Bulk modulus is a material property that measures a material’s resistance to compression. Solids have a high bulk modulus, while gases have a low one. It is important to consider bulk modulus in high-pressure situations, such as in hydraulic equipment, to ensure proper system design and operation. The speed of sound and energy storage in solids are also related to bulk modulus. It is expressed as the ratio of applied pressure to volume change per unit volume.
Materials generally can be compressed when subjected to external pressures applied to their surfaces. The reduction in volume of a material under a given pressure varies widely from material to material. Gases are generally more easily compressed under pressure while solids can be compressed relatively little and with great difficulty. Bulk modulus is a material property that indicates the degree of resistance of a material to compression. It can also be referred to by a number of other terms such as mass modulus of elasticity, modulus of compression, and others.
One way to think of it is as the reciprocal of compressibility. A high modulus of mass for a material indicates a relatively high compressive strength, meaning it is difficult to compress. A low value indicates relatively low compressive strength, meaning the material is compressed relatively easily. For example, the bulk modulus of steel is several orders of magnitude greater than that of air, which can be compressed relatively easily with an air compressor.
The mass modulus values for a material vary depending on factors such as the temperature of that material or the amount of air that is mixed with it. As a material heats up, its volume will generally expand, thus resulting in a more open physical structure that is easier to compress. Air trapped in a material also affects the physical structure of a material, thus affecting its bulk modulus.
Some fluids, such as water or hydraulic fluid, are sometimes casually referred to as incompressible fluids. This is not strictly accurate, but because their compressibilities are relatively low, the bulk modulus can be ignored in some engineering calculations. Under certain circumstances however, such as in some high pressure situations, this must be taken into account to ensure proper system design and operation.
For example, the performance of hydraulic equipment at very high pressures can be degraded if the mass modulus of the hydraulic fluid is not considered in the system design. This is because some of the energy is spent compressing the hydraulic fluid rather than going directly towards the work being done by the equipment. The fluid in the system must be compressed to the point where it will resist further compression before the equipment and load are applied. Energy diversion from the primary task can affect equipment location, power available for the intended function, response time, and so on.
Bulk modulus is less often a feature of interest than solids as they are typically extremely difficult to compress, but it is relevant in some circumstances. The speed at which sound travels through a solid depends in part on the material’s mass modulus. The amount of energy that can be stored in a solid is also related to this property so it is relevant to the study of earthquakes and seismic waves.
As a mathematical function, this material property is expressed as the ratio of the applied pressure to the volume change of the substance per unit volume. This produces a value expressed in the same units used to express pressure because the units of volume cancel out. In graphical form, it is the slope of the curve formed by plotting the pressures applied to a material versus the corresponding specific volumes of the material at those pressures.
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