Failure mode and strength of uni-directional composite single lap bonded joints with different bonding methods

To evaluate the effects of ethylene oxide and gas-plasma sterilization on the failure strength and failure mode of pre-tied monofilament ligature loops. A randomized block design was used by blocking for suture (size 1 polyglyconate, size 2 polydioxanone) and sterilization technique (control, ethylene oxide, gas plasma). Twenty replicates of each suture-sterilization technique combination. Ligature loops were pre-tied using a 4S-modified Roeder knot then exposed to 1 of 3 treatments (control, ethylene oxide, or gas-plasma sterilization). Ligature loops were mounted onto a mechanical testing machine to evaluate the effects of sterilization on failure strength and failure mode (disruption, untying). There were no differences between polydioxanone and polyglyconate suture materials in failure strength or mode within sterilization methods. Both sterilization methods significantly increased the likelihood that suture loops would untie compared with controls subjects. Ligature loops that disrupted had a greater breaking strength than those that untied, regardless of sterilization method or suture type. Sterilization with either ethylene oxide or gas plasma of pre-tied, polyglyconate, or polydioxanone ligature loops significantly increases the in vitro likelihood of a modified Roeder knot untying. Further studies need to be conducted to determine the in vivo requirements for ligature loops.

Failure analysis of bolted connections of composites with drilled and moulded-in hole

This paper presents an experimental investigation of the compression behaviour of Carbon Fiber Reinforced Plastics (CFRP) tubes with different off-axis ply orientations. A series of compression tests with effective end-reinforcement were conducted on [04/±theta] CFRP tubes, with θ equal to either 0°, 30°, 45°, 60°, or 90°. Various failure progressions and fracture morphologies were measured using a high-speed camera and a scanning electron microscope. The failure modes and mechanisms of CFRP tubes with different stacking sequences were analysed in detail. The results indicate that the off-axis ply orientation greatly influences the compression behaviour. The adopted end-reinforcement ensures that nearly all of the CFRP tubes fail within the gauge length. When θ < 45°, the tubes exhibit various failure modes, and the scatter of strength is large. However, when θ ≥ 45°, the sole failure mode is a shear fracture of the inner 0° layers, and their scatter of strength is minor. A new shear failure mechanism is instrumented: the shear fracture direction changes from inclining along the circumferential direction to inclining along the radial direction when θ varies from 45° to 90°. The failure strength and off-axis ply orientation display a complex non-linear relationship. When θ = 60°, the compression strength becomes maximum at an average value of 602 MPa, and the scatter of strength is 2.71%.

Based on the progressive failure analysis method, taking account of 7 kinds of failure modes: stitch thread failure mode, resin extend (or compress) failure mode, fibre extend (or compress) failure mode, extend (or compress) delamination failure mode, a new failure analysis FE model was proposed for predicting the tensile strength of stitch-reinforced composite T-stiffened structures. The 3D solid element and truss element were used to simulate laminate and stitch thread individually in the FE model. The Hashin failure criterion and stiffness degradation technology are used to analyze damage failure of the stitch T-stiffened structure. Comparing the initial and final damage failure strength with those of experiments on stitch composite T-stiffened structures validated the Python secondary development program for the FE model presented in the paper. There was good agreement between the failure strength from the damage failure analysis and that of the experiment. This indicated that the model was correct, and that it could be a reference for stitch-reinforced composite design and analysis.

Influence of single or multi-factor coupling of temperature, humidity and load on the aging failure of adhesively bonded CFRP / aluminum alloy composite joints for automobile applications

The mechanical properties of adhesives are sensitive to both temperature and humidity. A modified Arcan device was proposed to characterize the failures of adhesive joints under mixed mode loading. Aluminum alloy butt joints (BJs), 45°scarf joints (45°SJs) and thick-adherend shear joints (TASJs) were manufactured to investigate the effects of loading conditions. Moreover, these joints were subjected to various durations of high-temperature aging (80°C) and hygrothermal aging (80°C/95% RH): 0 (unaged), 6, 12, 18, 24, and 30 days. The adhesive joints were analyzed by Fourier transform infrared (FTIR) spectroscopy, and the load–displacement curves, failure strengths, failure modes were analyzed. The results showed under high-temperature conditions, humidity can accelerate the rate of decrease in the failure strength of the adhesive joints, which resulted from the fracture of the polymer chains and the influence of internal stresses. As the hygrothermal aging duration increased, the fracture surfaces of the joints changed significantly. The number of cracks and cavities in the fracture surface increased as the aging duration increased. After aging, the failure strength values of the adhesive joints conformed to the quadratic stress failure criterion. Two curved surface equations reflecting the relationship between the quadratic stress criterion and the aging duration were established.

