Impact Damage Area Research Articles

Comparison with Low-Velocity Impact and Quasi-staticIndentation Testing of Foam Core Sandwich Composites

The need for quasi-static indentation test method for modeling low-velocity foreign object impact events would prove to be very beneficial to researchers. In order to examine whether it is feasible, series of quasi-static indentation and low-velocity impact tests were carried out and compared. An analysis of the relationships between impact energy (or quasi-static indentation force) and damage area, dent depth indicates clearly that dent depth was selected as the damage parameter to set up damage relationship between the two tests. The knee points of dent depth appearing in the two tests curves were very close. The variation tendency of the dent depth, the process curves and the cross sectional damage views of the two tests were in similarity. Results show that no distinct differences could be seen between low-velocity impact and quasi-static indentation testing, indicating that quasi-static indentation testing can be used to represent low-velocity impact testing.

Effect of Core Thicknesses on Impact Performance of Thermoplastic Honeycomb Core Sandwich Structure under Low-Velocity Impact Loading

Low-velocity impact test on sandwich panels composed of aluminum face sheets and thermoplastic honeycomb cores have been performed to characterize the impact performance as a function of core thickness and drop heights. Impact parameters like maximum impact force, impact energy and impact damage area were evaluated and compared. Consequent damages were inspected visually on the impact surface as well as the rear surface. The experimental results found that panels with thicker core exhibited higher impact force than thinner core counterparts, allowing the panel to absorbed more energy. Higher degree of impact damage can be observed at elevated drop heights as most of the damage took the form of local core crushing, face sheet buckling and debonding between the face sheet and core,. Resulting damage area were different according to the core thickness as thicker core prone to absorbed more energy that lead to more damage propagation.

Engineering approach for predicting post-impact compressive strength of laminated composites

An engineering approach is presented to analyse the whole process of damage initiation and development for laminated composites under impact loading as well as compressive loading after impact using the three-dimensional progressive damage theory. The real impact damage status of laminated composites is employed to analyse the residual compressive strength instead of the artificial premises adopted by traditional methods. A comparison of impact damage area and residual compressive strength from numerical simulations and experimental tests is given, which shows a good agreement with each other. Moreover, large numbers of experiments for obtaining the impact damage status parameters can be avoided by this engineering approach. A parametric modelling programme package based on the analytical method has been developed.

ＣＦＲＰスカーフ継手の引張強度における衝撃損傷の影響

An experimental investigation on the mechanical properties of CFRP scarf joints with impact damage was carried out to understand the relationship between scarf angle and impact characteristics. This research shows the effect of the scarf angle on various characteristics, tensile strength, impact damage, and tensile after impact (TAI) strength. These characteristics were investigated experimentally about four scarf angles of 2o, 3o, 4.5o, and 6o. For the non-damaged specimen tensile test, it was found that the tensile strength decreases as the scarf angles increase. It was thought that the reason of decreasing of tensile strength was decrease of bonded area of scarf joint due to increase of scarf angle. Tensile fracture mode of non-damaged specimens were changed from net tension failure mode to adhesive failure mode as the scarf angles increase. As a result of the cross-sectional observation after impact test, impact damage area was increased by the increasing of scarf angle. At the higher scarf angle, debonding of adhesive layer was increased, but damage of laminate was decreased. It was thought that the damage decreasing of laminate was affected by the energy absorption due to significant debonding in the adhesive layer. For the damaged specimen tensile test, residual tensile strength decreased as the scarf angles increase. It was thought that the residual tensile strength of the scarf joints was affected by the increase of debonding in the adhesive layer at higher scarf angle.

