Stiffnesses materials

Materials respond to stress by straining. Under a given stress, a stiff material (like steel) strains only slightly a floppy or compliant material (like polyethylene) strains much more. The modulus of the material describes this property, but before we can measure it, or even define it, we must define strain properly. 

Basically, anything that can be done to reduce the temperature of the product by removal of heat generated by the cyclical stress will improve the possibilities of surviving the cyclical stress. If the heat transfer capability is limited, then the only alternative is to use stiff materials and low stress levels on the product compared with the strength capability of the material. The heavier products that result will be relatively inefficient in the use of material. In some cases when the load applied is an inertial load (such as an impeller on a pump) it may be that only a trade-off of weight for low stress level can cause failure. 

Glucose molecules can link together into chains, with each ring tethered to the next by a bridging oxygen atom. In one form, this is cellulose, the stiff material that gives the stalks of plants and the trunks of trees their structural strength. Chitin, a variation on cellulose, is an even stiffen material that forms the exoskeletons of crustaceans such as crabs and lobsters. 

Dinelli, F., Biswas, S. K., Briggs, G. A. D., and Kolosov, O. V. (2000a). Measurements of stiff-material compliance on the nanoscale using ultrasonic force microscopy. Phys. Rev. 5 61(20), 13995-14006. [300]... 

More complex expressions arise if the sphere is taken to be elastic, so that the above equation should only apply to soft materials indented by very stiff materials. 

To avoid stick-slip, one should try to make the spring constant high enough (using stiff materials and stable constructions). It can be shown, that stick-slip may also arise from the velocity dependence of the friction coefficient [460], When the friction coefficient decreases with sliding velocity, stick-slip is amplified. When the friction coefficient increases with velocity, stick-slip is damped out. The former is usually the case at low speeds, certainly for the transition from static to dynamic friction, whereas the latter prevails usually at high velocity. 

Also the curvature of the loading curve is an indication of the presence of a compressible material. The third curve in Fig. 29 can be fitted with a function in the form F=F0+az2 without any linear term, whereas the second curve can be fitted with a linear term. This means that, when the tip is placed on the residues of the border walls inside the rectangles, it pushes at the beginning on a soft material that becomes stiffer and stiffer as the tip compresses it. Consequently, the slope of the loading curve goes from 0 (very soft material) to kc, i.e. the elastic constant of the cantilever (very stiff material). 

Series I Acrylic Latex Emulsions. A series of four acrylic latex emulsions varying in glass transition temperature (Tg) (3) were applied first. Tg is the temperature at which the resin changes from a relatively flexible to a relatively stiff material. The acrylic latexes are made from water-insoluble monomers such as acrylates and alkyl acrylates polymerized in emulsion form to produce an aqueous dispersion or latex of the polymer. Upon drying, the emulsion is irreversibly broken so that the applied material becomes wash-fast. The application requires no catalyst or high temperature heating. 

This process is used to manufacture products with a combination of properties. For example, a ballpoint pen barrel needs to have the rigidity of a stiff material, but it is convenient to have attached a more easily gripped area consisting of an elastomeric material. Another example is the set of keys on a computer keyboard. To improve wear resistance, the characters are co-injected rather than printed onto the key top. Finally, co-injection can be used to encapsulate a low cost material that has poorer properties, such as regrind, inside a shell of prime material. 

A situation particularly easy to imagine is as follows let the space x < 0 be occupied by a stiff material (A, B)X let the plane x = 0 be an interface against a material also of formula (A, B)X but of much lower viscosity but let this material be just a thin layer, bounded at say x = A by another interface, and let the material in the space x > A match the material in the space X < 0. Then if atoms of A are larger than atoms of B or the combined... 

Figure 18.1 Cross-section through a cylinder of stiff material embedded in an infinite extent of less stiff material. The stress state in the host is homogeneous but anisotropic at all points, except insofar as it is modified by the inclusion.

Fig. 6.7-14 Typical robust double-screw extruder with integrally mounted pug sealer for the processing of stiff materials (courtesy ).C. Steele, Statesville, NC, USA)...

Fig. 6.7-13 Diagram of maxima of properties, such as strength, that are dependent on both porosity decreases and grain size increasing as a function of firing temperature or time [B.80]. The dashed line suggests that maxima may change with different material and fabrication parameters Fig. 6.7-14 Typical robust double-screw extruder with integrally mounted pug sealer for the processing of stiff materials (courtesy J.C. Steele, Statesville, NC, USA)...

---