D. Arola

Orthogonal cutting of fiber-reinforced composites : A finite element analysis

Abstract Orthogonal cutting of unidirectional fiber-reinforced polymer composites was analyzed using the finite element method. A dual fracture process was used to simulate chip formation incorporating both the maximum stress and Tsai—Hill failure criteria. All aspects of the cutting tool geometry are considered in the model including the tool rake and clearance angles, nose radius and wear land, as well as friction between the tool and work material. Predictions for the cutting forces from numerical simulations are verified with experimental measurements for orthogonal trimming of unidirectional graphite/epoxy. The principal cutting force predictions agree very well with those obtained from experiments. The influence of fiber orientation and tool geometry on the fracture stress are highlighted and their effects on the material removal process in orthogonal trimming of reinforced polymers are discussed.

Applications of digital image correlation to biological tissues

Optical methods are becoming commonplace in investigations of the physical and mechanical behavior of biological tissues. Digital image correlation (DIC) is a versatile optical method that shows tremendous promise for applications involving biological tissues and biomaterials. We present the fundamentals of DIC with an emphasis on the application to biological materials. An approach for surface preparation is described that facilitates its application to hydrated substrates. Three examples are presented that highlight the use of DIC for biomedical research. The first example describes the use of DIC to study the mechanical behavior of arterial tissues up to 40% elongation. The second example describes an evaluation of the mechanical properties of bovine hoof horn in the dehydrated and fully hydrated states. Uniaxial tension experiments are performed to determine the elastic modulus (E) and Poissons ratio (nu) of both the arterial and dermal tissues. Spatial variations in the mechanical properties are evident from the full-field characterization of both tissues. Finally, an application of DIC to study the evolution of loosening in cemented total hip replacements is described. The noncontact analysis enables measurement of the relative displacement between the bone/bone cement and bone cement/prosthesis interfaces. Based on the elementary optical arrangement, the simple surface preparation, and the ability to acquire displacement/strain measurements over a large range of deformation, DIC should serve as a valuable tool for biomedical research. Further developments will enable the use of DIC for in vivo applications.

ON THE R-CURVE BEHAVIOR OF HUMAN TOOTH ENAMEL

In this study the crack growth resistance behavior and fracture toughness of human tooth enamel were quantified using incremental crack growth measures and conventional fracture mechanics. Results showed that enamel undergoes an increase in crack growth resistance (i.e. rising R-curve) with crack extension from the outer to the inner enamel, and that the rise in toughness is a function of distance from the dentin enamel junction (DEJ). The outer enamel exhibited the lowest apparent toughness (0.67+/-0.12 MPam(0.5)), and the inner enamel exhibited a rise in the growth toughness from 1.13 MPam(0.5)/mm to 3.93 MPam(0.5)/mm. The maximum crack growth resistance at fracture (i.e. fracture toughness (K(c))) ranged from 1.79 to 2.37 MPam(0.5). Crack growth in the inner enamel was accompanied by a host of mechanisms operating from the micro- to the nano-scale. Decussation in the inner enamel promoted crack deflection and twist, resulting in a reduction of the local stress intensity at the crack tip. In addition, extrinsic mechanisms such as bridging by unbroken ligaments of the tissue and the organic matrix promoted crack closure. Microcracking due to loosening of prisms was also identified as an active source of energy dissipation. In summary, the unique microstructure of enamel in the decussated region promotes crack growth toughness that is approximately three times that of dentin and over ten times that of bone.

