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10 2.1 Physical Testing A comprehensive test program was conducted to evaluate the performance of elastomeric bearings under static and dynamic rotation. Seventy-eight bearings were tested in seven major test series.
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11 Series Type of test No. of specs.
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12 Table 2.2. Bearing test matrix.
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13 In order to isolate the effect of the shim edge shape from the effect of the nonstandard plan geometry of the shim, more bearings were ordered with standard rectangular shims, and edges prepared in three different ways: sharp and squarecut (as-sheared) , deburred (using a belt-sander)
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ment causes shear deformations in the elastomer that are too small to be of much consequence.) Second, if the shear displacements of the bearing are large, they may involve some roll-over at the end of the layer, which leads to a complex state of stress there, consisting of combined shear and vertical tension.
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15 is occupied by the cover, in which the displacement field is drastically different from that in the body of the bearing layer. It is necessary in the FE model to extrapolate the strain field from a point just inside the edge of the shim to the outer edge of the bearing, if strains in corresponding locations are to be compared.
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16 parameters. Following a suggestion by the project Advisory Group, the simplifying assumptions by Yeoh (1993)
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and making the coefficients such as Ba functions of λ. In Equation (2-3)
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bearing was taken as the average of the gross length and the shim length. Nonlinear effects also were found to become less important as the shape factor increased.
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2.2.4.1.6 Local effects at the ends of the steel shims. Various levels of mesh refinement were applied to study local effects near the end of the steel shims.
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the shear strain at the elastomer–steel interface close to the edge of the shim and are predicted by the linear theory. This permits the shear strain obtained from the linear theory to be used as a proxy for the local hydrostatic tension.
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precisely true, but especially at low loads, they both prove to be excellent approximations. They are particularly true for thin rubber layers, which correspond to high shape factors, which in turn are the conditions under which hydrostatic tension is likely to be critical.
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layer of θL = 0.006 rad./layer. An iso-error plot shows contours of constant error, or difference, between linear and nonlinear results.
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in Figure 2.13. The typical model error lies below 5%.
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basis of the procedure used for the design method used in this research, and is summarized here. It is approximate because it assumes a parabolic distribution of displacement through the thickness of the elastomer within a layer, but, as the FEA shows, that approximation proves to be remarkably good, and, for the geometries and stresses used in practical bearings, the errors are small compared with those arising from other sources, such as characterization of material properties.
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of both shear strain demand and capacity. This section addresses capacity.
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theory. In the second approach, the Linear Model, all strains were obtained from stresses using Gent's linear model.
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reason, the strain capacity was rounded up to 5.0. This value is still significantly less than the 6.7 corresponding with the start of debonding in the PMI series of monotonic tests.
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2.3.4 Evaluation of the Design Model The Linear Model for design was evaluated in the light of the findings described in Sections 2.3.1 to 2.3.3. It brought to light several issues summarized here and discussed in detail in Appendix F
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today is, on average, higher than it was 20 years ago. This can be seen by comparing the average compressive stress at which initial debonding of elastomeric bearings was observed.
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M251 now states that, "The properties of the cured elastomeric compound material listed in Table 1 shall be determined using samples taken from actual bearings." Preparing samples from a finished bearing imposes considerable extra costs, both for preparation time of the sample and in destruction and loss of a bearing. Although the changes only are just coming into force and no cost data are yet available, it is likely that the net effect will be to increase overall costs.
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definition of a batch also is important, and the definition differs between manufacturers that mix their own rubber and those that buy it premixed. Manufacturers that mix their own batch generally mean one mixer load, so the amount depends on the size of the mixer, whereas those that buy premixed material tend to mean a single purchase.
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From page 33... ...
even a single bearing in a bridge. Current QA/QC procedures are resulting in good quality elastomeric bearings, and relaxation of the test requirements appears to carry considerable risk without a corresponding price benefit.
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