Characterisation of plasticity-induced closure--crack flank contact force versus plastic enclave

This paper presents some results from a totally new and powerful fusion between automated, digital photoelastic techniques and mathematical analysis of a crack experiencing closure. In essence, real-time photoelastic stress field data is acquired and fitted to a Muskhelishvili complex potential model of a loaded crack experiencing closure. The fit entails optimisation of multiple variables via a memetic algorithm, which combines a genetic algorithm and a local search. From observations made in this work, it is proposed that the plastic enclave around a crack tip will shield the crack from the full influence of the applied stress field. Furthermore, the effects of this ‘back stress’ are overcome by the applied tensile load in a way that, often, cannot be readily correlated with crack contact length, wake contact force, or with compliance.     

Slip initiation on frictional fractures

Direct shear tests and biaxial compression tests are conducted to investigate the onset of slip along a non-homogeneous frictional surface and to determine the effect of specimen thickness and confining stress on slip initiation and propagation. The specimens are made of two and three acrylic blocks with the contact surfaces between blocks having on their upper half a frictional strength smaller than on their lower half. This creates a “weak” surface on the upper half and a “strong” surface on the lower half. The specimens are then loaded in direct shear or biaxial compression with confining pressures ranging from 0.7 to 3.5 MPa. The onset of slip, slip propagation, and the stress field generated at the front and center of the blocks interfaces are monitored using a photoelastic technique where a thin photoelastic film is placed at the location where observations are made. The onset of slip at the weak-strong zone interface is treated as propagation of a frictional crack under Mode II loading. The critical stress intensity factor, K_IIC, at the onset of slip is obtained from photoelastic techniques. The results show a weak dependency of K_IIC on the normal stress applied and no influence of the specimen size for specimens thicker than 25.4 mm; for thinner specimens the K_IIC values are smaller because the boundaries of the specimen prevent the full development of the stress field ahead of the crack tip. The experiments show a linear increase of the critical energy release rate with normal stress which is explained with linear elastic fracture mechanics theories.     

容器斜接管拐角裂纹应力强度因子K_1的光弹研究

本文利用三维光弹冻结切片法,对K_I沿容器斜接管(?)分别为15°、30°模型内拐角裂纹前缘的分布进行测定,同时利用实验所得的结果和(?)等于45°模型角裂纹自然扩展的情况,对某些疲劳扩展现象进行了定性解释.     

Contact stress distributions in a turbine disc dovetail type joint - a comparison of photoelastic and finite element results

Two dimensional photoelastic frozen stress techniques were used to investigate the stress distributions of an axially loaded dovetail joint as is found in the design of some turbine blade-disc fixings. The internal stress distributions were obtained by the shear difference method and were compared to results obtained by using the finite element method. It was found that steep principal stress gradients were present immediately below the contact surfaces in the zone adjacent to the fillet radius of the dovetail fixing, which could be an important consideration in the fatigue strength of the joint. The finite element model used gave the same trends of stress as the photoelastic model, predicted higher peak principal stresses and did not exhibit the same detailed variations of stress distribution.     

高速CCD在光弹材料中的应用

为了探讨高速CCD(电荷耦合装置)在光测力学中的应用,解决工程应用中的瞬间冲击接触的强度问题。由环氧树脂材料浇筑的光弹材料板材作为研究对象,研究对象下端固定,上部对其进行冲击,实验结果采用高速CCD对垂直对象进行实时采集,通过图像处理得到研究对象内部的较为清晰的应力波发生、发展、衰减的过程,通过实验结果得出将高速CCD摄像机与光弹材料制成的研究对象相结合的手段适用于港口、矿山、冶金锻压等领域的强度测试,能够有效获取瞬间接触内部应力。     

Evaluating crack tip stress field in a thin glass plate under thermal load

The stress field around a propagating crack tip in a quenched thin glass plate is discussed through experimental and theoretical analyses. Instantaneous phase-stepping photoelasticity using a CCD camera equipped with a pixelated micro-retarder array is used for measuring the crack tip stress field. From the successive phase maps of principal direction, the position and the velocity of the crack tip are evaluated. On the other hand, the fracture parameters, that is, the stress intensity factors and the T-stress are determined from the phase maps of the retardation. Experimental results obtained for a straight crack show good agreement with those obtained by theory of elasticity. The results also indicate that the direction of the crack propagation arising in the quenching process is not determined by the direction of the maximum principal stress. Furthermore, the results show that the T-stress criterion is inappropriate to evaluate the crack path instability in a quenched thin glass plate.     

