Manufacturing influence on the delamination fracture behavior of the T800H/3900-2 carbon fiber reinforced polymer composites

'Torayca' T800H/3900-2 is the first material qualified on Boeing Material Specification (BMS 8-276) which utilizes the thermoplastic-particulate interlayer toughening technology. Two manufacturing processes, the autoclave process and the fast heating rated Quickstep™ process, were employed to cure this material. The Quickstep process is a unique composite production technology which utilizes the fast heat transfer rate of fluid to heat and cure polymer composite components. The manufacturing influence on the mode I delamination fracture toughness of laminates was investigated by performing double cantilever beam tests. The composite specimens fabricated by two processes exhibited dissimilar delamination resistance curves (R-curves) under mode I loading. The initial value of fracture toughness GIC-INIT was 564 J/m2 for the autoclave specimens and 527 J/m2 for the Quickstep specimens. However, the average propagation fracture toughness GIC-PROP was 783 J/m2 for the Quickstep specimens, which was 2.6 times of that for the autoclave specimens. The mechanism of fracture occurred during delamination was studied under scanning electron microscope (SEM). Three types of fracture were observed: the interlayer fracture, the interface fracture, and the intralaminar fracture. These three types of fracture played different roles in affecting the delamination resistance curves during the crack growth. More fiber bridging was found in the process of delamination for the Quickstep specimens. Better fiber/matrix adhesion was found in the Quickstep specimens by conducting indentation-debond tests.

Effect of susceptibility to interfacial fracture on fatigue properties of spot-welded high strength sheet steel

While advanced high strength steels (AHSS) have numerous advantages for the automotive industry, they can be susceptible to interfacial fracture when spot-welded. In this study, the susceptibility of interfacial fracture to spot-weld microstructure and hardness is examined, as well as the corresponding relationships between fatigue, overload performance, and interfacial fracture for a TRIP (transformation induced plasticity) steel. Simple post-weld heat-treatments were used to alter the weld microstructure. The effect on interfacial fracture of diluting the weld pool by welding the TRIP material to non-TRIP steel was examined, along with the effect of altering the base material microstructure. Results show that weld hardness is not a good indicator of either the susceptibility to interfacial fracture, or the strength of the joint, and that interfacial fracture does not necessarily lead to a decrease in strength compared to conventional weld-failure mechanisms, i.e. button pullout. It was also found that while interfacial fracture does affect low cycle to failure behavior, there was no effect on high cycle fatigue.

The effect of osteoporotic vertebral fracture of predicted spinal loads in vivo

he aetiology of osteoporotic vertebral fractures is multi-factorial, and cannot be explained solely by low bone mass. After sustaining an initial vertebral fracture, the risk of subsequent fracture increases greatly. Examination of physiologic loads imposed on vertebral bodies may help to explain a mechanism underlying this fracture cascade. This study tested the hypothesis that model-derived segmental vertebral loading is greater in individuals who have sustained an osteoporotic vertebral fracture compared to those with osteoporosis and no history of fracture. Flexion moments, and compression and shear loads were calculated from T2 to L5 in 12 participants with fractures (66.4 ± 6.4 years, 162.2 ± 5.1 cm, 69.1 ± 11.2 kg) and 19 without fractures (62.9 ± 7.9 years, 158.3 ± 4.4 cm, 59.3 ± 8.9 kg) while standing. Static analysis was used to solve gravitational loads while muscle-derived forces were calculated using a detailed trunk muscle model driven by optimization with a cost function set to minimise muscle fatigue. Least squares regression was used to derive polynomial functions to describe normalised load profiles. Regression co-efficients were compared between groups to examine differences in loading profiles. Loading at the fractured level, and at one level above and below, were also compared between groups. The fracture group had significantly greater normalised compression (p = 0.0008) and shear force (p < 0.0001) profiles and a trend for a greater flexion moment profile. At the level of fracture, a significantly greater flexion moment (p = 0.001) and shear force (p < 0.001) was observed in the fracture group. A greater flexion moment (p = 0.003) and compression force (p = 0.007) one level below the fracture, and a greater flexion moment (p = 0.002) and shear force (p = 0.002) one level above the fracture was observed in the fracture group. The differences observed in multi-level spinal loading between the groups may explain a mechanism for increased risk of subsequent vertebral fractures. Interventions aimed at restoring vertebral morphology or reduce thoracic curvature may assist in normalising spine load profiles.

Manufacturing process effects on the mode I interlaminar fracture toughness and nanocreep properties of CFRP

Quickstep ™ is a fluid filled floating mould technology which was recently developed by an Australian company of the same name. The Quickstep and conventional autoclave manufacture of composites were compared by investigating the mode I interlaminar fracture toughness and nanocreep propeties of HexPly914 carbon epoxy composites. It was found that composites cured using the Quickstep technology had significantly higher fracture toughness (1.8 times) than the composites cured via autoclave for this system. DMTA (dynamic mechanical thermal analysis) results showed a higher Tg (glass transition temperature) for the material manufactured by the Quickstep than that cured by the autoclave. FTIR (Fourier transform infrared spectroscopy) spectra did not indicate any difference in cure chemistry between the two processes. Nanocreep experiments were performed to explore the viscoelastic properties of the epoxy matrix of composites. The KelvinVoigt three-element model was applied to analyse the indentation creep behaviour of both composites.

