Key Engineering Materials Vols. 488-489

Abstract: Intelligent materials have been studied in various fields. In this study, Shape Memory Composite (SMC) is produced as one of those intelligent materials. SMC consists of Shape Memory Polymer (SMP) and Shape Memory Alloy (SMA). These two materials possess different characteristics in temperature changes. Therefore, we investigated the possibility of the SMC’s actuator function as an intelligent material. The actuator function is achieved by applying a current to SMA fiber of SMC to grab and release objects. As a result of this investigation, meaningful data for actuator to design SMC is obtained.

Abstract: One of the primary applications of polymer based cellular solids is to act as an energy absorbing material during impact where compressive strain rates may reach 500-800/s. In reality, impacts occur over a wide range of temperatures and velocities at different angles of incidence. Understanding and modelling the behaviour of the polymer foams requires characterisation of the material response in detail. The stress-strain response that covers both compressive and tensile behaviour for a wide range of strain rates and temperatures are needed to characterize the mechanical performance of polymer foams as polymeric foams are highly nonlinear materials that undergo large deformation in crashworthiness related cases. It is reported in literature that any increase or decrease in temperature over the glass transition region can cause changes by order of magnitude in elastic modulus of polymeric foams. However, creation of cross linking at high temperature can affect the elastic modulus. In this work, the behaviour of two, polyamide-6 (PA-6) based closed cell foams at elevated temperatures were investigated covering the glass transition temperature. This work presents the variation of elastic and tangent modulus of two low densities PA-6 and PA-6/polyolefin (Nylon alloy) based foams. Empirical equations have been proposed to allow the prediction of modulus over a temperature range of 23°C to 120°C for these materials.

Abstract: The effect of slight surface REM finishing, that is a chemically accelerated polishing process, in order to reduce surface roughness after shot peening has been studied on different series of specimens shot peened by varied peening parameters. Room temperature rotating bending fatigue tests have been performed on the shot peened series and the series subjected to shot peening and successive REM finishing process. The results represent how REM finishing affects the final fatigue behavior of differently treated specimens in two ways: decreasing surface roughness and removing a thin layer of material with compressive residual stresses.

Abstract: The aim of the proposed studies is the development of the carbon/carbon composite with prescribed elastic properties. To achieve this, a microstructure optimisation problem for estimation of the microstructure with prescribed stiffness is formulated. The design variables of the posed problem are the local fibers distribution and porosity. The volume fractions of the fibers and pores in the whole microstructure are fixed. Material properties of the local microstructure of the composite are calculated using virtual models. Semi-analytical homogenization procedures were used for the development of these models. Modeling results are compared with elastic properties obtained experimentally by tension and compression test and ultrasonic studies of the bulk material. Approach to design microstructure for three point bending test is proposed.

Abstract: The determination of monotonic mechanical properties and high cycle fatigue parameters of high chromium steel (HCS) is presented. The monotonic mechanical properties (ultimate compressive and ultimate tensile strength) are determined using standardized testing procedures according to DIN 50125 standard. The high cycle fatigue parameters are determined using uniaxial fatigue test where the tests specimens are loaded with pure pulsating compression load (load ratio R=0 in compression) at different load levels. Therefore, a typical S-N curve and appropriate fatigue parameters (fatigue strength coefficient s_f’ and fatigue strength exponent b) are determined. The experimental results determined in this study can serve as a basis for the determination of service life of rolls using stress-life approach. However, a few guidelines for the further research work considering increased temperatures and multiaxial fatigue are given in the conclusions of this study.

Abstract: Ba_0.5Sr_0.5Co_0.8Fe_0.2O_{3 – δ} is a mixed ion-electron conductor with high application potential as high-temperature gas separation membrane. However, in practical use the integrity of this brittle perovskite is challenged by the mechanical boundary conditions of transient temperature exposure. Moreover, long term failure mechanisms such as static fatigue at room temperature and creep rupture at operation temperature might occur. The relevance of both effects for BSCF has been investigated. The slow crack growth at room temperature has been determined using bi-axial bending under different loading rates. The creep rupture at elevated temperature has been analyzed from three-point bending tests. The results indicate favourable behaviour of BSCF in both cases. A low risk of failure due to slow crack growth exists and the strain to failure in combined tensile - compressive mode reaches up to 40 %.

Abstract: High temperature low cycle fatigue behaviour of cast nickel-based superalloy Inconel 738LC in as-received condition and coated with an Al-Si diffusion layer was studied. The Al-Si protective layer was deposited on the gauge section of cylindrical specimens using the slurry technique. Fatigue tests were performed on cylindrical specimens under total strain control in symmetrical cycle at 800 °C in air. The coating has a beneficial effect on fatigue life in the low amplitude domain. The stress response of the coated material is higher for high amplitudes in comparison with the uncoated one. Fracture surfaces and sections parallel to the specimen axis have been examined to study fatigue damage mechanisms.

Abstract: The atigue strength of Friction Stir Welding is affected by the structure and the welding defect size. The welding defect sizes have a scatter. Therfore, the effect of Friction Stir Welding condition on fatigue limit of welded noncombustible Mg alloy was investigated with the specimen which has a controlled defect size. As a result, the good correlation between the fatigue limit and the hardness of the welding area was clarified.

Abstract: The paper presents relationships between strain level generated by creep process and two parameters determined form non-destructive tests, i.e. acoustic birefringence and amplitude of magnetoacoustic emission for three kinds of steel: 40HNMA, P91 and 13HMF. Moreover, the relationships between prior deformation level and selected mechanical parameters resulting from the standard tensile tests subsequently carried out at room temperature were established. As a consequence, this enabled to formulate mutual relationships between these mechanical parameters and parameters obtained from the ultrasonic/magnetic investigations.

Abstract: The aim of this study is firstly to investigate the applicability of a sharp notch with Focused Ion Beam (FIB) as a crack for fatigue limit evaluation. Secondly we investigate a condition in which artificial defects (drilled hole, FIB notch) can be used as a crack for fatigue limit evaluation. To achieve the aim, the growth behaviors and the non-propagating crack sizes of small fatigue cracks initiated from a FIB notch and a drilled hole are carefully compared with those of an annealed fatigue crack which imitates an ideally sharp crack. The results show that a FIB notch can be used as a crack for fatigue limit evaluation under some conditions. The results also show that the condition which controls the applicability of an artificial defect as an ideal crack for fatigue limit evaluation is strongly dependent on the relation between (i) the length of a non-propagating fatigue crack and (ii) the crack length when the small fatigue crack growth behaviors from an artificial defect and an ideal pre-crack become almost the same. It is found that the length of (ii) can be obtained by the analyses using the number of cycles from a certain crack length to failure.