University Of Jordan Faculty of Engineering and Technology

Industrial Engineering Dep.

Metrology and Instrumentation

Course Project

Instructor: Dr. Belal Gharaibeh

Part 1: Investigate the techniques used to measure strain using methods other than electrical circuit types discussed in class. Provide at least two methods explaining the name, method and techniques, application range, accuracy and comparison to other methods.

Method 1 Extensometer:

An extensometer is a device that is used to measure small/big changes in the length of an object. It is useful for stress-strain measurements and tensile tests. Its name comes from "extension-meter". It was invented by Dr. Charles Huston.

Types:

There are two main types of extensometers: contact and non-contact.

1. Contact Contact extensometers have been used for many years and are also subdivided into two further categories. The first type of contact extensometer is called a clip-on extensometer. These devices are used for applications where high precision strain measurement is required (most ASTM based tests). They come in many configurations and can measure displacements from very small to relatively large (less than an mm to over 100 mm). They have the advantage of lower cost and ease of use; however they can influence small / delicate specimens. For automated testing clip-on devices have been largely replaced by digital "sensor arm" extensometers. These can be applied to the specimen automatically by a motorized system and produce much more repeatable results than the traditional clip-on devices. They are counter balanced and so have negligible effect on the specimen. Better linearity, reduced signal noise and synchronization with the corresponding force data are big advantages due to the lack of analogue to digital converters and associated filters which add time lags and smooth the raw data. In addition these devices can remain on the specimen until failure and measure very high extensions (up to 1000 mm) without losing any accuracy. These devices typically have resolutions of 0.3 µm or better (the highest quality devices can read values as low as 0.02 µm) and have sufficient measurement accuracy to meet class 1 and 0.5 of ISO 9513.

2. Non-contact

For certain special applications, non-contact extensometers are beginning to bring advantages where it is impractical to use a feeler arm or contact extensometer.

Laser A laser extensometer is an extensometer capable of performing strain or elongation measurements on certain materials when they are subjected to loading in a tensile testing machine. The principle works by illuminating the specimen surface with a laser, the reflections from the specimen surface are then received by a CCD camera and processed by complex algorithms. When using a laser extensometer it is not necessary to attach marks to the specimen, bringing substantial time savings for material testing laboratories.

Resolutions less than one micrometer (typically 0.1 μm) and elongations up to 900 mm can be achieved, which renders these devices suitable for the most complex testing.

Laser extensometers are used primarily for materials which may damage a traditional "clip-on" extensometer, or where the mass of the clip on device affects the material properties, due to being physically attached to the specimen.

Laser extensometers can also be used for testing at elevated or sub zero temperatures.

Video

A video extensometer is a device that is capable of performing stress/strain measurements of certain materials, by capturing continuous images of the specimen during test, using a frame grabber or a digital video camera attached to a PC. The specimen of the material under test is usually cut in a specific shape and is marked with special markers (usually special stickers or with pens that distinguishes the marker from the specimen color and texture in the captured image).The pixel distance between these markers in the captured image are constantly tracked in the captured video, while the specimen under test is stretched / compressed. This pixel distance can be measured in real time and mapped against a calibration value to give a direct strain measurement, and to control the testing machine in strain control, if required.

With a proper calibration value and good image processing algorithms, resolution of much less than one micrometer (is) can be achieved. Proper calibration value also depends on the calibration specimen which is usually a specially etched material with great precision. To calibrate, pictures are first captured with the calibration specimen under the same testing conditions to be used for the new specimen.

Video extensometers are used primarily for materials which may damage a traditional contact or digital "feeler arm" extensometer. In some applications the video extensometer is replacing mechanical measurement units - but this is mainly clip-on devices.

When measuring the modulus of elasticity on 50 mm gauge length plastics to ISO 527 an accuracy of 1 µm is required. Some video extensometers cannot achieve this, whilst for production testing it is better to use automated motorized digital extensometry to avoid operator's manually attaching marks to the specimen, and spending time setting and adjusting the system. Note that some video extensometers have difficulty in achieving acceptable results when used to measure strain within temperature chambers.

For applications demanding high accuracy, non-contact strain measurement, and video extensometers are a proven solution. In certain test applications they are superior to other technologies, such as laser speckle because of the ability to measure strain over a large range. This allows measurements such as modulus to be determined as well as strain at failure.

