Mixed-mode Interlaminar Fracture Toughness Research Articles

Experimental and virtual testing of mode II and mixed mode crack propagation under dynamic loading

Under static loading, measuring experimentally the mode II and mixed mode fracture toughness of composite materials and adhesive joints is well standardised. However, under dynamic loading, no standard procedure has been defined yet. Therefore, this paper proposes an experimental methodology to measure the mode II and mixed mode interlaminar fracture toughness of composite materials and adhesive joints. The methodology is based on a modified split Hopkinson compression bar. Two different data reduction schemes are explored and compared, one based on measuring the crack length, and another based on measuring the force from the strains in the transmitted bar. The two data reduction methods provided considerably different results. By using the method based on measuring the force, the mode II and mixed mode fracture toughness for both interlaminar and adhesive joints decreased for higher strain rates, while the opposite was found with the other approach. The method based on the crack length measurement was deemed to be unreliable due to the difficulties in measuring it.

Temperature effects on mixed mode I/II delamination under quasi-static and fatigue loading of a carbon/epoxy composite

This paper addresses the effect of temperature on the mixed-mode interlaminar fracture toughness and fatigue delamination growth rate of a carbon-fibre/epoxy material, namely IM7/8552. Quasi-static and fatigue characterisation tests were carried out at −50°C, 20°C, 50°C and 80°C, using asymmetric cut-ply coupons. The experimental results show that temperature may have an accelerating or delaying effect on delamination growth, depending on the loading regime, i.e. either quasi-static or fatigue. Fractographic examinations were also carried out in order to assist the interpretation of the experimental data. A semi-empirical equation is introduced to describe the experimentally observed fatigue delamination growth rates at elevated temperatures.

필라멘트 와인딩된 카본/에폭시 복합재의 층간파괴인성에 미치는 온도 영향

본 논문은 필라멘트 와인딩 공법으로 제작된 구배 단면을 갖는 돔 분리형 복합재 압력용기 접착체결부의 온도영향에 따른 층간파괴인성 평가를 위해 모드 I, II 그리고 혼합모드의 시험적 평가를 수행하였다. 모드 I과 혼합모드 층간파괴인성은 DCB 시편을 사용하였으며, 모드 II 층간파괴인성은 ENF 시편을 사용하여 평가하였다. 와인딩 각도는 [±10°]SUB6/SUB, [±27°]SUB6/SUB 그리고 ([±10°]SUB3/SUB/FM73/[±10°]SUB3/SUB )이며 곡면 단면을 고려하였다. 시험 평가에 적용된 온도 환경은 환경 챔버와 전기로를 이용하여 -30℃ 에서 60℃로 조성하였다. 층간파괴인성에 온도가 미치는 영향을 평가한 결과, 층간파괴인성은 저온에서 높게 나타났으며, 온도가 증가함에 따라 감소하는 경향을 확인하였다. 시편 종류별 결과에서는, 돔부와 헬리컬부가 접착 체결된 [±10°]SUB3/SUB/FM73/[±10°]SUB3/SUB 와인딩 시편이 가장 높은 층간파괴인성을 가짐을 확인하였다.

To Improve Mixed-Mode Interlaminar Fracture Toughness of Composite Sub-Structures

The present paper investigates the influence of stitching on delamination resistance of laminated composite structures. The mixed-mode interlaminar fracture toughness, GI/IIC, of the stitched hybrid laminated composites is studied in order to investigate the resistance of the 3D-composites to the crack propagation in delaminated composite structures. To that end, the mixed-mode interlaminar fracture toughness was measured using the asymmetric double cantilever beam (ADCB) test method. The hybrid ADCB and stitched hybrid ADCB composite beams were laid-up in order to study the effect of stitching on the interlaminar fracture toughness. The test results showed that the resistance of stitched fibres against the crack propagation in stitched hybrid composites can significantly improve the mixed-mode interlaminar fracture toughness.

