Backface Strain Technique Research Articles

A novel approach for damage assessment in adhesively bonded composite joints using backface strain technique

In this study, the backface strain (BFS) is measured by both digital image correlation (DIC) and fiber optic sensors (FOS) to detect the crack initiation and propagation in adhesively bonded composite single-lap joints (SLJ). BFS measures the resultant strain deriving from the positive strain, due to tensile load, and negative strain related to the bending moment. A point, called zero-strain point (ZSP), can be detected on the substrate surface of SLJ due to the concurrent effect of these positive and negative strains. The experimental activity shows that the value of the ZSP changes when the crack starts to propagate. Thus, this point can be used to monitor the service conditions of adhesive joints. The effect of joint dimensions on the position of the ZSP is investigated when the joint is subjected to quasi-static loading. In addition, the applicability of the method is investigated under a cyclic loading condition. The work shows that the ZSP can be used as an index to monitor joint healthiness. Furthermore, FOSs can be used for an in-situ monitoring of the joint.

Comparison of cohesive zone and continuum damage approach in predicting the static failure of adhesively bonded single lap joints

The paper presents a comparison of the cohesive zone model (CZM) and the continuum damage mechanics approach in predicting the static failure of a single lap joint (SLJ). The effect of mesh size and viscosity were studied to give more understanding on the failure load and computational time. Both the load–displacement response and the backface strain technique were utilised to compare the validity of predictions. Peel and shear stress and damage distributions along with the damage progression are compared to understand the behaviour of the models in predicting the static failure response. In general, both approaches show good accuracy in predicting the failure load; however, the cohesive zone approach requires shorter computation time than the continuum damage approach. The continuum damage approach shows some mesh-dependency particularly for elements with high aspect ratios, whereas the cohesive zone approach is not. The continuum damage approach is less sensitive than the cohesive zone approach to the artificial damping required to achieve convergence. Another interesting finding is using the same ultimate stress level of damage in the continuum damage approach at the peak load is much lower than that in the cohesive approach; but the failure process in this approach is faster.

The static and fatigue responses of aged metal laminate doublers joints under tension loading

Experimental studies have been undertaken to investigate the static and fatigue responses of metal laminate doublers (MLD) joints under tension loading after ageing in deionised water at a temperature of 50 °C up to 2 years. It was found that absorbed water did not have a significant effect on the static and fatigue degradation of the MLD; however, corrosion pits located on the aluminium surfaces caused a reduction in fatigue life. Inevitably, the laminate contained butts where co-planar aluminium sheets were joined, and it was found that the position of the butt affected the static response of the MLD but, due to a restricted data set, it has not been possible to assess the effect under fatigue loading. The backface strain technique in conjunction with video microscopy has been utilised to monitor the damage of the adhesive bondlines, the butts and the aluminium layers and successfully identified both the localised and the global damage in the MLD. Most of fatigue failures initiated at the stringer bondline edge and at the aluminium layers where the butt was located. A careful design should be made to reduce the stress concentration at the stringer edge and to avoid positioning the butts at the upper layer of aluminium close to the stringer edge.

Modelling the static response of unaged adhesively bonded structures

A cohesive zone model has been used to model the progressive damage in adhesively bonded aluminium monolithic single lap joints and laminated doublers. The backface strain technique was used to monitor the damage process in the adhesive layer and was also key in the calibration of a unique set of cohesive zone properties in the single lap joint. Further, this backface strain technique has been successfully used to assess the effect of substrate plasticity, position of cohesive elements, traction and fracture energy, and adhesive fillet in a monolithic single joint. The calibrated cohesive properties have then been successfully used to predict the static strength and backface strain response of the doublers in bending.

The fatigue response of environmentally degraded adhesively bonded aluminium structures

Experimental studies have been undertaken investigating the effect of moisture on the fatigue response of adhesively bonded monolithic single lap joints and laminated doublers loaded in bending, both made of the same materials. The joints were aged in deionised water at a temperature of 50°C for up to 2 years exposure. The backface strain technique was employed to monitor damage initiation and propagation in the joints. The test results show that the fatigue life degraded with increasing moisture content and tended to level off when approaching saturation. The failure surfaces were cohesive in the adhesive. Numerical fatigue modelling has been undertaken to predict the fatigue response of these joints utilising a strain-based fatigue damage law integrated with a bilinear traction-separation cohesive zone model. The residual stresses due to thermal and swelling strain were included in the model. Good agreement was found between the predicted fatigue response and the experimental results.

