Continuous and intermittent stress relaxation studies on foamed polysiloxane rubber

The measurement of dynamic stress relaxation can reveal otherwise hidden weaknesses in rubber materials. The stress relaxation of the minimum of the restoring foce has the undesirable characteristic of being greater than the continuous stress relaxation. An increase in the amplitude of the deformation increases the decay in the minimum values of the varying retractive stress, and an increase in the prestrain makes it smaller. The normalized curve for continuous stress relaxation is found to lie somewhere between the normalized curves for the maximum and minimum of the retractive stress. The measurement of network density on specimens aged under continuous and dynamic stress relaxation indicate that the mechanism behind the degradation of the rubber network is the same in both cases. The relations between intermittent, continuous, and dynamic stress relaxation give a reasonable estimate of the dynamic stress relaxation from the other two when proper values of the coefficients are used. This relation says that results both of continuous and intermittent stress relaxation are important for predicting the life time of a dynamic seal. The relations offers an explanation of the differences between maximum and minimum values obtained during the measurement of dynamic stress relaxation and their relation to continuous stress relaxation. From these relations it can be concluded that a rubber intended for dynamic seals should contain neither reversion nor secondary crosslinking. Crosslink density should be high in order to overpower effects on crosslinking of factors in the surroundings.

A novel method to study recrystallization behavior: Continuous heating stress relaxation

The effect of antioxidant on the crosslinking reaction occurring during the oxidation of sulphur vulcanizates of natural rubber was investigated by using both continuous and intermittent stress relaxation measurements. Similarly, by means of continuous stress relaxation and equilibrium swelling measurements the cross-linking reaction in vacuum was followed. The presence of antioxidant or removal of air reduced considerably both continuous and intermittent relaxation and also the difference between them, which measures the crosslinking. The crosslink formation must therefore form part of the sequence of oxidative reactions.

Continuous and intermittent stress relaxation experiments were carried out on dicumyl peroxide cured cis-polybutadiene in high vacuum and in air at several temperatures. An extension of the “two-network theory” is proposed whereby the effectiveness of a cross-link at various extension ratios is experimentally defined. The effectiveness varies from unity at an extension ratio of unity to zero at very high extension ratios.

A Quantitative In Situ SEM Bending Method for Stress Relaxation of Microscale Materials at Room Temperature

Stress relaxation in compression in air has been studied for compounds of various polymers. However, rubber seals, gaskets, and other applications are widely used where swelling agents, such as oil, are encountered. It was thought desirable to measure the stress-relaxation properties of various rubbers to determine their performance under such conditions. The results show that continuous stress relaxation is inhibited by the presence of oil and that unconfined swelling measurements predict the degree of inhibition directly. The variables of temperature and type of oil were investigated.

Glass powder reinforced epoxy was used in the fields of plastic sealing of semiconductor devices. In such environment, the materials were exposed in the high temperature, and the visco-elastic behavior must occur in the materials. However, there are few reports about these materials focused on visco-elasticity. Therefore, the analysis of visco-elastic behavior was an important issue to be taken in account. In this study, the stress relaxation behavior and dynamic mechanical analysis of epoxy composites filled with different amounts of glass powder were investigated. On the dynamic mechanical analysis, the glass transition temperature decreased with the increment of the amount of glass powder contents. And the viscosity at high temperature also decreased when the contents were augmented. On the stress relaxation test, master curves of stress relaxation were obtained, and the time-temperature superposition principles was ascertained. The slopes of time-temperature shift factors were changed depending on the amount of glass powder. Therefore, the time-temperature shift factors were calculated by using the relationship between the slopes of the time-temperature shift factors and the amounts of glass powder. The effect of amounts of glass powder on stress relaxation was also investigated. When the amount of glass powder increased, then the stress relaxation curves shifted to short time side. Comparing the master curves of stress relaxation with the different amount of glass powder, it is understood that the stress relaxation behavior of each material is similar. The effect of glass powder contents on stress relaxation behavior showing the effect of reinforcement and heat resistance is also discussed.

