Spall

About: Spall is a research topic. Over the lifetime, 1787 publications have been published within this topic receiving 31614 citations.

Mechanisms controlling the durability of thermal barrier coatings

Abstract: The durability of thermal barrier coatings is governed by a sequence of crack nucleation, propagation and coalescence events that accumulate prior to final failure by large scale buckling and spalling. Because of differing manufacturing approaches and operating scenarios, several specific mechanisms are involved. These mechanisms have begun to be understood. This article reviews this understanding and presents relationships between the durability, the governing material properties and the salient morphological features. The failure is ultimately connected to the large residual compression in the thermally grown oxide through its roles in amplifying imperfections near the interface. This amplification induces an energy release rate at cracks emanating from the imperfections that eventually buckle and spall the TBC.

Computational modelling of impact damage in brittle materials

Abstract: A Lagrangian finite element method of fracture and fragmentation in brittle materials is developed. A cohesive-law fracture model is used to propagate multiple cracks along arbitrary paths. In axisymmetric calculations, radial cracking is accounted for through a continuum damage model. An explicit contact/friction algorithm is used to treat the multi-body dynamics which inevitably ensues after fragmentation. Rate-dependent plasticity, heat conduction and thermal coupling are also accounted for in calculations. The properties and predictive ability of the model are exhibited in two case studies: spall tests and dynamic crack propagation in a double cantilever beam specimen. As an example of application of the theory, we simulate the experiments of Field (1988) involving the impact of alumina plates by steel pellets at different velocities. The calculated conical, lateral and radial fracture histories are found to be in good agreement with experiment.

Effect of fire on concrete and concrete structures

Abstract: The behaviour of concrete in fire depends on its mix proportions and constituents and is determined by complex physicochemical transformations during heating. Normal-strength concretes and high-performance concretes microstructurally follow similar trends when heated, but ultra-high-performance concrete behaves differently. A key property unique to concrete amongst structural materials is transient creep. Any structural analysis of heated concrete that ignores transient creep will yield erroneous results, particularly for columns exposed to fire. Failure of structural concrete in fire varies according to the nature of the fire; the loading system and the type of structure. Failure could occur from loss of bending or tensile strength; loss of bond strength; loss of shear or torsional strength; loss of compressive strength; and spalling of the concrete. The structural element should, therefore, be designed to fulfil its separating and/or load-bearing function without failure for the required period of time in a given fire scenario. Design for fire resistance aims to ensure overall dimensions of the section of an element sufficient to keep the heat transfer through this element within acceptable limits, and an average concrete cover to the reinforcement sufficient to keep the temperature of the reinforcement below critical values long enough for the required fire resistance period to be attained. The prediction of spalling – hitherto an imprecise empirical exercise – is now becoming possible with the development of thermohydromechanical nonlinear finite element models capable of predicting pore pressures. The risk of explosive spalling in fire increases with decrease in concrete permeability and could be eliminated by the appropriate inclusion of polypropylene fibres in the mix and/or by protecting the exposed concrete surface with a thermal barrier. There are three methods of assessment of fire resistance: (a) fire testing; (b) prescriptive methods, which are rigid; and (c) performance-based methods, which are flexible. Performance-based methods can be classified into three categories of increasing sophistication and complexity: (a) simplified calculations based on limit state analysis; (b) thermomechanical finite element analysis; and (c) comprehensive thermohydromechanical finite element analysis. It is only now that performance-based methods are being accepted in an increasing number of countries.

The spall strength of condensed matter

Abstract: Two conditions are proposed which place constraints on the processes of dynamic spall in condensed media, and determine inequalities which bound the spall strength, fragment size, and failure time. Spall is defined as rupture within a body due to stress states in excess of the tensile strength of the material. The first is a horizon condition which establishes a domain of communication, consistent with the time to failure, within which spall must be independent of the surrounding environment. The second is an energy condition which requires that the potential and kinetic energy associated with the tensile loading process exceed the fracture energy of the material. Equality in the relations established from these conditions corresponds to energy-limited spall and provides specific analytic expressions for the spall properties. Inequality implies flaw-limited spall and requires more detailed material property information before spall can be characterized. Energy-limited spall is determined by the material fracture toughness in brittle solids and the material flow stress in ductile solids. Calculated spall properties, assuming energy-limited spall, compare well with experimental spall data for various materials. Under certain conditions, a transition from brittle to ductile spall (definition in text) with increasing strain rate is predicted. Comparison is made with spall data on 6061-T6 aluminum for which a brittle-to-ductile transition is predicted to occur at a critical strain rate of approximately 4 × 105 s−1. Energy-limited spall in liquids within their range of Newtonian fluid behavior is governed by surface energy and viscosity. Spall is predicted to be dominated by surface energy at low strain rates and viscous dissipation at high rates. Examples of each appear to exist within the scant experimental spall data available for liquids.

Load carrying capacity of concrete structures with corroded reinforcement

Abstract: Corrosion of reinforcing bars is one of the main causes which induces an early deterioration of concrete structures, reducing their residual service life. With regard to this subject, the Brite/Euram project BE-4062 started in 1992 and some models are being developed for the assessment of concrete structures affected by steel corrosion and other deterioration mechanisms. This paper summarizes and discusses the results of some research work carried out with corroded concrete beams, under the mentioned project. Reinforcement was corroded by means of adding calcium chloride to the mixing water and applying a current density of 100 μA/cm2. This value corresponds to ten tines the corrosion intensity Icorr measured in highly corroding concrete structures. It has been shown that corrosion increases both the deflections and the crack widths at service load, and reduces the strength at ultimate load. Beside, corrosion modifies the type of failure in concrete beams with usual ratios of reinforcement. Whereas sound tested beams failed by bending, deteriorated beams failed by shear. Pitting at links and cracking and spalling of top concrete cover, due to corrosion of reinforcement, have been shown as the most relevant damages in the tested beams. Finally, a conservative value of either the ultimate bending moment or the ultimate shear force can be predicted by using RC conventional models, as those included in Eurocode 2, and considering the reduced section of both steel and concrete due to corrosion of reinforcement.