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Constrained Layer Damping (CLD) is a process used on stiff structures, such as brake pads in a car, to reduce vibration and suppress noise. CLD is achieved by attaching a layer of both viscous and elastic materials known as the “viscoelastic” or “damping” layer to the base layer of the structure and then adding a third “constraining” layer on top of the viscoelastic layer. These three layers are stuck together with adhesives to form the CLD system. Often a fourth layer known as the "standoff" layer is introduced to further enhance the dissipation of energy in CLD.

History
Although the first scientific work in constrained layer damping took place in the early 1930’s, the problem of insufficient damping and unstable vibration was first recognized by the Roman Empire in the early centuries A.D. Roman officers would train their soldiers to march in a non-rhythmic pattern over bridges to avoid exciting resonant frequencies within the bridge, for these vibrations could have led to destruction in bridges and loss of soldiers’ lives. Scientists such as Foppl, Zener, and Davidenkoff were the first to investigate the damping materials of metals. Then as World War ll approached, there was definite development in area of CLD, but there were not many publications of these developments because of the rapid increase in the scientific community at the time. The first notable advancements in the CLD occurred in the 1950’s, beginning with Ross and Kerwin’s analytical method to layered damping treatments. Also, in 1952 Mycklestad published the first investigation of the complex modulus modeling of damping materials. Research in CLD significantly increase through the Aerospace industry. They were seeking a way to reduce vibration and resultant sound through aircraft fuselage panels without significantly increasing weight. Furthermore, as the mid 1960’s approached, NASA funded research for both thin and lightweight films that could be used to reduce vibration and sound in rocket housings. From the 1970’s to now there has been ongoing research on the properties of damping materials to enhance CLD systems. The most common material used for damping in the current market are viscoelastic materials. Viscoelastic materials allow a wide range of different compositions making them efficient materials for specific applications.

Process
Constrained Layer Damping begins with a simpler process known as free-layer damping. In this process, a layer of both viscous and elastic materials is attached onto a structure. As the base structure flexes from vibration, the function of the viscoelastic layer is to dissipate this energy by extending and compressing before the energy can build up and radiate as sound. In order to convert the free-layer damping system into a CLD system a third "constraining" layer is attached. CLD systems are much more favorable because by adding the constraining layer, larger shear strains form in the damping layer. Therefore the amount of energy dissipation significantly increases.

Constraining Layer
Advances in constrained layer damping have been accomplished by shifting attention off the viscoelastic layer and onto the constraining layer. If the motion of the constraining layer can actively be controlled, higher rates of energy dissipation would be achieved. This motion can be controlled by combining piezoelectric elements in addition to the passive constrained-layer damping materials.

Viscoelastic Layer
Viscoelasticity is the combination of perfectly elastic and viscous behavior. In an elastic material there is perfect energy conservation - all the energy stored is recovered. On the other hand, a viscous material does not recover any of the stored energy. The effectiveness of a viscoelastic material can be determined by the rate at which the material dissipates energy in the form of heat through shear.

Components
Mycklestad was one of the first pioneering scientists to investigate the complex modulus behavior of viscoelastic materials. The moduli of a viscoelastic material have two parts to them - real and imaginary. The imaginary part, also called the loss modulus, defines the viscous behavior of the material. The real part, known as the storage modulus, defines both the elastic behavior and the stiffness of the material.

Temperature Regions
There are three main temperature regions in which viscoelastic material can effectively operate: glassy, transition, and rubbery. The glassy region is representative of low temperatures where the storage moduli are much higher than for the other regions and it optimal for polymers operating below their brittle transition temperature. However, the temperature range which defines the glassy region of a polymeric material depends on the composition and type of viscoelastic material being used. The high values of the storage moduli correlates to low loss factors, which are mainly due to the stiffness of the viscoelastic material. Material used in the rubbery region is easily deformable, for it represents high material temperatures. In this region, the material takes much longer to reach equilibrium after a load is removed from the structure due to the low interaction between the polymer chains. The transition region falls between the glassy and rubbery regions and represents a medial temperature range.

Standoff Layer
In order to maximize damping performance, a standoff layer is often used to increase the shear deformation of the viscoelastic layer. When adding a standoff layer there are a few factors to consider. The material used in an effective standoff layer must be stiff in shear and flexible in bending. The standoff layer must also be lightweight in order to minimize the added weight on the CLD system. Ultimately the ideal material used in a standoff layer provides efficient energy dissipation under shear deformation. This must be true under a wide temperature range to accommodate the varying thermal environments a CLD system is exposed to during its operation. However this is difficult to accomplish because most viscoelastic materials have their highest loss factor limited to the glass transition temperature range. Finally, it is crucial to evaluate the strength of the adhesion between the layers in order to avoid a collapse of the CLD system.

Shear Strain and Deformation
As the constraining layer system flexes during vibration, shear strains - ratio of deformation to original length - form in the viscoelastic layer, then the vibrational energy is lost through shear deformations. The Shear Modulus - ratio of shear stress to shear strain - is also an important factor is CLD and can be divided into three regions: low, medium, and high. A low shear modulus will yield poor performance in both active and passive constrained layer systems whereas a high shear modulus results in large vibration reduction.

Partial Constrained Layer Damping
Rather than applying a viscoelastic layer, damping can be applied to specific points in a structure. This process is known as Partial Constrained Layer Damping and has two advantages over normal CLD. As described in the analysis of partially damped structures by Moreira et al., PCLD lowers additional mass and stiffness in structures while being just as effective as a complete damping treatment. In his analysis, Moreira determined that partial damping can be maximized if damping is applied to host structure ares where the highest surface strains are located.

Industry Applications
The applications of Constrained Layer Damping can be applicable to a wide range of industries and each industries applies the process of CLD in a different manner.

Computer Hardware
Reduce vibration in disk storage systems, top covers, and circuit board dampers.

Aerospace
Provide significant damping in commercial airplane fuselages and wing skins, satellite instrumentation platforms, and satellite fuselages.

Automotive
Reduce vibrations in disk and drum brake pads and reduce acoustical noise in passenger compartments.

Future Technology
Microcellular foams have attracted interest for being used in CLD systems. Originally, these foams were created to reduce the amount of polymer used in applications where the full strength of solid polymers was not needed. One of the desirable features of microcellular foam in CLD is its low weight penalty. The foam material consists of a large number of bubbles causing the density of the foam to be relatively low. It’s light-weight allows it to be used in CLD as a damping or standoff layer and the internal foam structure can be engineered to create different degrees of damping.