Experimental investigations of the influence of material and thickness on fracture under pure mode II loading

Experimental investigation to the effects of thickness and material on mode II fracture were performed. Tension-shear specimens made of aluminium alloy LC4CS and 7050-T7452 with thicknesses of 2, 4, 8 and 14 mm were used. All crack tip appearances and fracture profiles of the specimens were observed. Mode II fracture toughness were calculated. It is shown that material and thickness play an important role in mode II fracture. The fracture of LC4CS appears shear fracture under all kinds of thicknesses, however the fracture of 7050-T7452 is tensile fracture when thickness is larger or equal to 8mm, and shear initiation along the original crack plane, then turnaround and tensile failure when thickness is smaller than 8mm. Mode II fracture toughness is independent of thickness.


Introduction
It is well known that under mode I loading condition the specimen thickness have strong influence on the crack tip fields as well as fracture toughness of materials [1][2][3].However, the literature still lacks systematic studies of the relation between mode II fracture and specimen thickness.The aim of the present investigation is to study the effect of material and thickness on fracture under mode II loading by means of fracture experiments of various materials and thicknesses.a e-mail: huirudong@263.net 2 Experimental procedures

Specimen
Two kinds of high-strength aircraft structural aluminum alloy LC4-CS and 7050-T7452 were used in this study.Compact tension (CT) specimens were chosen for all fracture tests.The thickness range of specimens chosen for this research included 2, 4, 8, and 14 mm, reflecting various three-dimensional constraint of crack tip.Initial 2mm wide V-rooted notches in the CT specimens were mechanically cut.Before the fracture tests, fatigue precracks were produced under constant amplitude loading with stress ratio R slightly above zero.Fatigue precrack extensions in all the specimens are greater than 1.5mm to eliminate the influence of the V-rooted notches.The shape of a specimen is shown as Figure 1.

Experimental fixture and procedures
The special loading grip jaw designed by Richard [4] was used.The specimen was fixed by three bolts at each end to the three large inner holes of each grip in the manner shown in Figure 2.
Loadings were brought to bear on by means of the outer seven pairs of holes, 1-1' to 7-7'.The seven pairs of holes divide equally the 90°angle.Pure II loading was carried out using the No. 7 and 7′ holes, and Loading angle φ, the angle between the load line and the initial crack plane, is 0 degree.through the digital image acquisition system and into a computer for storage.The fracture morphologies were examined by scanning electron microscope HITACHI S-2700.Figure 3 shows the experimental procedures.

The effect of thickness on fracture
Fracture of LC4CS appears shear fracture under all kinds of thickness conditions.Fracture of 7050-T7452 is tensile fracture when thickness is larger or equal to 8mm, and shear initiation along the original crack plane, then turnaround and tensile failure when thickness is smaller than 8mm.The results are summarized in Table 1.
Pure II Shear fracture (shear fracture) Tensile fracture Shear initiation Tensile fracture Table 1.The effect of material and thickness on fracture type

The thickness effect on fracture toughness
For the CTS specimens used in this researches, mode II stress intensity factor KII is given by [4] The variation of KII with thickness under crack initiation loads is shown in Figure 6 for 7050-16008-p.4 14 th International Conference on Experimental Mechanics T7452.The curve reveals that mode II critical stress intensity factor KIIC does not undergo any significant with increasing thickness B.

Discussions
In LC4CS, yielding occurs at the crack tip and Linear Elastic Fracture Mechanics (LEFM) is no longer applicable.An elastic-plastic fracture toughness J-integral should be used.However, the load-COD curves of LC4CS under pure II condition could not be obtained for the COD clip-gauge dropping, J-integral of LC4CS was incapable of determining.Fortunately, researches of the thickness effect on JII were found in reference [5].In reference [5], mode II tests were carried out on an aircraft aluminium alloy, Al 7075-T7351; a series of specimens of six thicknesses between 5 and 16mm are employed for testing.A rigid plastic analysis and elastic-plastic finite element analysis were employed to show that for non dimensional crack length a/W of about 0.5 A is area under a load-crack sliding displacement record.B is specimen thickness, and (w-a) is uncracked ligament.
The measured JII values are shown in Table 2.It would appear that JIIC did not vary with thickness.
16008-p.5 The results in the paper and in references show that mode II fracture toughness is independent of thickness.

Conclusions
Pure mode II loading fracture experiments were made by various thickness CTS specimens made of aircraft structural aluminum alloy LC4CS and 7050-T7452.The effects of material and thickness on pure mode II fracture were analyzed.It is shown that material and thickness play an important role in mode II fracture.The fracture of LC4CS appears shear fracture under all kinds of thicknesses, however the fracture of 7050-T7452 is tensile fracture when thickness is larger or equal to 8mm, and shear initiation along the original crack plane, then turnaround and tensile failure when thickness is smaller than 8mm.Thickness has no effect on mode II fracture toughness.

Table . 2
J IIC values for different specimen thicknesses B