Material heterogeneity or stress concentration: the thermoelastic response from woven composite materials subjected to cyclic fatigue

A study of the growth of fatigue damage in 2 x 2 twill woven composite materials, subjected to cyclic tensile loading are described. Thermoelastic stress analysis (TSA) is used to monitor the stress field. As a result of the damage, a net reduction in the thermoelastic signal is observed. Laminates are found to be more resistant to fatigue

Development of thermoelastic stress analysis as a non-destructive evaluation tool

A modified methodology is proposed in which only a single transient load is used for the TSA measurement. Specimens with different damage severities are tested and it is shown that the modified TSA method has the potential to be applied in the field as a non-destructive evaluation tool

Thermoelastic stress and damage analysis using transient loading

Thermoelastic stress analysis (TSA) is often regarded as a laboratory based technique due to its requirement for a cyclic load. A modified methodology is proposed in which only a single transient load is used for the TSA measurement. Two methods of imparting the transient load are validated against calculations and the conventional TSA approach. Specimens with different damage severities are tested and it is shown that the modified TSA method has the potential to be applied in the field as a non-destructive evaluation too

Digital image correlation vibrometry with low speed equipment

A low-cost method is presented which enables digital image correlation (DIC) with conventional cameras (i.e. not high-speed) to be used for determination of vibration deflected shapes via the use of a stroboscopic lamp and some simple ancillary circuits. For each natural frequency of the structure under consideration, a sequence of images is captured asynchronously with the vibrations using the DIC system and the resulting displacement fields are correlated with the excitation signal driving the vibration using a least-squares approach. Three approaches for performing this correlation are outlined, one of which is developed into the algorithm used for processing the present results to obtain the amplitude and phase of the vibration at each point on the specimen, allowing the deflected shape to be reconstructed. This process is illustrated using the example of a vibrating aluminium plate. The resulting shapes and frequencies agree well with finite element modal analyses of the plate

The use of a lock-in amplifier to apply digital image correlation to cyclically loaded components

An approach for processing strain data obtained from Digital Image Correlation (DIC) that can be used directly on components subject to cyclic loading, such as in fatigue testing, is described. A key challenge addressed herein is obtaining an accurate and precise surface strain map without recourse to expensive high speed cameras to capture data to map the load cycle. The basis of the approach is the application of a lock-in amplifier to the strain data obtained from images taken from low-cost low-frame rate cameras. The technique enables images to be captured throughout a fatigue test and hence evaluate the effect of any damage on the strain field, without interrupting the test. The methodology is assessed using an aluminium disc in diametric compression. It is demonstrated that the approach enables accurate strain maps to be obtained using a range of loading frequencies that are greater than the camera frame rate. A realistic example application of the technique on a fatigue test with an evolving crack in a T-shaped specimen is presented

On the fatigue response of a bonded repaired aerospace composite using thermography

Lock-in thermography was employed to investigate the repair efficiency of a bonded repaired aerospace composite subjected to step-wise cycling mechanical loading. The studied component (substrate) was artificially damaged with a 5 mm circular notch and subsequently repaired with a tapered bonded patch. Critical and sub-critical damage of the repaired component was monitored via thermography during 5 Hz tension–tension fatigue. The examination of the acquired thermographs enabled the identification of the patch debonding propagation as well as the quantification of the stress magnification at the patch ends and the locus of the circular notch. It was found that fatigue mechanical loading yields both thermoelastic and hysterestic phenomena with the latter being more prominent prior to the failure of the studied repaired component

Flaw and damage assessment in torsionally loaded CFRP cylinders using experimental and numerical methods

CFRP structural elements are prone to failure initiating from defects. While defects are expected after damage has occurred, flaws and voids can already be present after manufacturing. To study the criticality of such defects, CFRP cylinders have been manufactured from a lay-up that was designed to predict damage mode and to allow for controlled damage growth under torsional load. FEA simulations of defect-free and flawed cylinder models were performed to first ply/interface failure. X-ray computed tomography revealed that cylinders manufactured with different finishing had a completely different void content and distribution. Simulations of failure, using finite element models, for the two classes of void distribution are corroborated by experimental results for the ultimate load, and damage initiation from manufacturing flaws is confirmed. Digital speckle pattern interferometry was used to identify flaws using thermal and mechanical loading, while infrared thermography and thermoelastic stress analysis were used to identify possible failure initiation sites and monitor the failure process and damage growth, whilst the specimen was loaded in torsion

