Review Question - QID 214020

The yield point is the transition point between elastic and plastic deformation. The yield strength is defined as the amount of stress necessary to produce a specific amount of permanent deformation.

Stress is the amount of force applied to a material and strain is the deformation resulting from that stress. This is graphically depicted as a stress-strain curve, where the X-axis represents strain and the Y-axis represents stress. The elastic modulus of a material is the linear region of the graph (rise over run/stress on strain). Remember, an elastic material is one that resists a change in shape (less strain or deformation under increasing stress). Non-linear regions include the toe region for some materials (tendons/ligaments) and the plastic zone, which occurs after the yield point.

Mantripragada et al. provide a review of recent advances in designing orthopaedic implants. Of note, they discuss modifications to metallic implants to reduce unwanted effects, such as nickel-free stainless steel. They also go over newer alloys with desirable mechanical and biological properties, such as tantalum, niobium, zirconium, and magnesium.

Kennedy et al. provide a classic in-vitro tension study of the human knee ligaments. They used an Instron Tension Analyzer to test the ultimate failure of the medial collateral, lateral collateral, anterior cruciate, and posterior cruciate ligaments at different loading rates. They found that the posterior cruciate ligament was the strongest (the other ligaments were all of comparable strength) and that microscopic failure occurred before macroscopic failure.

Illustration A represents a stress-strain curve.

Incorrect Answers:
Answer 1: The necking region occurs prior to failure/fracture of a material and is a phenomenon especially associated with a ductile material; the diameter of the material is diminished prior to fracture.
Answer 2: The ultimate strength is defined as the maximum stress that a solid material can withstand before failure.
Answer 3: Toughness is defined as the amount of energy per volume a material can absorb before fracture and is the area under the stress-strain curve.
Answer 5: The toe region is seen in materials such as ligaments and tendons and represents the straightening of the crimped ligament fibers.