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Hybrid bonded/bolted (HBB) joint has been widely used in engineering practice because it can overcome the potential weakness of pure bonded and pure bolted joints. However, studies on HBB joint are still at the initial stage. In this paper, tensile properties of a composite–metal single-lap HBB joint was investigated experimentally. And a detailed finite element model (FEM) was established to simulate the tensile behavior of the joint. The model was verified by the experimental results. Then the damage propagation and load transfer mechanism were explored based on the FEM. The results show that the HBB joint can provide multi-load transmission paths and resist damage propagation in the adhesive. The HBB joint has higher strength and energy absorption capacity than the pure bonded joint. And the HBB joint has greater initial damage load and tensile stiffness than pure bolted joint. Adhesive fillets can obviously improve the tensile performances of the HBB joint. Lateral stiffness of the joint boundary and testing machine show obvious effects on tensile performances of single-lap hybrid joints.

A kind of interlaminar film with carbon nanotubes inserted into polyether ketone with cardo was used for lightning strike protection of composite laminates. The distribution of the interlaminar film was investigated by experimental and numerical methods. Artificial lightning strike tests were conducted for 12-film carbon nanotube and traditional surface silver coating protected specimens. Then corresponding finite element models (FEMs) were established to analyze the lightning strike effect and validated by the experimental results. Based on the FEMs, the number, distribution and thickness of interlaminar film were investigated in order to obtain equivalent protection effect with the traditional surface silver coating. The results show that only the first two layers were damaged for the surface silver coating protected specimen, while 5 layers were ablated for the 12-film protected specimen. Lightning strike damage area of the laminate protected with 5-film carbon nanotube is almost the same as that of the laminate protected with 12-film carbon nanotube. Compared with traditional surface silver coating protection, one film protection with thickness of 360 μm can make the laminate to obtain equivalent damage depth, 54.8% smaller damage area and 58% less additional weight. And reparability of the laminate is better than that of the laminate protected with 5 interlaminar films.

Tensile experiments were conducted with single-bolted and multi-bolted countersunk composite-metal joints. The finite element models with Hashin criteria and modified Camanho degradation law were developed to predict the bearing load and to simulate the failure behavior. The models were validated with the experimental results, including load-displacement curves and composite damage profiles. Based on the verified models, bolt load distributions of multi-bolted joints were calculated and analyzed before and after damage occurs. Results show that composite damages can change the bolt load distributions to a certain extent, which needs to be considered during the structure design. To adjust the bolt load distribution, the effects of joint variables, including bolt-hole clearance, bolt spacing, countersunk height ratio, and bolt stiffness combination, have been studied. It is found that the increase of the pin clearance around the largest bearing bolt would decrease its corresponding bearing load. The countersunk height ratio affects the bolt load distribution difference that a greater ratio leads to a less difference. The bolt spacing and bolt stiffness can also be used to reduce the bolt load distribution difference with elaborately designed combinations.

Both single-face vacuum bag curing (SVC) and double-face vacuum bag curing (DVC) can be used in scarf repair of composite structures. But different curing conditions caused by the sealing state may affect the bonding quality of scarf-repaired structures. In this paper, the effect of curing condition on bonding quality of scarf-repaired laminates was experimentally investigated in terms of surface profiles, moisture absorption curves and section profiles. In order to further explore the moisture absorption mechanism, finite element model of the repaired laminates using DVC was established with moisture diffusion of both the adhesive and composite laminates considered. This model was verified by experimental results. Based on the model of DVC case, the model of SVC case was built by changing moisture absorption parameters of the adhesive. Results show that SVC reduces the bonding quality, mainly reflecting in more adhesive inner voids and patch-to-parent dislocation. And SVC increases moisture absorption rate and moisture equilibrium content of the adhesive, and its effect on the former is far greater than that on the latter.

Carbon nanotubes can be used to protect composites from lightning strike due to their high electrical conductivity. A kind of carbon nanotube film was developed to protect composite structures here. Its lightning strike protection effect was studied. Artificial lightning strike tests were conducted on carbon nanotube film protected specimen, traditional surface silver coating protected specimen and pristine specimen in this paper. Coupled thermal-electrical finite element models were established to study the lightning strike protection effect and damage mechanism of two kinds of methods. Numerical results agree well with experimental ones, illustrating the validity of the models. The results show that lightning strike damage in the carbon nanotube film protected laminate is restricted in 5 composite piles and 5 carbon nanotube films close to the lightning strike surface. Whereas lightning strike damage in surface silver coating protected specimen is restricted in coating and 2 composite piles close to the lightning strike surface. Compared with the pristine laminate, damage in the laminate with carbon nanotube films decreases by 77.6% and 68.0% in area and depth respectively, whereas damage in the laminate with surface silver coating decreases by 66.1% and 92.0% correspondingly. The inserted carbon nanotube films can increase the electrical conductivity and conduct the strike current and energy along both depth and in-plane directions quickly, while surface silver coating mainly conducts the energy along in-plane direction.