Elastomer seals are used in many industrial applications due to their excellent mechanical properties at a wide range of temperatures. Their high versatility and recovery potential under several load conditions make them well suitable for the application in containers designed for transport, storage and/or disposal of radioactive materials. In containers for low and intermediate level radioactive waste, elastomer seals are used as barrier seals, and as auxiliary seals in storage and transportation casks (dual purpose casks) for heat generating radioactive waste, such as spent fuel and high-level waste. While a seal exchange at defined intervals is typical in many conventional applications, it is impossible or at least hard to perform when principles of minimization of radiation exposure have to be considered and prohibit an unnecessary cask handling. An extensive knowledge of the change of the elastomer’s properties during aging and the availability of reliable end-of-lifetime criteria to guarantee the permanent safe enclosure of the radioactive material is mandatory. As BAM is involved in most of the national cask licensing procedures and in the evaluation of cask-related long-term safety issues, great efforts have been already made and are still planned to scientifically support this task. Compression stress relaxation and compression set were identified as key indicators of elastomer long-term performance and quantitatively investigated in comprehensive test programs. Among other representative types of elastomers, specimens made from ethylene propylene diene rubber (EPDM) were tested before, during and after aging to capture the most important of their complex mechanical properties. In the presented study, exemplary results were used to simulate the compression stress relaxation and the compression set of elastomer O-rings during aging. Regarding the influence of temperature, the time-temperature superposition principle is applied in the relaxation analysis of elastomer O-rings. The proposed model is implemented in the commercial finite element software ABAQUS/Standard® [1] with a sequential temperature displacement coupling. Numerical results match the experimental compression stress relaxation measurements well. The prediction of compression set values after long-term aging shows a relatively good agreement with the experimental results. Nevertheless, all input parameters derived from the specimen tests, additional assumptions concerning boundary conditions and modeling strategy are discussed with regard to the identified slight discrepancies. The possibility to extend the finite element model to represent the O-ring seal’s ability to recover after a (fast) partial release is taken into account.

Pultrusion manufacturing process is a well established technique for the cost-effective production of high-modulus and lightweight composite materials having constant cross-sectional profiles. The pultruded composites are widely used as structural members viz., beams, trusses and stiffeners, owing to the presence of high proportion of axial fibers necessary to sustain large tractive forces. These structural members are subjected to a combination of static and dynamic loading conditions at wide temperature ranges and over longer periods. Since polymeric composites exhibit viscoelastic behavior, the effectiveness of these materials as structural members must be thoroughly evaluated to ensure long-term stability. In previous research, the dynamic performance characteristics of pultruded glass-graphite/epoxy hybrids were evaluated at room temperature. The effects of temperature, frequency, post-curing, along with the type and placement of fibers on the dynamic flexural properties of glass/epoxy and hybrid glass-graphite/epoxy were also investigated. This paper focuses on the evaluation of creep and stress relaxation performance characteristics of pultruded glass-graphite/epoxy hybrid composites. Dynamic mechanical analysis technique was adopted for the accelerated creep and stress relaxation testing. Time-temperature superposition principle, which greatly reduced the experimental time, was effectively utilized for predicting the creep and stress relaxation properties of the hybrid composites. Results indicate that the type and amount of fibers as well as their lay-up sequence plays a significant role in determining the flow and load bearing characteristics.

Stress relaxation of elastomer compounds

We constructed a second generation micro tensile test apparatus (“bulge tester”) to study mechanical properties of free standing thin films. The apparatus is insensitive to the initial bulge height, allowing for measurements on films deposited under compressive stress. We have checked the performance of the set-up by measuring the elastic properties of thin films. We found that our set-up displays an excellent linearity without hysteresis. We measured the Young’s modulus of Al thin films with submicron thickness. We obtain a Young’s modulus of 62±4 GPa. We studied stress relaxation in thin Al films after furnace anneal. The membrane samples cool down quickly after removal from the furnace, inducing a high thermal stress. The measurements could be started about 3 minutes after removal from the furnace, which enables us to study the first stage of the stress relaxation, which is thusfar not achieved by stress relaxation studies using the substrate curvature method. In the set-up we are able to perform creep measurements. The applied stress can be kept constant in the 10–300 MPa range (δσ<0.2 MPa) and creep rates as small as 1.10−10/s can be measured. We show the first results of creep measurements on thin Al films. It is shown that primary and secondary creep stages can be distinguished.

