Key Engineering Materials Vol. 627

Abstract: Fracture behaviour of a three-layer polymer pipe subjected to nonhomogenous distribution of external pressure induced by soil embedding is studied in this paper. Both long term and additional short term loading is considered. Such loading induces tensile stresses in the inner pipe wall which can lead to crack initiation and further slow crack propagation. The material interface between a protective layer and the base pipe can contribute to crack deceleration and can prolong the residual lifetime of the pipe. The paper presents three-dimensional numerical analysis of a commercial three-layer pipe containing an internal semi-elliptical crack. The effect of soil load on the fracture behaviour of the cracked pipe is quantified and discussed.

Abstract: Two Cu samples: oxygen-free copper (99.99 wt% Cu) and deoxidized low-phosphorous copper (99.9 wt% Cu) were processed by equal channel angular pressing (ECAP). After the ECAP processing using 4 passes, equiaxed grains (~300 nm grain size) and elongated grains were formed for both samples. Fatigue strength of ultrafine grained Cu was enhanced by the addition of trace impurities. The formation behavior of surface damage and the change in surface hardness during stressing were monitored. A close relationship was observed between the change in hardness and the formation behavior of damage. The effect of trace impurities on the fatigue damage was discussed from the viewpoints of the grain coarsening and the crack initiation/growth behaviors.

Abstract: Structural integrity is crucial for the safety of power plants with higher efficiency to meet the increasing global energy consumption. High-temperature environment will demand not only improved high-temperature corrosion resistance but also a maintained sufficient toughness. This study investigates how long term high-temperature environment influence the impact toughness of two austenitic stainless steels (AISI 304 and Sandvik SanicroTM 28) and one nickel-bas alloy (Alloy 617). Alloy 617 has shown increasing impact toughness with both increasing temperature and time, up to 700°C and 3 000 hours, while the two austenitic stainless steels have shown the opposite for the same conditions. At 10 000 hours the impact toughness of Alloy 617 has decreased but the alloy still possess great toughness. Both austenitic stainless steels show embrittlement due to brittle σ-phase and Alloy 617 seems to gain good impact toughness performance from small evenly distributed precipitates.

Abstract: The oxidation, environmental corrosion and creep damage are limiting factors in the life assessment of engineering components operating at elevated temperatures. Experimental observations have shown that environmental effects (i.e. oxidation) during components operation time can harden the material [1] and lead to evolution of damage in the form of surface cracks (see Figure 1). The regions subjected to environmental damage which show higher hardness are structurally weaker and more brittle. When components operate in the creep regime, the initiated micro cracks may lead to accelerated creep crack growth in the material. The aim of this paper is to highlight the importance of oxidation effects and microcracks on creep in component life assessments.

Abstract: The implementation of the switch is the stepstone of a fully automated structural health monitoring (SHM) System. This paper attempted to tackle the hardware connectivity issues when dense transducer arrays present in the system with the switch that features the matrix topology. The program that controls the instruments and automates the data acquisition for SHM system was developed in LabVIEW, in which a scanning approach that sensorises the interrogation area and a hybrid programming technique are employed. Through interfacing with a top-level GUI of the program, it is easy to be controlled and employed. The solution showed a significant reduction in testing time comparing that with manual switch operation and a weight-saving solution as a minimum number of shielded cable needed. In addition, because only one channel each from the signal generator and digitiser is required, it is less demanding on the specification of the instruments. The prototype of the current system can be scaled up for testing on a larger structure due to its modular nature in both PXI express system and software design.

Abstract: This paper exploits the implementation of a delay-and-sum imaging method using Lamb wave signals to localise barely visible impact damage (BVID) in quasi-isotropic composite panels. The structural discontinuities, such as opening and stiffener, has also been tested to reflect the common structural features of an aircraft and to examine the feasibility of the proposed detection technique. The prediction results are compared with ultrasonic C-scan images, which indicate location error for three different panels –flat panel, flat panel with an opening and stiffened panel. The accuracy is believed to be improved by increasing the number of transducers. Overall the proposed damage detection technique, with the use of only four transducers, demonstrated sufficient accuracy and efficiency in impact damage detection and can be applied alongside the traditional NDT inspections for providing a priori information of the impact damage location.

Abstract: A single cantilever beam test was performed on joints constituted from unidirectional carbon fiber composite flexible beam bonded to a rigid polyamide/aluminium block. The loading was following a constant separation rate rule, for which steady-state fracture energy can be obtained. In addition to the standard resistance curve we have investigated, using backface strain monitoring technique, fiber bridging effect on load distribution in the vicinity of the crack front. A big impact of the fiber bridging on the overall R-curve behaviour was confirmed with local non self-consistency of the crack propagation. The results are backed up analytically.

Abstract: Delamination in layered materials is analyzed in a fracture mechanical framework. The work deals with quasi-static propagation of crack fronts along planar interfaces between different isotropic, elastic layers. Special focus is here on local effects at the sides of the layers which are assumed to be stress free. The interface crack front meets the free sides of the specimen at an angle which depends on the elastic mismatch in the system. Finite element calculations allowing the shape of the crack front to be arbitrary are carried out for double cantilever beam type specimens. An iterative procedure is formulated which adjusts the shape of the crack front so that an interface fracture criterion is satisfied locally along the front. Apart from the overall shape of the crack front, the angle of intersection with the free sides is in particular determined numerically by this procedure. Comparisons with analytical formulations and experimental results are performed.

Abstract: Fatigue strength prediction methods of blades of a high pressure steam turbine are the main topic of this article. Experimental approaches, as well as use of the experimental results for the verification of the finite element method (FEM) and fatigue models, were performed on two basic levels. First, verification by the fatigue tests of smooth and notched cylindrical specimens under room and service conditions was performed. Second, verification of the real blade fatigue limit prediction was conducted. These tests were carried out using special test stand under the typical combined blades loading. Appropriate uniaxial and multiaxial fatigue criteria were applied. Achieved results were finally used in the process of the fatigue strength prediction of rotor blades.

Abstract: A comparative study of the mechanical properties of the extruded and flattened nanostructured composites Al-C₆₀ has been made using two different methods of destructive and non-destructive testing: tensile and compression macro-tests and sub-micron range sclerometry (scratch test). Direct correlation was found between the dominant types of deformation during scratching and the type of “stress-strain” dependencies. The results are useful for understanding the extrusion process and quality control at different load scale.