Photoelastic investigation on the Crack-arrest capability of a pretensioned stiffened plate

Dynamic photoelasticity, employing a 16-spark-gap Cranz-Schardin camera system, was used for dynamic analysis of propagating cracks in stiffened panels. The method of finite elements was used for a corresponding static analysis. Photoelastic models included 0.009525×0.25×0.25 m Homalite-100 plates with 10- and 25-percent pin-joined and bonded stringers. Static and dynamic strain-energy release rates, kinetic-energy release rates and stringer-load concentration factors were determined in stiffened panels that were pretensioned and then impacted by a projectile. It was found that the arrest capability of a stiffened panel could be assessed through the kinetic-energyrate concept. Also bonded stiffeners were found to be more effective in arresting a propagating crack than a corresponding pin-joined stiffened panel.     

Photoelastic investigation of a shrink-fit problem

Stress analysis of the components of a shrinkfit assembly has been carried out by the three-dimensional photoelastic technique. Experimental results obtained from the analysis of a typical shrink-fit assembly, wherein a cylindrical wheel is shrink fitted onto a hollow cylindrical hub in an asymmetrical position. Photoelastic models made from hot-setting Araldite B were loaded at the critical temperature in a stress-freezing oven and the meridional and the hoop slices were observed in a transmission polariscope. It is concluded from this investigation that the bending induced in the components due to the asymmetrical placement of the wheel and the contact shear at the interface influences the stress distribution quite considerably. Also, the dimensions of the components in the axial direction have an influence on the mode and contribute to the magnitude of the variation of stress components.     

Photoelastic investigation of crack-inclusion interaction

An experimental procedure is presented for determining the mode I stress-intensity factor of an edge crack with a nearby rigid elliptical inclusion in a finite plate loaded in uniform tension. The rigid inclusion was modeled by bonding two identical steel inclusions on to the faces of a thin plate of polycarbonate. Models were constructed with edge cracks and various inclusion geometries so that the effect of parameters such as inclusion shape, orientation, and cracktip position on the stress-intensity factors of the crack could be determined. Photoelasticity experiments were used for this investigation and the results were compared to the results of a similar theoretical analysis of the interaction between a crack and an inclusion in an infinite plate. A good correlation was found between the experimental and theoretical models indicating that the results may help provide a better understanding of the toughening mechanisms in materials such as short-fiber-reinforced composites and ceramics. This experimental method is relatively easy to use making it an attractive candidate to be applied to similar problems involving cracks and inhomogeneities.     

Photoelastic investigation of a channel cover of bleed condenser

This paper describes the investigation on an epoxy model of a channel cover of bleed condenser, by three-dimensional photoelasticity. The channel cover is a thick plate with two nozzles which are connected on one side to pipes carrying water vapor/water under high pressure. A flat tube plate is connected on the other side by means of sixteen bolts around the periphery with high initial bolt forces. The photoelastic model of the channel cover was made by machining a cast epoxy block (Araldite CT200). The stress distribution along critical sections of the model and the regions of maximum stresses and their values were determined. The prototype stresses were calculated from experimentally obtained values of model stresses, using simiilitude laws.     

Photoelastic investigation of plane wedge-shaped drawing die

A photoelastic investigation is conducted for plane wedge-shaped dies with special reference to the distribution of pressure and friction on the contact surface. A softened celluloid strip is drawn through the gap of Araldite dies kept in the state of glassy elastic. The results are compared with the theoretical results by other investigators.     

Photoelastic analysis of a thick plate with an elliptical hole subjected to simple out-of-plane bending

A three-dimensional photoelastic analysis using the stress freezing and slicing techniques was employed to study the stress distribution and the stress-concentration factors around an elliptical hole in a plate of finite thickness. The plate was subjected to simple out-of-plane bending. A special bending device was designed to produce uniform bending moment at the two opposite free edges of the plate. Six plates with various elliptical holes were studied. The stress variation across the plate thickness at the periphery of the elliptical hole was also investigated. The experimental results were correlated with the existing theoretical solutions.     