The strength of shaft joints between carbon fiber reinforced plastics (CFRP)and stainless steel bonded with epoxy resin was analytically and experimentally investigated at room temperature and low temperature (-70°C). The distributions of stress for tensile load and thermal stress for cooling in the joint were analyzed by applying the elastic finite-element method. The strength of the joints was predicted by applying the strength laws of CFRP, stainless steel, adhesive layer and their interfaces to the calculated stress distributions. The predicted strength was compared with the experimental strength of the joints. The effects of the overlapped length and diameter ratio between the adherends on the joint strength were examined at both conditions of room and low temperatures. The joint strength for the initial failure is saturated by a certain overlapped length, but the strength decreases with increasing diameter ratio. The final joint strength at low temperature is larger than that at room temperature.

In this paper, the effect of macro nonmetallic fibers (i.e., polypropylene fibers and basalt fibers) on the impact response of basalt fiber‐reinforced polymer‐reinforced concrete (FRP‐RC) discs is experimentally investigated using a self‐developed drop‐weight impact test device. The plain concrete and conventional steel‐reinforced concrete samples are explored as references. The impact resistance and failure behaviors are analyzed. Statistical analyses for first‐crack strength and failure strength are performed. The composite effect of basalt FRP bars and macro nonmetallic fibers on the impact energy at failure is also compared. The results indicate that the behaviors under impact load, that is, failure strength, crack number, the indent diameter, and penetration depth of the shriveled area, are greatly improved by adding of macro nonmetallic fibers, in particular macro polypropylene fibers. Additionally, the incorporation of these fibers into the basalt FRP‐RC transforms the brittle failure mode into a well ductile failure mode. Two‐parameter Weibull models are fitted by graphical methods and used to characterize the first crack strength and failure strength distributions. Reliability functions for first crack strength and for failure strength are estimated and failure strength can be predicted from first‐crack strength by using a linear regress model. The hybrid use of basalt FRP bars and macro nonmetallic fibers demonstrates a positive synergetic effect on the impact energy at failure.

This paper presents detailed experimental information on mode-II delamination development in fibre/epoxy composite materials and provides observations about the process zone in the vicinity of the crack tip. It is shown that the energy dissipated in delamination propagation is spent on two ways (i) creating new fracture surfaces (delamination) and (ii) nonlinear shear deformation in the composite plies adjacent to the delaminating interface. Therefore, the nonlinear process zone is not restricted to the resin-rich interface between the layers, but also extends into the fibre/epoxy composite layers. This is different from the conventional assumption in modelling delamination using cohesive elements where the fibre/epoxy layers are fully linear-elastic and the process zone is lumped at the plane of fracture. Based on the accurately measured displacement field around the crack tip, the experimental traction-separation relation at the interface is found to be trapezoidal which is again different from the conventional bilinear cohesive law.

This paper summarises the authors' work on strength and failure mode estimation of self-piercing rivets (SPRs) for automotive applications. First, the static cross tension strength of an SPR joint is estimated using a lower bound limit load based strength estimator. Failure mode associated with the predicted failure strength can also be identified. It is shown that the cross tension strength of an SPR joint depends on the material and gage combinations, rivet design, die design and riveting direction. The analytical rivet strength estimator is then validated by experimental rivet strength measurements and failure mode observations from nine SPR joint populations with various material and gage combinations. Next, the estimator is used to optimise rivet strength. Two illustrative examples are presented in which rivet strength is improved by changing rivet length and riveting direction from the original manufacturing parameters.

Novel adherend laminate designs for composite bonded joints

Carbon fiber epoxy laminates are extensively used in the aerospace industry as they offer a combination of high performance with relatively low weight. However, damage may occur in these materials, either at the manufacturing level or during use, which may affect their structural integrity. Among the type of damage which may occur, delamination is a common defect which causes a reduction in compressive strength and thus limits the life of laminated composites. If undetected, delamination may propagate during use due to mechanical loads and in the worst case may cause fracture of the material. Therefore, delamination in composite materials needs to be efficiently detected and accurately quantified for safety and economic reasons. Because of their heterogeneity, multi-layered structure and anisotropy, the inspection of composite materials remains difficult with conventional NDT methods. A method making use of Foucault currents, and relying on the principle of electromagnetic induction, can be adapted for the detection and characterization of damage in composite materials. The use of Foucault currents in characterizing delaminated areas at interfaces between plies in carbon fiber/epoxy laminates is addressed in this paper. It is demonstrated that Foucault currents can accurately detect and quantify delamination in CFRP materials. Furthermore, the potential of this technique for monitoring defect propagation in composite materials is discussed.