Bolted composite joints are one of the most common joining methods used in aerospace industry for joining of parts and sub-assemblies, where there is a need to transfer high load with consistency a...

In this paper, moisture parameters of adhesive SY-14M-III and composite T800/AC531 were firstly measured, which were used for the following moisture diffusion investigations. Then, double cantilever beam (DCB) tests and end notched flexure (ENF) tests were conducted in four environments, and relevant progressive damage models considering cohesive zone models were established. The results show that the models are efficient by comparison of experimental and numerical results, and eight pairs of cohesive parameters (interface strength and interface stiffness) corresponding to the specified fracture energy release rates were obtained. Finally, a modified Tsai prediction method of cohesive zone model for simulating composite/adhesive delamination in hygrothermal environments was proposed based on an assumption of the composite/adhesive moisture diffusion process, which can be as a reference or basic data for numerical analysis of adhesively bonded composite structures.

Composite-to-metal 2D (two dimensional) scarf bonded joint (SBJ) specimens with 4scarf angles were tested to study their tensile behaviors. Based on the experimental results, progressive damage finite element models (FEM) were established in ABAQUS/Standard, and the effects of three variables: the metal material, composite stacking sequence and adhesive properties, with respect to the joint tensile properties were investigated. The results show that the metal plasticity and scarf angle can affect the tensile plastic behavior of the joint. Compared with Q235 (steel), 7075-T6 (aluminium) and 30CrMnSiNi2A (steel) plates, TC4 (titanium) plate makes the joint possess higher tensile strength. Changing the composite plate (laminate) stacking sequences can result in the variation of plastic behavior and failure location of the joints. The adhesive properties have significant effects on the joint plastic behavior and ultimate loads.

The over-ply covering the bondline in scarf-repaired laminates can avoid direct exposure of the adhesive to moisture environment which would decrease the properties of the adhesive. The effect of the over-ply on moisture absorption behavior of scarf-repaired composite laminate was studied in this paper. 3D finite element models (FEMs) of scarf-repaired laminates with and without over-ply were established to simulate the moisture absorption behavior and verified by experimental results. Then these models were used to investigate the effects of some factors, including over-ply type, configuration, direction and number, on moisture absorption behavior of the repaired structures, especially of the adhesive. The results show that the over-ply can efficiently decelerate moisture absorption of the adhesive and delay the time of its moisture absorption equilibrium. Over-ply type has effect on moisture absorption of the adhesive. Woven over-ply is more effective to protect the adhesive in repaired laminates from moisture absorption than unidirectional over-ply. For the laminates with only large-circle bonding surface covered, over-ply lap length, configuration and direction are not very important parameters to affect the moisture absorption.

A 3D progressive model with better convergence property is established to capture the mechanical properties and damage mechanism of a composite bolted π-joint under bending load and shown to closely match experimental results. This model is then used to investigate the effect of internal laminate stacking sequence and surface layers thickness on the bending performance of the joint. It was found that the stacking sequence and the surface layers thickness have a significant effect on the matrix crack and delamination distributions in the joint, respectively. Local surface layer thickness can be designed to control the delamination damage around the bolt hole. Finally, volume fraction of 0° layer in the internal laminate and surface layer thickness are used to improve the joint bending performances by proper design.

In this paper, a 3D progressive finite element model was established to simulate scarf repaired laminates subjected to low velocity impact, in which intralaminar damage evolution was captured by user-defined subroutine and interlaminar delamination was simulated via the failure of zero-thickness cohesive elements inserted between plies. By comparison, the numerical results are in good agreement with experimental results. Based on the validated model, the effects of patch stacking sequence and rotation angle on low velocity performances (impact loads and damages) of scarf repaired laminates were investigated respectively. Both of them have effects on the impact performance of scarf repaired laminates, but patch rotation angle has greater effect than patch stacking sequence.

