6 research outputs found
Bayesian Optimised Collection Strategies for Fatigue Testing : Constant Life Testing
This paper presents a statistical framework enabling optimal sampling and
robust analysis of fatigue data. We create protocols using Bayesian maximum
entropy sampling, which build on the staircase and step methods, removing the
requirement of prior knowledge of the fatigue strength distribution for data
collection. Results show improved sampling efficiency and parameter estimation
over the conventional approaches. Statistical methods for distinguishing
between distribution types highlight the role of the protocol in model
distinction. Experimental validation of the above work is performed, showing
the applicability of the methods in laboratory testing.Comment: 25 pages, 12 figures + 1 SI figur
Cold Creep of Titanium: Analysis of stress relaxation using synchrotron diffraction and crystal plasticity simulations
There is a long standing technological problem in which a stress dwell during
cyclic loading at room temperature in Ti causes a significant fatigue life
reduction. It is thought that localised time dependent plasticity in soft
grains oriented for easy plastic slip leads to load shedding and an increase in
stress within a neighbouring hard grain poorly oriented for easy slip.
Quantifying this time dependent plasticity process is key to successfully
predicting the complex cold dwell fatigue problem. This work uses a novel
approach of in situ synchrotron X-ray diffraction during stress relaxation
tests, to quantify the time dependent plasticity. Measured lattice strains from
multiple lattice families (21 diffraction rings) were compared with simulated
lattice strains from crystal plasticity finite element (CPFE) simulations. The
prism slip parameters were found to show stronger strain rate sensitivity
compared to basal slip, and this has a significant effect on stress
redistribution to hard grain orientations during cold creep
Non-contact, non-destructive mapping of thermal diffusivity and surface acoustic wave speed using transient grating spectroscopy
We present new developments of the laser-induced transient grating
spectroscopy (TGS) technique that enable the measurement of large area 2D maps
of thermal diffusivity and surface acoustic wave speed. Additional capabilities
include targeted measurements and the ability to accommodate samples with
increased surface roughness. These new capabilities are demonstrated by
recording large TGS maps of deuterium implanted tungsten, linear friction
welded aerospace alloys and high entropy alloys with a range of grain sizes.
The results illustrate the ability to view grain microstructure in elastically
anisotropic samples, and to detect anomalies in samples, for example due to
irradiation and previous measurements. They also point to the possibility of
using TGS to quantify grain size at the surface of polycrystalline materials.Comment: The following article has been submitted to Review of Scientific
Instruments. After it is published, it will be found at
https://aip.scitation.org/journal/rs
An in-situ synchrotron diffraction study of stress relaxation in titanium:Effect of temperature and oxygen on cold dwell fatigue
There is a long-standing technological problem in which a stress dwell during cyclic loading at room temperature in Ti causes a drastic fatigue life reduction. To better understand the material characteristics that control or exacerbate this behaviour, evaluation of the time dependent plasticity of the main prismatic and basal slip systems is critical. Incorporating the influence of operating temperatures and common alloying elements on cold dwell fatigue will be beneficial for future alloy design to address this problem. In this work, characterisation of the time dependent plastic behaviour of two commercially pure titanium samples (grade 1 and grade 4) with different oxygen content at 4 different temperatures (room temperature, 75 , 145 and 250 ) was performed during stress relaxation using synchrotron X-ray diffraction. Key parameters that govern the dislocation motion were determined for the major prismatic and basal slip systems as a function of temperature and oxygen content by calibrating a crystal plasticity finite element model with the measured lattice strain relaxation responses. From the temperatures assessed, 75 was found to be the worst-case scenario, where the macroscopic plastic strain accumulation was significant during a relaxation cycle due to the greatest activity of both prism and basal slip systems. As the temperature increases, the contribution of thermal energy becomes greater than mechanical energy for dislocation glide. Oxygen was found to have a stronger strengthening effect on prism slip over basal slip, through a significant change in their respective critical resolved shear stresses. This effect becomes more significant in high oxygen content commercially pure Ti
Weighted ellipse fitting routine for spotty or incomplete Debye-Scherrer rings on a 2D detector
We introduce a weighted ellipse fitting routine to measure Debye Scherrer
rings acquired on 2D area detectors and demonstrate its use in strain
determination. The method is relatively robust against incomplete rings due to
low number of grains in the diffraction volume (spotty rings), or strong
texture (intensity depletion in some azimuths). The method works by applying an
annular mask around each diffraction ring and fitting an ellipse, using all
pixel positions and their diffracted intensity as weights in the minimisation.
We compare this method to the more popular cake integration method, and show
that the weighted ellipse method works when the cake integration method fails
or works poorly. The lattice strain sensitivity from spotty diffraction rings
is in the order or 2E-5 or better. The algorithm has been made available for
public use and works with 2D diffraction patterns acquired in a laboratory
scale XRD equipment, TEM or a synchrotron