Table 10 lists the applied parameters for the nano-scratch tests. In order to obtain reliable average properties, five repetitions of the tests are performed.
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The wall thicknesses of the tube specimens are measured by a magnetostatic sensor called MiniTest FH 4 of the company Elektrophysik Cologne, Germany. The applied parameters of the tensile tests are shown in Table The local interphase width can be measured by means of nano-scratching, as shown in Figure 2.
In this case, the sample is moved at a constant speed in contact with the nano-indenter tip such that the tip scratches from the matrix over the interphase to the fiber. Nano-scratch with constant scratch depth following [ 35 ]: a representation of the scratch path at constant depth from the matrix to the fiber; b imaging of the PBT-GF20 sample after nano-scratching; c 3D-imaging of b. During the nano-scratch, the sample is moved at a constant speed of 0.
As shown in Figure 2 a, the penetration depth of the tip is kept constant by the normal force being controlled during the process. In order to avoid artefacts from the sample surface topography, a pre-scratch is first performed. For that purpose, the tip is initially immersed nm into the sample. Thereafter, the main scratch proceeds along an identical scratch path with nm of penetration depth to the surface. The uniformity of the material pile-up displays the advantage of this approach. In addition, the nano-indenter tip is continuously located at the same depth in the material.
Thus, the engaged contact area of the indenter tip and the sample is constant, whereby no correction factor is required to calculate the self-imaging effect of the tip. Figure 2 b,c displays the nano-scratch paths according to the described methodology. In order to determine the interphase width, different slopes are identified through the measured normal force progression during nano-scratching.
Figure 3 shows a representative normal force profile of a nano-scratch coming from the matrix to the glass fiber. A nearly constant normal force is measured over the scratch path in the area of the pure matrix and fiber material. Due to the presence of different slopes between the matrix and fiber phases, the width of the interphase area can be identified.
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The onset of the interphase starts with the deviation from the linear gradient from the matrix material. The subsequent linear increase in the normal force refers to the interphase. This interphase area ends with a further increase of the normal force.
The renewed rise of the normal force is referred to the initial contact of the indenter tip with the pure fiber material. Measured normal force over the scratch path, identification of the interphase width using the slope method. To determine the mean interphase width for each selected composite material, the nano-scratches are performed on different fibers within the cutting plane of the corresponding sample. Subsequent to the nano-scratch, an AFM image is taken of the sample surface as presented in Figure 2 b,c.
Table 10 lists the parameters used for the nano-scratch tests. The results of the nano-scratches for different sample types are presented in Figure 4 and Figure 5. Interphase width for the influence of the fiber sizing measured with the introduced nano-scratch method with sized fibers SC and unsized fibers UC. Interphase width for the influence of the processing parameters measured with the introduced nano-scratch method. Figure 4 shows the results of the nano-scratch tests of the various samples to investigate the influence of the fiber sizing on the interphase formation.
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At least five scratches are performed on different glass fibers for each sample type, respectively. Depending on the glass fiber sizing, the width of the interphase varies widely. It is noticeable that the values for the standard deviations vary significantly concerning the measured interphase width. The sample type containing unsized fibers has an extremely low standard deviation, whereas the specimen type with sized fibers, as well as the reference samples show higher standard deviations. A possible reason represents the applied sizing on the glass fibers, which is also measured in the nano-scratch test and included to the interphase width.
For this purpose, the quantity of the applied sizing of 0. On this occasion, a constant width of the applied sizing around the fiber is assumed. The calculated sizing thickness corresponds in a good approximation to the difference of the mean width of the interphase for a comparison of the unsized sample type nm and the sized sample type nm. The wider interphase area of the reference sample can be explained by unknown additives added to the industrially-manufactured material of the reference specimens.
These additives usually improve the flowability and the dispersibility. In addition, the adhesion between glass fiber sizing and the matrix material can be optimized by additives. The results of the nano-scratching tests for the specimens influenced by the processing parameters of the injection molding are shown in Figure 5.
The interphase widths are measured for every specimen type. Differences of approximately nm in the interphase widths are measured for the same material. To identify the injection molding parameter with the highest influence on the interphase formation, the experimental design has to be evaluated.
The main effects plot for the influences on the interphase is shown in Figures 14— The investigations in the present study mentioned above are performed with a semi-crystalline thermoplastic matrix material. In a semi-crystalline thermoplastic material, a crystallization occurs in a privileged place at the interfaces of the fibers, and the degree of crystallization depends on the process conditions of the crystallization. Amorphous thermoplastics are not able to crystallize because of their macromolecular structure, and correspondingly, it is not possible to form a long-range order like a semi-crystalline structure.
In amorphous thermoplastics, there is only a near-range order. On this basis, it is obvious to study the possibility of the formation and the detection of an interphase in amorphous fiber-reinforced thermoplastics.
