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When a test fails to meet the thickness and other test requirements that are in place to ensure plane strain conditions, the fracture toughness value produced is given the designation . Fracture toughness is a quantitative way of expressing a material's resistance to crack propagation and standard values for a given material are generally ...
The mode I fracture toughness for plane strain is defined as K I c = Y σ c π a {\displaystyle K_{\rm {Ic}}=Y\sigma _{c}{\sqrt {\pi a}}} where σ c {\displaystyle \sigma _{c}} is a critical value of the far field stress and Y {\displaystyle Y} is a dimensionless factor that depends on the geometry, material properties, and loading condition.
The advantages of the Watts-Ford test are that it is convenient for testing thin sheets or strips, it is similar to a rolling process (in manufacturing analyses), frictional effects may be minimized, there is no 'barrelling' as would occur in a cylindrical compression test, and the plane strain deformation eases the analysis. Stress-strain curve
This critical value determined for mode I loading in plane strain is referred to as the critical fracture toughness of the material. K I c {\displaystyle K_{\mathrm {Ic} }} has units of stress times the root of a distance (e.g. MN/m 3/2 ).
The major surface strain has a minimum value when plane strain deformation occurs, which means that the corresponding minor surface strain is zero. Forming limits are a specific material property. Typical plane strain values range from 10% for high-strength grades and 50% or above for mild-strength materials and those with very good formability.
This new material property was given the name fracture toughness and designated G Ic. Today, it is the critical stress intensity factor K Ic, found in the plane strain condition, which is accepted as the defining property in linear elastic fracture mechanics.
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The stress intensity factor at the crack tip of a compact tension specimen is [4] = [() / / + / / + /] where is the applied load, is the thickness of the specimen, is the crack length, and is the effective width of the specimen being the distance between the centreline of the holes and the backface of the coupon.