Stress Waves thermalization

A third source of stress waves derives from the expansion of any gases (CO, CN, N2, CH3 etc.) produced by thermal or photochemical decomposition within the substrate [104]. This factor, for instance, has been invoked to account for the transient stresses of about 0.1 MPa detected in the UV irradiation of polyimide below the ablation threshold [106]. In the case of doped PMMA, irradiation with 150-ps pulses at 1064 nm, Hare et al. [104] estimate that at the ablation threshold, the thermoelastic mechanism and the expansion of the decomposition products contribute about equally to the generated pressure. For specifically designed polymers that upon irradiation form a high enough concentration of volatile products, the generated pressure has been suggested to be the primary cause of material ejection [68-69]. 

Acoustic emission If a bond is mechanically or thermally stressed local perturbations of energy, or stress waves, may be released from discontinuities such as disbonds. The high frequency content of such stress waves may then be detected with a piezoelastic sensor. Unfortunately it is usually necessary to stress the joint to a considerable extent, which may often be impossible or inadvisable. 

If voids exist at the interface between the coating and substrate they may reduce the adhesion by decreasing the effective contact area, acting as stress concentration defects and providing an easy path for fracture initiation and propagation (Ch. 12). Interfacial voids also result in increased contact resistance between film and substrate and decreased thermal conductance across the interface, and present a discontinuity to stress wave propagation. 

Assuming that the cyclic waveform used in the previous section was sinusoidal then the effect of using a square wave is to reduce, at any frequency, the level of stress amplitude at which thermal softening failures start to occur. This is because there is a greater energy dissipation per cycle when a square wave is used. If a ramp waveform is applied, then there is less energy dissipation per cycle and so higher stresses are possible before thermal runaway occurs. 

The pressure is to be identified as the component of stress in the direction of wave propagation if the stress tensor is anisotropic (nonhydrostatic). Through application of Eqs. (2.1) for various experiments, high pressure stress-volume states are directly determined, and, with assumptions on thermal properties and temperature, equations of state can be determined from data analysis. As shown in Fig. 2.3, determination of individual stress-volume states for shock-compressed solids results in a set of single end state points characterized by a line connecting the shock state to the unshocked state. Thus, the observed stress-volume points, the Hugoniot, determined do not represent a stress-volume path for a continuous loading. 

A new rheo-photoacoustic Fourier transform infrared cell has been developed to perform stress-strain studies on polymeric materials. The rheo-photoacoustic measurements lead to the enhancement of the photoacoustic signal and allow one to monitor the effect of elongational forces on the molecular structure of polymers. Propagating acoustic waves are detected as a result of infrared reabsorption and the deformational and thermal property changes upon the applied stress. 

In all block copolymers investigated, the principal morphological wave length of the rubbery domains, i.e. sizes or mean spacings, is too short to concentrate the stress in a large enough volume element to nucleate crazes in the majority phase of PS. Furthermore, with the exception of perhaps the pure KRO-3 Resin with lamellar morphology, the combination of thermal stresses and the levels of applied stress are also insufficient to cavitate the rubbery domains without any further assist from more macro stress concentrations. Hence the available evidence indicates that in these polymers, crazes initiate entirely from surface stress concentrations. 

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