Low-velocity impact performance of scarf-repaired composite laminates was investigated by experimental and numerical methods. The numerical model was built using continuum damage mechanics and cohesive elements were used in all the interfaces of the adherends (i.e. the patch and the parent). The model was verified to be efficient. Based on it, damage profiles and damage mechanism were studied, and the effects of the scarf angle, the adhesive thickness and the patch off-axis angles were analyzed. Results show that the patch damage occurs in its center region through the thickness, while the damages in the parent and adhesive mainly occur around the back face. Damage area of the patch is obviously more serious than that of the parent, and the adhesive can protect the parent to some degree. The scarf angle, the adhesive thickness and the patch off-axis angles have effects on the impact properties. The scarf angles ranging from 7° to 8°, the adhesive thicknesses of 0.15 mm, the patch off-axis angles ranging from 135° to 160° are recommended for better impact performances.

Composite-metal joints with a metal insert are one kind of connecting structure. In this paper, tensile experimental tests were carried out to investigate tensile properties of a composite-metal joint with a novel metal insert design. Finite element models of the joint were established, and strain distribution and tensile strength were analyzed. The numerical results are in good agreement with the experimental results. Results show that the joint failure is dominated by shear properties of the resin layer. Increasing the resin layer thickness in a certain range will improve the tensile strength of the joint, while increasing the radius of the fillet on the ending side of the metal insert will decrease the joint strength. Increasing the resin layer plasticity will improve the joint strength. The effect of the embedded depth of the metal insert can be ignored.

Failure envelope method is comprehensively used in failure mode prediction of multi-bolt composite joints. The method is based on the assumption that bolt load distribution proportion is constant till failure. However, bearing damage and net-section damage must have some influence on the load distribution and then change structure failure mode. In this paper, tensile experiments of two-bolt and three-bolt joints were conducted to obtain the failure modes and other properties. Three dimensional (3D) finite element models involving damage were constructed, and the results were consistent with experimental data. Then, the models were used to study the effect of damage on failure mode of multi-bolt composite joints. The results show that failure mode prediction with failure envelope method should involve the effect of composite damage. Compared with net-section damage, bearing damage has greater influence on failure mode. Based on conventional failure envelope method, a new evaluation procedure involving damage was proposed.

In this paper, tensile behavior of box-joint structure fabricated by resin transfer moulding (RTM) was investigated. A finite element model (FEM) of the box joint based on cohesive zone model was established to study the joint tensile behavior. Hashin failure criterion and degradation rules were applied to simulate damage modes and to predict failure strength. The FEM results show a good agreement with experimental results. Based on the model, the influences of bolt location and lap length on the joint tensile performances were analyzed. The results show that the delamination between the L-shape part and box-shape parts is the main failure mode. Bolt location affects the damage modes, ultimate load and structure stiffness of the joint, and longer lap length has a tendency to promote ultimate load and structure stiffness.

In this paper, experiments were conducted to study the tensile performance of metal-to-composite bolted joints at room temperature and 250 °C. Furthermore, numerical models were developed to simulate tensile behavior of the joint. After achieving a decent validation between numerical models and experimental results, more detailed studies were conducted through finite element models to find out the influence of temperature, bolt material, the thickness of composite laminate and stacking sequence on tensile properties of the joint at room temperature and 250 °C. The research showed that temperature significantly affects the bearing strength of the joint. Around 42–50% decrease in bearing strength was observed at 250 °C as compared to room temperature. Stacking sequence variations have a little more effect on bearing strength at higher temperature than at room temperature. With the change of stacking sequences, around 11–24% variation in bearing strength was observed at 250 °C as compared to 8–20% at room temperature.

Experimental and numerical study are carried out to investigate hygrothermal effects on the mechanical behavior and damage propagation of scarf repaired carbon fiber reinforced plastic (CFRP) laminates. Experimental tests were carried out under four environment conditions: room temperature/dry, room temperature/wet, elevated temperature(95°C)/dry and elevated temperature(95°C)/wet. Results showed no decrease in compression strength after moisture absorption, whereas the reduction under elevated temperature were 15.5% and 17.2% for dry and wet specimens, respectively. Furthermore, a finite element model was developed to simulate the failure process and predict structural strength, in which moisture diffusion, hygrothermal stress analysis and progressive damage model were considered. Good agreement was achieved between the predicted compression strength and experimental results. It is revealed that the damage in the adhesive initiates adjacent to 0° plies and propagates in circumferential direction. Water absorption causes initial damage in the edge of the adhesive layer, while it does not reduce the compression strength. And elevated temperature leads to earlier damage in adhesive. After large part of the adhesive layer damaged, damages in parent laminate occur and propagate transversely to the free edge sides until the final structural collapse, which is similar with the experimental phenomenon. POLYM. COMPOS., 2016. © 2016 Society of Plastics Engineers