A study of mixed mode interlaminar fracture on nanoclay enhanced epoxy/glass fiber composites

Fiber reinforced laminate are widely used in aerospace, automobile and marine industries, despite its poor interlaminar fracture toughness (IFT), as consequence of the absence of fibers to sustain transverse load. One way recently explored with relative success in order to improve IFT is the use of nanoparticles to reinforce the matrix. Present paper intends to assess and discuss the fracture toughness on mixed mode loading of fiber glass mats/nanoclay enhanced epoxy matrix laminates. The matrix used was the epoxy resin Biresin® CR120 combined with the hardener CH120-3, the fiber glass was triaxial mats ETXT 450 and the nanoparticles were montmorillonite nanoclay (NC). The results were discussed in order to understand the effects of the percentage of nanoclay and the shear load quantified in terms of the GII/GI ratio on the total fracture toughness G. The incorporation of a small quantity of NC into matrices improves significantly mixed-mode IFT for all loading mode ratios GII/G. The total fracture toughness G increases with the mode II loading component and a linear mixed-mode fracture criteria reproduces the Gc against GII/G relationship.

An enhanced beam-theory model of the mixed-mode bending (MMB) test—Part I: Literature review and mechanical model

The paper presents a mechanical model of the mixed-mode bending (MMB) test used to assess the mixed-mode interlaminar fracture toughness of composite laminates. The laminated specimen is considered as an assemblage of two sublaminates partly connected by an elastic–brittle interface. The problem is formulated through a set of 36 differential equations, accompanied by suitable boundary conditions. Solution of the problem is achieved by separately considering the two subproblems related to the symmetric and antisymmetric parts of the loads, which for symmetric specimens correspond to fracture modes I and II, respectively. Explicit expressions are determined for the interfacial stresses, internal forces, and displacements.

Experimental and Numerical Investigation of the Mixed-Mode Delamination of PAN Based Carbon/Epoxy Composite Using Modified Arcan Fixture

The present research examines analytically and experimentally the mode-I and mode-II and mixed mode-Interlaminar fracture toughness of PAN based carbon/epoxy composite. A modified Arcan fixture, well-suited for the study of the behavior of used composite assemblies, was developed in order to focus on the analysis of the fracture behavior of the material. The edge effects are minimized by using an appropriate design of the substrates so that experimental results give reliable data. Also the mode-I and mode-II stress intensity factors were computed for different crack lengths and load orientation angles using finite element analysis. The numerical results show that the modified Arcan specimen is able to provide pure mode-I, pure mode-II and any mixed mode loading conditions. It is shown that the results obtained from the fracture tests are consistent very well with mixed mode fracture theories. Obtained results indicated that fracture toughness and stress intensity factor for sliding mode enhanced up when the loading angle increased. Mechanism of fracture and toughening were examined by using scanning electron microscopy.

Mixed-Mode Delamination Failure of Z-Pinned Hybrid Laminated Composites

The mixed-Mode interlaminar fracture toughness, GI/IIC, of z-pinned hybrid laminated composites is studied to investigate the effect of 3D-composites on the crack propagation resistance of delaminated composite structures. In this regard, the mixed-Mode interlaminar fracture toughness, GI/IIC, was measured using asymmetric double cantilever beam (ADCB) test method. The hybrid ADCB and z-pinned hybrid composite beams were laid-up from [G0/C0]4, [G0/C90]4, [G90/C0]4 and [G90/C90]4 to study the effect of z-pinning on the interlaminar fracture toughness. From the obtained results from test it was found that the resistance of z-pin fibres against the crack propagation in z-pinned hybrid composites can significantly increase the mixed-mode interlaminar fracture toughness.

Off-axis Crashworthiness Characteristic of Woven Glass/Epoxy Composite Box Structures

This study investigates the influence of off-axis loading on the fracture mechanism and specific energy absorption (SEA) of glass/epoxy twill/weave composite box structures. In this regard, the off-axis angles of 5°, 10°, 20°, and 30° of loading direction with respect to the composite box axis is studied experimentally under quasi-static crushing process. At the off-axis angle of 5°, the crushing process behavior is similar to that of the axial crushing and fracture mechanisms, such as bundle fracture and interlaminar crack propagation in Mode-II are observed. These are characteristic of brittle fracture crushing mode. For crushing at an off-axis angle of 10°, additional interwall crack propagation at one side of the box is also found. This increases the energy absorbing capability of glass/epoxy composite box at this angle. The sidewall crack is a mixed-mode crack. To characterize this crack, the mixed-mode interlaminar fracture toughness, G I/IIC, is measured using asymmetric double cantilever beam (ADCB) test method. For other angles, the non-symmetrical fracture mechanisms are found at four sides of the composite boxes. The arriving of sustained crushing stage is delayed by increasing the off-axis crushing angle. Owing to this fact, the energy absorbing capability is reduced by increasing the off-axis loading angle. An analytical solution is proposed to predict the mean force of axial crushing in brittle fracture crushing mode. The off-axis crushing process of composite boxes is also simulated by finite element software LS-DYNA and the results are verified with the relevant experimental results.

