Failure properties

Bulk powder flow properties can be used either as a qualitative comparison for product specifications or as quantitative numerical values in the design of plant. 

The parameters derived from shear tests generally are  

The internal angle of friction (the internal angle of friction is a two-dimensional projection of the critical state line) (5e) 

Failure properties are strongly influenced by humidity, temperature and time of consolidation. The above failure properties must, therefore, be measured under controlled conditions of temperature and humidity. For any prolonged storage, samples must be sheared on time consolidated samples. 

Powder flow is defined as the relative movement of a bulk of particles among neighboring particles or along the container wall surface (Peleg, 1977). The practical objective of powder flowability investigations is to provide both qualitative and quantitative knowledge of powder behavior, which can be 

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The blend exhibited reversible strain only up to 2%. Below 20%, the deformation appeared to be homogeneous. Beyond this region, the stress derivative with strain decreased significantly. The decrease in stress derivative was strain rate-sensitive. The stress-strain behavior for the blends containing different SBRs are given in Figure 11.18. Both modulus and yield stress increased with increase in PP content. Neither modulus nor yield stress varied strongly with SBR type. On the other hand, aU the failure properties increased with increase in PP content. 

For a consideration of filler-network breakdown at increasing strain, the failure properties of filler-filler bonds and filler clusters have to be evaluated in dependence of cluster size. This allows for a micromechanical description of tender but fragile filler clusters in the stress field of a strained mbber matrix. A schematic view of the mechanical equivalence between a CCA-filler cluster and a series of soft and hard springs is presented in Figure 22.9. The two springs with force constants... 

A combination of an energy criterion and the failure envelope has been proposed by Darwell, Parker, and Leeming (22) for various doublebase propellants. Total work to failure was taken from the area beneath the stress-strain curve, but the biaxial failure envelope deviated from uniaxial behavior depending on the particular propellant formulation. Jones and Knauss (46) have similarly shown the dependence of failure properties on the stress state of composite rubber-based propellants. 

Non-failure properties of Small deformation testing under Storage modulus (elastic Advantages Fundamental properties of UNIT H3.2 Steffe... 

In an initially ductile polymer, failure properties (ultimate elongation, fracture toughness, impact resistance) decrease rapidly during a chain-scission aging process, whereas elastic and yield properties are practically unaffected at the embrittlement point. 

For a given rate of chain scission, the failure properties of an initially brittle polymer decrease at a considerably slower rate than for an initially ductile polymer. 

For an initially ductile polymer, the rate of decay of failure properties decreases abruptly after the embrittlement point. 

Table 13 Worked example E5 failure properties of the powders...

For most rubber systems the network is formed after the polymer is compounded and molded into the desired final shape. Once cross-linked, the material no longer can be processed. If cross-linking occurs prior to compounding or molding, the material is referred to as gelled, and it cannot be used. Most rubber is used in a compounded and cured form. There is an optimum cross-link density for many failure properties such as tensile strength and tear which will be discussed in the next section. 

This crude distinction between adhesive and cohesive wear mechanisms is probably oversimplified in the sense that it neglects many aspects of the interactions between bulk deformation modes and interface rheology. It has, however, the merit of making a clear distinction between wear processes which can, to some extent, be related to known bulk failure properties and... 

Consider a master sheet of a given material from which tensile specimens can be cut at various directions as shown in Figure 6A. The master sheet is assumed to be both macroscopically homogeneous and isotropic insofar as its failure properties are concerned. The tensile samples will display no changes of statistical failure properties with their orientation... 

Figure 6. Top schematic of a homogeneous and isotropic master sheet containing individual samples showing no directional preference of failure properties. Bottom an enlarged view of a section of an individual sample snowing a distribution of flaw elliptici-ties and orientations.

The effects of molecular orientation on local failure properties have been neglected. The simultaneous effects of flaw orientation and molecular orientation should be accounted for in the model and delineated exprimentally. 

Practical uses of epoxies include load bearing applications such as structural adhesives and composite matrices. In these applications, their most detrimental feature is a characteristic low resistance to brittle fracture. The desire to improve this property has motivated studies on thermoset fracture behavior for the last two decades. Of particular interest is the relationship between the molecular structure and the failure properties of thermosetting epoxies, the subject of this chapter. 

Eqs. (8) and (11) are important because they predict a direct relationship between a failure property and network structure. If the Equations hold, then the following experimental results would be expected (1) l o for a series of networks of the same chemical composition will reflect only differences in crosslinking, and (2) 2 /n for chemically different networks of the same will reflect the differences in their backbone composition. Experiments such as these have been performed on a variety of elastomers e.g., polyurethanes,... 

We shall see that the situation is far from simple and that the data on molecular processes cannot be used to make quantitative predictions about macroscopic deformation and failure properties. In particular it will beconw evident that the kinetic theory of fracture initiation is an oveRimpliflcation. 

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