what is the difference between pressure and stress?

Answer 1

Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface. Mathematically:


where:


p is the pressure
F is the normal force
A is the area.
Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It is a fundamental parameter in thermodynamics and it is conjugate to volume.
Moments is the scientific name given to someting that turns on a pivot (fulcrum). The pressure of the moment can also be measured using a similar calculation to one given earlier in this article. The thing to consider or the relationship is that the further away the load (effort force) is from the pivot the larger the force. For example, a door, it has its handle (effort force) right at the end furthest away from the hinges (pivot). This allows the door to move with ease. The calculation used to measure force of moments is:

Distance from Pivot (mm/cm/m/km/M) X Force (N) =

Example: A door has its handle 70cm away from its pivot and the force exerted on the door is 8 (N)

Distance from Pivot (70) X Force (8) = 560 Ncm2
In the human body there are several types of pressure receptor. Baroreceptors monitor blood pressure in the carotid arteries, aortic arch and right atrium of the heart. Mechanoreceptors are part of the somatosensory system and are present in the dermis of the skin and in deeper tissues. They respond to different forms of touch and pressure; the main types of mechanoreceptor are Pacinian corpuscles, Meissner's corpuscles, Merkel cells and Ruffini endings.1 Pa = 1 N/m² = 10−5 bar = 10.197×10−6 at = 9.8692×10−6 atm ....etc.
Note: mmHg is an abbreviation for millimetres of mercury.

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m-2 or kg·m-1·s-2). This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N/m².

Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (psi) and the bar.

The cgs unit of pressure is the barye (ba). It is equal to 1 dyn·cm-2.

Pressure is still sometimes expressed in kgf/cm² or grams-force/cm² (sometimes as kg/cm² and g/cm² without properly identifying the force units). But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is expressly forbidden in SI; the unit of force in SI is the newton (N). The technical atmosphere (symbol: at) is 1 kgf/cm².

Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit millibar (mbar). Similar pressures are given in kilopascals (kPa) in practically all other fields, where the hecto prefix is hardly ever used. In Canadian weather reports, the normal unit is kPa. The obsolete unit inch of mercury (inHg, see below) is still sometimes used in the United States.

The standard atmosphere (atm) is an established constant. It is approximately equal to typical air pressure at earth mean sea level and is defined as follows.

standard atmosphere = 101325 Pa = 101.325 kPa = 1013.25 hPa.
Because pressure is commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e.g. inches of water). The most common choices are mercury (Hg) and water; water is nontoxic and readily available, while mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure. The press exerted by a column of liquid of height h and density ρ is given by the hydrostatic pressure equation as above: p = hgρ.

Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. When 'millimetres of mercury' or 'inches of mercury' are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. The water-based units still depend on the density of water, a measured, rather than defined, quantity.

Although no longer favoured in physics, these manometric units are still encountered in many fields. Blood pressure is measured in millimetres of mercury in most of the world, and lung pressures in centimeters of water are still common. Natural gas pipeline pressures are measured in inches of water, expressed as '"WC' ('Water Column'). Scuba divers often use a manometric rule of thumb: the pressure exerted by ten metres depth of water is approximately equal to one atmosphere.

Non-SI units presently or formerly in use include the following:

atmosphere.
manometric units:
centimetre, inch, and millimetre of mercury (Torr).
millimetre, centimetre, metre, inch, and foot of water.
imperial units:
kip, ton-force (short), ton-force (long), pound-force, ounce-force, and poundal per square inch.
pound-force, ton-force (short), and ton-force (long) per square foot.
non-SI metric units:
bar, millibar.
kilogram-force, or kilopond, per square centimetre (technical atmosphere).
gram-force and tonne-force (metric ton-force) per square centimetre.
barye (dyne per square centimetre).
kilogram-force and tonne-force per square metre.
sthene per square metre (pieze). Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure when forced to a standstill. Static pressure and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See Bernoulli's equation (note: Bernoulli's equation only applies for incompressible flow).

The pressure of a moving fluid can be measured using a Pitot probe, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressure.

Now Stress:

Stress (physics)
A mature tree trunk may support a greater force than a fine steel wire but intuitively we feel that steel is stronger than wood. To predict the forces which a structure of any size, made of any material, can sustain without failing, we need the concept of mechanical stress.

