Creeping bent

Creeping bent grass, Agrostis stolonifera, senescent leaf 10 DW vs. 1 DW 88... 

In the constant-strain method, the specimen is stretched or bent to a fixed position at the start of the test. The most common shape of the specimens used for constant-strain testing is the U-beud, hairpin, or horseshoe type. A bolt is placed through holes in the legs of the specimen, and it is loaded by tightening a nut on the bolt. In some cases, the stress may be reduced during the test as a result of creep. In the constant-load test the specimen is supported horizontally at each end... 

Fabricated TPs can be bent, pulled, or squeezed into various useful formed shapes. But eventually when heat is added, they return to their original form. This behavior can be annoying or provide interesting design possibilities for all types of fabricated products. When the change is unwanted it is called creep when the change is skillfully adapted to use in the overall design, it is referred to as plastic memory. 

Consider the polypropylene seat of a stackable chair. The seat sides are bent over , providing a place to grip the seat. These sides provide bending stiffness to the seat. You can prove this if you can cut off the side parts if you lean back in the chair, it flexes excessively at the back/seat junction. There is more about the bending creep of plastics in Chapter 7 and the bending stiffness of beams in Chapter 13. The seat surface has a moulded-in texture (Fig. 1.5), to increase the coefficient of friction with your clothes, and to disguise scratches. Note how dust build-up and scratches spoil the appearance of the hidden side, which is smooth. 

If a plastic is subjected to a constant high creep stress for a long time, the creep rupture failure that occurs may be by yielding, or by crazing and crack growth. Creep rupture is important for the design of gas or water pipes (Chapter 14), where the cylindrical shape is stable during creep. It is less important for products that are bent or twisted, where excessive deflection is likely before any rupture process starts. 

Under the influence of force and when heated, the adhesive layers of footwear materials bonds suffer plastic flow. The creep test at constant temperature serves to assess the behavior of shoe materials bonds when heated under the influence of a constant peeling force over a fixed time. Weight between 0.5 and 2.5 kg can be fixed on the lower holders outside a heating oven where the test pieces can be heated at 50 to 70°C. The unbounded ends of five test pieces are bent apart carefully and inserted in the holders. Temperature is generally raised to 60° C after a warm-up period of 1 h. The test pieces are loaded with the chosen weight constantly for 10 min. Then, the heating oven is opened and the separations of the bonds are marked. 

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