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The enthalpy recovery of polystyrene materials with narrow molecular weight distribution is studied by means of fast calorimetry. Two different molecular weights, Mw, have been investigated, namely 2000K and 212k. Intrinsic isotherm experiments at aging temperatures ranging from above to below the nominal Tg have been reported. The effect of the cooling rate on the glass transition temperature is also exanimated and it is showed that the cooling rate-dependence of the Tg is Vogel-Fulcher and not Arrhenius. The main finding is that the enthalpy recovery kinetics is not affected by the molecular weight showing that the contribution to free volume related due to chain terminals is negligible in the molecular weight range (212k-2000k) analysed in this paper.

Composite materials subjected to cyclic loading degrade their strength and/or stiffness due to the accumulation of different overlapping damage mechanisms. Three substantial regimes of damage are commonly recognized. At the early stage of the loading a diffuse matrix cracking and matrix/fiber debonding (Stage I) precipitate in a saturation state termed “characteristic damage state” (CDS) that represents the initiation of fibers rupture and incipient delamination phenomena (Stage II). The final collapse is debited to the coalescence of macro cracks with resulting ply ruptures and delamination (Stage III). In this paper, summarizing the capabilities of a recently developed two-parameter phenomenological model based on residual strength, it is shown that the strength degradation kinetics can be described by three distinct functions associated to the sequence of damage mechanisms. Despite the phenomenological prerogative of the model, from the analytical approach it results that the multiple damage mechanisms develop simultaneously even with different kinetics and manifest their effectiveness at different time scales, accordingly. This highlights the hierarchical nature of damage accumulation in composites, from diffuse matrix cracking, to fiber/matrix interface failure to fiber and ply rupture and delamination.

Fatigue and residual strength data available in literature were modeled with a modified two-parameter wear-out model based on strength degradation. The model explicitly accounts for the maximum applied stress and the stress ratio and requires a limited number of experimental data to predict with accuracy the fatigue life of a series of polymer-based composites. In this paper a substantial modification of the model is proposed in order to enhance its capability in predicting the residual strength kinetics with emphasis to the “sudden drop” of strength before catastrophic failure. It is argued that the strength degradation kinetics under given loading conditions can be obtained from the statistical distribution of cycles to failure under the same loading conditions. From the new approach no new parameters are introduced, limiting to a minimum the experimental data needed to predict the residual strength. The strength degradation law reliability is verified on three different materials data sets appeared in literature. The results indicate that both the fatigue life and the residual strength are related to the statistical distribution of the static strength.

A new methodology for the analysis of failure modes in composite materials by means of acoustic emission techniques has been developed. A single-carbon-fiber composite based on a polyester matrix, has been used as a simple model. The occurrence of fiber-breakage during tensile loading tests has been observed by a polarized light microscope and concurrently detected by a resonant acoustic probe. The resonant probe has been used as a trigger for the reading of fiber failure events. Single acoustic emission events from a wide-band probe has been recorded for FFT Analysis. The single-fiber specimen, having a unique failure mode, has advantages for the standardization of AE techniques for the quantitative analysis of failures in polymer-composite materials. The same procedure can be exploited to investigate other failure modes namely, fiber matrix solidus debonding and matrix cracking.

The structural relaxation process of an inorganic glass (Li2O · 2SiO2) at an ageing temperature of 703 K for an ageing timeof 1 h has been studied by differential scanning calorimetry. A four-parameter model—the Tool-Narayanaswamy-Moynihan (TNM)—model was applied to simulate the normalized specific heat curve measured. A set of optimized parameters, Δh*/R, β, In A, and x was obtained. Then the effects of variation of each adjustable parameter on the calculated specific heat were summarized.

Tension-tension fatigue tests were conducted on unnotched injection moulded poly(phenylene ether ketone) (PEK-C) specimens with two stress ratios, R. The fatigue behaviour of this material is described. The S-N curves (S = alternating stress, N = number of cycles to failure) for different R values have the same general shape, but the curve for bigger R is shifted to long cycles. A fatigue lifetime inversion is observed from constructed S-N curves. Examinations of failure surfaces and analyses of the fatigue data reveal that the fatigue failure mechanism of the material studied is crack growth dominated. But the manner of the fatigue crack initiation and propagation depends on the maximum cyclic stress applied. At higher stresses, the fatigue crack originates at the corner of the specimen and propagates inward; at lower stresses, the fatigue crack nucleates at an internal flaw of the specimen and propagates outward. The fatigue lifetime inversion corresponds to the transition of crack initiation and propagation from one mode to the other.

Structural relaxation is accompanied by a simultaneous change in all structure-sensitive physical mechanical properties. Naturally, a fundamental correlation between structural relaxation and the variation of the properties of the glass would be of great interest to scientists and engineers. Changes in mechanical and dielectric properties during the course of structural relaxation are of particular interest, but one must remember that their measurements involve the use of external stress (or strain), which, regardless of its magnitude, will affect the ongoing structural relaxation. The application of even the smallest stress or strain could shorten the relaxation time by one or more orders of magnitude and hence caution must be exerted in seeking correlations between volume and enthalpy relaxation on the one hand, and viscoelastic (mechanical or dielectric) measurements on the other. The two approaches can be viewed as manifestations of the same structural relaxation although one must bear in mind that they differ fundamentally in terms of the cause and the effect, and while the theoretical background of correlation between structural and viscoelastic relaxation has not been explored, in this paper an empirical correlation between the two will be reported.

In order to simulate the behaviour of fibres in relative motion in polymer fluids we measured, by a microbalance, the wetting force of intermediate modulus carbon fibres, both untreated and oxidized, by several fractions of fluorinated fluids of increasing viscosity and by polyacrylamide ( paa )-water solutions. The results showed that a further term must be included in the conventional wetting force equation, in order to take into account the contribution of the shear stress exerted by the viscous fluid on the fibre surface. Calculated wetting force curves were found in good agreement with experimental results.