A robust numerical approach for the simulation of skin–stringer debonding growth in stiffened composite panels under compression

In this paper, a numerical study on skin–stringer debonding growth in stiffened composite panels has been carried out. A novel numerical methodology is proposed here to investigate the compressive behaviour of a stiffened composite panel in the presence of skin–stringer partial separation. The novel numerical methodology, able to overcome the mesh size and time increment dependency of the standard Virtual Crack Closure Technique (VCCT), is an evolution of a previously developed and tested numerical approach for the circular delaminations growth. The enhancements, with respect to the previously developed approach, rely mainly in the capability to deal with the different defect shapes characterising a skin–stringer debonding. The proposed novel methodology has been implemented in a commercial finite element platform and tested over single stiffener composite panels. The effectiveness of the suggested numerical methodology, in predicting the compressive behaviour of stiffened panels with skin stringer debondings, has been preliminary confirmed by comparisons, in terms of load versus applied displacement and debonding size at failure, with literature experimental data and numerical results obtained with the standard VCCT approach.     

Use of CFD-DEM to evaluate the effect of intermediate stress ratio on the undrained behaviour of granular materials

This study investigates the effect of intermediate stress ratio (b) on the mechanical behaviour of granular soil in true triaxial tests. A CFD-DEM solver with the ability to model compressible fluid and moving mesh has been developed and calibrated based on existing experimental test results on Nevada sand. The effect of b on the undrained true triaxial test, which has been neglected in the literature, was investigated using a reasonable number of models. The effects of the initial confining stress and initial void ratio also have been studied. The developed model was used to calculate the hydrodynamic forces on the particles and evaluate the ratio of the particle–fluid interaction force to the resultant force on the particles. It has been demonstrated that, in numerical studies, the effect of these forces cannot be neglected.     

A mechanistic approach to critical-distance methods in notch fatigue

One approach to the prediction of notch fatigue limits is the Neuber method in which stresses are averaged over a critical distance ahead of the notch. In recent years this theory has been updated by the discovery of an analytical method for finding the critical distance for a given material, which shows that the appropriate distance is 2 a _o , a _o being El Haddad's short-crack constant. The present author has advocated this approach, which he has called the Line Method (LM) and has tested it extensively against experimental data. However the method still remains essentially empirical; the aim of the present paper was to try to link this approach to the known mechanisms of crack growth at a notch. It is proposed that the LM is successful because it expresses the conditions necessary for growth of a small crack located at the notch root. The argument is developed by starting from the resistance curves used to predict non-propagating cracks and by linking the LM with the expression for stress intensity, K , derived by the crack-line loading method. The results provide some mechanistic justification for the use of the LM for sharp, crack-like notches; its success for other types of notch (i.e. blunt notches and short notches) requires a different explanation.     

Limitations and potential improvement of the aircraft pavement strength rating system to protect airport asphalt surfaces

A pavement strength rating system is internationally adopted in order to protect aircraft pavements from inadvertent overload. The system has two elements. The primary element is designed to protect the pavement against subgrade rutting and the second is intended to protect asphalt pavement surfaces. The surface-protection element is arbitrary and empirical, placing category-based limits on aircraft tyre pressures. In 2008, increases in the tyre pressure limits were proposed by aircraft manufacturers and these were approved in 2013. The research reported in this paper assesses the impact of tyre pressure and individual wheel load increases on calculated flexible pavement stress indicators, as well as identifying an improved surface layer protection element. Stresses were calculated near the surface, at the surface layer interface and at the subgrade. Tyre pressure and wheel load combinations included current (18 t and 1.35 MPa), imminent (33 t and 1.75 MPa) and future (40 t and 2.15 MPa) aircraft. Surface layer stress increased significantly (20–30%) with increases in both tyre pressure and wheel load. The subgrade stress increased near-equally (97%) with wheel load but was insensitive (<1%) to tyre pressure changes. The ability of the current aircraft pavement strength rating system to protect pavements from the increasing demands of aircraft was demonstrated to be limited to the subgrade. It is recommended that the tyre pressure rating be amended to reflect the combined impact of both tyre pressure rating and individual wheel load. It is also recommended that ongoing efforts to incorporate additional asphalt surface failure modes into routine pavement design be given high priority. The importance of these issues is reinforced by the limited availability of remedies to counter any negative impacts of increased surface layer stresses, especially in hot climates.     