Load transfer at imperfect interfaces--dislocation-like model

A dislocation-like model describing the boundary conditions of a partially debonded interface is verified experimentally. The effect of the imperfect interface on the load transfer is studied by photoelastically measuring the elastic deformation in bimaterials due to an inclusion with dilatational misfit strain. The boundary conditions to be modeled are that the radial and the tangential tractions are continuous across the interface and the displacements may be discontinuous from one solid to another. The discontinuity of displacement across the interface is assumed linearly proportional to the displacement at the interface of the constituent where the stress source is. The maximum shear stress distributions measured from the isochromatic fringe patterns are in good agreement with the theoretical calculations. The results show that an imperfect interface could be viewed as a continuum entity with interface rigidity as proposed by the dislocation-like model.     

Progress in experimental stress analysis for Airbus aircraft design

Advanced experimental stress analysis techniques have been increasingly used on large aircraft structures to provide stress data for both design and numerical analyses validation. In the past these techniques tended to be limited to the structural test phase which is subsequent to the design and manufacture stage. Experimental stress techniques have had to become more sophisticated and utilised much earlier in the programme in order to meet the ever increasing demands for competitive, aircraft design, performance and reliability. Currently the techniques have applications from the earliest design stage through to final in-service support engineering. Photoelastic stress analysis was one of the earliest and most powerful experimental design tools in use at Filton, however, the range of techniques is continually being extended to meet the technological demands of new designs. This has involved keeping abreast of a wide array of sciences to identify and develop new physical procedures for stress related experimental analysis. Progress is reviewed in this paper as well as the vision for the future within the context of Airbus aircraft design. The economic and technical pressures to move towards more automation and image processing are discussed including the requirement to provide data that is more user friendly to conventionally trained design and stress engineers.     

Measurement of residual stress in EFG ribbons using a phase-shifting IR photoelastic method

This paper reports on the measurement of residual stress in EFG silicon ribbons for solar cell applications using the phase-shifting infrared (IR) photoelastic method. The samples analysed were wafers cut from EFG octagons with 100 mm face width and from EFG 125 mm face-width octagon under development. Experimental results show that the distribution of residual stress in both types of samples is similar, within measurement uncertainties. The average residual stress in the samples is about 8 MPa. Maximum stresses of around 30 MPa are associated with twin and grain boundaries. Significant variations of stress along the growth direction, possibly related to buckling, were also measured.     

Neural net based torque sensor using birefringent materials

Photoelasticity can be used to accurately measure surface strains or stresses in a part or structure. In this paper we describe the use of a photoelastic transducer and neural net image processing to estimate the torque of stationary and rotating shafts. A strain sensitive (photoelastic) plastic cylinder is attached to the shaft and illuminated by polarized light. As the shaft torque varies the photoelastic plastic displays the corresponding shaft strain as a two-dimensional fringe pattern when viewed through an optical polarizer. The strain that causes this observed optical pattern is a complex function of the torque applied to the shaft. In this paper, we describe the use of neural net image processing to estimate this function to realize an optical torque sensor. A CCD camera/image processing system was used to acquire and process the optical patterns. A neural net torque estimator was trained with these fringe patterns and tested against a laboratory strain gauge torque sensor. Our experiments show that the neural net torque estimator can accurately estimate (to within a few percent) the applied torque for both static and slowly rotating (<20 rpm) shafts.