J.A. Álvarez

Use of Small Punch Notched Specimens in the Determination of Fracture Toughness

In recent years an increasing interest has been shown in materials characterisation techniques based on the use of reduced dimension samples. One of these techniques, first developed in the early 80’s, is Small Punch testing, in which a punch acts over a plane small specimen deforming it until fracture. So far, this test has been used as a tool for the estimation of mechanical properties, for characterising ductilebrittle transition or for knowing the evolution of these properties with ageing phenomena as neutron irradiation. The Small Punch Test has even been used in the determination of fracture toughness by means of indirect correlations or complex methodologies based on damage models and Finite Element simulations. In this paper, a direct methodology for the determination of fracture toughness from Small Punch tests is proposed. The approach is based on the use of simply modified Small Punch specimens in which a lateral notch was machined. Attending to energy considerations, the J-Δa curve for a ship-building Grade A steel, as well as for a structural steel, E690, has been obtained using this type of samples and the results have been compared with the calculated results from conventional fracture tests.Copyright

THE GAP BETWEEN SUBSPACES AND PERTURBATION OF NON-SEMI-FREDHOLM OPERATORS

A classical result of Fredholm theory states that perturbing a semi-Fredholm operator T, (that is, an operator with closed range R(T) and finite dimensional kernel N(T) or cokernel Y/R(T)) by an operator A with norm ||A|| < j(T), the minimum modulus of T, or A compact, we obtain a semi-Fredholm operator T + A with dimN{T + A) < dimN(T) and dimY/R(T + A) ^ dimY/R(T). Hence, for semiFredholm operators the finite dimension of the kernel or the cokernel is stable under small or compact perturbations. However, for non-semi-Fredholm operators, we have the opposite case: Given such an operator we can find a compact operator with arbitrarily small norm such that the perturbed operator has infinite dimensional kernel and cokernel [6], (see also [7]).

Amelanotic lentigo maligna

\s=b\A 68-year-old woman had a pigmented lentigo maligna lesion excised from her left arm. During a seven-year follow-up period, the excision site gradually evolved into a hypopigmented macule with irregular areas of mild erythema. The clinical impression was neurodermatitis, but a biopsy specimen revealed lentigo maligna without melanin pigmentation. To our knowledge, this is the first case of amelanotic lentigo maligna reported in the English literature in which the clinical presentation was dermatitislike. Our experience suggests that biopsy should be done on all clinically changing sites of treated lentigo maligna, whether the changes are pigmented or not. (Arch Dermatol 116:82-83, 1980)

Effect of Cutting Method on Fatigue Crack Initiation and Fatigue Life of Structural Steel S355M

This paper analyses the effect of the cutting method on both the fatigue crack initiation and the fatigue life of steel S355M. The research covers three cutting methods (plasma, laser and oxy-fuel) and two specimen geometries: plain specimens with rectangular section and cut edges, and specimens with machined edges and a cut hole in the middle section. All the specimens were conducted to failure in a resonance machine by applying fatigue cycles, the stress ratio (R) being 0.1, and the corresponding S-N curves were obtained for each combination of cutting method and specimen geometry. The crack initiation time was estimated by analyzing the evolution of the resonance frequency on each specimen. The results show a significant influence of the cutting method on both the crack initiation and the fatigue life of this particular steel.Copyright

Evolution of mechanical behaviour in a structural steel subjected to high temperatures

Abstract In this work a study of how exposure to high temperatures can affect the mechanical properties of a structural steel is presented. To this end, several mechanical tests such as tensile, impact and microhardness tests have been carried out in order to compare the properties of the heat-exposed steel with those of an unaffected reference steel. Complementary compositional, metallographic and fractographic analyses have also been performed to try to cast some light on the problem. The results obtained allow us to extract some conclusions on the reutilization of the steel for structural applications.

Transition Region of Nuclear Vessel Steels: Master Curve Approach Using Small Punch Notched Specimens

The behaviour of the materials in ductile-brittle transition region must be known when performing structural integrity assessments of nuclear reactor vessels working under the effects of neutron irradiation. The characterisation of this region has been usually carried out by means of Charpy impact tests. Just during last few years new approaches based on direct fracture mechanics tests have begun to be used. In most of these cases, the Master Curve methodology, which allows the transition region to be characterised using only one parameter (T0 reference temperature), has been employed. In this paper the transition region of two materials –one vessel steel and one common structural steel-has been characterised by means of Small Punch Tests. First of all, this zone has been characterised using conventional specimens and the results were compared with those of Charpy impact tests. Finally a new approach based on the use of notched Small Punch samples together with Master Curve methodology has been proposed.

Using Small Punch tests in environment under static load for fracture toughness estimation in hydrogen embrittlement

In this paper, the response of three medium and high-strength steels to hydrogen embrittlement is analyzed by means of the quasi-non-destructive test known as the Small Punch Test (SPT). SPT tests on notched specimens under static load are carried out, applying Lacaclles methodology to estimate the fracture toughness for crack initiation, comparing the results to KIEAC fracture toughness obtained from C(T) precracked specimens tested in the same environment; SPT showed good correlation to standard tests. A novel expression was proposed to define the parameter KIEAC-SP as the suitable one to estimate the fracture toughness for crack initiation in hydrogen embrittlement conditions by Small Punch means, obtaining good accuracy in its estimations. Finally, Slow Rate Small Punch Tests (SRSPT) are proposed as a more efficient alternative, introducing an order of magnitude for the adequate rate to be employed.

Effect of Thermal Cutting Methods on the Fatigue Life of High Strength Structural Steel S690Q

This paper analyzes the effect of different cutting methods on the fatigue life of high strength steel S690Q. The research covers three cutting methods (oxy-fuel, plasma and laser) and two specimen geometries: plain specimens with rectangular sections and cut edges, and specimens with machined edges and a cut hole in the middle section. All the specimens were conducted to failure by applying fatigue cycles, the stress ratio (R) being 0.1, and the corresponding S-N curves were obtained for each combination of cutting method and specimen geometry. Measurements of roughness and hardness have been performed in order to explain the influence of the cutting method on the fatigue life of this particular steel. Fatigue results have been compared with the predictions provided by current fatigue standards, analyzing the possibility of extrapolating their S-N curves, focused on oxy-fuel cuts, to plasma and laser cuts.Copyright

New Equipment for Liquid Metal Embrittlement Characterisation for Vertical Axis Testing Machines

This text shows the design, validation and working of new experimental equipment. This set-up allows the performance of mechanical test with vertical axis universal testing machines when the specimens have to be horizontally immersed in aggressive liquids. A practical implementation to determine liquid metal embrittlement is showed in this paper. The design, validation process and the results obtained with the equipment for J tests using CT specimens immersed in liquid Zn at 450°C are explained in detail.

Structural Integrity Assessment of Componets with Low Constraint

When performing Structural Integrity assessments using Failure Assessment Diagrams, a point with coordinates Kr and Lr is representative of the component situation. The first one represents its situation against fracture and the second one against plastic collapse: