Browsing by Author "Hahner, Peter"

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An enhanced method to evaluate tensile yield stress by small punch tests using deflection curves
(Mdpi, 2020-06-01) Hahner, Peter; Soyarslan, Celal; Bargmann, Swantje; Çakan, Betül Gülçimen; GÜLÇİMEN ÇAKAN, BETÜL; Bursa Uludağ Üniversitesi/Mühendislik Fakültesi/Makine Mühendisliği Bölümü.; 0000-0003-1739-1143; AFD-6959-2022
While force-displacement curves are often preferred in Small Punch (SP) tests due to the ease of the experimental set-up, they encompass significant uncertainties arising from frame compliance. In this work, a methodology is presented to predict yield stresses from the force vs. deflection curves. The present method relies on determining different force levels from the initial part of the force-deflection curve to reflect both the slope and the curvature instead of using a single force level only. The predicted yield stresses for different types of materials, that is, low- and high-strength alloys, are found to be in good agreement with the actual proof stresses with a maximum error of 16%.
Determination of creep properties of a P91 weldment by small punch testing and a new evaluation approach
(Elsevier, 2013-12-20) Hahner, Peter; Çakan, Betül Gülçimen; Uludağ Üniversitesi/Mühendislik Fakültesi/Makine Mühendisliği Bölümü.; 36983839100
Local creep properties of a P91 weldment consisting of distinct zones, namely base metal, weld metal, as well as the fine-grained and the coarse-grained parts of the heat-affected zone were examined by Small Punch creep tests. Deflection vs. time graphs showed the zones' different creep resistances with that of the Heat Affected Zone-Fine Grained being lowest. Norton and Monkman-Grant type relations were established from Small Punch deflection curves for all zones. The evaluation of the Small Punch results was performed according to a new approach originating from the membrane stretching theory, as well as the method associated with the European Code of Practice for Small Punch Testing. The stress-rupture time results calculated by the new method were found to be very close to the Code of Practice's results with a similar amount of scatter, noting that with the new method, each Small Punch creep test is analyzed individually according to its observed deflection curve contrary to the Code of Practice method which disregards the actual deflection behaviour.
Determining tensile yield stresses from Small Punch tests: A numerical-based scheme
(Elsevier Science, 2019-06-24) Hahner, Peter; Soyarslan, Celal; Bargmann, Swantje; Çakan, Betül Gülçimen; Bursa Uludağ Üniversitesi/Mühendislik Fakültesi/Makina Mühendisliği Bölümü.; 57209831238
The Small Punch (SP) test serves the screening of mechanical material properties and their degradation in a virtually non-invasive way. It requires robust frameworks for the derivation of mechanical properties and microstructure-mechanical property correlation. The tensile yield stress sigma(y) is commonly associated with an elastic-plastic transition force F-e via sigma(y) = alpha F-e/h(2) with h denoting the SP disc thickness and a dimensionless coefficient alpha considered constant. Here it is shown that alpha cannot be taken as a constant. Instead a new self-consistent data reduction scheme is proposed for the determination of sigma(y) which is based on the curvature of the force-displacement curve rather than a single F-e force level. The scheme derives from finite element simulations of a wide range of strength coefficients C and hardening exponents n of power law flow sigma = C epsilon(n). To a good approximation the scheme depends only on the hardening exponent n, which depends on the curvature, whereas C and the elastic modulus barely matter. The method is validated by comparing the yield stress predictions with the actually implemented yield stresses in the simulations, using various types of hardening rules, as well as experimental data. The uncertainty of yield stress determination by SP tests is thereby largely reduced as compared to the traditional scheme.
Experimental and computational study of ductile fracture in small punch tests
(MDPI, 2017-08-24) Soyarslan, Celal; Bargmann, Swantje; Hahner, Peter; Çakal, Betül Gülçimen; Uludağ Üniversitesi/Mühendislik Mimarlık Fakültesi/Makine Mühendisliği Bölümü.; 36983839100
A unified experimental-computational study on ductile fracture initiation and propagation during small punch testing is presented. Tests are carried out at room temperature with unnotched disks of different thicknesses where large-scale yielding prevails. In thinner specimens, the fracture occurs with severe necking under membrane tension, whereas for thicker ones a through thickness shearing mode prevails changing the crack orientation relative to the loading direction. Computational studies involve finite element simulations using a shear modified Gurson-Tvergaard-Needleman porous plasticity model with an integral-type nonlocal formulation. The predicted punch load-displacement curves and deformed profiles are in good agreement with the experimental results.
Modeling of fracture in small punch tests for small- and large-scale yielding conditions at various temperatures
(Elsevier, 2015-12-10) Soyarslan, Celal; Bargmann, Swantje; Hahner, Peter; Gülçimen, Betül; Uludağ Üniversitesi/Mühendislik Fakültesi/Makine Mühendisliği Bölümü.; 36983839100
We present a systematic numerical study on temperature dependent fracture mode change in small punch tests. Following Needleman and Tvergaard (2000), we model the material as thermo-inelastic, where the ductile fracture mode, by void nucleation, growth and coalescence is accounted for by Gurson's porous metal plasticity (Gurson, 1977). The brittle fracture mode by cleavage is accounted for by Ritchie-Knott-Rice's deterministic maximum principal stress criterion (Ritchie et al., 1973). The well-known problem of mesh dependence associated with softening material behavior is remedied by using an integral type nonlocal formulation similar to that presented in Tvergaard and Needleman (1995). Two length scales are incorporated into the constitutive relations: the ductile fracture length scale is based on the average inclusion distance and associated with the nonlocal evolution equation for the porosity. The brittle fracture length scale is based on the average grain size and associated with the material region at which the maximum principal stress is averaged out. The material model is used to simulate small punch tests at -196 degrees C, -158 degrees C and 25 degrees C of notched and unnotched specimens of P91 steel representative for small- and large-scale yielding conditions, respectively. The simulated fracture modes and patterns show a very good agreement with experiments: for 196 degrees C brittle fracture propagating normal to the maximum (tensile) principal stress prevails. For 25 degrees C ductile fracture is governed by shear localization with voidage. The simulations also show that the deformation energy is considerably higher for the upper shelf tests compared to the lower shelf tests.