In this paper there is presented an experimental procedure used to determine the flexural rigidity for composite sandwich bars with polypropylene honeycomb core with various thickness values: 1, 1,5 and 2 cm. The composite bars will be reinforced with one layer of carbon fiber. The width value of the composite bars will be of 6 cm. In order to obtain the flexural rigidity the composite bars will be clamped at one end and left free at the other. An accelerometer will be placed at the free end used to record the free vibrations of these bars. The simplifying assumption of “bar” will be used in this research, so I have chosen several free lengths for the bars: 29, 32 and 35 cm. The eigenfrequency of the first eigenmode will be used to determine the flexural rigidity of the bars.
By applying a cyclic eccentrically tensile loading, oscillatory positive, determines at the crack peak that exist in a plate specimen CT type a compound loading of bending with tensile. The aim of the study is to analyze the equivalent stress variation σ, when the working temperature varies, namely: T= 293K (+20°C), T= 253K (-20C) and T= 213K (-60°C). The specimens are made from a stainless steel 10TiNiCr175 type, and were loaded with the asymmetry coefficient R= 0.1. There are drawn the variation curves of stress versus the crack length variation, σ(a), versus the material durability, σ(N), and respectively versus the stress intensity factor, σ(ΔK), for the three loading temperatures.
In the process of materials fracture, a very important parameter to study is the cracking rate growth da/dN. This paper proposes an analysis of the cracking rate, in a comparative way, by using four mathematical models:1 - polynomial method, by using successive iterations according to the ASTM E647 standard; 2 - model that uses the Paris formula; 3 - Walker formula method; 4 - NASGRO model or Forman - Newman - Konig equation, abbreviated as FNK model. This model is used in the NASA programs studies. For the tests, CT type specimens were made from stainless steel, V2A class, 10TiNiCr175 mark, and loaded to a variable tensile test axial - eccentrically, with the asymmetry coefficients: R= 0.1, 0.3 and 0.5; at the 213K (-60°C) temperature. There are analyzed the cracking rates variations according to the above models, especially through FNK method, highlighting the asymmetry factor variation.