A technique for evaluating the (steady-state) creep stress exponent (n) from indentation data has come into common use over recent years. It involves monitoring the indenter displacement history ...under constant load and assuming that, once its velocity has stabilized, the system is in a quasi-steady state, with Stage II creep dominating the behaviour. The stress field under the indenter, and the way in which the creep strain field is changing there, are then represented by "equivalent stress" and "equivalent strain rate" values. These are manipulated in a similar manner to that conventionally employed with (uniaxial) creep test data, allowing the stress exponent, n, to be obtained as the gradient of a plot of the logarithm of the equivalent strain rate against the logarithm of the equivalent stress. The procedure is therefore a very simple one, often carried out over relatively short timescales (of the order of 1h or less). However, concerns have been expressed about its reliability, regarding the neglect of primary creep (after a very short initial transient) and about the validity of representing the stress and strain rate via these "equivalent" values. In this paper, comprehensive experimental data (both from a conventional, uniaxial loading set-up and from instrumented indentation over a range of conditions) are presented for two materials, focusing entirely on ambient temperature testing. This is supplemented by predictions from numerical (finite element method) modelling. It is shown that the methodology is fundamentally flawed, commonly giving unreliable (and often very high) values for n. The reasons for this are outlined in some detail. An attempt is made to identify measures that might improve the reliability of the procedure, although it is concluded that there is no simple analysis of this type that can be recommended.
Equal biaxial residual stresses (of up to about 175
MPa) have been generated in thin copper foils via differential thermal contraction. These foils were subsequently indented, under displacement ...control, and the load–displacement–time characteristics were measured. The applied load required for penetration to a given depth (in a given time) was found to decrease with increasing (tensile) residual stress, in accordance with predictions from a finite-element model (incorporating both plasticity and creep). The main thrust of this paper concerns sensitivities. Relatively small changes in residual stress (of the order of a few tens of MPa) were observed to generate effects that should be detectable via their influence on the nanoindentation response. This is encouraging in terms of the potential of the technique for characterizing (in-plane) residual stresses in surface layers, particularly for mapping of point-to-point variations (as opposed to obtaining accurate absolute values). In contrast to this, it is shown that changes in the hardness, as a consequence of changes in residual stress level, are smaller and more difficult to analyse.
Background
There are a variety of approaches that can be employed for Hopkinson bar compression testing and there is no standard procedure.
Objectives
A Split-Hopkinson pressure bar (SHPB) testing ...technique is presented which has been specifically developed for the characterisation of hazardous materials such as radioactive metals. This new SHPB technique is validated and a comparison is made with results obtained at another laboratory.
Methods
Compression SHPB tests are performed on identical copper specimens using the new SHPB procedures at Imperial College London and confirmatory measurements are performed using the well-established configuration at the University of Oxford. The experiments are performed at a temperature of 20
∘
C
and 200
∘
C
. Imperial heat the specimens externally before being inserted into the test position (ex-situ heating) and Oxford heat the specimens whilst in contact with the pressure bars (in-situ heating). For the ex-situ case, specimen temperature homogeneity is investigated both experimentally and by simulation.
Results
Stress-strain curves were generally consistent at both laboratories but sometimes discrepancies fell outside of the inherent measurement uncertainty range of the equipment, with differences mainly attributed to friction, loading pulse shapes and pulse alignment techniques. Small metallic specimens are found to be thermally homogenous even during contact with the pressure bars.
Conclusion
A newly developed Hopkinson bar for hazardous materials is shown to be effective for characterising metals under both ambient and elevated temperature conditions.
•A methodology is presented for extraction of creep parameters from indentation data.•It is shown that primary creep often exerts a strong influence on the behaviour.•An explanation is provided of ...why a currently-common methodology gives poor results.
A methodology is presented for the extraction of creep parameters from nanoindentation data – i.e. data obtained from an indentation system with a high resolution displacement measuring capability. The procedure involves consideration of both primary and secondary creep regimes. The sensitivities inherent in the methodology are explored and it is concluded that, provided certain conditions are satisfied, it should be reasonably robust and reliable. In contrast to this, it is also shown that the methodology commonly used at present to obtain (steady state) creep parameters is in general highly unreliable; the effects responsible for this are identified.
