Dynamic Photoelasticity Research Articles

Explosively generated pulse propagation through particles containing natural cracks

An experimental study is conducted to study dynamic load transfer through particulate assemblies containing a damaged particle. The assembly is simulated by a one-dimensional array of circular disks, and the damage itself by a natural, zero-width crack. This assembly is explosively loaded and the dynamic load transfer process observed using dynamic photoelasticity and strain gages, as the stress wave pulse propagates down the assembly. Attention is focussed on the influence of damage size and orientation on the group wave velocity, wave dispersion and peak contact load attenuation of the stress wave pulse. Damage growth occurring from the interaction of the stress wave pulse with the closed natural crack tip is also studied. The results demonstrate that the inclusion of a damaged particle greatly influences the load transfer process, both locally and in the far field. The degree to which the damaged particle modifies the load transfer process is more dependent upon the orientation of the natural crack than upon the size of the crack. Moreover, damage growth within the damaged particles is also a strong function of the orientation of the natural crack.

Digital drum camera for dynamic recording

Currently, digital imaging and hard copy techniques are extensively used in photomechanics research. However, the slow speed of popular video systems limits their use in dynamic photomechanics. New devices and sensors are being introduced to meet the high-speed requirements in dynamic applications. We report the applications of a lowcost, high-speed time delay and integration (TDI) camera in some of the dynamic photomechanics experiments. TDI is a special operating mode built on the traditional full-frame imager architecture. In this mode, the charge-coupled device (CCD) camera functions similarly to a drum camera. The high sensitivity of the CCD enables the use of low-cost, lowpower semiconductor light sources. After a brief description and comparison of TDI operation with conventional recording schemes, two applications are demonstrated. First, dynamic photoelasticity experiments using streak and strobe recording are exemplified. In the second demonstration, imaging and inspection of curved objects rotating at high speeds are presented.

The interaction of a single laser-generated cavity in water with a solid surface

This paper reexamines the collapse of a cavity in water and its role in possible damage processes. Laser-generated cavities were formed near to a water–solid boundary and simultaneous observations made of the collapse processes and stresses on the solid surface. Using schlieren photography and a thin film transducer on the solid, the importance of fluid flow to induced material stress was determined. The stresses in the solid were also observed using dynamic photoelasticity. The cavity deformation in the collapse phase was observed with high spatial and temporal resolution. Two distinct surface stresses were observed associated with bubble collapse near the boundary. The results indicate that one was due to a shock wave. It is postulated that the second stress was associated with fluid flow through the bubble, set up by the so-called ‘‘liquid jet’’ as opposed to jet impact on its own.

DDGS high speed camera with some applications

DDGS high speed camera is an improved version of the multispark system. In this system the time delay and the frame interval can be set arbitrarily from 5 μs to 10 000 μs, and the intervals may be equal or not. The modified spark gaps can provide point flashes that are small enough in size and strong enough in light for both dynamic caustics and dynamic photo-elasticity. In addition, this system can be changed easily to observe the flow by schlieren method. Thus, this optical system is suitable to investigate many dynamic problems in the fields of solid and fluid mechanics.

USING DYNAMIC PHOTOELASTICITY TO EVALUATE THE INFLUENCE OF PARTING PLANES ON STRESS WAVES INTERACTING WITH STOPES

Dynamic photoelasticity is used as a means for visualizing the complex interaction process between elastic waves and geometrical discontinuities. The photoelastic experiments are back-analysed by the dynamic finite difference program WAVE, and the code is assessed in terms of its accuracy and modelling capabilities. Three model geometries are investigated: (i) a stope situated within a homogeneous medium; (ii) a stope surrounded by softened material, the interface between the softened and bulk material being bonded and (iii) a stope situated within softened material, with a non-cohesive material interface. Prominent waves resulting from the diffraction, refraction and reflection of incident waves, as well as normalized dynamic stress intensification factors at the stope face, stope back area and along the hanging-wall skin are analysed in this study. The parting planes are found to reflect a portion of the incident energy and thus shield the stope. However, a non-cohesive parting plane traps energy within the hanging-wall beam, and any shielding benefits are negated. WAVE has been proven to model accurately the diffraction, refraction and reflection of stress waves in a homogeneous medium and the interaction with cohesive and non-cohesive interfaces separating two material types.

