SIMULING AND DATA ANALYSIS
RESIDUAL LIFE OF WELDED COMPONENTS IN THE PRESENCE OF FLAWS
It is well known that in carbon steels and low-alloy steels, a defect 1.2 mm in size can degenerate into a crack when the structure is subjected to stress. Of course, the growth rate of the discontinuity depends upon the operating temperature and the presence of time-dependent load cycles. This approach enables an evaluation of the residual life of the component along with a monitoring of flaw growth over a long period of time during actual plant operation.
During the last decade N.D.E. has gained considerable experience in fracture mechanics, particularly with respect to defect characterisation by way of both non-destructive testing techniques as well as mathematical modelling of defect growth using finite element methods. This approach enables the determination of the component’s residual life such that a superior strategy may be adopted for plant shut-down since the correct time interval allowable between consecutive inspections is known with accuracy. Further advantages derive from increased component reliability and through a determination of the several operating conditions (pressure, temperature, load cycles) in guaranteeing a minimum failure-free time period in operation. The methodology developed allows defect analysis in both the fusion zone and the heat-affected zone.
EVALUATION OF STEEL AGING AND NON-DESTRUCTIVE EVALUATION OF MECHANICAL PROPERTIES
It is well known that the initiation of ductile fracture in metals and alloys frequently occurs due to void formation at inclusions and/or precipitates either by interface separation or by particle cracking. The voids so formed eventually either coalesce or else form fine cracks, thereby leading to failure. Recent work has suggested that particle fracture arises from stress concentrations generated by individual slip bands in the matrix. Dynamic strain-aging is known to be accompanied by heterogeneous macroscopic and microscopic deformations. This phenomenon involves a reduction in both yield stress and toughness; in view of this, a non-destructive comparative methodology has been developed to evaluate the variation of mechanical properties of the element. In particular, a core sample is extracted and subjected to metallographic and ultrasonic tests (multiple reflection echo technique). Metallographic examinations determine the grain size, characterise the precipitates and possibly the microvoids too (by means of scanning electron microscopy - SEM). Since the rate of diffusion of elements in supersaturation differs between the external surface and the material’s internal zones, yet another ultrasonic test is required to estimate the attenuation coefficient.
RESIDUAL LIFE EVALUATION IN ELEMENTS SUBJECTED TO CREEP
For many years, the phenomenon of creep has been under study both theoretically and experimentally since the behaviour of a material at high temperature is governed by many variables. Even more complex is estimating the residual life of a component, particularly because operating conditions tend to differ so much from the original design conditions; this is rendered all the more difficult through plant mis-operation or substantial deviations from the required chemical composition of the materials in use.
The mechanism of creep:
Creep - transgranular fracture
Creep - intergranular fracture
Nucleation of voids
Growth of voids
Evaluation criteria:
Naubauer method
Needleman-Rice criteria
FAILURE ANALYSIS
When serious damage in inflicted on, say, a ship or else a major structural failure goes under way, as is the case with a large fracture through a ship’s hull, damage investigations are sought at once. The interested parties usually include the ship owner, the company which originally built the ship, the operators, the competent court and the insurance company; moreover, a considerable measure of conflict tends to exist amongst all these particularly with respect to which of them is to bear the larger part of the costs involved in bringing the ship back in shape and ready for operations once again. Meanwhile, a substantial loss of revenue is being incurred which ought to be reduced to the unavoidable minimum level.
It is therefore required to determine with certainty what exactly went wrong and to decipher what really caused the rupture or other failure, as the case may be. To this aim, the nature of the tests required depend upon the type of failure/rupture being investigated and the relevant operating conditions under which it took place. In general, however, evaluation of failure generally takes the form of fractographic analysis, whereby metallographic examinations are necessary comprising sectioning within the fractured zone and adequate analyses with both optical microscopy (500x, 1000x) and scanning electron microscopy (over 10,000x). Microhardness testing in selected areas complements this as do specialised mathematical models which enable the determination of the stress and strain states in operation by simulating service conditions through finite element analysis. Tests of an even more specialised nature would need to be performed depending on the particular case and ancillary requests by the authorities involved.
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