Impact response of three-dimensional stitched sandwich composite

Abstract The paper aims at evaluating the damage resistance of sandwich structures composed of stitched foam core and glass facesheets subjected to low-velocity impact. To obtain a suitable baseline comparison, the equivalent set of properties was measured for an equivalent unstitched sandwich. Based on the force and energy histories, parameters have been introduced as following: load at incipient damage, maximum load, penetration depth at maximum load, total energy absorbed during impact and impact damage area. The impact resistance of the sandwich structure is greatly improved by the presence of the stitches. Skin/core delamination is limited and initial energy is used to degrade core’s stitches. Moreover the global behavior under impact is influenced by the stitching geometrical parameters.

새로운 z-피닝 기술로 제작된 복합적층판의 내충격 특성

본 논문에서는 복합적층판의 취약한 층간 특성을 보강할 수 있는 z-피닝 기술에 대하여 기존에 알려진 방법들을 소개하고, 최근 저자에 의하여 제안된 새로운 방법을 소개하였다. 새로운 방법을 적용하여 시제작된 z-피닝 시편들에 대하여 충격시험을 수행하고, 충격손상 영역을 측정하였다. z-피닝 기술이 적용되지 않은 일반 복합재 시편에 대해서도 동일한 시험을 수행하고, 두 결과를 서로 비교하였다. 결과적으로 새로운 z-피닝 기술은 기존의 기술에 비하여 양산에 적용하기가 매우 유리한 장점이 있으며, 새로운 z-피닝 기술이 적용된 복합적층판 시편들의 내충격 특성이 확실히 향상되고 있음을 알 수 있었다. In this paper, conventional z-pinning technology which can be used to reinforce inter-laminar property of laminated composites is introduced and new z-pinning technique recently proposed by author is also introduced. On some trial specimens manufactured by the new z-pinning technique, the low-velocity impact test was performed and impact damage area was measured. Similar impact test was performed on normal composite laminate specimens and those test results were compared to each other. Consequently, it can be seen that the new z-pinning technique is more useful in applying to mass production of z-pinned composite laminate structures than the conventional techniques and some clear improvement on impact resistance of z-pinned composite laminates manufactured by the new z-pinning technique is observed.

Experimental and Numerical Simulation Studies of Low-Velocity Impact Responses on Sandwich Panels for a BIMODAL Tram

This paper describes the results of experiments and numerical simulation studies on the impact and indentation damage created by low-velocity impact subjected onto honeycomb sandwich panels for application to the BIMODAL tram. The test panels were subjected to low-velocity impact loading using an instrumented testing machine at six energy levels. Contact force histories as a function of time were evaluated and compared. The extent of the damage and depth of the permanent indentation was measured quantitatively using a 3-dimensional scanner. An explicit finite element analysis based on LS-DYNA3D was focused on the introduction of a material damage model and numerical simulation of low-velocity impact responses on honeycomb sandwich panels. Extensive material testing was conducted to determine the input parameters for the metallic and composite face-sheet materials and the effective equivalent damage model for the orthotropic honeycomb core material. Good agreement was obtained between numerical and experimental results; in particular, the numerical simulation was able to predict impact damage area and the depth of indentation of honeycomb sandwich composite panels created by the impact loading.

Low-velocity impact analysis of composite laminates under initial in-plane load

In this paper, low-velocity impact response and damage of composite laminates is analytically investigated. A modified displacement field of the plate considering initially loaded in-plane strain is proposed. From the displacement field, a finite element equation on the structural behavior of composite laminate is newly induced and a computational program is coded. Numerical results using the FEM code is compared with the numerical ones from reference. Additional numerical analysis is performed on another impact condition, and the effect of initial in-plane load is investigated. Potential delamination damage area in the first inter-ply surface from the bottom of the laminate is approximately estimated, and the effect of the initial in-plane load and the impact condition are also investigated. Consequently, it may be concluded that the initial in-plane load of the laminate does not affect so much on the impact damage area of the laminate.

