Internal stresses relaxation

Non-reactive solution adhesives the solvent wets the surfaces to be assembled, then evaporates involving the cohesion of the parts to be assembled by the adhesive joint. The heat behaviour is generally moderate. If the solvent swells the materials to be assembled, there can be migration of materials and subsequent cracking by residual internal stress relaxation. 

High adhesion strength with simultaneous minimization of the internal stresses can be obtained in the formation of adhesive-bonded joints imder conditions in which the rates of internal stress relaxation and pol5rmer formation are compatible. This approach to decreasing... 

The authors of Ref. [21] supposed, that in orientational drawing process of poly(metyl methacrylate) (PMMA) the following structure changes occur the transition to more equilibrium structure owing to molecular package improvement and internal stresses relaxation. The quantitative structural model absence not allows the authors of Ref [21] to give direct proofs of their suppositions. In Ref [22] such treatment was fulfilled on the example of extruded amorphous polyarylates DV and DF-10 with the cluster model of polymers amorphous state structure using [12, 23],... 

Returning to the Maxwell element, suppose we rapidly deform the system to some state of strain and secure it in such a way that it retains the initial deformation. Because the material possesses the capability to flow, some internal relaxation will occur such that less force will be required with the passage of time to sustain the deformation. Our goal with the Maxwell model is to calculate how the stress varies with time, or, expressing the stress relative to the constant strain, to describe the time-dependent modulus. Such an experiment can readily be performed on a polymer sample, the results yielding a time-dependent stress relaxation modulus. In principle, the experiment could be conducted in either a tensile or shear mode measuring E(t) or G(t), respectively. We shall discuss the Maxwell model in terms of shear. 

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

Two other temperatures are important in the working of glass. At the annealing point (p = 10 poise) there is still enough fluidity to relax internal stresses in about... 

Internal stresses occur because when the melt is sheared as it enters the mould cavity the molecules tend to be distorted from the favoured coiled state. If such molecules are allowed to freeze before they can re-coil ( relax ) then they will set up a stress in the mass of the polymer as they attempt to regain the coiled form. Stressed mouldings will be more brittle than unstressed mouldings and are liable to crack and craze, particularly in media such as white spirit. They also show a characteristic pattern when viewed through crossed Polaroids. It is because compression mouldings exhibit less frozen-in stresses that they are preferred for comparative testing. 

Many designs incorporate the phenomenon of stress-relaxation. For example, in many products, when plastics are assembled they are placed into a permanently deflected condition, as for instance press fits, bolted assemblies, and some plastic springs. In time, with the strain kept constant the stress level will decrease, from the same internal molecular movement that produces creep. This gradual decay in stress at a constant strain (stress-relaxation) becomes important in applications such as preloaded bolts and springs where there is concern for retaining the load. The amount of relaxation can be measured by applying a fixed strain to a sample and then measuring the load with time. 

Most polymers are applied either as elastomers or as solids. Here, their mechanical properties are the predominant characteristics quantities like the elasticity modulus (Young modulus) E, the shear modulus G, and the temperature-and frequency dependences thereof are of special interest when a material is selected for an application. The mechanical properties of polymers sometimes follow rules which are quite different from those of non-polymeric materials. For example, most polymers do not follow a sudden mechanical load immediately but rather yield slowly, i.e., the deformation increases with time ( retardation ). If the shape of a polymeric item is changed suddenly, the initially high internal stress decreases slowly ( relaxation ). Finally, when an external force (an enforced deformation) is applied to a polymeric material which changes over time with constant (sinus-like) frequency, a phase shift is observed between the force (deformation) and the deformation (internal stress). Therefore, mechanic modules of polymers have to be expressed as complex quantities (see Sect. 2.3.5). 

Annealing in metals can first lead to stress relaxation in which stored internal strain energy due to plastic deformation is relieved by thermally activated dislocation motion (see Figure 5.18). Because there is enhanced atomic mobility at elevated temperatures, dislocation density can decrease during the recovery process. At still higher temperatures, a process known as recrystallization is possible, in which a new set of... 

There is an international standard, ISO 691455 which covers both the continuous and intermittent procedures plus the simplified intermittent method. Strip test pieces are used, 1 mm thick to minimise oxygen diffusion effects. Measurement at a series of temperatures is recommended and results are presented in graphical form but no consideration is given to interpretation. British Standards did not accept this revision of ISO 6914 and BS 903 Part A5256 is identical to the 1985 ISO method. The revision was not accepted in the UK because mistakes in handling comments resulted in inconsistencies. As an example, the title is now stress relaxation but a note says that this term is avoided ... 

Creep, Stress Relaxation, Elastic Recovery. Olefin fibers exhibit creep, or time-dependent deformation under load, and undergo stress relaxation, or the spontaneous relief of internal stress. High molecular weight and high orientation reduce creep. 

Stress relaxation tests Measurement of internal friction... 

Illers and Jenckel have discussed the internal frictions peak in the region of the crystal disordering transition. They analyzed the shape of the loss peak at various frequencies in terms of two different distributions of relaxation times existing above and below the transition. Nagamatsu, Yoshitomi, and Takemoto have also discussed this change in the distribution of relaxation times as revealed by measurements of stress relaxation. 

---