State of the art: interface shear resistance of asphalt surface layers

Interface shear resistance is a measure of the bonding between two layers under shear loading. Adequate interface shear resistance and long-term bonding of the surface to the underlying pavement are critical to the performance of pavement structures. Interface shear strength is a function of adhesion, friction and aggregate embedment or interlock and is commonly modelled as a Mohr–Coulomb type envelope. Measurement of interface shear resistance can be performed in the field on full-scale pavements, in the laboratory on cores recovered from the surface or in the laboratory using samples prepared in the laboratory. However, laboratory testing of cores recovered from the field is likely to be more reliable and repeatable than field testing. There is a large range of test methods and procedures for the measurement of interface bond. These test methods are generally grouped into three main loading mechanisms; axial tension, torsional shear and direct shear. Direct shear tests offer a more comprehensive assessment of the full interface strength. The interface’s resistance to shear can be characterised by its strength, modulus/stiffness or work/energy. The results are affected by the test protocol, tack coat type and application rate, test temperature, applied normal stress and rate of loading, interface condition and post-construction trafficking. Of these, the test temperature is the most influential factor. A number of studies have reported contradictory and conflicting conclusions with regard to the importance of various factors and conditions on the different measures of interface shear resistance. Such inconsistent findings likely stem from the complicated interaction between the various interface conditions and testing protocols. The fundamental factors affecting monotonic interface strength are now reasonably well understood. The focus of future research is expected to be on shear fatigue performance of interfaces.     

Analysis of the debonding of the stem-cement interface in intramedullary fixation using a non-linear fracture mechanics approach

Implant loosening is one of the most important cause of long-term failure of total hip replacements. Fatigue failure of the stem-cement interface and the bulk cement may cause aseptic loosening of the femoral stem. In this work, both mechanisms of failure were simulated using finite elements. The stem-cement interface failure was modelled by means of the cohesive surface theory that was implemented into a specific interface element, while damage accumulation and creep in bone cement were formulated through the theory of Continuum Damage Mechanics. Model parameters, such as, the mechanical characteristics of the interface, damage accumulation rules both for the cement and the interface, crack closure effect and friction evolution law, were determined to simulate the subsidence patterns of the stem in the cement mantle from experimental tests. A parametric analysis was also performed observing how each parameter of the model influences the micromotions and damage accumulation in the cement mantle. Results of this study support the importance of simulating the progressive debonding of the stem-cement interface on the prediction of long-term implant loosening.     

The flatwise compressive properties of Nomex honeycomb core with debonding imperfections in the double cell wall

Mechanical properties of Nomex honeycomb core are governed by not only its global dimensions, cell topology, material properties and proportion of the aramid paper and phenolic resin, but also possible manufacturing imperfections, such as the debonding between the two aramid paper sheets in the double cell wall. To account for the layered feature of the cell walls and the bonding conditions between aramid paper sheets, a three-dimensional unit cell model was proposed and developed in this study. The aramid paper sheets, the phenolic resin coating, the adhesive between the aramid paper sheets, and their bonding relationships were all explicitly modelled in accordance with their actual geometry and material parameters. The model was validated by comparing the predicted load-displacement curves and failure modes with the test results. The effects of representative bonding imperfections on both the collapse load and the related displacement of the honeycomb core under flatwise compression were evaluated. Through the analyses, it was found that the debonding imperfections have significant effects on the mechanical behaviour of the honeycomb core and that with the same debonding area the debonding at the outside edge of the adhesive printing line is the most critical. It was also found that debonding fracture may occur if adhesive is not strong enough or the debonding imperfection area is large.     

Using the local approach to evaluate scaling effects in ductile fracture

This paper uses a local model to predict ductile fracture in geometrically similar structures of different sizes containing either sharp cracks or blunt stress concentrators. Simple theoretical considerations suggest that when fracture occurs by quasi-isotropic void growth, fracture initiation at blunt notches follows replica scaling, whereas fracture initiation at sharp cracks does not. Simulations with a local fracture model of fracture events in (1) fatigue precracked compact specimens and (2) three-point-bend bars containing blunt notches confirm these conclusions. However, a comparison of simulations with actual experimental results with HY-130 steel specimens leads to mixed conclusions. Predicted and observed behaviors for fracture at sharp cracks agree well, but the discrepancy is considerable for fracture initiating at blunt notches loaded in bending. Significant scaling effects are observed in the experiments for the conditions of fracture initiation at blunt notches. Fractographic analysis reveals that the reason for this discrepancy is a difference in the micromechanisms controlling fracture at sharp cracks as opposed to blunt notches. At sharp cracks, quasi-isotropic void growth dominates, whereas fracture initiates at blunt notches by a shear localization process and the nucleation, growth, and coalescence of voids in a mixed shear and tensile deformation field. The transition from one mode to the other may be governed by the hardening rate and, if so, is material dependent. Therefore, when using local fracture models for predicting fracture under generalized geometric and loading conditions, care must be taken, that the micromechanisms of ductile fracture invoked in the actual material match those assumed by the local fracture model. If this correspondence is verified, local fracture models can be used to predict fracture conditions and associated scaling effects for situations not amenable to treatment by classical elasto-plastic fracture mechanics. However, new or expanded models that can treat ductile fracture in localized shear zones should be developed to realize the full potential of these local fracture methodologies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Investigation of the influence of surface-activated carbon fibres on debonding energy and frictional stress in polymer-matrix composites by the micro-indentation technique

