US20030203178A1

US20030203178A1 - Toughened, crack resistant fiber reinforced composite article and method for making

Info

Publication number: US20030203178A1
Application number: US09/907,981
Authority: US
Inventors: John Ravenhall; Bruce Busbey; Jack Baldwin
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2001-07-18
Filing date: 2001-07-18
Publication date: 2003-10-30

Abstract

A composite article, for example a blading member of a gas turbine engine, comprising a plurality of stacked layers of reinforcing fibers bonded together with a matrix resin is provided with enhanced resistance to impact cracking, material loss and/or delamination though use of a matrix resin including properties comprising a tensile strain property of at least 5% and a K_1ctoughness of at least about 850 psi·inch^1/2. A method for making such a composite article with such resin comprises providing the layers of reinforcing fibers in a substantially dry, unimpregnated condition. The dry layers are stacked as a preform in a mold cavity and impregnated with the resin to wet and impregnate the dry layers of the preform. Then the resin is cured as a matrix about the fibers and the stacked layers.

Description

BACKGROUND OF THE INVENTION

This invention relates to a fiber reinforced composite article and its manufacture, and more particularly, to a fiber reinforced composite blading member including an airfoil, for example a turbine engine fan or compressor blade.

Components for sections of gas turbine engines, for example a fan and/or a compressor, operating at relatively lower temperatures than sections downstream of the combustion section have been made of resin matrix composites including stacked, laminated plies. Generally such primarily non-metallic composite structures, which replaced heavier predominantly metal structures, include superimposed layers, sometimes called plies, reinforced with fibers, which include filaments in its meaning, in a variety of configurations and lay-up directions, sometimes about a core and/or with local metal reinforcement or surface shielding. For elevated temperature applications, a variety of materials are used for such fibers, including carbon, graphite, glass, boron, etc., as is well known in the art. Typical examples of such components made primarily of non-metallic composites are reported in such U.S. Pat. Nos. 3,892,612—Carlson et al. (patented Jul. 1, 1975); 4,022,547—Stanley (patented May 10, 1977); 5,279,892—Baldwin et al. (patented Jan. 18, 1994); 5,308,228—Benoit et al. (patented May 3, 1994); and 5,375,978—Evans et al. (patented Dec. 27, 1994).

As has been discussed in detail in such patents as the above-identified Evans et al. patent, such non-metallic composites in an aircraft gas turbine engine are subject to damage from ingestion into the engine and impact on components of foreign objects. Such objects can be airborne or drawn into the engine inlet. These include various types and sizes of birds as well as inanimate objects such as hailstones, sand, land ice, and runway debris. Impact damage to the airfoil of blading members, including fan and compressor blades, as well as damage to strut type members in the air stream, has been observed to cause loss of material and/or delamination of the stacked layers. Such a condition in a rotating blade can cause the engine to become unbalanced resulting in potentially severe, detrimental vibration.

The above identified and other prior art have reported various arrangements and structures to avoid such material loss and/or delamination of layers. Some arrangements, for example U.S. Pat. No. 3,834,832—Mallinder et al. (patented Sep. 10, 1974) and the above-identified Benoit et al. patent, include use of transverse seams or fastening devices to avoid delamination of laminated composite structures using ordinary commercial resin systems as the composite matrix.

BRIEF SUMMARY OF THE INVENTION

The present invention, in one form, provides a composite article comprising a plurality of stacked layers of reinforcing fibers, the layers bonded together with a matrix resin. The article is toughened and has enhanced resistance to cracking and layer delamination as a result of the matrix resin including properties comprising a tensile strain property of greater than 5% and a K _1ctoughness of at least about 850 psi·inch^1/2.

In another form, the present invention provides a method for making such a composite article comprising providing the plurality of layers of reinforcing fibers in substantially dry form, not impregnated with a partially cured matrix resin. Sometimes such a partially cured member is referred to as being in the “green state” or as a “prepreg”. According to a form of the present invention, the dry layers are stacked one upon another into a dry preform shape that then is impregnated with the above defined matrix resin.

