Small-scale Test Specimens Research Articles

Friction of polyoxymethylene homopolymer in highly loaded applications extrapolated from small-scale testing

Most studies on tribology of polymer materials are traditionally performed on small-scale test specimens. However, to obtain data relevant for practical design of polymer parts in highly loaded bearings or sliding systems one must simulate real working conditions as close as possible on laboratory scale. In present work, a large-scale test rig has been used for determinating friction of a commercial polyoxymethylene homopolymer (POM-H). Test samples with contact area 22500 mm 2 are submitted to a reciprocating motion with stroke 230 mm under different contact pressures from 8 to 150 MPa and sliding velocity of 5 mm/s. Test results are compared to those obtained on a traditional cylinder-on-plate configuration and reveal lower friction on large-scale tests. However, general laws predicting the coefficient of friction as a function of normal loads cannot be used for extrapolation. Bulk and flash temperatures are calculated to explain transitions in friction mechanisms on both testing scales. Local surface temperatures are higher under large-scale sliding and allow for surface melting, which is not observed on small-scale tests. Although, sliding conditions implying an identical flash temperature induce polymer transfer only on large-scale tests, while no transfer occurs on small-scale tests. Even an artificial increase in small-scale bulk temperatures allowing for polymer transfer, is not able to provide low friction as observed on large-scale. An appropriate combination of load and sliding velocity is used for linear extrapolation of friction values, indicating the additional importance of bulk temperatures. When the polymer softening point is exceeded, creep at the polymer surface contributes to low friction.

Friction and wear of acetal: A matter of scale

Most studies on polymer tribology are traditionally performed on small-scale test specimens. However, to obtain data relevant for practical design of polymer parts used in high load/low sliding velocity systems one must simulate real working conditions as close as possible on laboratory scale. A large-scale test rig has presently been used for determination of friction and wear behaviour of a commercial polyoxymethylene homopolymer (POM-H). Test results are compared to those obtained on a small-scale cylinder-on-plate configuration, investigating possibilities for extrapolation. For small-scale tests, a transition is found from pure adhesive/abrasive wear to deformation and softening when the calculated bulktemperature exceeds 90 °C, corresponding to stabilisation in friction. Overload conditions occur at higher temperatures due to the lack of polymer transfer. Softening and melting of large-scale polymer samples allow for huge transfer films, providing low to extremely low friction and stabilisation in wear rates. Although a thermal extrapolation model with a macroscopic geometry factor is evaluated, friction and wear rates cannot be estimated from small-scale tests because of differences in wear mechanisms, influenced by transfer ability, creep and contact area.

Finite element modelling of the behaviour of thin planar and curved plates

The finite element has been made increasingly more accessible to engineering analysts through the development of powerful user-friendly software. It is a widely used approach to the analysis of thin planar and curved plate structures for bending behaviour. This applications-type paper describes the role which the finite element technique played in an analytical and experimental investigation of the behaviour of thin plate elements under static loading.In this investigation, finite element analysis was used firstly to assess the validity of various assumptions used in the experimental setup and, secondly, to provide theoretical predictions for the cracking load of the small scale test specimens. One type of element examined was the thin doubly curved hyperbolic paraboloid form. Linear elastic analysis of such relatively complex geometrical shapes is now comparatively straight forward using computers.The paper highlights the value of the finite element method as a complementary tool to experimental analysis. The data provided helps to validate the use of discrete models for the class of problem studied. The discretization and refinement of the model is greatly faciliated by modern software and hardware.

Beam-leaded tantalum thin-film capacitors : W. E. Wesolowski and M. Tierman, Proc. 1969 Electron. Compon. Conf., Washington DC, April 30–May 2 (1969), p. 393

Size effects in metals have received considerable attention in literature in the last decades. For preparing specimens dedicated processing techniques, such as laser-cutting, micro-milling, turning, etc., are used. Most of these processing methods intrinsically damage crystals just below the worked surface. In macroscopic applications, the effect on the overall mechanical behaviour can safely be neglected in most cases. Upon miniaturisation, however, the influence of the affected region becomes more important and may induce a processing induced size effect, which is far from negligible. Processing induced size effects are analysed by carefully characterising the plastic yielding in uniaxial tension of rectangular, 300μm-thick aluminium sheet specimens, with a well-defined homogeneous microstructure containing through-thickness grains. The specimens are processed to different widths by three independent machining techniques: (1) laser-cutting, (2) mechanical cutting, and (3) extensive grinding from a larger width. These independent techniques all result in a distinct processing induced size effect upon miniaturisation, i.e. an increase of up to 200% in yield stress for a decrease from about 12 to 3 grains over the specimen width. Using a simple Taylor averaging model, it is shown that the yield stress in the affected edge region increased to 210–350% of its initial (or bulk) value. In addition, it is found that even a prolonged anneal near the melting temperature can only partially remove the processing induced size effect. The results clearly demonstrate that processing induced size effects have to be considered in the design of miniaturised devices and parts as well as in scientific research relying on the testing of manufactured small-scale test specimens.