An experimental assessment of the size effects on the strength and ductility of freestanding Cu films under macroscopically homogenous deformation

Metallic interconnects and circuitry has been experiencing excessive deformation beyond their elastic limits in many applications, ranging from micro-electro-mechanical systems (MEMS) to flexible electronics. These applications are creating needs to understand the extent of strength and ductility of free standing metallic films at scales approaching the micron and sub micron range. This work aims to elucidate the effects of microstructural constraint as well as geometric dimensional constraint on the strength and ductility of free-standing Cu films under uniaxial tension. Two types of films are tested, (i) high purity rolled films of 12.5-100μm thickness and average grain sizes of 11-47μm and (ii) electroplated films of 2-50μm thickness and average grain sizes of 1.8-5μm. Several experimental tools including residual electrical resistivity measurements, surface strain measurements and surface roughness measurements are employed to highlight the underlying deformation mechanisms leading to the observed size effects. The results show that thickness effects on the strength (yield and flow stress) of freestanding films are primarily an outcome of the competing contributions from two distinct dislocation mechanisms i.e. intragranular Frank-Read (IG) type dislocations and grain boundary (GB) dislocations. At large grain sizes, typically ~O(10μm) or larger, in bulk materials and foil/plate type specimen, IG dislocations dominate the overall response. In this regime, reducing the film thickness leads to a reduction of the effective microstructural constraint. Consequently, prominent thickness dependent weakening is observed. On the other hand, for small grain sizes, ~O(1μm) or smaller, GB dislocations play an increasingly prominent role. In such cases, reducing grain boundary area per unit specimen volume, with reducing thickness, results in a reduction of the available GB dislocation source density. As a result, plasticity commences under source-limited conditions leading to thickness dependent, source starvation strengthening. The role of film thickness on macroscopic ductility was more prominent for dg = 3.5μm and larger for the examined set of film microstructures. Generally, reducing ductility with reducing film thickness was observed. Surface scans showed that in specimens with many grains across the thickness, plastic deformation remains relatively uniform to higher level of macroscopic plastic deformation. On the other hand, for specimens with just one grain across the thickness, plastic deformation evolves into highly localized deformation bands at an early stage of deformation, leading to premature failure. It is speculated that local accommodation of deformation incompatibilities between neighboring grains are the primary driving mechanism for the observed trends. For films with the lowest dg = 1.8μm, the apparent ductility is inherently limited by the loss of strain hardening capability. These films show a limited ductility with maximum uniform strain of ~2%, irrespective of the thickness

A new tensile testing methodology for stable mechanical characterization of free-standing films

The advent of new technologies and applications employing structural components at the micron scale and their future development depends on the understanding of the fundamental behavior of materials at such small scale. The defining characteristic of these materials is the overlap and interaction between the structural length scales of the components and the intrinsic micro-structural length scale of the material. The objective of this work is to develop a new stable testing fixture that will facilitate quantitative understanding of the size-dependent material response and the heterogeneous deformation field at these length scales. The testing fixture utilizes compliant mechanisms which arrests deformation instabilities and premature failure. Detailed fixture stability analysis is performed to optimize the final fixture dimensions. The device design and performance is then calibrated with finite simulation and verified experimentally. The test methodology was implemented to test copper films with thickness between 10 -100[Mu].m. The copper microstructure was altered via heat treatment at different temperature to change the grain size to film thickness ratio at the same film thickness. Within the experimental window, the yield stress and strain to fracture showed strong dependence on the grain size i.e. the Hall-Petch effect. The experimental observations showed that the film ductility is a strong function of the testing methodology. For the same film microstructure and macroscopic dimensions, different level of ductility can be attained based on the relative stiffness of the test fixture and the specimen. The fundamental operative mechanism that has been identified is by consecutively stabilizing the longer wavelengths of shear band localization within the film and pushing them to shorter wavelengths. Thus, the film may attain higher levels of final strain to fracture. Detailed measurements of the in-plan finite strain field wilhin the film have revealed the structure of the shear bands and their interactions. The developed testing fixture was promising in the initial runs at the macro-scale. A micro-scale version is being developed for further analysis of plasticity and fracture at the microstructure length scale of the material

Sorting Logs and Lumber for Stiffness Using Director HM200

Acoustic tools such as the Director HM200 are able to sort logs and boards according to their intrinsic stiffness. Here, some uses of such tools are demonstrated. First, six logs are analyzed before sawing into cants and individual boards. Second, the same approach is reconsidered from basic principles, emphasizing the variety of investigative projects to match the species-specific responses to acoustics and the need to match local resources to local markets. In both instances, correlations between acoustic properties measured in logs are related to those from cants and boards. The offsetting effects of changes in stiffness and density in determining acoustic velocity are discussed. The intention is to emphasize future possibilities for commercial application

Probing thickness-dependent dislocation storage in freestanding Cu films using residual electrical resistivity