DETAILED DESCRIPTION OF THE INVENTION

Use of the above-defined toughened, high tensile strain resin as the matrix of a fiber reinforced composite article comprising stacked layers of reinforcing fibers, preferably aligned, provides an article resistant to cracking, crack propagation, and layer delamination. Such resin matrix obviates the need to provide transverse reinforcement for the purpose of resisting such delamination. [0007]
The method step, in a form of the present invention, of stacking dry rather than prepreg layers into a dry preform for subsequent impregnation provides the ability to pre-inspect the dry preforms for potentially detrimental ply assembly conditions such as wrinkles. Inspection early in the manufacturing method and prior to matrix impregnation and curing enables defect correction such as by repositioning, redraping and/or smoothing of individual dry layers without damaging previously placed layers. The known step of stacking of prepreg type of layers and then curing the stacked layers, as currently is common in the art, enables detection of such defects as wrinkles only after resin matrix curing at which point the defect is fixed in the article. Thus the method of the present invention, enabling such early defect detection and correction capability, improves the yield and reduces the cost of manufacturing a fiber reinforced resin matrix laminated composite article. [0008]
In addition, dry layers of reinforcing fibers used in a form of the method of the present invention does not require use of layer backing sheets or papers commonly used with individual prepreg layers. Currently, such sheets or papers used to maintain separation of individual, tacky prepreg layers prior to assembly must be removed as stacking is conducted. [0009]
The fibers defined herein to be in a dry condition are not impregnated with a matrix resin. However, in some examples, the individual fibers include a lightly tacky surface material to enable the individual fibers to maintain a position or relationship with adjoining fibers in a layer. However, such lightly tacky condition on the fibers is insufficient to bind individually stacked layers together, thereby enabling the above described defect correction in the dry preform, prior to matrix resin impregnation of the dry stacked layers. [0010]
During one series of evaluations of the present invention, heat curable epoxy forms of the above-defined type of toughened resin, included in forms of the present invention, was compared with other commercially available epoxy resin systems currently used in the art as matrix resins for laminated composite articles. Such a comparison was made with the matrix resin cured in stacked layers of aligned reinforcing fibers. Currently used cured resin systems, one example of which commercially is available as Dow Chemical TACTIX 123 epoxy resin system, have a tensile strain property of about 5% or less, for example in the range of about 2-5%, in combination with a K[0011] _1ctoughness of less than about 850 psi·inch^1/2, for example in the range of about 400-500 psi-inch^1/2. These properties both are typical of currently used epoxy resin systems. It was recognized that use of such current resin systems in certain applications resulted in insufficient resistance to impact damage and delamination caused by ingestion of such objects as birds, hail, and land ice into the engine. For example, one type of test impacted the cured laminated structure with an object equivalent to a 2.5 pound bird. The lower tensile strain of current systems was insufficient to resist impact damage or loss of material, and the lower toughness level provided too low a threshold at which cracking and layer delamination could be initiated. This lower threshold results in unacceptable component matrix loss causing performance and balance conditions detrimental to the engine. In contrast according to the present invention, a matrix, for stacked reinforcing layers, of a cured epoxy resin system with a tensile strain property of greater than 5%, for example about 7% or more, in combination with a toughness K_1cof at least about 850 psi·inch^1/2, provided resistance to degradation, such as material damage and delamination, from such an impact.
In one specific evaluation series, a plurality of shaped layers of dry, substantially unidirectional carbon fiber bundles, commercially available as IM-7 12K tow tape from Hexel Company, were disposed in a stack as layers of a preform in the cavity of a commercial resin transfer mold, with typical amounts of intermediate wicking felt as used in the art. The mold cavity was in the shape of a turbine engine blade including an airfoil. The preform and its layers were inspected prior to injection of the resin matrix and any detrimental wrinkles were removed by redraping or smoothing the individual layers while the preform was in the dry state. [0012]
After closing the cavity of the transfer mold, which included typical resin ports and vents, a vacuum was provided in the cavity to remove ambient air from the cavity and from about the preform. Then the above-defined type of high strain, toughened epoxy resin, identified as 3M PR520 epoxy resin system, was injected into the mold cavity about the stacked layers, wetting and saturating the preform with the resin. Properties of that resin system included a tensile strain of about 6.9% and a toughness K[0013] _1cof about 1380 psi·inch^1/2. The resin was injected into the evacuated mold cavity under a pressure in the range of about 25-100 psi.
After injecting the resin, the mold and its contents were heated in the range of about 350-400° F. for about 90-120 minutes to cure the resin into a matrix about the fibers into a carbon/epoxy composite blade. After curing, the mold and its contents were cooled. When removed from the mold cavity, the resulting article was a near net shape molded epoxy resin carbon fiber reinforced composite blade including a molded airfoil and molded base. The co-cured fiber reinforced composite blade using the above-defined high tensile strain, toughened epoxy resin as a matrix provided improved impact capability at the point of impact as well as away from the impact site while retaining blade spanwise and chordwise directional strength capability. [0014]
In one evaluation to demonstrate the benefit of the present invention, two series of geometrically identical flat panels were prepared in the above-described manner. A first series of panels was made with an ordinary epoxy resin system having a K[0015] _1ctensile toughness of 800 psi·inch^1/2. A second series of panels was made, according to a form of the present invention, with the above-described high strain, toughened epoxy resin system having a K_1ctensile toughness of 1380 psi·inch^1/2. Each series was tested using 2″ diameter reinforced hailstones shot at a velocity of 680 feet per second to impact each panel at an angle to the panel surface. As a result of such testing, the first series of panels, made with the low tensile toughness resin, exhibited significant through panel material loss and extensive delamination. The second series of panels, made with the high strain, toughened epoxy resin exhibited no material loss and minimal point of impact delamination.
Embodiments of the present invention, including a method for making, provide a tough, impact and delamination resistant fiber reinforced resin matrix composite article, for example a blading member of a gas turbine engine. Resin properties are tailored to provide high impact resistance while maintaining, for example in an airfoil, spanwise properties to resist centrifugal and bending loads as well as chordwise properties to resist gas and torsional bending loads. This combination of enhanced properties improves the overall operating capability of the article. The method form of the present invention improves the production cycle of the article by substantially eliminating yield problems associated with wrinkles which have been seen to occur in stacked prepreg layers of composite materials. [0016]
The present invention has been described in connection with specific examples and combinations of materials and structures. However, it should be understood that they are intended to be typical of rather than in any way limiting on the scope of the invention. Those skilled in the various arts involved, for example technology relating to gas turbine engines, to fiber reinforced composite structures, fibers and resins, etc, will understand that the invention is capable of variations and modifications without departing from the scope of the appended claims. [0017]

