Whereas many research have examined the conduct of adhesive bonds underneath quasi-static and fatigue loading circumstances, analysis centered on their response to affect loading is scarce. This hole is noteworthy as a result of, in service, bonded joints are sometimes uncovered to sudden or surprising high-rate occasions, similar to instrument drops throughout upkeep, particles affect throughout flight or driving, and even crash eventualities.
The dearth of ample understanding of adhesive joint efficiency underneath such dynamic circumstances can hinder their broader adoption in crucial load-bearing purposes, particularly the place crashworthiness or harm tolerance is paramount. Moreover, dynamic occasions typically contain a mixture of excessive pressure charges, speedy stress wave propagation, and sophisticated failure mechanisms, making it important to analyze these phenomena past conventional quasi-static approaches.
These pictures present the bonded take a look at coupons (a) and the equipment (b) during which the specimens are mounted earlier than the take a look at. Supply: Ankara Yildirim Beyazit College
Structural adhesives exhibit optimum load-bearing conduct underneath shear-dominated circumstances. One key distinction between affect and quasi-static loading is the rise in yield power with greater pressure charges. This phenomenon is well-documented in metals, however equally related in adhesives, the place molecular mobility turns into restricted at excessive deformation speeds, resulting in an obvious stiffening impact. Nevertheless, past a threshold pressure price, this yield power enhancement tends to plateau.
Incorporating Z-pins, or reinforcing fibers, into single-lap adhesive joints can improve their post-impact load-bearing capability and power absorption in contrast with standard joints. The improved residual power comes from the hybrid joint’s capacity to distribute and take in affect power, lowering delamination and enhancing structural integrity after affect.
The commonest strategies to check bonded joints at elevated loading charges are pendulum affect gadgets, drop weight testers, and Break up Hopkinson strain bar techniques. These setups are usually efficient for moderate-speed impacts, usually underneath 10 meters per second (m/s) for drop checks and about 5 m/s for pendulums.
Nevertheless, structural adhesive joints in real-world purposes could encounter a lot greater loading charges as a consequence of blast waves, high-speed particles, chicken impacts, or collisions with shifting objects. The problem is additional compounded by the truth that experimental replication of such excessive eventualities is advanced, typically requiring specialised high-speed gear and sturdy security measures. Consequently, analysis into ballistic or near-ballistic loading of adhesive joints has been restricted.
This lack of empirical knowledge represents a barrier for industries that need to use adhesives in safety-critical constructions uncovered to potential high-velocity impacts. With out such knowledge, predictive modeling of joint survivability underneath life like accident or menace eventualities stays unsure.
In search of fast solutions on meeting and manufacturing subjects?
Attempt Ask ASM, our new sensible AI search instrument.
Ask ASM
We got down to tackle this data hole. Our examine explores the dynamic response of SLJs subjected to high-strain-rate loading. A custom-designed ballistic equipment was employed to topic SLJs to affect from a lead projectile at a velocity of 288 m/s. Our take a look at joints consisted of two an identical aluminum coupons. Every coupon was 3, 4 or 5 millimeters thick. The overlap lengths was both 15 or 25 millimeters.
The broken joints had been subsequently characterised by quasi-static tensile checks to evaluate their residual load-bearing capability. To achieve a deeper understanding of the mechanical degradation, the impacted joints had been in contrast with non-impacted specimens.
Complementary to the experiments, we carried out detailed finite aspect (FE) simulations utilizing ABAQUS 2021. We employed the Johnson-Cook dinner materials mannequin to explain the plastic conduct of the metallic adherends and projectile, whereas the adhesive layer was represented utilizing a cohesive zone mannequin, enabling complete evaluation of harm initiation and propagation. This twin experimental-numerical method supplies a uncommon, holistic view of bonded joint efficiency underneath ballistic loading, bridging the crucial hole between laboratory testing and real-world service calls for.
The take a look at setup (a) consisted of a pressurized air gun, a high-speed imaging system, knowledge acquisition gear, and a {custom} clamping fixture to securely maintain the take a look at specimens. A high-speed digicam captured the bullet in flight (b). Supply: Ankara Yildirim Beyazit College
Experimental Work
The SLJs had been made out of 6061 aluminum alloy bonded with a modified epoxy movie adhesive. The coupons had been sanded and cleaned previous to bonding. The bonding course of was performed in a digitally managed oven at 125 C underneath a strain of two bars for one hour, in step with the producer’s pointers.
The take a look at setup consisted of a pressurized air gun, a high-speed imaging system, knowledge acquisition gear, and a {custom} clamping fixture to securely maintain the take a look at specimens. Measuring 5.5 millimeters in diameter, the projectiles had been round-nosed lead pellets, every weighing 1.25 grams. The gap from the projectile to the goal was 210 centimeters.