Effect of fiber angle orientation and stacking sequence on mixed mode fracture toughness of carbon fiber reinforced plastics: Numerical and experimental investigations

This paper focuses on the effect of fiber orientation and stacking sequence on the progressive mixed mode delamination failure in composite laminates using fracture experiments and finite element (FE) simulations. Every laminate is modelled numerically combining damageable layers with defined fiber orientations and cohesive zone interface elements, subjected to mixed mode bending. The numerical simulations are then calibrated and validated through experiments, conducted following standardized mixed mode delamination tests. The numerical model is able to successfully capture the experimentally observed effects of fiber angle orientations and variable stacking sequences on the global load–displacement response and mixed mode inter-laminar fracture toughness of the various laminates. For better understanding of the failure mechanism, fracture surfaces of laminates with different stacking sequences are also studied using scanning electron microscopy (SEM).

An enhanced beam-theory model of the asymmetric double cantilever beam (ADCB) test for composite laminates

The paper introduces a mechanical model of the asymmetric double cantilever beam (ADCB) test, usable to assess the mixed-mode interlaminar fracture toughness of composite laminates. The laminated specimen is represented as an assembly of sublaminates, each of which is modelled as an elastic beam partly connected to the other by a deformable interface, in turn considered to be a continuous distribution of elastic-brittle springs. Based on Timoshenko’s beam theory, a set of six differential equations, accompanied by suitable boundary conditions, governs the problem. By adopting the interfacial stresses as the main unknowns, the differential problem is solved analytically, and the contributions of the opening and sliding fracture modes are evaluated directly. Moreover, explicit expressions are determined for the interfacial stresses, internal forces, and displacements, as well as for the compliance, energy release rate, and mode-mixity angle. The predictions of the model are to some extent similar to those of analogous mechanical models in the literature and appear in good agreement with both numerical and experimental results.

Analysis of mixed-mode interlaminar fracture and damage behavior of GFRP woven laminates at cryogenic temperatures

The objective of this work is to investigate the interlaminar fracture and damage behavior of glass fiber reinforced polymer (GFRP) woven laminates loaded in a mixed-mode bending (MMB) apparatus at cryogenic temperatures. The finite element analysis (FEA) is used to determine the mixed-mode interlaminar fracture toughness of MMB specimen at room temperature (RT), liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). A FEA coupled with damage is also employed to study the damage distributions within the MMB specimen and to examine the effect of damage on the mixed-mode energy release rate. The technique presented can be efficiently used for characterization of mixed-mode interlaminar fracture and damage behavior of woven laminate specimens at cryogenic temperatures.

Experimental and numerical investigation of the mixed-mode delamination in Arcan laminated specimens

This paper investigates mixed-mode interlaminar fracture behaviour in woven carbon fibre/polyetherimide (CF/PEI) thermoplastic composite material based on experimental and numerical analyses. Experiments were conducted on modified Arcan specimens using the special test loading device. By varying the loading angle from 0° to 90°, pure mode-I, pure mode-II and a wide range of mixed-mode data were obtained experimentally. Using the finite-element results, correction factors were applied to the CF/PEI fracture specimen. By employing experimentally measured critical loads and the aid of the finite-element method, mixed-mode interlaminar fracture toughness for the composite under consideration determined. The failure response of CF/PEI composite was compared to the different mixed-mode failure criteria, and the best criterion was selected. The fracture surfaces of the CF/PEI composite under different mixed-mode loading conditions were examined by optical and scanning electron microscopy to gain insight into the failure responses.

