Evaluating the localised through-thickness load transfer and damage initiation in a composite joint using digital image correlation

Experimental analysis of the stresses and strains in a glass fibre polymer composite double butt strap joint using digital image correlation (DIC) is described in the paper. Initially the strain fields through the thickness of the joint are derived from DIC measurements, capturing the effect of the initiation and development of cracks in the joint up to failure. It is demonstrated that the relatively small strains developed in the through-thickness direction are critical in the development of damage in the joint at the geometric discontinuity between the adherends. An experimental methodology is established to perform DIC at the mesoscopic scale, enabling accurate, high spatial resolution analysis of the small through-thickness strains around the discontinuity. The DIC enables the strains to be evaluated, their development monitored and hence establish their contribution to the failure process. To determine the full-field stresses in the joint experimentally derived materials properties are used. From component stress maps principal stresses maps are derived, which clearly show where the damage is initiating and its subsequent growth in the composite adherends until final failure of the joint.     

Confirmation of Principal Residual Stress Directions in Rectilinear Components by Neutron Diffraction

Abstract: Large magnitude residual stresses in precipitation hardened aluminium alloys are a consequence of inhomogeneous plastic deformation arising from thermal strains. Thermal gradients develop because of the metallurgical requirement to quench the material rapidly from the solution treatment temperature. Characterisation of the resulting residual stresses can be usefully performed by neutron diffraction. For rectilinear shapes, it is usually assumed that the orthogonal directions of the sample are coincident with the principal stress directions. To test this assumption, residual strains were measured in a rectilinear block of the aluminium alloy 7449 using the SALSA neutron strain analyser. The strains at a single point in the forging were measured as a function of orientation; that is the forging was rotated around the three orthogonal axes of a coordinate system with its origin at the measurement point. Analysis of the neutron diffraction data allowed the determination of the full three‐dimensional strain and stress tensors which confirmed that the orthogonal directions of the forging did coincide with the principal stress directions.     

Critical plane approach for the assessment of the fatigue behaviour of welded aluminium under multiaxial loading

ABSTRACT Multiaxial stress states occur in many welded constructions like chemical plants, railway carriages and frames of trucks. Those stresses can have constant and changing principal stress directions, depending on the loading mode. Latest research results on welded steel joints show a loss of fatigue life for changing principal stress directions simulated by out‐of‐phase bending and torsion compared to constant directions given by in‐phase loading. However, aluminium welds reveal no influence of changing principal directions on fatigue life compared to multiaxial loading with constant principal stress directions. This behaviour is not predictable by any conventional hypothesis. A hypothesis on the basis of local normal and shear stresses in the critical plane has been developed and applied to aluminium weldings.     

Application of Digital Image Correlation to Address Complex Motions in Thermoelastic Stress Analysis

A motion compensation method for thermoelastic stress analysis (TSA) is described that uses digital image correlation (DIC) to capture the displacement field on the surface of the specimen. The displacement field is used to correct the infrared (IR) images to remove the effect of the motion of the specimen from the TSA. As the DIC displacements are obtained with a relatively high spatial resolution, sharp displacement gradients and discontinuities can be corrected. The feasibility of the motion compensation method for TSA is investigated firstly by validating the approach using data obtained from an aluminium alloy plate with a central circular hole loaded in tension and comparing the results with a finite element model. It is shown that the motion compensation approach significantly improves the accuracy of TSA, particularly when high magnification optics are used. Next, the feasibility of simultaneous capture of IR and white light images is investigated. It is shown that by using the correct combination of paints, a speckle pattern can be applied to the surface to provide contrast in the white light spectrum for the DIC but have a uniform emissivity in the IR spectrum so that there is no effect on the TSA. Thus, it is possible for the motion compensation to be conducted on data collected during fatigue tests. Finally, it is demonstrated that the motion compensation technique can be applied to discontinuous motion produced by face sheet debonding in a foam cored sandwich structure loaded in a double cantilever beam (DCB) configuration. It is shown that the motion compensation technique is capable of correcting the complex and non‐uniform motion for TSA in the DCB test, thereby enabling detailed thermoelastic data to be obtained from the vicinity of the crack tip.     

