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denis-greek [22]
3 years ago
9

Find the True statement

Engineering
2 answers:
dmitriy555 [2]3 years ago
6 0

Answer:

Option A is correct ( Stress relaxation is time- and temperature-dependent).

Explanation:

Luda [366]3 years ago
6 0

Answer:

Option A is correct.

The only completely true statement in all of the available options is that "Stress relaxation is time- and temperature-dependent"

Explanation:

Stress relaxation is the constant decrease of stress with time in a material which is exposed to a constant strain at a constant temperature.

It is similar in concept to vaccination; in that, the material is being prepared to be able to undergo a particular amount of stress at a specific temperature in order to make the stress levels in the material lower than it usually should be.

It is evident that stress relaxation, (the extent), in a material completely depends on the amount of time that the constant strain is applied and the temperature at which this whole process occurs.

Polymers always have a melting temperature.

While it is true that almost all materials can melt. For Polymers, it is not necessarily true that the melting takes place at a constant temperature. In very complex Polymers, melting occurs over an appreciable range of temperature, not sharply as in some simple polymers.

Hence, polymers don't always have a melting temperature, some polymers melt over a range of temperature. So, this statement isn't completely true.

Polyethylene becomes weaker if H is replaced with other radicals inside branches.

This also is a statement that isn't always true. Replacing the H in some of the branches of polyethylene with radicals such as the carboxylic group, the hydroxyl group etc., have been shown to seriously strengthen the polyethylene. Polymers formed by replacing the H with ion-Like radicals are known as ionomers.

These groups have a polarity, hence, they are drawn together by their charges, come together in microdomains, toughening and strengthening the polyethylene without taking its ability to be cast to permanent shapes away.

This statement isn't always true too.

Crazing indicates cracks inside the polymer.

This is also false. Crazing doesn't indicate cracks in materials. What it does is that Crazing precedes the cracks.

Crazing is a phenomenon that entails the formation of microvoids in a material in response to excessive tensile stress being applied to the material. The microvoids (which forms in a plane normal to the tensile stress that caused them) often leads to cracks or fracture later.

So, Crazing doesn't indicate cracks, rather, it precedes them in most cases.

Hope this Helps!!!

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At a point on the free surface of a stressed body, the normal stresses are 20 ksi (T) on a vertical plane and 30 ksi (C) on a ho
victus00 [196]

Answer:

The principal stresses are σp1 = 27 ksi, σp2 = -37 ksi and the shear stress is zero

Explanation:

The expression for the maximum shear stress is given:

\tau _{M} =\sqrt{(\frac{\sigma _{x}^{2}-\sigma _{y}^{2}  }{2})^{2}+\tau _{xy}^{2}    }

Where

σx = stress in vertical plane = 20 ksi

σy = stress in horizontal plane = -30 ksi

τM = 32 ksi

Replacing:

32=\sqrt{(\frac{20-(-30)}{2} )^{2} +\tau _{xy}^{2}  }

Solving for τxy:

τxy = ±19.98 ksi

The principal stress is:

\sigma _{x}+\sigma _{y} =\sigma _{p1}+\sigma _{p2}

Where

σp1 = 20 ksi

σp2 = -30 ksi

\sigma _{p1}  +\sigma _{p2}=-10 ksi (equation 1)

\tau _{M} =\frac{\sigma _{p1}-\sigma _{p2}}{2} \\\sigma _{p1}-\sigma _{p2}=2\tau _{M}\\\sigma _{p1}-\sigma _{p2}=32*2=64ksi equation 2

Solving both equations:

σp1 = 27 ksi

σp2 = -37 ksi

The shear stress on the vertical plane is zero

4 0
3 years ago
What are the optical properties of steel
dezoksy [38]

Answer:

A selective surface with large absorption for solar radiation and high reflectance for thermal infrared radiation was produced by use of surface oxidation of stainless steel. The surfaces were studied for use with concentrated light in a solar power plant at temperatures of 400°C and higher.

In order to investigate the relation between surface treatment and optical properties, stainless steels (AISI 304 and 430) which were submitted to different chemical and mechanical surface treatments, were used. To increase the spectral selectivity, these surfaces were treated in air and in vacuum at different temperatures and times. The optical properties of these films were investigated. Visual and infrared spectral absorptances were measured at room temperature. The thermal hemispherical emittance and absorptance were obtained by a calorimetric method at 200°C. It was noticed that these chemically and mechanically treated stainless steel surfaces have good spectral properties without further oxidations. This is very important for high temperature uses. The best values are found for samples 7 and 8 under vacuum and air. These two samples with mechanically ground surfaces retained their selectivity and specularity after several hours oxidation. One can conclude that the surface ground treatment confers good selectivity on the steel surfaces for use in concentrating solar collectors with a working temperature of 500°C.

Sample surfaces were subjected to long temperature ageing tests in order to gain some idea of the thermal stability of the surfaces. The results promise better-performing surface and the production of durable selective finishes at, possibly, lower cost than competing processes.

Explanation:

3 0
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