I. Peñuelas

Damage modeling in Small Punch Test specimens

Ductile damage modeling within the Small Punch Test (SPT) is extensively investigated. The capabilities of the SPT to reliably estimate fracture and damage properties are thoroughly discussed and emphasis is placed on the use of notched specimens. First, different notch profiles are analyzed and constraint conditions quantified. The role of the notch shape is comprehensively examined from both triaxiality and notch fabrication perspectives. Afterwards, a methodology is presented to extract the micromechanical-based ductile damage parameters from the load-displacement curve of notched SPT samples. Furthermore, Gurson-Tvergaard-Needleman model predictions from a top-down approach are employed to gain insight into the mechanisms governing crack initiation and subsequent propagation in small punch experiments. An accurate assessment of micromechanical toughness parameters from the SPT is of tremendous relevance when little material is available.

Inverse Methods on Small Punch Tests

The characterization of the mechanical behaviour of structural materials, with the exception of material hardness, is a destructive procedure which requires direct extraction of test specimens from the component to analyse. Because this component needs to be operative, these specimens have to be as small as possible, in order not to affect the behaviour of the component and in order to allow easy reparation of the ‘damaged’ component. However, tests with miniaturized specimens are not defined in standards. Thus, the results obtained with these tests have to be interpreted in order to obtain the actual properties of the components from which the specimens have been extracted (Lucas et al., 2002). The small punch test (SPT) is very useful in all applications that require the characterization of the mechanical behaviour of structural materials or operational components without compromising their service (Lucon, 2001), as in the case of nuclear or thermal plants. Another application is the study of small testing zones. Thus, this test has been recently applied to the mechanical characterization of metallic coatings (Penuelas et al, 2009) or the heat affected zone of welds (Rodriguez et al, 2009), which are practically impossible to characterize by means of the conventional mechanical tests. Advance constitutive models frequently include parameters that have to be identified through numerical simulation of tests and mathematical optimization of variables, because they cannot often be directly measured in laboratory. In this paper, an inverse methodology for the identification of the mechanical and damage properties of structural steels has been developed. Thus, from the load-displacement curves obtained during the non-standard SPT, the mechanical and damage properties will be obtained. Moreover, this methodology also allows simulating the SP test with numerical methods. Structural steels may exhibit creep behaviour and behave according to the Hollomon’s law (σ = K·epn). Besides, ductile fracture of metallic materials involves micro-void nucleation and growth, and final coalescence of neighbouring voids to create new surfaces of a macro-crack. The ductile failure process for porous materials is often modelled by means of the Gurson model (Gurson, 1977), which is one of the most widely known micro-mechanical models for ductile fracture, and describes the progressive degradation of material stress capacity. In this model, which is a modification of the von Mises one, an elastic–plastic matrix material is considered and a new internal variable, the void volume fraction, f, is introduced. Although the original Gurson model was later modified by many authors, particularly by Tvergaard and Needleman (Tvergaard, 1981; Tvergaard, 1982; Tvergaard & Needleman, 1984), the resultant model is not intrinsically able to predict coalescence, and is only capable of