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Crack growth resistance plays a different role in crashworthiness analysis since the progressive energy absorption is based on controllable fracture mechanisms. In this regard, the present paper studies the efficient crack growth resistance in off-axis crushing of composite tubular structures by implementing natural fiber yarns. One of the through-to-thickness reinforcement methods known as stitching has been chosen to influence the axial and off-axis crushing process. Improving the crack growth resistance and appropriate fiber breakage at different stages of crushing process can significantly improve the resistance force and consequently the energy absorption capability of composite absorbers in axial and off-axis crushing. This analysis will be applied to non-stitched and stitched CFRP composite boxes which showed brittle fracture and transverse shearing crushing modes under off-axis loading of 10 degrees. The analytical methods are also implemented to analysze the effect of various failure mechanisms such as bending, friction, bundle fracture, and interlaminar crack growth for the observed crushing modes. The proposed model is able to predict the crushing load and crush force efficiency in close agreement from experimental studies.

Many of composite absorbers are designed to dissipate large amounts of impact energy under axial crushing, bending or combined loading with controllable collapse manner in the collision event. Extensive research during the last decade focused on the axial impact of crashworthy components. Understanding this particular failure mode has a very high significance in the health and integrity of automotive and aircraft structures. Various researches have studied the energy absorption capability of composite tubular structures. The most recent works are reviewed as follow.

Mahdi and Hamouda [_{3} glass/polyester, [0 ± 75]_{2} glass/epoxy, [±15]_{3} glass/polyester and [±15]_{3} glass/ epoxy under off-axis loadings to examine how the ply pattern and off-axis angle affected the energy absorption performances. More recently, Othman and Jailani [

In previous studies the off-axis crushing process of tubular structures was studied based on the various parameters related to geometry, laminate design and loading conditions. However, the effective interwall crack propagation namely controllable crack propagation is another factor which can significantly influence the energy absorption capability in off-axis loading. This research studied a controlling process to reinforce the laminate through the thickness using natural fibre yarns to improve the crack resistance during the crushing process known as effective crack growth.

As it was discussed before the off-axis angle of 10˚ showed the highest energy absorption capability in comparison with other crushing angles. In this regard, this angle was chosen in this work to investigate the failure mechanisms of stitched composite plates involved in off-axis crushing process. Details of manufacturing and specimen preparation are explained in detail in [

the adjustable bolt. The whole clamped fixture together with the gripped specimen was placed on the lower crushing plane testing machine and was tilted to a predetermined off axis angle with a certain spacer load cell at a vertical cross-head crushing rate of 2 mm/min (see

In contrast with axial crushing load the off-axis failure is non-symmetrically process. Load increased initially to the value of 37kN at the displacement of 5mm (see

In previous work of authors [_{m}, in off-axis crushing of square composite box is obtained in two levels of triggering stage and sustained crushing stage. The external work of crushing platen is dissipated by mechanisms of friction, bending, fracture and shear deformation. The idealised crush zone for our analytical model is shown in

The energy dissipated by friction between crushing platen and the debris inside the internal and external fronds in brittle fracture crushing mode is assumed into two stages, first is related to triggering stage and second one is related to sustained crushing stage and it can be obtained from,

where

The energy dissipated by frond bending was calculated by assuming that the whole cross section of the frond will reach to the flexural bending stiffness, s_{b}.

The flexural strength, s_{b}, for each lay-ups of CFRP was measured σ_{b} = 570 MPa from 3 PB experiment. The energy dissipated in bending for brittle fracture crushing mode at stationary hinge lines is

where b = width of box and t = thickness of wall,

The energy dissipated by interlaminar crack propagation in Mixed-Mode I/II is calculated from interlaminar fracture energies. In brittle fracture mode there is one interwall Mixed-Mode I/II crack at sides of the box as shown in

The Mixed Mode I/II interlaminar fracture toughness is measured experimentally by ADCB test. Dissipated energy by axial splitting, shear deformation and bundle fracture for brittle fracture crushing mode are calculated as follow:

where E_{z} is Young’s modulus in axial direction of the crush box.

where

The energy balance for the brittle fracture crushing process during a single stroke crush distance is

Substituting from Equations (1) to (6) in Equation (8), the energy dissipated by brittle fracture crushing mode, UBF, is

Therefore, the mean oscillatory crushing force in a stable brittle fracture progressive crush is:

A single stroke crush distance for brittle fracture mode can be found from Equation (11) by setting

The energy balance for the transverse shearing crushing process during a single stroke crush distance is

Substituting from Equations (1), (2), (3), (5), (6) and (7) in Equation (12), the energy dissipated by transverse shearing crushing mode, UTS, is

Furthermore, the mean oscillatory crushing force in a stable transverse shearing progressive crush is:

A single stroke crush distance for transverse shearing mode can be found from Equation (15) by setting

The mean force during the progressive crushing can be found by calculating l from Equations (11) and (15) substituting in Equations (10) and (14) (see

Off-axis specimen | Cushing failure mode | F_{m}(Exp.) kN | SEA (Exp.) kJ/kg | F_{m}(Anal.) kN | SEA (Anal.) kJ/kg | Error (%) |
---|---|---|---|---|---|---|

Stitched_{ } | Transverse Shearing (TS) | 57 | 12.5 | 54 | 11.8 | 5 |

Non-stitched_{ } | Brittle Fracture (BF) | 118 | 25.8 | 116 | 25.4 | 1.5 |

In this paper experimental analysis was performed to study the effect of stitching on controllable crack growth in off-axis crushing behaviour and energy absorption of carbon/epoxy twill-weave composite box composite. The two types of specimens, stitched and non-stitched composite boxes, were subjected to quasi-static off-axial compression load. Two fracture mechanisms of bundle fracture and interlaminar crack propagation in Mixed- Mode I/II were found for all composite boxes at off-axis crushing process. Owing to crack propagation in Mixed-Mode I/II and more friction and bending resistance at one side of the composite box, which first contacted the crushing platen, the amount of SEA at off-axis loading of 10˚ was higher than the equivalent value in axial loading. However, the CFE of non-stitched composite boxes at off-axis loading is higher than the CFE of stitched boxes. It was observed that in off-axis loading the energy absorption of stitched boxes was less than the energy absorption of non-stitched boxes. The main problem was related to the position of stitched area in the box wall. It is assumed that lower position from top of the box could allow all sides of box firstly get involved in crush process, and then the stitched area increases the load resistance and consequently energy absorption capabilities. Various mechanisms based on corner fracture, interlaminar fracture, friction, bending and shear deformation were performed to predict the off-axis crushing mean force and CFE of composite absorbers. The analytical method was also carried out to study the mechanisms involved in crushing process which could show good agreement with the relevant experimental results.

N.Ghafari-Namini,H.Ghasemnejad, (2015) Controllable Crack Propagation in Off-Axis Crushing of Stitched Composite Absorbers. Open Journal of Composite Materials,05,93-100. doi: 10.4236/ojcm.2015.54012