Laminates Characterization

Tunneling n. A condition occurring in incompletely bonded laminates, characterized by release of longitudinal portions of the substrate and deformation of these portions to form tunnel-like structures. 

Table 12.4 summarizes all of the test procedures. TABLE 12.4 Laminate Characterization Test Plan... 

O Brien, T.K., Characterization of Delamination Onset and Growth in a Composite Laminate in Damage in Composite Materials, ASTM STP 775, p. 140-167,1982 Poursartip, A. Ashby, M. F., Beaumont P.W.R., The Fatigue Damage Mechanics of Fibrous Laminates in Proceedings of the European Workshop on Nondestructive Evaluation of Polymers and Polymer Matrix Composites, Polymer NDE (edited by Khg. Ashbee), Technomic Publishing, p. 250-260, 1984... 

Fig. 2 X-ray refraction topographs of a series of /OyPOj/s samples of different impact energies. The total damage of the laminates is characterized by addition of all debonded layers of0° and 90° fiber direction.

Composites. The history of phenoHc resin composites goes back to the early development of phenoHc materials, when wood flour, minerals, and colorants were combined with phenoHc resins to produce mol ding compounds. In later appHcations, resin varnishes were developed for kraft paper and textile fabrics to make decorative and industrial laminates. Although phenoHcs have been well characterized in glass-reinforced composites, new developments continue in this area, such as new systems for Hquid-injection molding (LIM) and sheet-molding compounds (SMC). More compHcated composite systems are based on aramid and graphite fibers. 

Mechanical Properties. The performance of various polyester resin compositions can be distinguished by comparing the mechanical properties of thin castings (3 mm) of the neat resin defined in ASTM testing procedures (15). This technique is used widely to characterize subtle changes in flexural, tensile, and compressive properties that are generally overshadowed in highly filled or reinforced laminates. 

Some engineers have tried to characterize laminates with effective laminate stiffnesses, E, Ey, v y, and G y, and indeed such properties can be determined for a laminate by the usual measurements. However, it is crucial to recognize that with an effective laminate stiffness approach... 

For plate problems, whether the specially orthotropic laminate has a single layer or multiple layers is essentially immaterial the laminate need only be characterized by 0 2, D22. and Dgg in Equation (5.2). That is, because there is no bending-extension coupling, the force-strain relations, Equation (5.1), are not used in plate analysis for transverse loading causing only bending. However, note that force-strain relations are needed in shell analysis because of the differences between deformation characteristics of plates as opposed to shells. 

Symmetric angle-ply laminates were described in Section 4.3.2 and found to be characterized by a full matrix of extensional stiffnesses as well as bending stiffnesses (but of course no bending-extension coupling stiffnesses because of middle-surface symmetry). The new facet of this type of laminate as opposed to specially orthotropic laminates is the appearance of the bend-twist coupling stiffnesses D. g and D2g (the shear-extension coupling stiffnesses A. g and A2g do not affect the transverse deflection w when the laminate is symmetric). The governing differential equation of equilibrium is... 

Obviously, the classical lamination theory stresses in Pagano s example converge to the exact solution much more rapidly than do the displacements as the span-to-thickness ratio increases. The stress errors are on the order of 10% or less for S as low as 20. The displacements are severely underestimated for S between 4 and 30, which are common values for laboratory characterization specimens. Thus, a practical means of accounting for transverse shearing deformations is required. That objective is attacked in the next section. 

The Onnagawa Formation conformably overlies the Nishikurosawa Formation and is composed of siliceous shale and shale. The rocks are characterized by organic-rich laminated diatomaceous deposits, siliceous microfossils, and fish bones, while foraminiferal fossil is poor in amounts. The total thickness is about 3(X) m. The age of base of the Onnagawa Formation is estimated to be 12.9 Ma based on diatoms (Koizumi and Matoba, 1989). The age of the top of the Formation is 5.8 Ma. 

NA isolation and molecular characterization will be important to define the origin and functions of these proteins. At this time, infected cell nuclei offer the only source of these proteins, and NA have proved resistant to classic nuclear extraction methods (Yao and Jasmer, 1998). NA can be solubilized under conditions that co-extract nuclear lamins a/c and b (4 M urea, pH 8.0). Despite these similar physical properties, NA do not co-localize with lamins in the nucleoskeleton. However, both disulphide bonds and ionic interactions appear to contribute to nuclear complexes containing NA. In addition, NA can be cross-linked within host nuclei with protein cross-linking reagents. The foregoing properties represent current information available for the development of strategies to isolate and characterize these proteins and to investigate host proteins with which NA interact. 

Delamination represents the weakest failure mode in laminated eomposites, and is considered to be the most prevalent life-limiting crack growth mode in most composite structures. As such, ever-increasing attention has been directed toward the understanding and characterization of delaminations of various natures, and at... 

The EDT specimen shown in Fig 3.34(a) has been used to characterize the interlaminar failure of the composite in opening mode I (Whitney and Knight, 1985). The straight-sided tensile specimen has starter cracks placed along the free edges at the laminate mid-plane (Fig 3.34(a)). Due to the low interlaminar shear... 

The intimate contact data shown in Figure 7.16 were obtained from three-ply, APC-2, [0°/90o/0o]7- cross-ply laminates that were compression molded in a 76.2 mm (3 in.) square steel mold. The degree of intimate contact of the ply interfaces was measured using scanning acoustic microscopy and image analysis software (Section 7.4). The surface characterization parameters for APC-2 Batch II prepreg in Table 7.2 and the zero-shear-rate viscosity for PEEK resin were input into the intimate contact model for the cross-ply interface. Additional details of the experimental procedures and the viscosity data for PEEK resin are given in Reference 22. 

The methodology presented is generic and can be applied to other types of materials, laminate constructions, and processing methods. In all cases a characterization of the cure state is needed first. As a result, models must be developed to predict the mechanical response as a function of the cure state. Mechanics models can then be developed for the specific application of interest. 

The same test method, although not standardized, can be used to characterize the laminate in-plane shear behavior. This is accomplished by winding a multiorientation (hoop/helical and/or helical only) tube. Other test methods that can be used to measure in-plane shear stiffhess/strength of filament wound composites are discussed by Tamopol skii and Kinds [45]. These methods include schemes for torsion of intact rings and split rings. Both of these ring test methods are used to evaluate the in-plane shear modulii G0r and G0z for a filament-wound laminate. 

Other characterization methods that are of value are dynamic scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). A sample DSC is shown in the middle of Figure 15.2. Most cure reactions are exothermic, and the heat generated by cure can cause excessive heat to build up in the polymer if control is not exercised. DSC measures the generation of heat as a function of time and temperature. This can be used to predict the temperature at which the laminate will begin to cure (the onset of the peak in Fig. 15.2) and the temperature or time at which cure will be complete, further improving the selection of cure cycles to try. 

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