Shankar Kalyanasundaram

Mode I and mode II delamination properties of glass/vinyl-ester composite toughened by particulate modified interlayers

Abstract Various vinyl-ester (VE)/poly(acrylonitrile-butadiene-styrene) (ABS) blends were used for interlayer-toughening of a glass/VE composite to increase delamination resistance of the base material under mode I and mode II loading. Dry ABS powder was mixed with the liquid resin in four weight ratios: 3.5, 7, 11 and 15 phr (parts per hundred parts of resin) while the layer thickness was varied within the range of 150–500 μm. Firstly, mode I fracture toughness and tensile properties of the VE/ABS blends were assessed. By using the Raman Spectroscopy technique a chemical reaction was discovered which occurred during ABS–VE mixing: i.e. butadiene transition from the ABS particles to the VE. A butadiene saturation was discovered to occur in the VE beyond 7% ABS particle content. Both mode I and mode II fracture toughness were significantly improved with application of the interlayers. Mode I fracture toughness was found to be a function of layer thickness and particle content variations. The latter dominated G Ic after the saturation point. On the other hand mode II fracture toughness was found to be independent of the layer thickness (within the used layer thickness range) and only moderately influenced by the particle content. Important Toughening mechanisms were plastic deformation and micro-cracking of the layer materials. Evidence of both mechanisms has been found using optical and scanning electron microscopy (SEM).

On crack-initiation conditions for mode I and mode II delamination testing of composite materials

Abstract An investigation of the influence of pre-crack conditions on the fracture behavior of a unidirectional vinyl-ester/E-glass composite has been undertaken. For this investigation, recommendations from three different testing standards for preparing double-cantilever beam (DCB) specimens were followed. For this purpose two sets of specimens were tested: a set of specimens with a blunt starter crack and the one with a fatigue pre-crack (i.e. sharp). The two sets of specimens exhibited similar fracture behavior during testing, with the crack propagation being stable and not influenced by the different pre-crack conditions. As expected, propagation values of the strain-energy release rate were unaffected by the pre-crack conditions and, owing to the extensive fiber bridging, propagation G Ic prop values were significantly larger than initiation G Ic ini values. However, the influence of the pre-cracking on the initiation values of the G Ic was noticed. A similar investigation was also conducted for mode II, with ENF specimens. No influence of the pre-crack condition on the fracture toughness of the composite under the mode II loading was noted. Scanning electron micrographs were taken to provide additional explanations for the observations made during the testing.

Low Energy Impact Damage Modes in Aluminum Foam and Polymer Foam Sandwich Structures

The energy absorption of an aluminum foam sandwich structure and a conventional polymer foam sandwich structure is similar for impacts ranging from 5 to 25 J. The polymer foam-based samples exhibit localized damage in the form of skin fracture and core crushing, but with negligible permanent out-of-plane deformation. In contrast, the aluminum foam-based samples show little fracture but exhibit extensive out-of-plane deformation radiating from the impact point. This deformation suggests that the impact damage could be more easily detectable in the aluminum foam sandwich structure. Surface strains are lower in the aluminum foam sandwich samples during post-impact loading in a single cantilever beam test, suggesting improved damage tolerance.

WallGen, software to construct layered cellulose-hemicellulose networks and predict their small deformation mechanics

We understand few details about how the arrangement and interactions of cell wall polymers produce the mechanical properties of primary cell walls. Consequently, we cannot quantitatively assess if proposed wall structures are mechanically reasonable or assess the effectiveness of proposed mechanisms to change mechanical properties. As a step to remedying this, we developed WallGen, a Fortran program (available on request) building virtual cellulose-hemicellulose networks by stochastic self-assembly whose mechanical properties can be predicted by finite element analysis. The thousands of mechanical elements in the virtual wall are intended to have one-to-one spatial and mechanical correspondence with their real wall counterparts of cellulose microfibrils and hemicellulose chains. User-defined inputs set the properties of the two polymer types (elastic moduli, dimensions of microfibrils and hemicellulose chains, hemicellulose molecular weight) and their population properties (microfibril alignment and volume fraction, polymer weight percentages in the network). This allows exploration of the mechanical consequences of variations in nanostructure that might occur in vivo and provides estimates of how uncertainties regarding certain inputs will affect WallGens mechanical predictions. We summarize WallGens operation and the choice of values for user-defined inputs and show that predicted values for the elastic moduli of multinet walls subject to small displacements overlap measured values. “Design of experiment” methods provide systematic exploration of how changed input values affect mechanical properties and suggest that changing microfibril orientation and/or the number of hemicellulose cross-bridges could change wall mechanical anisotropy.