탄소섬유직물/에폭시 복합재의 혼합모우드 층간파괴 거동

MMF 시험을 적용하여 혼합모우드 비율을 20%～90%의 범위 내에서 변화시키면서 탄소섬유직물/에폭시 복합재의 혼합모우드 층간파괴 거동을 조사하였다. 혼합모우드 층간파괴 거동을 예측하기 위해 NL점과 5% offset점에 근거한 혼합모우드 층간파괴 결정식을 제시하였다. 파단면 양상과 균열진전 거동은 이동식 현미경과 전자현미경을 통해 조사하였다. 연구결과에 따르면 혼합모우드 층간파괴 거동은 NL점에 근거한 경우 매개변수 m=1.5와 n=0.5, 5% offset점에 근거한 경우 매개변수 m=2와 n=3인 혼합모우드 층간파괴 결정식에 의해 잘 예측되어진다. 파단면 양상과 균열진전 거동은 혼합모우드 비율에 매우 민감하게 변하며 MMF 시험은 혼합모우드 층간파괴인성의 평가에 성공적으로 적용됨을 알 수 있었다. Mixed mode interlaminar fracture behaviors of carbon fabric/epoxy composites were investigated through MMF (Mixed Mode Flexural) test by varying mixed mode ratio ranging from 20% to 90%. Mixed mode interlaminar fracture criteria based on NL point and 5% offset point were also suggested in order to predict mixed mode interlaminar fracture behaviors. Fracture surfaces and crack propagating behaviors were examined through a travelling scope and a scanning electron microscope. According to the results, mixed mode interlaminar fracture behaviors can be predicted by mixed mode interlaminar fracture criterion with m=1.5 and n=0.5 on the basis of NL point or mixed mode interlaminar fracture criterion with m=2 and n=3 on the basis of 5% offset point. Fracture surfaces and crack propagating behaviors are sensitive to mixed mode ratios. MMF test can be successfully applicable in evaluating mixed mode interlaminar fracture toughness of carbon fabric/epoxy composites.

A new mixed mode test for carbon/epoxy composite systems

The present research examines analytically and experimentally the mixed mode interlaminar fracture toughness of a resin film infused (RFI) carbon fiber/epoxy laminate, namely a IM7-AS4/3501-6 hybrid composite system. The inability to develop representative interlaminar failure in composites with current mixed mode test configurations motivated this particular investigation. The paper is part of a more extensive research effort concerned with the effects of stitching upon the mixed mode fracture toughness of a RFI composite. A new mixed mode test configuration is suggested, the Single Leg Four Point Bend (SLFPB), which provides a robust method with small specimens and a simple apparatus. Closed form fracture mechanics-based strain energy release (SERR) calculations have been established for this configuration. Finite element analysis was conducted to validate the closed form solution. Results show a very good agreement between analytical solutions, numerical simulation and the newly designed SLFPB experimental test.

The effect of matrix toughness and loading rate on the mode-II interlaminar fracture toughness of glass-fibre/vinyl-ester composites

Glass-fibre-reinforced composites are increasingly used for structural applications. However, like high-performance carbon-fibre composites, they are susceptible to mode-II-dominated delamination. In response to this problem, this paper investigates the effect of matrix toughness and loading rate on the mode-II interlaminar fracture toughness ( G IIc) of unidirectional glass-fibre composites with brittle and rubber-toughened vinyl ester matrices. Mode II tests were conducted σn end-notch-flexure (ENF) specimens at test rates ranging from 1 mm/min to 3 m/s. The G IIc results were compared to the order of matrix G Ic. There was no significant effect of loading rate or matrix toughness on G IIc. The absence of a loading rate effect is consistent with the bulk of the experimental data in the literature, but the absence of a matrix effect is not. Microscopic examination of fracture surfaces shows similar matrix deformation in each composite. The through-thickness matrix deformation zone size is also similar. These observations suggest similar energy absorption in each composite and hence support the G IIc test results. It is concluded that failure is interface controlled, whereby unstable fracture is initiated after a similarly short period of crack growth in each composite, and before an increase in G IIc as a result of increased matrix toughness becomes apparent. The G IIc results indicate that the use of rubber-toughened vinyl ester matrices in glass-fibre composites will not improve resistance to impact-induced mode II-delamination. However, through-thickness impact damage in composite structures is likely to result from mixed-mode (I/II) loading. Therefore, suggestions for future work include investigation of the matrix effect on mixed-mode (I/II) interlaminar fracture toughness, and on delamination resistance of plate structures subjected to transverse low-velocity impact.

