What is "packaging stiffness"?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
1

Is the sensor too rigid, and will it affect the deformation of the object being measured?

2

It does.

When the stiffness of the sensor is too large relative to the local stiffness of the object being measured, a significant Reinforcement Effect will occur, thereby altering the original force field and strain distribution of the object being measured in the measurement area.

I. Analysis of Physical and Mechanical Mechanisms

When a sensor is attached to the surface or embedded within, the sensor and the object being measured together form a composite stressed system. The influence of the sensor on the deformation of the object being measured is mainly controlled by the following physical mechanisms:

1. Local Stiffness and Constraint Effect

When the axial tensile stiffness of the sensor (represented by the product of its elastic modulus E and cross-sectional area A, EA) or its bending stiffness (EI) is added to the structure being measured, it is equivalent to introducing a section of high stiffness constraint in the measurement area. If the sensor is too rigid, when the object being measured deforms under load, the sensor will generate a reaction force resisting the deformation, causing the actual strain at the installation location of the object to be suppressed, i.e., lower than the original strain before the sensor was installed.

2. Relative Stiffness Ratio (Stiffness Matching Principle)

The degree to which the sensor affects the deformation of the object being measured depends on the relative ratio of the stiffness of the object being measured to the stiffness of the sensor:

  • Rigid Substrates (e.g., heavy steel structures, large concrete components):
    The sectional stiffness of the structure being measured is much greater than the stiffness of the sensor (EA_{\text{substrate}} \gg EA_{\text{sensor}}). In this case, the reinforcement effect of the sensor can be ignored, and the sensor can faithfully record the true deformation of the substrate.
  • Flexible/Thin-walled/Low Elastic Modulus Substrates (e.g., composite materials, films, polymer materials, rubber, biological tissues):
    The stiffness of the structure being measured is low or extremely thin. If a sensor with a large cross-section or encapsulated with a high-modulus material is used, the sensor will forcibly restrict the local deformation of the object being measured, resulting in a measured strain that is significantly less than the true strain before the sensor was attached, leading to a large measurement error.

3. Shear Stress Concentration and Strain Transfer

Stiffness mismatch between the sensor and the object being measured can also lead to high shear stress concentration at the interface (adhesive layer). Excessive stiffness not only causes gradient attenuation of strain during transmission, but severe shear stress can also lead to early cracking or debonding of the adhesive layer.

II. Solutions in Engineering and Product Selection Principles

To minimize the impact of the sensor on the original deformation of the object being measured, optical engineering typically follows the design principles of “reducing the physical cross-section” or “matching the substrate stiffness”:

  1. Flexible and Thin-walled Structure Scenarios:
    Select sensors with small cross-sectional areas and high flexibility, such as polymer-encapsulated or miniaturized encapsulated sensors, to minimize the added stiffness.

  2. General Metal Structure Scenarios:
    Select a thin-diameter elastic alloy tube encapsulated structure, so that the sensor has good linear strain transfer performance while maintaining a small physical cross-sectional area.