Compression After Impact Strength Research Articles

Compression after impact (CAI) failure mechanisms and damage evolution in large composite laminates: High-fidelity simulation and experimental study

This study focuses on developing and validating a high-fidelity finite element model for predicting damage evolution and residual strength in fiber-reinforced composite panels. Impact and compression after impact (CAI) tests were conducted at both barely visible impact damage (BVID) and clearly visible impact damage (CVID) levels. The ASTM D7137 standard 100 mm × 150 mm CAI coupons were inadequate to cover the range of experimental studies required for model validation. Therefore, larger 254 mm × 304.8 mm laminates were investigated under two CAI testing conditions: one a scaled-up version of ASTM standard coupon, and the other with additional anti-buckling support plates to reduce unsupported areas to 127 mm × 177.8 mm. The model captured inter- and intra-laminar failure modes, including fiber breakage, splitting, kinking, pull-out, and crushing as well as matrix cracking, delamination, and their interactions. This was achieved by cohesive zone modeling technique and enhancement of the LaRC05 failure criteria through modeling the fiber damage evolution and utilizing an efficient search algorithm to determine the matrix fracture plane and fiber kink band angle. This study underscores the efficacy of the high-fidelity modeling approach in accurately predicting both impact damage and CAI strength in typical aircraft impact damage scenarios. Additionally, it provides insights into complex CAI failure mechanisms and energy release associated with various damage modes and highlights the effect of global buckling on the failure behavior and compressive strength of composite laminates. Furthermore, it shows that the proposed fixture with support plates is suitable for testing a broader range of impact scenarios without experiencing global buckling.

Experimental investigation on the low‐velocity impact response and the residual strength of CFRP tubes

AbstractWith the help of the high‐speed camera and self‐designed fixture, it was investigated that the effects of the impactors in a variety of shapes on the mechanical behavior and failure mode of carbon fiber‐reinforced polymer (CFRP) tubes with a range of inner diameter during the low‐velocity impact (LVI) of four impact energies. Then, the three‐dimensional digital image correlation (3D‐DIC) system was carried out to monitor the damage evolution of the LVI‐damaged specimens under compressive and flexural loading. In addition, the residual compressive and flexural strength of all kinds of cases were compared and discussed. The results show that the increase in wall thickness can improve the LVI, compression‐after‐impact (CAI), and flexure‐after‐impact performance. The failure mode of compression changes to brittle fracture failure at the impact circumferential region. The CAI strength of the specimen damaged by a flat impactor is 23.89% less than that of a hemispherical one, but there is little difference in flexural loading.Highlights Mechanical behavior on in & post LVI of CFRP tubes varying wall thickness and impactor. Due to point‐line contact for flat impactor, two peak forces are in the F‐D curves. LVI property and residual strength are improved with the increase of wall thickness. Compressive failure mode is brittle fracture at the impact circumferential area. Flexure‐after‐impact strength mainly depends on the no‐impact surface of tubes.

Fracture mechanism of carbon fiber-reinforced thermoplastic composite laminates under compression after impact

Thermoplastic carbon fiber-reinforced plastics (CFRPs) are increasingly utilized in the aerospace industry owing to their beneficial properties and enhanced formability relative to thermoset CFRPs. Despite the extensive use of these materials, studies focusing on compression after impact (CAI) tests with impact energies exceeding 20 J for thermoplastic CFRPs remain scarce. This study examines CAI tests on quasi-isotropic laminates of thermoplastic CFRP, subjected to a low-velocity impact energy of 27.04 J. For comparison, quasi-isotropic laminates of thermoset CFRP were subjected to a low-velocity impact energy of 36.5 J. These tests reveal that the CAI strength of both materials is comparable, notwithstanding the lower fiber volume fraction in the thermoplastic CFRP. Further, this research incorporates a finite element (FE) analysis to investigate the damage mechanisms in thermoplastic CFRP. The FE model, integrating interlaminar damage observed during the impact tests, accurately predicted the relationship between compressive stress and strain, correlating closely with the experimental outcomes. It was observed that both interlaminar and intralaminar damage propagation were constrained until the point of maximum compressive stress. Prior to reaching this maximum, a region of elevated compressive stress in the fiber direction was noted in the 0° layer near the non-impacted side. These findings indicate that the compressive stress in the fiber direction in the 0° layer adjacent to the non-impacted side is pivotal in dictating the final failure, which determines the CAI strength of thermoplastic CFRP laminates.