This paper was aimed to investigate the hygrothermal effects on the mechanical behavior of the double lap shear joints of Carbon/Epoxy composite laminates, which were bonded using epoxy adhesive film SY14. First, experimental method was used to evaluate the hygrothermal dependent properties of the adhesive and the static response of composite laminate double lap joints at various environment conditions. The specimens were grouped into room temperature/dry (RD), room temperature/wet (RW), elevated temperature/dry (ED) and elevated temperature/wet (EW). The wet specimens were immersed in deionized water at temperature of 90 °C for 60 h and the elevated temperature is 95 °C. Tensile tests were carried out at room temperature and elevated temperature (95 °C) in a controlled chamber. Results showed that the elastic modulus and tensile strength decreased about 24% and 27% respectively after exposure to humidity environment. And a large degradation was found when exposure to high temperature while the plasticity became notable. The failure modes of the double lap shear joints were studied by visual inspection. It was found that both adhesive and cohesive failure happened for dry specimens at room temperature. While failure modes were dominated by cohesive failure after moisture exposure and adhesive failure at elevated temperature. Second, a finite element model was utilized to simulate the damage evolution of the double lap joints. The moisture diffusion process, swelling stress and thermal stress were included. And hygrothermal dependent cohesive law was considered to study the damage evolution. It revealed that failure modes were highly dependent on the weaker strength of adhesive and cohesive. Moisture absorption caused large degradation in cohesive strength, so the failure modes were mostly cohesive failure. Elevated temperature decreases strength of adhesive seriously, so adhesive failure dominates. Good agreement was achieved between the predicted failure loads and experimental results. And the predicted failure modes were also consistent with the experimental phenomenon.

In this paper, tensile experiments were conducted into single-lap bolted joint between composite laminate and steel with countersunk fastener, and two three-dimensional explicit finite element models were developed using Abaqus/Explicit. Continuum shell elements (SC8R) were used to model in-plane ply failure with built-in damage model while cohesive elements were selected to predict interlaminar damage in model I. 8-node reduced integration solid elements (C3D8R) were chosen to model all the parts with 3D Hashin-type initiation criteria and Camanho degradation law implemented in a VUSDFLD subroutine in model II. The results simulated by both models accurately predicted the elastic loading response and captured overall damage profile compared with the experimental results. The numerical analysis using explicit solver provided acceptable accuracy which highlighted the robust nature and high computational efficiency without convergence issues compared with implicit solver. In addition, the effect of cohesive elements on computational efficiency was investigated to simplify the countersunk joints analysis.

In this present investigation, a novel design scheme for composite bolted π-joint is proposed, which is mainly improved in layup design. A set of joints are manufactured by resin transfer moulding (RTM) process based on the design above. A detailed investigation by experimental and numerical analysis is conducted on them to study their mechanical behavior and failure mechanism under bending load. In this work, computational models are developed by ABAQUS/Standard, with 3D progressive damage model to predict multiple damage modes in two components of joint. By validation of experimental results, it is indicated that the numerical analysis captures the failure mechanism and provides reasonable accuracy in predicting ultimate load. It is shown that the shear-out failure of lug bolt holes is the failure mode of joints. Besides that, decreasing the thickness of base panel properly will probably cause additional damage to base panel, though it has no significant effect on ultimate load of joints. Also, the location of lug bolt hole is proved to be one of the factors that will influence the load carrying ability of joints.

Analysis on hybrid bonded/bolted (HBB) joint is difficult due to its complex failure mechanism influenced by multiple design parameters. The effect of design parameters on HBB joint mechanical performances has not been systematically investigated. In this paper, a detailed finite element model (FEM) of a composite-steel single-lap HBB joint was established and verified by experimental results. Based on the validated model, ten major design parameters of the joint related to assembly relationship, joint geometry and material property were discussed. And the effect of design parameters on the joint tensile performances, including strength, stiffness and energy absorption, was investigated and compared comprehensively by the methods of uniform design and multiple linear regression analysis. The results indicate that increasing values of all the discussed design parameters except for bolt pre-torque, free part length ratio S/D and adhesive modulus show positive effect on tensile performances of HBB joint. Using high strength adhesive with low modulus is a key method to improve initial damage load and bonding strength of the joint. High strength bolt can effectively improve bolt hole bearing strength and energy absorption capacity of the joint. Larger width ratio W/D is effective to improve tensile stiffness of hybrid joint with single bolt.

