L. W. Zachary

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.

Optical response and yield behavior of a polyester model material

A mixture of flexible and rigid polyester resins is one of the materials that has been used in the past to model elasto-plastic prototype behavior. As a result of recent curtailments in plastic production, one of the constituents of the mixture is no longer available. A different flexible resin of the same family is available, however, and has been shown in this program to be suitable for optical-model studies involving deformations into the plastic range of material response.In this paper, complete mechanical and optical properties for the new model material are presented for both uniaxial tensile and compressive loadings. Results from a series of thin-walled cylinder tests under internal-pressure loadings are also presented which provide some information on the optical and yield characteristics of the polyester model material under a biaxial state of stress.Results of the study indicate that stress-strain curves for the material can be modified significantly by changing the mixture ratio or the test temperature. Optical data from the study indicate that the fringe order in the material is a function of the instantaneous principal-strain difference.Data from the uniaxial tension and compression tests, together with limited data from the cylinder tests, indicate that the polyester material may follow a modified von Mises yield criterion which accommodates differences in tensile and compressive yield strengths of a material.

Designing an All-purpose Force Transducer

Many design and analysis situations require the determination of loads being transmitted to a structure. Since it is not always possible to insert a load cell, it is proposed that the structure itself and a set of strain gages be used to determine each of the applied loads (forces and moments) responsible for the measured strains. In essence, the structure becomes the transducer.Although a set of loads will uniquely deform a structure, the measurements can only be made at a finite number of points. However, in most situations, these data are sufficient to reveal the combination of loads that caused the deformation.The development of a technique for determining these loads is described. Limitations of the technique are discussed along with the associated error estimates. A worked example is given at the end.

Use of photoelasticity to determine orthotropicK I stress-intensity factor

A new experimental method of obtaining orthotropic stress-intensity factor,KI, is presented. The orthotropic photoelasticity and orthotropic linear-elastic fracture-mechanics laws are combined. The combined set of equations is used along with half-fringe photoelasticity to determineKI in a compact-tension specimen made of a transparent unidirectional fiberglass-epoxy material. The results are compared with finite-element-method solutions.

A new technique for the determination of stress-optical constants using the shadow spot method

A new technique has been developed to determine both the transmitted and the reflected stress-optical constants. The reflected optical constant is associated with the light reflected from the rear face of a transparent material. The stress-optical constant related to the light reflected from the front surface is not considered in this paper since it is equal to Poissons ratio divided by Youngs modulus. In this new method, a monochromatic light beam emitted from a He−Ne laser impinges on the area surrounding a circular hole under load in a plate. A digital image-analysis system, EYE-COM III, is used to determine the points of the highest light intensity of the caustic that results from the light transmitted or reflected from the rear face of the plate. The effect of the hole size in optically isotropic materials is investigated.

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.

Damping characterization of a filled epoxy used for dynamic structural modeling

This paper presents information on the usefulness of a particular aluminum-particle-filled epoxy for dynamic modeling. Damping characteristics in terms of the loss factor are presented for this epoxy and some structural metals. Though the damping of the epoxy was larger than that of any metal tested, it can still be considered small. Portal frames were modeled using the epoxy. Natural frequencies of vibration for the metal frames and epoxy models were determined. The scaled-epoxy-model frequencies accurately predicted the metal prototype frequencies. A possible area of error is pointed out with respect to the modeling of the shear modulus.

Zero-shift values of automatic and inexpensive strain-gage instrumentation systems

Computer-controlled data-acquisition systems are being used extensively for gathering strain-gage data. This paper explores the relative merits of using modern solidstate digital multi-meters (DDM) to measure the strain-gage resistance directly rather than using a conventional Wheatstone bridge. Both a direct-resistance measurement scheme and a reversed current scheme are compared over long measurements terms of 6 and 12 days using a 6 1/2-digit multimeter. The results show that the reversed current method is superior in maintaining the zero-gage resistance reading at the cost of using several meters. Possibly the direct-resistance method can be improved so that the operation and equipment can be simplified.

Application of photoelasticity to a weld-penetration problem

The need for more information on the “initiation period” in fatigue tests of weld specimens with penetration defects is discussed and the literature which relates the elasticity stress-concentration factor and Irwins stress-intensity factor is reviewed.A series of photoelasticity tests on two-dimensional plane-stress models of typical penetration defects is described. In particular a method for casting “ready to use” very narrow defects is explained.The results are presented in a graph of stress-concentration factor against defect length. This graph has a “knee” at defect length-to-plate thickness ratios around 0.2. Below the “knee”, the stress-concentration factor changes very little with changes in defect length but, for lengths beyond the knee, i.e., ratios larger than 0.2, the stress concentrations increase linearly with defect length. It is concluded that such a critical defect length should have a strong effect on fatigue life of defective welds and that it may constitute a first approach to the specification of an “acceptable” level of penetration defects for production processes.