The relationship between repeated impacts and CAI strength of sandwich structure considering different impactor shape in individual impact

Sandwich structures are susceptible to repeated impact loads, and each individual impact is not nearly identical to the previous one, especially for repeated impacts under different impactor shapes such as hail impact. Therefore, the repeated impact behavior using different impactor shape in individual impact of polymethacrylimide (PMI) foam sandwich structure is studied and discussed through experiments and finite element (FE) simulations in this paper. The representative mechanical characteristics such as impact responses, damage modes, and energy absorption are investigated until penetration occurs, and the mechanism of impact damage accumulation between impact events with different impactor shapes is revealed. Besides, the compression after impact (CAI) strength is utilized to assess the residual strength of sandwich structure under repeated impacts. It indicates when the contact area between the specimen and impactor is larger, it can result in a higher impact force, a shorter contact duration, and potentially reduce the extent of damage. The matrix damage is the main damage mode, and the proportions of matrix tension and compression are nearly constant, with a ratio of approximately 2:1. In addition, the proportion of the impact energy absorbed by foam core increases as the number of impacts increases. When the impact energy is low, the CAI strength is sensitive to the number of impacts, the shape of impactor, and the sequence of impactors, but the similar phenomenon does not occur at higher impact energy. The present study provided a series of experimental and numerical results, which could be utilized as design reference guidance in the designing of composite sandwich structures.

Unleashing the potential of CFRP laminate: enhancing mechanical characteristics with STF-Impregnated woven carbon fabric

The objective of this study is to evaluate the potential for enhancing the mechanical characteristics of carbon fiber reinforced polymer (CFRP) composites through the embedding of STF-impregnated woven carbon fabric within their structure, referred to as STFFC. The dynamic and static characteristics of the proposed STFFC were evaluated through a series of experimental tests including tensile, modal, dynamic mechanical analysis (DMA), low-velocity impact (LVI), and compression after impact (CAI) tests. The shear thickening fluid (STF), which was prepared by dispersing nano-silica particles in a solvent of polyethylene glycol (PEG), exhibited an abrupt increase in storage and loss modulus upon exceeding a critical shear strain. The result of the modal test revealed that the addition of STF-impregnated fabric into the CFRP composite led to greater damping ratios of 17.71–417.86%, despite the degradation of tensile strength by 10.81% and Young’s modulus by 12.49%. DMA results show that the STFFC has a larger tan δ except for between intersections of 118 °C, and 161 °C. For impact responses, the STFFC resulted in a lower contact force, with a reduction of 2.41–3.30%, and an increase in absorbed energy of 2.87–5.00%. Meanwhile, the CAI strength of the STFFC was degraded by 3.62 to 6.20%. Despite the reduction of tensile, impact, and CAI characteristics of the STFFC, it is noteworthy that the CFRP composites exhibited relatively minor degradation even with the application of STF. The results of the study suggest that embedding STF-impregnated woven carbon fabric can enhance the potential of CFRP composites, making them even more promising solutions for various applications.

Experimental study on the impact resistance and damage tolerance of thermoplastic FMLs

This study aimed to enhance the impact resistance of fiber metal laminates (FMLs) and achieve lightweight optimization by incorporating a new thermoplastic resin, a titanium alloy and ultra-high-molecular-weight polyethylene (UHMWPE) fiber to produce a novel type of FMLs (PEFMLs). The impact resistance of PEFMLs was analyzed through low-velocity impact tests conducted at different energy levels. Subsequently, the residual compression-after-impact (CAI) strength of the PEFMLs was evaluated through compression tests on the impacted specimens. The experimental findings revealed that PEFMLs exhibited subcritical failure when subjected to impact energies less than 35 J, with a penetration energy threshold of 55 J. Higher impact energies resulted in larger damage areas and increased plate buckling of PEFMLs, consequently leading to reduced CAI strength. The presence of metal, thermoplastic resin and UHMWPE in the PEFMLs effectively dissipated a substantial amount of impact energy while maintaining their structural integrity during both the impact and compression processes.