Estimating the fatigue stress concentration factor of machined surfaces

Abstract In this study the effects of surface texture on the fatigue life of a high-strength low-alloy steel were evaluated in terms of the apparent fatigue stress concentration. An abrasive waterjet was used to machine uniaxial dogbone fatigue specimens with specific surface quality from a rolled sheet of AISI 4130 CR steel. The surface texture resulting from machining was characterized using contact profilometry and the surface roughness parameters were used in estimating effective stress concentration factors using the Neuber rule and Arola–Ramulu model. The steel specimens were subjected to tension–tension axial fatigue to failure and changes in the fatigue strength resulting from the surface texture were assessed throughout the stress–life regime (103≤Nf≤106 cycles). It was found that the fatigue life of AISI 4130 is surface-texture-dependent and that the fatigue strength decreased with an increase in surface roughness. The fatigue stress concentration factor (Kf) of the machined surfaces determined from experiments was found to range from 1.01 to 1.08. Predictions for the effective fatigue stress concentration ( K f ) using the Arola–Ramulu model were within 2% of the apparent fatigue stress concentration factors estimated from experimental results.

Evaluating the Mechanical Behavior of Arterial Tissue using Digital Image Correlation

In this study, digital image correlation (DIC) was adopted to examine the mechanical behavior of arterial tissue from bovine aorta. Rectangular sections comprised of the intimal and medial layers were excised from the descending aorta and loaded in displacement control uniaxial tension up to 40 percent elongation. Specimens of silicon rubber sheet were also prepared and served as a benchmark material in the application of DIC for the evaluation of large strains; the elastomer was loaded to 50 percent elongation. The arterial specimens exhibited a non-linear hyperelastic stress-strain response and the stiffness increased with percent elongation. Using a bilinear model to describe the uniaxial behavior, the average minor and major elastic modulii were 192±8 KPa and 912±40 KPa, respectively. Poissons ratio of the arterial sections increased with the magnitude of axial strain; the average Poissons ratio was 0.17±0.02. Although the correlation coefficient obtained from image correlation decreased with the percent elongation, a correlation coefficient greater than 0.8 was achieved for the tissue experiments and exceeded that obtained in the evaluation of the elastomer. Based on results from this study, DIC may serve as a valuable method for the determination of mechanical properties of arteries and other soft tissues.

Chip formation in orthogonal trimming of graphite/epoxy composite

The mechanisms of chip formation in edge trimming of graphite/epoxy laminates with polycrystalline diamond tools were studied. Characteristics of chip formation were found to be primarily dependent on fibre orientation, with only secondary effects from tool geometry and operating conditions. Although discontinuous chips were noted with all tool/material combinations, variation in the chip geometry depicted a change in chip formation mechanisms with fibre orientation. Spectral analysis of the cutting force signatures and machined surface profiles were used to distinguish changes in chip geometry with fibre orientation and tool geometry. An increase in the rake angle of the cutting tool insert was found to localize the extent of fracture from the tool nose, resulting in smaller discontinuous chips and giving rise to a higher machined surface quality.

Dental primer and adhesive containing a new antibacterial quaternary ammonium monomer dimethylaminododecyl methacrylate

OBJECTIVES The main reason for restoration failure is secondary caries caused by biofilm acids. Replacing the failed restorations accounts for 50-70% of all operative work. The objectives of this study were to incorporate a new quaternary ammonium monomer (dimethylaminododecyl methacrylate, DMADDM) and nanoparticles of silver (NAg) into a primer and an adhesive, and to investigate their effects on antibacterial and dentin bonding properties. METHODS Scotchbond Multi-Purpose (SBMP) served as control. DMADDM was synthesized and incorporated with NAg into primer/adhesive. A dental plaque microcosm biofilm model with human saliva was used to investigate metabolic activity, colony-forming units (CFU), and lactic acid. Dentin shear bond strengths were measured. RESULTS Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the new DMADDM were orders of magnitude lower than those of a previous quaternary ammonium dimethacrylate (QADM). Uncured primer with DMADDM had much larger inhibition zones than QADM (p<0.05). Cured primer/adhesive with DMADDM-NAg greatly reduced biofilm metabolic activity (p<0.05). Combining DMADDM with NAg in primer/adhesive resulted in less CFU than DMADDM alone (p<0.05). Lactic acid production by biofilms was reduced by 20-fold via DMADDM-NAg, compared to control. Incorporation of DMADDM and NAg into primer/adhesive did not adversely affect dentin bond strength. CONCLUSIONS A new antibacterial monomer DMADDM was synthesized and incorporated into primer/adhesive for the first time. The bonding agents are promising to combat residual bacteria in tooth cavity and invading bacteria at tooth-restoration margins to inhibit caries. DMADDM and NAg are promising for use into a wide range of dental adhesive systems and restoratives.

