DYNAMIC PROPERTIES OF FILLED RUBBER. PART II: PHYSICAL BASIS OF CONTRIBUTIONS TO THE MODEL

A relatively simple time-domain model is proposed with the scope to capture those aspects of the uniaxial stress–strain behavior of filled rubber that are most significant in engineering applications, and is discussed in the context of a literature review. Its performance is investigated in simple shear using analytical expressions. Attention has been given to assembling the model from separate physical contributions, each already established in the literature, so that not only is the number of parameters small but also they may be at least semi-quantitatively related to the formulation of the elastomer. The small number of parameters helps to keep tests for fitting them simple, while their connection to mechanisms also enables a degree of utility of the model even when extrapolated to situations beyond those covered by tests.

Effect of energy and protein levels on feedlot performance and carcass characteristics of Mehraban ram lambs

Fifty four Mehraban ram lambs (6-to 8-month old, initial live weight 35.4 +/- 4.2 kg) were assigned to a completely randomized design consisting of 9 groups and were fed for 70 days with diets containing three levels of energy (2.3, 2.5 and 2.7 Mcal metabolizable energy per kg dry matter) and three levels of protein (10.5, 12.5 and 14.5 percentage in dry matter). Either energy or protein levels alone significantly affected most of the parameters of lamb performance, but their interaction effect was significant only for feed conversion ratio, cold carcass weight, tail weight, flap weight and back fat (subcutaneous fat) depth. The lowest level of energy (2.3 Mcal ME per kg DM) resulted in a significant decrease in lamb performance as compared with other energy levels. Increasing energy concentration of the diet resulted in significant increases in fat percentage, but significantly decreased the moisture and protein content of the Longissimus dorsi muscle. Increased dietary protein level increased the daily DMI and ADG and at the same time improved the FCR. Hot and cold carcass weights increased significantly with increasing dietary CP levels, but dressing percentage was similar amongst the dietary protein densities. Dietary CP levels had no significant effect on the chemical composition of the Longissimus dorsi muscle. At the lowest energy level (2.3 Mcal ME per kg DM), dietary protein level had a significant effect on FCR (Table 4); with the diet containing 10.5% protein having the highest FCR. At the medium and low energy levels the lowest level of dietary protein concentration resulted in smaller carcasses. The highest level of protein along with the medium energy concentration resulted in smaller tail weights. Flap weight was significantly smaller at low energy concentration along with medium and low protein level. The lowest back fat depth was found in lambs fed on the low energy diet containing medium to high levels of protein.

Dynamic Properties of Filled Rubber — Part I: Simple Model, Experimental Data and Simulated Results

A simple “viscoplastic” model is used to capture the stress-strain behavior of a filled SBR vulcanizate; a key objective is to predict dynamic properties, in particular the Fletcher-Gent or Payne effect, from non-cyclic stress-strain data. A simple fitting procedure is described to obtain the parameters of the viscoplastic model from the stress relaxation data and stress-strain loading curves at constant rate. Special attention is given to keeping the numbers of parameters and of characterization tests small. Elastic models are incapable of representing several aspects of the material behavior whereas it is confirmed that the proposed “viscoplastic” approach captures the essence of the behavior.