Adhesives have been widely used in aircraft composite bonded structures, which are required to stand severe environment conditions involving high temperature and moisture during the service. Current relevant investigations focus on the environment effects on adhesive specific properties with little attention to adhesive material selection in different environment conditions. In this paper, the properties of two types of adhesives from the same material system, SY14M (an elevated temperature curing epoxy adhesive) and SY24C (a medium temperature curing epoxy adhesive), were studied in different environment conditions. An adhesive selection method based on property matching between adhesive tensile and shear strength in different environment conditions was proposed. Results show that hydrolysis reaction can cause adhesive color variation, moisture absorption acceleration and plasticity decrease. Thermal effect on tensile properties of the adhesives is greater than that on shear properties, while moisture effect is opposite. High temperature can extremely increase adhesive plasticity, especially for tough adhesive. For the two adhesives, the matching consistency between tensile and shear strength is different in different environment conditions, so environment conditions must be considered in the adhesive selection.

An investigation based on experimental and finite element (FE) studies was carried out to understand the failure mechanisms of scarf-repaired T700SC/NY9200GA composite laminates under tensile load. Test specimens were categorized into 4 groups based on the type of scarf patch (pre-preg and pre-formed) and scarf angles (6°and 10°). A 3-D progressive damage accumulation model was developed to understand the damage mechanisms and predict the ultimate strength of repaired laminates. Damage characteristics of specimens were observed by optical microscope and scanning electron microscopy. Test results have shown that the laminates repaired with pre-preg scarf patch and 6° scarf angle have higher failure strength. It was also found that the fracture morphology of failed laminate is different but failure mode was found to be the same. The crack initiated from highly stressed adhesive regions and then circumferentially propagated throughout the adhesive interface thus causing adhesive failure. FE results were found to be in good agreement with experimental results.

Single lap bolted joints subjected to tensile load are essential joining methods in engineering design. Lateral displacement of the grip in single lap bolted joint experiment was examined and analyzed. This was different from early investigations that considered the joint fully gripped with no lateral degree of freedom. The lateral stiffness of the grip was tested to obtain the test machine properties. Tensile experiments on single lap composite-to-aluminum bolted joints were conducted, and the lateral displacement of the grip measured. Other measurements included tensile displacement, out-of-plane displacement, and surface strains. By allowing for lateral displacement, the novel finite element model predicted good results. The influence of lateral displacement of the grip was analyzed using the novel and traditional models. The novel model predicted 13 % smaller joint stiffness than traditional model. The parameters related to secondary bending were found different and the failure load of the joint was delayed because of lateral displacement of the grip.

This present work investigated the failure mechanism of a novel composite bolted π-joint subjected to bending load by experimental and finite element simulation. A test sample manufactured by resin transfer moulding process (RTM) was tested. A 3D progressive damage model developed in ABAQUS/Standard was used to simulate the failure of the π-joint. Based on good correlation of failure load and damage distribution between experimental results and FE prediction, further investigation was extended to the effect of two primary assembly clearances on mechanical behavior of the π-joint. The study results reveal that delamination of the fillet region in L-preform is the π-joint's failure mode. Moreover, the assembly clearances have little effect on the failure load of the joint.

The damage characters of scarf repaired composite laminates subjected to low-velocity impact with various energy levels at different locations are studied experimentally. The results are compared with those of the original laminates which have no initial damage and don’t need repair. The impact load-time history of the specimens, the velocity-time curves of the impactor, the post impact compressive strength of the specimens and the C-scan photographs of the damaged regions are obtained. The delamination threshold load and damage character of the specimen section at impact point are also studied. The results have shown that the impact response of a repaired composite laminate is sensitive to the location of the impact. The impact load and the delamination threshold load have shown different characters for specimens with different impact locations. The debonding characters of the adhesive and compressive strength after impact of the specimens are also influenced by impact locations.

Tensile mechanical behaviors of two kinds composite laminates G803/5224 and G827/5224 specimen after high-speed impact were investigated separately. The influences of open-hole on the specimen’s net tensile strength were analyzed by comparing with standard specimen’s experimental results. An FEM model was established to simulate the damage growth in the laminates. Different equivalent zones were applied to model different velocity impact damages, and different damage criteria and modulus attenuation rules were used for two different ply forms.