The definition of stress originates in two observations of the behaviour of a one-dimensional body subject to uniaxial loading, for example, a steel wire under uniaxial tension:

When the wire is pulled, it stretches. Up to a certain load, the stretch ratio (current length / initial length) is proportional to the load divided by the cross-sectional area of the wire. We therefore define the stress as σ = F/A.
Failure occurs when the load exceeds a critical value for the material, the tensile strength multiplied by the cross-sectional area of the wire, Fc = σt A.
These observations suggest that the quantity that effects the deformation and failure of materials is stress.

In the more general setting of continuum mechanics, stress is a measure of the internal distribution of force per unit area that balances and reacts to the external loads or boundary conditions applied to a body. Stress is an example of a type of quantity called a second-order tensor. In 3-dimensions, a second-order tensor can be represented by a 3x3 square matrix containing nine components. However, in the absence of body moments, the stress tensor is symmetric and can be fully specified by six components. These six components are sometimes written in Voigt notation as a 6x1 column matrix which is often, misleadingly, called the stress vector.

In N-dimensions, the stress tensor is defined by:


where the are the components of the resultant force vector acting on a small area which can be represented by a vector perpendicular to the area element, facing outwards and with length equal to the area of the element. In elementary mechanics, the subscripts are often denoted x,y,z rather than 1,2,3.

In a 1-dimensional system, such as a uniaxially loaded bar, stress is simply equal to the applied force divided by the cross-sectional area of the bar (see also pressure). The 2-D or 3-D cases are more complex. In three dimensions, the internal force acting on a small area dA of a plane that passes through a point P can be resolved into three components: one normal to the plane and two parallel to the plane (see Figure 1). The normal component divided by dA gives the normal stress (usually denoted σ), and the parallel components divided by the area dA give shear stresses (denoted τ in elementary textbooks). If the area dA is finite then, strictly, these are average stresses. In the limit, when dA approaches zero, the stresses become stresses at the point P. In general, stress varies from point to point and so is a tensor field.

The components of the stress tensor depend on the orientation of the plane that passes through the point under consideration, i.e on the viewpoint of the observer. This leads to the ridiculous conclusion that the stress on a structure, and hence its proximity to failure, depends on the viewpoint of the observer. However, every tensor, including stress, has invariants that don't depend on the choice of viewpoint. The length of a first-order tensor, i.e a vector, is a simple example. The existence of invariants means that the components seen by one observer are related, via the tensor transformation relations, to those seen by any other observer. The transformation relations for a second-order tensor like stress are different from those of a first-order tensor, which is why it is misleading to speak of the stress 'vector'. Mohr's circle method is a graphical method for performing stress (or strain) transformations.

Stress can be applied to all types of materials, including solids, liquids and gases. Static fluids can support normal or hydrostatic stress (pressure) but will flow under shear stress. Moving viscous fluids can support shear stress ('dynamic pressure'). Solids can support both shear and normal stress, even when static. Brittle fracture of solids is controlled by the maximum principal stress, which is a normal stress. Ductile fracture of solids is controlled by the Mises stress, which depends on the shear stresses. Some materials are however difficult to pigeonhole and their reaction to stress may be rate-dependent, i.e depend on the speed with which the stress is applied or temperature dependent. Toffee, for example, is a brittle solid at room temperature when the stress rate is high, a ductile solid when the stress rate is low or the temperature is higher and a viscous fluid when the temperature is raised even more.
All real objects occupy 3-dimensional space. However, if two dimensions are very large or very small compared to the others, the object may be modelled as one-dimensional. This simplifies the mathematical modelling of the object. One-dimensional objects include a piece of wire loaded at the ends and viewed from the side and a metal sheet loaded on the face and viewed up close and through the cross-section.

For 1-dimensional objects, the stress tensor has only one component and is indistinguishable from a scalar. The simplest definition of stress, σ = F/A, where A is the initial cross-sectional area prior to the application of the load, is called engineering stress or nominal stress. However, when any material is stretched, its cross-sectional area reduces by an amount that depends on the Poisson's ratio of the material. Engineering stress neglects this change in area. The stress axis on a stress-strain graph is often engineering stress, even though the sample may undergo a substantial change in cross-sectional area during testing.

True stress is an alternative definition in which the initial area is replaced by the current area. In engineering applications, the initial area is always known and so calculations using nominal stress are generally easier. For small deformation, the reduction in cross-sectional area is small and the distinction between nominal and true stress is insignificant. This isn't so for the large deformations typical of elastomers and plastic materials when the change in cross-sectional areas can be significant.