After affect, the joints had been subjected to quasi-static tensile loading utilizing a common testing machine with a most load capability of 100 kilonewtons. To evaluate the extent of efficiency degradation, the outcomes obtained from the impacted joints had been in comparison with these from pristine, unimpacted specimens.
This illustration exhibits the 3D finite aspect mannequin of the lap joint subjected to a high-speed affect adopted by quasi-static tensile loading. Supply: Ankara Yildirim Beyazit College
Numerical Modeling
Numerical analyses had been performed utilizing ABAQUS to analyze the dynamic response of SLJs underneath affect circumstances. We developed an in depth 3D FE mannequin. The aluminum coupons had been represented by eight-node linear brick parts incorporating decreased integration and hourglass management to enhance each computational effectivity and numerical stability.
To make sure correct prediction of stress focus and progressive harm, a extremely refined mesh was utilized within the overlap zone, that includes aspect dimensions of roughly 150 by 150 by 150 microns for the adherends and barely coarser parts of 150 by 200 by 150 microns for the adhesive layer, leading to roughly 16,666 parts throughout the adhesive for an overlap size of 15 millimeters.
This simulation picture exhibits the deformed form of the SLJ six microseconds after the bullet affect. Supply: Ankara Yildirim Beyazit College
Past the overlap area, a graded mesh with a bias ratio of two alongside the joint size was employed for the coupons. Mesh convergence was validated by evaluating the expected force-displacement curves, which exhibited deviations of lower than 5 p.c relative to outcomes obtained from finer mesh. This mesh configuration was additionally examined for the broken space after the affect take a look at, and the distinction in comparison with a finer mesh was discovered to be lower than 3 p.c.
The adhesive layer—generated by separating the overlap part from the adherends—was simulated utilizing linear cohesive parts.
The simulation process was carried out in two consecutive levels. Within the first stage, the SLJ was impacted by a deformable projectile modeled with strong parts and assigned an preliminary velocity of 288 m/s.
This simulation picture exhibits the change within the distributions of von Mises stress on the bonded joint 4 microseconds after high-speed affect with the bullet. Supply: Ankara Yildirim Beyazit College
The second stage simulated the tensile loading of the deformed configuration obtained from the previous evaluation. Throughout each levels, one finish of the SLJ was totally constrained in all levels of freedom, whereas the other finish remained mounted through the affect section and was subsequently launched to permit axial displacement within the tensile loading step.
To mannequin the coupons and the projectile, the Johnson-Cook dinner materials mannequin was employed, since it’s well known for its suitability in capturing the conduct of supplies subjected to high-strain-rate deformations. This constitutive mannequin encapsulates pressure hardening, pressure price sensitivity, and thermal softening results inside its formulation.
For simulating the adhesive layer, a cohesive zone modeling method was adopted. On this methodology, floor tractions are ruled by the relative displacements throughout cohesive parts. A triangular bilinear traction-separation regulation was applied to explain this conduct.
Harm initiation is assumed to happen when a quadratic nominal stress criterion—outlined as a mixture of the stress ratios throughout completely different failure modes—reaches unity. As soon as harm is initiated, a linear softening conduct is triggered, lowering the traction in every path to zero upon reaching a crucial separation. This crucial displacement is derived from the corresponding fracture toughness and interfacial power. The evolution of harm is additional ruled by a second quadratic interplay involving power dissipation.
This graph exhibits the force-displacement curves from tensile checks obtained experimentally (experiments 1–3) and from simulations. Our finite aspect mannequin was very near precise outcomes. Supply: Ankara Yildirim Beyazit College
Within the harm evolution section, the constitutive relationships for mixed-mode loading make use of a single harm variable, SDEG, which characterizes the discount in materials stiffness throughout all loading modes. Within the evaluation, this harm variable is calculated utilizing an equal traction–separation curve.
The development of harm throughout the adhesive layer was tracked within the simulations utilizing this parameter, which ranges from 0, indicating no harm, to 1, indicating full harm at an integration level throughout the cohesive parts. As soon as the variable reaches a worth of 1 in any respect integration factors of a cohesive aspect, the aspect loses its load-bearing capability completely and is faraway from the mannequin.
It is very important emphasize that each stiffness and fracture power values stay fixed throughout completely different loading charges. Accordingly, materials properties obtained underneath quasi-static take a look at circumstances had been utilized uniformly in all numerical simulations.
These pictures present the highest view of the bonded joint after affect with the bullet. The left picture (a) exhibits the precise joint; the best picture (b) exhibits the numerical mannequin. The underside picture (c) exhibits the deformation of the joint after tensile testing. Supply: Ankara Yildirim Beyazit College
Outcomes
For our FE evaluation, we created two fashions of the bonded joint: quasi-static and dynamic. Our simulation with the quasi-static mannequin indicated that the adhesive layer would fail utterly if subjected to affect loading. That was not the case in our real-world testing.
