Brittle material behavior

Chip Formation (Abrasive Process), Fig. 2 Brittle material behavior versus ductile material behavior in grinding (based on Salje and Mohlen 1987 Klocke 2009)... 

Visible damages can be found for specimens with a fiber orientation of 90° and —45°, while the surface for 0° and +45° appears to be very consistent and nearly free of cracks or other damages. Specimens with 90° fiber orientation are showing cracks extending frequently from the milled surface at an angle of 18° into the material at intervals of about 200 pm. These cracks are caused by the brittle material behavior... 

If dimensioning is performed consistently and systematically, only the characteristic data acquired during short-term tests are suitable as characteristic measurement data (K values). They include yield point Oy, ultimate strength Of for brittle material behavior, and the threshold for a particular non-elastic deformation Ogg. 

Formulations of isotropic quasi-brittle materials behavior consider, generally, different inelastic criteria for tension and compression. The new model introduced in (Lourengo et al. 1997), extended to accommodate shell masonry behavior (Lourengo 2000), combines the advantages of modem plasticity concepts with a powerful representation of anisotropic material behavior, which includes different... 

Most adhesives are polymer-based materials and exhibit viscoelastic behavior. Some adhesives are elastomer materials and also exhibit full or partial rubberlike properties. The word elastic refers to the ability of a material to return to its original dimensions when unloaded, and the term mer refers to the polymeric molecular makeup in the word elastomer. In cases where brittle material behavior prevails, and especially, when inherent material flaws such as cracks, voids, and disbonds exist in such materials, the use of the methods of fracture mechanics are called for. For continuum behavior, however, the use of damage models is considered appropriate in order to be able to model the progression of distributed and non-catastrophic failures and/or irreversible changes in material s microstructure, which are sometimes described as elastic Hmit, yield, plastic flow, stress whitening, and strain hardening. Many adhesive materials are composite materials due to the presence of secondary phases such as fillers and carriers. Consequently, accurate analysis and modeling of such composite adhesives require the use of the methods of composite materials. 

Molding compounds based on epoxy or phenolic resins mixed with fillers are used for the conventional printed-circuit board technologies widely employed in electronics production. These molding compounds belong to the group of thermoset plastics. They consist of tightly crosslinked macromolecules, as reflected in their extremely hard and brittle material behavior at room temperature. 

In this chapter, we will review the effects of shock-wave deform.ation on material response after the completion of the shock cycle. The techniques and design parameters necessary to implement successful shock-recovery experiments in metallic and brittle solids will be discussed. The influence of shock parameters, including peak pressure and pulse duration, loading-rate effects, and the Bauschinger effect (in some shock-loaded materials) on postshock structure/property material behavior will be detailed. 

While the structure/property behavior of numerous shock-recovered metals and alloys has received considerable attention in the literature to date, the response of ceramics, cermets, and other brittle solids (including geological materials) to shock loading remains poorly understood [9], The majority of shock-recovery studies on brittle materials have concentrated on examining... 

Polyurethane adhesives are formed by the reaction of various types of isoeyanates with polyols. The polar urethane group enables adhesion to various surfaees. Depending on the raw materials, glue lines with rubber-like elastic to brittle-hard behavior ean be aehieved. The presence of reactive terminal groups provides a ehemieally hardened adhesive. When polymerized to a high enough molecular weight, the adhesive ean be physically rather than chemically hardened, i.e. a hot melt. 

Material behavior have many classifications. Examples are (1) creep, and relaxation behavior with a primary load environment of high or moderate temperatures (2) fatigue, viscoelastic, and elastic range vibration or impact (3) fluidlike flow, as a solid to a gas, which is a very high velocity or hypervelocity impact and (4) crack propagation and environmental embrittlement, as well as ductile and brittle fractures. 

Consequently, changing the temperature or the strain rate of a TP may have a considerable effect on its observed stress-strain behavior. At lower temperatures or higher strain rates, the stress-strain curve of a TP may exhibit a steeper initial slope and a higher yield stress. In the extreme, the stress-strain curve may show the minor deviation from initial linearity and the lower failure strain characteristic of a brittle material. 

Brittleness Brittle materials exhibit tensile stress-strain behavior different from that illustrated in Fig. 2-13. Specimens of such materials fracture without appreciable material yielding. Thus, the tensile stress-strain curves of brittle materials often show relatively little deviation from the initial linearity, relatively low strain at failure, and no point of zero slope. Different materials may exhibit significantly different tensile stress-strain behavior when exposed to different factors such as the same temperature and strain rate or at different temperatures. Tensile stress-strain data obtained per ASTM for several plastics at room temperature are shown in Table 2-3. 

In terms of the mechanical behavior that has already been described in Sections 5.1 and Section 5.2, stress-strain diagrams for polymers can exhibit many of the same characteristics as brittle materials (Figure 5.58, curve A) and ductile materials (Figure 5.58, curve B). In general, highly crystalline polymers (curve A) behave in a brittle manner, whereas amorphous polymers can exhibit plastic deformation, as in... 

Majerus (61, 62) has approached the failure behavior of highly filled polymers by a thermodynamic treatment in which the ability to resist rupture is related to the propellant s ability to absorb and dissipate energy at a certain rate. An energy criterion which requires failure to be a function of both stress and strain was originally stated by Griffith (36) for brittle materials and later adapted to polymers by Rivlin and Thomas (80). Williams (115) has applied an energy criterion to viscoelastic materials such as solid propellants where appropriate terms are included for viscous energy dissipation. 

Strain rate sensitivity of (or the effect of press speed on) the formulation is of primary concern in scale-up. Whether the product development work was performed on a single-stroke press or a smaller rotary press, the objective in operations will be to increase efficiency, in this case the tablet output rate and, therefore, the speed of the press. For a material that deforms exclusively by brittle fracture, there will be no concern. Materials that exhibit plastic deformation, which is a kinetic phenomenon, do exhibit strain rate sensitivity, and the effect of press speed will be significant. One must be aware that although specific ingredients (such as calcium phosphate and lactose) may exhibit predominately brittle fracture behavior, almost everything has some plastic deformation component, and for some materials (such as microcrystalline cellulose) plastic deformation is the predominant behavior. The usual parameter indication is that target tablet hardness cannot be achieved at the faster press speed. Slowing the press may be the only option to correct the problem. 

Mechanical Criteria. There is a big difference in the behavior of initially ductile and initially brittle materials. Ductility is sharply linked to the macromolecular scale structure, whereas in brittle materials (or more generally in the brittle regime for any polymeric material), the properties (including ultimate ones) depend essentially on the molecular scale structure and on the size of eventual defects. This difference can be easily illustrated in the case of an amorphous linear polymer, but the reasoning would be the same for a thermoset. 

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