Failure loads

Next, the straight-sided tension specimen has a transition region that is created by thickness change (instead of the width change of the ASTM D 638 specimen) as shown in Figure 2-22. Failures typically occur either in the bonded tabs or in the gage section. If the bonded tabs fail, then the failure load is never a measure of the subject material s strength ... 

Suppose we replace the 90° layers with a laminae in an attempt to increase the axial stiffness and to increase the first-ply failure load as in Figure 7-61. The load-deflection curve slope after first-ply failure also increases when a laminae replace the 90° layers. However, the energy absorption decreases with such a stacking sequence change. The associated fatigue lives are not known unless both laminates are made and subjected to fatigue loading. 

Assessments of the comparative safety of the different modes of transport should take into account both die risk to die public and die risk to terminal operators and tnuisport crew. The transporter may be imohed in a crash or denuhnent drivers may be injured or killed. Thus, die events that can give rise to liazards include container failure, loading and unloading operations, and accident impact. Hazardous materials are moved by ... 

Picherit et al., 2001 7 month old OVX adult rats 3 doses of isoflavone extract (20, 40, 80 mg/kg body wt/d) for 91 d Isoflavones reversed femoral failure load, total femoral, diphyseal or metaphyseal BMDs and normalized bone turnover markers induced by OVX. Optimal dose without uterotrophic effect was 40 mg/kg body wt... 

Sun. C.T. and Luo, J. (1988). Failure loads for notched graphite/epoxy laminates with a softening strip. Composites Sci. Techno . 22, 121-133. 

In the USA, disopyramide is only available for oral use. The typical oral dosage of disopyramide is 150 mg three times a day, but up to 1 g/d has been used. In patients with renal impairment, dosage must be reduced. Because of the danger of precipitating heart failure, loading doses are not recommended. 

Fig. 20 Critical scratching load L, macro-indentation failure load F and residual stresses SiC in the triple layer system, e. corresponds to the thickness of the Ta interlayer.

This paper presents results from a study of assemblies composed of glass fibre reinforced epoxy composites. First, tests performed to produce mixed mode fracture envelopes are presented. Then results from tests on lap shear and L-stiffener specimens are given. These enabled failure mechanisms to be examined in more detail using an image analysis technique to quantify local strain fields. Finally the application of a fracture-mechanics-based analysis to predict the failure loads of top-hat stiffeners with and without implanted bond-line defects is described. Correlation between test results and predictions is reasonable, but special attention is needed to account for size effects and micro-structural variations induced by the assembly process. 

The paper is presented in three parts. First, the tests employed to determine the mixed mode fracture envelope of a glass fibre reinforced epoxy composite adhesively bonded with either a brittle or a ductile adhesive are briefly described. These include mode I (DCB), and mixed mode (MMB) with various mixed mode (I/II) ratios. In the second part of the paper different structural joints will be discussed. These include single and double lap shear and L-specimens. In a recent European thematic network lap shear and double lap shear composite joints were tested, and predictions of failure load were made by different academic and industrial partners [9,10]. It was apparent that considerable differences existed between different analytical predictions and FE analyses, and correlation with tests proved complex. In particular, the progressive damage development in assemblies bonded with a ductile adhesive was not treated adequately. A more detailed study of damage mechanisms was therefore undertaken, using image analysis combined with microscopy to examine the crack tip strain fields and measure adherend displacements. This is described below and correlation is made between predicted displacements and failure loads, based on the mixed mode envelope determined previously, and measured values. 

FRACTURE MECHANICS APPROACH TO PREDICT FAILURE LOAD... 

Figure 2. Schematic illustration of procedure to determine failure load, Gc=Gi + Gii, (non-linearity (NL) values shown).

A first step in the validation of this approach is to test simple specimens under controlled conditions and to compare predictions with measured failure load values. First lap shear geometries were examined, then an L-geometry was studied in more detail. The bond-line in these small specimens was very similar to that in the quasi-unidirectional fracture specimens as the small dimensions allow panels to be pressed uniformly after assembly (which is not the case for industrial top-hat stiffeners). 

The correlation is quite good for the SRI500 resin, while for the more ductile adhesive resin the predictions overestimate the measured failure loads. However, in the latter case an extensive damage zone develops before final failure and the non-linear elastic fracture model is no longer appropriate. It is interesting to note however, that when a fillet is left at the end of the overlap the test values are much closer to the predictions. 

Noting that the specimen is statically determinate, then the failure load P3 is related to P from Figure 4 via P = P3/ (1 + L1/L2). Figure 11 shows a comparison between the predicted (P3) and experimental failure loads as a function of the effective crack length. In general, failure loads are underestimated using this procedure, though there is some scatter in results. 

Figure 11. Comparison between predicted and experimental failure loads (P3) for L type specimen as a function of effective crack length. Using Gic = 240 J/m. ...

The results above suggest that it may be possible to apply fracture mechanics data to determine failure loads of more complex structures, provided that (i) the adhesives used are not too ductile, (ii) bondline thickness is known and controlled, (iii) non-linear behaviour due to adherend and interface damage is limited, and (iv) the specimens employed to determine... 

Transverse pull-off tests induce mainly mode 1 loading, provided the base panel is sufficiently rigid. Finite element analyses have been performed to look at this geometry in more detail, and will be reported elsewhere, but here a simple analytical beam theory expression is used to predict the pull-off failure load [21] ... 

While the results from this first attempt to use fracture data from standard specimens to predict failure loads are quite encouraging there are a number of aspects which have been neglected and these must be examined in more detail if the work is to be extended in the future. 

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