The influence on fibre/matrix adhesion of the acidic and basic nature of surface-activated carbon fibres in epoxy and PPS matrices has been investigated by means of the micro-indentation method. The fibre ‘push-out’ and ‘push-in’ techniques were used for this study. The debonding energy and frictional stress are calculated, and the adhesion behaviour is compared with the calculated thermodynamical work of adhesion (as derived from the fibre surface tension) and surface oxygen content. Influence of surface activation on the interfacial frictional stress is discussed.     

Multiple shear step approach to determine the rotational viscosity of asphalt emulsions and its correlation with water content of original and diluted emulsions

Viscosity determination of thixotropic emulsions with good repeatability has always been a major challenge. Currently, Saybolt Furol viscometer (SFV) is used to determine the viscosity of the emulsion, but the main drawback of the SFV is that it cannot simulate the behaviour of emulsion under different shear rate. Rotational viscometer (RV) can measure viscosity at different shear rates. Due to the thixotropic behaviour of the emulsions, getting repeatable results by following the hot binder specification is a problematic task. In this study, a new multiple shear step approach is used to determine the viscosity of the emulsified asphalt using RV. Three low viscous (SS-1, SS-1H and SS-1L) and two high viscous (CRS-2 and CRS-2P) emulsions were used in this study. Shear stress is gradually stepped up to different levels after certain time interval to determine the viscosity. In this manner, emulsion undergoes a known shear state and each reading is preceded by a certain repeatable shear history. It was observed that with the progression of time and simultaneous increase in shear rate, the viscosity results are much more stable and repeatable with less than 5% coefficient of variance. The final specifications proposed are 220–730 and 5–90 cP at 50 rpm and 30 °C for high and low viscous emulsions, respectively, which are based on 98% probability. Viscosity measured by this approach also showed strong correlation with water content (R² > 0.94). The correlation between viscosity and water content is even stronger after dilution. With different dilution water content, viscosity of CRS-2 and CRS-2P exhibited R² values of 0.97 and 0.98, respectively.     

A new approach to triaxial residual stress evaluation by the hole drilling method

A new analysis technique is introduced to resolve the triaxial residual stress profiles from measurements using the conventional strain gauges for the hole drilling or ring core method extended with a displacement transducer to measure the out of plane displacement. The new technique uses an inverse formulation with wavelets.     

Illustration of stress/strain behavior and yarn fragmentation in a pseudo-hybrid composite system

To study the stress/strain behavior and the fragmentation of fibers in a hybrid composite in tensile loading, a model strand was constructed with cotton yarns as the low-elongation component and polyester yarns as the high-elongation component. The effect of interfacial shear strength on the performance of a hybrid composite was simulated by varying the twist multiplier of the strands. It was observed that a strand with a lower twist level was more likely to show a typical hybrid stress/strain behavior and a multi crack fragmentation pattern. The same was found for strands with the highest twist levels but with cotton contents between 22 and 33%.     

Modelling of critical fibre length and interfacial debonding in the fragmentation testing of polymer composites

Micro-mechanical models based on a unidimensional load transfer approximation are used to predict the critical fibre length as a function of applied strain in the fragmentation testing of polymer matrix composites. Conditions of perfect adhesion, partial debonding, and total debonding are considered in turn. Situations are identified where the critical length cannot be viewed as a material constant, i.e. where it remains strain dependent as the applied strain increases. Numerical results based on the partial debonding model are given for the critical fibre length and the extent of the debonding zone as a function of applied strain. The prediction of the total debonding model is recovered asymptotically for large strains. We find, however, that the critical length predicted by the partial debonding model can be lower than the one predicted by the total debonding model if the interfacial bond strength is sufficiently larger than the frictional shear stress. These theoretical results show that both bond strength and frictional shear stress must be taken into account in the interpretation of the fragmentation test data.     

A single parameter to evaluate stress state in rail head for rolling contact fatigue analysis

Based on the Smith‐Watson‐Topper (SWT) method, a phenomenological approach for multiaxial fatigue analysis, the maximum SWT parameter is proposed as a single parameter to evaluate the stress state in the rail head for assessing the fatigue integrity of the structure. A numerical procedure to calculate the maximum SWT parameter from a finite element analysis is presented and applied in a case study, where the stress and strain fields due to wheel/rail rolling contact are obtained from a three‐dimensional finite element simulation with the steady‐state transport analysis technique. The capability of the SWT method to predict fatigue crack initiation in the rail head is confirmed in the case study. Analogous to von Mises stress for strength analysis, the maximum SWT parameter can be applied to evaluate the fatigue loading state not only in rail head due to rolling contact fatigue but also in a generic structure subjected to a cyclic loading.     

