shear strength of phenolic laminates with different reinforcements data scatter is indicated by broken line (Sundaram, Tavakoli and Orpin).

These results suggest that for the initial design stages, similar levels of properties can be assumed for the main thermosetting laminating resins. However the interlaminar shear strength of phenolic composite can be appreciably less than polyester or epoxy ones and so the level of shear stresses may need to be checked. 

Table 9.3 Effect of preconditioning on interlaminar shear strength of reinforced epoxy laminates...

It is prudent to make a test laminate by the selected fabrication route to check the compatibility of the individual constituents. Properties like interlaminar shear, strength, modulus, should be measured and compared with one or more of the following ... 

For resins this property could be the gel time and for fibres the properties of the size or their interlaminar shear strength when incorporated into a laminate. 

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