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Time ME/EMA Dependent Deformations Lab #5
1. Review True Stress and Strain Lab 2. Definitions Creep - Deformation that occurs under a constant load/stress. Relaxation - Decrease in stress under constant deformation/strain. Recovery - Disappearance of creep strain with the removal of the load. Time dependent deformation occurs mostly in metals at high temperatures or to metals with low melting points, also to some glasses and polymers at low temperature. 3. Creep Graphs
Figure 1 Label Regions and points O A _________________ B D _________________ C ____________________ Strain Hardening Region__________ Creep__________ Necking Region _________________
Figure 2
A B _______________ D E _______________ E __________________ Steady State
4. Recovery Graphs
Figure 3
Figure 4
Figure 3 For relaxation, the stress decreases and the strain is a constant. For recovery, the load is removed so the stress is zero and the strain decreases with time. Figure 4 - Polymers often recover a large amount of strain when they are unloaded, however, most metals do not. (Gain in height of acrylic after compression test, Lab # 3.) Creep Very dependent, temperature see Figure 6 in the Lab Manual. Figure 7 shows the stress level dependence of creep. 5. Maxwell Model - Simple model used to predict time dependent deformations. Dashpot Like a shock that displaces at a constant rate, represents creep strain and is the second term in the equation, which grows linearly with time. . Spring Represents initial strain, which can be both elastic and plastic and is the first term in the equation, assumes instant loading. =/E+ ( /)*t
Maxwell Model
6. Lab & Assignment Individually turn in two graphs as listed below, no report or write-up. Due next week. Creep test of lead A constant load ( ___ lbs) is applied; the deform...