Question

Constant stress test is used for a biomaterial to measure

• A. Ductility
• B. Toughness
• C. Creep
• D. Fatigue

Hint:

When considering mechanical properties of biomaterials, remember that 'creep' is related to deformation under constant stress over time, while 'fatigue' is related to failure under cyclic or fluctuating stress. These are crucial for predicting the long-term performance and failure of implants in the human body.

Answer 1

The Correct Option is C

Definition of Constant Stress Test

A constant stress test involves applying a fixed stress (force per unit area) to a biomaterial over an extended period and observing its response, typically deformation or failure over time. This test is used to evaluate the long-term mechanical behavior of materials, particularly under sustained loads. For biomaterials, such as those used in implants or scaffolds, this test is critical to understanding how they perform under prolonged physiological conditions. The question asks what a constant stress test measures, with options: ductility, toughness, creep, and fatigue. Let’s evaluate each option.

1. Ductility

Ductility is the ability of a material to undergo significant plastic deformation before fracturing, often measured as the percentage of elongation or reduction in area during a tensile test. A constant stress test, which applies a sustained load over time, does not primarily focus on measuring deformation to failure. Instead, it examines time-dependent behavior under constant stress, making ductility an incorrect choice for this test.

2. Toughness

Toughness is the ability of a material to absorb energy before fracturing, calculated as the area under the stress-strain curve from a tensile or impact test. It reflects a material’s resistance to sudden failure. A constant stress test does not involve varying stress to measure energy absorption or fracture; it maintains a steady load to observe gradual deformation over time. Therefore, toughness is not measured by this test.

3. Creep

Creep is the slow, time-dependent deformation of a material under constant stress, typically at elevated temperatures or over long periods. In biomaterials, creep is relevant for applications like implants (e.g., bone screws or joint replacements) that experience sustained loads in the body. A constant stress test is specifically designed to measure creep, as it tracks how the material deforms gradually under a fixed load, often producing a creep curve (strain vs. time). This makes creep the most relevant option.

4. Fatigue

Fatigue is the weakening of a material due to repeated, cyclic loading, leading to crack formation and eventual failure. Fatigue tests involve applying cyclic stresses (not constant stress) to assess a material’s endurance limit or crack growth over many cycles. A constant stress test, with its steady load, does not simulate the cyclic conditions needed to measure fatigue, so this option is incorrect.

Why Creep is the Correct Answer

The constant stress test is used to measure creep in a biomaterial. Creep is the gradual deformation that occurs when a material is subjected to a constant stress over time, which is exactly what the constant stress test evaluates. For biomaterials, creep is critical in applications like orthopedic implants, where sustained loads (e.g., body weight on a hip implant) could cause deformation over months or years. The test monitors strain over time, producing data on primary, secondary, and tertiary creep stages. For example, a polymer-based scaffold in tissue engineering might exhibit creep under constant pressure, affecting its long-term performance. The other options—ductility, toughness, and fatigue—require different testing conditions (e.g., tensile stretching, impact, or cyclic loading) and are not measured by a constant stress test.

Why Not the Other Options?

• Ductility: Measured in tensile tests focusing on plastic deformation to failure, not time-dependent deformation under constant stress.
• Toughness: Assessed through energy absorption in tensile or impact tests, not under sustained constant stress.
• Fatigue: Requires cyclic loading to evaluate failure under repeated stress, not a constant load.

A Simple Way to Remember

Think of a constant stress test as a “patience test” for a biomaterial:

• Creep: The material slowly stretches or deforms under a steady push, like dough slowly sagging under weight.
• Ductility: How far the material stretches before breaking, tested by pulling it quickly.
• Toughness: How much energy the material absorbs before snapping, tested by hitting or stretching.
• Fatigue: How the material holds up under repeated pushing and pulling, not a steady push.

Final Answer

A constant stress test is used for a biomaterial to measure Creep.