In one dimension, true stress is related to nominal stress via σtrue = σ(1 + ε) where ε is nominal strain and σ is nominal stress. In uniaxial tension, true stress is then greater than the nominal stress. The converse holds in compression.

Example: a steel bolt of diameter 5 mm, has a cross-sectional area of 19.6 mm2. A load of 50 N induces a stress (force distributed over the cross section) of σ = 50/19.6 = 2.55 MPa (N/mm2). This can be thought of as each square millimeter of the bolt supporting 2.55 N of the total load. In another bolt with half the diameter, and hence a quarter the cross-sectional area, carrying the same 50 N load, the stress will be quadrupled (10.2 MPa).

The ultimate tensile strength is a property of a material and is usually determined experimentally from a uniaxial tensile test. It allows the calculation of the load that would cause fracture. The compressive strength is a similar property for compressive loads. The yield stress is the value of stress causing plastic deformation.


All real objects occupy 3-dimensional space. However, if one dimension is very large or very small compared to the others, the object may be modelled as two-dimensional. This simplifies the mathematical modelling of the object. Two-dimensional objects include a piece of wire loaded on the sides and viewed up close and through the cross-section and a metal sheet loaded in-plane and viewed face-on.

Notice that the same physical, three-dimensional object can be modelled as one-dimensional, two-dimensional or even three-dimensional, depending on the loading and viewpoint of the observer.


Plane stress is a two-dimensional state of stress (Figure 2). This 2-D state models well the state of stresses in a flat, thin plate loaded in the plane of the plate. Figure 2 shows the stresses on the x- and y-faces of a differential element. Not shown in the figure are the stresses in the opposite faces and the external forces acting on the material. Since moment equilibrium of the differential element shows that the shear stresses on the perpendicular faces are equal, the 2-D state of stresses is characterized by three independent stress components (σx, σy, τxy). Note that forces perpendicular to the plane can be abbreviated. For example, σx is an abbreviation for σxx. This notation is described further below.





Cauchy was the first to demonstrate that at a given point, it is always possible to locate two orthogonal planes in which the shear stress vanishes. These planes are called the principal planes, while the normal stresses on these planes are the principal stresses. They are the eigenvalues of the stress tensor and are orthogonal because the stress tensor is symmetric (as per the spectral theorem). Eigenvalues are invariants with respect to choice of basis and are the roots of the Cayley–Hamilton theorem (although the term 'the' invariants usually means (I1,I2,I3)). Mohr's circle is a graphical method of extracting the principal stresses in a 2-dimensional stress state. The maximum and minimum principal stresses are the maximum and minimum possible values of the normal stresses. The maximum principal stress controls brittle fracture.







Engineers use Mohr's circle to find the planes of maximum normal and shear stresses, as well as the stresses on known weak planes. For example, if the material is brittle, the engineer might use Mohr's circle to find the maximum component of normal stress (tension or compression); and for ductile materials, the engineer might look for the maximum shear stress.


Augustin Louis Cauchy enunciated the principle that, within a body, the forces that an enclosed volume imposes on the remainder of the material must be in equilibrium with the forces upon it from the remainder of the body.

This intuition provides a route to characterizing and calculating complicated patterns of stress. To be exact, the stress at a point may be determined by considering a small element of the body that has an area ΔA, over which a force ΔF acts. By making the element infinitesimally small, the stress vector σ is defined as the limit:


Being a tensor, the stress has two directional components: one for force and one for plane orientation; in three dimensions these can be two forces within the plane of the area A, the shear components, and one force perpendicular to A, the normal component. Therefore the shear stress can be further decomposed into two orthogonal force components within the plane. This gives rise to three total stress components acting on this plane. For example in a plane orthogonal to the x axis, there can be a normal force applied in the x direction and a combination of y and z in plane force components.

The considerations above can be generalized to three dimensions. However, this is very complicated, since each shear loading produces shear stresses in one orientation and normal stresses in other orientations, and vice versa. Often, only certain components of stress will be important, depending on the material in question.

The von Mises stress is derived from the distortion energy theory and is a simple way to combine stresses in three dimensions to calculate failure criteria of ductile materials. In this way, the strength of material in a 3-D state of stress can be compared to a test sample that was loaded in one dimension.