A rapid technique to evaluate the oxidative stability of a model drug

The objective of the current study was to investigate the oxidative induction time (OIT) as a measurement of the stability of an oxygen-sensitive model drug. The OIT was determined by differential scanning calorimetry and represents the time required for oxidative decomposition to occur at a given temperature. Samples were heated to a specific temperature under a nitrogen blanket then held isothermal while exposed to oxygen. The experiment proceeded until oxidative degradation of the sample was apparent from the real-time heat flow graphs. Variables investigated in this study included different lots and suppliers of a model drug as well as the addition of antioxidants. Results demonstrated that the stability of the drug was dependent on the supplier. All antioxidants investigated in this study improved oxygen stability of the model compound, as evidenced by a longer OIT. Butylated hydroxyanisole (BHA) was found to better stabilize the drug than butylated hydroxytoluene at equivalent concentrations. The combination of ascorbic acid and BHA provided the greatest protection against oxidation of the model compound. The results of this study demonstrate the usefulness of OIT to investigate the oxygen stability of pharmaceutical compounds.     

Application of ground-penetrating radar in measuring the density of asphalt pavements and its relationship to mechanical properties

The ground-penetrating radar (GPR) is a proven technology that is used typically to determine the thicknesses of pavement layers. This paper explores the applicability of the GPR to assess the density of the asphalt layer in pavements. The measurements were conducted on three test sections that were constructed using different asphalt mixtures. Each of the test sections was divided into sub-test sections that were compacted using different compaction methods and number of roller passes in order to achieve a range of asphalt mixture densities. The results showed that there was very good correlation between the GPR results and density of extracted field cores. Consequently, the paper examines the correlation between density and mechanical properties of asphalt mixtures. The results of the mechanical tests provided valuable information on the effect of density on performance.     

A novel method to evaluate the setting process of cement and asphalt emulsion in CA mortar

Cement and asphalt mortar (CA mortar) is the key component in the structure of Shinkansen slab track and serves as the elastic shock-absorber. A new method was put forward to evaluate the setting process of cement and asphalt emulsion (CAE) in CA mortar. It was noted that the setting process was governed by several factors such as cement types, cement/asphalt emulsion ratios (C/AE ratio). Results also indicated that the setting process of CAE was faster, the higher proportion of cement content was; the early age strength and the separation rate of CA mortar could be improved by using cement of high early age strength and rapid hydration rate, or a blended cement with ordinary Portland cement partially replaced by sulfoaluminate cement, or by increasing C/AE ratio. Nevertheless, the replacement ratio of ordinary Portland cement by sulfoaluminate cement should not exceed 15% and C/AE ratio should be not less than 0.8.     

A fracture mechanics and mechanistic approach to the failure of cortical bone

The fracture of bone is a health concern of increasing significance as the population ages. It is therefore of importance to understand the mechanics and mechanisms of how bone fails, both from a perspective of outright (catastrophic) fracture and from delayed/time‐dependent (subcritical) cracking. To address this need, there have been many in vitro studies to date that have attempted to evaluate the relevant fracture and fatigue properties of human cortical bone; despite these efforts, however, a complete understanding of the mechanistic aspects of bone failure, which spans macroscopic to nanoscale dimensions, is still lacking. This paper seeks to provide an overview of the current state of knowledge of the fracture and fatigue of cortical bone, and to address these issues, whenever possible, in the context of the hierarchical structure of bone. One objective is thus to provide a mechanistic interpretation of how cortical bone fails. A second objective is to develop a framework by which fracture and fatigue results in bone can be presented. While most studies on bone fracture have relied on linear‐elastic fracture mechanics to determine a single‐value fracture toughness (e.g., K_c or G_c), more recently, it has become apparent that, as with many composites or toughened ceramics, the toughness of bone is best described in terms of a resistance‐curve (R‐curve), where the toughness is evaluated with increasing crack extension. Through the use of the R‐curve, the intrinsic and extrinsic factors affecting its toughness are separately addressed, where ‘intrinsic’ refers to the damage processes that are associated with crack growth ahead of the tip, and ‘extrinsic’ refers to the shielding mechanisms that primarily act in the crack wake. Furthermore, fatigue failure in bone is presented from both a classical fatigue life (S/N) and fatigue‐crack propagation (da/dN) perspective, the latter providing for an easier interpretation of fatigue micromechanisms. Finally, factors, such as age, species, orientation, and location, are discussed in terms of their effect on fracture and fatigue behaviour and the associated mechanisms of bone failure.