Tensile testing fracture analysis

Forming Limit Analysis. The ductihty of sheet and strip can be predicted from an analysis that produces a forming limit diagram (ELD), which defines critical plastic strains at fracture over a range of forming conditions. The ELD encompasses the simpler, but limited measures of ductihty represented by the percentage elongation from tensile tests and the minimum bend radius from bend tests. 

An appropriate cure cycle was established based on the results obtained from the thermal analysis and cure rheology studies of the resin and cured BCB bar and dogbone shaped samples were fabricated for testing. Bar shaped specimens had the dimensions of 3.5 x 0.5 X 0.125 and were used to stake compact tension specimens for fracture toughness studies and for dynamic mechanical analysis of a torsion bar. Dogbone shaped specimens for tensile tests had a gauge area of 1 x 0.15 and were approximately 0.040 thick. 

Figure 5 presents the results of tensile tests for the HPC/OSL blends prepared by solvent-casting and extrusion. All of the fabrication methods result in a tremendous increase in modulus up to a lignin content of ca. 15 wt.%. This can be attributed to the Tg elevation of the amorphous HPC/OSL phase leading to increasingly glassy response. Of particular interest is the tensile strength of these materials. As is shown, there is essentially no improvement in this parameter for the solvent cast blends, but a tremendous increase is observed for the injection molded blend. Qualitatively, this behavior is best modeled by the presence of oriented chains, or mesophase superstructure, dispersed in an amorphous matrix comprised of the compatible HPC/OSL component. The presence of this fibrous structure in the injection molded samples is confirmed by SEM analysis of the freeze-fracture surface (Figure 6). This structure is not present in the solvent cast blends, although evidence of globular domains remain in both of these blends appearing somewhat more coalesced in the pyridine cast material.

Abstract The effects of the amount of rubber, the concentration of fibres and the state of the fibre/matrix interface upon the mechanical behaviour of short glass fibre-reinforced rubber-toughened nylon 6 ternary blends are described. First, tensile tests were carried out on different intermediate materials and then on the ternary blends to derive the stress-strain curves and document the damage mechanisms. Fracture toughness tests were implemented on compact tension specimens and the results were correlated to fractographic observations and acoustic emission analysis to assess the role of the different constituents. 

Very recently, in our group, enhanced self-reinforced PP composites based on commercial PP fabrics were obtained by the film stacking method followed by compression molding introducing different contents of micron-sized quartz particles in the matrix films (unpublished results). Simultaneous improvements of composite tensile strength, ductility and fracture toughness were observed from the addition of quartz to the polymer matrix (Table 14.1). Enhanced degree of consohdation was obtained for the composites with quartz as evidenced from the improved mechanical properties and by SEM observations. This was also confirmed by acoustic emission analysis in situ in tensile tests. 

In this chapter, brittle fracture is first discussed for the simple case of a uniaxial and homogeneous stress field (as in a tensile test), and for a single crack-like flaw which is oriented perpendicular to the stress field (in so-called mode I loading). These simple conditions are suflficient to explain the basic features of brittle fracture. later, the analysis will be extended to more complex and more general situations, including multiple cracks, the arbitrary orientation of cracks (mode II and mode III loading), and multi-axial stress fields. 

The importance of statistical analysis, and censoring of volume and surface flaws were demonstrated. Simple bend tests did not provide sufficient volume fracture data for this analysis. To obtain more volume fracture data, tensile testing may be required. 

The problem of secondary flexure in axial tensile tests is considered in view of the tension-softening process. The procedures aimed at elimination of that parasite effect were studied by Akita et al. (2001) and Novak et al. (2006) who have shown possibilities of nonlinear fracture mechanics simulation in analysis of the experimental data, including post-peak descending branch of the load-displacement curves, taking into consideration material imperfections and heterogeneities. 

Previous thermogravimetric analysis had established that under such processing conditions no appreciable thermomechanical degradative effects were detected. To perform tensile and fracture tests the sheets were successively cut by a mill. 

A series of panels were made with differing number of PIP cycles with the expectation that this would result in different overall porosity levels. The optical and x-ray porosity analysis did not confirm this trend and some of the properties did not follow the expected trend with increasing number of infiltration cycles. It was found fiom fast fracture studies that the overall diffusivity is controlled by the microstructure and therefore the porosity correlates with tensile properties. No clear correlation between porosity and properties was observed from the durability testing done. Durability appears to be controlled by the location and distribution of the porosity. Additional testing and analysis is needed to shed insight on the material performance. 

Test Procedures - Generally, a displacement-control or strain-control test mode is used to avoid run-away tests that sometimes occur in force-control. A test mode rate is chosen so as to produce test specimen failure in 5-50 s. Failure within 1 minute or less should be sufficient to minimize slow-crack growth (SCG) effects in the tensile test. If slow crack growth is observed (e.g. under slow test mode rates), subsequent tests can be accelerated to reduce or eliminate slow crack growth. Extensometers and/or strain gages are used to determine gage length extension and/or local strain. The test specimen is tested in flexure to fracture. The test specimen is retained for failure analysis and post-test dimensional measurement. 

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