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It can be seen that an interphase is detectable by using the applied nano-scratch method. Interphase width for the influence of the amorphous material measured with the introduced nano-scratch method. Despite the amorphous structure of the ABS matrix, the formation of an interphase occurs between the matrix and the glass fiber or rather the glass fiber sizing. The absolute values of this interphase width deviate compared to the values of the semi-crystalline samples.
Since the structure of the macromolecules from the ABS differs from the macromolecular chain structure of the PBT material, a comparison of the overall width of the interphase is not appropriate. The formation of the interphase in an amorphous thermoplastic shows that the interphase is not only a phenomenon of crystallization. Similar to semi-crystalline thermoplastics, the amorphous thermoplastics form a three-dimensional interphase due to a thermodynamically-controlled interdiffusion of the macromolecules and the molecules of the glass fiber sizing [ 8 ].
In the present contribution, the characterization of the macroscopic mechanical properties of the presented short glass fiber-reinforced thermoplastics is performed by means of tensile tests. Therefore, tube specimens introduced by Kaiser et al. Figure 7 shows the tube specimen geometry with a 3D representation of the specimen in a and the connecting dimensions in b. Besides the possible application of multi-axial loading cases, the main feature of the specimen is a unidirectional fiber orientation in the central measurement section.
The mechanical testing of the tube specimens is performed by uniaxial tensile tests to determine the mechanical properties in the fiber direction of the specimens and by internal pressure tests to determine the mechanical properties perpendicular to the fiber direction.
Tube specimen geometry for experimental investigations: a 3D representation; b connecting dimensions [ 36 ]. To convert the measured loads of the tensile tests into the required true strains for a stress-strain diagram, the following formula is used [ 37 ]. While the load is applied to the specimen, the wall thickness of the tube specimen is reduced due to the necking of the specimen, and therefore, the effective cross-sectional area decreases.
To measure the wall thickness during the tests, a magnetostatic sensor is used. The effective cross-sectional area is calculated using Formulae 2 — 7. The symbols used in Formulae 2 — 7 are displayed in Figure 8. The mechanical material properties perpendicular to the fiber orientation are measured by means of internal pressure tests. This load case is realized by a constant flow of a fluid into the sealed, parallel test area of the tube test specimen.
Analogous to the calculation of the true stresses of the tensile tests, the true stresses of the internal pressure tests are calculated using Formulae 8 and 9.
The pressure p is measured in bar. Geometric variables to calculate the effective cross-sectional area of the tube specimens. Figure 9 represents the performed tensile and internal pressure load cases in and perpendicular to the fiber orientation of the tube samples.
It is well known that the mechanical properties, especially the stiffness, of a composite are influenced by its fiber content. The results of the matrix incinerations are given in Table The fiber weight fractions for the two granulate types are averaged for three measurements in each case.
The average glass fiber contents of the granulate types deviate slightly from the aspired 20 weight percent of the reference sample material. The influence of the glass fiber sizing on the mechanical properties is tested by at least ten tensile tests and ten internal pressure tests per specimen type. The applied parameters of the tensile testing are given in Table The results are shown in Table 13 and Table 14 , as well as in Figure 10 and Figure Average stress-strain diagram for the tensile tests of samples with SC and without UC glass fiber sizing and reference samples R.
Average stress-strain diagram of the internal pressure tests for the samples influenced by the injection molding parameters. Averaged tensile strengths and elongations at break of the tensile tests of samples with SC and without UC glass fiber sizing and reference samples. Average tensile strengths and elongations at break of the internal pressure tests of samples with SC and without UC glass fiber sizing and reference samples.
The results of the tensile tests are given by Figure 10 and Table It is recognizable that the stress-strain behavior is influenced by the presence of a glass fiber sizing. In additional, differences in the stiffness of the sample types are determined for the specimen types.
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The tensile tests with loads in the fiber direction of the tube specimens show that the tensile strength and the stiffness of the specimens with unsized glass fibers are lower than the tensile strength and stiffness of the samples with sized glass fibers, although the fiber content of the samples with unsized glass fibers is higher. In comparison, the stress-strain plot of the tensile tests from the reference specimens and the samples with the sized glass fibers show similar tensile strengths and stiffnesses, but the elongation at break is higher for the samples with sized glass fibers.
Considering these results, it is evident that the glass fiber sizing has an enormous impact on the fiber-matrix adhesion, because the samples with unsized glass fibers break at significantly lower loads. The load cannot be transferred through the fibers because of a low adhesion between the matrix material and the glass fibers resulting from the absent sizing. The results of the internal pressure tests for the samples influenced by the glass fiber sizing, as well as the reference specimens are listed in Table 14 and Figure The determined stress-strain diagrams of the internal pressure tests with loads perpendicular to the fiber direction of the manufactured tube specimens show similar stiffnesses for every sample type.
The similarity of the stiffnesses results due to the reduced influence of the fibers on the mechanical composite properties when loads are applied perpendicular to the fiber orientation. In the internal pressure test, the fibers are oriented perpendicular to the applied load, and the load cannot be transferred through the fibers.