The site of chain scission and crosslinking in vulcanized natural rubber irradiated with 4 MeV electrons has been determined by analysis of stress relaxation data. Sulfur and peroxide vulcanizates of different crosslink densities were prepared and the crosslink densities determined from stress—strain measurements. Stress relaxation was measured during irradiation using modified commercial relaxometers. The specimens were maintained in an atmosphere of nitrogen to minimize oxidative side effects. Scission is deduced to take place in the vicinity of crosslinks, since the rate of continuous stress relaxation is independent of crosslink density. Scission may be associated both with crosslinks initially present and with those subsequently introduced by irradiation. Crosslinking by radiation is largely a random process. However, there is a crosslinking reaction dependent to a slight extent on crosslink density as well as a small contribution from random scission reactions. G values for the random reactions are given.

The human skin is composed of several layers, each with an unique structure and function. Knowledge about the mechanical behavior of these skin layers is important for clinical and cosmetic research, such as the development of personal care products and the understanding of skin diseases. Until today, most research was performed in vivo and focused on the mid-layer, the dermis. However, clinical and cosmetic applications require more detailed knowledge about the skin layers at the skin surface, the viable epidermis and stratum corneum, and the deeper lying hypodermis. Studying these layers in an in vivo set up is very challenging. The different length scales, ranging from µm for the stratum corneum to cm for the hypodermis, the interwoven layered structure and the inverse relation between penetration depth and resolution of non-invasive measurement techniques form major problems. As a consequence, hardly any data are available for the viable epidermis and hypodermis and reported data for stratum corneum are inconsistent. The aim of this thesis was therefore to characterize the mechanical behavior of individual skin layers in vitro and, for that, to develop the required experimental procedures. It was considered essential to perform experiments with samples of consistent quality in an accurate measurement set-up in a well-controlled environment. Various isolation and preservation methods were investigated on tissue performance, reproducibility and ease of handling. Because of the inhomogeneous layered structure of the upper skin layers, mechanical properties of the stratum corneum and viable epidermis were determined for various loading directions. First, the stratum corneum and epidermis were subjected to shear over a wide frequency range and with varying temperature and humidity. The typical geometry of the upper skin layers required preliminary testing series in order to define the right experimental conditions to ensure reliable results. Subsequently, micro-indentation experiments were applied using a spherical tip with a relatively large diameter. The Young¿s moduli were derived via an analytical and numerical method. Because of the complexity of measuring those skin layers, it was decided to focus on small deformations first. For both types of loading, result were highly reproducible. The shear tests demonstrated that the shear modulus is influenced by humidity but not by temperature in the measured range. If the skin is compressed with an indenter, the stiffness of the epidermis and stratum corneum, which is about 1-2 MPa, is about a factor 100 higher than for shear. No significant differences in stiffness between the stratum corneum and viable epidermis were observed per loading type. The results of these tests prove that it is essential to take into account the highly anisotropy of the tissue in numerical models. Rheological methods were developed to study the mechanical response of the subcutaneous adipose tissue. In the small linear viscoelastic strain regime, the shear modulus showed a frequency- and temperature-dependent behavior and is about 7.5 kPa at 10 rad/s and 37°C. Time-Temperature Superposition is applicable through shifting the shear modulus horizontally. A power-law function model was able to describe the frequency dependent behavior at constant temperature as well as the measured stress relaxation behavior. Prolonged loading at small strains results into a dramatic stiffening of the material. Loading-unloading cycles showed that this behavior is reversible. In addition, various large strain history sequences showed that stress-strain responses are reproducible up to 0.15 strain. When the strain further increases, the stress is decreasing for subsequent loading cycles and, above 0.3 strain, the stress response becomes stationary. These results showing time and strain effects indicate that adipose tissue likely behaves as an (anti-)thixotropic material, meaning that a constitutive model should contain parameters to describe the build-up and breakdown of the material structure. However, further experimental research is needed to fully understand the thixotropic behavior before such a model can be worked out in detail. In conclusion, this thesis evaluates the mechanical behavior of stratum corneum, epidermis and hypodermis using various in vitro set-ups. It was proven that for all skin layers reproducible results can be obtained. The research was aimed at developing reliable methods to determine the mechanical behavior of individual human skin layers. Future work should be focused on the relationship between mechanical properties and tissue deformation using imaging techniques and heading to the determination of the skin¿s failure behavior in relation to clinical and cosmetic treatments.