Characteristics of the contraction twins formed close to the fracture surface in Mg-3A1-1Zn alloy deformed in tension

Characteristics of the “contraction” twins, formed close to the fracture surface in Mg–3Al–1Zn alloy deformed in tension approximately perpendicular to the grain c-axes, are investigated using transmission electron microscopy. The grain c-axis contractions were largely accommodated by {1011}-{1012} source double-twins in a variant characterized by 38° ⟨1210⟩ source twin/matrix misorientation in conjunction with dislocation slip. A possible interpretation of the observed preference for this variant formation is given and some crystal plasticity modelling is performed to elucidate the matter.

Experimental and numerical evaluation of forming and fracture behaviour of high strength steel

Car manufacturers are under pressure to reduce vehicle mass while maintaining comfort and passenger safety for current and future vehicles. To meet this demand the steel industry has developed Advanced High Strength Steels (AHSS) that promise higher strength and improved formability compared to conventional steel grades. Even though significant research has already been performed to evaluate the material properties and forming behaviour of most AHSS types, only a limited literature is available on their necking and fracture behaviour and the effect on formability. This paper examines and compares the thinning, necking and fracture behaviour of two AHSS and one conventional steel type, namely TRIP, DP and HSLA. Uniaxial, plane and biaxial strain conditions are investigated by tensile, cup drawing and stretch forming tests and by using numerical methods. The test results indicate that significant differences exist in necking and fracture behaviour between all three steel types.

Influence of helium atmospheric plasma treatment on surface and fracture-mechanical behaviour of quickstepTM cured bio-composites

This work investigated the potential of improving flexural properties of natural fiber (jute) reinforced biocomposites by atmospheric pressure helium plasma treatment. Composites were made by the use of combined hand lay-up and vacuum bagging technique followed by newly developed Australia patented Quickstep^TM curing. The physical properties of helium plasma modified fibers were investigated by means of wettability time, coefficient of friction (COF), atomic force microscopy (AFM) and chemical nature of the surface with ATR-FTIR and XPS. There was found a logical correlation between physical and chemical characteristics of the surface of fiber with the fracture mechanical behavior of their resulting biocomposites. In addition, the use of helium atmospheric plasma treatment prior to Quickstep^TM process has proved to be a potential way to positively alter the fracture-mechanical behavior of biocomposites. This study will lead to new commercial applications of natural fiber jute for the composite industry that go beyond wrapping and packaging.

Fractal analysis of bone texture : a screening tool for stress fracture risk?

Background The aim of this study was to identify specific bone characteristics of stress fracture (SF) cases in sportswomen. To date, no tool is able to distinguish individuals who are at risk, limiting preventive measures.

Material and methods We investigated the skeletal system of sportswomen who did sustain SF in the past (n = 19) and compared it with that of female controls (C) with a similar sporting history but without any fracture history (n = 20).

Bone mass and body composition were measured using dual-energy X-ray absorptiometry. Bone micro-architecture was investigated by calcaneal ultrasound and fractal analysis of calcaneus radiographic images. Oestradiol levels were measured by E.I.A, and IGF-1 by R.I.A. Menstrual characteristics, nutrient intake, and training were assessed using questionnaires.

Results The result of the fractal analysis, expressed by the Hmean parameter, was significantly lower in the SF group, reflecting a more complex structure of the trabecular micro-architectural organization (P < 0·005). Body mass index (BMI) at birth was also found to be lower in the SF cases as compared with their C (P < 0·03).

Multivariate analysis revealed that the fractal parameter Hmean, bone mineral content (BMC) at Ward's triangle and the BMI at birth correctly assigned 84·85% of the female athletes into their respective SF or C groups (P = 0·001).

Conclusion These results suggest that the fractal parameter and the BMI at birth may be able to identify female athletes most at risk for this overuse bone injury, as their low indexes might reflect a greater skeletal sensitivity.

Fracture analysis of spot-welds with edge cracks using 2-D hybrid special finite element

This paper employed a systematic analysis using a 2-D hybrid special finite element containing an edge crack in order to describe the fracture behavior of spot-welds in automotive structures. The 2-D hybrid special finite element is derived form a mixed formulation with a complex potential function with the description of the singularity of a stress field. The hybrid special finite element containing an edge crack can give a better description of its singularity with only one hybrid element surrounding one crack. The advantage of this special element is that it can greatly simplify the numerical modeling of the spot welds. Some numerical examples demonstrate the validity and versatility of the present analysis method. The lap-shear, lap-tension and angle-clip specimens are analyzed and some useful fracture parameters such as the stress intensity factor and the initial direction of crack growth are obtained simultaneously.

Prediction of ductile fracture in the cold forging of steel

The thesis describes the development of a model of the cold forging process that accurately predicts the initiation of ductile fracture. The effect of the deformation history of the material during multiple forming operations is considered. Both two- and three-dimensional numerical models of the forging process were combined with a ductile fracture criterion to predict the material ductility and damage distribution in the workpiece.

Fracture analysis of spherical shells : application to cracking of macadamia nuts

The main consideration in recovering the macadamia kernal is to crack the spherical nutshell without damaging the kernal. Five mechanical cracking tools were tested, and the fracture mechanisms of nutshells, under various cracking loads, were studied. A classical theoretical approach and a numerical method were both used to investigate the influence of crack face closure on the stress intensity factor for a cracked spherical shell subjected to membrane forces and bending moments.