Experimental assessment of mixed-mode partition theories

The propagation of mixed-mode interlaminar fractures is investigated using existing experimental results from the literature and various partition theories. These are (i) a partition theory by Williams (1988) based on Euler beam theory; (ii) a partition theory by Suo (1990) and Hutchinson and Suo (1992) based on 2D elasticity; and (iii) the Wang–Harvey partition theories of the authors based on the Euler and Timoshenko beam theories. The Wang–Harvey Euler beam partition theory seems to offer the best and most simple explanation for all the experimental observations. No recourse to fracture surface roughness or new failure criteria is required. It is in excellent agreement with the linear failure locus and is significantly closer than other partition theories. It is also demonstrated that the global partition of energy release rate when using the Wang–Harvey Timoshenko beam or averaged partition theories or 2D elasticity exactly corresponds with the partition from the Wang–Harvey Euler beam partition theory. It is therefore concluded that the excellent performance of the Wang–Harvey Euler beam partition theory is either due to the failure of materials generally being based on global partitions or that for the specimens tested, the through-thickness shear effect is negligibly small. Further experimental investigations are definitely required.     

Elastic wave propagation in cylindrical bars after brittle failures: Application to spalling tests

The effect of brittle failures on the elastic wave propagation along cylindrical bars is analysed. From experimental observations provided by spalling tests of ceramic materials, a theoretical analysis is carried out based on a finite elements simulation of the experiments and a mathematical analysis of the pulses by means of Fourier Transform techniques. Differences between the propagating waves before and after the material failure are revealed. After failure, the pulse is influenced by dispersion effects and its shape changes during propagation. To correct this effect, a procedure based on Bancroft's curves is suggested. Finally, some clues about the way to get consistent results from spalling tests are given.     

The spalling strength of ultra-fiber reinforced cement mortar

This is a research report about the effects of polypropylene fiber and wood fiber on mechanical properties of cement mortar. First, using advanced Hopkinson pressure bar (HPB) tests, it investigates the wave propagation in cement mortar comprised polypropylene fiber and wood fiber. Second, according to the experiment, the spallation position is recorded by high-speed camera. Thirdly, it analyzes the test data of ultra-fiber reinforced and common cement mortar by numerical method. Finally, it deduces the spalling strength of all kinds of cement mortar by integrating all experimental data above. The results indicate that, compared with the strength of common cement mortar, the dynamic spalling strength of ultra-fiber especially that of the polypropylene fiber reinforced cement mortar increases evidently. However, adding too much fibers will deteriorate the dynamic spalling strength of cement mortar specimen. So the results will provide a test basis for further optimizing performance of cement mortar.     

On the use of British standard 7910 option 1 failure assessment diagram to non‐metallic materials

This paper provides a structural integrity assessment methodology for the analysis of non‐metallic materials. The approach uses the British standard 7910 option 1 failure assessment diagram, originally proposed for the fracture‐plastic collapse assessment of metallic materials. The methodology has been applied to 60 fracture specimens, combining 12 different materials and covering polymers, composites, and rocks. The results obtained validate the proposed assessment methodology and demonstrate its safety for the materials analysed here.     

A failure criterion for brittle elastic materials under mixed-mode loading

The failure criterion of Leguillon at reentrant corners in brittle elastic materials (Leguillon 2002, Eur J Mech A/Solids 21: 61–72; Leguillon et al. (2003), Eur J Mech A—Solids 22(4): 509–524) validated in (Yosibash et al. 2004, Int J Fract 125(3–4): 307–333) for mode I loading is being extended to mixed mode loading and is being validated by experimental observations. We present an explicit derivation of all quantities involved in the computation of the failure criterion. The failure criterion is validated by predicting the critical load and crack initiation angle of specimens under mixed mode loading and comparison to experimental observations on PMMA (polymer) and Macor (ceramic) V-notched specimens.     

Modeling for fracture in materials under long-term static creep loading and neutron irradiation. Part 1. A physico-mechanical model

The paper presents a physico-mechanical model for predicting creep rupture in neutron-irradiated materials. The model is based on the approach whereby damage is described as voids on grain boundaries. The equations for void nucleation and growth which were proposed by the authors earlier are augmented to include neutron irradiation of material. Constitutive equations are derived to describe viscoplastic deformation of material including void propagation. A criterion for plastic stability of a unit cell is employed as a fracture criterion. __________ Translated from Problemy Prochnosti, No. 3, pp. 5–22, May–June, 2006.     

Bulk metallic glasses: A new class of engineering materials

Bulk glass-forming alloys have emerged over the past fifteen years with attractive properties and technological promise. A number of alloy systems based on lanthanum, magnesium, zirconium, palladium, iron, cobalt and nickel have been discovered. Glass-forming ability depends on various factors like enthalpy of mixing, atomic size and multicomponent alloying. A number of processes is available to synthesise bulk metallic glasses. The crystallisation behaviour and mechanical properties of these alloys pose interesting scientific questions. Upon crystallisation many of these glasses transform to bulk nanocrystals and nanoquasicrystals. A detailed study of the structure and the crystallisation behaviour of glasses has enabled the elucidation of the possible atomic configuration in liquid alloys. Their crystallisation behaviour can be exploited to synthesise novel nanocomposite microstructures and their mechanical properties can be enhanced. A broad overview of the present status of the science and technology of bulk metallic glasses and their potential technological uses is presented.     