Impact damage resistance of sandwich structure subjected to low velocity impact

The paper aims at evaluating the damage resistance of sandwich structure, composed of Nomex ® honeycomb core (thickness: 10 mm and 20 mm) and two kinds of facesheets (carbon/epoxy and glass/epoxy laminates) subjected to low velocity impact. The impact tests are performed using the instrumented impact-testing machine and resulting impact damages are inspected by scanning acoustic microscope (SAM). Based on the force and energy histories, six parameters have been introduced as following: load at incipient damage, energy absorbed at incipient load, maximum load, total energy absorbed during impact, plastic energy absorbed by damage, and impact damage area. The impact resistance of the sandwich structure is greatly influenced by the facesheet type and core thickness . And their impact damages are mainly delamination in the facesheets, which is peanut-shaped with major axis along their lower fiber orientation, and their behavior is dependent on the facesheets.

THE EFFECTS OF STACKING SEQUENCE ON THE IMPACT DAMAGE SHAPE IN DISSIMILAR LAMINATED PLATES

A systematic investigation has been performed regarding the mechanisms that lead to damage of dissimilar laminated construction, with a different stacking sequence. Also, such construction of materials involving glass/polymer is used in blast-proof safety screens or bullet-proof windows or windshields in automobiles, architectural glazing, aerospace and aeronautical fields. In many cases, the laminated plate can be effectively used if it has properties such as high-performance and a low specific weight. In this work, the impact induced damages in glass/PMMA and PMMA/glass, and laminated circular plates with an upside-down stacking sequence are investigated by using an instrumental long bar impact tester in a biaxial bending state. In order to improve the damage tolerance of other types of laminated circular plates, various geometric (ratio of inner to outer layer thickness) and stacking sequence parameters were considered in determining their effect in absorbing an impact. The measured impact force profiles and the impulse of the force explained the impact damage behavior that is induced in dissimilar laminated circular plates. We have shown that a stacking sequence could have an effect on the initiation of radial cracks and in the impact damage area of laminated plates, in inner and outer layers. Also, the greater the inner layer thickness, the less damage the areas sustain. A design guideline of a stacking sequence is proposed for effective dissimilar laminated plate construction.

Improvement of low-velocity impact and compression-after-impact performance by z-fibre pinning

Z-fibre ® pinning is a new method of through-thickness reinforcement of laminated composites. This paper presents an experimental test and theoretical analysis on how and why these pins can improve the resistance to impact loading and post-impact compression. Specimens were made of carbon/epoxy T300/914C with nominal thicknesses of 2, 4, and 6 mm. For the specimens tested in this study, z-pinning reduced impact damage area by 19–64% depending on the specimen thickness and impact energy. Experimental results obtained also indicate that z-pins can significantly increase the compression-after-impact (CAI) strength by about 45%. In this paper all of these observations are discussed in the context of theoretical and numerical models that have been developed previously to predict the critical impact force and z-pinning performance in terms of the laminate in-plane stiffness and mode I delamination fracture toughness with the z-fibre bridging effect.

Impact damage resistance of buckled carbon/epoxy panels

Results are presented for impact tests conducted on postbuckled, thin, orthotropic carbon/epoxy laminates manufactured by the RTM process from preforms of uniweave fabric, some of which were stitched with Kevlar sewing thread. They were impacted by a 13 mm diameter 8.9 g steel projectile at velocities ranging to 90 m/s on their convex and concave faces. For impact velocities above a critical value, either penetration occurred or the specimens failed catastrophically. Their corresponding critical velocity boundaries were established. Their projected impact damage area increased with increased impact velocity, however, it was independent of preload. Their postimpact properties remained largely unchanged. Stitching reduced significantly the projected damage area, however none of their other impact damage resistance and tolerance characteristics were effected, including their critical velocities for penetration and catastrophic damage. Impacting on the concave face, compared to impacting on the convex face, reduced the projected damage area and the penetration critical velocity boundary. No catastrophic damage was observed for these specimens.