The purposes of this study are two folds: one is to attempt to develop a new, suitable test method for differentiating the factors that affect the variability of die strength, and the other is to investigate the failure and fatigue strength of silicon dies. In this study, a new test method, a plate-on-elastic-foundation test (PEFT) associated with point-or line-loading has been proposed and evaluated. It is found that the PEFT can provide not only a simple, chipping-free test for dummy or real IC chips without limitation of size, but also a stress field similar to the temperature loading. The test data obtained from the current test are presented for those failed on IC and ground surfaces in real IC chips. It is found that the good consistency of the die strength data with minor scatter from both the point- and line-load tests for the specimens failed on IC surfaces, but not for the ones failed on the ground surfaces. The reason for this is that the inconsistency of strength data from both tests for failure on ground surfaces is due to edge chipping involved, and the larger scatter is resulted from the combined factors of the angle of grinding marks, planes of weakness of material, and loading stress state. The surface roughness of the dies measured by atomic force microscopy is correlated with the failure mode and strength from the tests. It is found that the silicon die strength is dominated by the roughness on failure surface, and their failure modes always appear along the parallel and normal to the edges of the die, which might be the weak plane of the crystal lattice of silicon. The specimens with artificial cracks have been further tested. It has been proved that the die strength dominated by the crack initiation depends on the most severe defect and its failure mode is controlled by a special weak plane after the crack initiation. This study concludes that there are four factors to influence die strength: the surface conditions of the die (including grinding-line direction and surface roughness), the edge crack of the die (so-called edge chipping created during the cutting process), the weak planes of the crystal lattice of silicon, and, sometimes, the cause from different loading types. The fatigue strength of the dies is also determined to be about 25% lower than static one.

The research presented in this paper is an experimental study and numerical analysis on mechanical behavior of the adhesively-bonded joint between FRP sandwich bridge deck and steel girder. Generally, there are three typical stress states in the adhesively-bonded joint: shear stress, tensile stress, and combination of both. To realize these stress states in the adhesively-bonded joint during tests, a specific loading device is developed with the capacity of providing six different loading angles, which are 0°(pure tension), 18°, 36°, 54°, 72° and 90°(pure shear). Failure modes of adhesively-bonded joints are investigated. It indicates that, for the pure shear loading, the failure mode is the cohesive failure (near the interface between the adhesive layer and the steel support) in the adhesive layer. For the pure tensile and combined loading conditions, the failure mode is the combination of fiber breaking, FRP delamination and interfacial adhesion failure between the FRP sandwich deck and the adhesive layer. The load-bearing capacities of adhesive joints under combined loading are much lower than those of the pure tensile and pure shear loading conditions. According to the test results of six angle loading conditions, a tensile/shear failure criterion of the adhesively-bonded joint is obtained. By using Finite Element (FE) modeling method, linear elastic simulations are performed to characterize the stress distribution throughout the adhesively-bonded joint.

This study developed a kind of non-woven carbon tissue reinforced adhesive film (NWCTRAF) with high bonding strength and reliability by interleaving the NWCT into the adhesive film. The manufacturing process, bonding strength, Weibull distribution, metallography of cross-section, and fracture surface morphology of NWCTRAF specimens were investigated. The results showed that the average bonding strength and Weibull characteristic strength (WCS) of single lap joint (SLJ) were 27.4a and 28.2 MPa, which increased by 17.6% and 15.1%, respectively compared with the blank specimen, and the Weibull modulus risen from 9.51 to 18.72, which indicated that NWCTRAF specimens had higher bonding reliability. The mechanism of micro-crack formation was obtained by analyzing the cross-section of loading 95% WCS without visible damage. The metallographic section showed that the microcracks of the blank specimen originated from the interface between carbon fiber-reinforced polymer (CFRP) and adhesive layer, while the microcracks of the NWCTRAF specimen originated from the resin-rich regions between the carbon fiber fabric layers in CFRP. Typical failure modes were gained from optical microscope and SEM. Failure modes of blank specimen included more interface failure of CFRP-adhesive and Al-adhesive, while NWCTRAF specimen had more carbon fiber tearing and delamination of CFRP. The fracture morphology of adhesively cohesive failure included the pull-out of short carbon fibers and the appearance of rough and multi-stage resin in the NWCTRAF specimen, which improved the toughness of the adhesive layer. The bridging effect of short carbon fibers in the NWCTRAF disperse and transfer stress to CFRP and fail at resin-rich regions of CFRP, decrease the bonding interface's direct failure, and improve the bonding strength and reliability.