A new fracture mechanics theory for orthotropic materials like wood

A new fracture mechanics theory is derived based on a new orthotropic-isotropic transformation of the Airy stress function, making the derivation of the Wu-“mixed mode I-II” fracture criterion possible, based on the failure criterion of a flat elliptic crack. As a result of this derivation, the right fracture energy and theoretical relation between mode I and II stress intensities and energy release rates are obtained.     

Polycrystal models for the analysis of intergranular crack growth in metallic materials

In polycrystal materials the intergranular decohesion is one important damage phenomena that leads to microcrack initiation. The paper presents a mesoscale model, which is focused on the brittle intergranular damage process in metallic polycrystals. The model reproduces the crack initiation and propagation along cohesive grain boundaries between brittle grains. An advanced Voronoi algorithm is applied to generate polycrystal material structures based on arbitrary distribution functions of grain size. Therewith, the authors are more flexible to represent realistic grain size distributions. The polycrystal model is applied to analyze the crack initiation and propagation in statically loaded samples of aluminium on the mesoscale without the necessity of initial damage definition.     

Overview of metallic materials for heat exchangers for ocean thermal energy conversion systems

Candidate materials for use in ocean thermal energy conversion systems, OTECS, heat exchangers include aluminium, Cu-Ni, stainless steel and titanium alloys. These are considered in this review, and their advantages and disadvantages are highlighted and discussed. Aluminium alloys have shortcomings for the anticipated long life span of OTEC heat exchangers; however they may still offer an economic alternative in the form of short-life, disposable units of low initial capital cost. The long-term effects of exposure to ammonia and to erosion pose questions about the suitability of Cu-Ni alloys. Although stainless steel alloys provide a strong challenge for OTECS use, titanium exhibits better seawater performance, has good fabricability, and has an excellent service history in marine environments. These factors, together with current and projected developments promise major savings in materials usage, reduced OTECS structure sizes and increased efficiencies consequent on the use of titanium and its alloys.     

A new statistical local criterion for cleavage fracture in steel. Part II: application to an offshore structural steel

In the first part of this paper, a new model for cleavage fracture in steel was presented, based on a new statistical local criterion, which expresses the necessity of simultaneously fulfilling the conditions for both cleavage microcrack nucleation and propagation. In this second part, the assumptions and predictive capabilities of the new model are assessed using a modern offshore structural steel plate (Grade 450EMZ). It is shown that the model assumptions are consistent with the cleavage fracture behaviour of the steel and that the new model has the potential of correctly quantifying the effects of size, constraint, temperature and strain rate on cleavage fracture risk.     

A critical plane-based fracture criterion for mixed-mode delamination in composite materials

A new critical plane-based mixed-mode delamination failure criterion is proposed in this study. First, many existing models are reviewed and their capability to handle the mixed-mode fracture of general anisotropic materials are discussed. Following this, a previously developed critical plane approach is extended to analyze the interfacial fracture of composite materials by considering the anisotropic fracture resistance under mixed-mode loadings. Next, comparison with extensive experimental data available in the literature is performed to demonstrate the validity of the proposed criterion. A general good agreement is observed between the model's predictions and experimental observations. Finally, some conclusions and future work are drawn based on the proposed study.     

Dynamic fracture of berylium-bearing bulk metallic glass systems: A cross-technique comparison

We report on a detailed comparison between two different experimental techniques used to measure the dynamic initiation fracture toughness of a bulk metallic glass system (Vitreloy-1) and its β-phase composite. Both the coherent gradient sensing interferometry (CGS) and one-point impact techniques reveal very similar trends in the relationship for Vitreloy-1. A drastic increase in initiation toughness with the stress intensity rate is observed. By contrast, the one-point impact method shows a relative rate-insensitivity for the of the β-phase composite. The results are rationalized through a detailed characterization of the failure mechanisms.     

A multiparameter approach to the prediction of fatigue crack growth in metallic materials

A multiparameter approach is proposed for the characterization of fatigue crack growth in metallic materials. The model assesses the combined effects of identifiable multiple variables that can contribute to fatigue crack growth. Mathematical expressions are presented for the determination of fatigue crack growth rates, d a /d N , as functions of multiple variables, including stress intensity factor range, Δ K , stress ratio, R , crack closure stress intensity factor, K _cl , the maximum stress intensity factor K _max , nominal specimen thickness, t , frequency, Ω , and temperature, T . A generalized empirical methodology is proposed for the estimation of fatigue crack growth rates as a function of these variables. The validity of the methodology is then verified by making appropriate comparisons between predicted and measured fatigue crack growth data obtained from experiments on Ti–6Al–4V. The effects of stress ratio and specimen thickness on fatigue crack growth rates are then rationalized by crack closure considerations. The multiparameter model is also shown to provide a good fit to experimental data obtained for HY-80 steel, Inconel 718 polycrystal and Inconel 718 single crystal. Finally, the implications of the results are discussed for the prediction of fatigue crack growth and fatigue life.     