Photoelastic and strain-gage investigation of penetration of thin plates

An experimental investigation of the central normal impact and penetration of plates of Columbia Resin No. 39, simply supported at the corners, was conducted using both photoelastic and strain-gage techniques. Both conically-pointed and blunt-nosed steel projectiles of 1/4-in. diam were employed at initial velocities ranging from 191 to 272 fps. Sequences of fringe photographs were obtained by repeating shots on different plates, using various predetermined time intervals between impact and photograph. A dynamic value of Young's modulus has been calculated from the velocity of propagation of tension waves, and a strain-optic coefficient of 398 microinches per fringe for the material was determined from a quasi-static test. Both fringe order and strain histories for various impact conditions have been ascertained and are compared.Paper was presented at 1961 SESA Spring Meeting held in Philadelphia, Pa., on May 10–12.     

Photoelastic investigation of large deflections by low-modulus photoelastic materials

Information is given to enable the experimental analysis of large deformations by application of photoelastic techniques. Promising photoelastic materials having low elastic modulus and high optical sensitivity were obtained for practical use at room temperature. The time and temperature dependence of the stress-fringe order and stress-strain relations of one of these viscoelastic materials is discussed. Photoelastic investigations of large deflected straight and circular beams are reported. Photoelastic technique seems to be a useful method for analysis of large deformations of models with complicated shapes.     

Photoelastic investigation of metal-forming processes using new sapphire dies

Experimental Techniques -     

Photoelastic stress analysis of an elliptical hole in a thick plate subjected to uniform in-plane compressive Loading

A three-dimensional photoelastic analysis was conducted to determine the stress distribution and concentration around the periphery of a centrally located elliptical hole in a plate of finite thickness. The edge of the plate was subjected to a uniformly distributed compressive uniaxial in-plane load. The principle of superposition was employed to study the effect of uniform biaxial loading.Elliptical holes with five different major/minor axis ratios () ranging from 1.0 to 2.64 were investigated. Among the results of this study, it was established that the variation of the principal stresses at the edge of the hole is not linear across the plate thickness. It was also found that in loading the plate in a direction parallel to the major axis of the ellipse, the value of the maximum tangential principal stress () occurs in a plane other than the middle plane of the plate. However, in loading the plate in a direction either parallel or perpendicular to the major axis, the maximum transverse stress ( _z ) occurs at the middle plane. In addition, the maximum value of ( _z ) was about 20 percent of the maximum value of the tangential stress for all models tested. Furthermore, the effect of the bixial loading has reduced the value of the maximum tangential stress at the periphery of the hole as compared with uniaxial loading.As a three-dimensional theoretical solution does not exist for this problem, the present findings were correlated with the well established two-dimensional solutions.     

Photodot investigation of plastic-strain pattern in flat sheet with a hole

The strain pattern was measured, using the photodot method, in a thin flat 24ST-3 aluminum sheet with a central hole, unidirectionally loaded into the plastic region of the material. Test results are presented and the accuracy of the photodot method is discussed.     

Photoelastic investigation of stresses in bars with reinforced semicircular notches under bending

The object of the work described was to obtain information about the stresses in reinforced plastic bars. Tests were carried out under bending on a bar with a composite reinforced semicircular notch. The bars were of epoxy resin reinforced with duralumin. The reinforcements are the concentric, crescent and arch types. The relations between the maximum shear stress and breadth of bars are obtained. The most suitable reinforcements are discussed.     

Photoelastic study of the stresses near a cleavage crack

Photoelastic analysis has been used to determine the stress distribution near the tip of a cleavage crack. The crack bisects a slender rectangular bar along its length, and forces are applied at one end of it tending to open it out. The region of high stresses is localized in a region of the order of the half-width of the bar in size. Very near the crack tip, the stresses decrease in proportion to the inverse square root of the distance from the crack tip. The maximum gradient of the principal tensile stress lies at an angle of 70 to 80 deg from the plane of the crack. Contour maps of the principal stresses, the 45-deg shear stress and the maximum tensile stress are presented.     

Photothermoelastic investigation of stresses around a hole in a plate subjected to thermal shock

This paper contains a three-dimensional photothermoelastic study of stresses generated around the edge of a hole in a flat unrestrained plate subjected to a thermal shock uniformly applied to one face of the plate. The approach taken is experimental in nature, utilizing a newly developed three-dimensional, non-destructive photoelastic technique. An extrapolation procedure is formulated in order to determine transient fringe orders at the thermally shocked surface. For the case considered, the thermal-stress-concentration factor at the edge of the hole was found to be 1.28. Paper was presented at 1964 SESA Annual Meeting held in Cleveland, Ohio, on October 28–30. Work reported herein was conducted by the Douglas Missile and Space Systems Division under company sponsored Research and Development funds.