Because the behavior of a body does not depend on the coordinates used to measure it, stress can be described by a tensor. In the absence of body moments, the stress tensor is symmetric and can always be resolved into the sum of two symmetric tensors:

a mean or hydrostatic stress tensor, involving only pure tension and compression; and
a shear or deviatoric stress tensor, involving only shear stress.
In the case of a fluid, Pascal's law shows that the hydrostatic stress is the same in all directions, at least to a first approximation, so can be captured by the scalar quantity pressure. Thus, in the case of a solid, the hydrostatic (or isostatic) pressure p is defined as one third of the trace of the tensor, i.e., the mean of the diagonal terms.



In the generalized stress tensor notation, the tensor components are written σij, where i and j are in {1;2;3}.

(caution: subscript notation in this section is different from the rest of the article - the order of subscripts is reversed)

The first step is to number the sides of the cube. When the lines are parallel to a vector base , then:

the sides perpendicular to are called j and -j; and
from the center of the cube, points toward the j side, while the -j side is at the opposite.


σij is then the component along the i axis that applies on the j side of the cube. (Or in books in the English language, σij is the stress on the i face acting in the j direction -- the transpose of the subscript notation herein. But transposing the subscript notation produces the same stress tensor, since a symmetric matrix is equal to its transpose.)



This generalized notation allows an easy writing of equations of the continuum mechanics, such as the generalized Hooke's law:


The correspondence with the former notation is thus:

x → 1
y → 2
z → 3
σxx → σ11
τxy → σ12
τxz → σ13
...


The fact that the Newtonian stress is a symmetric tensor follows from some simple considerations. The force on a small volume element will be the sum of all the stress forces over the surface area of that element. Suppose we have a volume element in the form of a long bar with a triangular cross section, where the triangle is a right triangle. We can neglect the forces on the ends of the bar, because they are small compared to the faces of the bar. Let be the vector area of one face of the bar, be the area of the other, and be the area of the "hypotenuse face" of the bar. It can be seen that


Let's say is the force on area and likewise for the other faces. Since the stress is by definition the force per unit area, it is clear that


The total force on the volume element will be:


Let's suppose that the volume element contains mass, at a constant density. The important point is that if we make the volume smaller, say by halving all lengths, the area will decrease by a factor of four, while the volume will decrease by a factor of eight. As the size of the volume element goes to zero, the ratio of area to volume will become infinite. The total stress force on the element is proportional to its area, and so as the volume of the element goes to zero, the force/mass (i.e. acceleration) will also become infinite, unless the total force is zero. In other words:


This, along with the second equation above, proves that the σ function is a linear vector operator (i.e. a tensor). By an entirely analogous argument, we can show that the total torque on the volume element (due to stress forces) must be zero, and that it follows from this restriction that the stress tensor must be symmetric.

However, there are two fundamental ways in which this mode of thinking can be misleading. First, when applying this argument in tandem with the underlying assumption from continuum mechanics that the Knudsen number is strictly less than one, then in the limit , the symmetry assumptions in the stress tensor may break down. This is the case of Non-Newtonian fluid, and can lead to rotationally non-invariant fluids, such as polymers. The other case is when the system is operating on a purely finite scale, such as is the case in mechanics where Finite deformation tensors are used.


The state of stress as defined by the stress tensor is an equilibrium state if the following conditions are satisfied:






σij are the components of the tensor, and f 1 , f 2 , and f 3 are the body forces (force per unit volume).

These equations can be compactly written using Einstein notation in which repeated indices are summed. Defining as the equilibrium conditions are written:


The equilibrium conditions may be derived from the condition that the net force on an infinitesimal volume element must be zero. Consider an infinitesimal cube aligned with the x1, x2, and x3 axes, with one corner at xi and the opposite corner at xi + dxi and having each face of area dA. Consider just the faces of the cube which are perpendicular to the x1 axis. The area vector for the near face is [ − dA,0,0] and for the far face it is [dA,0,0]. The net stress force on these two opposite faces is


A similar calculation can be carried out for the other pairs of faces. The sum of all the stress forces on the infinitesimal cube will then be


Since the net force on the cube must be zero, it follows that this stress force must be balanced by the force per unit volume fi on the cube (e.g., due to gravitation, electromagnetic forces, etc.) which yields the equilibrium conditions.

