J. R. Yates

Dynamic properties of high structural integrity auxetic open cell foam

This paper illustrates various dynamic characteristics of open cell compliant polyurethane foam with auxetic (negative Poissons ratio) behaviour. The foam is obtained from off-the-shelf open cell polyurethane grey foam with a manufacturing process based on mechanical deformation on a mould in a temperature-controlled oven. The Poissons ratio is measured with an image processing technique based on edge detection with wavelet methods. Foam samples have been tested in a viscoelastic analyser tensile test machine to determine the Youngs modulus and loss factor for small dynamic strains. The same samples have also been tested in an acoustic impedance tube to measure acoustic absorption and specific acoustic resistance and reactance with a transmissibility technique. Another set of tests has been set up on a cam plastometer machine for constant strain rate dynamic crushing analysis. All the tests have been carried out on auxetic and normal foam samples to provide a comparison between the two types of cellular solids. The results from the experimental tests are discussed and interpreted using microstructure models for cellular materials existing in the literature. The negative Poissons ratio foam presented in this paper shows an overall superiority regarding damping and acoustic properties compared to the original conventional foam. Its dynamic crushing performance is also significantly superior to the normal foam, suggesting a possible use in structural integrity compliant elements.

Dynamic crushing of auxetic open-cell polyurethane foam

Abstract A high strain rate compression test with a constant speed of 1.5 m/s has been performed on samples of negative Poissons ratio and normal open-cell polyurethane foam. The tests show that the transformation of the normal foam into the auxetic phase greatly increases the crashworthiness qualities of the open-cell foam.

The stress intensity of mixed mode cracks determined by digital image correlation

A generalized approach for determining the stress intensity factors (SIFs) K I and K II for any mode mixity directly from displacement fields obtained by digital image correlation is presented using a centre fatigue cracked aluminium plate as an example problem. It was found that the crack-tip position could be determined on average to within 50 per cent of the displacement vector spacing (60 μm). The approach has been shown to be fairly robust, both in terms of the stability of the SIFs thus obtained and their sensitivity (less than 0.07 MPa

A model for sliding mode crack closure part II: mixed mode I and II loading and application

Abstract The physical model proposed in Part I has been extended to quantify sliding mode crack closure (SMCC) under mixed mode I and lI loading conditions, with predominantly mode lI loading. The results reveal that a small nominal mode I component tends to reduce the local wedging mode I stress intensity and consequently reduces friction attenuation to increase the effective mode II stress intensity. Local stress fields are significantly disturbed due to crack morphology with a clear increase of mixed mode ratio, leading to a predominantly mode I loading condition at the crack tip. The extended model has been used to rationalize the mixed mode fatigue threshold results under predominantly mode II loading conditions for a structural steel BS4360 50D. The effect of load ratio observed experimentally is discussed in the light of fretting-which is believed to be the main mechanism contributing to sliding mode crack closure.

Modeling and optimal design of machining-induced residual stresses in aluminium alloys using a fast hierarchical multiobjective optimization algorithm

The residual stresses induced during shaping and machining play an important role in determining the integrity and durability of metal components. An important issue of producing safety critical components is to find the machining parameters that create compressive surface stresses or to minimize tensile surface stresses. In this article, a systematic data-driven fuzzy modeling methodology is proposed, which allows constructing transparent fuzzy models considering both accuracy and interpretability attributes of fuzzy systems. The new method employs a hierarchical optimization structure to improve the modeling efficiency, where two learning mechanisms cooperate together: the Nondominated Sorting Genetic Algorithm II (NSGA-II) is used to improve the models structure, while the gradient descent method is used to optimize the numerical parameters. This hybrid approach is then successfully applied to the problem that concerns the prediction of machining induced residual stresses in aerospace aluminium alloys. Based on the developed reliable prediction models, NSGA-II is further applied to the multiobjective optimal design of aluminium alloys in a “reverse-engineering” fashion. It is revealed that the optimal machining regimes to minimize the residual stress and the machining cost simultaneously can be successfully located.

The formation and propagation of mode I branch cracks in mixed mode fatigue failure

Abstract Fatigue crack propagation behaviour under mode II and mixed mode I and II loadings has been investigated in a weldable structural steel BS4360 50D, using an asymmetrical four point bend arrangement. The results show that the conditions for the formation and propagation of mode I branch cracks are of decisive importance for fatigue failures under combined loading conditions. Various aspects of the subject, including fatigue thresholds, branch angle, path and branch crack growth rate, have been studied and the results are reported in this paper. While cracks have been observed to propagate in the maximum ΔKI direction as widely reported, complications associated with the conditions to initiate such branch cracks are far less well understood. In addition, simple tools are in demand to measure and predict branch cracking in this well tried test system. Attempts towards these objectives have been made in this work.

Some aspects of fatigue thresholds under mode III and mixed mode III and I loadings

Abstract Fatigue threshold behaviour under mode III and in-phase mixed mode III and I loading conditions has been studied in a 3.5% NiCrMoV steel, using V-notched and precracked cylindrical bar specimens at load ratios R = 0.1 and −1. Comparisons have been made between the results from V-notched, precracked specimens and the results from earlier work using slit specimens. While little influence of R -ratio was found for slit specimens, R -ratio appears to be a decisive component in defining mixed mode fatigue thresholds in precracked specimens. Ambiguities arise as to the best strategy for mixed mode fatigue tests. Crack surface interference appears to be the predominant force to modify fatigue crack growth behaviour under mixed mode loading conditions. In such circumstances the measured mixed mode fatigue thresholds are generally geometry dependent and failure analyses based on LEFM can not be applied directly.

T-stress determination using thermoelastic stress analysis

T-stress and mixed-mode stress intensity factors have been determined experimentally using thermoelastic stress analysis and using a finite element method. Pure mode I, strong mixed-mode I and II, and interacting cracks have been used as the case studies. A new technique has been proposed to identify the crack tip from thermoelastic images. It has also been shown that using three terms of Williamss stress field formulation to determine the T-stress, yields a more accurate solution than using only the first two terms of the expansion.

Fatigue thresholds under mixed-mode (I+III) loading

Abstract A model is presented to describe the propagation of mode-l fatigue cracks under mixed-mode (I+III) fatigue conditions in angled-crack three-point-bend specimens. The onset of mode-I crack growth is governed by the orientation and the crack opening displacement of the fatigue cracks.

A mission synthesis algorithm For fatigue damage analysis

This paper presents a signal processing based algorithm, the Mildly Nonstationary Mission Synthesis (MNMS), which produces a short mission signal from long records of experimental data. The algorithm uses the discrete Fourier transform, orthogonal wavelet transform and bump reinsertion procedures. In order to observe the algorithm effectiveness a fatigue damage case study was performed for a vehicle lower suspension arm using signals containing tensile and compressive preloading. The mission synthesis results were compared to the original road data in terms of both the global signal statistics and the fatigue damage variation as a function of compression ratio. Three bump reinsertion methods were used and evaluated. The methods differed in the manner in which bumps (shock events) from different wavelet groups (frequency bands) were synchronized during the reinsertion process. One method, based on time-synchronized section reinsertion, produced the best results in terms of mission signal kurtosis, crest factor, root-mean-square level and power spectral density. For improved algorithm performance, bump selection was identified as the main control parameter requiring optimization.