Surface quality effect

The surface quality effect can be switched on for the LESA method. There are 4 options how to cope with it in the Calculation Methods menu:

If no other influence is active, the surface quality influence factor FS modifies the actual fatigue strength:

The solution is described in [ECS4.4]. The surface quality factor is FS = 1, if the evaluated material is of the cast-iron type. If the tensile strength Su is less than 1200 MPa, the formula proposed by Siebel and Gaier [SG56] is used:

There is no suggestion concerning materials with higher tensile strength, the last equation is thus used in PragTic even behind this limit.

The surface roughness Rz,C of the relevant node/element/isolated point is defined in the Properties of Structure dialogue. The default setting valid for any new FE-model or set of isolated points is the value Rz,C = 1. The Rz,S surface roughness is related to the specimen, for which the appropriate S-N curve was derived.

The solution adopted here utilizes the proposal defined in [SMB03]. The number of defined categories is much lesser - see the table below. Since the intersection with the other two definitions is rather small, the use of the roughness defined in the Properties of Structure dialogue is not applied here. The user defines the appropriate surface finish as valid for the whole component in the Calculation Methods menu.

Surface finish	aS	bS
ground	1.58	-0.085
machined or cold-drawn	4.51	-0.265
hot-rolled	57.7	-0.718
as-forged	272	-0.995

Note: The non-detailed characterization of the surface allows only very small credibility in this method. It should be understood as being here only for study and research reasons.

The solution is described in [FKM03], here only its transcription is done. The surface quality influence is related to an influence acting on the normal stress. The torsion variant is also given there. Because of the solution used in LESA works with an equivalent tensile stress, only this variant is used in PragTic:

The material variables C1 and C2 are defined in dependency of the material type:

class of material	C1 [-]	C2 [MPa]
steels	0.22	400
cast steels	0.2	400
nodular cast iron	0.16	400
malleable cast iron	0.12	350
gray cast iron	0.06	100
wrought aluminum alloys	0.22	133
cast aluminum alloys	0.20	133

The values given in the table above are hard-coded to the material classes in PragTic, but the user can change these default settings in the Material dialogue. The roughness of the examined structure Rz,C is defined in the Properties of Structure dialogue, whereas the roughness of the specimen Rz,S used for the S-N curve derivation is defined also within the Material dialogue.