Catálogo de publicaciones - libros

The usual finite elements have complicated stiffness matrices and present particular difficulties in handling nonlinear problems (Argyris [ 1 ]). The bar of a truss is the finite element with the simplest possible local stiffness matrix (Absi [ 2 ], Fraternali et al. [ 3 ], Papadopoulos and Xenidis [ 4 ], Slaich and Schäfer [ 5 ]), which can be written, in 2D, as: where κ_E elastic stiffness, κ_G geometric stiffness, E elasticity modulus, A cross-section area and ℓ_o undeformed length of the bar. The $$ \bar c = \{ c_x {\text{ }}c_y \} $$ are direction cosines of bar axis, N axial force, present length of the bar and I_2 is the unit matrix in 2D.

Meshing issues in the context of the finite element method (FEM) are often difficult ones to deal with, as in the case of modelling helix-shaped cracks in rotor shafts (Andrier et al . [ 1 ]). As a matter of fact, 3D automatic meshing programs often generate a large number of badly-shaped elements which are not reliable and imply ill-conditioned stiffness matrix. Creating an adequate mesh requires a considerable amount of user-time compared to the total computing time and these meshing procedures are relatively costly. Besides, remeshing is necessary at each propagation step.

Computer simulation of fracture processes remains a challenge for many industrial modelling problems. In a classical finite element method, the non-smooth displacement near the crack tip is captured by refining the mesh locally. The number of degrees of freedom may drastically increase, especially in three dimensional applications. Moreover, the incremental computation of a crack growth needs frequent remeshings. Reprojecting the solution on the updated mesh is not only a costly operation but also it may have a troublesome impact on the quality of results.

For economic reasons, the power industry is now operating its 500 MW coal-fired plants on a two-shift cycle in which the turbines are on-load for 16 hours per day and off-load overnight and at weekends. The concern with ‘two-shifting’ is the impact on environment assisted cracking of the associated transients in stress, water chemistry and temperature. On-load, with well-controlled water chemistry, the condensate on the low-pressure turbines will be free of oxygen with chloride and sulphate levels both up to about 300 ppb. Off-load, the condensate would essentially be pure water but aerated unless there is nitrogen blanketing. The stress off-load would be zero. Ideally, to fully simulate two-shifting in laboratory testing, the combined influence of transient stress and water chemistry would be evaluated but there are technical difficulties in synchronising the changes in the stress, temperature, oxygen, and anion (chloride and sulphate) concentrations. For the purpose of assessing the impact of transient stress on crack propagation, the environment was held constant, viz. deaerated 300 ppb Cl^− +300 ppb SO^2−_4 solution at 90 °C. Separate measurement to examine the effect of transient water chemistry at constant stress are underway.

β - 21S titanium alloy is ranked among the most important advanced materials for a variety of technological applications, due to its combination of a high strength/weight ratio, good corrosion behavior and oxidation resistance. However, in many of these technological applications, this alloy is exposed to environments which can act as sources of hydrogen, and consequently, hydrogen-induced cracking and property degradation, hydrogen-induced ductile-to-brittle transition associated with a change in the fracture mode from ductile, micro-void coalescence to brittle, cleavage have to be considered[ 1 – 4 ]. In the aged β-21S alloy, the susceptibility to hydrogen induced cracking and the decrease in the alloy’s strength has been attributed to the -phase precipitated during the aging and the hydrogen-induced stabilization of the β-phase[ 1 , 2 , 5 ]. Hydrogen-induced intergranular cracking in the cathodically pre-charged β-21S alloy was significantly influenced by the preferential α precipitation at β grain boundaries[ 5 ]. Even without hydriding, the α-β interfaces could provide trapping sites and the accumulation of hydrogen at these interfaces could result in fracture. Pound[ 6 ] revealed that in aged β-Ti alloys the relationships among the trapping constants, resistance to hydrogen embrittlement and grain boundary are critical in determining the role of trapping in hydrogen embrittlement of these alloys.

The layer-like structures are widely used for corrosion protection of hull constructions in a power engineering and refinery industry. The work presents the corrosion and corrosion fatigue studies of three-layered metallic material with the aim to assess the possible corrosion damaging and corrosion fatigue crack growth behaviour under operating conditions.

Key input data for SIM calculations are the service stresses and the crack/defect size. For the offshore industry the Alternating Current Field Measurement (ACFM) technique [ 1 ] was initially developed for subsea and topside inspection but is now used universally. ACFM has the capability to both detect and size cracks. A recent innovation for ACFM has been the introduction of array probes. These can collect crack depth information from various sites along the crack in one placement of the probe. The array probe is an area inspection and hence can also be used for crack size monitoring if left in place on the structure.

The use of poly methyl methacrylate (PMMA) based bone cement as a grouting agent for the invivo fixation of orthopaedic implants has been in practice for nearly fifty years. Fatigue failure of the bone cement has been identified as the primary cause of cement failure. Implant loosening due to the failure of the cement is one of the major reasons necessitating revision surgery. The need for a more fatigue resistant bone cement is well documented in the literature [ 1 , 2 , 3 , 4 ]. One method of producing a more fatigue resistant bone cement is to reinforce it with short fibers [ 2 ].

Most human body tissues are composites in nature. Using natural tissues as templates, “designer” bioactive ceramic-polymer composites are developed for tissue replacement and regeneration [ 1 ]. This biomimicking concept has now been extended into developing bioactive composites with metals or ceramics as matrices. Furthermore, with the recent emergence of tissue engineering, bioactive composite scaffolding materials are under active development [ 2 ]. The composite approach is therefore being established as one of the most important and viable ways in developing new biomaterials.

A surprisingly wide range of metals, polymers, and ceramics meet the requirements of biocompatibility for use in orthopaedic implants [ 1 ]. Three metals are by far the most commonly used: stainless steel, chromium cobalt alloys, and titanium and its alloys. Oxide ceramics (Al_20_3, ZrO_2) are used for bearing surfaces and calcium phosphate bioceramics and glass ceramics are used as coatings to invoke integration with host tissues. Regarding polymers, two are ubiquitous in orthopaedic implants: ultra-high molecular weight polyethylene (UHMWPE) for bearings and polymethylmethacrylate (PMMA) as a grouting to ‘cement’ implants into bones [ 2 ].