A modification of the mixed-mode bending test apparatus

Abstract In this article, the mixed-mode bending test appartaus was modified in order to reduce the influence of the weight of the lever on the mixed-mode fracture toughness during the measurement. A unidirectional glass fibre reinforced polyamide 12 composite laminate was measured at G I / G II ratios of 3/1 and 1/1, by using the original and the modified MMB apparatus respectively, and the results of the mixed-mode interlaminar fracture toughness data were compared. It has been found that the modified MMB apparatus can be used to avoid the preloading on the specimens caused by the weight of the lever, and the fracture toughness can directly be calculated by applying the beam theory or the modified beam theory equations without the lever-weight correction.

Interlaminar fracture toughness: the long and winding road to standardization

Abstract The on-going efforts to achieve internationally accepted test standards for interlaminar fracture toughness over the past two decades will be reviewed. The status of current efforts to obtain an ISO standard for the Double Cantilever Beam test for mode I interlaminar fracture toughness will be highlighted. Current round robin activity underway to develop standards for mode II, mode III, and mixed-mode interlaminar fracture toughness will also be discussed. Differences in the interpretation of interlaboratory “round robin” test results will be highlighted. The influence of the different roles of standardization bodies in the US, Europe, and Japan on achieving international standards for interlaminar fracture toughness will be discussed.

Effect of the Interfacial Interleaf to the Interlaminar Fracture and Intralaminar Fracture of a New BMI Matrix Composites System

Systematic studies were conducted to investigate the effect of the introduc tion of a soft thermoset adhesive layer at appropriate interface to the interlaminar fracture and intralaminar fracture of the BMI matrix based composite laminates. Tests include DCB and ENF tests for measuring interlaminar fracture toughnesses, quasi static tension test on the crossply laminates and edge delamination test for measuring matrix crack initi ation, edge delamination onset, mixed-mode interlaminar fracture toughness and the ulti mate laminate strength. The improvements of the interlaminar fracture toughnesses of the GFRP and CFRP laminates are significant. An elastic foundation beam model is intro duced to analyze the interleaved DCB specimen. Results show stress concentration at the crack-tip of the interleaved DCB specimen is reduced. Besides the factor of the possible formed plastic zone in interleaf, the modified interlaminar bond strength of the adhesive/ composites interface may lead to higher interlaminar fracture toughness. The edge delamination strength and the mixed-mode interlaminar fracture toughness in interleaved laminate are raised significantly. However, the interleaf has no significant effect on the laminate strength because the interfacial interleaf only improves the interlaminar fracture feature while the final laminate fracture is controlled by the intralaminar properties of the lamina. On the matrix cracking in interleaved laminate, both the matrix crack growth rate and its final saturation crack density decrease in the interleaved multidirectional laminate. The matrix crack was found to initiate early in the interleaved crossply laminate. This*Author to whom correspondence should be addressed Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 13-June 1994 0731-6844/94/06 0509-32 $6.00/0 @ 1994 Technomic Publishing Co., Inc.result agrees with the analysis of the one-dimensional shear-lag model. But in the edge delamination specimen, matrix crack initiation is raised by the interleaf.

Interaction between matrix cracking and edge delamination in composite laminates

Matrix cracking and edge delamination are two main damage modes in continuous-fibre composite laminates. They are often investigated separately, and so the interaction between two damage modes has not yet been revealed. In this paper, a simple parallel-spring model is introduced to model the longitudinal stiffness reduction due to matrix cracking and edge delamination together. The energy release rate of edge delamination eliminating the matrix crack effect and the energy release rate of matrix cracking in the presence of edge delamination are then obtained. Experimental materials include carbon- and glass-fibre-reinforced bismaleimide composite laminates under static tension. The growth of matrix cracks and edge delaminations was recorded by means of NDT techniques. Results show that matrix cracks may initiate before or after edge lamination. This depends on the laminate layup, and especially on the thickness of the 90° plies. Edge delamination may also induce matrix cracking. Matrix cracking has a significant effect on the stiffness reduction in GRP laminates. The present model can predict the stiffness reduction in a laminate containing both matrix cracks and edge delaminations. The mixed-mode delamination fracture toughness obtained from the present model shows up to 50% differences compared with O'Brien's model for GRP laminates. However, matrix cracking has a small effect on the mixed-mode interlaminar fracture toughness of the CFRP laminates.