Analysis of Contact Singular Stresses with Relief Notch by Using Dynamic Photoelasticity(II)

The dynamic photoelastic technique had been utilized to investigate the possibillity of relieving the large local singular stresses induced at the corner of a right- angle- indenter. The indenter compressed a semi-infinite body dynamically with an impact load applied on the top of the indenter. The effects of the geometric changes of the indenter in terms of the diameter (d) and the location (1) of the stress relieving notch on the behavior of the dynamic contact stresses were investigated. The influence of stress relieving notches positioned along the edge of the semi-infinite body on the dynamic contact stresses were also studied by changing the diameter (D) and the location (L) of the notch. A multi-speak-high speed camera with twelve sparks were used to take photographs of full field dynamic isochromatic fringe patterns. The contact singular stresses were found to be released significantly by the stress relief notches both along the indenter and the edge of the semi-infinite body. The optimal position and geometry of the stress relieving notches were obtained with the aid of limited experimental results.

Dynamic Photoelasticity Using LED and Polaroid Film

Dynamic photoelasticity relies on high-speed photographic systems to record the fringes in motion. Several systems are being used, but the delayed microflash is widely popular in non-destructive studies due to the high-contrast recording and low cost. Flash lamps and lasers are routinely employed as the light source in these setups. The use of LED as a microflash in such systems is presented. Capability of the system is demonstrated by recording fringe patterns from a photoelastic model under dynamic loading. A high-speed Polaroid film is used for recording the dynamic fringe patterns. Low cost, low power, and simplicity in the experimental setup make it possible to adapt the system to undergraduate laboratories.

Effect of flaws on the stress wave propagation in particulate aggregates: Near and far field observations

An experimental study is conducted using dynamic photoelasticity and strain gage techniques to investigate transient stress fields in aggregate assemblies of circular disks. In particular, attention is focused on the effect of damage and damage growth in the particles on the dynamic load transfer process. The experimental data are analyzed to obtain the wave velocity, contact load profiles, peak load attenuation, wave dispersion and other changes in the stress fields as the dynamic load transfer process takes place. Both near and far field effects due to the presence of a flaw on the load transfer process are studied. The results show that the size and the orientation of the flaw greatly influence the load transfer process locally. However, in the far field the flaw only affects the attenuation of the peak loads without changing the wavelength of the loading pulse.

On the use of strain gages in dynamic fracture mechanics

An experimental study is conducted to show the application of strain gages in studies of dynamic fracture. In particular, strain gages have been used to evaluate the dynamic stress intensity factors in different specimen geometries in a variety of materials. A direct comparison of the results obtained from strain gages is made with those obtained using dynamic photoelasticity. A detailed evaluation of the effect of the position and orientation of the strain gage on the determination of the fracture parameters is also presented.

Multiple LED camera for dynamic photoelasticity

Dynamic photoelasticity involves the high-speed recording of rapidly moving fringe patterns in synchronization with loading. Cranz Schardin cameras are routinely utilized in the recording of dynamic photoelastic patterns. There are no moving components in these cameras, making the setup simple and attractive. Amultiple LED camera based on the Cranz Schardin format is presented. Highspeed instant polaroid film is used for recording the photoelastic fringes. Low cost, simplicity in the experimental setup, and rapid repeatability are the advantages of the camera.

Low-cost Dynamic Photoelasticity

Low-cost Dynamic Photoelasticity

Low-cost digital polariscope for dynamic photoelasticity

A low-cost dynamic polariscope is presented. The experimental arrangement is based on the delayed-microflash technique. A LED is used as the flash source. Dynamic photoelastic patterns are recorded after different time delays from the loading point using a CCD camera operating in the time-delay and integration mode. Unlike the conventional photographic techniques, the experiments need not be performed in total darkness. Digital recording provides instantaneous visualization of the patterns and rapid repeatability.

DYNAMIC MATERIAL PROPERTIES IN BIREFRINGENT MATERIALS

Dynamic photoelasticity is a full field technique which has been used by various researchers as an aid to improve understanding of problems that involve stress wave propagation. Birefringent materials that exhibit sufficient sensitivity to loading for stress wave propagation studies are typically polymers. Glass, which is not nearly as viscoelastic as polymers does not exhibit enough sensitivity for easy measurements, thus it is unsuitable for most dynamic studies. The effect of loading rate on material properties is more pronounced in polymers, hence accurate measurement of these properties is essential in a dynamic photoelastic study. The problem associated with the determination of the dynamic material properties of a polymer is the determination of the exact location of the wavefront. Digital image analysis can be used to locate the wavefront with a great deal of accuracy. The results from the digital image analysis can then be used to determine dynamic material properties for the polymer.