Einfluss der Werkstoffzähigkeit auf das Festigkeitsverhalten unter mehrachsiger Beanspruchung am Beispiel von Schweißverbindungen aus Stahl und Aluminium

Influence of Ductility on the Multiaxial Fatigue Behaviour by the Example of Welded Joints of Steel and Aluminium The multiaxial fatigue behaviour of materials with different ductility under constant and changing principal stress directions is also applicable to welded joints of different materials. For this, welded flange tube connections of the fine grained steel StE 460 and the artificially aged aluminium alloy AlSi1MgMn T6 were investigated under constant amplitude combined bending and torsion. Out‐of‐phase loading, i. e. changing principal stress directions, of the steel joints led to a decrease of fatigue life, which is observed at ductile material states. However, for the aluminium joints out‐of‐phase loading resulted same behaviour as in‐phase loading, which indicates a semi‐ductile material behaviour. The results for the welded steel joints were evaluated on basis of local stresses by the integral hypothesis of the Effective Equivalent Stress EES (WVS). This hypothesis for ductile material states takes into account the life decreasing influence of out‐of‐phase loading by considering the interaction of the shear stresses on different planes. The fatigue behaviour of the aluminium welds is described by the critical plane based combination of shear and normal stresses (KoNoS), which is valid for semi‐ductile material states.     

Alternative calibration techniques for quantitative thermoelastic stress analysis

In recent years thermoelastic stress analysis has been established as an important experimental technique. The SPATE (Stress Pattern Analysis by Thermal Emissions) system, marketed by Ometron Ltd., is the standard equipment for thermoelastic stress analysis. The signal obtained from the SPATE equipment is directly proportional to the sum of the principal stress on the surface of the structure under evaluation. For quantitative stress analysis accurate calibration constants must be obtained; these are dependent on the radiometric properties of the SPATE system detector and the material properties and condition of the surface in question. This paper describes a number of calibration techniques commonly in use. The factors which affect the accuracy of each technique are discussed in detail. An experimental calibration study of a "steel" is also included.     

Elasto-plastic stress distributions in adhesively bonded double lap joints

Elasto-plastic stress analysis is an important solution for a used ductile adhesive in single or double lap solutions. After loading, the joint strength can be increased by residual stresses. In this study, an elasto-plastic stress analyses of the shear and peel stresses are carried out in a double-lap joint. The adhesive material is chosen as a ductile material which is DP460. Elasto-plastic solution of the shear stress is obtained in a simple and accurate form in one dimensional case. Then, the solution of the peel stress is performed by considering the shear stress in the elasto-plastic form. In this solution, the boundary conditions are satisfied by the Newton–Raphson method. A good agreement is obtained between these analytical and numerical (ANSYS 10) solutions. The values are calculated highest at the ends of the adhesive.     

Automatic Determination and Evaluation of Residual Stress Calibration Coefficients for Hole‐Drilling Strain Gauge Integral Method

Abstract: Residual stress calibration coefficients are used to calculate residual stresses from the measured strains relieved during hole‐drilling. The current residual stress measurement practice interpolates the published non‐dimensional coefficients for a given measurement condition. Errors are always introduced from the interpolation. In addition, the calibration coefficients vary with respect to factors such as sample geometry dimensions, radius, offset and incline of the drilled hole, and material properties as shown in our sensitivity studies and other researchers’ work. This paper presents a better solution that is to calculate the calibration coefficients for each specific measurement. A set of routines coded in Python language for Finite Element software ABAQUS is developed to address our sensitivity studies of these factors. With these automatic routines, a technician who is not familiar with Finite Element and programming can conveniently obtain the calibration coefficients for his measurement conditions and residual stresses automatically. Because coefficients are determined directly by Finite Element Analysis (FEA), dimensionless coefficients are not needed anymore; instead, a modified integral method is proposed and implemented. An experiment is conducted to demonstrate the practical procedures of measuring residual stresses using resistance strain rosette and calibration coefficients obtained with this set of routines. Bending stresses on a narrow and thin beam are measured using this set of routines and compared to the theoretical results and the stress obtained by interpolating non‐dimensional coefficients.     