Because of this, the corresponding parameters had been decided by FE based mostly calibration. The yield power of epoxy adhesives will increase markedly with rising pressure price—reaching practically threefold underneath compressive loading and as much as fivefold underneath tensile loading. Importantly, this enhance in power happens whereas the adhesive’s elastic modulus stays practically unchanged between high-strain-rate and quasi-static loading circumstances.
Thus, the elevated cohesive power values included in our dynamic mannequin are in keeping with strain-rate sensitivity developments. The dynamic mannequin efficiently preserved joint integrity through the affect occasion.
Within the simulation, the bullet penetrates the joint throughout the first 15 to twenty microseconds after which rebounds after present process extreme deformation. Notably, the force-displacement curves from post-impact tensile checks, obtained each experimentally and thru simulation, confirmed robust settlement.
Because of this, we selected the parameter set from our dynamic mannequin for subsequent analyses. It is usually essential to focus on that the conventional cohesive power utilized in our dynamic mannequin is 240 megapascals, which is roughly 5 occasions better than the worth reported for quasi-static circumstances.
Because the extremities of the overlap area are predominantly subjected to peeling stresses, preliminary harm usually initiates at these ends and propagates inward underneath quasi-static tensile loading. Subsequently, harm confined to the center part of the adhesive had restricted affect on total joint efficiency.
These load-displacement curves present the distinction in tensile power between the bonded joint with a 15-millimeter overlap (a) and the joint with a 25-millimeter overlap (b). Not surprisingly, the latter was stronger. Supply: Ankara Yildirim Beyazit College
Affect of Overlap and Substrate Thickness
Our experimental outcomes indicated that the post-impact tensile power of the meeting with a 15-millimeter overlap exhibited important degradation, with its load-bearing capability lowering by roughly 33 p.c. In distinction, the meeting with a 25-millimeter overlap largely maintained its post-impact structural integrity. These findings recommend that elevated overlap size mitigates the hostile mechanical results of affect loading on the joint’s tensile efficiency.
These pictures present harm distribution on the adhesive layer for the specimens instantly after affect (prime), simply earlier than failure throughout tensile testing (center), and for the management specimens simply earlier than failure throughout tensile testing (backside). Supply: Ankara Yildirim Beyazit College
The harm distribution throughout the adhesive layer throughout tensile testing confirmed a major distinction between the impacted and non-impacted assemblies. Within the non-impacted specimens, harm steadily developed from the central area towards the ends of the adhesive layer, exhibiting a comparatively uniform development alongside the width of the joint. Nevertheless, within the impacted samples, the presence of pre-existing harm localized on the middle—ensuing from the projectile strike—led to a extra heterogeneous harm sample, with outstanding harm concentrated in each the central and finish areas of the adhesive layer.
The FE evaluation of harm development within the adhesive layer indicated that bullet affect induced harm on the ends for shorter overlap lengths, whereas for longer overlap lengths, harm was confined to the middle. Because the ends of the overlap size bore extra tensile loading in comparison with the central area, joints with an overlap size of 15 millimeters skilled mechanical degradation, whereas these with a 25-millimeter overlap size didn’t.
After we checked out how substrate thickness affected bond power after affect, our outcomes had been counterintuitive. We discovered that the tensile power of the assemblies post-impact decreased because the thickness of the substrates elevated. After affect, the tensile power of assemblies made with 3-millimeter-thick coupons decreased by 17 p.c in contrast with assemblies that had not been hit. In distinction, the tensile power of assemblies made with 4-millimeter-thick coupons decreased by 34 p.c, whereas the tensile power of assemblies made with 5-millimeter-thick coupons decreased by 50 p.c.
This may be attributed to the rising mismatch in bending stiffness between the adhesive layer and the substrates because the latter change into thicker. The adhesive layer possesses considerably decrease stiffness in contrast with the substrates, and this disparity intensifies because the thickness of the substrates will increase.
In brief, the harm of bonded assemblies after affect is ruled by two competing elements: the rising stiffness mismatch between the substrate and adhesive layers, and the tendency towards extra homogenized deformation patterns within the adhesive layer at better substrate thicknesses.
Wanting ahead, future research will discover the harm mechanisms related to repeated impacts, consider the progressive degradation of mechanical efficiency underneath various affect energies and loading sequences, and study how completely different overlap lengths affect the residual power of the joints.
Editor’s be aware: This text is a abstract of a analysis paper co-authored by Murat Demiral, Ph.D., and Ali Mamedov, Ph.D., of the American College of the Center East Faculty of Engineering and Expertise in Eqaila, Kuwait. To learn the complete paper, click on right here.
For extra info on adhesive bonding, learn these articles:
Automated Meting out in Aerospace
PSAs for Aerospace Meeting
Structural Adhesives for Automotive Meeting