Experimental and numerical investigation of the static response of environmentally aged adhesively bonded joints

The aim of this research is to investigate the effect of moisture on the static response of adhesively bonded monolithic single lap joints and laminated doublers loaded in bending. All joints were made of aluminium alloy Al 2024-T3 bonded using epoxy film adhesive FM 73M OST. The joints were aged in deionised water at a temperature of 50°C for up to 2 years exposure. The use of different widths of specimen (5mm for monolithic single lap joints and 15mm for laminated doublers) allowed both full and partial saturation of the adhesive layer. The bulk adhesive has been characterised to obtain the coefficient of moisture diffusion, the coefficient of thermal and moisture expansion and the moisture dependent mechanical properties. The testing results showed that the mechanical properties degraded in a linear way with the moisture content. The residual strength after exposure decreased with increasing moisture content (exposure time) and tended to level off towards saturation. The damage evolution and failure of the joint has been successfully monitored using the backface strain technique and in-situ video microscopy. Progressive damage finite element modelling using a moisture dependent, bilinear traction-separation law has been undertaken to predict the residual strength. Residual stresses due to thermal and swelling strains in the adhesive layer have been included; however their effect on the predicted static strength was not significant. Good agreement was found between the predicted residual strength and the experimental result.

Predicting fatigue damage in adhesively bonded joints using a cohesive zone model

A reliable numerical damage model has been developed for adhesively bonded joints under fatigue loading that is only dependant on the adhesive system and not on joint configuration. A bi-linear traction–separation description of a cohesive zone model was employed to simulate progressive damage in the adhesively bonded joints. Furthermore, a strain-based fatigue damage model was integrated with the cohesive zone model to simulate the deleterious influence of the fatigue loading on the bonded joints. To obtain the damage model parameters and validate the methodology, carefully planned experimental tests on coupons cut from a bonded panel and separately manufactured single lap joints were undertaken. Various experimental techniques have been used to assess joint damage including the back-face strain technique and in situ video microscopy. It was found that the fatigue damage model was able to successfully predict the fatigue life and the evolving back-face strain and hence the evolving damage.

Mixed-mode crack growth in bonded composite joints under standard and impact-fatigue loading

Carbon fibre reinforced polymers (CFRPs) are now well established in many high-performance applications and look set to see increased usage in the future, especially if lower cost manufacturing and solutions to certain technical issues, such as poor out-of-plane strength, can be achieved. A significant question when manufacturing with CFRP is the best joining technique to use, with adhesive bonding and mechanical fastening currently the two most popular methods. It is a common view that mechanical fastening is preferred for thicker sections and adhesive bonding for thinner ones; however, advances in the technology and better understanding of ways to design joints have lead to increasing consideration of adhesive bonding for traditionally mechanically fastened joints. In high-performance applications fatigue loading is likely and in some cases repetitive low-energy impacts, or impact fatigue, can appear in the load spectrum. This article looks at mixed-mode crack growth in epoxy bonded CFRP joints in standard and impact fatigue. It is shown that the back-face strain technique can be used to monitor cracking in lap-strap joints (LSJs) and piezo strain gauges can be used to measure the strain response of impacted samples. It is seen that there is significant variation in the failure modes seen in the samples and that the crack propagation rate is highly dependent on the fracture mode. Furthermore, it is found that the crack propagation rate is higher in impact fatigue than in standard fatigue even when the maximum load is significantly lower.

Experimental Study on Fatigue Life of Steel Beams Strengthened with a CFRP Plate

The adhesive bonding between the steel beam and carbon fibre reinforced polymer (CFRP) plate is the weakest link and fatigue performance is a major consideration. This paper gives details of a fatigue test programme of a series of small-scale steel beams bonded with a CFRP plate. Two phases of the fatigue life, including crack initiation life and crack propagation life, are considered. Backface-strain technique was applied to monitor crack initiation. An S-N curve was developed from the test results. The curve correlates the maximum principal interfacial stress at the plate end to the crack initiation life. The fatigue limit of the S-N curve was found to be about 30% of the ultimate static failure stress. In accordance with Paris Law, moreover, an equation was developed to predict the number of cycles during the crack propagation. The empirical coefficients of the equation were obtained from the fatigue test results. This equation can correctly predict the crack propagation life. The fatigue load range affects the fatigue life, but its significance is much less than the magnitude of the maximum load in the load range.

Monitoring Fatigue Crack Growth in Compact Tension Specimens Using Piezoelectric Sensors

This paper aims to report the results of an experimental study on the application of piezoelectric dynamic strain sensors for crack length measurement in fracture mechanics specimens. The performance of the piezoelectric sensors was assessed through fatigue crack propagation tests in compact tension (CT) specimens. Sensors of polarized polyvinilidene fluoride polymer (PVDF) were bonded to the back face of CT specimens, in the same manner as the electrical resistance strain gages installed for crack length measurement in the back face strain technique. The results showed that, mainly due to its high sensitivity to strain, the use of piezoelectric materials as dynamic strain sensors can contribute to the experimental investigation in the field of fracture mechanics.

