Mechanical responses specimens

Tnucleation is less effective with a low molecular grade than with a high molecular one. The temperatures at which the ductile-brittle transitions, Tdb, occur for selected non-nucleated and -nucleated sets support our conclusion (Fig. 5) /J-nucleation lowers the Tdb by about 50 °C independently of the flowabihty of the neat polymer. Although these tests were performed with single notched specimens, they can be correlated to a certain extent to those carried out on double-edge notched specimens. Sect. 3.2.4 deals in more detail with the influence of the local stress concentration on the mechanical responses of P-iPP. 

Typically, these instruments measure dynamic mechanical responses to sinusoidal input. To characterize the viscoelastic properties of a material, these tests must be repeated over a range of temperatures and frequencies. This is sometimes done at a fixed frequency while the polymer specimen is heated or cooled and... 

The control of dynamic effects at impact rates up to 1 m/s (in some instances somewhat higher) frequently makes use of mechanical damping in the load transmission by placing a soft pad (elastomer or grease) between the striker tup and the specimen [3,5], Above about 1 m/s inertia effects overshadow the true mechanical response of the specimen. Due to such dynamic effects, the applicability of FBA is limited to loading rates up to about 1 to 2 m/s for bending type fracture specimens. 

Results of studies of tensile deformation at constant e in samples of PMMA containing from 0 to 2.2% H-O indicate that mechanical response is relatively insensitive to e when considered in terms of reduced variables at intermediate strains. At short times or just before failure, the specimens with higher H2O... 

Therefore, in all the polymer crystal specimens studied to date the principal loss tangent maximum found for melt-formed samples is greatly reduced in size if present at all, and a maximum at higher temperatures either appears or is more in evidence. The exact mechanism (or mechanisms) responsible for this crystalline loss process is still in doubt. The principal loss mechanism for melt-formed samples is believed to be associated with segmental motion in disordered regions which could possibly be those regions containing tie molecules between lamellae, the surface folds of the crystals, or both. 

Our discussion of the viscoelastic properties of polymers is restricted to the linear viscoelastic behavior of solid polymers. The term linear refers to the mechanical response in whieh the ratio of the overall stress to strain is a function of time only and is independent of the magnitudes of the stress or strain (i.e., independent of stress or strain history). At the onset we concede that linear viscoelastie behavior is observed with polymers only under limited conditions involving homogeneous, isotropie, amorphous samples under small strains and at temperatures close to or above the Tg. In addition, test conditions must preclude those that ean result in specimen rupture. Nevertheless, the theory of linear viseoelastieity, in spite of its limited use in predicting service performance of polymeric articles, provides a useful reference point for many applications. 

The above work concentrated on GFRP beam components (subjected to bending, with one side - the fire side - in tension), this section focuses on column or wall components (with both sides - also the fire side - in compression) [17]. The column specimens were pultruded web-flange sandwich sections with four cells as shown in Figure 6.25) and the geometric parameters and mechanical properties at ambient temperature are summarized in Table 7.2. The experimental details and results of thermal responses were reported in Chapter 6. The mechanical response and time-to-failure are introduced in this section. Again both noncooled and water-cooled scenarios were investigated. 

Temperature-dependant material property models were implemented into stmc-tural theory to establish a mechanical response model for FRP composites under elevated temperatures and fire in this chapter. On the basis of the finite difference method, the modeling mechanical responses were calculated and further vaUdated through experimental results obtained from the exposure of full-scale FRP beam and column elements to mechanical loading and fire for up to 2 h. Because of the revealed vulnerabihty of thermal exposed FRP components in compression, compact and slender specimens were further examined and their mechanical responses and time-to-failure were well predicted by the proposed models. 

In this chapter, the post-fire behavior of FRP composites was evaluated and modeled on the stmctural level. Results from the models compared well with results from fuU-scale post-fire experiments on cellular GFRP beam and column specimens that had been subjected to mechanical and thermal loading up to 120 min with inclusion of different thermal boundary conditions. On the basis of the previously proposed thermal and mechanical response models, existing approaches for post-fire evaluation can be applied. Predicted temperature profiles and the conversion degrees of decomposition can be used to estimate the post-fire stiHhess from existing two- and three-layer models. The borders between different layers can be determined either by a temperature criterion or a RRC criterion. 

The very nature of polymers makes their mechanical response very sensitive to variations in the rate at which the polymer solids are deformed. The strain rates considered in this chapter are well above these for standard tensile or flexural tests and well below the ballistic deformation rates at which bullet-proof vests and other polymer applications are tested (78). Impact speeds vary from 1 to 10 m/s compared to 10 m/s for standard tensile tests. The actual strain rates within the loaded solid body depend on the loading and specimen geometries. 

Figure 3. Illustration of mechanical response of soil specimen with flexible boundaries for 2D biaxial test simulation under 100 kPa, 200 kPa, 300 kPa confining pressures, (a) deviatoric stress versus axial strain (b) volume strain versus axial strain.

In order to study the influence of rock block shape on the mechanical response of S-RM specimen, the biaxial compression test of specimens contain circular and polygonal rock block respectively are simulated under 100 kPa, 300 kPa and 500 kPa confining pressures. Two kinds of specimens have the same size, rock content and flexible lateral boundary. 

The mechanical response, especially the deformation characteristics of soil-rock mixture is fundamentally determined by the microstructural changes, thus the quantitative description and analysis of the S-RM specimen s microstructure during the process of test will deepen the understanding of soil-rock-mixture s particularity. 

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