Tensile stress-strain behaviour of cementitious composites at high loading rates

The influence of loading rate on the tensile stress-strain behaviour of cementitious composites was studied experimentally. The project was undertaken to obtain an insight into the possible relation between internal structure parameters of composites and their loading-rate sensitivity. Five different types of cementitious composites were applied. Composite structure data were obtained by testing porosity and by quantitative observation of fracture surfaces. Direct tensile tests were performed at four different loading rates within the range 0.001–1000 MPa s^–1. The tensile stress-strain behaviour was significantly influenced by the loading rate and structure parameters of composites. The relative tensile strength increase due to an increase of loading rate was found to be higher for composites with higher total porosity. Recorded stress-strain diagrams obtained at various loading rates are presented and discussed with the aid of continuous damage mechanics.     

Crack resistance curves of alumina at high temperatures

Ceramic three-point bend specimens were pre-cracked in a displacement-controlled test in air at room temperature to form sharp cracks of different lengths. Critical stress intensity factors (K _lc) were then measured as a function of sharp crack length in a fast-fracture, load-controlled test at 900, 1000 and 1100° C. By means of these fast fracture tests, crack resistance curves (K _lc against crack length) were determined for two commercially pure aluminas of different grain size and for a debased alumina containing a glassy phase. The crack resistance curve for the pure, fine grained alumina proved to be flat at 900° C, as was found for room temperature. A steeply rising crack resistance was, however, observed for the pure coarse-grained alumina at 1100° C and for the debased alumina at 1000 and 1100° C. This rise in KR curves is explained by friction effects of the cracked microstructure behind the crack front for the coarse grained alumina and by adhesive forces caused by the second phase behind the crack front for the debased alumina. These facts are proved by comparison to experiments on notched specimen and by annealing experiments. From the annealing experiments the size of the adhesive zone is estimated for the debased material.The death of Dr R. F. Pabst is sadly recorded. (On leave of absence from Max-Planck-Institut für Metallforschung, Seestraße 92, 7000 Stuttgart 1, Federal Republic of Germany).     

Analysis of the low frequency damping observed at medium and high temperatures

Isothermal internal friction measurements performed versus frequency exhibit often a large damping rise at very low frequency (10⁻² to 10⁻⁴ Hz). This increasing is found for experiments at very high temperature above 0.7T_M, (T_M=melting point) similarly to the so-called high temperature background obtained during measurements at fixed frequency and often described in the literature. In some cases, as for instance in cold work metals or metallic alloys, such low frequency background occurs at lower temperature (0.3 T_M<T<0.7T_M) and disappears after high temperature sample annealing. This exponential background depends on the sample microstructure and can be analysed with the Schoeck’ model. On the contrary, experiments performed at temperature close to the melting point or the liquidus temperature for metallic alloys exhibit a peak. The high temperature background measured for instance at 1 Hz, is only the lower part of a peak and does not correspond to an exponential increase. This fact explains why none of the different expressions proposed in the literature is able to describe properly the experimental high temperature background.     

The heat capacity of cadmium telluride at medium and high temperatures

The modified Schomeit method for processing enthalpies (H _T ⁰ -H _298.15 ⁰ ) is suggested, which provides for agreement between high-temperature data on C _p both with a reliably measured value of C _{p, 298.15} ⁰ and with derivative $\left( {\frac{{\partial C_p^0 }}{{\partial T}}} \right)_{298.15} $ . The latter gives a “smoother” joining of dependences C _p (T) obtained experimentally with different accuracy. The suggested procedure is applied to analysis of experimentally obtained distributions of heat capacity of cadmium telluride as a function of temperature, and a new approximating equation is obtained for the most reliable data in the range from 298 K to the melting point of CdTe. This procedure is compared to other known methods of simultaneous processing of C _p (T) data. It is demonstrated that the values of heat capacity of CdTe recommended by us agree with reliably determined coefficients of thermal expansion (CTE) of this compound. The characteristic Debye temperature of cadmium telluride is estimated in a wide temperature range, and the pattern of its variation with temperature is analyzed.     

Fragmentation of brittle materials at high rates of loading

Experiments involving the fragmentation of SiC during both quasistatic and high-rate compressive loading are described. Measured fragment size distributions are considered in terms of current ideas regarding high strain rate damage and microfracture. It is concluded that both crack dynamics and crack nucleation via plastic flow mechanisms may be important elements in the failure process.     

