Load displacement analysis

Linear elastic fracture mechanics (LEFM) approach can be used to characterize the fracture behavior of random fiber composites. The methods of LEFM should be used with utmost care for obtaining meaningful fracture parameters. The analysis of load displacement records as recommended in method ASTM E 399-71 may be subject to some errors caused by the massive debonding that occurs prior to catastrophic failure of these composites. By using the R-curve concept, the fracture behavior of these materials can be more accurately characterized. The K-equa-tions developed for isotropic materials can be used to calculate stress intensity factor for these materials. 

The results from 12 pull-off tests on QX/epoxy specimens with implanted defects are shown in Figure 15. Both measured and predicted values are shown. Different criteria may be used to compare top hat pull-off and fracture test values. These include various acoustic emission parameters (first acoustic events, first events above a certain amplitude), visual or image analysis parameters or values on the load-displacement plots. Several criteria have been examined, here non-linear values are shown (Gic = 240 J/m, the lower, dashed line). 

A first comparison of the three specimen types ([0°]24, [0°/90°]6s, and [0790°]i2) is shown in Fig. 4. The load-displacement values used in the analysis are plotted for all specimens tested in one laboratory (testing from the Mode-I pre-crack). Both cross-ply laminates show larger displacements for comparable delamination lengths, much more scatter and somewhat higher but comparable maximum load values compared with the unidirectional lay-up. There is no clear difference between the cross-ply laminate types, except that the symmetric lay-up yields the lowest load values and hence the largest scatter. In the data from the other laboratory, this is reversed, i.e., the non-symmetric lay-up showing the larger scatter. 

The comparison of the different initiation points in the three laminate types (Tables 2-4) raises the question which definition shall be used for initiation in the cross-ply laminates. Since visual initiation (VTS-point) and probably also non-linearity of the load-displacement plot (NL-point) yield values similar to initiation values in the corresponding unidirectional laminate, the maximum load or 5% offset in compliance (MAX/5%-point) seems to reflect the higher delamination resistance of cross-ply compared with unidirectional laminates better. Further analysis of additional data from the 3" round robin may allow a better assessment of this question. 

The trends seen in the present analysis seem to support the conclusion that, if the type of fracture is considered, a meaninghil relative ranking of cross-ply lay-ups (symmetric or non-symmetric) with respect to a unidirectional lay-up of the same material can be achieved. The load-displacement plots and R-curves show that cross-ply materials will )deld a larger scatter but, if effects from changing fracture surfaces are recognised and those specimens are... 

The SN-curve is a line in the Oamp-N-plane (N is a measure of number of cycles to failure, S is a general symbol referring to stress, strain, load, displacement etc. In this case, S denotes aamp). When constant-amplitude fatigue data are available for the material, an SN-curve can be found using regression analysis of the test data. Usually, it is appropriate to... 

Figure 4.14. Calibration factors H for use in the analysis of load-displacement records in plane strain fracture toughness tests (after ASTM STP 410) [ ] ...

This analysis shows that as far as the load-displacement curve is concerned, the dominant friction coefficient is that between the laminates, except towards the end of the transition region where the washer-laminate coefficient becomes significant. This exercise also shows the insight that can be gained with finite element analysis, since this load breakdown would be very difficult to obtain experimentally. 

A number of cruciform joints of rectangular hollow section were fabricated with adhesive gusset plates (Fig. 8.15). Hollow sections of two different sizes were used, and some of the specimens were subjected to accelerated ageing prior to testing in tension. The structural behaviour of these joints was also predicted by non-linear, three-dimensional, finite-element analysis. Very good agreement was obtained between the experimental and predicted load-displacement behaviour. 

Using the permissible stress approach, both the demanded stresses from loading and the ultimate stress capacity of the foundation are evaluated. The foundation is considered to be safe as long as the demanded stresses are less than the ultimate stress capacity of the foundation. A factor of safety (FS) of 2-3 is usually applied to the ultimate capacity to obtain various allowable levels of loading in order to limit the displacements of a foundation. A separate displacement analysis is usually performed to determine the allowable displacements for a foundation, and for the bridge structures. Design based on the permissible concept is still the most popular practice in foundation design. 

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. 

The same expression can be obtained for a fixed grip displacement loading and a compliant loading device. This simple result is quite powerful because it can be used to determine energy release rates directly from load-displacement records of fracture toughness tests without the need for any further stress analysis. Furthermore, in the event that laminated beams are used for fracture tests, the... 

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