Testing methods fiber orientation

Figure I represents a two-dimensional damage distribution of an impact in a 0/90° CFRP laminate of 3 mm thickness. Unlike in ultrasonic testing, which is usually the standard method for this problem, there is no shadowing effect on the successive layers by delamination echos. With the method of X-ray refraction the exact concentration of debonded fibers can be calculated for each position averaged over the wall thickness. Additionally the refraction allows the selection of the fiber orientation. The presented X-ray refraction topograph detects selectively debonded fibers of the 90° direction.

Injection-molded plaques of liquid crystalline polymers exhibit a multilayered structure through the thickness of a molding, where in each layer there is a high degree of orientation. Accordingly the fracture test methods of continuous fiber composites can be adopted for the testing of liquid crystalline polymers [176]. 

In lock-in thermography (154), the selected frequency of the heat wave to which the detector is tuned (locked) is important. Heat waves are diffusion waves that yield a quadratic dependence on time for the observation of indications at a certain depth of the test object. For thin-walled PMC shells (a few millimeters) this does not constitute a severe limitation (157). Limited operating temperature, pronounced temperature-dependent properties, and thermal degradation may pose limits on the application of active thermography to polymers and PMC. A recent example of a possible application of active thermography in PMC is the determination of the fiber orientation of CFRP (158) that, in the future, may replace the conventional destructive methods. 

In practice, only in very few cases do materials work in tensile mode more often they are subjected to flexure or impact. On the other hand, fiber-reinforced composites are usually applied as laminates with different orientation and alignment of the fibrous reinforcement. That is why the CPC laminates were used to study their flexural stiffness and impact resistance. The flexural tests were performed by the threopoint support test method used by Nunes et al, as shown in Figure 14.6 [74]. The support was mounted in the same Instron machine used for the tensile tests, this time operating in compression mode. Rectangular samples (155 x 100 mm) were cut out from the CPC MFC plates and placed upon the sup-... 

Fiber orientation measurement on short fiber reinforced PUR-RIM components combination of nondestmctive testing methods for optimization of simulation and production processes. Tech. Mess., 7J, 617. 

The procedure to calculate fiber orientation is the same as explained above, but their implementation into explicit solvers and non-linear material models is more complex than it is for quasi-static load-cases and purely elastic material models. The fiber orientation is characterized by a so called orientation distribution function (ODE) that describes the chance of a fiber being oriented into a certain direction. For isotropic, elastic matrix materials an integral of the individual stiffness in every possible direction weighted with the ODE provides the complete information about the anisotropic stiffness of the compound. However, this integral can not be solved in case of plastic deformation as needed for crash-simulation. Therefore it is necessary to approximate and reconstruct the full information of the ODE by a sum of finite, discrete directions with their stiffness, so called grains [10]. Currently these grains are implemented into a material description and different methods of formulation are tested. 

Flexural creep behavior of nylon 6/6 based long fiber thermoplastics (LFT) was determined nsing transient and dynamic testing methods. While the eflect of increasing fiber volume fi action reduced creep, there was only a negligible effect of flow orientation effect. The creep data generated by dynamic mechaitical analysis (DMA) tests was consistent with the transient tests. 

Finally, as in macro-Raman experiments, orientation-insensitive spectra can also be calculated for spectromicroscopy. A method has been developed recently for uniaxially oriented systems and successfully tested on high-density PE rods stretched to a draw ratio of 13 and on Bombyx mori cocoon silk fibers [65]. This method has been theoretically expanded to biaxial samples using the K2 Raman invariant and has proved to be useful to determine the molecular conformation in various polymer thin films [58]. 

If the materials are anisotropic, they will present different properties in the different directions. Examples of these polymeric materials are polymer fibers, such as polyethylene terephthalate, PET, nylon fibers, injection-molded polymers, fiber-reinforced composites with a polymeric matrix, and crystalline polymers where the crystalline phase is not randomly oriented. A typical method for measuring the modulus in tension is the stress-strain test, in which the modulus corresponds to the initial slope of the stress-strain curve. Figure 21.4 shows typical stress-strain curves for different types of polymeric materials. 

From the phase diagram the regions of stabihty of the different polymorphic forms of iPP in oriented fibers are defined as a function of stereoregularity and degree of deformation e. The values of the critical strain at which the polymorphic transitions start and at which the transformation is complete depend on the stereoregnlarity. The critical values of the stress instead depend also on other parameters as, for instance, the degree of crystalhnity of the sample, the amonnt of strnctnral disorder present in the crystals and on the method of preparation of the test specimens. 

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