Hardness Test
Simply stated, hardness is the resistance of a material to permanent indentation. It is important to recognize that hardness is an empirical test and therefore hardness is not a material property. This is because there are several different hardness tests that will each determine a different hardness value for the same piece of material. Therefore, hardness is test method dependent and every test result has to have a label identifying the test method used.
Hardness is, however, used extensively to characterize materials and to determine if they are suitable for their intended use. All of the hardness tests described in this section involve the use of a specifically shaped indenter, significantly harder than the test sample, that is pressed into the surface of the sample using a specific force. Either the depth or size of the indent is measured to determine a hardness value.
Why Use a Hardness Test?
Easy to perform
Quick - 1 to 30 seconds
Relatively inexpensive
Non-destructive
Finished parts can be tested - but not ruined
Virtually any size and shape can be tested
Practical QC device - incoming, outgoing
The most common uses for hardness tests is to verify the heat treatment of a part and to determine if a material has the properties necessary for its intended use. Establishing a correlation between the hardness result and the desired material property allows this, making hardness tests very useful in industrial and R&D applications.
Hardness Scales
There are five major hardness scales:
Brinell - HB
Knoop - HK
Rockwell - HR
Shore - HS
Vickers - HV
Each of these scales involve the use of a specifically shaped diamond, carbide or hardened steel indenter pressed into the material with a known force using a defined test procedure. The hardness values are determined by measuring either the depth of indenter penetration or the size of the resultant indent. All of the scales are arranged so that the hardness values determined increase as the material gets harder. The hardness values are reported using the proper symbol, HR, HV, HK, etc. indicating the test scale performed.
Five Determining Factors
The following five factors can be used to determine the correct hardness test for your application.
Material - grain size, metal, rubber, etc.
Approximate Hardness - hardened steel, rubber, etc.
Shape - thickness, size, etc.
Heat Treatment – through or casehardened, annealed, etc.
Production Requirements - sample or 100%
Saturday, 31 January 2009
Thursday, 29 January 2009
Nondestructive testing
The need for NDT
It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture.
During their service lives, many industrial components need regular non-destructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:
Aircraft skins need regular checking to detect cracks;
Underground pipelines are subject to corrosion and stress corrosion cracking;
Pipes in industrial plants may be subject to erosion and corrosion from the products they carry;
Reinforced concrete structures may be weakened if the inner reinforcing steel is corroded;
Pressure vessels may develop cracks in welds;
The wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important.
Finished machined parts, such as bearings, that have newly been assembled can be tested for missing pieces, such as a ball or roller bearing, or grease within the housing non-destructively with a checkweigher. A roller motor for a conveyor can be tested for the proper level of oil, without disassembling the finished product. Thousand of manufactured products can benefit from this form of testing.
Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by fatigue — a function that is now carried out by instrumentation and referred to as the acoustic impact technique.
Use of X-rays for NDT is a common way of examining the interior of products for voids and defects, although some skill is needed in using radiography to examine samples and interpret the results. Soft X-rays are needed for examining low density material like polymers, composites and ceramics.
Methods and techniques
NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT.
Shopping
It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture.
During their service lives, many industrial components need regular non-destructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:
Aircraft skins need regular checking to detect cracks;
Underground pipelines are subject to corrosion and stress corrosion cracking;
Pipes in industrial plants may be subject to erosion and corrosion from the products they carry;
Reinforced concrete structures may be weakened if the inner reinforcing steel is corroded;
Pressure vessels may develop cracks in welds;
The wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important.
Finished machined parts, such as bearings, that have newly been assembled can be tested for missing pieces, such as a ball or roller bearing, or grease within the housing non-destructively with a checkweigher. A roller motor for a conveyor can be tested for the proper level of oil, without disassembling the finished product. Thousand of manufactured products can benefit from this form of testing.
Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by fatigue — a function that is now carried out by instrumentation and referred to as the acoustic impact technique.
Use of X-rays for NDT is a common way of examining the interior of products for voids and defects, although some skill is needed in using radiography to examine samples and interpret the results. Soft X-rays are needed for examining low density material like polymers, composites and ceramics.
Methods and techniques
NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT.
Shopping
Monday, 26 January 2009
Sunday, 25 January 2009
Concrete slump test
Concrete slump test
In construction and civil engineering, the Concrete Slump Test (or simply the Slump Test) is an in situ test or a laboratory test used to determine and measure how hard and consistent a given sample of concrete is before curing.
