T.C. Bor

Influence of physical aging on impact embrittlement of uPVC pipes

Abstract Most failures of unplasticised poly(vinyl chloride) (uPVC) pipes used in the Dutch gas distribution network originate from third party damage. Brittle pipes should therefore be replaced to ensure safe operation of the network. In this study, the relation between physical aging and embrittlement of uPVC is investigated using instrumented falling weight impact tests. The ductile to brittle transition temperature was first measured for a water pipe grade uPVC at different stages of aging. As a hypothesis, a critical stress criterion is proposed above which failure is brittle. The evolution of the ductile to brittle transition temperature that followed from the use of this hypothesis and a model for the polymer yield stress agrees qualitatively with the experimental data. A minor increase in transition temperature was observed for the water pipe grade with aging. Applying the same hypothesis to a uPVC gas pipe grade shows a more pronounced influence of physical aging.

Modeling of Stress Development During Thermal Damage Healing in Fiber-reinforced Composite Materials Containing Embedded Shape Memory Alloy Wires:

Fiber-reinforced composite materials are susceptible to damage development through matrix cracking and delamination. This article concerns the use of shape memory alloy (SMA) wires embedded in a composite material to support healing of damage through a local heat treatment. The composite material contains a thermoplastic matrix that allows healing at elevated temperatures. The woven in SMA wires, oriented in the out-of-plane direction of the composite material, are used to close the delamination upon heating. Several case studies were performed. The influence of the SMA wire fraction, the degree of prestraining of the SMA wires, the glass transition temperature of the amorphous thermoplastic matrix, and the healing temperature have been considered. The delamination is compacted while heating up to the healing temperature. The thermal expansion coefficient of the thermoplastic matrix is much larger than that of the SMA wires causing a compressive thermal stress in the composite material upon heating. Prestraining of the SMA wires is not a priori required to obtain a compressive stress at the delamination interfaces at the healing temperature. After cooling to room temperature residual stresses occur in the composite material and SMA wires if the SMA wire composition at the start of the heat treatment differs from that at the end. The conditions under which no residual stresses develop have been determined. SMA wire fractions have been calculated to achieve a preset stress level in the composite material at the healing temperature and a stress-free state after healing. Finally, the use of high strength wires to replace SMA wires has been considered and shown to be a worthwhile alternative.

Cladding of Advanced Al Alloys Employing Friction Stir Welding

Friction stir welding (FSW) is a relatively new solid-state joining technology for metals. It shows no solidification-related joint imperfections which makes it utmost suitable for hard-to-weld highly alloyed aerospace aluminium grades, like AA 2xxx and AA 7xxx. These alloys are often cladded with a thin layer of pure aluminium for corrosion protection. Friction stir welding of such materials requires removal of the clad layer prior to welding to prevent weakening of the joint by the soft clad material. This leaves the welded region vulnerable to corrosion after the joining process. Post-weld restoration of the clad layer is required to restore the protective action of the clad layer and as such to enhance the life expectancy of the welded construction. In this work the deposition of thin layers of pure aluminium on AA 2xxx and AA 7xxx alloys is studied employing an innovative FSW tool. The tool shoulder is equipped with strategically placed internal channels that allow delivery of filler type of material into the weld zone. Depending on the channel architecture used, filler material can be deposited on top of the work piece surface and/or mixed with the work piece surface region. The cladding is done in the solid state avoiding many problems with solidification and interface reactivity often observed with other surface modification techniques, such as laser surface engineering, plasma spraying or casting. Here, the filler material is deposited on top of the work piece; the modified tool is not equipped with a tool pin. The work comprises an in depth study of the influence of process conditions on the microstructural changes in the underlying work piece and on the quality of the bonding of the clad material (99.5 % aluminium) to the work piece material. Apart from the usual process conditions, such as tool rotation speed, translation speed, down force and tool angle also the delivery pressure and rate of the filler supply system can be varied. The influence of the usual process conditions on the microstructure of the underlying work piece is similar to that observed with “traditional” FSW. Changes in hardness can be related to the amount of heat generated by the welding process. Shape and dimensions of the microstructural zones found are typical for welds made without a tool pin. The effect of the small amount of clad material deposited on top of the work piece on the temperature distribution is small. The amount of heat required to heat it up is negligible to the heat required to heat up the work piece and the tool. The quality of the bonded clad layer is dependent on the amount of heat and plastic deformation generated at the interfaces between the tool, the filler material and the work piece. Tool angle, tool shape and supply rate of the filler supply system determine the layer thickness.

Modeling of the Austenite-Martensite Transformation in Stainless and TRIP Steels

The transformation of austenite to martensite is a dominant factor in the description of the constitutive behavior during forming of TRIP assisted steels. To predict this transformation different models are currently available. In this paper the transformation is regarded as a stress induced process based on the thermodynamic action of the local stresses during transformation. A threshold for the thermodynamic action, above which transformation will occur, can be easily measured in a properly instrumented tensile test. The martensitic transformation is a diffusionless lattice shear. It is characterized by a habit plane normal n and a shear vector m, which are both defined with respect to the austenite lattice coordinate system. Therefore the thermodynamic action in each material grain strongly depends on the orientation of the grain with respect to the applied stress. Uniaxial tensile tests on both a non-textured austenitic stainless steel and one with a strong crystallographic texture were performed in both the rolling and the transverse directions. Both materials show mechanically induced phase transformation from austenite to martensite. When a strong texture is present in the austenite, differences between transformations during deformation in different directions can be observed clearly. The stress induced transformation theory, in combination with the textures measured before and after deformation, is used to explain and model the difference in transformation behavior when straining in various directions. During deformation the texture changes. This can have consequences for modeling of the transformation during non-proportional deformation.

Free surface modeling of contacting solid metal flows employing the ALE formulation

In this paper, a numerical problem with contacting solid metal flows is presented and solved with an arbitrary Lagrangian-Eulerian (ALE) finite element method. The problem consists of two domains which mechanically interact with each other. For this simulation a new free surface boundary condition is implemented for remeshing of the boundary elements. It uses explicitly that the integral of the convective velocity along a boundary element remains zero. Steady state solutions are obtained only if the integral of the convective velocities along each free surface boundary element remains zero. The new remeshing option for the free surface is tested on a cladding problem employing friction stir welding (FSW). The problem describes two elasto-viscoplastic aluminum material flows which mechanically interact.