Effect of stitch density on the damage inhibition and compression strength after high-velocity impact of UHMWPE fiber composites

This paper investigates the effect of stitch density on the high-velocity impact damage inhibition and CAI (compression after impact) strength of UHMWPE (Ultra-High Molecular Weight Polyethylene) fiber composites. The damage response and compression behavior after impact were quantified and analyzed using CT scanning and DIC (Digital Image Correlation) techniques. The findings reveal that the delamination area of the composite plate with the stitch space of 1 cm reduces by 49.14 % compared to the unstitched sample, and exhibits a 73.05 % higher compressive load than the plate with a 4 cm stitch space. Notably, the 4 cm stitch composite plates demonstrate a significant advantage in energy absorption. However, the sparse stitch structure destroys the original structure inside the composite, resulting in a degradation of CAI performance. The study provides guidance for the practical design of composite materials with enhanced energy absorption and damage resistance.

Experimental investigations on the repeated low velocity impact and compression‐after‐impact behaviors of woven glass fiber reinforced composite laminates

AbstractIn this paper, the repeated low velocity impact responses and compression‐after‐impact (CAI) behaviors of woven glass fiber‐reinforced composite laminates were studied at the same total impact energy. Three types of impact energies, 30, 40, and 60 J, were chosen for the repeated impact tests relative to the single 120 J impact event. The impact behaviors including impact contact force–displacement and energy–time curves during the impact testing were recorded. The variations of the impact mechanical characteristics such as peak impact force, maximum displacement, and energy absorption were evaluated. The methods of visual inspection and stereo microscope were applied to identify the damage morphology of the impacted laminates. The damage accumulation was evaluated employing the absorbed energy fraction and normalized maximum displacement. It was found that the influences of impact energy on the repeated impact mechanical response are more remarkable than that of the impact number, and the impact events with fewer impact numbers cause more damage to the laminates for the same accumulative impact energy. Furthermore, the load–displacement and the CAI strength versus impact number curves were compared for the CAI testing. The ultimate load and the resulted CAI strength decrease with the increase of impact number owing to the influence of damage accumulation in repeated impacts.Highlights Repeated impact responses were studied at the same total impact energy. The impact energy makes significant influence on the impact dynamic responses. The impact events with fewer impact numbers cause more damage to the composites. The CAI behavior was explored considering the effect of damage accumulation. The CAI strength of damaged laminates decreases with impact number increasing.

The relationship between the impact position interference and CAI strength of composite sandwich structures under double impacts

Sandwich structures are susceptible to multiple-impacts, and such multiple-impacts will typically occur at certain distances from each other. Therefore, the influence of five different distances between impact positions (DBIP) and three impact energies on compression after impact (CAI) strength are considered and discussed for honeycomb sandwich structure with composite face sheets in this paper. By comparing two typical impact characteristics, i.e., maximum impact force and displacement, the trend of CAI strength shows a turning-point, which is closely related to the size of the impactor. However, high impact energy (15 J) leads to damage in the bottom face sheet during the second impact, and the presence of damage in both top and bottom face sheet reduces CAI strength, resulting in the turning point of CAI strength not exist. It was observed that there is obvious buckling failure, and some cracks along the width direction from the impact point to the edge after CAI. To provide a more comprehensive understanding of double impacts and CAI behavior, a finite element (FE) model considering different DBIP is developed. In addition, the predicted fraction of energy absorbed by top face sheet, honeycomb core, and bottom face sheet is also obtained to reveal the relationship between the impact position interference and CAI strength.

Numerical investigation of the low-velocity impact damage resistance and tolerance of composite laminates with preloads

Composite materials employed in engineering services are often subjected to preloads such as pre-tension and pre-compression, necessitating a comprehensive understanding of their effects on the Low Velocity Impact (LVI) damage resistance and tolerance. Driven by this, a Finite Element Analysis (FEA) model, accounting for fibre damage, matrix damage, and delamination, has been developed to capture the LVI behaviors of composite laminates subjected to various preloads. Furthermore, the developed model has also been employed to predict the Compression After Impact (CAI) strength of these composite laminates which have been impacted under pre-loading conditions. By comparing the modelling results with experimental data, the fidelity of the model is demonstrated, especially revealing a deviation of less than 10 % between the measured and predicted CAI strengths. The validated model is then employed to further explore the relationships between LVI behaviors, CAI strength, and different types and magnitudes of preloads under varying impact energies. The findings indicate that a monotonic transition in preloading from compression to tension leads to a reduction in LVI delamination area and energy dissipation, while for CAI strength, it is related with the stacking sequence.