Fatigue performance of metallic beam strengthened with a bonded CFRP plate

This paper gives details of a fatigue test programme of a series of small-scale steel beams bonded with a carbon fibre reinforced polymer (CFRP) plate. Backface-strain technique was used to detect crack initiation and monitor crack growth. Results show that this technique could detect crack initiation and track the deterioration of the adhesive layer. The strains recorded show that crack initiates and grows in Mode I earlier than in Mode II in the bonded joints, and this finding agrees with the observations on double-lap joints published in literature. The strains recorded also show that the adhesive spew fillet at the end of the CFRP plate is beneficial to the fatigue performance, although the improvement is not significant. An S– N curve was developed from the test results. The curve correlates the maximum principal interfacial stress at the plate end to the number of cycles to crack initiation. This maximum stress can be calculated from the authors’ previous analytical work. The fatigue limit of the S– N curve was found to be about 30% of the ultimate static failure stress.

The prediction of crack growth in bonded joints under cyclic-fatigue loading I. Experimental studies

The performance of adhesively-bonded joints under monotonic and cyclic-fatigue loading has been investigated using a fracture-mechanics approach. The joints consisted of an epoxy film adhesive which was employed to bond aluminium-alloy substrates. The effects of undertaking cyclic-fatigue tests in (a) a ‘dry’ environment of 55% relative humidity at 23°C, and (b) a ‘wet’ environment of immersion in distilled water at 28°C were investigated. In particular, the influence of employing different surface pretreatments for the aluminium-alloy substrates was examined. In addition, single-lap joints were tested under cyclic fatigue loading in the two test environments, and a back-face strain technique has been used which revealed that crack propagation, rather than crack initiation, occupied the dominant proportion of the fatigue lifetime of the single-lap joints. In Part II, the data obtained in the present Part I paper will be employed to predict theoretically the lifetime of the adhesively-bonded single-lap joint specimens.

Investigating Fatigue Damage Evolution In Adhesively Bonded Structures Using Backface Strain Measurement

Predicting the service life of adhesive joints under fatigue loading remains a major challenge. A significant part of this task is to develop laws that govern the crack initiation phase. This paper contributes to this area through the development and application of the backface strain technique. A numerical study was carried out to investigate the effect of key parameters on the technique and to determine optimum gauge specification and location. Calibration curves were then produced relating the change in strain to the extent of damage. These numerical studies were then validated by undertaking a series of fatigue tests on both aluminium and GRP (glass-reinforced polymer)-bonded joints. Following various degrees of predicted damage the joints were carefully sectioned, polished, and studied using optical microscopy. The predicted and observed damage showed close correlation. The fatigue tests have also indicated that, for unmodified joints (intact fillets), even at high loads (50% static failure load) there was an initiation phase that accounted for about half the fatigue life of the joint. Removal of the adhesive fillet has been found to eliminate the initiation phase and consequently reduce fatigue life.

Fatigue Crack Initiation in Solder Joints

A backface strain technique is introduced to examine fatigue crack initiation in solder lap joints. The technique detects the fatigue crack initiation by monitoring the backface strain at the end of the overlap. Variation of the backface strain with the development of a crack was simulated by finite element method. The simulation indicated that the backface strain at the end of the overlap reached a peak value when a fatigue crack initiated. Experimental verification was carried out in 63Sn-37Pb solder joints. The backface strain was recorded as a function of stress cycle to demonstrate the applicability of this technique. Experimental results showed that fatigue crack initiation took about half of the fatigue lifetime of the solder joints.

A Backface Strain Technique for Detecting Fatigue Crack Initiation in Adhesive Joints

Abstract A new backface strain technique was developed to detect fatigue crack initiation in adhesive-bonded lap joints. The technique was based on the special strain distribution in single lap joints and detected the fatigue crack initiation by the switch in the direction of the strain variation. Use of this technique not only permits the determination of fatigue crack initiation life in the joint, but also allows the site of crack initiation to be located. With the assistance of this new backface strain technique, a fatigue crack was found to initiate in the adhesive but to propagate towards the interface to continue its growth on the interface and to cause the final separation of the joint along the interface. Measurements of fatigue crack initiation lives at different stress levels indicate that the adhesive-controlled crack initiation took an increasingly greater proportion of the total fatigue life as the stress decreased, so that the lifetime in the long-life regime was dominated by the resistance ...

Finite element analysis of roughness and oxide induced crack closure in compact specimen.

The role of crack surface roughness and oxide debris on crack growth characteristics was examined by finite element method for a compact specimen. Several asperities were attached on the crack surfaces. The analysis shows the influence of rigidty and strength of the asperities, asperity length, asperity thickness and the distance from the crack tip. The results were interpreted in termes of the opening stress intensity Kop and the effective CTOD, Δσo*. It is concluded that the strength of asperities has a marked influence on Kop. On the other hand, rigidty is a controlling factor of Δσo*. The analysis also gives a support to resolve the uncertainty concerning the experimental determination of Kop using back face strain technique.