Patterns of Change in Collaboration Are Associated with Baseline Characteristics and Predict Outcome and Dropout Rates in Treatment of Multi-Problem Families. A Validation Study

Objective: The present study validates the Multi-Problem Family (MPF)-Collaboration Scale), which measures the progress of goal directed collaboration of patients in the treatment of families with MPF and its relation to drop-out rates and treatment outcome. Method: Naturalistic study of symptom and competence-related changes in children of ages 4-18 and their caregivers. Setting: Integrative, structural outreach family therapy. Measures: The data of five different groups of goal directed collaboration (deteriorating collaboration, stable low collaboration, stable medium collaboration, stable high collaboration, improving collaboration) were analyzed in their relation to treatment expectation, individual therapeutic goals (ITG), family adversity index, severity of problems and global assessment of a caregiver's functioning, child, and relational aspects. Results: From N=D 810 families, 20% displayed stable high collaboration (n=162) and 21% had a pattern of improving collaboration. The families with stable high or improving collaboration rates achieved significantly more progress throughout therapy in terms of treatment outcome expectancy (d=0.96;r=0.43), reaching ITG (d=1.17;r=0.50), family adversities (d=0.55;r=0.26), and severity of psychiatric symptoms (d=0.31;r=0.15). Furthermore, families with stable high or improving collaboration maintained longer treatments and had a bigger chance of finishing the therapy as planned. The odds of having a stable low or deteriorating collaboration throughout treatment were significantly higher for subjects who started treatment with low treatment expectation or high family-related adversities. Conclusion: The positive outcomes of homebased interventions for multi-problem families are closely related to "stable high" and an "improving" collaboration as measured with the MPF-Collaboration Scale. Patients who fall into these groups have a high treatment outcome expectancy and reduce psychological stress. For therapeutic interventions with multi-problem families it seems beneficial to maintain a stable high collaboration or help the collaboration, e.g., by fostering treatment expectation

Assessment of fatigue damage evolution in woven composite materials using infra-red techniques

Thermoelastic stress analysis (TSA) is used to study the growth of fatigue damage in single and twoply, 2 x 2 twill woven composite materials. Test specimens were subjected to a uniaxial tensile cyclicloading with maximum stresses of 10, 15 and 20% of the ultimate failure stress. The development offatigue damage locally within the weft yarns is monitored using high resolution TSA. The specimenswere subsequently inspected using optical microscopy to evaluate the location and extent of cracks.Cracks were found in the weft fibres, running transverse to the loading direction. It is demonstrated thatthe lighter weight fabric is more resilient to damage progression. A signature pattern is identified in theTSA phase data that indicates the onset and presence of fatigue damage in the composite material

Thermomechanical interaction effects in composite sandwich structures – initial experimental analysis and derivation of mechanical properties

An experimental study of the effect of increasing temperature on the tensile modulus of PVC foam is presented. The focus is on obtaining reliable measurements of specimen deformation using optical techniques based on digital image correlation (DIC). A FE model that accounts for the 3D strain distribution in the specimen was used to inform the test specimen design. A new testing methodology and test rig design is devised and described in the paper. Firstly, tests are conducted at room temperature using DIC to obtain the strain on two opposite surfaces of the specimen simultaneously, to assess the effect of any specimen misalignment and devise a correction approach referred to as a ‘misalignment ratio’. In the elevated temperature test only one face was optically accessible through an optical window in a thermal chamber. Therefore the experiments are performed over a temperature range of 30°C to 90°C using only a single camera and the misalignment ratio applied. It is shown that the proposed methodology provides precise measurements and the thermal degradation of the tensile modulus can be obtained