Key Engineering Materials Vols. 452-453

Abstract: This paper analyses the differences in fracture behaviour of two cold drawn pearlitic steels with different degree of strain hardening: a slightly drawn bar and a heavily drawn wire. The load-displacement curve F-u was analysed in the two cases, with special emphasis in the characteristic points of the plot: the load defining the end of linear behaviour (Fe), the final fracture load (Fmax) and, in the heavily drawn steel, the pop-in load (FY). Results demonstrate that slightly drawn steels exhibit isotropic fracture behaviour with crack propagation along its own plane (mode I propagation). On the other hand, heavily drawn steels exhibit a markedly anisotropic fracture behaviour with crack deflection (mixed mode propagation), and vertical fracture embryos suddenly appear associated with the pop-in instant. This special fracture mode is a consequence of the markedly oriented microstructure of the heavily drawn steel.

Abstract: The use of structural adhesive joints to join carbon fibre reinforced polymer (CFRP) adherends is now well established in the aerospace industry. These joints are subjected to varied load spectra, of which one of the most damaging forms of loading is fatigue with intermittent low energy impacts, which is termed combined standard and impact fatigue (CISF) in this paper. It is seen that the rate of crack growth in impact fatigue is greater than that in standard fatigue for a given value of the strain energy release rate, moreover, it is seen that the fatigue crack growth rate (FCGR) in standard fatigue (SF) increases after a block of impact fatigue. In this paper a model is proposed to predict crack growth in bonded joints subjected to CISF. The model is based on numerical crack growth integration (NCGI) with a method of accounting for the accelerated crack growth in SF following IF. The model was seen to provide a good prediction of the fatigue crack growth in CISF.

Abstract: In this paper, a finite element sub-modelling method is used to estimate the stress distribution and the crack propagation lifetime in Aluminium alloy, Al7075-T6 specimens. Different contact geometries, i.e. cylinder-on-flat and flat-on-flat, and different contact span width’s with different pad’s compression forces are analysed. It is found that fretting fatigue life for two based cylindrical pads is shorter than that for two based flat pads and in two based flat pads is shorter than that for perfectly flat pads. In the case of two based flat pads, stress distribution in the pad with width of 3.1 mm is about 14% higher than that in the pad with the width of 6.2 mm and about 21% higher than that in the pad with the width of 9.3 mm. Similarly, stress in cylindrical pads with width of 3.1 mm is 36% higher than that in the pad with width of 6.2 mm and 130% higher than that in the pad with width of 9.3 mm. Stresses increased by 21% for pad force of 1800 N and around 40% for pad force 2400 N compared with 1200 N pad force. Also it is observed that by increasing pad width for both flat and cylindrical pads, crack propagation lifetime increased. Furthermore, fretting fatigue crack growth rate raises when pads compression force increases.

Abstract: The fatigue lifetime of the aluminum alloy 7075-6 depends upon the cyclic stress amplitude and the environment. At an R value of 0.05, and a maximum stress of 400 MPa the lifetime in vacuum is an order of magnitude greater than that in air, but interestingly at a maximum stress of 275 MPa, the lifetime in vacuum is only three times that in air. It was noted that well defined striations were observed in air, whereas striations were absent in vacuum, instead indications of ductile rupture were observed. The mechanism of fatigue crack initiation and propagation behavior as influenced by the environment is discussed.

Abstract: In this research, the experimental works were conducted to study the lap strength of the headed reinforcements with confinement provided by stirrups or tie-down bars. Five lap splice specimens were tested. Wide beam specimens were fabricated with a lap splice at mid-span. They were loaded in flexure, placing the lap splice in tension. The variables in this study were: confinement details (stirrups, tie-down bars with stirrups) and lap length (10, 14 times the diameter of reinforcement). Specimens were instrumented to measure the load on the specimen, strain along the reinforcements with in the lap zone, and mid-span deflection. The cracking behavior was also observed and recorded. The following conclusions were drawn from the test results in this study; 1) The initial crack of the specimens lapped developed at the heads of reinforcements. The specimens failed by the crack with the struts acting between the opposing heads of lapped reinforcements. 2) Bond stress and confinement details contributed to increase strength and deformation. 3) CCD method evaluated the maximum strength as 4.95∼6.87 times the test maximum strength. The ratios of the test maximum strength and the maximum strength assumed by ACI 318-08 for headed deformed bars were 1.38∼2.01.

Abstract: The linear notch mechanics (LNM) was proposed by H. Nisitani in 1983. The concept of LNM can assure the occurrence of the same phenomena in a notched specimen and a real object. The effectiveness of LNM under the condition of small scale yielding has not been confirmed sufficiently. In this paper, the effectiveness of LNM under the condition of small scale yielding is discussed based on the results of FEM elastic-plastic analyses.

Abstract: The present investigation attempts to evaluate the improvement of working by bolt on the fatigue limit of structural steel sheets with drilling a circular hole. The material used in this study is structural steel (JIS SM400A). And the specimens are identified to two types of the non-worked specimen and the worked specimen. The results obtained in this study can be summarized as follows: (1) The fatigue limit of worked specimens increases than that of non-worked specimen. Futhermore, there has a suitable value of the torque for improving the fatigue limits. (2) The reasons of enhancing the fatigue strength of the worked specimens are due to the elastic deformation, the plastic deformation and the changing of stress concentration part. Our investigations confirm the advantages of this working method which repaired the structural steel sheets by bolt.

Abstract: The concept of damage detection using sensors in composite panels is of growing interest, due to the reduction in maintenance activity and increasing of the aircraft safety. Piezoelectric materials have been widely used to build a variety of transducers for activating and receiving Lamb waves which are used for delamination/damage detection. The aim of this paper is to use smart structures for detecting and locating damage in composite panels, by using piezoelectric patches as both sensors and actuators.

Abstract: In the contribution solution of special fracture mechanics problems connected with multi-layer plastic pipes was investigated. The assumptions of linear elastic fracture mechanics were accepted. A complex three-dimensional numerical model of multi-layer pipe system consisting of main (functional) pipe and protective layers has been suggested and numerically solved by finite element method. Two basic problems connected with lifetime expectation of multilayer pipe system have been considered and discussed, namely: question of fracture mechanics description of multilayered pipe system and corresponding measurements of the material properties. The suggested approach can help for more accurate estimation of the multilayer pipe damage.