The influence of human resource management practices on occupational health and safety in the manufacturing industry.

This study provides an overview of the influence of applying two human resource management (HRM) practices on workers' occupational health and safety (OHS). These practices are selection and recruitment, and development and training. Furthermore, this study highlights the observed activities to define hazards, processes and OHS measures in the manufacturing industries. It also assesses the importance of workers' knowledge and awareness of OHS programs. A structured questionnaire was administered to those working in the manufacturing sector in several countries. The responses were analyzed statistically. The results showed that the demographic variables had a significant impact on workers' knowledge and awareness of OHS programs. In addition, HRM practices played a major role in OHS. Therefore, it is recommended to top management to highly consider OHS practices by integrating them into the workplace, starting from worker selection, all the way to production, operations and management, by establishing a strong HRM system.

The effect of resistance grooves on the fracture toughness of zirconia-based crowns from mono and cyclic loading.

OBJECTIVE: Prosthetic molar crowns in service are subjected to chewing loads, which cause a shift or dislodgment. The objective of this study is to investigate whether the addition of resistance grooves to the proximal surfaces of the abutment teeth would enhance the fracture resistance of the zirconia crowns and to compare between the patterns of cracks development on the zirconia crowns after the application of mono loading versus cyclic loading forces. MATERIALS AND METHODS: Thirty-six all-ceramic zirconia cored crowns were prepared on the same abutment. Resistance grooves were added to the mesial and distal surfaces of 16 abutments. Before testing, all specimens subjected to thermal aging. Two groups of crowns were then subjected to cyclic axial and lateral forces for 1,250,000 cycles in aqueous conditions. Two groups of samples were also tested in monoloading fashion. RESULTS: The crack pattern between mono and cyclic loading were compared. The crown fracture resistance was compared in the two types of abutments, with and without grooves. The results confirmed that the grooves addition had no effect on critical conditions to initiate failure in the case of mono loading. In cyclic loading, grooves addition increased the critical loads in the order of two. Failure patterns and location were obtained. CONCLUSIONS: The results showed that the location of retention grooves halted the failure in the surfaces where it was located in all loading mechanisms used in this study.

Margin failures in crown-like brittle structures: off-axis loading.

The effect of off-axis loading of compliant indenters on the initiation of cracks at the margins of dental crown-like dome structures consisting of glass shells back-filled with an epoxy resin is examined. As in previous studies on similar structures but with strictly axial loading, cracks can be made to initiate and propagate from the margins around the dome faces into a "semi-lunar" fracture pattern characteristic of some all-ceramic crown failures. In this study, balsa wood and teflon disk indenters are used to provide the off-axis loading, at 45 degrees to the dome axis. The soft indenters, considered representative of food bolus, spread the contact at the top surface, suppressing otherwise dominant radial cracks that ordinarily initiate at the dome undersurface directly along the load axis beneath harder indenters. Finite element modeling is used to show that off-axis loading dramatically increases the tensile stresses at the near-side dome margin, strongly diminishing the loads required to generate the lunar fracture mode.

The role of skirt geometry of dental crowns on the mechanics of failure: experimental and numerical study.

Dental crown structures were modelled using a curved bi-layer system consisting of a brittle glass coating on a compliant polymeric substrate, to illustrate the role of skirt geometry on the mechanics of failure evolution. The geometries of the samples were varied by tapering the skirts of the glass shells in different lengths and thicknesses. The failure of these samples was investigated under single-cycle axial loading tests using an indenter of low elastic modulus. The onset of fracture was observed in situ using a video camera. A relationship between the height and thickness of the taper and the critical load required for a crack to appear in the sample was observed. Margin cracks were observed to propagate from flaws near the margins. Experimental trends suggested that critical loads increased with increasing taper thickness, and decreased with increasing taper length. Finite element modelling was also used to evaluate the stress distribution in the glass coating. Peak maximum principal stresses at the margins decreased with increasing taper thickness, and increased with increasing taper length, consistent with the experimentally determined critical loads. It is concluded that long, narrow tapers should be avoided in order to maximise the load bearing capacity of dental crowns.

Margin failures in brittle dome structures: relevance to failure of dental crowns.

Margin cracks in loaded brittle dome structures are investigated. Dome structures consisting of glass shells filled with polymer resin, simulating the essential features of brittle crowns on tooth dentin, provide model test specimens. Disk indenters of diminishing elastic modulus are used to apply axisymmetric loading to the apex of the domes. Previous studies using hard indenters have focused on fractures initiating in the near-contact region of such specimens, including radial cracks at the glass undersurface directly below the contact axis. Here, we focus on fractures initiating at the remote support margins. Margin cracks can become dominant when loading forces are distributed over broad contact areas, as in biting on soft matter, here simulated by balsa wood disks. Cracks preinitiated at the dome edges during the specimen preparation propagate under load around the dome side into segmented, semilunar configurations reminiscent of some all-ceramic crown failures. Finite element analysis is used to determine the basic stress states within the dome structures, and to confirm a shift in maximum tensile stress from the near-contact area to the dome sides with more compliant indenters.

Effect of off-axis concentrated loading on failure of curved brittle layer structures.

A study is made of the effects of superposed tangential force by off-axis indentation loading on curved bilayers consisting of brittle shells filled with polymer support material. Such loadings are pertinent to all-ceramic crown structures on tooth dentin in occlusal function. Layer flexure places the ceramic undersurfaces in tension, leading to fracture by initiation and propagation of radial cracks. Following an earlier study, model specimens with curved surfaces are prepared by pressing glass plates 1 mm thick onto steel spherical dies with radius of curvature 20 mm to 8 mm at elevated temperatures, and bonding the resultant hemispherical shells onto an epoxy support base. The specimens are tested by indentation with spheres loaded vertically but off-center, with the contact center located at 30 degrees to the hemisphere axis. The applied loads to initiate radial cracks are little affected by the resultant tangential component, but the loads to propagate the same cracks to the specimen edges are substantially reduced. Finite element calculations are used to evaluate stress states in the specimens for correlation with the experimental data.

Contact damage in brittle coating layers: influence of surface curvature.

Fracture from indentation by a hard sphere on bilayer systems composed of curved brittle coating layers on compliant polymeric substrates is investigated, in simulation of dental crown structures. Glass plates 1 mm thick are used as representative of enamel/crown layers, and epoxy filler substrates as representative of support dentin. Specimens with curved surfaces are prepared by pressing the glass plates onto steel sphere dies with radius of curvature down to 4 mm, to reflect common occlusal geometries. The influence of curvature on the conditions to initiate and propagate subsurface "radial" cracks, widely believed to be the principal failure mode in ceramic-based dental crowns, is studied. Finite element calculations are used to evaluate stress states in the specimens. It is shown that surface curvature can play an important role in the radial crack evolution, initially by inhibiting initiation but subsequently, in the case of convex curvature, by strongly enhancing propagation to failure. Implications concerning the design of ceramic-based dental crowns are considered.