Changing of ambient light conditions during the test can affect the test results if the video extensometer does not utilize appropriate filters both over the lighting array and lens. Systems with this technology remove all effects of ambient lighting conditions.

Mining

In the mining environment, extensometers are used to measure displacements on batters/high walls. Plotting displacement vs. time enables Geotechnical engineers to determine if wall failures are imminent. For complicated failures, further equipment such as radar or laser scans are used enabling 3-dimensional and ultimately 4-dimensional analysis.

Method 2 Photoelastic Strain Gages: A strip which is made from optically stressed active material exhibits an isochromatics fields a result of a “frozen”, continually increasing stress. The isochromatics become displaced as a result of the strain. The degree of displacement which is read off a scale is a measure of the strain. Strain gages of this type are made in the USA. They have achieved no practical significance and are no longer commercially available. Method 3 Semiconductor Strain Gages:

Apart from metal strain gages there are other types of electrical resistive strain gages. Semiconductor strain gages Belong to this group and they extend the range of applications in strain gage technology. The measurement principle is based on the semiconductor piezoresistive effect discovered by C.S. Smith In 1954. Initially germanium was used, which was later superseded by silicon. In construction, semiconductor strain gages are substantially the same as metal strain gages. The measuring element consists of a strip a few tenths of a millimeter wide and anew hundredths of a millimeter thick which is fixed to an insulating carrier foil and is provided with connecting leads. Diode effects are prevented by using a thin gold wire as connection between the semiconductor element and the connecting strips. B) A)

The gage factor for normally available semiconductor strain gages, i.e. the ratio between the measured strain and the signal given by the strain gage, is about fifty to sixty times that of metal strain gages. They are therefore mainly used in transducer manufacture for the measurement of other physical quantities, being supplemented by simple electronic devices to form transmitters. Semiconductor strain gages are not widely used in experimental stress analysis and there are a number of reasons for this: - The non-linear characteristics of the semiconductor strain gage call for measurement correction demand high accuracy. - Semiconductor strain gages are substantially more expensive than metal types

Even when the greater sensitivity is taken into account, the adverse temperature dependent effects are more severe with semiconductor strain gages than with metal ones and these effects are more difficult to compensate. - Handling is more difficult due to the semiconductor's brittle nature.On the other hand the high sensitivity is a reason for using semiconductor strain gages forth measurement of very small strains.

The large signal given by this type of strain gage is of particular advantage in the presence of strong interference fields. Apart from conventional strain gages, there are other types which are only mentioned here for the sake of completeness and are not treated in great detail.

Method 4 Vapor-deposited (thin-film) strain gages. Electrical resistive strain gage is provided by vapor deposition techniques.Here the measuring element is directly deposited onto the measurement point under avacuum by the vaporization of the alloy constituents. The range of applications is restricted to the production of transducers.

A “measuring grid”

B carrier material

C intermediate gold conductor

D connecting strips

Part 2: Estimate the type of fit for the following four drawings. Use the tables in the lecture slides provided earlier or any other tables for the fit type specified for each shaft/hole combination. Full grades only for solutions made on drawing software like Pro-E or Auto-Cad.: A-

B-

30H7r6 Hole H7 Shaft s6 U.T +0.021 +0.048 L.T 0.000 +0.035

Basic Si 30 30 U.L 30.021 30.048 L.L 30.000 30.035

Fit Type Interference Fit

30.035

30.035

30.021

30.000

30H8e6 Hole H8 Shaft e6 U.T +0.033 +0.048 L.T 0.000 +0.035 B.S 30 30 U.L 30.033 29.977 L.L 30.000 29.960

Fit Type Clearance Fit

30.033

30.000

29.977

29.960

C-

30F8r6 Hole F8 Shaft r6 U.T +0.053 +0.041 L.T + 0.020 +0.028 B.S 30 30 U.L 30.053 30.041 L.L 30.020 30.028

Fit Type Transition fit

D-

30G7r6 Hole F8 Shaft r6 U.T +0.028 +0.041 L.T + 0.007 +0.028 B.S 30 30 U.L 30.028 30.041 L.L 30.007 30.028

Fit Type Interference Fit