A finite element simulation of micro-mechanical frictional behaviour in metal forming

Friction is a critical factor for sheet metal forming (SMF). The Coulomb friction model is usually used in most finite element (FE) simulation for SMF. However, friction is a function of the local contact deformation conditions, such as local pressure, roughness and relative velocity. Frictional behaviour between contact surfaces can be based on three cases: boundary, hydrodynamic and mixed lubrication. In our microscopic friction model based on the finite element method (FEM), the case of dry contact between sheet and tool has been considered. In the view of microscopic geometry, roughness depends upon amplitude and wavelength of surface asperities of sheet and tool. The mean pressure applied on the surface differs from the pressure over the actual contact area. The effect of roughness (microscopic geometric condition) and relative speed of contact surfaces on friction coefficient was examined in the FE model for the microscopic friction behaviour. The analysis was performed using an explicit FE formulation. In this study, it was found that the roughness of deformable sheet decreases during sliding and the coefficient of friction increases with increasing roughness of contact surfaces. Also, the coefficient of friction increases with the increase of relative velocity and adhesive friction coefficient between contact surfaces.

Comparison of surface strain for stamp formed aluminum and an aluminum-polypropylene laminate

Laminate structures incorporating thin layers of metal and polymer, or polymer composite, can offer significant weight savings for engineering structures, while retaining excellent mechanical and impact performance. Laminates based on thin layers of aluminum and glassfiber/polypropylene thermoplastic have been the subject of recent study [1, 2], and have exhibited excellent specific mechanical properties and superior specific impact behavior compared to monolithic aluminum. Such materials, therefore, have great potential for widespread application in engineering structures. One such potential area is the automotive industry where weight reduction and impact performance are pertinent issues. Lighter vehicles will result in improved fuel efficiency, and greater energy absorption capability may contribute to improved crash performance. However, for the automotive industry it is necessary to produce components using a high-volume manufacturing process such as stamping. Thermoplastic-based materials and sandwich structures are good candidates for stamp forming as they can be heated to conform to the mold, and then rapidly cooled for removal from the mold. Mosse et al. [3, 4] investigated the effects of blankholder force, laminate preheat temperature, tooling temperature, and tool radii on FML formability. It was found that significantly lower levels of springback could be achieved over aluminum, and forming defects could be eliminated by restricting process variables to a given range. In particular, it was found that delamination at the bimaterial interface and within the composite layer was eliminated when the laminate was pre-heated to 160 ◦C then formed in a heated die. This is significant as delamination would adversely affect the mechanical performance of a formed component. Further, Kim and Thomson [5] found that high forming speed increased the transverse stiffness of polymer-metal laminates, in turn reducing the inter-laminar shear and the degree of springback. They also found that laminates forming at elevated temperatures decreased the rigidity but improved the springback characteristics. This letter presents some preliminary results from research into stamp-forming aluminum-thermoplastic sandwich materials. Here, the permanent strain on the surface of a channel-formed aluminum-polypropylene laminate is compared to monolithic aluminum. Characterization of the strain is significant as it provides insight into the behavior of the material during formation and assists in the production of parameters for subsequent formation methodologies. The materials used in this study were 5005-H34 aluminum and a self-reinforced polypropylene (Curv, BP). An aluminum-Curv laminate was made in a 2/1 configuration in a 200 × 200 mm picture frame mold. A 0.9 mm thick layer of Curv was sandwiched between two layers of 0.5 mm thick aluminum cleaned with a solvent (isopropanol). A 50 μm thick layer of a hot-melt polypropylene adhesive (Gluco Ltd., UK) was placed at each bi-material interface. The laminate was consolidated by heating to 160 ◦C in a platen press followed by rapid water cooling under a pressure of approximately 1 MPa. The nominal laminate thickness was 2.2 mm. Samples of 19 mm width were sectioned from the laminate and from a plain sheet of 2 mm thick aluminum. A 3 mm circular grid etched onto the surfaces enabled post-forming major strain measurements, that is in the direction of the sample length, to be made. Channel sections were stamped in an open die. Plain aluminum was stamped cold whereas the aluminumCurv laminates were pre-heated to 160 ◦C then immediately transferred to the die, which was pre-heated to 80 ◦C. This enabled a temperature window of 125– 140 ◦C to be maintained during the stamping operation. The channel sections were stamped in an Enerpac 30 tonne press using two tool radii of 3 and 7 mm. The blank holder force was 3.5 kN. Surface strain measurements were taken from ten grids around the mid-point of the sidewall area of the channel section, shown in Fig. 1, using an optical microscope with a graticule scale of 20 μm resolution. Measurements were taken from the sidewall area as it is likely to undergo significant tensile strain during formation. Microscope examination of the sidewall edge, prior to taking the strain measurements, confirmed the absence of delamination. The average major surface strain for the aluminum and aluminum-Curv samples is plotted in Fig. 2. (The