Through-thickness strain field measurement in a composite/aluminium adhesive joint

This paper presents an experimental procedure, which enables us to assess the shear strain field in an adhesive joint between composite and aluminium. In practice, this strain field is representative of the progressive stress transfer between a loaded structure and a composite patch used for reinforcement purposes. Digital image correlation (DIC) is used to measure the displacement field through the thickness of a patched specimen subjected to a tensile test. The shear strain field derives from the measured displacement field. The shear strain clearly decreases when the distance from the free edge of the adhesive increases, as predicted by numerical and analytical models of the joint. These measurements are used to estimate the in situ shear modulus of the adhesive. It is observed that the shear modulus decreases when the shear stress increases, thereby illustrating the non-linear response of the adhesive.     

Separating Stresses Thermoelastically in a Central Circularly Perforated Plate Using an Airy Stress Function

Abstract: Thermoelastic stress analysis (TSA) is a contemporary full‐field, non‐contacting method of experimental stress analysis. In a cyclically loaded structure which experiences adiabatic and reversible conditions, the measured local change in temperature is proportional to the change in stress. Under isotropy, the technique measures information on the sum of the principal stresses. As engineering analyses often necessitate knowing the individual components of stress, additional experimental methods or information are frequently required to ‘separate the stresses’. The ability to evaluate individual stresses reliably in a uniaxially loaded finite plate with a central circular hole from TSA‐recorded information without supplementary experimental data is demonstrated here. Measured temperature data are combined with an Airy stress function and some limited traction‐free conditions. The present inverse technique does not presuppose knowledge of the external geometry or boundary conditions, overcomes the traditional difficulties of unreliable edge data, and reduces the number of coefficients needed by satisfying the traction‐free conditions analytically on the edge of the hole. Particular attention is paid to determining a realistic value for the needed number of Airy coefficients.     

The development of simultaneous thermo- and photo-elasticity for principal stress analyses

A technique is developed for determining the individual principal stresses by measuring experimentally the sum and the difference of the principal stresses over a surface of an engineering component. This application, which results from previous research on the thermoelastic response of polycarbonate coatings typically used in reflection photoelasticity, is based upon the simultaneous use of thermoelastic stress analysis and automated photoelasticity. Full field patterns of the individual principal stresses can be evaluated, even over complex geometric surfaces, by properly processing the data obtained by the independent experimental techniques.     

Beginnings and Early Development of Thermoelastic Stress Analysis

Abstract:  Starting with Gough's first recorded observation of the thermoelastic effect in 1803, through to the 1990s, the principal steps in the evolution of the thermoelastic stress analysis (TSA) technique are identified and reviewed. The coverage includes the contributions of Weber, who first described the effect in metals, Kelvin who provided the classical theoretical treatment, Turner and Coker who saw the potential of the effect as the basis of a stress analysis technique, Belgen who demonstrated the value of infrared technology in this context and Mountain and Weber whose foresight and skills resulted in the first commercially available equipment (SPATE) for TSA. Comments on the personal qualities and attributes of those involved are also included.     

Dynamic Deformation of Clamped Circular Plates Subjected to Confined Blast Loading

In this paper, the dynamic deformation of thin metal circular plates subjected to confined blast loading was studied using high‐speed three‐dimensional Digital Image Correlation (3D DIC). A small‐scale confined cylinder vessel was designed for applying blast loading, in which an explosive charge was ignited to generate blast loading acting on a thin metal circular plate clamped on the end of the vessel by a cover flange. The images of the metal plates during the dynamic response were recorded by two high‐speed cameras. The 3D transient displacement fields, velocity fields, strain fields and residual deformation profiles were calculated by using 3D DIC. Some feature deformation parameters including maximum out‐of‐plane displacement, final deflection, maximum principal strain and residual principal strain were extracted, and the result was in good agreement with that simulated by AUTODYN. A dimensionless displacement was introduced to analyse the effects of plate thickness, material types and charge mass on the deflection of metal plates. DIC is also proven to be a powerful technique to measure dynamic deformation under blast loading.     