Looking Back& Looking Forward

In this study, the effect of impact loading upon fatigue behavior of hybrid composite pipes has been investigated. The composite pipes were subjected to tangential pre-stress as 3–48 MPa according to ANSI/AWWA C950 standard than, subjected to low velocity impact at 20 J. The impact pre-damaged pipes then subjected to cyclic loading at 50% σult in accordance with ASTM D 2992-06. It is also observed that increasing the tangential pre-stress decreases impact damage area and increases fatigue life of the pipe. It is also showed that impact pre-damage are not effective on the fatigue life of the pipe at the pre-stress level of 24 MPa.

Characterization of the residual mechanical properties of woven fabric reinforced composites after low-velocity impact

An investigation is undertaken to examine the residual mechanical properties of crowfoot-weave carbon/epoxy laminates subjected to a transverse central low-velocity impact load. It is found that the residual strength and stiffness of impacted laminates decrease with increasing impact damage area. Experimental data indicates that flexural stress constitutes the basis of a failure criterion that also describes damage severity, if localized damage at the region of impact is not serious. By formulating a simple model involving the motion of a rigid impactor, together with fundamental stress analysis of a transversely loaded plate, the effects of impactor mass, impact velocity, impactor tip radius, laminate thickness and lay-up on low-velocity impact damage are identified. Based on the experimental observations, it is found that the residual mechanical properties can be approximated by a linear relationship with a single damage severity parameter Q, where Q is a function of incident impact energy, impactor tip radius and laminate thickness. The theoretical results are verified by experimental data.

Compression Failure Mechanisms of 3-D Angle Interlock Woven Composites Subjected to Low-energy Impact

Instrumented drop weight impact and compression-after-impact tests (CAI) were conducted to assess the impact performance, compression failure mechanisms and residual compression strength of three-dimensional (3-D) interlock carbon woven/epoxy composites. Comparative evaluations were also contrasted with those for conventional two-dimensional (2-D) plain woven/epoxy laminated composites. The results show that the 3-D interlock woven composites exhibited a smaller impact damage area and higher % residual strength than the 2-D woven laminated composites. The improvements were attributed to the larger crimp-angle and interlacing structure of the 3-D interlock woven composites, which inhibits delaminated propagation along the plane direction. Besides, the fact that the 3-D interlock woven structure prevents buckling of the fibre tows under compressive load also contributes to the CIA performance.

충격손상을 받은 섬유 금속 적층판의 잔류 강도 연구

Residual strength of fiber metal laminates after impact was studied. 3/4 lay up FML was fabricated using 4 ply prepreg, 2 ply aluminum sheets, and I ply steel sheet. Quasi isotropic ([0/45/90/-45]s) and orthotropic ([0/90/0/90]s) FRP were also fabricated to compare with FML Impact test were conducted by using instrumented drop weight impact machine (Dynatup, Model 8250). Penetration load and absorbed energy of FML were superior to those of FRPs. Tensile tesls were conducted to evaluate the residual strength after<br/> impact. Strength degradation of FML was less than that of FRP. This means that the damage tolerance of FML is excellent than that of FRP'. Residual strength of each specimen was predicted by using Whitney and Nuismer(WN) Model. Impact damage area is assumed as a circular notch in WN model. Damage width is defined as the average of back face and top face damage width of each specimen. Average stress and point stress criterions were used to calculate the characteristic length. It is supposing that a characteristic length is a constant. The distribution of characteristic length shows that the assumption is reasonable. Prediction was well matched with experiment under both stress criter ions.