An investigation based on experimental and analytical approaches was conducted to evaluate the behavior of the stitched laminates under hygrothermal conditions. Tensile strength of laminates under different environmental conditions was predicted using FEM model. Effects of stitching parameters upon strength performance of laminates were studied using FEM model and validity of results was checked by comparing with the experimental results. Test results have shown that hygrothermal environment condition has no significant effect on the tensile strength of unstitched laminates, but improved the strength of stitched laminates significantly. Stitching decreased the tensile strength of laminates under 20°C dry environment; however, it improved the tensile strength under 20°C wet environment. It was also found from analytical results that, the failure strength of stitched laminates is higher for smaller thread radius (R

Experimental and analytical investigation is conducted to explore the effects of stitching on plain (without hole) and open-hole compressive and tensile strength of uniweave T300/QY9512 laminates under different environmental conditions (20 °C/dry and wet, 150 °C/dry and wet). Strength performance of stitched composite laminates is also studied using finite element analysis (FEA) model and compared with the experimental results to validate the model. It is found that under similar environmental conditions, the open-hole compressive strength of stitched laminate is decreased and open-hole tensile strength increased as compared to the unstitched laminates. Predicted tensile and compressive strengths are found to be in a good agreement with the test results and the relative error in all cases is less than 15%.

A robust multi-objective wing design optimization procedure using CFD is presented in this work. Instead of directly applying CFD to wing design optimization, a Kriging aerodynamic model that appro...

Based on the concept of the damage-tolerance and durability design, the total fatigue life of titanium alloys is divided into three phases: crack initiation (0–0.3 mm), short crack growth (0.3–2 mm) and long crack growth (2 mm–aC). Among these three phases, different prediction models are accepted due to different failure mechanisms. A computer program was developed to predict the total fatigue life of the titanium alloy structure. Fatigue testing is also conducted for two types of ELI grade titanium alloy to verify the prediction models. The predicted fatigue life agrees well with experimental results.

This paper proposes a new analytical solution to predict the elastic modulus of a 2D plain weave fabric (PWF) composite accounting for the interaction of orthogonal interlacing strands. The two orthogonal yarns in a micromechanical unit cell are idealized as curved beams with a path depicted by using sinusoidal shape functions. The modulus is derived by means of a strain energy approach founded on micromechanics. Four sets of experimental data pertinent to four kinds of 2D orthogonal PWF composites have been implemented to validate the new model. The calculations from the new model are also compared with those by using four models in the earlier literature. It is shown that the experimental results correlate well with predictions from the new model.

This paper seeks to address a novel model for predicting the compression modulus of a 2D plain weave fabric (PWF) composite accounting for the interaction of orthogonal interlacing strands. The two orthogonal yarns in a micromechanical unit cell are idealized as curved beams with a path depicted by using sinusoidal shape functions. The modulus is derived by means of a strain energy approach founded on micromechanics. In order to validate the model, experiments have been performed to investigate the mechanical properties of two-dimensional (2D) orthogonal EW220/5284 PWF composites fabricated by resin transfer moulding (RTM). In-plane tensile, compressive and shear moduli have been measured. It is shown that the experimental results correlate well with predictions from the new model.

Tensile tests are conducted on stitched and unstitched uniweave T300/QY9512 laminates (with a hole) in hygrothermal environment and normal environment. It is shown that in a normal environment stitching reduces the tensile strength of laminates and does not significantly alter the open-hole tensile strength. The hygrothermal environment has a beneficial effect on the tensile strength of laminates and open-hole laminates, especially of stitched specimens. It is also noted that stitching direction has no effect on the tensile properties.

Experimental investigation was conducted in order to understand the hygrothermal properties of stitched and unstitched uniweave T300/QY9512 laminates (with a hole) in this paper. The tests were conducted in a hygrothermal environment and a normal environment, respectively. Experimental results for compression, transverse impact and compression after impact of stitched and unstitched laminates were presented in this paper. The results showed that a hygrothermal environment degrades the compression properties of stitched and unstitched laminates (with a hole), and hygrothermal environment had a beneficial effect on low-velocity impact property.

Experimental studies on the compressive behavior of composite laminates after low velocity impact was carried out with two test methods. One is SACMA Standard, and the other is a small dimensional specimen test method. Impact damage distributions, compressive failure process after impact, quasi-static indentation and compression of laminates with a hole were brought into comparison between these two test methods. The results showed that there is a great difference between these two test methods. Compressive behavior of laminates after impact varies with different test methods. Residual compressive strength of laminates after low velocity impact measured with SACMA Standard can reflect stiffness properties of composite resins more wholely than that measured with the other method can do. Small dimensional specimen test method should be improved on as an experimental standard of compressive strength after impact.