Stress development

3 Thermally and Pressure-Induced Stresses 6.3.1 Stress Development 

In the injection molding process, frozen layers develop from the outer surface toward the core each layer solidifies at a different time under different pressures. While all layers undergo the tendency of thermal contraction that generates 

At each time step, the integral of stresses over the thickness is balanced with the external forces applied to the part by the mold walls. When the part is ejected from the mold, the external forces applied to the part surface are suddenly removed. The product can thus deform according to force balance. Some residual stresses can remain within the part. The equilibrium of the residual stresses determines the 

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Dehydration. Residual liquid and physisorbed moisture on particle surfaces can be eliminated on beating to - 200° C. Temperatures ia excess of 1000°C may be requited to eliminate cbemisorbed water (29). Kaolin must be beated to 700°C to Hberate tbe water of crystallisation and produce tbe desired dehydrated aluminosiUcate. As with biader burnout, rapid gas evolution from rapid dehydration can result ia catastrophic stress development within a body. 

Fig. 7. (a) Relative thermal expansion of porcelain enamel on sheet steel, (b) Stress development in the enamel layer. 

This form of corrosion can be prevented in some instances by ehm-inating high stresses. Stresses developed during fabrication, pai4icu-larly during welding, are frequently the main source of troiible. Of course, temperature and concentration are also important factors in this type of attack. 

Good wetting is of course not a sufficient criterion for good contact adhesion because it takes no account of the factors that influence the mechanical loss factor, C, in Eq. 8, nor does it account for residual stress development during cure. But aside from these factors, one might inquire into the validity of the correlation between practical contact adhesion and VEa beyond 0° contact angle , i.e. can any distinction be made based on VEa between different adhesives, all of which perfectly wet the adherend ... 

Stress relieving at 600-700°C for steels decreases operation at temperatures some 20°C lower. Unfortunately, suitable furnaces generally are not available, and local stress relieving of welds, etc., is often not successful because further stresses develop on cooling. 

Speed cannot be increased indefinitely due to mechanical stresses developed in the rotating impeller and due to the limit of Mach 1 for the tip speed and gas velocity. The limitations will vary according to the impeller designs of the various manufacturers. 

Rate of change of temperature, as rapid temperature changes can cause some heavy-duty finishes to crack up due to the high stresses developed by thermal shock ... 

Object in this section is to review how rheological knowledge combined with laboratory data can be used to predict stresses developed in plastics undergoing strains at different rates and at different temperatures. The procedure of using laboratory experimental data for the prediction of mechanical behavior under a prescribed use condition involves two principles that are familiar to rheologists one is Boltzmann s superposition principle which enables one to utilize basic experimental data such as a stress relaxation modulus in predicting stresses under any strain history the other is the principle of reduced variables which by a temperature-log time shift allows the time scale of such a prediction to be extended substantially beyond the limits of the time scale of the original experiment. 

When a plastic material is subjected to an external force, a part of the work done is elastically stored and the rest is irreversibly (or viscously) dissipated hence a viscoelastic material exists. The relative magnitudes of such elastic and viscous responses depend, among other things, on how fast the body is being deformed. It can be seen via tensile stress-strain curves that the faster the material is deformed, the greater will be the stress developed since less of the work done can be dissipated in the shorter time. 

Cyclic stress developing from on-off and peak-load boiler operations... 

The shear stress Is uniform throughout the main liquid slab for Couette flow ( ). Therefore, two Independent methods for the calculation of the shear stress are available It can be calculated either from the y component of the force exerted by the particles of the liquid slab upon each reservoir or from the volume average of the shear stress developed Inside the liquid slab from the Irving-Kirkwood formula (JA). For reasons explained In Reference (5) the simpler version of this formula can be used In both our systems although this version does not apply In general to structured systems. The Irvlng-Klrkwood expression for the xy component of the stress tensor used In our simulation Is... 

The nip width (or gap between the rolls) is critical. Increasing the width decreases the shear stress developed and less dispersion occurs. 

The viscosity of the polymer or mix also controls the level of the shear stresses developed in the nip region. The level of temperature in the rubber mass, by its effect on viscosity, will also influence the level of shear stresses developed. 

In this section we deal with perhaps the most conceptually difficult of all the responses observed in linear viscoelastic materials. This is the response of a material to an oscillating stress or strain. This is an area that illustrates why rheological techniques can be considered as mechanical spectroscopy. When a sample is constrained in, say, a cone and plate assembly, an oscillating strain at a given frequency can be applied to the sample. After an initial start-up period, a stress develops in direct response to the applied strain due to transient sample and instrumental responses. If the strain has an oscillating value with time the stress must also be oscillating with time. We can represent these two wave-forms as in Figure 4.6. 

These designs avoid the possible stress developed in stepped horns. The amplification factor for either a linear or an exponential horn is the ratio of the end diameters (not areas as with stepped). The linear taper is the easier design to manufacture but its potential magnification is normally restricted to a factor of approx. 4-fold. The exponential taper offers higher magnification factors than the linear taper. Its shape makes it more difficult to manufacture but the small diameter of the working end and its length make it particularly suited to micro applications. 

The thermal expansion coefficient of silicon is approximately seven times larger than that of Si02, as given in Table 2. When Si02 is deposited, typically at temperatures of several hundred degrees C, an in-plane compressive stress develops in the oxide layer as the Si wafer is cooled by AT to room temperature. For a uniform 2-dimensional thin film deposited on a substrate, the in-plane stress obtained from Equation 2 is ... 

Shear stresses develop in fluids when adjacent particles have different velocities. This causes the fluid to deform and undergo turbulent mixing. 

Kim, K.S. and Hahn, H.T. (1989). Residual stress development during processing of graphite/epoxy composites. Composites Sci. Technol. 36, 121-132. 

Thermal Stress Development in Thick Epoxy Coatings... 

Thermal Stress Determination. The method selected to determine the thermal stress developed at the epoxy-aluminum interface was the bending beam technique utilized by Dannenberg (9), Shimbo, et al. (10) and others (12-13,17). The exact apparatus configuration is that of Dannenberg s except that thicker coatings were applied to the beam. 

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