Elasto-static and elasto-plastic stress analysis of adhesive bonded tubular lap joints

The problem of an adhesive bonded lap joint between two dissimilar orthotropic circular cylindrical laminated shells is considered. The principal directions of orthotropy do not have to coincide with the principal directions of curvature, and the external loads are allowed to be of non-axisymmetric type. The adhesive layer is modelled in two ways. The first approach assumes the adhesive layer to behave as a linear elastic material. The second, more realistic approach takes into account the predominantly inelastic behaviour of many polymeric adhesives, and it is shown that the non-linear behaviour affects the adhesive stress distribution even at low levels of external loading. The developed numerical solution procedures have been used to conduct a parametric study, and a few general design recommendations are given.     

A single camera three‐dimensional digital image correlation system for the study of adiabatic shear bands

We describe the capability of a high‐resolution three‐dimensional digital image correlation (DIC) system specifically designed for high strain‐rate experiments. Utilising open‐source camera calibration and two‐dimensional DIC tools within the MATLAB framework, a single camera three‐dimensional DIC system with submicron displacement resolution is demonstrated. The system has a displacement accuracy of up to 200 times the optical spatial resolution, matching that achievable with commercial systems. The surface strain calculations are benchmarked against commercially available software before being deployed on quasi‐static tests showcasing the ability to detect both in‐ and out‐of‐plane motion. Finally, a high strain‐rate (1.2×10³ s^?1) test was performed on a top‐hat sample compressed in a split‐Hopkinson pressure bar in order to highlight the inherent camera synchronisation and ability to resolve the adiabatic shear band phenomenon.     

Shear modulus estimation of the polymer polydimethylsiloxane (PDMS) using digital image correlation

Single lap joint under tensile load is used as a standard test specimen for characterizing adhesive properties and is considered the simplest form of adhesive joints. This paper presents an experimental investigation of full-field displacement in single lap adhesive joint. For the single lap joint specimen, steel adherends are bonded using a flexible rubber elastic polymer. The experimental procedure is carried out using the digital image correlation (DIC) method. This method consists of an optical-numerical experimental approach developed for full-field and non-contact measurements. The principal main is to describe a new testing methodology to analyze and estimate the adhesive shear modulus of polydimethylsiloxane (PDMS), which is a commercially available silicone elastic rubber.     

Thermoelastic Determination of Individual Stresses in a Diametrally Loaded Disk

Abstract: In a cyclically loaded structure which experiences adiabatic and reversible conditions, the measured change in temperature is associated with the change in the stresses. For proportional loading and isotropy, the technique measures information on the sum of the principal stresses. As engineering analyses frequently necessitate knowing the individual components of stress, supplementary experimental methods or information is sometimes required to ‘separate the stresses’. This need is circumvented here in that individual stresses are evaluated throughout a loaded disk without additional measured data by hybridising the TSA information with an Airy stress function and some known boundary conditions. Although demonstrated here to a circular aluminium disk under diametral compression, the approach is applicable to more complicated shapes, under more complex loadings and inverse problems. The technique does not pre‐suppose knowing the details. While TSA information has been displayed previously for a loaded disk, the authors are unaware of the stresses being thermoelastically separated for this case.     

Data-rich characterisation of damage propagation in composite materials

A novel methodology for the synchronised capture of high resolution white-light and infra-red (IR) images during a fatigue test is described. The approach allows digital image correlation (DIC) and thermoelastic stress analysis (TSA) to be applied practically simultaneously without the requirement to pause the cyclic load. The methodology is demonstrated on cross-ply carbon-epoxy specimens that have experienced damage induced by intermediate strain rate loading. Similar undamaged specimens are studied and the results from each compared. Various damage types are identified which include transverse cracking, delaminations and longitudinal splitting. The results are verified using X-ray computed tomography (CT).     

A new method for measurement of three dimensional residual stresses in a multi-pass butt welded joint

In this paper, a new method for the measurement of three dimensional residual stresses in a multi-pass butt welded joint is presented. The method involves the measurement of strain changes at a through hole surface as a coupon of material with the hole is separated from the body of interest and split. The finite element calculations are used to relate the strain changes at the measurenent points to the initial residual stresses at the hole location. The accuracy of the method is demonstrated by using it to measure a known stress field in a bent beam. The method has been used to measure the through thickness distribution of three dimensional residual stresses in a multi-pass butt welded joint. It could also be applied to the measurement of residual stresses in other welded joints.