Residual electrical resistivitymeasurement is employed to study dislocation storage under tensile loading of freestanding electroplated Cufilms (1–5μm grain size and 2–50μm thickness). The results indicate that the nature of thickness effects (strengthening or weakening) depends on the underlying deformation mechanisms via the average grain size. A threshold grain size of about dg=5μm is identified to distinguish grain size effects in thicker films from those in thinner films. For dg\u3e5μm, diminishing microstructural constraint with reduced thickness weakens the films due to dislocation annihilation near the free surface. For dg\u3c5μm, reduction of film thickness leads to strengthening via grain boundary-source starvation

Changes in colour and mechanical properties of wood polypropylene composites on natural weathering

This comparative study focused on understanding the effect of coupling agent and particle size on weathering behaviour of wood polypropylene composite. Two coupling agents, namely maleic anhydride grafted polypropylene and m-TMI (m-Isopropenyl-a,a- dimethylbenzyl isocyanate) grafted polypropylene were used in preparation of the composites. The composites were exposed to outdoor conditions for one year and changes in surface colour and mechanical properties were measured after 2, 4, 8 and 12 months of natural weathering. During the initial four months of weathering considerable colour change was observed with increase in lightness.  Mechanical properties were unaffected largely for the initial four months and thereafter started declining.  Overall, tensile strength decreased by about 15 % and flexural strength decreased by about 8 % after one year of weathering. The flexural modulus also decreased by about 10 %. Wood particle size was found to affect the aesthetic and strength of the composites after natural weathering. Coupling agents had a positive impact on mechanical properties however their influence on weathering degradation was not noticeable

Effect of borax-boric acid and ammonium polyphosphate on flame retardancy of natural fiber polyethylene composites

Wood fiber filled high density polyethylene composites (WPCs) were prepared using twin screw extruder and maleated polyethylene as a coupling agent. Bamboo fibers were initially treated with alkali (NaOH), boric acid - borax (Ba-Bx) and borax (Bx). The treated and untreated fibers were used in combination with ammonium polyphosphate (APP) to investigate their synergistic effects on thermal stability, flame retardancy and mechanical properties. Alkali pretreatment (5 % NaOH) of fibers showed significant improvement in performance of APP by increasing thermal stability in WPCs. The derivative thermogravimetric (DTG) results indicate significance of Ba-Bx in promoting char induction at lower temperatures (340 ºC) and thereby, improved the thermal stability in WPCs. Flammability decreased with addition of flame retardant additives. As compared to pure WPCs, composites containing APP 10 % / Ba-Bx 5 % exhibited maximum reduction in average heat release rate (HRR) by 69 %, peak heat release rate (PHRR) by 59 %, total heat released rate (THR) by 48 % and also increased time to ignition (TTI) by 62 %. However, no significant difference was found among the combinations i.e., APP with or without compounds towards reducing the flammability of WPCs. The strength properties also reduced significantly when boron compounds were added along with APP. In general, APP alone (15 %) is enough for imparting thermal stability and flame retardancy in WPCs.   &nbsp

Selecting and/or processing wood according to its processing characteristics

In this study, three issues associated with the segregation of wood according to their processing characteristics have been addressed. In the first part of the study, the influence of inhomogeneity on the acoustic velocity measured by a resonance based acoustic tool "WoodSpec" and a transit-time based tool "Fakopp-2D" was investigated. Four laminated panels with varying degree of inhomogeneity were prepared and acoustic velocities were measured by both tools. The acoustic velocity measured by the Fakopp-2D tool was always higher than that measured by the WoodSpec tool. The difference in the two velocities was found to differ with the magnitude of inhomogeneity in the panel. It was demonstrated that the velocity measured by the resonance frequency obeys the Law of Mixtures and is controlled by the volume-weighted average stiffness of wood. In contrast the velocity measured by the Fakopp-2D tool was influenced by the amount of energy propagating at the dilatational speed. The progression of the wave-front in panels was investigated using Fakopp-2D tool by picking up the arrival time of the signal at off-axis probes. The wave-front profile was found to be determined by the stress-wave speed (stiffness) at different angles from the axis symmetry. In logs, the transit-time velocity was found to be sensitive to the localised stiffness of the wood lying in between the measurement probes with in the propagation path. A strong correlation was observed between the velocities measured by WoodSpec and the Fakopp-2D tool in young radiate pine and eucalyptus logs; however the acoustic velocity by Fakopp-2D was higher than the WoodSpec velocity. The results imply that Fakopp-2D could be used to rank young trees according to their stiffness. In the second part of the study, two trials were conducted to test the hypothesis that acoustics can be used to segregate pulp logs into categories which will require different amounts of energy during mechanical pulping and will produce pulps with different strengths. Pinus radiata logs of varying age, length, SED, LED, taper, and volume were measured for acoustic velocity, segregated into four different velocity groups and chipped separately. It was shown that acoustics could segregate logs into groups that perform very differently in terms of pulp and paper properties when refined to a given freeness or at a certain energy input. At a given target freeness there was a 20% difference in energy requirement between the lowest and highest velocitylogs. Similarly there was a 17% difference in tensile strength between the lowest and highest velocity logs for a given specific energy. In the third part of the study, investigations were made to explore the potential of acoustic velocity in ranking young Eucalyptus nitens trees according to growth stress level. Longitudinal growth strain was measured in 155 selected trees at the breast height and acoustic velocity was measured in the same trees using the Fakopp tool. Measurements in the first 34 trees showed some relationship between growth strain and acoustic velocity which eventually vanished as the measurements were progressively extended to all 155 trees. The results indicated the dangers of drawing possible inferences on the basis of small sample sizes. A large variation observed in growth strains along the tree height and on the two opposite sides in logs suggested that a single strain measurement is not sufficient enough to assess the mean tree strain level even in young and small diameter eucalypts trees and measurements on two opposite sides at a specific height is an approach for the screening purpose. The relationships between longitudinal growth strain and certain key wood properties were also investigated. Green density, green moisture content, basic density, radial shrinkage, outerwood and corewood densities, volumetric shrinkage and dynamic MoE at 12% me and length-weighted fibre length were determined. Amongst all the studied wood properties, only shrinkage-related properties were found to have some association with the mean growth strain in trees. The mean growth strain was moderately but significantly related to the volumetric shrinkage of the outerwood while the corewood shrinkage was not related. However, the volumetric shrinkage differential (difference between outerwood and corewood shrinkage) was strongly related to the growth strain (r=0.70) suggesting that the growth stress gradient might be related to the shrinkage property variations within the stem. Fisher's LSD test indicated statistically significant lower volumetric shrinkage, lower outerwood MoE and less collapse in the wood from trees with the lowest growth strains as compared to those from the highest growth strains. The results suggest that Eucalyptus nitens trees with low strains could exhibit a lower degree of the drying defects like collapse and internal checking during processing The large variation in wood quality characteristics in plantation grown timbers makes some screening for wood quality necessary for the effective management of wood resources and for the allocation of logs according to "Fitness for Purpose" to capture greater value of forest product value chain

