Material Testing

Materials Analysis

Lecture Overview

  • Lecture 3 from UMass Amherst focusing on materials analysis, particularly in the biomedical context.

Key Take-aways

  • Overview of the primary types of materials tests in various scenarios.
  • Significance of measuring and understanding the properties and performance of materials.
  • Methods to calculate the modulus from plotted data.
  • Consideration of caveats in testing methods which can include physiological, physical, or technical factors.

Understanding the Need for Material Testing

  • Purpose of Testing Materials:
    • Essential for characterizing material behaviors under various conditions.
    • Tissues must sustain different forces such as:
    • Compression
    • Tension
    • Shear
    • Torsion
    • Bending
  • Important Concept: Anisotropy
    • Definition: Anisotropy refers to the variation in material properties based on direction.
    • Structural differences exist in tissues adapted to different stresses (compression vs tension).

Mechanical Performance at Different Scales

  • Variations in mechanical performance can happen at multiple scales:
    • Bulk
    • Web
    • Fibre network
    • Fibre
    • Fibrils

Considerations During Measurement

  • Removal of tissues for testing can introduce confounding factors which may alter results.
  • Results may vary based on the testing methods utilized.
  • Testing parameters should ideally mimic biological environments for accuracy.
  • The importance of each test might differ based on specific applications.

Testing Material Performance

  • Different testing types based on material properties:
    • Mechanical Tests
    • Conduct tests for:
      • Tension
      • Compression
      • Shear
      • Fatigue and wear (biotribology focus)
    • Chemical Analysis
    • Includes:
      • Chemical structure assessment
      • Compositional analysis
    • Biomedical Tests
    • Evaluates:
      • Cytocompatibility
      • Hemocompatibility
      • Suture retention
      • Gelation properties

Types of Mechanical Testing

  • Mechanical testing methods include:
    • Tension
    • Compression
    • Shearing
    • Torsion
    • Bending
    • Buckling

Example of Tensile Testing

  • Mention of standard specimen shape, referred to as a “dogbone.”
  • Purpose of Tensile Testing:
    • Characterizes both the elasticity and strength of materials.
    • Involves uniaxial stretch and can show:
    • Plastic deformation
    • Brittle failure

Calculating Modulus from Stress-Strain Curves

  • When tensile stress is applied, it generates strain in materials.
  • The Linear Slope derived from the stress-strain curve is known as:
    • Elastic Modulus (also referred to as Young's Modulus).
  • Additional important metrics include:
    • Yield strength
    • Toughness

Performance of Different Materials

  • Notable that all subjects are of polymer class.
  • Material selection depends on the application requirements:
    • Higher moduli needed for orthopedic applications.
    • Stretchy materials preferred for soft tissues.

Viscoelasticity in Biological Systems

  • Definition: Viscoelasticity refers to time- or rate-dependent behavior.
  • It illustrates the ability of materials to adjust to load over time.
  • Example: The stress-strain character of bone is influenced by the loading rate:
    • Bone that is loaded quickly fails at a lower stress level compared to slow loading.

Strain and Material Behavior Over Time

  • Various types of material responses to loading include:
    • Elastic Materials: Feature a return to original shape post-load.
    • Viscous Materials: Display time-dependent deformation.
    • Viscoelastic Materials: Exhibit both elastic and viscous characteristics, like biological tissues.

Compressive Testing

  • Involves uniaxial compression to determine the load resistance of a material.
    • Important to keep the shape and thickness consistent throughout testing.
    • More common than tensile tests for very soft materials.

Calculating Compression Modulus

  • The stress curve for compressive testing is similar to tensile testing.
  • Confusion arises as this is sometimes referred to as compressive elastic modulus.

Shear Testing via Rheology

  • Shear Rheology: Measures resistance to opposing forces.
  • Two primary measures obtained:
    • Storage modulus (
    • Loss modulus (G”), which is indicative of viscous properties.
  • Types include:
    • Linear shear
    • Rotational shear

Hardness Tests

  • Purpose: Assesses resistance to indentation, relevant under specific circumstances.
    • Method is quick, easy, and relatively non-destructive.
    • Hardness can be quantified through:
      extHardnessnumber=F<em>appliedA</em>indentext{Hardness number} = \frac{F<em>{applied}}{A</em>{indent}}

Tribology Testing

  • Crucial for understanding how an implant performs through extended interaction within the body.
  • Tribology Definition: The science and engineering surrounding interacting surfaces in relative motion.
  • Important factors include:
    • Fatigue – performance over time,
    • Abrasion and wear – leading to surface degradation.

Fatigue Testing

  • Fatigue testing measures the relationship between stress and the cycle number, showing the maximum cycle fatigue.
  • Considerations include use stress values at 10610^6 cycles.

Abrasion and Wear Testing

  • Conducted through cycles of rubbing to evaluate wear resistance and suitability for applications.
  • Standardized wear testing is required for device approval by regulatory bodies like the FDA.
  • Real surfaces exhibit curves and textures that impact performance.

Cytocompatibility Testing

  • Also known as cytotoxicity testing; involves incubating cells with the material and counting live cells post-exposure.
  • Useful for targeting specific cell types:
    • For example, assessing impacts on bacteria or cancer cells.

Compositional (Molecular) Analysis

  • Aids in determining, validating, or reevaluating material composition before and after implantation.
  • Utilizes various spectroscopy methods:
    • Measurement of species produced when matter interacts with or emits radiation/light.
  • Distinct from spectrometry:
    • Spectrometry specifically measures certain spectra, commonly mass.

Representative Tissue Moduli

  • Various tissue types with respective material moduli measured in MPA (Young’s modulus), kPa (Compressive modulus), and kPa (Shear storage modulus):
    • Cortical Bone: Young's modulus = 17,000-24,000 MPa, Compressive modulus = 135,000-161,000 kPa, Shear storage modulus = 50,000 kPa.
    • Cancellous Bone: Young's modulus = 1,000-4,500 MPa, Compressive modulus = 4,000-12,000 kPa, Shear storage modulus = 300 kPa.
    • Tendon: Young's modulus ≈ 560 MPa.
    • Muscle: Young's modulus = 480 MPa, Compressive modulus ≈ 7 kPa.
    • Skin: Young's modulus ≈ 30 MPa, Compressive modulus ≈ 85 kPa, Shear storage modulus ≈ 32 kPa.
    • Organs (Lungs/Kidney): Young's modulus ≈ 10 MPa, Compressive modulus ≈ 190 kPa, Shear storage modulus = 0.5-1 kPa.
    • Cornea: Young's modulus ≈ 3 MPa.
    • Cartilage: Compressive modulus between 100-1000 kPa, Shear storage modulus = 50,000 kPa.
    • Spinal Cord: Young's modulus ≈ 2 MPa, Compressive modulus ≈ 3 kPa, Shear storage modulus = 0.2 kPa.