Prediction of Impact Information of Composites Laminates Considering the Practicality

Recently, carbon fiber reinforced plastics (CFRP) have been used in various applications, including aircraft. Because they are vulnerable to out-of-plane loads, internal and externaldamage occurs when foreign objects impact them. Internal damage that can affect residual properties is difficult to find and judge from the outside without special devices, which arehighly costed and are sometimes difficult to conduct in some locations. In this study, surface contour information was obtained from impact tests on CFRP laminates, and thepredictability of compression after impact (CAI)strength was investigated using a conventional single-task random forest model, and a decision tree-based multi-task learning model with other objective variables related to impact tests. The models estimated CAI strength with around 75% R2, and the conventional single-task learning model showed the highest value. The importance of each model indicated that factors that contribute to impact-related objective variables (impactor shape, delamination area, and delamination length) and those to CAI strength do not have a strong relationship.

The effects of impactor shape on the compression after impact strength of carbon/epoxy face sheet foam core sandwich structure

This study presents experimental results of compression after impact (CAI) strength testing of foam core sandwich structure with carbon/epoxy face sheets impacted at three different energies with either a blunt or sharp tip impactor. The impact energies used were chosen to span the barely visible impact damage (BVID) thresholds for the sharp (4.1 J) and blunt (12.9 J) impactors with an impact energy approximately halfway between these (8.1 J) also used. While most impact testing on composites utilize a hemispherical (blunt) impactor, actual damage to a part may be due to an object impacting the part that is not blunt, but sharp and the impact response and resulting CAI strength values may be different for a given impact energy level. In this study, with regards to the impact response, the sandwich specimens showed larger transverse displacements (by about a factor of two) during the impact event when impacted by a sharp impactor versus a blunt impactor. The maximum load of impact was larger for the blunt impactor by about a factor of three. The barely visible impact damage (BVID) threshold energy was lower by about a factor of three for the sharp impactor. The absorbed energy of impact was higher for the sharp impactor. The CAI strength results showed that the sharp impactor gave lower average CAI strength values at the lowest impact energy level used, slightly lower average CAI strength values for the medium impact energy used and about the same average CAI strength values for the highest impact energy used.

Experimental investigation on Compression-After-Impact (CAI) response of aerospace grade thermoset composites under low-temperature conditions assisted with acoustic emission monitoring

Low-velocity impact (LVI) can provoke catastrophic failure depending on the impact energy, lay-up configuration, and the operating temperature in fiber-reinforced composite structures via damage accumulation during the service. Additionally, extreme operating temperature is an important factor affecting the damage initiation and growth mechanism and understanding the compression-after-impact (CAI) response is a promising way to predict the damage-growth dynamics. For this purpose, this study investigates the LVI response and CAI strength of composite laminates under the −55 °C temperature considering the low-temperature (LT) service conditions of aerospace applications. The experiments are performed on cross-ply ([0/90°]4s) and angle-ply ([±45°]4s) lay-up composites for impact energy levels of 1.4 J and 10 J. After LVI tests, the occurred damages are examined by infrared thermography (IRT). Furthermore, the effect of the impact damage on the failure mechanism under compression loading is elaborated using acoustic emission (AE) inspection. The CAI test results reveal that the LT increases the compressive residual strength. However, this change is directly related to the load and lay-up configuration. The alternation in the damage growth mechanism of angle-ply laminates under LT operating temperature is more significant compared to the cross-ply laminates.

