Podcast
Questions and Answers
What is the relationship between the gauge factor (GF) and the strain (ε) when resistivity (ρ) remains constant?
What is the relationship between the gauge factor (GF) and the strain (ε) when resistivity (ρ) remains constant?
Why is a high gauge factor (GF) desirable in practical applications?
Why is a high gauge factor (GF) desirable in practical applications?
Which of the following is NOT a necessary condition for securely bonding a strain gauge?
Which of the following is NOT a necessary condition for securely bonding a strain gauge?
How can temperature sensitivity be reduced in strain gauge bridges?
How can temperature sensitivity be reduced in strain gauge bridges?
Signup and view all the answers
What limits the frequency response of bonded strain gauges?
What limits the frequency response of bonded strain gauges?
Signup and view all the answers
What is the primary feature of unbonded resistance strain gauges?
What is the primary feature of unbonded resistance strain gauges?
Signup and view all the answers
Which statement is true regarding the excitation voltage in strain gauges?
Which statement is true regarding the excitation voltage in strain gauges?
Signup and view all the answers
What defines the strain sensitivity of a strain gauge?
What defines the strain sensitivity of a strain gauge?
Signup and view all the answers
What is the key role of strain gages in measuring stress within components?
What is the key role of strain gages in measuring stress within components?
Signup and view all the answers
In the context of uniaxial loading, which equation correctly represents the relationship between stress and strain?
In the context of uniaxial loading, which equation correctly represents the relationship between stress and strain?
Signup and view all the answers
What is the primary purpose of resistive strain gauges?
What is the primary purpose of resistive strain gauges?
Signup and view all the answers
Which of the following best describes bonded strain gauges?
Which of the following best describes bonded strain gauges?
Signup and view all the answers
What is the primary purpose of a strain gage rosette?
What is the primary purpose of a strain gage rosette?
Signup and view all the answers
For a thin-walled pressure vessel, which stress is referred to as the axial stress?
For a thin-walled pressure vessel, which stress is referred to as the axial stress?
Signup and view all the answers
What causes the change in resistance of a strain gauge conductor?
What causes the change in resistance of a strain gauge conductor?
Signup and view all the answers
Which of these statements is true regarding the orientation of principal stresses?
Which of these statements is true regarding the orientation of principal stresses?
Signup and view all the answers
Which equation describes the hoop stress in a thin-walled cylindrical pressure vessel?
Which equation describes the hoop stress in a thin-walled cylindrical pressure vessel?
Signup and view all the answers
The gage factor (GF) can be defined in terms of which of the following?
The gage factor (GF) can be defined in terms of which of the following?
Signup and view all the answers
What determines the quadrant in which the orientation angle of principal stresses lies?
What determines the quadrant in which the orientation angle of principal stresses lies?
Signup and view all the answers
What characteristic of a strain gauge affects its resistance when stressed?
What characteristic of a strain gauge affects its resistance when stressed?
Signup and view all the answers
Which loading condition involves using a strain gage to determine stress based on measurements from multiple directions?
Which loading condition involves using a strain gage to determine stress based on measurements from multiple directions?
Signup and view all the answers
In the gage factor formula, what does the term 'ε' represent?
In the gage factor formula, what does the term 'ε' represent?
Signup and view all the answers
What is a typical value for Poisson's ratio for many materials?
What is a typical value for Poisson's ratio for many materials?
Signup and view all the answers
What occurs when a load W is applied to a strain gauge wire?
What occurs when a load W is applied to a strain gauge wire?
Signup and view all the answers
How does temperature affect the resistance of strain gauge wires?
How does temperature affect the resistance of strain gauge wires?
Signup and view all the answers
Study Notes
Resistive Strain Gages
- Bonded gages: Fine wire or conducting film cemented to a structural member to measure strain.
- Unbonded gages: Wires not bonded to any structure, but might have initial tension. They change resistance in response to mechanical loading.
- Changes in resistance are measured using a Wheatstone bridge circuit.
- Elongation or compression of the gage wires leads to a change in the resistance of the gage conductor (wire), which can be accurately sensed.
Gage Construction Details
- Composition: Thin metal (e.g., constantan) wire or film (approximately 0.025 mm thick), deposited on a base (e.g., polyimide), for protection and electrical insulation.
- Arrangement: The strain gauge wire or film might be sandwiched between protective transparent layers, with solder tabs exposed. The package is bonded to the surface with an adhesive.
- Connection: Strain gauges are wired and soldered using a connector (bondable) terminal.
Resistance of a Wire
- General Formula: Resistance (R) = (p * L) / A, where p = resistivity, L = length, and A = cross-sectional area.
- Stress: Stress (s) = (Force (W)) / Area (A).
- Strain: Strain (ε) = (Change in length (ΔL)) / Length (L) = (stress / Young's modulus).
- Young's Modulus (Y/E): The material's elasticity.
Gage Factor
- Definition: A measure of how much the resistance changes for a given amount of strain. The formula varies but often involves changes in resistance (ΔR/R), resistivity (Δp/p), and length (ΔL/L)
- Constant: For most gages, the gauge factor (GF) is constant over a wide range of strains.
Unbonded Strain Gage
- Initial Condition: Two pairs of wires initially under tension.
- Strain Application: When force is applied, one pair is strained more and the other less in equal amounts. This is helpful, in part, because it gives more consistent and reliable measurements.
- Filaments: Fine-wire filaments are stretched around the mounting pins.
Strain Gage Mounting Considerations
- Surface Preparation: Surfaces must be clean, e.g., using emery cloth and acetone.
- Drying: Adequate time is required for the cement to dry and harden.
- Bond Strength: The resistance element should be securely bonded to its mounting (strong bond). Any slip causes errors.
Types of Bonded Strain Gages
- Wire Gage
- Foil Gage
- Semiconductor Gage (high GF)
Strain Gage Bridge Output Considerations
- Amplification: Strain gage bridge outputs usually need amplification (e.g., using op-amps).
- Temperature Compensation: Using two matched gauges on one side of the bridge can reduce temperature sensitivity. At least one gauge is unstrained while the other gauges are actively measuring. Both gauges must be at the same temperature.
Strain Gage Rosettes
- Types: Single element, two element rosette, three element rosette, special purpose rosettes.
- Applications: For complex loading or geometry. Strain gage rosettes are used to obtain the state of stress when one gage is not enough, i.e., in biaxial or more complex situations.
Other Important Considerations
- Frequency Response: The frequency response is dependent on the structure to which it's bonded.
- Power Excitation: Power excitation voltage is limited by dissipation in the wire and heating of resistors.
- Strain Sensitivity: The minimum deformation that can be indicated by the gage per unit base length.
- Pressure Vessels: Formulas are given to calculate pressure in cylindrical thin-walled pressure vessels based on strain measurements.
Different States of Stress
- Uniaxial Loading: Tension or compression, a single gauge can determine the state of stress.
- Biaxial Loading: Stress is applied in orthogonal directions, more than one gauge and complex calculations are required to determine the state of stress.
- Types of Rosettes: Rectangular strain gage rosettes and equiangular (delta) and T-delta strain gauge rosettes determine the state of stress under complex loading.
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
Related Documents
Description
This quiz covers the fundamentals of resistive strain gages, including the differences between bonded and unbonded gages, and their construction details. Learn how changes in resistance are measured using a Wheatstone bridge and discover the materials used in gage construction. Perfect for students studying electronics or materials science.
