James Lankford

High strain rate compression of closed-cell aluminium foams

Abstract The compressive deformation behavior of open- and closed-cell aluminum foams was assessed under static and dynamic loading conditions. High strain rate experiments were conducted in our laboratory using a split Hopkinson pressure bar system at strain rates ranging from 400 to 2500 s−1. A strain rate effect was demonstrated for Alporas, a closed-cell aluminum. foam. The strain-rate effect was more significant for a higher density (i.e. 15% relative density) Alporas foam and is attributed to the kinetics of gas flow through the cell structure. The experimental results are discussed in reference to recent findings by other investigators on the dynamic behavior of similar open- and closed-cell aluminum foams.

The role of plasticity as a limiting factor in the compressive failure of high strength ceramics

Abstract The behavior of aluminum oxide under compressive loading is investigated over a wide range in strain rate and degrees of confinement. It is shown that plastic flow can be generated in Al 2 O 3 at all strain rates if confinement is sufficient to prevent premature failure via microfracture. Moreover, plastic flow is itself a source of microfracture, and the threshold for multiple slip apparently constitutes the practical ultimate strength for the ceramic. Thus, for sufficiently fine-grained alumina tested under optimum conditions, no confinement is required to generate plastic flow, at which stress the material fails via dislocation-induced general microfracture.

The crack-initiation threshold in ceramic materials subject to elastic/plastic indentation

The threshold for indentation cracking is established for a range of ceramic materials, using the techniques of scanning electron microscopy and acoustic emission. It is found that by taking into account indentation plasticity, currant theories may be successfully combined to predict threshold indentation loads and crack sizes. Threshold cracking is seen to relate to radial rather than median cracking.

Compressive strength and microplasticity in polycrystalline alumina

The compressive strength of polycrystalline alumina at 23° C is found to be strain-rate sensitive, but insensitive to environment. Scanning electron microscopy of specimens loaded to near failure indicates the origin of the strength-strain-rate dependence to be localized plasticity in the form of twinning and, possibly, slip. The interaction of these deformation bands with grain boundaries causes the initiation of microcracks. Higher stresses produce still more twin/slip-nucleated microcracks, which finally coalesce at failure. It is suggested that twinning also may be related to the tensile failure of alumina.

Machine vision photogrammetry: a technique for measurement of microstructural strain in cortical bone

Understanding local microstructural deformations and strains in cortical bone may lead to a better understanding of cortical bone damage development, fracture, and remodeling. Traditional experimental techniques for measuring deformation and strain do not allow characterization of these quantities at the microstructural level in cortical bone. This study describes a technique based on digital stereoimaging used to measure the microstructural strain fields in cortical bone. The technique allows the measurement of material surface displacements and strains by comparing images acquired from a specimen at two distinct stress states. The accuracy of the system is investigated by analyzing an undeformed image set; the test image is identical to the reference image but translated by a known pixel amount. An increase in the correlation sub-image train parameter results in an increase in displacement measurement accuracy from 0.049 to 0.012 pixels. Errors in strain calculated from the measured displacement field were between 39 and 564 microstrain depending upon the sub-image train size and applied image displacement. The presence of a microcrack in cortical bone results in local strain at the crack tip reaching 0.030 (30,000 microstrain) and 0.010 (10,000 microstrain) near osteocyte lacunae. It is expected that the use of this technique will allow a greater understanding of bone strength and fracture as well as bone mechanotransduction.

The influence of crack tip plasticity in the growth of small fatigue cracks

In order to rationalize observed differences in the growth behavior of large and small cracks, local crack tip opening micromechanics have been characterized for both crack size regimes in a high strength aluminum alloy. It is found that crack tip opening displacement, crack tip opening load, and crack opening mode all differ widely for large and small cracks at equivalent cyclic stress intensities (ΔK). High crack tip opening displacements and relatively low, approximately constant, crack opening loads for microcracks account both for their rapid rate of growth relative to large cracks and the absence of a microcrack threshold stress intensity. Crack tip plastic zone sizes also were measured, and it was found that the ratio of plastic zone size to crack length for small cracks is ∼ 1.0, while for large cracks the same ratio is ≪ 1. Simple empirical corrections to ΔK are found inadequate to correlate the growth of large and small cracks. It is concluded that for small cracks, linear elastic fracture mechanics similitude does not apply, and that an alternative crack driving force must be formulated.

Measurement of microstructural strain in cortical bone.

It is well known that mechanical factors affect bone remodeling such that increased mechanical demand results in net bone formation, whereas decreased demand results in net bone resorption. Current theories suggest that bone modeling and remodeling is controlled at the cellular level through signals mediated by osteocytes. The objective of this study was to investigate how macroscopically applied bone strains similar in magnitude to those that occur in vivo are manifest at the microscopic level in the bone matrix. Using a digital image correlation strain measurement technique, experimentally determined bone matrix strains around osteocyte lacuna resulting from macroscopic strains of approximately 2,000 microstrain (0.2%) reach levels of over 30,000 microstrain (3%) over fifteen times greater than the applied macroscopic strain. Strain patterns were highly heterogeneous and in some locations similar to observed microdamage around osteocyte lacuna indicating the resulting strains may represent the precursors to microdamage. This information may lead to a better understanding of how bone cells are affected by whole bone functional loading.

Strain rate effects in porous materials

The behavior of metal foams under rapid loading conditions is assessed. Dynamic loading experiments were conducted in their laboratory using a split Hopkinson pressure bar apparatus and a drop weight tester; Strain rates ranged from 45 s{sup {minus}1} to 1200 s{sup {minus}1}. The implications of these experiments on open-cell, porous metals, and closed- and open-cell polymer foams are described. It is shown that there are two possible strain-rate dependent contributors to the impact resistance of cellular metals: (i) elastic-plastic resistance of the cellular metal skeleton, and (ii) the gas pressure generated by gas flow within distorted open cells. A theoretical basis for these implications is presented.

Temperature-strain rate dependance of compressive strength and damage mechanisms in aluminium oxide

The results of compression tests of Al3O3 performed over a wide range of temperatures and strain rates are interpreted in terms of dominant damage mechanisms. It is shown that compressive failure in Al2O3 is caused by one of three different mechanisms, each based on tensile (Mode I) growth of predominantly axial microcracks, and each characteristic of a specific temperature-strain rate regime. The concepts developed should be applicable to other strong ceramics.

The role of microstructural dissimilitude in fatigue and fracture of small cracks

Abstract The use of the stress intensity factor, K , as the characteristic driving force parameter for crack extension requires that the conditions of small scale yielding and microstructural similitude be met. The specific effect of microstructural similitude, or its lack, on the crack driving force of both small and short, versus large, cracks is examined in this article. The concept of microstructural dissimilitude is introduced, and its relevance to small crack behavior illustrated by considering the dependence of the number of grains interrogated by the crack front on crack size, and the resulting yield strength variation within the crack tip process zone. It is demonstrated that microstructural dissimilitude can lead to characteristic small crack behavior (anomalous rapid growth) by altering (1) the intrinsic fracture toughness and cyclic crack growth resistance of the process zone, and (2) the local crack driving force. The latter effect is examined in detail by using a modified Barenblatt-Dugdale model to deduce the dissimilitude-induced local crack driving force. The proposed model is then applied for predicting the crack growth and fracture behavior of short and small cracks on the basis of large crack data for a number of alloys. It is shown that the approach may be relevant to subcritical crack growth and fracture in brittle materials like ceramics, as well as to fatigue crack growth in metals.