Wood polymer composite bonded veneer based hybrid composites

Wood veneer based composites have a great demand in present market as the material can  utilize small diameter plantation timbers grown at short rotation cycle. This paper presents preparation and characterization of hybrid composites made of wood veneer and wood polymer composite. The study explored utilization of wood polymer composite as an adhesive for bonding veneers replacing formaldehyde-based adhesives. Wood polymer composite containing 40 % bamboo particles embedded in the matrix of polypropylene was used in sheet form to bind the veneers of Melia dubia wood. The composites were prepared in both laminated veneer lumber and plywood configurations. The assessment of physical and mechanical properties indicated that the properties of wood polymer composite contribute significantly to the properties of the hybrid composites. The density of the resultant composites was significantly higher (0,69 g/cm3 – 0,75 g/cm3) than conventional plywood or laminated veneer lumber. Among mechanical properties, there was no statistical difference in tensile and flexural strength of plywood and laminated veneer lumber configuration. Modulus of elasticity and compressive strength of laminated veneer lumber configuration were significantly higher than plywood. Glue shear strength and internal bond strength of the composites indicated acceptable bonding properties of wood polymer composite which suggests the potential application of these composites as a binding agent for wood veneers. These composites could be a special class of laminated composites with no formaldehyde emission hazards

Differences in dynamic modulus of elasticity determined by three vibration methods and their relationship with static modulus of elasticity

Dynamic modulus of elasticity was determined in clearwood samples of eight tropical hardwood species using longitudinal vibrations, flexural vibration and ultrasonic transit-time methods. These samples were subsequently subjected to three point static bending test to determine static modulus of elasticity and modulus of rupture. Acoustic velocity and wood density were found to be independent parameters as the velocity was nearly the same in wood with distinctly different densities. Among the three dynamic measurements, modulus from the ultrasonic method was the highest followed by the longitudinal vibration and flexural vibration. Any of three vibration methods could be used to predict static modulus as they exhibited a near perfect correlation with static MoE. However, the dynamic modulus determined by different vibration methods were found to diverge with increasing static modulus. Wood density was the dominating factor influencing both modulus of elasticity and modulus of rupture

Clinical and radiological outcome of distal femoral fractures treated by distal femoral locking compression plate

Background: Distal femoral fractures represent a challenging problem in orthopaedic practice. Open reduction with internal fixation has replaced previous trends of closed conservative management and external fixation. Distal femoral locking compression plate (DF-LCP) provides both locking and compression screw fixation of the femur shaft. This study was conducted to assess the clinical and radiological results of distal femoral fractures treated with DF-LCP.Methods: It was a prospective study on 90 cases. Fracture patterns AO type A and C were considered. Lateral approach was used as standard surgical technique. The total follow up period was 6 months .Functional and radiological results were evaluated using Neer’s score.Results: The series consisted of 64 males and 26 female patients with mean age 40.56 years. Road traffic Accident (68%) was the commonest mode of injury. Most were closed fractures. Wound infection was seen in 4 patients. Average flexion at knee joint was 117°. 100% union rate was observed with an average union time of 14.3 weeks. NEER’s score was excellent in 44, good in 32, fair in 10 cases and poor in 4 cases.Conclusions: DF-LCP is an important armamentarium in treatment of distal femur fractures especially when fracture is closed, severely comminuted and in case of osteoporosis