The microdroplet test: experimental and finite element analysis of the dependance of failure mode on droplet shape

The microdroplet technique is usually designed as a fibre embedded in a drop of resin and subsequently pulled out while the drop is being supported by two knife edges, resulting in either debonding of the droplets from the fibres, or breakage of the fibres before debonding can occur. In this study, the microdroplet technique was performed using a platinum ring with a 40 μm hole instead of the usual two knife edges, giving an axisymmetric geometry, load and stress distribution. Glass/phenolic and glass/polyester composite systems were tested experimentally and subsequent finite element modelling studies were performed to assess the variation of droplet size, and contact angle between the droplet and fibre. It was found that contact angle is of major influence in the proposed failure model. This study characterizes the influence of the contact angle between the droplet and the fibre on the subsequent stress distribution in the microdroplet specimen.

Effect of membrane stress on surface roughness changes in sheet forming

Friction plays an important role in sheet metal forming (SMF) and the roughness of the surface of the sheet is a major factor that influences friction. In finite element method (FEM) models of metal forming, the roughness has usually been assumed to be constant; even though it is commonly observed that sheet drawn under tension over a tool radius results in the surface becoming shiny, indicating a major change in surface morphology. An elastic–plastic FEM model for micro-contact between a flat surface and a single roughness peak has been developed. The model was used to investigate the effect of the membrane stress in the sheet on the deformation of an artificial roughness peak. From the simulation results, the change in asperity, or deformation of the local peak, for a given nominal tool contact stress is significantly influenced by the local substrate stress. The height of the asperity decreases with increasing substrate stress and the local pressure is much higher than the nominal pressure. In addition, the local contact stress decreases with an increase in the substrate stress levels.

Identifying variation in sheet metal stamping

This work looks at two different “Design of Experiments”(DoE) methods for defining an operating window in the sheet metal stamping process. The first involves the use of replicates at the different experimental points, while the second is a nonreplicated method. The two methods are compared by looking at the relationship results produced and the indication of variation in the process. It is found that the results from both the methods are very similar. However, the replicated method provides a greater level of confidence in the results. In the stamping process, where performing large numbers of replicates is expensive in both time and money, the nonreplicated method provides a cost effective way of understanding the process.

Lateral drill holes decrease strength of the femur: an observational study using finite element and experimental analyses

BackgroundInternal fixation of femoral fractures requires drilling holes through the cortical bone of the shaft of the femur. Intramedullary suction reduces the fat emboli produced by reaming and nailing femoral fractures but requires four suction portals to be drilled into the femoral shaft. This work investigated the effect of these additional holes on the strength of the femur.MethodsFinite element analysis (FEA) was used to calculate compression, tension and load limits which were then compared to the results from mechanical testing. Models of intact femora and fractured femora internally fixed with intramedullary nailing were generated. In addition, four suction portals, lateral, anterior and posterior, were modelled. Stresses were used to calculate safety factors and predict fatigue. Physical testing on synthetic femora was carried out on a universal mechanical testing machine.ResultsThe FEA model for stresses generated during walking showed tensile stresses in the lateral femur and compression stresses in the medial femur with a maximum sheer stress through the neck of the femur. The lateral suction portals produced tensile stresses up to over 300% greater than in the femur without suction portals. The anterior and posterior portals did not significantly increase stresses. The lateral suction portals had a safety factor of 0.7, while the anterior and posterior posts had safety factors of 2.4 times walking loads. Synthetic bone subjected to cyclical loading and load to failure showed similar results. On mechanical testing, all constructs failed at the neck of the femur.ConclusionsThe anterior suction portals produced minimal increases in stress to loading so are the preferred site should a femur require such drill holes for suction or internal fixation.