A crack extension force correlation for hard materials

Increasingly, the essential, robust character of many nanoscale devices requires knowledge of their fracture toughness. For most brittle materials the technique of choice has been indentation mechanics but little insight into the fracture mechanism(s) has resulted since these have generally been treated as brittle fracture dominated by the true surface energy. Linear elastic fracture mechanics approaches have been invoked to describe indentation fracture but do not address why the surface energy from fracture toughness is most often slightly or even substantially greater than the true surface energy. In the present study we invoke a crack extension force correlation that demonstrates why this is the case at least in fracture measurements based on indentation mechanics. The proposed correlation is different from previous ones in that it focuses on observations of indentation-induced dislocation activity prior to fracture. Allowing the resistance side of the crack extension force analysis to incorporate small amounts of plasticity gives a relationship that is consistent with 22 relatively brittle intermetallics, semiconductors and ceramics. This explains why measured strain energy release rates can be 2 to 5 times as large as surface energies measured in vacuum or calculated by pseudopotentials using the local density approximation.     

Non-local modeling of thermal shock damage in refractory materials

A non-local damage framework has been coupled with heat transport to model transient thermo-mechanical damage (in particular thermal shock) in refractory materials. The non-locality, to be dealt with to obtain an adequate problem formulation, is introduced by terms accounting for micro-structural strain gradients induced by transient temperature gradients. The parameters figuring in the evolution law for elasticity-based damage are temperature dependent. Damage due to isotropic thermal expansion has been accounted for by proposing a new evolution law. A single variable for the total damage is obtained by combining both damage mechanisms. The influence of non-locality and transient temperature gradients within non-locality is investigated in numerical examples. The phenomenological relevance of the framework is verified by modeling of experiments, which simulate thermal shock under process conditions.     

A hard diffusion boride coating for ferrous materials

A pack cementation method is described for forming a hard dense boride coating on ferrous materials. The coating produced on different steels was characterized by metallography, X-ray diffraction and electron microprobe analysis. The coating on all the steels studied consisted predominantly of iron borides, though small amounts of chromium could also be incorporated in the coating by varying the pack chemistry. The adherence of the coating was observed to depend on its thickness and the particular base material; greater thicknesses and greater amounts of alloying elements in the base material resulted in a generally greater tendency for the coating to spall. The coating exhibited good resistance to corrosion in slightly acidic water as long as it remained free of any microcracks; it also showed a much better resistance against erosion by flowing pressurized water than either tungsten carbide or Stellite did.     

Anomalous behavior of thermal conductivity and diffusivity in polymeric materials filled with metallic particles

Thermal properties of two composites prepared by using a polypropylene matrix filled with aluminum (slightly oxidized) and copper fillers were investigated. For each of these fillers two different particle sizes were used. We have shown an anomalous thermal behavior when these metallic fillers are slightly oxidized, i.e. higher thermal transport is obtained for PP/Al composites when using the largest particles. So, the PP/Al composites thermal behavior is not consistent with the PP/Cu ones and with the literature results reported for dielectric or conducting filler particles in a polymeric matrix. Thus, these PP/Al composites exhibit higher thermal transport properties than the homopolymer matrix where as electrical insulating properties of PP are preserved. This kind of composite structure might be of great interest in some technological applications where both good electrical insulation and high thermal conduction are required.     

Effect of microstructure on thermal expansion behaviour of nanocrystalline metallic materials

Materials properties, among which thermodynamic ones, are influenced by microstructural features. This is so also in the case of nanocrystalline materials, featuring average grain size below 100 nm. A reduced grain size involves that significant fractions of atoms are localised in grain boundary regions and this has remarkable effects on the resulting thermodynamic properties, like heat capacity, transition temperatures, coefficient of thermal expansion, etc. In the present work we consider the thermal expansion behaviour of ball-milled nanocrystalline metallic powders using dilatometric measurements. High-energy ball-milling, that is capable to achieve extremely high deformation degrees, induces in the milled powders microstructural defects, like vacancies, antisites, dislocations and planar faults. Another effect of milling is the reduction of the crystallite size, that, in the long run, may reach the nanometric range. In view of the microstructural changes that can be brought about by milling and of the numerous transformations occurring during the dilatometric runs, a comparative study has been conducted on intermetallic, NiAl and Ni₃Al, and on a pure metal, nickel, powders. The results emerging from the experimental investigation are quite complex, owing to the complex defect structures that are present in the ball-milled powders. It turns out that the thermal expansion coefficient of the nanocrystalline powders increases as the average grain size is reduced. However, when the average grain size achieves very low values, the strain relaxation of the crystalline lattice and the rearrangement of grain boundary regions result in a reduction of the thermal expansion coefficient. Another aspect that emerges from the dilatometric curves is the interplay between recrystallization and reordering, i.e. the re-establishment of the long-range order in the intermetallic powders, that had been partially or fully eliminated by ball-milling.