Both the hardness and modulus values obtained by nanoindentation depend on knowledge of the area of contact between the test material and the indenter. Significant errors can be introduced into the ...measurement if the projected area of the indentation contact is not known with sufficient accuracy. The state-of-the-art for user validation of indenter area function is to use materials of known Young’s modulus in the ‘two reference materials method’. However, the area function is best measured directly by traceably calibrated metrological atomic force microscopy. An area function for a standard Hysitron indenter was generated, using both atomic force microscopy and indentation. The results are compared, and the effect of data fitting techniques and measurement uncertainty are discussed.
A methodology is presented for inferring the yield stress and work-hardening characteristics of metallic coatings from indentation data. It involves iterative use of FEM modelling, with predicted ...outcomes (load–displacement relationships and residual indent shapes) being systematically compared with experimental data. The cases being considered are ones in which the indenter penetration depth is a significant fraction of the coating thickness, so that the properties of the substrate, and possibly of the interface, are of significance. The methodology is thus suitable for the testing of thin coatings. In the present work, the coatings were in fact relatively thick (hundreds of microns) and the (spherical) indenter penetration was a substantial fraction of this. In this way, the basic validity of the methodology could be investigated with minimal complications from effects related to microstructure, oxide films, surface roughness etc. Furthermore, the properties of both coating and substrate (in the through-thickness direction) were established separately via conventional compression testing. The systems studied were copper (yield stress ∼15 MPa) on stainless steel (yield stress ∼350 MPa) and vice versa. Both exhibited significant work hardening. It is concluded that the methodology is basically reliable, with relatively good sensitivity and resolution, although this does depend on several factors, which are highlighted in the paper. It is unlikely to be suitable for very thin (sub-micron) films, but should be reasonably accurate for coatings of thickness down to a few microns.
Austenitic stainless steels like 316L are amongst the most commonly selected structural alloys for use in corrosion environments. Unfortunately, their resistance to surface degradation caused during ...sliding contacts with other materials, in such environments is poor. Here, a synergistic combination of mechanical (wear) and chemical (corrosion) processes, known as corrosion–wear processes, are responsible for causing surface material loss. Accordingly, efforts are being made to identify surface treatments that can enhance the corrosion–wear resistance of 316L and similar alloys. One plausible solution is to apply thin hard coatings (∼5–10
μm thick) using various plasma-based technologies. In practice, this is often fraught with difficulty because of the complex nature of the pervading corrosion–wear mechanisms. This paper presents our recent work that has identified three major corrosion–wear mechanisms that must be minimised if a successful surface engineering design is to be achieved for corrosion–wear protection. These are: Type I—the removal of the coating passive film during sliding contact; Type II—galvanic attack of the substrate resulting in blistering of the coating and; Type III—galvanic attack of the counterface material leading to abrasion of the coating during subsequent sliding contact.
The response of 316L austenitic stainless steel when untreated or sputter coated with CrN and S-phase (nitrogen supersaturated austenitic stainless steel) has been evaluated under sliding contact ...corrosion-wear situations. The lowest friction coefficient was recorded for the CrN coated material which also gave the best resistance to corrosion-wear. The S-phase coating reduced corrosion-wear compared to untreated 316L, but despite exhibiting the most electrochemically passive behaviour, was inferior to CrN. The corrosion-wear of the uncoated and S-phase coated 316L was dominated by mechanical wear, which in the latter case involved brittle fracture of the S-phase coating and/or fracture along the S-phase–substrate interface.
Ductile damage in metallic materials is caused by the nucleation, growth and coalesce of voids and micro-cracks in the metal matrix when it is subjected to plastic strain. A considerable number of ...models have been proposed to represent ductile failure focusing on the ultimate failure conditions; however, only some of them study in detail the whole damage accumulation process. The aim of this work is to review experimental techniques developed by various authors to measure the accumulation of ductile damage under tensile loads. The measurement methods reviewed include: stiffness degradation, indentation, microstructure analysis, ultrasonic waves propagation, X-ray tomography and electrical potential drop. Stiffness degradation and indentation techniques have been tested on stainless steel 304L hourglass-shaped samples. A special interest is placed in the Continuum Damage Mechanics approach (CDM) as its equations incorporate macroscopic parameters that can represent directly the damage accumulation measured in the experiments. The other main objective lies in identifying the strengths and weaknesses of each technique for the assessment of materials subjected to different strain-rate and temperature conditions.
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