Claims

What is claimed is:

1. A composite article comprising a plurality of stacked layers of reinforcing fibers, the layers bonded together with a matrix resin, wherein:

the matrix resin includes properties comprising a tensile strain property of greater than 5% and a K_1ctoughness of at least about 850 psi·inch^1/2.

2. The article of claim 1 in which the reinforcing fibers in a stacked layer are substantially aligned with one another.

3. The article of claim 2 in which:

each of the stacked layers predominantly includes substantially aligned reinforcing fibers; and,

the matrix resin is an injectable epoxy resin.

4. The article of claim 3 in which the fibers comprise at least one selected from the group consisting of carbon, graphite, glass, and boron fibers.

5. The article of claim 1 in the form of a turbine engine blading member including an airfoil in which at least the airfoil comprises a plurality of shaped, stacked layers of reinforcing fibers impregnated and bonded together with the matrix resin.

6. The blading member of claim 5 in which the reinforcing fibers in a stacked layer are substantially aligned with one another.

7. The article of claim 5 in which:

the fibers comprise at least one selected form the group consisting of carbon, graphite, glass, and boron fibers; and,

the resin is an injectable epoxy resin.

8. A method for making a composite article comprising a plurality of stacked layers of reinforcing fibers bonded together with a matrix resin comprising the steps of:

providing a plurality of layers of substantially dry, unimpregnated reinforcing fibers,

stacking the layers one upon another into a preform shape; and,

impregnating the preform shape with a resin that includes properties comprising a tensile capacity of greater than 5% and a K_1ctoughness of at least about 850 psi·inch^1/2.

9. The method of claim 8 in which the reinforcing fibers in a stacked layer substantially are aligned with one another.

10. The method of claim 8 in which:

the layers of the preform shape are stacked in a cavity of a mold;

the mold cavity is closed;

a vacuum is provided within the mold cavity about the layers of the preform shape;

the matrix resin is provided in the cavity about the fibers and the layers to wet and impregnate the layers and fibers; and,

the resin is cured about the fibers and layers.

11. The method of claim 10 in which:

the resin is an injectable epoxy;

after providing the vacuum in the mold cavity, the resin is injected into the mold cavity under a pressure in the range of about 25-100 psi; and,

the resin is cured at a temperature in the range of about 350-400° F.