Equilibrium also requires that the resultant moment on the cube of material must be zero. Taking the moment of the forces above about any suitable point, it follows that, for equilibrium in the absence of body moments

σij = σji.
The stress tensor is then symmetric and the subscripts can be written in either order.


As with force, stress cannot be measured directly but is usually inferred from measurements of strain and knowledge of elastic properties of the material. Devices capable of measuring stress indirectly in this way are strain gages and piezoresistors.


The SI unit for stress is the pascal (symbol Pa), the same as that of pressure. Since the pascal is so small, engineering quantities are usually measured in megapascals (MPa) or gigapascals (GPa) In US Customary units, stress is expressed in pounds-force per square inch (psi). See also pressure.



Residual stresses are stresses that remain after the original cause of the stresses has been removed. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment. Heat from welding may cause localized expansion. When the finished weldment cools, some areas cool and contract more than others, leaving residual stresses. Castings may also have large residual stresses due to uneven cooling.

While uncontrolled residual stresses are undesirable, many designs rely on them. For example, toughened glass and prestressed concrete rely on residual stress to prevent brittle failure. Similarly, a gradient in martensite formation leaves residual stress in some swords with particularly hard edges (notably the katana), which can prevent the opening of edge cracks. In certain types of gun barrels made with two telescoping tubes forced together, the inner tube is compressed while the outer tube stretches, preventing cracks from opening in the rifling when the gun is fired. These tubes are often heated or dunked in liquid nitrogen to aid assembly.

Press fits are the most common intentional use of residual stress. Automotive wheel studs, for example, are pressed into holes on the wheel hub. The holes are smaller than the studs, requiring force to drive the studs into place. The residual stresses fasten the parts together. Nails are another example.


Hope this helps

Answer 2

Stress (mechanics), the average amount of force exerted per unit area. It is the internal resistance a material offers to being deformed and is measured in terms of the applied load. The effects of mechanical stress on a polymer can be measured using dynamic mechanical analysis. Strain (mechanics) geometrical measure of deformation representing the relative displacement between particles in a material body, i.e. a measure of how much a given displacement differs locally from a rigid-body displacement. Pressure (symbol: P) is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure. As an example of varying pressures, a finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a thumbtack can easily damage the wall. Although the force applied to the surface is the same, the thumbtack applies more pressure because the point concentrates that force into a smaller area. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. Unlike stress, pressure is defined as a scalar quantity.

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Answer 3

Pressure is the cause and stress is the effect.

If I put 400 lbs on a table that weighs 50 lbs it would be so much pressure that it would stress the wood of the table and cause it to bend or break.

If I have too many things to do and not enough time I feel the pressure and it causes me to become stressed.

Answer 4

Pressure is the amount of weight or strain forced into matter, while stress is the result or ending effect of what matter looks like after it has been strained. Effects will show. For instance, take a steel beam, the beam can only manage 14,000 pounds of pressure throughout its dimensions. Take three separate beams and place three each on either side of this main girder. Each other beam weighs 20,000 pounds a piece. Over a period of about five hours, the stress in the middle will show that the tempered steel has weakened considerably and cannot be used to build with.

Answer 5

Pressure may be a lot of demands on your time or energy but you can handle it. Pressure is an external demand to you.

Stress is when the pressure gets to you and your body and mind begin to feel the results of the pressure. This can cause health problems if it's over a long period of time.

For example, I can put pressure on you to study harder. You may just reject that and say "whatever". Even if you ackowledge my request and you do decide to study harder, it may still be very easy for you. But stress is when you now internally feel that you have reached point where you "fee" the pressure and are worried that you need to keep up with the demands.

Make sense? Sort of?

Answer 6

Mathematically, there is not a difference; both are measurements of force distributed across area. Typically, though, pressure refers to the force exerted by a fluid, and stress refers to tensile and compressive stresses in solid objects. Both can apply to the same problem, as when the pressure of a contained fluid causes stresses in its container.

Answer 7

In work you can be under pressure to complete a task but have no stress in doing it. Without stress you are able to complete the task effectively and efficiently. If you are under stress you can not complete the task in the given time. You may even mess it up.

Answer 8

pressure causes stress

Answer 9

I think pressure can make you more competitive, but stress sends your body into a negative cycle that is harmful.

Answer 10

Pressure is measured in weight per unit area while stress
is strain multiplied by its elasticity...