Low-velocity impact and compression-after-impact behaviors of twill woven carbon fiber/glass fiber hybrid composite laminates with flame retardant epoxy resin

Composite structures suffer from significant residual strength reduction as a result of invisible damage caused by impacts, which can cause severe harm without any prior warning. Fibers play a crucial role in improving the impact resistance of composite materials by providing strength, toughness, and energy absorption. This work explores the dynamic response and residual compressive strength of composite laminates reinforced with twill woven carbon fiber, glass fiber, and carbon fiber /glass fiber hybrid, subjected to low-velocity impact (LVI) and compression-after-impact (CAI) testing. First, the performance of woven carbon fiber composite laminates infused with flame retardant epoxy resin and general epoxy resin was studied. The results indicated a lower load-bearing capacity and elongation of the former. To enhance the impact resistance of the laminates, we explored the effects of incorporating glass fibers into the woven carbon fiber composite laminates. Our findings demonstrated that the introduction of glass fibers alteres the impact damage mode of the laminates and effectively improved their impact resistance and CAI strength. Further, several techniques were adopted to characterize the corresponding damage morphologies and failure mechanisms of the woven carbon fiber composite laminates, which provides valuable insights for their structural design and performance improvement.

The effects of off-axis loading on the compression after impact strength of quasi-isotropic face sheet honeycomb core sandwich structure

This study presents experimental results of compression after impact (CAI) testing of aluminum honeycomb core sandwich structure with face sheets made of co-cured T1100/3960 quasi-isotropic carbon/epoxy when tested at +22.5⁰ and −22.5⁰ with respect to the 0⁰ fibers. In a previous study examining the CAI strengths of honeycomb sandwich structure, it was found that specimens had different CAI strengths, based on a [−45/90/+45/0]S layup, depending on whether they were tested in the 0⁰ direction (face sheet layup of [−45/90/+45/0]S) or 90⁰ direction (face sheet layup of [+45/0/-45/90]S). The CAI strength results showed that the specimens tested in the 90⁰ direction had a 19% drop in CAI strength compared to specimens tested in the 0⁰ direction. This was attributed to the 0⁰ load bearing plies in the 0⁰ direction specimens being “tucked in” at the center of the specimen thus providing more stability against microbuckling. This raised the question as to what CAI strength would specimens tested at +22.5⁰ (face sheet layup of [−22.5/−67.5/+67.6/+22.5]S) and −22.5⁰ (face sheet layup of [−67.5/+67.5/+22.5/-22.5]S) have compared to specimens tested in the 0⁰ and 90⁰ direction. Results presented in this study show that the specimens loaded at +22.5⁰ and −22.5⁰ have a similar average CAI strength compared to the specimens loaded in the 0⁰ direction. The specimens loaded in the 90⁰ direction exhibit 16% lower average CAI strength. Additional specimens were tested in the +45⁰ direction to put the 0⁰ load bearing fibers on the outside of the specimen to see if this would decrease the strength as has been documented for undamaged strength. These specimens have average CAI strength values between the 0⁰ direction average CAI strength values and the 90⁰ direction average CAI strength values.

Modelling low-velocity impact damage and compression after impact of 3D woven structures considering compaction

A novel finite element modelling approach is presented which incorporates representative binder yarn compaction, for simulating the low-velocity impact (LVI) and compression after impact (CAI) response of 3D woven layer-to-layer carbon/epoxy composite architectures. Simulations of out-of-plane drop-weight impact tests were performed at energies of 32 J and 42 J. Warp and weft layers were modelled as continuous plies and three different approaches were explored to model the binder reinforcement; (i) with a rectangular cross-section and non-compacted, (ii) with an elliptical cross-section and non-compacted, and (iii) an elliptical cross-section which accounts for compaction. Predictions were compared with experimental results from literature and it is shown that modelling the binder reinforcement as an elliptical cross-section with compaction leads to a predicted damage area, on the impacted side, which is within 2%, and the non-impacted side within 6% of experimental measurements. The predicted CAI strength is within 11% of the experimental values.