The influence of abrasive waterjet cutting conditions on the surface quality of graphite/epoxy laminates

Abstract An experimental investigation was conducted to determine the influence of cutting parameters on the surface roughness and kerf taper of an abrasive waterjet machined graphite/epoxy laminate. Experimental design was used to systematically measure the influence of cutting parameters on the surface roughness and kerf taper of laminate specimens. Stylus prolifometry was used to measure the surface roughness and a visual inspection including scanning electron microscopy (SEM) was conducted. Profilometry measurements supplemented with microscopy analysis suggests that three regions of surface topography are evident on the machined surface of the laminate specimens. ANOVA techniques indicate that the influence of cutting parameters on the surface roughness changes as a function of cutting depth. Mathematical models were developed to predict the surface roughness and kerf taper in terms of the cutting parameters of a graphite/epoxy laminate to cutting depths of 16 mm.

Material removal in abrasive waterjet machining of metals. Surface integrity and texture

An experimental study was conducted to determine the influence of material properties on the surface integrity and texture that results from abrasive waterjet (AWJ) machining of metals. A microstructure analysis, microhardness measurements, and profilometry were used in determining the depth of plastic deformation and surface texture that result from material removal. Models now available for dry abrasive erosion were adopted and found useful in understanding the influence of material properties on the hydrodynamic erosion process. It was found that the depth of subsurface plastic deformation is inversely proportional to a metals strength coefficient and extends the greatest depth near jet entry in the initial damage region (IDR). Furthermore, surface skewness in AWJ machining of metals increases with ductility and the corresponding critical strain for lip formation.

Fatigue and human umbilical cord stem cell seeding characteristics of calcium phosphate–chitosan–biodegradable fiber scaffolds

Calcium phosphate cement (CPC) has in situ-setting ability and bioactivity, but the brittleness and low strength limit CPC to only non-load-bearing bone repairs. Human umbilical cord mesenchymal stem cells (hUCMSCs) can be harvested without an invasive procedure required for the commonly studied bone marrow MSCs. However, little has been reported on hUCMSC delivery via bioactive scaffolds for bone tissue engineering. The objectives of this study were to develop CPC scaffolds with improved resistance to fatigue and fracture, and to investigate hUCMSC delivery for bone tissue engineering. In fast fracture, CPC with 15% chitosan and 20% polyglactin fibers (CPC-chitosan-fiber scaffold) had flexural strength of 26mPa, higher than 10mPa for CPC control (p<0.05). In cyclic loading, CPC-chitosan-fiber specimens that survived 2x10(6) cycles had the maximum stress of 10MPa, compared to 5MPa of CPC control. CPC-chitosan-fiber specimens that failed after multiple cycles had a mean stress-to-failure of 9MPa, compared to 5.8MPa for CPC control (p<0.05). hUCMSCs showed excellent viability when seeded on CPC and CPC-chitosan-fiber scaffolds. The percentage of live cells reached 96-99%. Cell density was about 300cells/mm(2) at day 1; it proliferated to 700cells/mm(2) at day 4. Wst-1 assay showed that the stronger CPC-chitosan-fiber scaffold had hUCMSC viability that matched the CPC control (p>0.1). In summary, this study showed that chitosan and polyglactin fibers substantially increased the fatigue resistance of CPC, and that hUCMSCs had excellent proliferation and viability on the scaffolds.