Linear viscoelastic behaviors for diglycidyl ether of bisphenol-A epoxy (DGEBA) resins cured with 4,4’-methylenedianaline (MDA) have been studied. For a particular DGEBA/MDA material with a glass transition temperature (Tg) of 102 °C, dynamic shear moduli were measured from 10-2 to 101.7 Hz for 41 temperatures between −150 °C and 180 °C. Because DGEBA/MDA epoxies are thermorheologically complex, construction of master curves via time-temperature superposition is not applicable. Stress relaxation experiments were performed from 90 °C to 112.5 °C using the same specimen. A combination of Fourier transformed transient data and dynamic data extended the frequency window of an isotherm to 6 orders of magnitude. A new methodology was developed for determining the relaxation spectrum, where the spectral density changes under the constraint that all spectral components have the same strength. This new method accommodates thermorheological complexity. The analysis was performed throughout the entire temperature range, including sub-Tg low-temperature glassy state (i.e., γ region), sub-Tg “excess wing” (i.e., β region), glass-to-rubber transition zone (i.e., α region), and above-Tg plateau and terminal zones. In sub-Tg γ and β regions, dielectric relaxation data were analyzed using the same method. Spectral analysis was applied to a variety of glassy-forming materials, including linear polymers, crosslinked polymers, oligomers and small molecules, with a focus on the relaxation processes in glass-to-rubber transition region (i.e., near and above α region). Spectra were constructed from (i) experimental data of a series of linear and crosslinked DGEBA polymers with different crosslink densities, and (ii) digitized literature data consisting storage and loss isotherms near and above Tg. A second relaxation process was found at higher temperatures and longer relaxation times than the main α peak, which is designated as the “α+” process, where the molecular mechanism of α+ is unknown. For molecular glass formers no α+ process is observed. For other materials the α+ process is present as a shoulder on α peak. In addition to temperature, deformation also affects mobility, where in the absence of deformation the mobility will decrease as the materials evolves towards the equilibrium state in a process called “physical aging”. Two major ideas how mobility is affected by deformation: rejuvenation and accelerated aging. In accelerated aging, the molecular mobility is decreased after deformation. In rejuvenation, the mobility will increase. To discriminate these two models, a set of 4-step non-linear tensile experiments were performed on a crosslinked DGEBA polymer at Tg – 16 °C. The effect of deformation history was evaluated by comparing the mobility before and after the large deformation, where the mobility was quantified as the magnitude of stress overshoot on a stress-strain curve during reloading in the 4-step experiment. The experimental results at Tg – 16 °C are predominantly due to aging effect but the aging is not accelerated by the presence of deformation history.

With a view to isolating the secondary component from the complexed stress relaxation, a method originally proposed by Tobolsky and used for the study of the chemical stress relaxation in covalently bonded networks was applied to ion-containing polymers to which time-temperature superposition had been previously found inapplicable. A few examples were chosen from the two different groups of ionomers to demonstrate the applicability of this method. Although the secondary component in. one of styrene–sodium methacrylate copolymers (PS–NaMA) with 7.7 mol% of ions cannot be separated by this method at low temperatures, the secondary mechanism can be isolated at high temperatures. In other ionomers with more ions, the secondary mechanism is isolated by this method for the entire temperature ranges. The secondary relaxation is due to the yielding of the ionic structure, and these ionomers studied here approximately follow Maxwellian decay of stress. The activation energy obtained for the secondary mechanism of each ionomer, which ranges from 26 to 31 kcal mol−1, may correspond to the energy required to break the ionic interactions in the ionic clusters.