C. P. Burger

Half-fringe photoelasticity: A new approach to whole-field stress analysis

This paper presents a new method for whole-field stress analysis based on a symbiosis of two techniques—classical photoelasticity and modern digital image analysis. The resulting method is called ‘half-fringe photoelasticity (HFP)’.Classical photoelasticity demands materials with high birefringence, which leads to extensive use of plastics as model materials. Since the behavior of these materials is often different from that of the prototype materials, their use distorts the similitude relationships. In many contemporary problems this distortion is untenable. HFP offers a way out of this dilemma. It permits materials and loads to be chosen so that no more than one half of a fringe order appears in the area of interest. Thus, for example, glass, which behaves linearly up to high stress levels and over a wide range of temperatures, could be used as model material. Alternatively, models from polymeric materials could be used under very low load in order to stay within the linear part of the stress-strain diagram and to prevent large deformations. The half-fringe-photoelasticity system, which is described here, utilizes the resulting low levels of birefringence for effective stress analysis.This paper describes the system. It outlines a calibration routine and illustrates its application to two simple problems using glass models.

Fractional Moiré strain analysis using digital imaging techniques

A new method for analyzing low-order moiréfringe patterns of displacement fields is presented. This method adapts the techniques of half-fringe photoelasticity to moiré and extracts continuous displacement information in the regions between integral fringes. The effectiveness of the technique is illustrated with three examples: a uniform uniaxial field, a tapered specimen in tension, and a disk in diametral compression.

Dynamic wave propagation in a single lap joint

Dynamic photoelasticity was used to study the stresses when an impulse wave propagates through a single lap joint. An explosive was detonated in one of the adherends. The resulting stress wave propagated down the adherend and through the joint area. At the junction between the adherends, the stress wave interacted with the ends of the adherends and significant changes in the stress wave occurred. The stress wave then propagated through the joint and, in doing so, interacted with the two square corners at the adhesive/adherend interface. It was found that a significant biaxial tensile stress occurs at one of the corners.With only the usual black and white photographs, the ordering and interpretation of the photoelastic-fringe orders was difficult. The paper indicates how color photographs were used to simplify identification of the fringes.

Material properties in thermal-stress analysis

With simple techniques and not too-costly laboratory equipment, the significant material properties in modeling transient thermal stresses by photothermoelasticity are evaluated. The results are presented for a room-temperature-cured epoxy, a hot-cured epoxy and a polycarbonate. The materials tested are also evaluated on their applicability to a transient-thermal-stress analysis.

Nonlinear photomechanics: A review of progress in photoplasticity and its recent applications to studies of metal-forming processes

The history of developments in photoplasticity can be traced from the work of Filon at the beginning of this century to the most recent attempts to model large-scale flow in metal-forming processes. The results of some of the studies on strain fields in upset forging and hot rolling are presented.

Photothermoelastic analysis of transient thermal stresses

Photothermoelasticity by means of heating is used to obtain an experimental solution to the quasi-static problem of transient thermal stresses around elliptical holes. Empirical equations are obtained for the maximum stresses due to step temperature changes at the edges of the plates.

Transient thermal stress-intensity factors for short edge cracks with equal depth of crack tips

Abstract Stress intensity factors were determined for very short edge cracks with different inclinations but equal depths of crack tips from the heated edge of a plate under transient thermal stresses. The critical orientation of edge cracks was found experimentally.

Three-dimensional strain distributions in upset rings by photoplastic simulation

The changes in internal diameter that occur when a hollow disk, or ring, is plastically deformed by axial compression have been proposed as a method for measuring interface friction during metal forming. The adoption of this test method has been delayed because the three-dimensional strains in such rings are not known.This paper describes how an experimental method using photoplastic simulation was used to obtain the three-dimensional strains for a particular ring geometry. It presents proot of highly nonuniform strains and also shows that the division between flow inward and flow outward occurs at different radii through the thickness of the ring.

Three-dimensional strains in rolled slabs by photoplastic simulation

Small polyester billets are used to model the deformations that occur during the hot rolling of slabs of aluminum. The billets are rolled to a retained reduction of around 11 percent. They are then sliced along principal planes and the three-dimensional strains are plotted over a transverse cross section of the deformed billet. The extent of this nonuniformity in the strains is clearly shown.

Dynamic photoelasticity as an aid in developing new ultrasonic-test methods

A prerequisite for the development of quantitative ultrasonic-inspection techniques for surface flaws is a thorough understanding of the ways in which elastic waves interact with defects. Analytical and numerical approaches are presently inadequate. Experimental methods are needed for a better understanding of wave interactions with real geometries.This paper describes how dynamic photoelasticity was used to study the interaction between Rayleigh waves and slots. To fully interpret the interactions between an incident Rayleigh wave and a surface slot, the problem was subdivided as follows: first, the reflections and mode conversions of a Rayleigh wave at a corner were studied. This simulated the Rayleigh-wave interaction with a slot opening. Then, the interaction when a Rayleigh wave ran off the tip of a slot was observed, and, finally, the total interaction with slots perpendicular to the surface was studied.The results for these three cases are presented. It is suggested that the most important property of a Rayleigh wave that can be used to size surface and near-surface defects is the subsurface particle motions. These motions persist up to a depth of the order of a wavelength. The shape (that is, the frequency spectrum of the transmitted wave) should, therefore, be affected by the depth of the slot.Spectroscopic analysis is applied to the photoelastic data to develop a simple method for sizing slots. Results from ultrasonic tests on slots in steel confirm the validity of the suggested method. By applying contemporary concepts of signal processing to photoelastic data, a powerful new area of experimental investigation is introduced. It promises to overcome the current inability of scatter theories to predict the interactions between real-life defects and acoustic waves as used in ultrasonic testing. Applications of this approach will improve the quantitative ability of ultrasonic-inspection methods.