Influence on fiber inclination and interfacial conditions on fracture in composite materials

Dynamic photoelasticity has been used to study the effect of the fiber-matrix interface and fiber orientation on dynamic crack growth in fiber composites. Two types of fiber-matrix interfaces are considered: well bonded and partly debonded. The fiber-matrix interface is characterized by conducting fiber pullout tests. Partly debonded fibers aligned with the loading direction, result in higher fiber debonded lengths, lower dynamic stress-intensity factorKID and lower fracture surface roughness compared to well bonded fibers. Orientation of brittle fibers, with respect to the loading direction, impairs their ability to lowerKID, while oriented ductile fibers produce no significant change inKID. Misalignment of fibers from the loading direction reduces the fiber debonded length due to kinding of the fiber at the crack face.

Separation of principal stresses in dynamic photoelasticity

A new and effective method used to separate the transient principal stresses for dynamic photoelasticity is proposed. This is a hybrid method combining the optical method of dynamic caustics and the boundary element numerical method. Firstly, a modified Cranz-Schardin spark camera is used to record simultaneously the isochromatic fringe patterns of photoelasticity and the shadow spot patterns in the dynamic process. By means of the isochromatic fringe patterns, the difference between transient principal stresses in the whole domain and the principal stresses along the free boundary can be solved. In addition, the method of caustics is a very powerful technique for measuring the concentrative load. Then, the sum of the principal stresses is calculated by the boundary integral equation obtained from the Laplace integral transform of the wave equation. So, the transient principal stresses can be determined from the experimental and numerical results. As an example, the transient principal stresses in a polycarbonate disk under an impact load are resolved.

Dynamic photoelasticity method using three-layer striker and its application to key/keyway structure : Journal of JSNDI, vol. 42, no. 11 (November 1993) p. 617 Inoue, Atsushi; Takeishi, Hiroyuku; Futakawa, Masatoshi

Dynamic photoelasticity method using three-layer striker and its application to key/keyway structure : Journal of JSNDI, vol. 42, no. 11 (November 1993) p. 617 Inoue, Atsushi; Takeishi, Hiroyuku; Futakawa, Masatoshi

Digital dynamic photoelasticity

A new experimental system for dynamic photoelasticity is presented. The technique makes use of a pulsed diode laser and a digital scanning camera and is suitable for repeatable dynamic photoelastic experiments. The experimental arrangement is rather simple and less tedious compared to the conventional wet film processing techniques. The digital recording makes it possible to repeat the experiments at a faster rate and displays the recorded pattern instantaneously. The capability of the system is demonstrated by recording dynamic photoelastic patterns from a simply supported beam after loading with a steel ball impacting at the midpoint between the supports.

Elastic impact loading of notched beams and bars

Four types of impact specimen, Charpy, Izod, Slender Cantilever and Double Notched Bar, are studied using the technique of dynamic photoelasticity. The growth of fringes with time in low modulus photoelastic material is analysed for each speciment and it is concluded that the stress wave behaviour of the Cantilever geometries and Double Notched Bar offer advantages over the traditional Charpy specimen.

Role of particle shape and contact profile on the dynamic response of particulate materials

Experimental and numerical studies have been conducted to investigate the effect of particle shape and particle surface roughness on intergranular load transfer, velocity and wavelength of a stress wave transmitted through granular media subjected to explosive loading. Dynamic photoelasticity was used as the experimental technique to study stress-wave propagation through granular media simulated by assemblies of elliptical and circular particles with varying surface roughnesses. Numerical studies employed a computational scheme based on the distinct element method. Results indicate that the shape of the particle affects both the velocity of the transmitted stress-wave pulse and the load-transfer characteristics. The wavelength shows no appreciable dependency on the particle shape. Particle surface roughness seems to affect the load-transfer characteristics and the wave length, and to a limited extent, the wave velocity.

Extraction of fracture mechanics parameters from steady-state dynamic crack-tip fields

A procedure for determining the fracture mechanics parameters of interest from full-field optical mechanics data is presented. Examples are presented for crack-tip stress fields recorded using dynamic photoelasticity. Stress-field representations in series form, when combined with local collocation procedures, are shown to be a powerful tool for reliable and accurate parameter determination. The methodology is illustrated and applied to two cases of dynamic crack propagation, one pertaining to opening-mode crack propagation and the other to a smoothly curving crack. Emphasis has been placed on giving guidance to a user on how best to approach implementation of the methodology from a practical standpoint.