Photoelastic strain gages

New photoelastic strain gages are described which are self-contained and give an accurate direct indication of magnitudes and directions of principal stresses. Large numbers of gages may be observed simultaneously and critical areas of members may be readily located. Various types of gages are applicable to high-termperature problems, dynamic measurements or direct photographic recording.     

Bendable strain gages?

Experimental Techniques -     

Bonding strain gages in small openings

Narrow openings have often complicated the bonding of strain gages because of the physical restrictions of the working area. A simple method has been devised to overcome this limitation by using a mock-up of the surface to which the gages will be bonded. The gages are bonded to the mock-up and then transferred to the surface of the member to be stressed. The technique was developed for use inisde pipes; however, it is directly applicable to many types of irregular configurations with limited access.     

Transverse sensitivity of bonded strain gages

Transverse sensitivity of strain gages usually causes unavoidable errors in two-dimensional stress analyses. Formulas have been derived for correcting these errors but involve transverse-sensitivity values which, in most cases, were not available to the user. This paper describes a method for determination of transverse-sensitivity values using a testing apparatus specially built for this purpose. Results from testing a wide variety of wire and foil strain gages are tabulated and mechanisms of the foil-gage transverse sensitivity discussed.     

Using strain gages to measure both strain and temperature

A transducer is proposed that measures both temperature and strain by using two different strain gages. The theoretical basis for designing such a transducer is developed. Experimental results indicate that the temperature signal can be measured satisfactorily. Paper was presented at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 10–13.     

Glass-bonding techniques for semiconductor strain gages

Conventional organic-epoxy adhesives outgas when exposed to ultra-high vacuum and, as operating temperatures are increased, they begin to exhibit plastic behavior causing hysteresis and zero instability in the transducer. The use of an inorganic glass as the bonding material has resulted in a significant advance in transducer-fabrication technology for the following reasons:

1. The outgassing of transducers in high-vacuum applications is minimized.
2. Mechanical properties of the transducer such as hysteresis and repeatability are improved.
3. The electrical isolation of the strain gages from the metallic elements of the transducer is increased at high temperatures over that provided by epoxy. Also, the glass bond can survive and operate in severe radiation environments, wherein the epoxy adhesive will suffer either temporary or permanent loss of its dielectric strength.
4. Glass-bonding techniques are particularly useful for the extension of the temperature range of operation of silicon-strain-gage transducers.

Nispan C and 440-C stainless-steel substrates were successfully used with glass-bonded silicon strain gages to fabricate transducers for evaluation.     

Transient response of bonded strain gages

Transient-response characteristics of bonded strain gages have been studied by measuring the elastic step wave produced in a steel bar. A quenched-steel bar with a circumferential notch is stretched statically along its axis until it fractures at the notch. Thus a new cross section is suddenly created in a bar under static tension. At this moment, a sharp step wave of zero stress is produced from the new section, and it travels in the bar at the velocity of sound. By measuring this step wave with strain gages, it is shown that the rise time of the gage itself is less than 0.5 μs+0.8L/c, whereL is the gage length andc is the velocity of longitudinal elastic wave in the bar.     

Biaxial gage factors for piezoresistive strain gages

Theoretical considerations of piezoresistive strain gages show that the change in electrical resistivity depends on the biaxial state of strain at the surface of the specimen to which the gage is bonded. In particular, whenV is the initial voltage across the gage and ( \( \in _{11} , \in _{22} , \in _{12} \) ) is the surface-strain state at the point of attachment, the gage-voltage change ΔV is given by \(\frac{{\Delta V}}{V} = G_{11} \in _{11} + G_{22} \in _{22} + G_{12} \in _{12} \) whereG ₁₁,G ₂₂ andG ₁₂ are the biaxial gage factors. Experiments were conducted on a nominally one-dimensional gage. Kulite type DLP-120-500, bonded to a standard ASTM flat tensile specimen of CR 1018 steel. For this gage, typical values were found to beG ₁₁?26,G ₂₂??1.4 andG ₁₂??1.1. SinceG ₂₂ andG ₁₂ are less than 6 percent ofG ₁₁, it is concluded that contributions from these two factors (called transverse and shear sensitivities) will be significant only when the gage is oriented such that \( \in _{11}<< \left( { \in _{22} , \in _{22} } \right)\) . However, in the interest of completeness and accuracy, all biaxial gage factors should be reported.     

The interferometric strain gage

The interferometric strain gage consists of two very shallow grooves ruled on a highly polished surface. The grooves are cut with a diamond and are 4×10^?5 in. deep and 5×10^?3 in. apart. Coherent, monochromatic light from a He?Ne gas laser incident upon these grooves will produce fringe patterns. A fringe pattern with the fringes parallel to the grooves is formed on each side of the impinging beam. The position of these patterns in space is related to the distance between the two grooves. As this distance changes, the fringes shift. Measurement of these fringe shifts enables one to determine the local strain of the specimen. In this paper, the theory of the measurement is developed first. The strain, ∈, is given by ∈=ΔFλ/d _o sin α _o where ΔF is the average fringe shift of the two patterns, λ is the wavelength of light,d _o is the initial distance between grooves, and α _o is the angle between the incident light beam and the fringe patterns. A procedure for making static measurements with the interferometric strain gage is presented. The sensitivity for these measurements is 0.5 percent strain per fringe shift, and the maximum strain is 4 percent. The method is evaluated by comparing its results with other accepted means of measuring large plastic strain. These other techniques are: post-yield foil gages, a 2-in. clip gage, and an Instron testing machine. The average percent difference among these techniques is less than 0.4 percent based on a full-scale measurement of 4-percent strain. The interferometric strain gage has the following features: a gage integral with the specimen surface, a very short gage length, relatively easy application, and the ability to measure large strains.     

Strain gages for long-term high-temperature strain measurement

The capabilities of resistance and capacitance strain gages for use in the temperature range of 1100–1400°F for durations up to 10,000 h are reviewed. Characteristics are given in summary and tabular form along with a list of references. Resistance strain gages offer some capability under these conditions, but the newer capacitance gages show promise for both laboratory and field use at high temperatures.     

The response of strain gages to longitudinally sweeping strain pulses

The current state-of-the-art for estimating the maximum rise time of a strain gage which is subjected to an axially sweeping strain pulse ist _rg≤0.8L/c+0.5μsec wheret _rg is the 10/90 rise time of the strain gage,^* L is the gage length andc is the strain-pulse velocity. This paper shows that the effect of the 0.8L/c term can be significantly reduced by utilizing an analytical compensation technique. In addition, it is shown that the 0.5 μsec additive constant can be reduced to approximately 0.1 μsec by reinterpreting data published by a previous author.