Weibull distribution-based prediction model for compression after impact (CAI) strength of CFRP laminates

The aim of this research was to assess the appropriateness of the two-parameter Weibull distribution-based model to predict the compression after impact (CAI) strength and to develop both efficient and secure prediction scheme for laminated composite. Four levels of impact energy were considered (10 J, 25 J, 40 J and 55 J) and CAI tests were conducted. The scanning electron microscope were used to evaluate the representative damage pattern. Two-parameter Weibull function was utilized for statistically evaluating the CAI strength results. Experiment results showed that the wildest variance was observed at the impact energy range of 25–40 J for both layups. It was found fiber damage accompanied with high variance while the matrix and delamination damage corresponded to lower variance. To predict the CAI strength more efficiently, the time to first failure (TTFF) concept was used to draw CAI strength versus impact energy graphics under different reliabilities for reference. It was verified that the safe design strength curve under 0.99 reliability level was capable and secure enough to predict the residual strength in actual application considering the variance of composite.

Compression after multiple impact strength of composite laminates prediction method based on machine learning approach

The intelligent structural health monitoring system that can evaluate the structural safety online is the future development trend, in which the strength online prediction is the key step. This study developed a machine learning (ML) method to predict the compression-after-impact (CAI) strength of carbon/glass hybrid laminates subjected to multiple impacts at different impact positions online, which can help to find and replace damaged materials quickly to prevent irreversible disasters caused by accidental impact. Firstly, a finite element model verified by experiments was established to obtain the data of training ML model. Secondly, the eXtreme Gradient Boosting (XGBoost) model was utilized to predict the CAI strength of the composites subjected to multiple impacts at different distances between impact positions (DBIP). In addition, the feature importance of impact parameters based on the SHapley Additive exPlanations (SHAP) method was also studied. The results showed that the prediction accuracy and efficiency of ML-based method were better than that of FEM. Impact energy was the most significant factor affecting CAI strength, and DBIP cannot be ignored. The proposed method has great potential in online structural integrity monitoring systems of high-performance composite structures.

Impact and Compression-After-Impact Performance of a Thin Z-Pinned Composite Laminate

Impact and compression-after-impact (CAI) tests were carried out on [02/ ± 45]s carbon/epoxy samples to characterize the effect of z-pinning on the delamination resistance and damage tolerance properties of the laminate. Unpinned and z-pinned samples were subjected to impacts between 2 and 36 J to produce damage conditions that extend from barely visible impact damage (BVID) to complete penetration. The damage induced by impact and the damage modes leading to ultimate CAI failure were examined by X-radiography and by direct observations of the sample faces during CAI loading. The analyses indicate that the role of z-pins on the impact and CAI response of the laminate is dependent on the size and features of the damage. Z-pins do not modify the structural response to impact of the laminate, but they are effective in reducing the extent of damage for impact energies above a threshold value. Z-pinning is also effective in improving the CAI strength of the laminate for impact energies above this threshold value, even though it degrades the residual compressive strength for lower impact energies. Reductions in impact delamination size of up to 50% and improvements in CAI strength of about 20% were achieved by z-pinning for high-energy impacts. The mechanisms by which the z-pins affect the CAI response of the samples are illustrated and examined in detail for different impact damage severities.

Compression-After-Impact analysis of carbon/epoxy and glass/epoxy hybrid composite laminate with different ply orientation sequences

This article is a continuation study of the LVI and CAI response of the plain carbon fiber composite laminates and explores the experimental & numerical study of the post-impact damage propagation in carbon/epoxy and glass/epoxy hybrid laminate under compression. LVI (Low-Velocity Impact) tests are first carried out at three different energy levels, and the same specimens are put under a displacement-controlled compression test. The damage initiation and propagation in both impact tests and compression tests are studied using X-ray computed tomography. The results matched well with the results obtained from the finite element model, simulated using ABAQUS/explicit. The effect of the changing impact energy and the influence of the stacking orientation on post-impact damage propagation is studied. The strain maps during the compression tests are studied through Digital Image Correlation (DIC) technique based on which the stress–strain histories are plotted by using well-calibrated strain values. Upon completing the study, it is observed that the CAI (Compression-After-Impact) strength depends on the factors like properties of the constituent material, ply orientations and stacking sequence of the laminate. It is observed that even when the damage is barely visible on the surface of the specimen, compressive strength has been reduced drastically. The observed mode of damage propagation in CAI testing is buckling of the sub-laminates in the vicinity of the damage zone formed by the impact event. The size of the impact damage zone depends on the impact energy, which actually governs the CAI strength.