Low-energy impact effects on candidate automotive structural composites

The objective of the work described in this paper was to experimentally characterize the susceptibility of three candidate automotive structural composites to incidental, low-energy impact damage. The composites, each of which had the same urethane matrix, were produced by a rapid molding process suitable for high-volume automotive applications. The reinforcement for the first composite was a random chopped-glass fiber, while the remaining two were reinforced with stitch-bonded carbon-fiber mats, one in a crossply layup, and the other in a quasi-isotropic layup. A pendulum device, representative of events such as tool drops, and a gas-gun projectile, representative of events such as kickups of roadway debris, were used to impact plate specimens. Brick-drop tests were also performed to assess the applicability of the baseline pendulum and gas-gun data to other events. Following the impacts, the damage areas were measured and the plates were cut into either tensile, standard compressive, or compression-after-impact specimens for determining strength degradation. The glass-fiber composite was least susceptible to damage, followed by the crossply carbon-fiber laminate, which had the same thickness. The quasi-isotropic carbon-fiber composite, which was thinner than the other two, sustained the most damage. While compressive strength was significantly degraded by moderate damage in the random-glass-fiber composite, tensile strength was not. On the other hand, both tensile and compressive strengths were degraded in the crossply carbon-fiber laminate (only compressive strength loss was measured in the quasi-isotropic laminate). Compressive strength degradation for a given damage area was similar in the two carbon-fiber laminates. Both showed lesser degradation than did the glass-fiber composite. For the quasi-isotropic carbon-fiber laminate, it was shown that strength degradation produced by an open circular hole provides a reasonable lower bound to the degradation due to an impact damage area of the same size.

Post-impact compressive strength of curved GFRP laminates

Low velocity impacts to fibre reinforced plastic composites cause a pattern of damage consisting in general of delamination, fibre breakage and matrix cracking. Such damage is accidental and may go unnoticed; therefore composite structures must be designed assuming impact damage exists. Previous work on flat composite laminates has resulted in a reasonable understanding of the mechanisms of compressive strength reduction. There are, however, many instances where curved laminates are used in structures where impact is likely. Furthermore, due to the mechanisms of strength reduction, it may be expected that curvature would have a significant effect on the behaviour of the laminates. The work described here consists of experimental measurement of the post-impact compressive strength of curved GFRP laminates. The laminates were of 8 plies of 0.3 mm thick pre-impregnated glass fibre/epoxy tape in a (0, ±45, 0°) s lay-up. Each laminate was 200 mm in length by 50 mm wide with the plane of curvature normal to the length. Laminates were impacted on the convex surface of the laminate by dropping a steel mass from 1 m vertically above it. Impacted laminates were loaded in compression and the out-of-plane displacements of the top and bottom surfaces were recorded. Final failure was typically due to fibre breakage occurring through the centre of the impacted area of the laminate. Possible differences in the impact response, and measurable differences in the sizes of the impact damage area, were found to arise from these curvatures, and differences were observed in their post-impact buckling behaviour. However, perhaps unexpectedly, the post-impact compressive strength for a curved laminate was found to be similar to that for a flat laminate. The failure loads for the impact damage laminates are shown to be comparable with those for laminates containing artificial delaminations.

K-0818 耐熱ファイバーメタル積層材の衝撃後圧縮強度評価(S11-5 構造健全性評価(5))(S11 構造健全性評価)

Hybrid titanium composite materials are expected to be applied to the structures of the future supersonic transports for their excellent properties of lightness, heat-resistivity and durability. In this report, fiber/metal composites of titanium foil laminated with heat-resistant thermoplastic (PIXA) reinforced graphite fiber plies were prepared to evaluate the adverse effects of out-plane impact damage. Impact damage area was investigated by ultrasonic inspection, cut section inspection and impact post-compressive strength test.

Low velocity impact and delamination buckling behavior of composite laminates with embedded optical fibers

A drop-weight impact test and delamination buckling test were performed to investigate the development of impact damage and post-impact compression behavior of cross-ply composite laminates with optical fibers embedded in the interface of two different angle plies. It was found that the presence of optical fibers had little or marginal effect on the shape and size of damage area of the laminate when the optical fibers were embedded parallel to the host reinforcing fibers. Non-parallel optical fibers promoted unstable crack growth through the formation of resin rich regions, which in turn increased slightly the impact damage area. It was also shown that a large number of parallel optical fibers may have an ameliorating effect on delamination buckling strength as they promote the waviness of the delamination fracture surface.