The employment of 0/90 unsymmetric samples for the characterisation of the thermo-oxidation behaviour of composite materials at high temperatures

The present paper explores the possibility to employ 0/90 unsymmetric plates as a tool for characterising the thermo-oxidation behaviour and the environmental ageing of composite materials at high temperature. The continuous monitoring of the out of plane displacements and the curvatures of such samples can be used for the identification of the thermo-oxidation induced irreversible chemical strain/stress build up and material property changes. A model has been developed to take into account the effects of thermo-oxidation on the time-dependent plate curvatures. Simulations show that 0/90 unsymmetric plates are very sensitive to thermo-oxidation induced changes, thus effective for capturing the searched thermo-oxidation effects. The predictions of the present model may serve for parametric studies and design of tests.     

Effect of prestressing at elevated temperatures on the cracking resistance of structural materials

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 7–14, May–June, 1992.     

Adiabatic flow curves of metallic materials at high strain rates

Dynamic compression tests are carried out on Armco iron, Cr-V-steel, Ni-Cr-Mo-V-steel, an austenitic Ni-Cr-Mo-steel, tantalum, nickel and Ni₃Al and magnesium. The flow curves are analysed to determine the influence of the deformation energy which is tranformed into heat on the flow behaviour and mechancial stability. Not only the material properties but also the conditions of friction between the specimen and the compression tool are found to have a greate influence on the flow stress reduction and stability. High frictional forces promote mechanical instability of materials with low strain hardening and low strain rate sensitivity.     

Deformation of PC/ABS alloys at elevated temperatures and high strain rates

The objective of this paper is to experimentally study the deformation behavior of the alloys of polycarbonate (PC) and acrylonitrile–butadiene–styrene (ABS) at elevated temperatures and high strain rates. Four kinds of PC/ABS alloys with the ratio of PC to ABS being 80:20, 60:40, 50:50 and 40:60 and three different strain rates 8.0 × 10² s⁻¹, 2.7 × 10³ s⁻¹ and 1.0 × 10⁴ s⁻¹ are considered. The Split Hopkinson Pressure Bar (SHPB) experiments are carried out at 293 K and 343 K, respectively. The curves of engineering stress and engineering strain and true stress and true strain are obtained for the PC/ABS alloys at different temperatures and different strain rates, respectively. The effects of temperature, strain rate and the fraction of ABS on the deformation behavior of PC/ABS alloys are discussed in details, and then a temperature and strain rate-dependent phenomenological constitutive model for PC/ABS alloys is developed.     

Change in characteristics of the band structure of oxide materials at high temperatures

The integral emissivity of oxide materials at high temperatures are evaluated by means of the band model; its behavior is explained. A simplified model of the band structure at high temperatures is proposed, and the expression for the calculation of the relative population of the states under thermodynamic equilibrium is obtained. The urgency of the measurements of absorption and emission of oxide materials for the determination of the density-of-states distribution and the band structure of substances at high temperatures and melting is demonstrated.     

Main considerations for the determination and evaluation of the acid resistance of cementitious materials

The paper reports on the development of an accelerated test method for determining and evaluating acid resistance of highly alkaline cementitious materials. The test method was derived through extensive laboratory experiments on hardened cement pastes, mortars and concretes, in order to determine the acid specific modes of action and to evaluate the related deterioration mechanisms for cementitious materials when subjected to acid attack. From the experimental study it can be concluded that the pH value of the acid solution is not the only decisive parameter. For example organic acids, such as acetic acid, cause a higher damaging effect compared to so-called “strong” acids (such as sulphuric or hydrochloric acid) because of the acid buffer action and the high solubility of the reaction products. In order to enable a constant deterioration rate the pH value during testing conditions has to be regulated at a stationary level, the degree of saturation in the acid solution needs to be regularly examined, and in particular for organic acids, the acid solution needs to be continuously stirred. The degree of degradation can be quantified by using visual methods or through measuring the residual mechanical properties, e.g. the compressive and/or flexural tensile strength. It should be noted that different damage mechanisms are possible, which are strongly dependent on the applied acid solution and constituents of the cementitious materials.     

Dielectric anomaly and low frequency dispersion in ferroelectric materials at high temperatures

A dielectric anomaly related to the observed departure from the Curie–Weiss law in several Aferroelectric materials has been characterized in this work in ferroelectric BaTiO₃. The electrical properties of the material were investigated using the a.c. analysis technique. From the analysis of the permittivity versus frequency curves collected at various temperatures, we discuss the anomalous behaviour as a result of a low frequency dispersion phenomenon due to the characteristics of the conduction processes at the material inhomogeneities.