The Concrete Slump Test is, in essence, a method of quality control. For a particular mix, the slump should be consistent. A change in slump height would demonstrate an undesired change in the ratio of the concrete ingredients; the proportions of the ingredients are then adjusted to keep a concrete batch consistent. This homogeneity improves the quality and structural integrity of the cured concrete
The Concrete Slump Test is, in essence, a method of quality control. For a particular mix, the slump should be consistent. A change in slump height would demonstrate an undesired change in the ratio of the concrete ingredients; the proportions of the ingredients are then adjusted to keep a concrete batch consistent. This homogeneity improves the quality and structural integrity of the cured concrete
Purpose
The goal of the Concrete Slump Test is to measure the consistency of concrete. Many factors are taken into account when satisfying requirements of concrete strength, and to make sure that a consistent mixture of cement is being used during the process of construction. The test also further determines the “workability” of concrete, which provides a scale on how easy is it to handle, compact, and cure concrete[3]. Engineers use the results to then alter the concrete mix by adjusting the cement/water ratios or adding plasticizers to increase the strength of the outcome concrete mix.M
Procedure
The Concrete Slump Test has witnessed many technological advances, and some countries even perform the test using automated machinery. The simplified, generally accepted method to perform the test is as follows:
Apparatus
Large pan
Trowel to mix concrete mixture
Steel tamping rod
Slump cone
Ruler
Concrete (Cement, water, sand & aggregates).
Steps
Place the mixing pan on the floor and moisten it with some water. Make sure it is damp but no free water is left.
Place the sand in the pan. Add the cement and mix it with the sand.
Add the coarse/fine aggregate and thoroughly mix.
Mix the water and dry cement ingredients thoroughly using the trowel.
Firmly hold the slump cone in place using the 2 foot holds.
Fill one-third of the cone with the concrete mixture. Then tamp the layer 25 times using the steel rod in a circular motion, making sure not to stir.
Add more concrete mixture to the two-thirds mark. Repeat tamping for 25 times again. Tamp just barely into the previous layer(1")
Fill up the whole cone up to the top with some excess concrete coming out of top, then repeat tamping 25 times. (if there is not enough concrete from tamping compression, stop tamping, add more, then continue tamping at previous number)
Remove excess concrete from the opening of the slump cone by using tamping rod in a rolling motion until flat.
Slowly and carefully remove the cone by lifting it vertically (5 seconds +/- 2 seconds), making sure that the concrete sample does not move.
Wait for the concrete mixture as it slowly slumps.
After the concrete stabilizes, measure the slump-height by turning the slump cone upside down next to the sample, placing the tamping rod on the slump cone and measuring the distance from the rod to the ORIGINAL DISPLACED CENTER.
The goal of the Concrete Slump Test is to measure the consistency of concrete. Many factors are taken into account when satisfying requirements of concrete strength, and to make sure that a consistent mixture of cement is being used during the process of construction. The test also further determines the “workability” of concrete, which provides a scale on how easy is it to handle, compact, and cure concrete[3]. Engineers use the results to then alter the concrete mix by adjusting the cement/water ratios or adding plasticizers to increase the strength of the outcome concrete mix.M
Procedure
The Concrete Slump Test has witnessed many technological advances, and some countries even perform the test using automated machinery. The simplified, generally accepted method to perform the test is as follows:
Apparatus
Large pan
Trowel to mix concrete mixture
Steel tamping rod
Slump cone
Ruler
Concrete (Cement, water, sand & aggregates).
Steps
Place the mixing pan on the floor and moisten it with some water. Make sure it is damp but no free water is left.
Place the sand in the pan. Add the cement and mix it with the sand.
Add the coarse/fine aggregate and thoroughly mix.
Mix the water and dry cement ingredients thoroughly using the trowel.
Firmly hold the slump cone in place using the 2 foot holds.
Fill one-third of the cone with the concrete mixture. Then tamp the layer 25 times using the steel rod in a circular motion, making sure not to stir.
Add more concrete mixture to the two-thirds mark. Repeat tamping for 25 times again. Tamp just barely into the previous layer(1")
Fill up the whole cone up to the top with some excess concrete coming out of top, then repeat tamping 25 times. (if there is not enough concrete from tamping compression, stop tamping, add more, then continue tamping at previous number)
Remove excess concrete from the opening of the slump cone by using tamping rod in a rolling motion until flat.
Slowly and carefully remove the cone by lifting it vertically (5 seconds +/- 2 seconds), making sure that the concrete sample does not move.
Wait for the concrete mixture as it slowly slumps.
After the concrete stabilizes, measure the slump-height by turning the slump cone upside down next to the sample, placing the tamping rod on the slump cone and measuring the distance from the rod to the ORIGINAL DISPLACED CENTER.
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Saturday, 24 January 2009
Cable-Stayed Bridge
Radial attachment pattern
Parallel attachment pattern
Cable-Stayed Bridge
Cable-stayed bridges may look similar to suspensions bridges—both have roadways that hang from cables and both have towers. But the two bridges support the load of the roadway in very different ways. The difference lies in how the cables are connected to the towers. In suspension bridges, the cables ride freely across the towers, transmitting the load to the anchorages at either end. In cable-stayed bridges, the cables are attached to the towers, which alone bear the load.The cables can be attached to the roadway in a variety of ways. In a radial pattern, cables extend from several points on the road to a single point at the top of the tower. In a parallel pattern, cables are attached at different heights along the tower, running parallel to one other.
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